Safe Design and Operation of Process Vents and Emission Control Systems

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Safe Design and Operation of Process Vents and Emission Control Systems

Safe Design and Operation of Process Vents and Emission Control Systems

Center for Chemical Process Safety of the American Institute of Chemical Engineers

CCPS

CENTER FOR CHEMICAL PROCESS SAFETY

An AlChE industry Technology Alliance

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright 0 2006 by John Wiley &. Sons, Inc. All rights reserved. A joint publication of the Center for Chemical Process Safety of the American Institute of Chemical Engineers and John Wiley & Sons, Inc. Published by John Wiley &. Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted i n any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 ofthe 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 1 I 1 River Street, Hoboken, NJ 07030, (201) 748-601 1, fax (201) 748-6008, or online at http://www.wiley.comlgo/permisc;ion. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neilher the publisher nor author shall be liable for any loss of profit or any other commercial damages,, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic format. For information about Wiley products, visit our web'site at www.wiley.com. Library of Congress Cataloging-in-Publication Data:

Safe design and operation of process vents and emission control systems i Center for Chemical Process Safety, p. cm. Includes index. ISBN-13: 978-0-471-79296-3 (cloth) ISBN-10: 0-471-79296-9 (cloth) 1. Chemical industry-Fume control. 2. Chemical plants-Heating and ventilation-Safety measures. 3. Chemical industry-Fires and fire prevention. 4. Air-Pollution. 5. Chemical plants-Piping. 6. Air ducts-Design and construction. 1. American Institute of Chemical Engineers Center for Chemical Process Safety. TH7684.C44.S24 2006 660'.2804-&22 2005033607 Printed in the United States of America. I 0 9 8 7 6 5 4 3 2 1

DISCLAIMER It is sincerely hoped that the information presented in this document will lead to an even more impressive safety record for the entire industry; however, neither the American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Committee and Subcommittee members, their employers, their employers' officers and directors, nor [Contractor name] and its employees warrant or represent, expressly or by implication, the correctness or accuracy of the content of the information presented in this document. As between (1) American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Committee and Subcommittee members, their employers, their employers' officers and directors, and Risk, Reliability and Safety Engineering, LLC, and its employees, and (2) the user of this document, the user accepts any legal liability or responsibility whatsoever for the consequence of its use or misuse.

V

CONTENTS Preface Acknowledgment

XIV

xv

Introduction 1.1 1.2 1.3 1.4 1.5 1.6

Objective Relationship to Other CCPS Publications Industnes and Operations Covered Intended Audience How to Use this Book References

Management Overview 2.1.

Impact on Vent Header Systems

Normal Process and Emergency Systems 3.1

3.2

Types of Vent Header Systems 3.1.1 Normal Process Vent Header Systems 3.1.2 Emergency Vent Header Systems 3.1.3 Combined Vent Header Systems 3.1.4 Considerations Design Philosophy 3.2.1 Design Sequence 3.2.2 Hazards Associated with Combining Vent Streams 3.2.3 Inherent Safety 3.2.4 Flammability and Combustibility

13 14 15 16 16 19 19 21 21 23

Contents

3.3

3.2.5 Toxicity 3.2.6 Reactivity 3.2.7 Regulatory Issues Reference

25 27 29 32

Combustion and Flammability 4.1

4.2 4.3 4.4 4.5

4.6

Flammable Limits 4.1.1 Mixture Stoichiometry 4.1.2 Factors Influencing Flammable Limits 4.1.3 Flammable Limit Variability 4.1.4 Effects of Temperature on Flammable Limits 4.1.5 Effects of Pressure on Flammable Limits 4.1.6 Flammable Limits of Combined Gas Streams 4.1.7 Cool Flame 4.1.8 Hybrid Mixtures Limiting Oxidant Concentration Deflagrations Pressure Piling Detonation Phenomena 4.5.1 Deflagration to Detonation Transition (DDT) and Run-Up Distance 4.5.2 Overdnven and Stable Detonations 4.5.3 Detonation Cell Size References

36 36 38 39 39 40 42 42 43 44 45 46 47 48 50 51 56

UNDERSTANDING REQUIREMENTS

5.1

viii

Understanding the Sources 5.1.1 Identify Vent Sources 5.1.2 Identify Normal Process Vent Streams 5.1.3 Normal Process Vent System, Design Case Scenario 5.1.4 Define Interface Requirements 5.1.5 Identify Hazard Scenarios That Could Result in Emergency Venting 5.1.6 Vent Gas Characteristicsfor Emergency Venting 5.1.7 Emergency Venting Design Case Scenario 5.1.8 Liquid Entrainment or Condensation in Normal Process Vent Headers 5.1.9 Two-Phase Venting

60 60 60 60 60 60 61 61 62 63

Contents

5.2

5.3 5.4 5.5 5.6 5.7

5.1.10 Flammable Gases and Vapors 5.1 .11 Toxic and Noxious Materials 5.1.12 Reactive Systems Regulatory Issues 5.2.1 Historical Background 5.2.2 Brief Review of Laws and Regulations 5.2.3 Improved Air Quality At-Source Treatment Options Combining Vent Streams End-of-Line Treatment Systems Specify Design Requirements References

64 66 68 74 74 76 78 79 80 82 82 82

DESIGNAPPROACH 6.1 6.2

6.3

6.4

6.5 6.6

ix

Design Basis Merging Vent Streams 6.2.1 Features Favorable for Merging Steams 6.2.2 Features that do not Favor Merging Streams Vent Header Systems Handling Flammable Materials 6.3.1 Explosion Prevention 6.3.2 Operating Fuel Lean 6.3.3 Operating Inerted 6.3.4 Operating Fuel Rich 6.3.5 Oxidizers Other Than Oxygen 6.3.6 Explosion Protection 6.3.7 Ignition Sources Vent Header Systems Handling Toxic Gases 6.4.1 Operating Pnnciples for Header Systems Handling Toxic Gases 6.4.2 Piping Design 6.4.3 Combined Relief Valve and Rupture Disk Devices Reactive Systems 6.5.1 Reactive Systems Design Considerations Mechanical Design Considerations 6.6.1 Vent Header Pipe Specifications 6.6.2 Vent Header Supports 6.6.3 Stresses on Vent Header Piping 6.6.4 Shock Waves Downstream of Rupture Disks 6.6.5 Corrosion 6.6.6 Header Operating Pressure and Pressure Drop

85 86 87 87 88 89 90 97 104 107 108 117 118

1 19 120 121 121 121 123 123 123 124 125 125 125

Contents

6.7

6.6.7 Thermal Stresses and Low Temperature Embrittlement 6.6.8 Liquid Knock-Out and Drainage 6.6.9 Expansion Joints and Flexible Connections 6.6.10 Valves in the Vent Header System References

126 126 128 129 130

Treatment and Disposal Systems Selection of Treatment and Disposal Methods Collection 7.2.1 Containment 7.2.2 Collection with Venting 7.2.3 Dump and Catch Tanks 7.2.4 Blowdown Drums and Tanks 7.2.5 Quench Drums 7.2.6 Quench Pools 7.2.7 Advantages and Disadvantages - Collection Systems Physical Separation 7.3.1 Vapor-Liquid Gravity Separators 7.3.2 Knock-Out Tanks and Drums 7.3.3 Mist Eliminators 7.3.4 Cyclones 7.3.5 Advantages and Disadvantages - Physical Separators Absorption 7.4.1 Spray Towers 7.4.2 Tray Towers 7.4.3 Packed-Bed Scrubber 7.4.4 Venturi Scrubbers 7.4.5 Advantages and Disadvantages - Absorption Systems Adsorption 7.5.1 Advantages and Disadvantages - Carbon Adsorption Recovery 7.6.1 Condensing Systems 7.6.2 Gas Recovery Systems 7.6.3 Advantages and Disadvantages - Recovery Systems Thermal Destruction 7.7.1 Flares 7.7.2 Thermal and Catalyhc Oxidizers 7.7.3 Process Heaters Used for Thermal Destruction 7.7.4 Advantages and Disadvantages -Thermal Destruction Systems

133 137 137 139 139 141 142 145 147 147 147 149 151 151 152 153 154 154 154 155 155 156 158 158 159 160 165 165 165 172 174 175 X

Contents

7.8

7.9

Dispersion of Vent Gas 7.8.1 Design and Safety Considerations 7.8.2 Atmospheric Dispersion Design 7.8.3 Advantages and Disadvantages- Dispersion to Atmosphere References

176 176 177 178 179

HAZARD ANALYSIS AND CONSEQUENCE ASSESSMENT 8.1 8.2

8.3 8.4

Hazard Analysis Methods Hazard Analysis Process 8.2.1 Identification of Causes 8.2.2 Development of Consequences 8.2.3 Estimation of Hazard Scenario Risk Consequence Assessment Techniques References

184 185 186 188 189 189 192

Operations and Maintenance 9.1 9.2

9.3 9.4

Daily Inspections Scheduled Inspections and Maintenance 9.2.1 Materials Build-Up 9.2.2 Pressure Relief Valves and Rupture Disks 9.2.3 Conservation Vents 9.2.4 Explosion Prevention Systems 9.2.5 Fast Acting Valves and Chemical Isolation Systems 9.2.6 Explosion Relief Panels 9.2.7 Inemng Systems 9.2.8 Instrument and Controls 9.2.9 Low Point Drains 9.2.10 Corrosion and Erosion 9.2.1 1 Structural Supports for Vent Headers 9.2.12 Insulation and Heat Tracing Management of Change References

196 196 196 199 200 200 200 20 1 20 1 20 1 20 1 20 1 202 202 202 202

Contents

Acronyms and Abbreviations

203

Glossary

207

Selected US Environmental Air Pollution Control Regulations

21 5

Vent Header Design Checklist

225

Normal Vent Header Source Control and Configuration Examples

233

PHA HAZOP Deviation Table

243

Worked Examples G1.

Inerted Flammable Liguid Storage

247

G2.

Flamable Liquid Process Operating Fuel Lean

254

G3.

Flamable Liquid Process Operating Fuel Rich

259

G4.

Flamable Liquid Process Operating Fuel Rich

263

G5.

Refinery Example: Crude and Vacuum Units

267

G6.

Refinery Example: Coker Unit and Gas Processing Plant

27 1

G7.

Reactive System

275

Past Incidents H1.

Combustion Incidents

283

H2.

Reactive Chemical Incidnets

290

H3.

Vacuum Failures

294

H4.

References

295

Contents

Historical Perspective on Air Pollution Control 11. Historical Background on Air Pollution

12. Brief Review of Laws and Regulations 13. Improved Air Quality 14. References

297 299 301 307

PREFACE The American Institute of Chemical Engineers (AIChE) has helped chemical plants, petrochemical plants, and refineries address the issues of process safety and loss control for over 50 years. Through its ties with process designers, plant constructors, facility operators, safety professionals, and academia, AIChE has enhanced communication and fostered improvement in the high safety standards of the industry. AIChE's publications and symposia have become an information resource for the chemical engineering profession on the causes of incidents and means of prevention. The Center for Chemical Process Safety (CCPS), an Industry Technology Alliance of AIChE, was established in 1985 to develop and disseminate technical information for use in the prevention of major chemical accidents. CCPS is supported by a diverse group of industrial sponsors in the chemical industry and related industries who provide the necessary funding and professional guidance for its projects. The CCPS Technical Steering Committee and the technical subcommittees oversee individual projects selected by CCPS. Professional representatives from sponsoring companies staff the subcommittees and a member of the CCPS staff coordinates their activities. Since its founding, CCPS has published many volumes in its "Guidelines" series and in smaller "Concept" series texts. Although most CCPS books are written for engineers in plant design and operations and address scientific techniques and engineering practices, several guidelines cover subjects related to chemical process safety management. A successful process safety program relies upon committed managers at all levels of a company who view process safety as an integral part of overall business management and act accordingly. A team of experts from the chemical industry drafted the chapters for this concept book and provided real world exarr.ples to illustrate some of the tools and methods used in their profession. The subcommittee members reviewed the content extensively and industry peers evaluated this book to help ensure it represents a factual accounting of industry best practices. xiv

ACKNOWLEDGEMENTS The American Institute of Chemical Engineers wishes to thank the Center for Chemical Process Safety (CCPS) and those involved in its operation, including its many sponsors whose funding made this project possible; the members of its Technical Steering Committee who conceived of and supported this concept book project, and the members of its Process Vent and Emission Control Systems Subcommittee. The members of the CCPS Process Vents and Emissions Control Subcommittee were: Christopher Lowe, Chair, Syngenta Crop Protection, lnc. Danny Bice, The Dow Chemical Company James Case, Air Products and Chemicals,Inc. David Kirby, Baker Engineering and Risk Consul tants Peter Lodal, Eastman Chemical Company Ray Mendelsohn, DuPont Edward Zamejc, BP John Davenport was the CCPS staff liaison and was responsible for overall administration of the project. Risk, Reliability and Safety Engineering (RRS), of League City, Texas was contracted to write this concept book. The principal RRS authors of this guideline were: John Birtwistle Tim McNamara Christy Franklyn Additional RRS staff that supported this project includes Donna Hamilton and Cathy Malek. CCPS also gratefully acknowledges the comments and suggestions received from the following peer reviewers; their insights, comments, and suggestions helped ensure a balanced perspective to this concept book: John Alderman, Risk, Reliability and Safety Engineering James Case, Air Products and Chemicals,lnc. Stan Grossel, Process Safety and Design, Inc. xv

Acknowledgments

Neil McNaughton, Innovene William Olsen, Merck and Company, Inc. Tony Thompson, Monsanto The members of the CCPS Process Vent and Emission Control Systems and the peer reviewers wish to thank their employers for allowing them to participate in this project.

xv i

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

INTRODUCTION The American Institute of Chemical Engineers (AIChE) has long been involved with process safety and loss control for the chemical, petrochemical, and hydrocarbon processing industries. The institute has developed strong ties with process designers, equipment builders, constructors, operators, safety professionals, and the academic community. AIChE has enhanced communications and improved safety standards for industry. Its publications are important information resources for the process industries. In 1985, AIChE established the Center for Chemical Process Safety (CCPS) to serve as the focal point for a continuing program to support and advance process safety. Since that time, CCPS has sponsored and published a number of documents, including proceedings of technical conferences and a series of books to improve process safety. This concept book, Safe Design and Operation of Process Vents and Emission Control Systems, is one of that series.

The simplest process vent system is one that consists of one vent device with minimal piping discharging directly to atmosphere at the nearest safe location. Numerous such simple systems exist in industry and satisfy the appropriate safety, health, and environmental requirements; particularly, where the quantities are small and the materials are less hazardous or non-hazardous. In practice, a number of factors have encouraged or required the collection of individual process vents into often complicated systems to collect these streams and treat, disperse, or dispose of them in order to meet increasingly demanding safety, health, environmental, and property protection requirements.

1

Safe Design and Operation of Procem Vents and Emission Control System

1.1

Objective

The objective of this book is to provide guidance for the design, evaluation, and operation of systems to collect and handle effluent gases and vapors vented from processes. These systems may consist of headers and manifolds of piping or ductwork and include other components that route or treat the effluent gases and vapors from their origin in process vessels, equipment, and storage tanks to the ultimate disposal or destruction system. Names for these systems include vent manifolds, vent collection systems, emission control systems, blowdown systems, vapor control systems, or vent header collection systems, as well as other descriptions and names. In this book, these systems are collectively referred to as vent header systems. This book addresses the concepts associated with the design and operation of vent header systems and provides guidance on: Designing vent header systems Preventing fires and explosions Controlling releases of toxics Maintaining safe operations Normal process operations, such as intentional routine controlled venting Emergency operations, for example, overpressure relief End-of-line treatment devices and their effects on the vent header system, including devices such as scrubbers, flares, thermal oxidizers, etc. This book focuses on vent header systems that handle gases, vapors, and entrained liquids that are vented from process tanks, vessels, and equipment. This book does not provide guidance on liquid-full systems, systems primarily intended for the removal, extraction, and collection of dust from otherwise innocuous air streams, or systems intended primarily to exhaust air from or ventilate working spaces. This book does not address the details of selection or computational aspects of sizing vent header piping systems or individual venting devices either for emergency overpressure relief venting or for normal process venting.

2

Chapter 1 -Introduction

A S M E B31.3 - Process Piping [Ref. 1-11contains details for design of piping systems for vent headers. For details on venting devices for emergency overpressure relief, refer to Guideline for Pressure Relief and Efluent Handling Systems [Ref. 1-21, Additional detailed design and sizing guidance for devices that may handle multi-phase flow is available from the Design Institute for Emergency Relief Systems (DIERS) [Ref. 1-31. DIERS operates under the auspices of AIChE as a users group currently comprised of representatives from 210 companies that cooperatively assimilate, implement, maintain, and upgrade the DIERS methodology. The group's purpose is to reduce the frequency, severity, and consequences of overpressure incidents and develop new techniques to improve the design of emergency relief systems. The venting devices for normal process venting are part of each specific process design and are typically standard process control valves and other components. 1.2

Relationship to Other CCPS Publications

Guidelines for Vapor Release Mitigation [Ref. 1-41 contains practices for controlling accidental releases of hazardous vapors and preventing their escape to the atmosphere. Its focus is primarily on pre-release factors. The 1988 guideline remains useful since it focuses on practical engineering design of mitigation systems and post-release mitigation methods. Since the 1988 guideline was published, substantial progress and improvements were made in many areas of mitigation design. To collect and update this progress, CCPS published Guidelines for Post-Release Mitigation Technology in the Chemical Process Industry [Ref. 1-51, The primary focus of the 1997 guideline is the mitigation of accidental releases of toxic or flammable materials and, in particular, countermeasures following a release. These guidelines make limited mention of collecting releases )+om process vent devices into vent header systems. In later chapters, this book discusses prevention of the propagation of fire and explosion within vent header systems. The following two books by CCPS provide useful background information. DefZagration and Detonation Flame Arresters [Ref. 1-61 provides guidance on the selection and proper application of fire and explosion arresting devices used within vent header system lines or at end-of-pipe locations. The book, Understanding Explosions [Ref. 1-71, published in 2003 provides a concise treatise on fires and explosions. 3

Safe Design and Operation of Process Vents and Emission Control Systems

Pertinent to the topic of vent header systems, the book also covers deflagration and detonation basics within closed equipment and purging and inerting of systems. Following extensive research into emergency venting, including large-scale tests involving reactive materials and two-phase venting, The Design Institute for Emergency Relief Systems PIERS) of AIChE published Emergency Relief System Design Using DIERS Technology [Ref. 1-31. It provides essential methodology for the design and sizing of emergency relief devices, but does not provide guidance on vent header systems.

Guidelines for Engineering Design for Process Safety [Ref. 1-81 includes information on flame arresters, pressure relief systems, effluent disposal systems, and provides some information on vent header systems. Guidelines for Pressure Relief and Effluent Handling Systems [Ref. 1-21 contains guidance and information on widely used codes and standards and their application in the detailed design of emergency overpressure relief devices and systems. It also includes the selection and design of systems and equipment to handle vent gases. These previous books and guidelines focused primarily on preventing releases, the detail design of overpressure relief devices, and the mitigation of the effects of releases to the atmosphere. They were primarily involved with emergency overpressure relief scenarios. The previous books and guidelines were not intended to provide guidance for the design and operation of vent header systems intended to collect vent gases from multiple sources during normal process operations, as well as during emergency overpressure conditions. This current book incorporates and consolidates information specific to vent header systems from these and other existing sources, as well as provides new information and learnings where possible. 13

Industries and OperationsCovered

Vent header systems are employed in one form or another in many facilities across numerous industry sectors. The industry sectors most commonly using vent header systems are: Oil and Gas Production and Processing Petroleum Refining Petrochemicals Synthetic Organic Chemicals 4

Chapter 1 - introduction

Agricultural Chemicals Specialty Chemicals Inorganic Chemicals Pharmaceuticals Polymers and Plastics Resins, Coatings and Adhesives Paints Synthetic Fibers The processes employed in these industry sectors vary greatly in complexity and scale. They may be continuous processes from raw materials to finished products, operate in a batch mode, or be a combination of batch and continuous processes. The vent header systems associated with these processes are similarly diverse in complexity and scale. Some of these vent header systems are simple, involving only one vent gas stream routed to a treatment device. Others may collect vent gas streams from multiple sources within a process unit or from several process units. Most vent header systems only handle the normal routine release of gases and vapors from the process. Some are intended to only handle emergency overpressure relief. A limited number are combined vent header systems that handle both normal process vent streams and provide the critical emergency function of safely venting effluent from overpressure relief devices. Many of these vent header systems are environmentally required to treat the vent gases before their release to the atmosphere. 1.4

Intended Audience

This book should be of interest to persons responsible for: Design of new or modification of existing processes that may require the use of a vent header system, including project managers and process design engineers Process safety or hazard analysis of processes with vent header systems Operation of process units or facilities with vent header systems, including operating management and staff and unit process or manufacturing engineers Maintenance, inspection, or testing for process units or facilities with vent header systems 5

Safe Design and Operation ofprocess Vents and Emission Control Systems

This book also provides useful reference for anyone interested in the subject of vent header systems used in the process industries.

How to Use this Book

1.5

This book is organized to meet the needs of those readers new to the issues associated with vent header systems, as well providing experienced readers specific references and design considerations. The organization and content is illustrated in Figure 1-1.

I I

I

Chapter 1 introduction

Chapter 2 Management Overview

Chapter 3 Normal Process and Emeraencv Svstems Chapter 4 Combustion and

Understanding Reauirements

I

introduction and objective Relationship to other CCPS publications Industries and operations covered Intended audience

I

Environmental and societal stewardship concerns History and impact of US environmental air regulations The vanety of purposes and applications for vent header systems Cost implications and business interruptions issues

I

Types of vent header systems Considerations for normal and emergency vent header systems Design Philosophy General Design Flammable limits Hybnd mixtures Deflagrations, pressure piling and detonation phenomena

Underjtandlng the process conditions Vent stream charactedstics Fiammable gases and vapors, toxic and noxious materials, reactive systems Objectives and design concepts for normal, emergency and combined sys!ems

Chapter 6 Design Approach

Combining vent header systems Systems handling flammables and toxics Reactive systems Mechanical design considerations

Chapter 7 Treatment and Disposal Systems

Selection of treatment and disposal methods Coiiection, separation, absorption, adsorption and recovery Thermal destruction Dispersion

Chapter 8 Hazard Analysis and Consequencr

Hazard analysis Hazard Identification anaiysis method Consequence assessment techniques

Chapter 9 Operations and Maintenance

Potential failure modes and concerns Pressure reiief devices

Figure 1-1. Guideline Organization and Content 6

Chapter 1 - Introduction

1.6

References

1-1

American Society of Mechanical Engineers. 2002. B32.3 - Process Piping. New York, New York.

1-2

Center for. Chemical Process Safety (CCPS). 1998. Guidelines for Pressure Relief and Efluent Handling Systems. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Engineers.

1-3

The Design Institute for Emergency Relief Systems PIERS). 1992. Emergency Relief System Design Using DIERS Technology. New York, New York. American Institute of Chemical Engineers.

1-4

Center for Chemical Process Safety (CCPS). 1988. Guidelines for Vapor Release Mitigation. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

1-5

Center for Chemical Process Safety (CCPS). 1997. Guidelines for Post-Release Mitigation Technology in the Chemical Process Industry. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Engineers.

1-6

Grossel, Stanley S. 2002. Deflagration and Detonation Flame Arresters. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

1-7

Crowl, D. A. 2003. Understanding Explosions. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Engineers.

1-8

Center for Chemical Process Safety (CCPS). 1993. Guidelines for Engineering Design for Process Safety. New York, New Y ork: Center for Chemical Process Safety of the American Institute of Chemical Engineers.

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

2 MANAGEMENT OVERVIEW A progression of societal drivers for air pollution control have prompted the process industries to control process effluents and emissions. Laws and regulations regarding protection of the natural environment and public health have exerted an increasing impact on the refining, chemical, and other process and related industries. In particular, the requirements regarding air pollution controls have resulted in an increase in the use of vent header systems. The development of regulations in the United States and, in particular, the requirements of the Clean Air Act amendments and related air pollution control regulations have greatly influenced the need for vent header systems and their design and operation. Further discussion of regulatory issues may be found in Chapter 3 of this book; an historical perspective on air pollution control laws and regulations may be found in Appendix I. The importance of safe design and operation of vent header systems has also been emphasized by recent incidents in the process industries. Some of these incidents are described in Appendix H. Impact on Vent Header Systems Environmental air pollution control regulations over the past several decades have arguably reduced the number of discrete emission points and increased the number of emission vent collection systems. The economics of emission reduction and treatment clearly encourage facilities to collect similar vent streams into vent headers for processing in common treatment or disposal systems before the final release to atmosphere. Current air pollution control regulations have extended coverage to more industry sectors and in many cases further restricted allowable end-ofpipe post treatment releases in terms of either or both quantity or concentration. Future regulations should be anticipated to continue this general trend. 2.1.

9

Safe Design and Operation of Process Vents and Emission Control Systems

So, what is the impact of these air pollution controls? Vent header systems of some type have become commonplace in many industry sectors for a wide range of processes. For the most part, they have been added on to processes to meet the environmental requirements. Possibly because of being viewed as an add-on, vent header systems have often been treated similar to a utility service rather than as an extension of the process operation. More often than not, utility systems are not accorded the same level of safety technical review as would be given to a section of the process. An objective of this book is to encourage the design and operation of vent header systems and their treatment/disposal components with an equivalent level of safety review as would be given the process itself. Increasingly, regulations and operating permits have made the availability of a functioning vent collection and treatment/disposal system a requisite for the process to continue to operate within its approved legal limits. This presents another reason to treat the vent header system as a part of the process. The regulations have had an impact on process economics as the number of emission sources that require treatment have increased. As a result, vent header collection systems have increased the number of connected vents. Clearly, it is more cost-effective to treat a larger number of vent streams in a common vent header system than to do so individually. An increasing trend is the collection of vent streams from different processes. However, the connection of multiple processes to a common vent header system increases the probability of unsafe conditions due to differing process start-up and shutdown schedules, vent header ownership issues affecting maintenance or monitoring, and other factors such as: Incompatible vent streams that could result in pluggage, violent reaction, fire, or explosion in the header or treatment equipment Unwanted flow of materials via the vent header from one process vessel to another Vent header and treatment system flow capacity or restriction issues resulting from simultaneous multiple vent streams Increasing difficulty of identifying hazards as the system becomes more complex

10

Chapter 2 - Management Overview

Current and future regulations may also have an impact on the complexity of the treatment systems used. To meet certain requirements, it may be necessary to add additional intermediate or end-of-pipe treatment and disposal systems, such as a scrubber on the combustion products stream from a thermal oxidizer. The addition of multi-step treatment and disposal systems can increase the complexity of the overall vent header system and may increase the probability of creating unsafe conditions. Certainly there are other factors that impact decisions by a facility regarding the use of vent header collection and treatment systems for control of their emissions, including their public image and preservation of the rightto-operate in their communities, but the compelling impact has been from the air pollution control regulations.

11

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

3 NORMAL PROCESS AND EMERGENCY SYSTEMS This chapter describes the functions of normal process, emergency, and combined vent header systems, and provides an overview of their overall design philosophy. The interface between major processing vessels and sources, and the vent header itself, is often unclear or ill defined. Precise definitions of both the physical design and administrative control boundaries between the main processing vessels and the vent collection system are necessary. This should include such issues as the impact of vent header system pressure fluctuations on process equipment, especially distillation systems. 3.1

Types of Vent Header Systems

As a general rule, process equipment, tanks, and other vessels have two complementary venting requirements: Normal Process - to provide normal venting while the process is functioningas intended in one of the normal operating phases. Emergency - to protect equipment and personnel from the effects of excessive pressure or vacuum caused by an abnormal condition that cannot be controlled by the basic process controls or the safety instrumented system (interlocks). These situations include events such as fires or runaway reactions. The regulatory and economic implications for normal and emergency vent header systems can differ significantly. Normal practice is to provide two separate vent systems; however, there are some situations when a single combined system can effectively satisfy both needs.

13

Safe Design and Operation of Process Vents and Emission Control Systems

3.1.1

Normal Process Vent Header Svstems

Normal process vent header systems handle vent gas produced during normal operating phases, including start-up, shutdown, and certain maintenance activities. The sources of these vent gases include, but are not limited to: Vessel breathing (either in or out) due to changes in liquid level, temperature, or variations in the atmospheric pressure Off-gases associated with the process chemistry Inert gas purging or other intentionally introduced gas flows that are not consumed in the process In some instances, vent gases have a commercial or economic value such as refinery fuel gas. In these cases, it may be cost-effective to recover or recycle the flows. When evaluating off-gas streams for recycling it is important to ensure the gases will be compatible with one another and any other process streams they could come in contact with. For example, they should not form flammable mixtures (fuel and oxidizer), build-up solids that could restrict flow in the vent header, or react with each other. It is also important to recognize the potential for trace components to build-up in the system where recycling does not provide an outlet. Normal vent header systems should be sized to handle maximum predicted flows from the equipment while the facility is operating as intended in one of the normal operating phases (start-up, shutdown, etc.). These maximum predicted flows must be achieved without the pressure increasing to a level that could restrict venting from other vessels connected to the vent header system or otherwise creating the potential for undesirable reverse flow to source vessels. Failures due to low temperature brittle fracture and vessel collapse due to vacuum are not uncommon, demonstrating the need to address both the minimum and maximum values of temperature and pressure, as well as other process variables. In addition, normal vent header systems should be able to withstand the worst-case conditions of temperature, pressure, composition, etc., they could be exposed to during an emergency venting incident.

14

Chapter 3 -Normal Process and Emergency Systems

3.1.2

Emereencv Vent Header Svstems

Emergency vent header systems are safety systems that safely dispose of vent gases resulting from unintended events outside the range of normal operations. These can include external fire, runaway reactions, human error, equipment, and instrumentation failures. The design of an emergency vent header should be conservative, taking into account the worst-credible overpressure scenario for the equipment it is protecting. This typically results in flow rates that are considerably higher than during normal venting and may involve a greater range of temperatures, pressures, and compositions. Selection of the ultimate disposal route for the emergency relief vent discharge depends on several factors, such as the: Physical properties of the vent gases, such as density, pressure, temperature, etc. Maximum flow rate and quantity of vent gases that could be discharged Toxicity and combustibility properties of the vent gases Historical weather information and topographical features affecting dispersion Proximity of the local community Nuisance issues, such as the odor and noise caused when venting occurs In some cases, non-toxic emergency vent gases can be discharged directly to atmosphere. Other materials may need treatment to address state or city permitting requirements and other regulatory issues. For further details on the treatment of vent gases, see Chapter 7. Most countries have developed or adopted codes defining the requirements for pressure relief systems. Within the United States, many states and local city authorities have adopted Section VIII of the ASME Boiler and Pressure Vessel Code (BPVC) [Ref. 3-11 for tanks and vessels with design pressures above 15 psig. Vessels with operating pressures between 2.5 and 15 psi are typically designed in accordance with API 620 [Ref. 3-21, Tanks that operate at, or close to atmospheric pressure, can be designed using API 650, which also includes an appendix identifying additional design requirements for tanks operating up to 2.5 psig [Ref. 3-31. Vacuum ratings for API 620 and 650 tanks generally do not exceed 1 oz of vacuum. Chapter 6 of this book discusses several requirements for pressure and vacuum relieving devices on vessels that have implications for vent headers systems [Ref.3-11, 15

Safe Design and Operation of Process Vents and Emission Control Systems

3.1.3

Combined Vent Header Svstems

Normal and emergency vent header systems have different purposes. If they are to be combined, the requirements of both must be satisfied without compromising either one. Emergency relief vents are typically the final safety devices protecting equipment from overpressure after the basic process controls and safety instrumented systems have been unable to provide the necessary protection. Although they are required to operate infrequently, they must be designed with a high reliability and meet code requirements. The ASME BPVC, Parts UG 125 through 137 [Ref. 3-11! identifies requirements for pressure relief devices. Generally, it requires any pressure relief device isolation valve to be locked or sealed open to assure there will always be an open vent path for emergency venting, except for maintenance or inspection as outlined by Appendix M of the code, (see Chapter 6 for further details).

In contrast, normal process vent header systems are primarily provided for environmental, health, and process reasons, and allow for the routine (sometimes continuous) emissions of off-gas from equipment. Tanks and vessels generally have separate systems for normal process and emergency vents; however, on occasion it may be cost-effective to combine them. Examples of separate and combined systems are illustrated in Figures 3-1 and 3-2. Figure 3-1 illustrates separate normal process and emergency vent systems, where the normal process vent system serves multiple sources. 3.1.4

Considerations

Vent header systems often collect off-gases generated from multiple equipment items, which inevitably means that gases leaving one vessel can mix with any combination of gases from other vessels connected to the same header. The chemical and physical properties of all streams must be mutually compatible and not form mixtures that are ignitable (i.e., mixtures where both fuel and air are present) or react when combined together. These gases range from being relatively innocuous to flammable, reactive, toxic, and/or corrosive. A process facility may contain multiple vent headers routed to a common treatment device, completely separate systems, or mixed systems where headers are joined after an intermediate treatment step. Several equipment items may also share a common vapor space representing a single vent source to the vent header system.

16

Chapter 3 - Normal Process and Emergency Systems

Figure 3-1.

Equipment with Separate Normal and Emergency Vent Header Systems

Safe Design and Operation of Process Vents and Emission Control Systems

Pilot Flame Detector

+--

Pilot Fuel Gas

Fuel Gas Header

Flare

Combined Normal and Emergency Vent Header

i Combined Vents From Other Process Vessels

Figure 3-2.

18

<

1

Knock-Out Water Seal Tank

Combined Normal Process and Emergency Vent Header Systems

Chapter 3- Normal Processand Emergency System

Normal process vent header systems typically involve one or more of the following: Treat and release Recover for material reuse Bumasfuel Depending on the potential consequences of a release, emergency vents may be routed directly to atmosphere or to a treatment device.

3.2

Design Philosophy

Depending on the process, vent header systems may need to be designed to handle streams that are flammable, toxic, reactive, corrosive, or prone to cause blockages. This section discusses the general design approaches that can be employed when one or more of these hazardous conditions are present. It does not address the computational methods for designing pressure relief systems, which are addressed in other CCPS and DIERS publications [Ref. 3-4 and Ref. 3-51,

3.2.1

Design Seauence

The vent header system design sequence is similar for both modifying existing processes and new project designs. Figure 3-3 illustrates the typical steps in vent header system design. Characterization of the various normal process vents (flow rates, compositions, temperatures, etc.) should be conducted as part of the process design. Once this information is available, the implications of bringing them together in the proposed header should be investigated by the design team and subsequently evaluated as part of a Process Hazards Analysis (PHA) for the facility, Identification of the venting scenarios for emergency vent systems should begin as early as practical, taking into consideration issues such as code requirements, preliminary safety reviews, good engineering design, etc. By beginning to address the emergency venting requirements early in the design, it may be possible to identify approaches to minimize the venting requirements, for example by reducing the inventories of reactive materials and process changes to avoid producing hazardous intermediates (See Section 3.2.3 on Inherent Safety). At this stage in the design, the possibility of combining the normal process and emergency vent header systems should be evaluated.

19

Safe Design and Operation of Process Vents and Emission Control System

Normal Process Vent Requirements

Chapter 5

Chapter 5

Understand and Develop Emergency Venting Scenarios

t

t Determine if Vent Headers Should be Combined

i

Preliminary Hazard Assessments and Design Reviews

Chapter

Options

:;$;

Determine if Intermediate Treatment is Required

Chapter

Define Vent Header System Preliminaly Design

1 Chapter

Finalize Design of Vent

Figure 3-3.

-

-

t-

Process Hazards

Chapter 8

Analysis and Final Design and Operation Reviews

Chapter 9

Steps in Vent Header System Design

When the preliminary design is available, a PHA should be conducted to identify other scenarios that could result in equipment venting to the emergency vent system and potential failure modes of vent header systems.

20

Chapter 3 - Normal Process and Emergency Systems

As a minimum, PHAs should be conducted during the design phase and revalidated on a routine basis throughout the lifetime of the facility. Accident experience shows a disproportionately large number of vent header explosions have occurred during non-standard operations, such as start-up, shutdown, and maintenance. Therefore, the PHAs should include these nonroutine operations, as well as equipment malfunction, process upsets, and human error during normal operations. See Chapter 8 for information on conducting hazard evaluations. 3.2.2

Hazards Associated with Combininv Vent Streams

Typically, there are many factors that should be taken into consideration when designing a vent header system. To assist in capturing them, it is good practice to develop a "vent design basis" identifying all items of equipment that will be connected to the header, the circumstances that could cause them to vent, potential compatibility issues, common failure modes resulting in more than one vessel venting simultaneously, etc. Determining which vent streams can be manifolded should be based on the ability to combine them in a safe and economic manner, while also meeting treatment requirements. The use of an interaction matrix is normally used to evaluate hazards with mixing vent streams (See Chapter 5). Table 3-1 identifies hazards and operability issues associated with combining vent streams. 3.2.3

Inherent Safety

The term inherently safer process refers to the approach proposed by Dr. T. Kleb [Ref. 3-61 in which emphasis is placed on designing or modifyng facilities to eliminate or minimize hazards. This can reduce the severity of worst-case events, permitting the use of smaller, less complex, and lower cost vent header systems. In turn, this inherent safety approach can lead to vent header systems that are less dependent on equipment reliability and the correct operator responses.

21

Safe Design and Operation of Process Vents and Emission Control Systems

Table 3-1. Typical Hazards and Operability Issues Associated with Combinii ; Vent Headers issue Flow Considerations & Physical Changes

Potential Cause Excessive pressure drop in the vent header creates backpressure on the relieving devices, reducing their capacity High and low pressure vessels discharge into the same header.

Varying flows or high pressure discharges through vent systems. Condensation of liquid as a result of wmbining hot and cold streams or due to heat losses to atmosphere from a hot vent header.

Reactivity Concerns

Flammability Issues Toxicity Considerations

22

Incompatible gases or entrained liquids may mix in the vent headers

Incompatible process materials may overRow from one vessel to another through the vent header. Air or other oxidizing gas streams mix with flammable gas. Combining a highly toxic stream with one that is relatively nontoxic can increase the volume of toxic gas that must be handled. Combining relatively non-toxic materials may react forming more toxic ComDounds.

Concern High pressure in the vent header restricts flow from other vessels. This wuld cause high pressure in these vessels, potentially resulting in damage or failure. If the vent header system becomes limiting, gas discharged from a highpressure vessel may be applied to a low-pressurevessel, causing wntamination or vessel failure. Fluctuatingbackpressure in the vent header affects stability of process operation. Liquid pools in the vent header restrictingits capacity for pressure or vacuum relief. Partial vacuum in the system can occur, which could allow unexpected flows between the vessels, air to be sucked in, and drainage lines to atmosphere to be blown backwards. Solid or liquid reaction products may be formed and restrict the flow of vent gases, Potentially hazardous reaction products may be formed. Combining vents may result in a significantly more comosive mixture. Potentially hazardous or undesirable reactions, such as runaway polymerizationsor decompositions, may occur in the contaminatedvessels. Ignitable mixture formed that could result in an explosion or fire. Personnel exposure if there is a loss of containment. Personnel exposure during maintenance operations.

Chapter 3 - Normal Process and Emergency Systems

Examples of this approach include: Substituting a flammable solvent with another that has a higher flash point or one that is totally noncombustible. An example of tlus approach can be seen in the paint industry, where environmental concerns have been the driving force to replace organic solvents with aqueous based solvents. In addition to reducing the quantity of volatile organic vapors being released to the environment, the change has also greatly reduced fire and explosion hazards at these facilities. If an aqueous based solvent is not a practical alternative, substituting flammable solvents with combustible liquids (i.e. materials with flashpoints above 100°F (37.8"C) can significantly reduce the potential for explosions occurring in vent header systems. Substituting established materials with others that are less toxic. For example, using sodium hypochlorite @leach) instead of liquid chlorine for effluent treatment systems. Developing continuous processes instead of batch operations. At the beginning of a batch reaction, there typically is a large inventory of reactive material present. Conditions continuously change as the batch proceeds; increasing the potential for process upsets. h the case of continuous reactions, after initial start-up the processes can be operated at steady conditions. For a given production rate, the equipment required for a continuous process is generally considerably smaller than for a batch operation. As a result, the emergency vent header system for a continuous process can be smaller and less complex than for a corresponding batch system. Time and effort spent evaluating a process to make it as inherently safe as practical can be very productive in reducing the venting requirements. Thus, it should be among the first steps taken when evaluating an existing facility or a new process. For further information on inherent safety, refer to Reference 3-7. 3.2.4

Flammabilitv and Combustibilitv

In many instances, normal process and emergency vent header systems handle gas streams that, if mixed with air and ignited, will bum causing a fire or explosion.

23

Safe Design and Operation of Process Vents and Emission Control Systems

Fires and the related explosions caused by combustion have three basic requirements:

1. A fuel present, such as a flammable gas within its flammable limits 2.

An oxidizer such as the oxygen in air

3. An ignition source of sufficient energy The flammable limits, minimum oxygen concentrations, and minimum ignition energy can be measured or obtained from literature sources. These values are specific to the conditions tested and will vary with conditions, such as temperature and pressure. Consequently, published values may not be applicable to plant conditions. If the combustion takes place in an unconfined location, the energy evolved by the reaction heats the combustion products causing them to expand creating a fireball many times greater than the initial combustible mixture. Alternatively, if the combustion is in a closed vessel the rise in temperature will cause the pressure to increase. Explosions can occur if pressure is sufficient to cause the vessel to rupture. For further information on the subject, see Chapter 4. The term "gas" refers to a material that has a vapor pressure in excess of 14.7 psia (1.01 bara) at standard temperature. Vapor refers to the gaseous phase from a volatile liquid, which has a vapor pressure below 14.7 psia (1.01 bara) at standard temperature. Gases and vapors have similar combustion properties, e.g., flammable limits, flame speeds, etc., and they are typically measured using similar, if not the same, test methods. Unless there is some other reason to identify the material type, the term flammable gas is used in this book when discussing either flammable gases or the vapors from flammable or combustible liquids that are above their flashpoint. Liquids are considered "flammable" if they produce sufficient vapor to form a mixture that will bum if ignited. Vapor pressure is dependant on temperature. Organizations have selected different criteria for determining if a liquid is flammable. For example, within the United States, OSHA [Ref. 3-11] defines any liquid with a flash point
24

Chapter 3 -Normal Process and Emergency Systems

3.2.4.1

O v e r v i m of Explosion Prevention and Protection

Vent headers typically interconnect several equipment items, increasing the likelihood of air entering the system. Consequently, from time-to-time there may be sufficient air present to support combustion. If flammable gases or liquids are handled in equipment that vents to the header, measures should be taken to prevent explosions or to protect against their consequences. Examples of methods used for explosion prevention and protection include: Designing the process to operate below the lower flammable limit Operating inerted, i.e. by controlling the oxygen concentration below the minimum level capable of sustaining combustion of the flammable materials being handled, for example by providing nitrogen purge systems Operating fuel rich, i.e. by maintaining the flammable gas composition above its upper flammable limit, such as by adding fuel gas Designing the process to include explosion venting, explosion isolation valves, flame arrestors, detonation arrestors, or chemical isolation (explosion suppression) Where practical, potential ignition sources should be eliminated. Although it is desirable to eliminate all ignition sources, this is not considered a reliable layer of safety on its own merit due to the number of potential sources and practical aspects such as assuring the integrity of grounding systems. Consequently, elimination of ignition sources only is not adequate explosion prevention in operating areas. To allow for the imprecision inherent in test methods and variability in operations, safety margins should be provided, as discussed in Chapter 5. 3.2.5

Toxicity

In most cases, the atmosphere present in vent header systems cannot support life and, therefore, in the broadest term the gases present are toxic. More typically, the term toxic gases refer to materials that can cause physiological harm other than asphyxiation and that are immediately dangerous to life and health and can be fatal at relatively low concentrations, such as phosgene or hydrogen sulfide.

25

Safe Design and Operation of Process Vents and Emission Control Systems

3.2.5.1

Toxic Hazard Assessment

As part of the original facility's design and the management of any subsequent changes, vent header systems should be evaluated to determine if the materials discharged from them could pose a health or environmental hazard. For normal vent header systems, this assessment should be based on the most severe combination of the flow rate and composition of the vent gas that can occur while the facility is in one of its operating phases (start-up, normal operations, etc.). Normal vent header systems receive vent gas regularly as part of day-to-day plant operations. Consequently, the composition and flow of gases exiting the vent system must not affect the environment, create potential personnel hazards, or create a public nuisance. Therefore, normal process vents containing toxic off-gases are generally sent to an effluent treatment system prior to being discharged to atmosphere (See Chapter 7). Factors contributing to the hazards of toxic gas releases include the: Degree of confinement in the faality restricting the ability of the toxic gas to disperse 0 Odor detection level of the gas Physiological properties of the gas Toxic gas releases inside buildings are more likely to form hazardous concentrations than if they occur outdoors. Releases of materials that have no odor, such as carbon monoxide, present an increased risk of injury as compared with gases with similar toxicity and strong unpleasant odors. All three factors (confinement, odor, and toxicity) should be evaluated when determining the potential hazard and the appropriate measures of protection. 3.2.5.2

Protective Measures

The potential for toxics to leak from vent headers can be greatly reduced by operating the system at sub-atmospheric pressure and by designing the system to resist leaks, for example in accordance with ASME B31.3, Piping For Category M Fluid Service [Ref. 3-13].

26

Chapter 3 - Normal Process and Emergency Systems

Vent header systems may handle both high or low concentrations of toxics: High concentrations of toxic gases are typically treated through destruction devices with the residual gases vented to atmosphere from an elevated vent stack providing adequate dispersion Minimal concentrations of some toxic gases may be vented directly to atmosphere from an elevated vent stack providing adequate dispersion

In locations where releases from vent headers could result in hazardous concentrations of toxic gas in areas where personnel may be present, gas detection systems should be provided. For further information on these effects, see Reference 3-5, pages 428 - 430, and Reference 3-14. 3.2.6

Reactivity

By their very nature, process industries handle a wide-range of materials, many of which can react energetically, either as self-reactives or with other materials. Depending on the process chemistry, off-gases may be formed that need to be collected and disposed of via an appropriate treatment device. It may also be necessary to design the emergency vent system to provide protection against a runaway reaction involving reaction rates and gas flows that may be significantly higher than normal process conditions.

Reactivity Hazard ldent$cation Identifying potential reactivity hazards typically involves a team that includes personnel knowledgeable in the process, the manufacturing operations, and the chemistry. Chapter 5 describes interaction matrixes, which can be an effective means to identify and document the materials and conditions that could result in a reactive hazard. Reactive chemical incidents can be categorized as either: Self-reactive- polymerizing, decomposing. isomerizing Reactive with combinations of materials - where the material may be stable by itself, but reactive with other chemicals In the case of self-reactive materials, incidents have often been initiated by relatively small amounts of contaminants acting as catalysts; although conditions such as elevated temperatures, pH changes, or the depletion of an inhibitor may also act as initiators. 3.2.6.1

21

Safe Design and Operation of Process Vents and Emission Control System

Reactivity concerns for combinations of materials tend to involve bulk mixing of incompatible materials or reaction with ubiquitous substances, such as air or water. Mechanisms can include process control failures, such as adding the wrong material to a reactor or feeding reactive materials to equipment with its agitator stopped. As in the case of selfreactive materials, these events have frequently involved a combination of materials and conditions. Therefore, the hazard identification should not be limited to single failures (See Chapter 8). 3.2.6.2

Runaway Reactions

Runaway reactions occur when the heat generation rate from a reacting mass exceeds the rate at which heat can be removed, causing an uncontrolled rise in temperature. In the absence of adequate overpressure relief, if the heat of reaction exceeds the cooling capacity, the reaction rate can accelerate (runaway) and may result in a gas evolution rate that overwhelms the vent header system. Initiating events can include malfunctions such as contamination by an incompatible material, mis-charging reactants, a delay in starting the agitator, failure of the cooling system, external fire, etc. Provided there is volatile liquid present, emergency vents can mitigate the runaway reaction by permitting the liquid to boil and for its latent heat of vaporization to “temper” the reaction. The set pressure of the relief device will determine the temperature of the material in the vessel and the reaction rate at the time when the venting begins. Consequently, setting the relief device’s opening pressure as low as practical reduces the flow the vent system must handle and the size of the vent header. 3.2.6.3

Mixing Implications

When vents from different vessels are directed to a single vent header system, there is the opportunity for any combination of materials present in them to mix in the header or to flow from the header into the vessels. The changing conditions from the process to the vent header may introduce new reactivity concerns. When identifying potential reactivity hazards, the vent headers should be evaluated to identify all potential combinations of materials and conditions that could occur in the equipment, including the utilities, cleaning materials, and by-products, as well as the normal process chemicals.

28

Chapter 3 - Normal Process and Emergency Systems

3.2.6.4

Reactive Hazards, Protective Measures

Based on the severity of the potential event, it may be necessary to implement one or more of the following control or mitigation measures: Design features, such as interlocks or safety instrumented systems, etc. Instrumentation and automatic responses to detect and respond to incipient runaway conditions in the event that mixing occurs An emergency vent header system adequate to handle the maximum vent flow rate resulting from a worst-credible event Separate vent header systems exclusively for the equipment handling materials that are compatible with each other Measures to minimize or manage potential solid build-up to ensure that vent header systems are available when required During runaway reactions, the temperature can rise sigruficantly, which may favor different reactions. If this occurs, the composition may shift to produce a more toxic off-gas, as occurred at Seveso, Italy (See Appendix H). If there is the potential for a runaway reaction, the composition of off-gases should be understood and an appropriate treatment selected. 3.2.7

Remlatorv Issues

When normal process vent gases are released to atmosphere, environmental regulations may require them to be treated. These regulations may limit the quantities and concentrations of released vent gases and may effectively dictate the types of treatment devices required. Normal process vent gases must also be included as part of the air emissions for the facility in compliance with the Environmental Protection Agency (EPA) Clean Air Act [Ref. 3-81 and other Federal, State or local environmental requirements. Other regulations, such as the EPA's Risk Management Program Rule (40 CFR Part 68) [Ref. 3-91 or the Occupational Safety and Health Administration (OSHA) Process Safety Management Standard (29 CFR 1910.119) [Ref. 3-10], may also apply if the facility is handling greater than the threshold quantity of a hazardous material. The reader is advised to refer to local regulations, as applicable. Within the United States, regulations may differ from those cited above. Facilities located outside the United States should address applicable country or international laws and regulations.

29

Safe Design and Operation of Process Vents and Emission Control Systems

Emergency releases of vent gases often do not require treatment. Emergency releases should be infrequent occurrences. If emergency releases happen more frequently, they may require treatment prior to release due to regulatory requirements. Emphasizing their commitment to the communities they operate in, facilities with toxic materials often route emergency releases to treatment systems, regardless of regulatory requirements. Environmental legislation and regulations have had and will likely continue to have a broad direct and indirect impact on the engineering design and selection of emission control devices and vent header systems. Federal and state air pollution regulations for VOCs and NOx have required or encouraged many facilities to collect VOC-containing process vent streams into vent header systems for common treatment. The Clean Air Act of 1990 increased the number of listed Hazardous Air Pollutants (HAPS), established threshold release quantities, and defined the sources that release air pollutants as follows: Point Sources - Individually identifiable stationary sources, such as electric power plants or chemical plants, that discharge air toxics through stacks, vents, equipment leaks, and fugitive emissions or during material transfers and are classified as:

-

-

Major Sources - Sources or groups of sources located within a contiguous area and under common control, that have the potential to release, considering air pollution controls, more than: 1

10 tons/year (9.07Mg/yr) of one pollutant

1

25 tons/year (22.68 Mg/yr) of a mixture

Minor Sources - Stationary sources that release less than the above

Non-Point Sources - Stationary non-point or area sources, such as dry cleaners and gas stations, have release limits that are the same as for Minor Sources above. Emissions from these are typically small, but can be of concern when concentrated in heavily populated areas. Mobile Sources - On-road highway vehicles and non-road or offroad vehicles or equipment, construction equipment, boats, and aircraft. 30

Chapter 3 - Normal Process and Emergency Systems

The facilities of concem in this book fall into the Major and Minor Point Sources category. The non-point and mobile source categories are shown only for completeness. The National Emission Standards for Hazardous Air Pollutants (NESHAP) rules require that all covered “major sources” in the regulated industry sectors reduce their emissions to limits based on a Maximum Achievable Control Technology (MACT) for each listed pollutant. The industry-specific NESHAP-MACT rules are written as an emissions limit that regulated sources must achieve, i.e., as a percent reduction in emissions or a concentration limit based on technology or other practices already in use in that industry. Facilities have some latitude to achieve the required performance level in whatever way is most cost-effective. There are 89 NESHAP-MACT final rules in effect as of 2004 that include industry sectors in approximately 175 different Standard Industrial Classifications (SIC). Additional MACT rules will be proposed by EPA in the future.

In the course of identifying those industry sectors that were the major contributors to airborne pollution, it was apparent to the EPA and to industry that the sector collectively identified as the Synthetic Organic Chemical Manufacturing Industry (SOCMI) would be one of the major source categories that release HAPs to the atmosphere with a large number of facilities involved. The regulation developed for the SOCMI sector was named the Hazardous Organic National Emission Standards for Hazardous Air Pollutants or the Hazardous Organic NESHAP or more simply called the “HON rule”. This regulation with its MACT requirements was developed collaboratively between the EPA and members of the industry and is considered a “negotiated regulation” and a possible model for future EPA rule-making. Other industry sectors not actually in the SOCMI sector were also included by EPA under the requirements of the rule due to their similarity of process operations and/or materials released. The HON rule has special significance since it regulates emissions of 111 of the 188 listed HAPs. The regulation can be found in 40 CFR Part 63 Sub-parts F, G, H and I. The rule contains specific control, monitoring, reporting. and recordkeeping requirements and regulates emissions from five possible normal or routine sources of releases from a process operation to the atmosphere: 31

Safe Design and Operation of ProcessVents and Emission Control Systems

Process vents Transfer operations Storage vessels Air emissions from wastewater streams, collection, and treatment operations Fugitive emissions from equipment leaks The fugitive emissions from equipment leaks would not typically appear to be of direct concern to the subject of this book. However, the regulations regarding fugitive emissions have encouraged many facilities to add such items as leak detection systems for double pump seals on larger pumps that are designed to capture leaks and rout them into a vent header system. The future could see more captured fugitive emissions directed into vent header systems.

Reference

3.3

3-1

I

3-2.

3-3.

3-4.

3-5.

3-6. 3-7.

3-8.

32

American Society of Mechanical Engineers. 2004. Boiler and Pressure Vessel Code. New York, New York ASME. American Petroleum Institute. 2002. API 620, Design and Construction of Large, Welded, and Law-Pressure Storage Tanks. Washington D.C.: API. American Petroleum Institute. 1998. APl Std. 650, Welded Steel Tanks for Oil Storage. Washington D.C.: API. Center for Chemical Process Safety. 1998. Pressure Relief and Efluent Handling Systems. New York, New York American Institute of Chemical Engineers. The Design Institute for Emergency Relief Systems of the American Institute of Chemical Engineers. 1992. Emergency Relief System Design Using DIERS Technology. New York, New York American Institute of Chemical Engineers. JSletz, T. 1998. Process Plants: A Handbook of l n h e n t l y Safer Design. London, UK: Taylor and Francis, Inc. Center for Chemical Process Safety. 1996. Inherently Safer Chemical Processes, a Life Cycle Approach. New York, New York: American Institute of Chemical Engineers. Environmental Protection Agency. 1990. The Clean Air Act Amendments. Waslungton D.C.

Chapter 3 -Normal Process and Emergency Systems

3-9.

3-10.

3-11. 3-12. 3-13. 3-14.

Environmental Protection Agency. 2004. Accidental Release Prevention Requirements: Risk Management Program Requirements Under Clean Air Act Section 222(rX7). 40 CRF Part 68. Washington D.C. Code of Federal Regulations Occupational Safety and Health Administration. 1992. Process Safety Management of Highly Hazardous Chemicals. 29 CFR 1910.119. Washington D.C. Code of Federal Regulations Occupational Safety and Health Administration. 1920.206, Flammable and Combustible Liquids. Washington: OSHA. Department of Transportation. 173.120 Class 3 - Definitions. Washington: DOT. American Soaety of Mechanical Engineers. 2002. B31.3 - Process Piping. New York, New York. Crowl, D. A and Louvar, J, F, 2002. Chemical Process Safety: Fundamentals with Applications, 2'' Edition. New Jersey: Prentice Hall.

33

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

COMBUSTION AND FLAMMABILITY This chapter discusses combustion and flammability phenomena that may be encountered in the design and operation of vent header systems handling flammable gases and vapors. Combustions are exothermic reactions involving fuels and oxidizers, where the energy release is sufficient to cause a rapid rise in temperature. In most cases, this is accompanied by a visible flame or glow. Whether a material will bum and how energetic the combustion will be depends on the combined effects of the: Properties of the fuel 0 Properties of the oxidizer 0 Conditions present at the time of igrution, such as the temperature, pressure, and turbulence Dimensions of the enclosure Ignition energy Combustion technology has been studied widely for many years, both with respect to the hazards associated with fires and explosions and as a source of energy for fired heaters and steam raising boilers, etc. These studies have identified several combustion characteristics for evaluating and classifying the properties of fuel/oxidizer mixtures. One of the first to be studied was the flammable limits of fire damp (methane), a gas that can build-up in coal mines and, during the early MOOS, caused a series of serious coal mine explosions in the United Kingdom. In 1816, Sir Humphrey Davy [Ref. 4-11 reported the upper and lower flammable limits of methane to be 14.3% and 6.2% respectively; values that compare favorably with the currently accepted limits of 15% and 5% [Ref. 4-21. Subsequently, many other characteristics have been identified, including limiting oxygen concentration, flash point, and minimum ignition energy. These properties are used to assess hazards and design systems handling flammable materials. 35

Safe Design and Operation o f P m e s Vents and Emission Control Systems

4.1

FlammableLimits

Starting within the flammable range, as the concentration of a flammable gas in air is progressively reduced, it reaches the lower flammable limit (LFL) or the lower explosive limit (LEL), below which flames will cease to propagate if igruted. Similarly, as the concentration is progressively increased, most flammable materials reach the upper flammable limit (UFL) or upper explosive limit (UEL), above which normal combustion ceases. However, there are some exceptions. For example, materials that decompose very energetically, such as acetylene and ethylene oxide, have a UFL of 100%. In addition, certain materials take part in “cool flame combustion” above their UFLs. For further information, see Section 4.1.7. Values of the LFL and UFL in air are available in various publications [Ref. 4-21, [Ref. 4-31! [Ref. 4-41 and are normally included on Material Safety Data Sheets (MSDS) sheets. For mixtures where the ratio of oxygen to nitrogen can differ from that of air, the flammable range of any possible composition of fuel, oxidizer, and inert gas can be presented graphically as shown in Figure 4-1. In Figure 41, points A, B, and C correspond to 100% methane, oxygen, and nitrogen, respectively. Combustion of fuel lean (LFL) mixtures is fuel limited and insensitive to changes in the oxygen concentration. Conversely, fuel rich UFL mixtures are highly dependant on the oxygen concentration. For example, the UFL for methane increases by a factor of more than 4.0 when air is replaced by oxygen. Flammability diagrams for other materials are available in the Bureau of Mines Bulletin 627 [Ref.4-51, Crow1 [Ref. 4-61 and Lees [4-7)also contain further discussions on flammability. 4.1.1

Mixture Stoichiometrv

Combustion properties of flammable mixtures vary depending on the ratios of fuel to oxidizer. A mixture is stoichiometric (Cst) if the combustion is balanced and all of the fuel and oxidizer present are consumed, forming completely oxidized products, such as H20 and C02. Typically, the most energetic combustion occurs when the mixture is approximately stoichiometric and it becomes less reactive as the composition moves towards the upper or lower flammable limit. The composition of a stoichiometric mixture can be calculated for methane and air by: Equation Moles

36

CH4 + 202+ 2(79/21)Np = CO2 + 2 H20 + 2(79/21)N2 1 + 2 + 7.52 = 1 + 2 + 7.52

Chapter 4 - Combustion and Flammability

.............. Methane and air mixtures - The points where this line intersect the flammable

--A

-B

mixture envelope are the upper and lower flammability limits of methane in air.

-

Minimum oxygen concentration The point where this line is tangent to the *nose' of the flemmabiiity envelope corresponds to the limiting oxygen concentration of 12.1%. Any mixture containing less than this amount of oxygen will not bum with a flame, although catalytic oxidation may be possible.

-

Methane and oxygen mixtures The points where this line intersect the flammable envelope are the upper and lower flammable limits of methane in oxygen (61.0% and 5.1%, respectively). This compares with the flammable limits of 14.0% and 5.3% for methane in air.

Figure 4-1. Methane/Oxygen/Nitrogen Flammability at 25OC and Ambient Pressure

31

Safe Design and Operation of Pmcess Vents and Emission Control Systems

Thus, a stoichiometric concentration (Cst) contains 100 x 1/(1+2+7.52)= 9.5%methane by volume. The stoichiometric concentration can be used when evaluating the potential hazards of a process. The LFL for many organic gases can be estimated using the following equation: LFL25.c = 0.55Cst (Equation 4-1) This relationship was initially developed for hydrocarbons [Ref. 4-51; however, if test data is not readily available it can also provide a useful first estimate for predicting the LFL for other organic materials. It should be noted that even with hydrocarbons, there are some materials such as ethylene and benzene, where the method overestimates the LFL, i.e., it underestimates the flammability envelope. In the case of nonhydrocarbons, there are some materials such as ammonia and hydrogen sulfide where the results predicted are grossly in error. If the information is required for an important safety purpose, the method should principally be used to assist in initial screening purposes and subsequently confirmed by testing. The stoichiometric concentration can also be used to estimate the UFL using the following equation: UFLISoC= 4.8CS10’ (Equation 4-2)

As with Equation 4.1, this equation was also developed for use with hydrocarbons, however, it is more prone to errors than the LFL method. Consequently, it is normally only used for processes handling hydrocarbons and it should not be used if the material can decompose exothermically. Flame limits have also been related to calculated adiabatic flame temperatures by numerous investigators [Ref.4-81. 4.1.2

Factors Influencing Flammable Limits

The values obtained for flammable limits are equipment and condition specific, depending on factors such as: Direction of flame propagation (up, down, or horizontal) Dimensions of the combustion chamber and the criteria used for detecting if combustion has occurred Testmethod Temperature and pressure at the time of ignition

38

Chapter 4 - Combustion and Flammability

4.1.3

Flammable Limit Variability

Values of the upper and lower flammable limits of pure materials are available in various publications. This information has come from a variety of sources, which have used different tests, such as ASTM E681 [Ref. 4-91 and the Bureau of Mines [Ref. 4-51 method. As a result, some variability can be expected between results measured in different equipment. Consequently, when explosion protection relies on controlling the combustible concentration, it is good practice to evaluate all available published values and to design using the most conservative values, i.e., the lowest value of LFL and the highest value for the UFL. Alternatively, testing can be conducted replicating conditions that will be present in the equipment. 4.1.4

Effects of Temperature on Flammable Limits

The LFL corresponds to the concentration of fuel below which the energy released when it oxidizes is insufficient to heat the unburned gases to the temperature ( T m i n ) required for a propagating combustion. Similarly, the UFL corresponds to the maximum concentration of fuel above which there is insufficient oxygen to sustain combustion. If the initial temperature of the unburned gas is increased, the sensible heat required to raise it to Tmm will reduce. Thus, less gas needs to bum to reach Tmin and, as the initial temperature of the mixture rises, its flammable limits will widen. The specific heat of gases is relatively constant between ambient and flame temperatures; as a result the flammable limits increase approximately linearly as the initial temperature rises. Figure 4-2 [modified from Ref. 4-10, page 191 illustrates the influence of temperature on the limits of flammability of hydrogen in air. The effects of temperature on flammable limits can be estimated using the modified Burgess-Wheeler Law [Ref. 4-51 shown in Equations 4-3 and 4-4. LFL, = LFLzs - 0.75 (t - 25)

(Equation 4-3)

mc UFL, = UFL25 + 0.75 (t - 2 5 ) M

(Equation 4-4)

C

39

Safe Design and Operation of ProcessVents and Emission Control Systems

where LFL, LFLzs t

AH,

UFL, 4.1.5

is the lower flammable limit at temperature t"C is the lower flammable limit at 25°C is initial the temperature of the mixture before combustion, "C is the heat of combustion of the fuel, K Calimole is the upper flammable limit at temperature t

Effects of Pressure on Flammable Limits

As a general rule, LFLs are only marginally affected as pressure increases above ambient conditions, In contrast, the LFLs normally increase significantly with increasing pressure (Figure 4-3) [Ref. 4-10, page 561. While these are the typical effects of elevated pressures, they are not universal for all gases. For example, increasing the pressure of carbon monoxide increases the LFL and decreases the UFL, resulting in narrower flammable limits as illustrated in Figure 4-4 [Ref. 4-10, page 331. Similar effects can also be seen with hydrogen up to about 20 atmospheres. Limits of Individual Gases and Vapors

5

10 I 70

75

80

Hydrogen (%)

Figure 4-2. Influence of Temperature on Limits of Flammability of Hydrogen in Air (Downward Propagation of Flame) 40

85

Chapter 4 -Combustion and Flammability

5.000

p

4,000

E E

3,000

@

2,000 p!

a

1,000

0

Hydrocarbons, YO

Figure 4-3. Influence of Pressure on Limits of Some Paraffin Hydrocarbons (Downward Propagation of Flame) 140

120

6

7 00

5 0

tf

80

6

60

a g: 40

20

0 0

10

20

30

40

50

60

70

80

Carbon Monoxide, Percent

LEGEND Downward propageuon, cylinder, 37 x 8 an 0 Side or central ignition. sphere. 7 6 un diameter

Figure 4-4.

Effects of Pressures Above Normal on Limits of Carbon Monoxide in Air 41

Safe Design and Operation of ProcessVents and Emission Control Systems

Reducing pressure below atmospheric pressure initially has little effect on flammable limits until reaching about 300 mm Hg. Below 300 mm Hg, the LFL and UFL converge and typically meet at about 50 - 150 mm Hg and, with further reductions, the mixture ceases to be ignitable (Figure 4-5) [Ref.4-10, page 421. 4.1.6

Flammable Limits of Combined Gas Streams

Many industrial process streams consist of a mixture of several materials with different flammable limits. In some cases, the upper or lower flammable limit can be predicted based on the concentration and flammable limit of each component, using the method developed by Le Chatelier [Ref. 4-11] and modified by Crow1 and Louvar [Ref.4-12]. For example, the LFL of a mixture can be estimated using the following equation: LFL = IOOE(2i L,) (Equation 4-5) where f,

L,

is the mole % of fuel component “i” is the lower flammable limit of fuel component “i”

A similar method can also be used to predict the UFL of mixtures.

It should be noted, however, that the Le Chatelier method is not a precise calculation and, therefore, if the information is required to prevent serious hazards, safety margins should be added or the limits should be determined through testing, particularly where non-hydrocarbon fuels or water vapor are present. 4.1.7

Cool Flame

Certain materials, such as ethyl ether, acetaldehyde, and heptane are capable of cool flame combustion at concentrations above their UFLs. These combustions result in relatively low temperatures and pressures, and are typically observable as a faint glow. The low temperatures and pressures associated with the cool flames are unlikely to cause damage. However, if the oxidant concentration is increased, bringing the mixture into the flammable range, the cool flame can transition to a normal combustion, creating the potential for a fire or explosion [Ref.4-13].

42

Chapter 4 - Combustion and Flammability

800 700 600

r”

500

E

E

$ 9 a

400

ln

300 200 100

0

2

4

6

8

10

12

14

16

Methane, Percent

LEGEND Initial Temperature of Flammable Mixtures X-5OOC

0-25OC *-2OC

Figure 4-5. Limits of Flammability of Methane in Air (Downward Propagation), Showing Mluence of Pressure (Below Normal) and Temperature

Hvbrid Mixtures Flammable and combustible materials that take part in explosions include: Flammable gases (including vapors) Combustible dusts Aerosols of combustible liquids If two separate fuel phases, such as a flammable gas and a dispersed combustible dust are present, both will take part in the combustion and can contribute in establishing the LFL of the mixture (Figure 4-6) [Ref. 4-14, page 51). As a result, a mixture of a flammable gas and a dispersed combustible dust may together form a flammable mixture despite the fact that neither one

4.1.8

43

Safe Design and Operation of Process Vents and Emission Control Systems

of them are present at or above their individual LFLs. Similarly, flammable hybrid mixtures can occur with mixtures of combustible liquid aerosols and a flammable gas. Consequently, if more than one phase of fuel can be present, the combined effects should be taken into consideration and will generally involve some testing to determine the limits of flammability.

0

1

2

3

4

5

Hydrogen Content in Air (VOl.

%)

Figure 4-6. Hydrogen Content in Air 4.2

Limiting Oxidant Concentration

Limiting Oxidant Concentration (LOC), also referred to as Minimum Oxygen Concentration (MOC), is the minimum concentration of oxidant that will support combustion in any possible combination of a given fuel, oxidant and inert gas. The LOCs for flammable materials are influenced by temperature and pressure. Based on limited information [Ref. 4-51, increasing the temperature or pressures will decrease the LOC. LOCs are also affected by the inert gas’s properties, increasing the specific heat of the inert gas typically increases the LOC. For example, the LOC for an air/methane mixture diluted with nitrogen is 12% oxygen. By comparison if carbon dioxide, which has a higher specific heat than nitrogen, is used to inert an air/methane mixture the LOC increases to 14.5%. Other inert gases with lower specific heats such as helium result in correspondingly lower LOC Values. See Figure 6-3 for examples. 44

Chapter 4 -Combustion and Flammability

Figure 41 illustrates the flammability diagram for methane, oxygen, and nitrogen. The LOC corresponds to the point where a line of constant oxygen concentration is tangential to the "nose" of the flammability curve. Any mixture containing less than the LOC will not burn with a flame if ignited. As with most other combustion characteristics, LOC values are affected by the test equipment and test conditions, consequently testing may be needed for specific applications such as elevated temperatures and pressures. NFPA 69, Chapter 5, [Ref. 4-15] provides guidance on explosion protection by oxidant concentration reduction (inerting). When the oxygen concentration is continuously monitored, it recommends that either: A safety margin of 2% by volume is required below the lowest credible LOC Or if the LOC is less than 5%, the equipment shall be operated at no more than 60% of the LOC

In applications where the oxygen concentration is not continuously monitored: The facility shall be designed to operate at no more than 60% of the LOC, or 40% of the LOC if the LOC is less than 5% The oxygen concentration shall be checked on a regularly scheduled basis When inerting is being used to provide explosion protection for vent headers that terminate at a potential continuous igrution source such as a flare or incinerator, an additional flash back prevention device should be provided. Examples of flash back devices that may be appropriate include flame arresters, seal tanks and automatic fast acting isolation valves. To avoid damage, or a possible failure, these devices should be installed within the manufacturers recommended distances from the potential ignition source. Also, a flame arrester (detonation arrester) may have to be protected from the flame radiation heat, and one flame arrester manufacturer recommends installing a heat shield between the arrester and the heat source (flame). 4.3

Deflagmtions

Deflagrations are combustions in which the flame propagates into the unburned mixture by processes of heat and mass transfer at speeds less than sonic velocity (typically in the order of 1 to 100 ft/sec). The pressure developed by deflagrations depends on the degree of confinement and the 45

Safe Design and Operation of Process Vents and Emission Control Systems

geometry of the enclosure, as well as the combustion properties of the fuel and oxidizer. In most cases, if a deflagration occurs in an open unconfined location, little or no pressure will occur. Alternatively, if it occurs in a closed vessel, high pressures can develop. Optimum compositions of most gas/air mixtures typically have constant pressure flame temperatures ranging between 2,OOO"C (3,632"F) and 2,300"C (4,172"F). Depending on the composition of the flammable mixture, the number of moles formed by the combustion may be greater, equal, or less than present in the original gas mixture. Neglecting the effects of heat losses, the pressure developed by combustion in an approximately spherical closed vessel can be estimated using the general gas law: (pV = nRT) (Equation 4-6) where

P

v n R T

is the absolute pressure is the vessel volume is the number of moles is the gas constant is the absolute temperature

For most optimum gadair mixtures ignited in approximately spherical vessels, the peak explosion (deflagration) pressure will be between seven and ten times the initial absolute pressure, or between 6 and 9 bar gauge (90 and 135 psig) for a mixture ignited at atmospheric pressure. However, when combustion occurs in pipes or elongated vessels, the expanding gases can cause pressure piling or detonations that may result in significantly higher pressures. 4.4

pressurepiling

When flammable gases ignite in pipes, such as vent header systems, the expanding products of combustion can compress the gas mixture ahead of the burning material. The magnitude of this pressure rise depends on the pipe geometry, its surface roughness, the location of the ignition source, and the volume of any vessel connected to the pipe in which ignition could occur. Under optimum conditions, the unburned gases can be precompressed by a factor of eight to nine. However, more typically it results in the unburned gas mixture being pre-compressed to approximately two to four times the initial absolute pressure prior to ignition. Test work conducted with a methane/air mixture, ignited at ambient pressure in a 5 m3 (178 f3) vessel, resulted in a peak deflagration 46

Chapter 4 - Combustion and Flammability

pressure of 7.4 bar (107 psi). Subsequently, when the test was repeated with the 5 m3 (178 f3) vessel interconnected to a 1 m3 (35.3 f3) vessel using a 400 mm (15 inch) diameter pipe, depending on the location of the ignition point, pressures up to 23 bar (333 psig) were measured [Ref. 4-16, page 231. It should be noted that the conditions that favor pressure piling are also the conditions that lead to deflagrations transitioning to detonations. The possibility of pressure piling should be thoroughly considered when manifolding multiple tanks into a common vent header system. The greater the size difference between the large tank and the small tank, the larger the pressure increase, up to about a factor of four. However, a volume ratio of at least 5 to 1 seems to be necessary to observe maximum pressure piling [Ref. 4-17]. Pressure piling also occurs in pipework where the expanding products of combustion pre-compress the unburned gases ahead of the flame front. It can be prevented by explosion prevention and explosion protection methods, such as operating inerted or installing flame arresters in the vent headers. 4.5

Detonation Phenomena

Combustible gases can bum by one of two mechanisms: deflagrations or detonations. Deflagrations can occur in vessels, pipework, or in the open. Gas phase detonations almost exclusively occur in pipes or other enclosures with high length-to-diameter (L/D) ratios. Under certain circumstances, it is possible for detonations to propagate from a pipe into a vessel or for detonations to occur in vapor cloud explosions. However, in practice these events rarely occur. Unlike deflagrations, which propagate by heat and mass transfer, detonations propagate by compression heating caused by a combustion driven shock wave as it travels at or above the speed of sound through the un-reacted gas mixture. In industrial facilities, gas phase detonations typically occur as a result of deflagration flames accelerating to high velocities in pipes and then transitioning to a detonation. Prior to the transition, pressure waves from the deflagration travel ahead of it so that initially the detonation wave propagates into a pre-compressed gas. As a result, immediately after the transition there is a period when the detonation is “overdriven” after which it reduces to a “stable” detonation. The rate of rise to the detonation pressure and the subsequent decay to the deflagration pressure are extremely rapid, so much so that pipework typically does not fully respond to it. After the detonation wave has passed, the equipment will be at the deflagration pressure, which subsequently will drop comparatively slowly to ambient pressure by heat 47

Safe Design and Operation ofProcess Vents and Emission Control Systems

loss or venting to the environment. Fuel/oxidizer mixtures have detonation limits similar to, but narrower than, the limits for deflagrations (Table 4-1) [Ref. 4-18, page 701. Other information can be found in Ref. 4-7 and 4-19. Detonations can be initiated directly using a powerful explosive. This method is used in laboratories to investigate detonation phenomena. However, ignitions that occur in industry are typically not sufficiently energetic to initiate a detonation by this mechanism. 4.5.1

Deflaaation to Detonation Transition IDDT) and Run-UP Distance

Flame speeds are highly dependant on the degree of turbulence. If the unburned gas is stationary or traveling at a low velocity, the level of turbulence will be minimal and laminar burning can occur. As gas velocities increase, either due to the normal process flow or as a result of expanding products of combustion, flow will become turbulent. The turbulent burning velocity is the flame speed relative to a fixed reference point and is a function of the fundamental burning velocity and the displacement velocity caused by the expansion of the products of combustion. Turbulence causes the flame to wrinkle and fold, increasing its surface area and the rate at which products of combustion are produced. This can cause further flame acceleration, developing a positive feedback loop, and may cause the flame to accelerate to the point where it transitions to a detonation (Figure 4-7) [Ref. 4-18, page 631. The distance required for a deflagration to run-up to a detonation provides some indication of the likelihood of detonations occurring. Materials that detonate readily, such as hydrogen and acetylene, have run-up distances of only a few feet. Optimum mixtures of materials, such as propane, require considerably longer distances. Table 4-2 summarizes the results of a test investigating the detonation to deflagration transition (DDT) of a 4.3% propanelair mixture in a 3-inch diameter straight pipe, initiated at 23 psig, After 19 feet, the flame accelerated to 400 ft/second and was developing 48.3 psi overpressure. After the flame had traveled a further 5 feet, the DDT occurred, resulting in an overdriven detonation with a flame speed of 7,360 ftjsecond and a pressure of 2,044 psi. Although not shown on the table, if the detonation traveled further down the pipe after initial DDT, the detonation would have rapidly transitioned to a "stable detonation" propagating at or slightly above the speed of sound in the unburned mixture, and developing approximately half the overdriven detonation pressure.

48

CHGCH) Petrol CH30H C2H50H CzHsOC2Hs Cy& C3H6 Cycb &Hi2 C6H6 Xylene CH3COCH3 CH3CHO H2

64.5

>40

29.0 36.0 26.5 40.0

90.0

9.50

2.6

1.4 1.55 1.05 3.3

15.0 18.3

1.60

5.1 2.8

-5.6

58.9

5.55

9.8 4.5

-9.4

13.0

6.7

4.7

48.0

39.0

29.0

6.7 3.3 1.9 2.4 0.57 1.3 1.1 2.6 4.0 4.0

36.0 19.0 36.0 10.4 7.8 7.9 6.4 13.0 60.0 75.0

36.0 11.0 80.0

Upper 12.4 9.5 8.4

Table 4 1 . DetonationLimits (Vol %) for Confined and Unconfined Explosionsand FlammabilityLimits (Vol X) in Oxygen and Air

I

---+--

66.0

Upper

I

30

Lower

02

Flammability Limit

Safe Design and Operation of Process Vents and Emission Control Systems

Table 4-2. Effect of Run-up Length on Propagation Velocity and

Overpressure Run-up Length (A)

Propagation Velocity

Detonation

1

15

No

6.9

6

250

No

16.1

(W

Overpressure (psi)

19

400

No

48.3

24

7360

Yes

2044

Note: The above data are from tests in a straight 3-inch diameter pipe using a test gas consisting of 4.3 volume percent propane mixture in air, initially at 23 psia. Ignition at closed end of pipe, with an arrester and a rupture disk at the other end [Ref. 4-19, page 3751.

Features that promote a rapid transition include: Increasing flame speed, either due to the materials being more reactive (Figure 4-8) [Ref. 4-20, page 2881 or as the mixture approaches its run-up distance (Figure 4 9 ) [Ref.4-21, page E-271 The presence of turbulence inducing items such as surface roughness, fittings, and bends Increasing igrution energy Increasing pressure or reduced temperature of the un-burnt mixture Increasing pipe diameter, which typically reduces the run-up distance as expressed as length-to-diameter &/DDDT)ratio, but generally not in terms of the absolute length (Table 4-3) [modified from Ref. 4-20, page 2921 Table 4-3. Distance to Deflagration to Detonation Transition Pipe Diameter in (cm) Propane

Pipe Length ft (m) Ethylene

Hydrogen

36(11)

33 (10.1)

29.5 (9)

10 (25.4)

59.2(18)

57.5 (17.5)

49.2 (15)

16 (40.6)

82.7 (25.2)

58.7 (17.9)

49.3 (15)

6(15.2)

Chapter 4 - Combustion and Flammability

As a result, at the moment when the deflagration flame transitions to a detonation, the gas can already be at an elevated pressure. Detonation pressures are directly proportional to the initial absolute pressure of the unburned mixture (Po). Thus, when the pre-compressed gas detonates, the pressure will be increased correspondingly causing what is referred to as an ”over-driven” detonation. Figure 4-10 illustrates the effect of run-up distance on the relative overpressure, where relative overpressure (AP/Po) is defined as the ratio of the pressure rise caused by the explosion, to the absolute pressure prior to ignition. Overdriven detonations can have side-on pressures in the order of 50 - 100 times the initial absolute pressure and their reflected pressures may be significantly higher. Factors that can influence this value include the presence of turbulence inducing items, the pipe diameter, and the reactivity of the mixture. Detonation flames travel at or above the speed of sound and, as a result, the gas ahead of the detonation wave will not be precompressed by the detonation wave. Following the Deflagration to Detonation Transition @DT), the pressure will fall to the stable detonation pressure which typically will have side on pressures 18 - 30 times the initial pressure (Figure 4-10) [Ref. 4-22, page 881 and can have reflected pressures up to 100 times the initial pressure. 4.5.3

Detonation Cell Size

Detonations consist of a three dimensional shock wave followed by a reaction zone. The shock waves form a structure that is characteristic of the particular gas mixture and can be observed, indirectly, by creating an imprint on a “target” covered with a thin soot layer. Table 4-4 and Figure 4-11 [modified from Ref. 4-18, page 681 illustrate the pattern that can be obtained. Of particular significance is the cell size, which provides a measure of the mixture’s reactivity and is a function of the specific fuel and oxidizer as well as its stoichiometry. In addition, cell size decreases with increasing temperature and pressure. Reducing cell size corresponds to increased reactivity and an increased potential for a deflagration to transition to a detonation. The cell size can also be used to estimate the minimum diameter pipe in which a given material can detonate.

51

Safe Design and Operation of Process Vents and Emission Control Systems

Figure 4-7. Differences Between Deflagration and Detonation Flame Fronts

52

Chapter 4 - Combustion and Flammability

- - - 4

-1-

40

- 4

-I

2o .. 0 0

.-

.........

.

-.

- ....

..

I

I

1

I

I

I

I

I

2

4

6

a

10

12

14

16

Pipe Diameter

Legend: Flammable Gas (Flame Speed)

-.-. ...........

---

Methane (40 cmls) Propane (46 crnls) Ethene (80 cmk) Hydrogen (313 cmis)

Figure 4-8. Deflagration to Detonation Transition Distances

18

Safe Design and Operation of Process Vents and Emission Control System

0

I

I

100

200

Diameter (rnrn)

I

I

300

400

Figure 4-9. Run-Up Distances

54

Chapter 4 - Combustion and Flammability

“1

Overdriven Transition

I

20

AP -

8.0

=I

Po

0.6

0.4 I

Run-Up Distance (ft.)

Figure 4-10. Relative Overpressure Versus Run-Up Distance

Table 4-4. Detonation Cell Widths of Some Gases GasNapor

Cell Width (cm)

Acetylene

I Hydrogen

0.98

I

Ethylene

1.5 2.8

n-Butane

5.8 to 6.2

Ethane

5.4 to 6.2

GasNapor

Cell Width (cm)

Propylene

I Propane

5.4

I

Hydrogen Sulfide

6.9 10.0

Methane

28.0

I

55

Safe Design and Operation of Process Vents and Emission Control Systems

I

__

I b

Unburned Gas

--

-J

Shock Wave

Figure 4-11. Pattern of a Detonation Cell 4.6

References

4-1.

D a y , Humphrey. 1816. On the Fire Damp of Coal Mines and on Methods of Lighting so as to Prevent its Explosion. London, UK: Trans. Royal Society.

4-2.

National Fire Protection Association (NFPA). 2002. Fire Protection Guide to Hazardous Materials. (Previously NFPA 325M and 49). Quincy, Massachusetts.

43.

Department of Health and Human Services, Centers for Disease Control and Prevention. 2003. NlOSH Pocket Guide to Chemical Hazards. Washington, DC: National Institute for Occupational Safety and Health.

4-4.

Center for Chemical Process Safety (CCPS). 1998. Estimating the Flammable Mass of a Vapor Cloud. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

4-5.

Bureau of Mines. 1965. Bulletin 627, Flammability Characteristics of Combustible Gases and Vapors. Washington, D.C. United States Department of the Interior.

56

Chapter 4 -Combustion and Flammability

4-6.

Crowl, D. A. 2003. Understanding Explosions. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

4-7.

Lees, F. P. 1980. Loss Prevention in the Process Industries, Hazard Identification, Assessment and Control. London, UK: Butterworth & Co. Ltd.

48.

Hansel, J. 1992. Predicting and Controlling Flammability of Multiple Fuel and Multiple Inert Mixtures. Plant/Operations Progress, Vol. 11. New York, New York. American Institute of Chemical Engineers.

4-9.

American Society for Testing and Materials. 2005. A S T M E681-04, Standard Test Method for Concentration Limits of Flammability of Chemicals (Vapors and Gases) ASTM International.

4-10, Coward, H.F. and Jones, G.W. 1952. Bulletin 503, Limits of Flammability of Gases and Vapors. Washngton, D.C. Bureau of Mines. United States Department of the Interior. 411.

Le Chatelier, H. 1891. Estimation of Firedamp by Flammability Limits. Ann. Mines.

4-12.

Crowl, D. and J. Louvar. 1990. Chemical Process Safety Fundamentals with Applications. Englewood Cliffs, New Jersey: Prentice-Hall.

4-13.

W.A. Affens and R.S. Sheinson. Autoignition: The importance of the Cool Flame in the Two-Stage Process. Paper presented to 1999 Loss Prevention Symposium, (Houston Texas), American Institute of Chemical Engineer, New York.

4-14.

Eckhoff, Rolf K. 1997. Dust Explosions in the Process Industries. Wobum, MA: Butterworth Heinemann.

4-15.

National Fire Protection Assodation (NFPA). 2002. NFPA 69, Standard on Explosion Prmention Systems. Quincy, Massachusetts.

4-16.

Bartknecht, W. 1981. Explosions Course Protection and Prevention. Berlin, Heidelberg. New York Springer-Verlag.

4-17.

Fitt, J. S. May 1981. Pressure Piling: A P r o b h for the Process Engineer, The Chemical Engineer, Institution of Chemical Engineers.

4-18.

Grossel, Stanley S. 2002. Deflagration and Detonation Flame Arresters. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

4-19.

Center for Chemical Process Safety (CCPS). 1993 Guidelines for Engineering Design for Process Safety. New York, New York Center 51

Safe Design and Operation ofProcess Vents and Emission Control Systems

for Chemical Process Safety of the American Institute of Chemical Engineers 4-20.

Chatrathi, K., Going, J.E., Grandestaff, B. December 2001. Flame Propagation in Industrial Scale Piping. Process Safety Progress, Volume 20, No. 4. New York, New York: American Institute of Chemical Engineers.

4-21

Steen, H. and Schampel, K. 1983. Experimental Investigations on the Run-Up Distance of Gaseous Detonations in Large Pipes. 4h International Symposium on Loss Prevention and Safety Promotion in the Process Industries, Volume 3 - Chemical Process Hazards. The Institution of Chemical Engineers. Elmsford, New York Pergamon Press Inc.

4-22.

Roussakis, N. and Lapp, K. April 1991. A Comprehensive Test Method for Inline Flame Arresters. Plant/Operations Progress, Volume 10, No. 2. New York, New York: American Institute of Chemical Engineers.

I

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

5 UNDERSTANDING REQUIREMENTS Chapter 5 describes the approach for collecting essential information, identification of requirements and preliminary selection of vent system options, whch includes the following steps: 1. Understand sources - Identify conditions required to address vent gas characteristics, flammability, toxicity, reactivity, plugging (Chapter 5) 2. Identify compatible vent streams to combine (Chapter 5) - Consider physical process arrangement and materials issues (Chapter 6) - Evaluate common treatment for sub-headers (Chapter 6) 3. 4.

5. 6.

Identify objectives for end-of-pipe treatment, i.e., environmental regulatory requirements (Chapter 7 ) Identify appropriate at-source and intermediate treatment, i.e., scrubbers, entrainment separators, condensers, etc. (Chapters 5 and 7 ) Select end-of-line treatment system (Chapter 7) Specify design requirements (Chapter 6) - Understand operational requirements, e.g., startup and shutdown requirements, maintenance needs, operational needs for running parts of the system, and behavior during upset conditions - Specify management system requirements to address residual risks, e.g., special operating procedures, operating controls on valves, routine monitoring requirements and controls on work 59

Safe Design and Operation of Process Vents and Emission Control Systems

Appendix D, Vent Header Design Information Checklist, contains steps to identify and evaluate design criteria and options that are discussed in the following sections. 5.1

Understanding the Sources

5.1.1

Identify Vent Sources

All normal process vent source equipment should be identified utilizing available process description, drawings, and material and energy balances. 5.1.2

Identifv Normal Process Vent Streams

The composition and properties of normal vent streams that could enter the vent header during any normal operations should be defined, including startup, shutdown, process upset, etc. For each vent stream entering the header, the range of flows should be established that could occur during any of the normal operations. In addition, other characteristics for these streams, including temperatures and pressures, should be identified as described in the Vent Header Design Checklist in Appendix D. 5.1.3

Normal Process Vent Svstem, Design Case Scenario

Based on the normal flow characteristics and an understanding of the process, the combination of normal process vent flows that produce the most severe set of venting conditions should be identified. These conditions, with appropriate safety margins for uncertainty in the design, should be used as the ”design case” for the normal process vent system. 5.1.4

Define Interface Reauirements

The interface requirements between source vessels and vent header should be defined. This will include instrumentation for pressure control, inerting or purging, and also address failure positions for control devices. Appendix E contains examples of normal vent header source conpols and configurations. 5.1.5

Identifv Hazard Scenarios that Could Result in Emergency Venting

Scenarios that could result in emergency venting should be identified and it should be established if more than one source vessel could be venting simultaneously.

60

Chapter 5 -Understanding Requirements

In the past, failure to recognize and characterize the most severe venting scenarios (normal and emergency) has caused numerous incidents (See Appendix H). Among the most common is vacuum collapse of atmospheric storage tanks or other low pressure equipment. These incidents can occur as a result of failing to take into account situations such as: Solids build-up caused by polymer or ice forming Liquid accumulating at low points in vent header systems The development of sub-atmospheric pressure when steam, which has been used for cleaning purposes, condenses Vent systems that have not been designed for the maximum credible liquid flow out of the vessel, for example when "dumping" wash water from a tank being prepared for maintenance Gross liquid discharge into vent header system Vessel failures from overpressure are less common than vacuum collapse. However, when they occur the consequences are typically more severe, resulting in loss of mechanical integrity including spill or release of contents, possible toxic exposures, fire, explosion, or rupture with blast and missile damage. Causes have included: Failure to identify, or fully understand and design for, runaway reaction hazards Failure to keep vent header systems free of solids build-up Transfer of fluid from a process vessel operating at high pressure to a low pressure hold tank Vent Gas Characteristics for Emergencv Ventins 5.1.6 For each vent stream entering the header, the maximum credible flow into or out of the header should be established. Also, other flow characteristics should be identified, such as temperature, entrained liquids, etc., as described in the Vent Header Design Checklist in Appendix D. 5.1.7

Emerzencv Venting.Design Case Scenario

To develop an emergency venting design case, the following should be identified: Potential worst-case emergency venting scenarios from each source vessel. When designing the vent header, it should be recognized that the initial vent flow from a safety relief device is determined by the actual capacity of the device, not the design flow rate. 61

Safe Design and Operation ofProeess Vents and Emission Control Systems

Emergency venting scenarios that may occur simultaneously from different source vessels. For connected source vessels potentially exposed in a common fire zone, the emergency vent header should be sized for the combined total flow. A Fire Risk Assessment is typically used to identify fire concerns and potential mitigation measures. Refer to Fire Protection in the Chemical, Petrochemical, and Hydrocarbon Processing Industries, for steps on conducting a Fire Risk Assessment [Ref. 5-11. Similar analyses should be conducted for other sources of emergency venting such as runaway reactions resulting from common cause failures, including utility failures, contamination, etc. 5.1.8

Liquid Entrainment or Condensation in Normal Process Vent Headers

Most equipment is designed to avoid excessive liquid entrainment during normal process venting operations. Consequently, for the purpose of calculating pressure-drop, the normal process vent gases can usually be treated as a single phase gas or vapor. Despite this, it is not uncommon for vent gases to include some liquid droplets entrained from the equipment or that have formed by condensation in the vent header system. 5.2.8.2 Liquid Build-Up Implications

The design should identify if there is the potential for liquid to be present in the vent header causing a hazard, for example by accumulating at low points where it may: Restrict gas flow Block gas flow if material can freeze or solidify Cause water hammer during routine higher rate venting or when emergency venting occurs Provide a site for reactions to occur leading to solids build-up Experience suggests that liquids are likely to enter most vent headers at some point over their lifetime. Therefore, unless this can be excluded, it is good practice to provide drainage as discussed in Chapter 6. 5.1.8.2 Solids Accumulations

If the equipment or vessels connected to the vent header system handle particulate solids, for example as a feed being added to the vessel, consideration should be given to the consequences of dust being carried into the vent header. There may be the potential for these particulate solids 62

Chapter 5 -Understanding Requirements

to build-up and restrict vent gas flows (especially on flame arresters) or they may introduce, fire, dust explosion, or reactivity hazards. In cases where particulate solids are being fed to equipment or are produced by the process, where practical the design should: Provide headspace in equipment to enable the dust to disengage before it enters the vent header Maintain gas velocity in normal vents sufficiently high so that dust that has entered the vent header remains in suspension Facilitate inspection and cleaning 5.1.9

Two-Phase Venting

Circumstances should be identified that could result in two-phase venting, taking into account the following: Two-phase venting occurs when gas bubbles form in a liquid more rapidly than they can rise to its surface. This creates a liquid phase-continuous mixture containing gas bubbles that can expand and overflow into the vent system. The pressure drop caused by this "bubbly" mixture flowing through the vent system is considerably greater than it would be for the volume of gas by itself. As a result, the vessel pressure increases, potentially to the point where its ultimate strength is exceeded and failure occurs. Reactive materials are particularly vulnerable to two-phase venting due to the fact that bubbles form uniformly throughout the volume promoting a large liquid swell. As the vessel pressure increases, the boiling required for the "tempering" process is suppressed allowing the reaction rate to rise, potentially resulting in a runaway reaction. Factors that may influence two-phase venting include: High gas evolution rate (due to reaction gas evolution or heat input rate) Increased fill percent of liquid in vessel Increased height to diameter ratio of vessel Foamy liquids Viscous liquids High levels of agitation Splashing at liquid entry 63

Safe Design and Operation of Process Vents and Emission Control Systems

Two-phase venting was the subject of a major study conducted by The Design Institute for Emergency Relief Systems PIERS), which consists of a consortium made up from industry under the auspices of American Institute of Chemical Engineers (AIChE). For further details, see the AIChE publication Emergency R e k f System Design Using D I E X S Technology [Ref. 521 The potential for two-phase venting should always be considered when evaluating runaway reactions; however, it may also need to be considered for external fires, rapid depressurizing of superheated liquids, or if gas could be sparged at high rates into a liquid. Liquid properties, such as surface tension and viscosity, also play a part in determining if two phase venting will occur. 8

As part of the DIERS study, a vessel was filled 95% with water, heated to 150°C (302°F) under its own vapor pressure, and rapidly vented. Venting occurred as a two-phase mixture of steam and water with approximately 28% of the water originally in the vessel being discharged. Subsequently, the test was repeated except that 1,000 ppm of a liquid detergent was added to the water. When the water and detergent material was vented, 96% of the mixture originally in the vessel discharged as a twophase flow. These results are consistent with reports from industrial incidents where it is not uncommon for greater than 90% of the material originally in a vessel to be discharged during incidents where two phase venting has occurred. Tests conducted during the DIERS study also demonstrated that when two-phase venting occurred, the combined effects of liquid and gas substantially increased the resistance to flow in the vent system, with 2 to 10 times larger vents being needed to maintain the same pressure drop as for a “vapor only” venting [Ref.5-3, page 4321. When two phase venting is considered to be credible the design should be conducted by persons with experience in DIERS technology, and appropriate entrainment separation (e.g., knockout tanks) provided. 5.1.10 Flammable Gases and Vapors Processes that handle flammable gases or combustible liquids that could bum explosively in a vent header (see Chapter 4) should be identified.

64

Chapter 5- Understanding Requirements

Mixtures of flammable gases (including vapors) and oxidizers (e.g., air) have a range of compositions within which, if they are ignited, a propagating combustion will occur. These combustions typically begin as deflagrations; however, in pipe work they may transition to detonations. Optimum hydrocarbon and air mixtures ignited at ambient pressure in spherical vessels develop a maximum deflagration pressure of approximately 7 to 10 times the initial absolute pressure, or between about 90 and 130 psig (6.2 and 9 barg).

If the same mixtures are ignited in a pipe or duct, flame acceleration and pressure piling can result in peak pressures that are several times higher. If a deflagration-to-detonation transition occurs, the initial "over-driven" detonation pressure can be up to 100 times the initial absolute pressure (See Chapter 4, Section 4.5.2). Techniques used to address these potentially hazardous situations fall into two general classifications: Explosion prevention - where the gas composition is maintained outside the flammable region Explosion protection - where devices are provided to minimize pressure development if ignition occurs The process may also be designed for explosion containment, where the vessel is designed for the explosion overpressure. 5.2.20.1 Explosion Prevention

Explosion prevention can be achieved by: Maintaining the flammable gas concentration below its LFL, i.e., operating the system "lean" Controlling the oxidizer concentration to maintain it below the limiting concentration, i.e., operating the system "inerted" Operating with the fuel concentration above the UFL, i.e., operating the system "rich Theoretically, fires and explosions can be prevented if all possible igrution sources are totally eliminated. In most facilities, however, the number of potential sources ( e g , from human error, static electricity, frictional heating, etc.) is so great that it is not realistic to assume every one could be identified and completely prevented. Consequently, although eliminating ignition sources is a useful layer of protection, it is typically not considered adequate by itself.

65

Safe Design and Operation of Process Vents and Emission Control Systems

5.1.10.2 Explosion Protection

Explosion protection methods include: Flame and detonation arresters High speed isolation valves Explosion relief vents Chemical suppressant systems Explosion containment For further information on these approaches, see Section 6.3.1. 5.1.11 Toxic and Noxious Materials The presence of toxic or noxious materials should be identified during vent header system design. The maximum acceptable ground level concentration and the maximum quantity of material that could be released to the atmosphere should be established. Depending on the materials being handled and the facility’s location, factors that should be taken into consideration include: Local, state, and federal regulations Permitting requirements Company guidelines Industry initiatives, e.g., Responsible Care The proximity to populated areas Detectable odor for noxious materials Threshold concentrations, e.g., IDLH, ERPG, AEGL Emergency vent header systems should be designed based on the worst credible release rates identified during the hazards analysis, coinciding with atmospheric conditions that are credible for the location and that would result in the most hazardous dispersion conditions. In the case of toxic materials, treatment systems will normally be required for normal process vents and may be needed on emergency vent header systems. In addition, some materials, such as mercaptans, may have very low odor thresholds requiring treatment systems to prevent the releases from becoming a public nuisance.

66

Chapter 5- Understanding Requirements

Plants typically have alarm systems, wind directors, etc., to provide warning of a toxic gas release. Plant personnel receive emergency response training, have rapid communication systems and often have access to facilities which allow shelter-in-place, Consequently, the design basis for many plants permits higher airborne concentrations onsite than would be acceptable beyond the facility's fenceline. Sources of information to assist in predicting how personnel respond to varying concentrations of toxic materials include the Emergency Response Planning Guidelines (ERPGs) levels developed by the American Industrial Hygiene Association (AHA) which are defined in Table 5-1 [Ref. 5-41, Table 5-1. ERPG Levels

1

I

ERPGV~I~~

Human Response

1

The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing other than mild, transient adverse health effects or without perceiving a clearly defined objectionable odor.

2

The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action.

3

The maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing life-threatening health effects.

Example ERPG levels for some materials are shown in Table 5-2. Companies typically establish their own criteria for using these values; for example, by designing the emergency vent header systems to ensure the maximum offsite concentration following a worst-case release to be no greater than ERPG-1, while permitting up to ERPG-2 within the plant boundary. Note, appropriate concentrations for normal process vents will typically be considerably lower, particularly if the material has a noticeable odor.

61

Safe Design and Operation of Process Vents and Emission Control Systems

ERPG Concentration (ppm)

Material

Ammonia Chlorine Formaldehyde Hydrosen Cyanide Suhr Dioxide

Level 1

Level 2

Level 3

25

750

1

150 3

20

1

10

25

Not Appropriate

10

25

0.3

3

15

In addition to being toxic, some materials may also be flammable or reactive, such as hydrogen cyanide and acrolein. In these cases, the vent header system may also need to address explosion and reactivity considerations. 5.1.12 Reactive Svstems

Combining the vent streams from multiple source vessels into a single vent header provides the opportunity for any combination of the materials being handled in the vessels to mix together and react. This may result in a variety of undesirable, and in some cases potentially hazardous, reactions. For example, solids may form and restrict the vent header. Also, the reaction products may be thermally unstable, toxic, or corrosive. Vent headers may also provide a path by which material in one vessel can overflow to another causing an unintended reaction in that vessel. The design team should identify potential reactivity hazards. Scenarios that should be considered include cases where a material selfreacts, e.g., by polymerizing or decomposing, or when combinations of materials and conditions may cause a runaway reaction. Theoretically, this latter case includes combustion; however, the general popular perception, as well as most regulations, industry guidance, etc., treat it separately. Consequently, this book considers combustion as a topic independent from other reactivity issues. A major concern involving vessels handling reactive materials is the potential for runaway reactions causing vent rates that may be several times greater than the normal process vent flows. These reactions can be caused by: Malfunctions directly involving the vessel, e.g., mis-charged feeds, external fire, etc. 68

Chapter 5 -Understanding Requirements

An incident elsewhere in the plant resulting in contamination of material in the vessel via the vent header In the case of reactions in the vent header system, concerns include: The formation of unstable materials, such as peroxides, when materials with certain structural groups are exposed to air for prolonged periods of time. Solids build-up restricting the flow of gases in the vent header system causing excessive pressure or vacuum in the equipment. The formation of pyrophoric materials, such as phosphine and certain iron sulfides. Corrosion causing a loss of integrity, allowing toxic or flammable materials to leak into the work area or produce reaction products that may act as catalysts capable of initiating runaway reactions if mixed with process materials. Water or other heat transfer fluids may leak from condensers, heat exchangers, or other heat transfer devices resulting in chemical reactions within the vent header system or in the source equipment. Table 5-3 [Ref. 5-5, Page 321 provides a list of reactive chemical structures that can make molecules energetic. These structures may provide sites for the material to polymerize or to react with other materials, or in some instances the molecule may decompose exothermically. 5.2.12.1 Chemical Interactions

Gases and vapors generated by energetic chemical reactions may set the design requirements of a vent header system. A first step in defining a vent header system's design basis is identification of any combination of materials and conditions that could react together. Typically, even for a relatively simple system, there can be many causes of possible upset conditions that could result in reactions, some of which may involve chemistry that is significantly different from the intended process. Interaction matrices provide an organized approach for identifying material combinations that could result in unwanted reactions and scenarios that should be evaluated to establish the most severe venting case. Vent header systems often accept materials from multiple sources, and they may contain materials from previous batches and materials not present in the processes such as cooling water, corrosion products, etc. Therefore, interaction matrices should be developed both for processes and for their vent header systems. An example of an interaction matrix involving a vent header incident is shown in Table 5-4. 69

C=N-O-M HO-(O=) N= -KN-Q -N-NQ F-C-NO2 NO, -Nh -N--N;No,

met9 fulminates h i t o salts

dluoro amino canpounds N,N,NBiRuwoalkylirniines

N-azdiurnnboimidates

fluro d

i

m mmpounds

Kflbnampounds

Knitrosoampounds N-nltroso

~~~

-C-C 0

1,2epoxffles

acyl or dkyl nilrites

acyi or dkyi niimtes

+NO, NO, -GONG -GO-N=O

pdynii alkyi ampounds; pciynltloaryl c€Hrqwnds

Table 5-3. Typical Hi& Energy Molecular Structures

I

bis-arenediazosulfides

bis-arenediazooxldes bfs-arenedlaz0 oxides

arenediazoaryl sulfides

arenediamtes

I hydroxyamroniurnsdts hydroxyamroniurnsdts

I hdeaarylmetal ampounds

Nmetal derivates

daroniurn carbaxylatesand salts amine metal 0x0 salts

duoniurn sulfides and derivalies. "Xanthates"

CN=NSN=NC

CN=KSG GN=N-O-N=W

-"-OH z C-NzN-OC-

k-WX

-KM

+

-GN=N' z (N M)'Z~

G N = NS

I 1

5

4

3

2

4 FP 12'F NIA

4 FP 19O'F NIA

I

NR Vaporizes NJA

5 NIA NIA

un-re~e

vnm others NR

I

Water

1

15

I Toxic

I Oxidizer I

Nox

alkaline

NMes c*n

NJA

acidic conditions.

unstable in

NIA

Oxidizer

In alkaline conditions ammonia and NOx can react forming ammonium nitrite. This material is unstable and decomposes violently in add condions Hghly soluble in water. Can result in subatmosphericpressures. Form combustiblemixture. Form potenbaliyexplasivemixture. Fomsanrosivemixture.

NR

NR

EX? - Reaclivityunknown. further studies required FP - Flashpoint NJA - Not applicable NR - No reactiw

ESY

Low Temp.

1

NR

2

Water

NR

I NR

Ex?

NR

1

Flammable

1 Liquid

Benzene

NOx

Combustible

Nitrobenzene

CcinbustibleToxicVapor

Ammonia (gas)

Ammonia (gas)

conditions

In Vesselsl

Materials

Table 5-4. Interaction Matrix for Historical Incident (Appendix H, Case 2.2)

Safe Design and Operation of Process Vents and Emission Control Systems

In processes involving reactive materials, hazardous situations may be caused by situations that reduce or delay the desired reaction, such as under-charging catalyst, low temperature, loss of agitation, etc. In these cases, reactive materials may accumulate in the system and subsequently react violently when conditions change or when the material is transferred to another vessel with less cooling capacity. These situations can significantly increase potential process venting flow rates and can be critical to the design of the vent header system. The following interactions should be considered: All chemicals present in the process, including raw materials, catalysts, intermediates, products, and by-products Interactions between process materials and utilities, e.g., cooling water, steam, nitrogen, and plant air making note of any contaminants that may be present in it Other materials that could be present, e.g., the materials of construction, rust, cleaning chemicals, gaskets, or lubricants Materials that could inadvertently replace or contaminate raw materials, e.g., a road tanker being unloaded into the wrong storage tank or if the wrong raw material is delivered to an operating area Materials present in the environment, e.g., air, water, carbon dioxide Abnormal conditions, e.g., high or low temperature, pressure, pH, etc. Loss of agitation If all of the above interactions were included in a matrix, it would become large and difficult to use. To develop an effective interaction matrix, potential interactions should be evaluated and those that do not present a credible hazard should be eliminated. Table 5-4 illustrates the conditions that lead to the incident described in Case 2.2 of Appendix H. This incident occurred after a process upset had caused the pH in the vent header to change from addic to alkaline. This created conditions that resulted in an unstable compound forming in the header and subsequently the material decomposed violently when the pH returned to normal.

12

Chapter 5- Understanding Requirements

While the interaction matrix does not identify the specific scenario that led to the incident, it would have provided a tool that could have assisted the hazards analysis team in predicting the event that occurred (Note shaded portion of matrix). For further discussion on the use of interactive matrixes, see CCPS Guidelines far Hazard Evaluation Procedures [Ref. 5-6, page 2441. Additional details regarding chemical structures and reactivity can be found in Essential Practicesfor Handling Chemical Reactivity Hazards [Ref.5-71. The primary reason for constructing an interaction matrix is to identify combinations of materials, and conditions (temperature, pressure, pH, etc.) that could lead to chemical incompatibility. As the matrix is developed, there will typically be some combinations where there is no potential for a hazardous event. In others, the consequences may be unknown or clearly there may be a potential hazard. Table 5-4 can be helpful as an initial step in identifying materials that may be incompatible. In addition, publications such as Bretherick [Ref. 5-81, Sax and Lewis [Ref. 5-91, NFPA 497 [Ref. 5-10] and Pohanish and Greene [Ref. 5-11] can provide useful information on the stability and reactivity of many materials. Other approaches include the Chemical Reactivity Worksheet developed jointly by NOAA and EPA [Ref 5-12], which enables the user to enter a combination of materials and establish if the mixture can react exothermically. The Chemical Reactivity Worksheet is a free program that can be used to evaluate the reactivity of substances or mixtures. It includes: Database of reactivity information for more than 6,000 common hazardous chemicals. The database includes information about the intrinsic hazards of each chemical and about whether a chemical reacts with air, water, or other materials. Method to virtually "mix" chemicals to find out what dangers could arise from accidental mixing. Finally, in some cases the MSDS for individual materials may provide some general information on a material's reactivity.

In most cases, the published information is qualitative or at best semi-quantitative. Consequently, it may be necessary to conduct tests to identify the worst credible case. Typically, testing will be needed to provide the information required for designing the vent system. For further information on this subject, refer to References 5-2 and 5-5.

73

Safe Design and Operation of Process Vents and Emission Control Systems

As the reactivity hazards become more clearly defined, it may be beneficial to reevaluate the process and determine if there are changes that can be implemented to minimize hazards to make the process inherent safer. 5.1.12.2 Efects of Relief Device Set Pressure on Reactive System Venting

Overpressure protection for runaway reactions is effective when the materials being handled permit "tempering" to occur, i.e., where the reacting mass contains sufficient volatile liquid so that the latent heat of vaporization "absorbed" as it boils will control the runaway. In these situations, the set pressure of the relief device will determine the temperature required to open the relief device. For many reactions, the reaction rate, the vent size, and the capacity of any vent header and treatment system, will increase exponentially with increasing reaction temperature. Consequently, it is good safety practice and economically attractive to specify the emergency relief device for reactive systems to open at as low a pressure as practical. Following the same logic, the vent system design should be reevaluated before increasing the set pressure of the relief device, or modifying the header in a way that could increase its pressure drop, to confirm that tempering will still be effective. For more information, see Reference 5-2.

5.2

Regulatory Issues

From the beginning of the Industrial Revolution through the mid-l900s, it is a fair criticism to note that industry in general did not have protection of the environment or public health as a significant philosophical or operating concern. In more recent times, the refining, chemical processing, and related industries have been required to meet higher societal expectations in the form of new laws and regulations. The business and industrial community has become increasingly aware of the need to ensure their right-to-operate in the eyes of the general public and the specific communities in which they operate. 5.2.1

Historical Background

Air pollution from human activities began long before the Industrial Revolution. To be sure, the advent of the Industrial Revolution resulted in increasingly larger scale manufacturing and industrial operations that significantly increased the air pollution levels.

74

Chapter 5- Understanding Requirements

These operations required increasingly larger sources of energy, usually from the burning of fossil fuels. They released gases, vapors, and particulates to the atmosphere. Process facilities were frequently concentrated geographically, thus creating new or exacerbating existing local and regional air pollution problems.

In time, power generation from the burning of coal and the emissions from ore processing, metal foundries and mills, cement plants, glass manufacturing, chemical plant processes and other industries became the dominant sources of air pollution. Current United States environmental laws and regulations to improve air quality have resulted from a lengthy list of historical and more recent air pollution events.

In London, England, air pollution was a serious problem beginning as early as the 1300s as low grade coal replaced wood for heating and cooking. A few of the milestone events are: In 1306, major smoke and soot pollution prompted King Edward I to proclaim a ban on burning sea coal in London. In 1873, a particularly dense coal-smoke saturated fog in London resulted in an estimated 268 deaths. In 1909, winter inversions and smoke accumulations in Glasgow, Scotland killed over 1,000 persons. In a report about the incidents, Dr. Harold Antoine Des Voeux coined the term "smog" as a contraction for smoke-fog. In 1952, a severe sulfur-laden fog killed an estimated 4,000 Londoners and spurred Parliament to enact the 1956 Clean Air Act to reduce coal burning and begin serious air-pollution reform in England. In the United States, concern for the air quality in and around large cities was increasing during the latter 1800s and resulted in local laws and regulations followed ultimately by federal air pollution control regulations. Some of the noteworthy events included: By 1881, a few cities, such as Chicago and Cincinnati, enacted limited municipal smoke abatement laws and regulations to reduce smoke and ash from factories, railroads, and ships. In 1928, the United States Public Health Service began checking air pollution in eastern cities and reported that sunlight was reduced by 20 to 50 percent in New York City.

75

Safe Design and Operation of Process Vents and Emission Control Systems

In November 1939, the city of St. Louis experienced nine days of extreme smoke air pollution with near zero visibility at midday even with street lights on. City officials and community, business, and industry leaders developed and implemented controls and regulations; St. Louis was the first major U. S. city to limit the use of soft, low quality coal. During the late 1940s, serious smog incidents in Los Angeles further heightened public awareness and concern about this issue. In 194.8, an air pollution inversion event in Donora, Pennsylvania, killed 20 people and sickened about 40 percent of the town's 14,000 inhabitants. In November 1953, a smog incident in New York City resulted in the death of between 170 and 260 people. In 1963 and 1966, regional weather patterns resulted in air inversions that trapped local air pollutants in the New York City area, resulting in 405 and 168 deaths, respectively. More recently, international signal events involving toxic chemical releases at Bhopal, India and Seveso, Italy brought an even sharper focus on prevention of catastrophic releases and their impact on people and the environment. 5.2.2

Brief Review of Laws and Realations

These and other air pollution events led the U. S. Congress to pass the Air Pollution Control Act of 1955 that established the federal government as having preeminent control over air pollution control matters. More important for the subject matter of this book, the Clean Air Act amendments in 1967 (also called the Air Quality Control Act) required the setting of national emission standards for pollutants. These emission standards were applied across the country to all stationary sources and recommended some control technologies. The setting of one common standard for each listed pollutant triggered decades of debate between those insisting on a monolithic singular approach to regulating air pollutants and those favoring the more pragmatic approach of regulating on an industry-byindustry basis.

16

Chapter 5 - Undentanding Requirements

In 1970, Congress re-wrote the original Clean Air Act adding these major features: Established National Ambient Air Quality Standards for the most hazardous high volume pollutants, called "criteria" pollutants: - Airborne particulates (PM) - Sulfur oxides (SO) Carbon monoxide (CO) - Nitrogen oxides (NOx) Ozone(0) - Lead(Pb) Established New Source Performance Standards (NSPS) to regulate emissions from new facilities. Required identification of "other" hazardous air pollutants (HAPS)and development of standards to reduce their emissions Empowered the newly created Environmental Protection Agency (EPA) to set these standards.

-

These latter two points are noteworthy since they required the EPA to significantly reduce day-to-day "routine" emissions of those air pollutants known or suspected to cause serious health problems.

In the 1970s and 1980s, the EPA attempted to regulate air pollutants using the mandated chemical-by-chemical approach based on health risk. There were numerous legal, scientific, and policy debates over which pollutants to regulate and how stringently to regulate them. Debates focused on risk assessment methods and assumptions, the amount of health data needed to justify regulation, analyses of costs to industry, and benefits to human health and environment. This risk-based decision process ran into the inevitable risk quandary question - what level of risk is acceptable or "how safe is safe". The regulatory process proved difficult and minimally effective at reducing emissions. During the 20 years preceding 1990, the EPA was only able to implement regulations for seven hazardous air pollutants: asbestos, benzene, beryllium, inorganic arsenic, mercury, radionuclides, and vinyl chloride. Collectively, the EPA estimates that these seven standards cut annual air toxics emissions by an estimated 125,000 tons.

77

Safe Design and Operation ofProces Vents and Emission Control Systems

A new strategy was adopted by Congress with the passage of the Clean Air Act of 1990, EPA was directed to use a "technology-based'' and performance oriented approach to sigruficantly reduce emissions from major sources of air pollution, Section 112(b) of this act established a list of hazardous air pollutants (HAPs). The current list of these Hazardous Air Pollutants (HAPs) contains 188 chemicals or groups of chemicals (for details see EPA's website at http://www.eua.prov). The 1990 act required EPA to develop regulations termed National Emission Standards for Hazardous Air Pollutants (NESHAP). EPA was directed to identify the principal source industry sectors and develop regulations for each, called Maximum Achievable Control Technology (MACT) standards. These standards require the covered facilities to meet specific emission limits based on levels already being achieved by similar emitting sources in that industry sector. The 1990 act also further strengthened the National Ambient Air Quality Standards for the "criteria" pollutants established in 1970 particularly regarding the ozone precursors, NOx and Volatile Organic Compounds (VOCs). Much of this authority was delegated to the states to allow regulatory control specific to the local and regional needs for "criteria" pollutant reductions. 5.2.3

Improved Air Oualitv

Air pollution data collected by EPA indicates that this new "technologybased" approach has produced real, measurable reductions. EPA periodically reports the levels of the criteria pollutants in the air and the amounts of emissions from various sources to see how both have changed over time and to summarize the current status of air quality. These air quality trends are generated using measurements from monitors located across the country. Table 5-5 shows that the air quality based on measured concentrations of the principal air pollutants has improved and that reported emissions for these pollutants have been significantly reduced nationally over the 20-year period 1983@2002 [Ref. 5-13].

Chapter 5 -Understanding Requirements

Table 5-5. Improvement In Air Quality and Reduced Emissions 1983-2002 Air Quality Percent Change 1983. 1993.2002 Pollutant (Ambient 2002 Measured)

NOz

-21

1-hour

-22

03 &hour

-14

I so2

I

-54

Emissions Percent Change Pollutant (Reported Emissions)

-11

NOX

-21

voc

-39

I so2

(retat& to 03 formation)

4a

I

Particulate

1983. 2002

1993. 2002

-15

-12

40

-25

I

-33

I

-31

I

I

-93

I

-5

I

ParticulateMatter?

I

-94

I

-57

I PP

Note: Negative numbers indicate improved air quality or reduced emissions. Positive numbers indicate worsened air quality or increased emissions. NA: a: b: C: d: e:

Trend data not available. Not statistically signhcant. Based on percentage change from 1999. Includes only directly emitted particles. Based on percentage change from 1985, prior estimates uncertain. Lead emissions are for 1982-2001.

Based on the 1996 National Toxics Inventory data, those industry sectors defined as Major Sources accounted for about 26 percent of air toxics emissions, smaller Area Sources and other sources (such as forest fires) for 24 percent, and Mobile Sources for 50 percent. Accidental releases, which obviously contribute air toxics to the atmosphere, are not included in these estimates. Clearly, the processing and related industries for which this book is intended are major contributors to airborne pollution in the U. S., although they are not the largest source. 5.3

At-Source Treatment Options

A review of the individual process vent sources may indicate an advantage or necessity to treat the vent stream prior to entry to a common vent header. At-source treatment may be indicated if 0 Vent temperature is significantly higher than other vent streams Solids are present 19

Safe Design and Operation ofProcess Vents and Emission Control Systems 0

Liquids are present

0

Vent streams are corrosive Condensable vapors near their dew point High toxicity or reactivity

0

Recoverable product

0

Opportunities for treatment of vented streams immediately at their release source should be identified that may reduce the volume and hazards of the vent stream and allow for more economical and efficient design. Common at-source treatment options may include: Blowdown tanks

0

Condensers Entrainment separator Scrubbers

Refer to Chapter 7 for futher details on at-source treatment. Figure 5-1 provides an example of at-source treatment provided by a condenser on the normal process vent from a source vessel and a blowdown tank in its emergency vent header. 5.4

Combining Vent Streams

Identify vent sources with common or compatible characteristics and evaluate. Properties to consider include: Temperature Pressure Composition Wet vs. dry Air rich vs. fuel rich vs. inerted Toxic vs. non toxic Reactive vs. non-reactive Corrosive vs. non-corrosive Common pH sensitivity

80

Figure 5-1.

I if

;

1

Blowdown Tank

T

Condenser

Emergency Vent Header --.---------------

-b

Normal Vent Header

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -b

Example of a Blowdown Tank and Condenser Installed as At-Source Treatment Devices

:------

Safe Design and Operation of Process Vents and Emission ControlSystems

The appropriate combination of compatible vent streams may offer a number of advantages, including: A more cost-effective design, i.e., lower initial costs Lower operating and maintenance costs A reduction in the number of atmospheric release points, each of whch may require regulatory air permitting. Consequently, the regulatory permitting process and periodic reporting may be simplified For further information on the subject and a summary of potential drawbacks of combining vent streams, see Section 3.2.2 and Table 3.1. 5.5

End-of-LineTreatment Systems

The selection of the end-of-line treatment systems is largely dictated by the nature of vented materials and their quantities, as required by applicable federal, state and local regulations. Other facility specific considerations may include the desire to minimize community impact due to odor or visible plumes resulting from vented streams. End-of-line treatment systems are described in Chapter 7. 5.6

Specify Design Requirements

The requirements presented in this Chapter provide the design basis necessary to begin the detailed design, as described in Chapter 6. 5.7

References

5-1.

Center for Chemical Process Safety (CCPS). 2003. Fire Protection in the Chemical, Petrochemical, and Hydrocarbon Industries. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

5-2.

The Design Institute for Emergency Relief Systems (DIERS). 1992. Emergency Relief System Design Using DIERS Technology. New York, New York. American Institute of Chemical Engmeers.

5-3.

Center for Chemical Process Safety (CCPS). 1993. Guidelines for Engineering Designfor Process Safety. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engneers.

5-4.

American Industrial Hygiene Association. Emergency Response Planning Guidelines. 2004. http://www.aiha,org-/Committees/docnts/erpelevels.pdf

82

Chapter 5 - Understanding Requirements

5-5.

Center for Chemical Process Safety (CCPS). 1995. Chemical Reactivity Evaluation and Applications to Process Design. New York, New York: Center for Chemical Process Safety of the American Institute of Chemical Enpeers.

5-6.

Center for Chemical Process Safety (CCPS). 1992. Guidelines for Hazard Evaluation Procedures, 2"* Edition. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

5-7.

Center for Chemical Process Safety (CCPS). 2003. Essential Practices for Managing Chemical Reactivity New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

5-8.

Urben, P. G. and Pitt, M. J. 2000. Bretherick's Handbook of Reactive Chemical Hazards. Amsterdam. Elsevier Science & Technology Books.

5-9.

Sax, N. I. and Lewis, R.J. 1975. Dangerous Properties of Industrial Materials, 4th Edition. New York, New York. Van Nostrand Reinhold.

5-10.

National Fire Protection Association (NFPA). 2004. NFPA 497, Recommended Practice fur the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (Classified) Locations for Electrical lnstullations in Chemical Process Areas. Quincy, Massachusetts.

5-11.

Pohanish, R. P. and Greene, S. A. 1997. Rapid Guide to Chemical Incompatibilities. New York, New York. Van Nostrand Reinhold.

5-12.

Office of Response and Restoration, National Ocean Service, NOAA. Chemical Reactivity Worksheet. 2004. httv:l/res~onse.restoration.noaa.gov/chem~dslreact.h~

5-13.

U. S. EPA Office of Air Quality, Planning and Standards. August 2003. Latest Findings on National Air Quality - 2002 Status and Trends. EPA DOC.454/K-03-01.

83

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

DESIGN APPROACH The chapter describes design approaches for vent header systems handling flammable, toxic, or reactive materials and includes a review of mechanical design features that should be addressed as part of the total design. The design of vent header systems is determined by the: Requirements for the individual source vessels, e.g., limitations on operating pressure and temperature Individual vent streams Interactions between vent gases from individual streams The design of a vent header system should address its worst-case venting scenario; for example, an emergency venting situation involving an external fire. The design should also take into consideration other relevant factors, such as corrosion or solids build-up in the vent header. To assist in managing these requirements, it is common practice to develop a design basis that can then be used to communicate the information to members of the design team, and serve as a baseline for any future changes or modifications.

6.1

DesignBasis

Development of the design basis should begin during the preliminary design phase of the project and be updated as additional information becomes available. Typically, it will include a description of relevant venting scenarios. The vent header system should be designed to handle the conditions in the vent stream (flows, temperatures, pressures, etc.), as well as the stream’s chemical properties.

85

Safe Design and Operation of ProcessVents and Emission Control Systems

Depending on the process, other considerations may include: Presence of entrained liquids Presence of entrained solids Flammability characteristics Reactivity considerations such as polymerization Toxicity Corrosivity Discharge cold liquids into the vent header The design basis can also include at-source treatment devices to remove materials from the vent streams that might otherwise cause corrosion, solids build-up, and other operability difficulties. Equipment commonly used for at-source treatment includes condensers, scrubbers, and entrainment separators. These devices may reduce the initial cost of the vent header system and improve its operability by reducing the: Concentration of corrosive materials in the vent stream, allowing less expensive materials of construction to be used Amount of liquid entrained into the vent header Purging requirements in vent header systems that operate fuel lean Quantity of toxic gases that are handled by the end-of-line treatment devices For detailed guidance on design of pressure relief systems, refer to Guidelinesfor Pressure Relief and EffZuent Handling Systems [Ref. 6-11, Emergency Relief System Design Using DlERS Technology [Ref. 6-21! and Code for Process Piping [Ref.6-31, 62

Merging Vent Streams

Merging normal vent gas streams from several sources into a single vent header system can result in significant cost savings, as well as simplifying air permitting for the facility. Establishing which vent gases should be directed to the same vent header primarily depends on their mutual compatibilities. The economic and operational benefits of reducing the complexity of the vent header system should also be considered.

86

Chapter 6 - Design Approach

Vent streams that are incompatible by virtue of their chemical or physical properties should not be mixed, for example: Streams containing materials that react rapidly together Fuel rich and fuel lean streams that could create a flammable gas and air mixture when combined Situations where viscous liquids or solids form as a result of adding hot and cold streams Mixing streams that produce a corrosive material

In these and similar situations, separate headers should be provided for compatible streams, which can then be routed individually to the treatment system. If this is not practical, the vent headers should have separate treatment systems. 6.2.1

Features Favorable for Mereinz Streams

Process designs that favor merging vent streams are: Interconnected vessels handling the same materials connected to a common vent header, e.g,, a reactor and its downstream equipment 0 Vent streams containing compatible materials connected to a common vent header, e.g., materials that do not form flammable, reactive, or corrosive mixtures when combined Vent streams that can be handled in the same treatment device Situations where environmental air permitting may be simplified by use of combined headers with fewer end-of-pipe treatment devices and emission points 6.2.2

Features that do not Favor Merging Streams

Merging vent headers provides a path for incompatible materials to flow from one vessel to another. Potentially hazardous situations may also occur in the vent headers if incompatible gas streams are mixed. For example, if fuel rich and fuel lean gas streams are combined in a way that produces more than a very small volume of gas within its flammable limits, a significant hazard may be created. Other factors that may make it difficult or impossible to merge vent streams include situations where: Potential reactivity concerns may occur from combining streams, such as formation of: - Liquids or solids, pluggmg the header for example, due to polymerization 87

Safe Design and Operation of Process Vents and Emission Control Systems

-

Thermally unstable materials Toxic materials Corrosive materials, e.g., by mixing anhydrous HC1 and a moist gas Volume of hazardous gases may be increased by combining toxic with relatively innocuous streams, resulting in the need for prohibitively large treatment devices Volume of corrosive gases may be increased by combining corrosive with relatively non-corrosive materials Liquid condensation may occur when vent streams of different temperatures are combined Interconnecting emergency vents from vessels with widely different desigdoperating pressures may result in high backpressure in the header, restricting flow from low pressure vessels Potential for more extensive and complex cleaning requirements for combined vent systems Geographic separation may significantly influence the practicality of interconnecting vent headers

In addition to safety and operability issues, economic factors also impact the decision of combining vent streams. Economic factors that may make merging vent streams less desirable are: Additional complexity in selecting appropriate intermediate and end-of-line treatment systems may be caused by combined streams Additional instrumentation may be required to monitor conditions in the header or sub-headers, e.g., to detect pressure, temperature, purge gas flows, etc. Mergng streams of different compositions may make it difficult or impossible to recover solvents or other valuable materials 6.3

Vent Header Systems Handling Flammable Materials

This section discusses methods for addressing hazards associated with the combustion of gas/air mixtures in vent headers, including approaches involving explosion prevention and explosion protection.

88

Chapter 6 -Design Approach

6.3.1

Explosion Prevention

Explosion prevention is typically achieved by controlling the concentration of fuel, oxidizer, and inert gas to ensure the vent gas composition remains outside the flammable envelope. Provided this is maintained and there are no significant changes in the temperature or pressure, combustion will not propagate even if ignition sources are present. When possible, credible ignition sources should be eliminated; in cases where this is not practical, explosion isolation, e.g. flame or detonation arresters, should be provided. Referring to the flammability diagram in Figure 6-1, three zones exist in which combustion will not propagate. Explosion prevention in vent header systems can be achieved by maintaining the vent header gas composition in any one of these zones, as discussed below: Operatingfuel lean with the fuel concentration maintained below the lower flammable limit (LFL) corresponding to the zone below Section X to Y of the flammability curve lnerted vent header systems operating below the limiting oxygen concentration needed to support combustion, corresponding to compositions containing less oxygen than is present at Point Y Operating fuel rich with fuel concentration above the upper flammable limit (UFL), corresponding to the region above the Section Z to Y of the flammability curve

Explosion protection includes systems to mitigate the explosion after ignition has occurred. Examples include: Explosion (deflagration)venting Deflagration protection by isolation Containment of explosion Within the United States, the National Fire Protection Association (NFPA), NFPA 69 Standard on Explosion Prevention Systems [Ref. 6-41 has been widely adopted in building and fire codes. It addresses the explosion (deflagration) prevention measures discussed above, as well as the use of high speed isolation valves and chemical suppression systems. Explosion venting may also provide appropriate protection for vent header systems. For guidance, refer to NFPA 68 Guide for Venting of Deflugrations [Ref. 6-51. Further information on this topic can be found in Section 6.3.6.1.

89

Safe Design and Operation of Process Vents and Emission Control Systems

*............ Methane and air mixtures - The points where this line intersect the flammable

- .- -

mixture envelope are the upper and lower flammability limits of methane in air.

Figure 6-1.

6.3.2

-

Minimum oxygen concentration The point where this line is tangent to the "nose" of the flammability enveiope corresponds to the limiting oxygen concentration of 12.1%. Any mixture containing less than this amount of oxygen will not burn with a flame, although catalytic oxidation may be possible.

Methane-Oxygen-Nitrogen Flammability at 25°C and Ambient Pressure

Operatine:Fuel Lean

Explosions can be prevented by operating vent headers fuel lean, in other words by maintaining the flammable gas concentration below its LFL. The LFL for many gases falls in the region of 1 - 3 percent and, typically, the remaining gases are mainly air. Thus, the oxygen concentration of fuel lean systems is normally close to that of ambient air.

90

Chapter 6 - Design Approach

Mixtures containing less than the LFL do not burn because there is insufficient heat released when they oxidize to reach the minimum temperature required to sustain combustion. As a result, apart from the area close to the "nose" of the curve where oxygen is also beginning to limit, LFL values do not vary greatly with changes in the oxygen concentration. When operating fuel lean, the following criteria should be applied [Ref. 6-41: The concentration of flammable gases should not exceed: - 25% of the LFL, or

-

60% of the LFL, where control instrumentation with safety interlocks are provided. Flammability analyzers for explosion protection should be able to respond quickly to allow the controls (interlocks) to operate before a flammable gas concentration is reached. Gas concentrations may change rapidly. Different types of flammability analyzers have a range of response times. For more information, see Section 6.3.2.2,Flammable Gas Detection. Redundant analyzers are typically provided for systems that operate above 25% of the LFL with the explosion protection interlocks operating from the instrument with the highest reading. Initial instrumentation and maintenance costs can be significant. Allowance should be made for possible variations in the temperature, pressure, and composition of vent gases during any operating phase, such as start-up, shutdown, etc. Relatively large changes in temperature or pressure are required before the flammable limits change significantly. For information concerning the effects of temperature and pressure on flammable limits, see Sections 4.1.4 and 4.1.5. The design should consider the variability inherent in combustion data. Instrumentation should be provided to monitor the control of the concentration of combustibles, such as air dilution flowrate, flammable gas detectors (possibly including its internal diagnostics). In practical terms, processes can often be operated fuel lean by selecting appropriate process design conditions, such as choosing solvents with high flash points, using dilute aqueous solutions of flammable materials, and controlling the temperature of combustible liquids to maintain the vapor concentration sufficiently below the LFL. Provided these 91

Safe Design and Operation of Process Vents and Emission Control Systems

conditions can be met and maintained, no further explosion protection is normally required. If non-condensable gases could be present or the flammability criteria cannot be met, the header may need to be purged with a nonflammable diluent gas, such as air. Alternatively, a different form of explosion protection, such as inerting or operating above the UFL may need to be considered. 6.3.2.1 Principles of A i r Purging In some instances it is not practical to operate the source vessels in a way that assures the vent gases are always outside the flammable zone. In these cases an alternative may be to provide an air purge to rapidly dilute flammable vent gases to below their LFL. This approach inevitably causes the composition of the gas mixture to pass through the flammable zone at some point in the system (see the “aii‘ line on Figure 6.1). To minimize the volume of flammable gadair mixture formed while the gas is being diluted, the airflow and pipe work should be designed to provide rapid mixing. To be reliable and effective, air purging requires: Sufficient air flow through the header, reducing the flammable gas concentration to 25% (or 60% if appropriate controls and interlocks are provided) A minimum velocity of gases in the vent header to ensure flammable and dilution gases will mix rapidly Sufficient mixing distance downstream of the point where flammables enter the vent header. The paper, Ensure Process Vent Collection System Safety [Ref 6-91 suggests minimum distances that should exist between the point where vent gases are mixed with air and potential ignition sources, as follows: - There should be a minimum of 20 pipe diameters separation between the air purge “mixing point” and any potential source of ignition if either of the following apply: The gas stream entering a vent header can exceed 25% of the LFL but will not exceed 60% of the LFL The oxygen concentration will be at least 2%below the LOC (or 60% of the LOC if the LOC is less than 5%). Note, if the gas streams entering a vent header are always below 25% of the LFL, explosion protection is not required. In these cases, it is recommended that the vent streams should be monitored periodically to confirm the concentration has not changed

.

92

Chapter 6 - Design Approach

There should be a minimum of 30 pipe diameters separation between the air purge "mixing point" and any potential source of ignition if either of the following apply: The gas stream entering a vent header can exceed 60% of the LFL The oxygen concentration could be higher than 2% below the LOC In these cases, the design should include: The air/vent gas mixing zone should be within 30 feet of the source vessel There should be a detonation arrester within 20 feet of the source vessel(or closer if specified by the manufacturer) Dilution air that is not contaminated with flammable gases or other undesirable components Monitors and alarms to verify that purge gas is above the required rate. Where a utility failure could result in the loss of air flow, consideration should be given to providing an alternative purge gas supply or back-up electrical power It is common practice to locate air addition points to prevent forming dead pockets. If the header is branched, purge gas should be introduced at the end of each header If more than one flammable gas could be present, the flammable limits of the mixture should either be determined experimentally or estimated using the Le Chatelier method described in Section 4.1.6 [Ref.6-61

. .

Figure 6-2 shows a typical vent header system employing air purges for explosion protection. Features that should be considered in the design include: Dilution air source and flow monitoring Flashback prevention Sufficient distance and turbulence to provide thorough mixing of air with the flammable streams

93

Safe Design and Operation of ProcessVents and Emission Control Systems

The dilution air should be taken from a source that is not subject to contamination with flammable gases. In cases where flammable gas contamination cannot be excluded, instrumentation and interlocks should be provided to prevent combustible gas/air mixtures occurring in the vent header. Moisture in the dilution air may cause corrosion in the header or adversely affect product quality. To minimize these effects, the air inlet should be located to avoid sources of moisture, such as cooling tower plumes, steam vents, etc. The dilution airflow should be monitored using instrumentation. Alarms and interlocks should be provided if loss of flow could result in a flammable mixture forming in the header. Flashback prevention, such as flame arresters or seal drums should be provided at the inlets to flares, thermal oxidizers, and carbon adsorption beds. Carbon adsorption beds should be evaluated to determine if they represent a fire hazard [Ref.6-7 and 6-81. If so, temperature alarms should be provided. See Section 6.3.7.1, Carbon Adsorption Beds, for more detail. Some key instrumentation considerations regarding flashback prevention devices are: Flame arresters should be equipped with temperature sensing instruments to provide warning if a flashback has occurred and burning continues on the face of the arrester element. For further details, see Section 6.3.6.4, Flame Arresters. Seal tanks should have instrumentation to monitor their liquid level to ensure they function as designed and freeze protection for cold weather installation. The design should ensure dilution gas and the vent streams mix before being exposed to a potential ignition source, such as a fan or a combustion treatment system. This is typically achieved by providing at least twenty (20) pipe diameters between the point where the vent joins the header and potential ignition sources [Ref. 6-91.

94

Chapter 6 - Design Approach

Mercaptan Blend

FP 80°F

I

Loading

Figure 6-2.

Example of a Vent Header System With Air Purges

Emergency relief vent flows are usually substantially higher than the flows from normal process vents and may occur at times when utilities have been lost. Consequently, air purging is not normally used to maintain an emergency vent header in the lean regime for explosion prevention. 6.3.2.2

Flammable Gas Detection

The most common type of flammable gas detector uses a heated catalytic bead to oxidize the flammables present in the gas stream being sampled. The oxidation causes a temperature rise proportional to the amount of flammable gas in the gas stream. From this data the flammability can be determined, and is typically displayed directly as % of LFL. When selecting this type of detector the following should be taken into consideration:

95

Safe Design and Operation ofProcw Vents and Emission Control Systems

The gas sample being measured must contain enough oxygen to completely oxidize all the flammable gas present. If the amount of oxygen is insufficient due to an inert gas being present, or if the mixture is fuel rich, h s type of detector will give inaccurately low readings. If there could be insufficient oxygen present there are flammable gas detectors available that automatically add a measured amount of ambient air enabling them to operate in oxygen deficient atmospheres. The hot catalytic bead can act as an i p t i o n source. Flammable gas detectors are typically constructed to meet Class 1, Division 1, Group B through G, National Electrical Code requirements. Generally they are not suitable for Class 1 Division1 Group A materials such as acetylene. The catalyst loses activity over time and if not recalibrated the detector will give a low, potentially unsafe reading. This loss of activity can be accelerated by certain materials, or if the gas stream goes "rich for a prolonged period. It is therefore important to check the calibration of the detectors on a regular basis. Flammable gas detectors using other operating principals, such as infrared absorption and thermal conductivity are available. These devices can be very material specific and therefore they are only applicable when the vent stream composition is known and where any change to it will not influence the instrument's calibration. Regardless of which type of detector selected, its applicability for the type of use should be reviewed with the manufacturer. 6.3.2.3

Advantages and Disadvantages ofFuel Lean Vent Header Systems

When operating fuel lean, any additional air will move the composition away from the flammable region. Consequently, fuel lean vent systems can be operated below atmospheric pressure to reduce the potential for leaks to the operating area as any inflow of air will not create a safety hazard. Systems that require dilution gas to remain lean may transition to the explosive region if the flow of dilution gas stops. The advantages and disadvantages of fuel lean vent header systems are summarized in Table 6-1.

96

Chapter 6 - Design Approach

Table 6-1. Advantages and Disadvantages of Fuel Lean Vent Header Systems

6.3.3

Operating Inerted

Inerting provides explosion protection by replacing oxygen with nitrogen or some other gas that will not support combustion. As the concentration of oxygen is reduced, a value referred to as the limiting oxidant Concentration (LOC) will be reached, below which the combustion reaction can no longer generate sufficient energy to produce a propagating flame. As discussed earlier, Figure 6-1 illustrates the flammable envelope for mixtures of methane, oxygen, and nitrogen, where the LOC (sometimes referred to as the minimum oxygen concentration) is the oxygen concentration at point "Y". Below this oxygen concentration, no mixtures of methane, oxygen, and nitrogen can burn. The LOC is not a "state property" for a given flammable gas, in other words, it changes with different inert gases and varying initial temperatures and pressures. Figure 6-3 [Ref. 6-10] compares the effectiveness of different inert gases to prevent combustion of air/methane mixtures. Differences between their inerting effectiveness are largely a function of their heat capacities. Carbon dioxide has the highest heat capacity and is also the most effective. Argon and helium have the same heat capacities; however, helium is a more effective inerting agent, due to its higher thermal conductivity. With this exception, the inerting effectiveness of the remaining gases corresponds to their respective heat capacities.

91

Safe Design and Operation ofProcess Vents and Emission Control System

LIMITS OF INDIVIDUAL GASES AND VAPORS Oxygen in Original Atmosphere (X)

0

5

10

15

20

25

30

35

40

45

50

55

Gases In Original Atmosphere (%)

Legend

.............. Argon

Figure 6-3.

98

Helium

- .- .-

---

Nitrogen Water Vapor Carbon Dioxide

Limits of Flammability of Methane in Separate Mixtures of Air with Carbon Dioxide, Water Vapor, Nitrogen, Helium, and Argon

Chapter 6 - Design Approach

NFPA 69: Standard on Explosion Prevention Systems [Ref. 6-41 specifies criteria for providing explosion protection by operating below the LOC; these include: If the oxygen concentration is continuously monitored, the concentration should be controlled to maintain a safety factor of at least 2% by volume below the LOC, unless the LOC is less than So/, in which case the concentration should be no greater than 60% of the LOC. If the oxygen concentration is not being monitored continuously, the system should be designed to operate at no more than 60% of the LOC or 40% of the LOC if the LOC is below 5%. Consideration should be given to the worst credible upsets that could ocmr during all phases of operation, such as start-up, shutdown, etc., and should include the effects of possible changes to the temperature, pressure, and composition of the vent streams. The inert gas should be compatible with the process materials. In facilities where the oxygen concentration is not being monitored continuously, it should be checked on a regularly scheduled basis. It is good practice to install a flashback prevention device on the inlet to any treatment system that could be an ignition source. Specifically, flashback arresters are required on all inlets to catalytic oxidation treatment systems [Ref.6-4, Section 6.3.21. 6.3.3.1 Principles of lnerting Figure 6-4 shows an example of an inerted vent header system designed to operate below the limiting oxidant concentration. Features that should be considered in the design of an inerted vent header system operating below the LOC include: Reliability of the inert gas supply Compatibility of the inert gas with the vented streams Temperature and dew point considerations for the inert gas Maintaining vent header at a positive pressure, including provision for vacuum relief for connected vessels Satisfying the oxygen concentration requirements throughout the vent header system Personnel hazards of inert gases 99

Safe Design and Operation of Process Vents and Emission Control Systems KO Tank

From PCV -Vessel 2

From PSV - Vessel 2

Figure 6-4.

100

Example of an Inerted Vent Header System

Chapter 6 - Design Approach

The inert gas supply should be reliable and compatible with any materials received by the vent header, for example carbon dioxide reacts with amines forming solid deposits and both moisture and carbon dioxide may promote corrosion of carbon steel. The temperature of the inert gas should be compatible with the process being protected. Excessively high inert gas temperatures may adversely affect product quality and low temperatures may cause condensation. If liquid nitrogen is being used as a supply of inert gas, measures should be taken to prevent it cooling the steel below its ductile/brittle transition temperature, making it vulnerable to brittle failure.

To prevent moisture condensing, the inert gas should have a dew point below the minimum temperature it could be exposed to in its distribution system. The inert gas pressure control valve should be located above the vessel and the line from it sloped continuously to the vessel. This is particularly important for atmospheric storage tanks that typically operate at 3 to 5 inches of water gauge positive pressure. In these applications the back pressure caused by a few inches of liquid accumulating in the inert gas supply line can result in pressure cycling or may completely stop the inert gas flow to the tank. Lowering the oxygen concentration in a flammable gas mixture to below its LOC will prevent combustion occurring. In most applications, the amount of flammable gas in inerted systems is more than enough to form flammable mixtures if air is added. Consequently, if air enters there is the potential to form a combustible fuel/air mixture that could burn explosively. It is desirable to operate inerted vent headers with a small positive pressure to minimize the potential for air entering the system. The potential for large amounts of air ingress is particularly important from source vessels associated with vacuum operations. The capacity of vacuum systems (vacuum pumps, steam ejectors, etc.) is normally based on vent rates as well as an air leakage rate dependant on the size and type of equipment the vacuum is provided for. Vacuum systems typically have the capacity to introduce large amounts of air, which must be addressed in considering the safety of the process as well as the vent header. Some typical measures to address this risk are monitoring vent rates and oxygen content with appropriate controls and interlocks to provide inerting and a safe shutdown when problems arise. 101

Safe Design and Operation of Process Vents and Emission Control Systems

The design should address vacuum relief for vessels to prevent air from being drawn into the vent header system. Provision should be made to introduce inert gas into vessels when they are emptied. In the case of a batch operation where the vessels are emptied frequently, this can be achieved by installing an automatic inert gas make-up system. Consideration should be given to the potential for flammable gas/oxidizer mixtures to form in a vessel and subsequently to enter the vent header system and ignite. Potential mechanisms include air entrained with particulate solids being fed to a vessel and oxygen formed by the decomposition of peroxides. If there is the potential for oxidizers to accumulate, an inert gas purge should be provided.

To ensure that the atmosphere in all parts of a vent header system is below the LOC, particularly in sub-headers or branches, inert gas purges should be installed at the following locations: At the upstream ends of the main header(s) At the equipment end of each sub-header, as a minimum to be used to purge the header prior to start-up Immediately upstream of the seal tank providing backflow protection from the treatment device Equipment and headers or sub-headers removed from service and opened to atmosphere should be purged and air-freed before they are returned to service. By design, an inerted vent header system operated at a positive pressure provides a means for inert gases to be present in any connected vessels and equipment. The use of inert gas for explosion protection introduces the hazard of personnel being asphyxiated if they enter vessels that have not been adequately ventilated to replace the inert gas with air. Procedures and training should be in place to address vessel entry, sampling, etc., when inerting is being used.

To ensure that a non-inerted zone does not exist in vent header systems, a minimum flow rate should be maintained. One suggested value for a minimum flow is 0.1 ft/sec for 6 inch (15cm) and smaller vent headers. AS with any vent header system, liquid build-up in the vent header pipework can be both a safety and operability concern.

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6.3.3.2

Advantages and Disadvantages of lnerted Vent Header Systems

Unlike flammable limits where there is a great deal of data available, there is little published information on LOC values. Consequently, testing may be required unless it is intended to operate significantly below an estimated LOC value. One company uses a value of 5% of the LOC. See Chapter 4 for further information. Operating below the LOC will prevent gas phase deflagrations; however, if there are dust layers present they may be able to smolder in atmospheres containing less oxygen than required for gases to burn (except a small number of gases that decompose exothermically, such as acetylene and ethylene oxide). Operating below the LOC will not provide explosion prevention for a small number of flammable gases, such as ethylene oxide and acetylene, which can decompose with sufficient energy to produce a flame in the absence of an oxidizer. Operating in an oxygen deficient atmosphere may allow potentially hazardous materials to form and accumulate, for example, carbon steel equipment handling sulfur containing materials in oxygen deficient conditions may form pyrophoric sulfides. Provisions should be made to prevent these materials from becoming an ignition source [Ref. 6-11, Section 154.8089(j)]and [Ref.6-12). The advantages and disadvantages of inerted vent header systems are summarized in Table 6-2. Table 6-2. Advantages and Disadvantages of Inerted Vent Header Systems Advantages Operating below the LOC will prevent gas phase deflagrations. Further additions of nitrogenwill move its composition away from the flammable zone. Increased concentrations of flammable gases in the vent header will not move the composifion into the flammable region.

Disadvantages Limited published information on LOC values. If air leaks into an inerted system, a flammable mixture may form. Some "inert gases" can react with the materials of construction used for vessel and piping forming additional hazards. A small number of flammable gases can decompose exothermically with sufficient energy to produce a flame in the absence of an oxidizer.

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Safe Design and Operation ofProcess Vents and Emission Control System

6.3.4

Operating Fuel Rich

Explosion protection can be achieved by operating above the UFL identified as the Section Y-Z of the flammability chart in Figure 6-1. Approaches include operating with at least 25% by volume of natural gas or methane in the vent header, or not less than 170% of the UFL when mixtures of other gases are involved [Ref. 6-13, page 194). If the latter approach is applied, testing should be conducted to establish the LiFL as a function of the stream compositions for worst credible case conditions. Other considerations for the operation of fuel rich vent header systems include: Operating above the UFL is not applicable for gases that can decompose energetically, such as ethylene oxide and acetylene. Systems involving other gases that decompose less energetically may require increased safety factors. The addition of air to a fuel rich system can produce a mixture that can bum explosively if ignited. Consequently, the vent headers should be operated at a small positive pressure to minimize the potential for air ingress and, if the system includes fans or other devices that could create a sub-atmospheric pressure, there should be instrumentation and interlocks to prevent this occurring. Nitrogen is a less effective diluent than hydrocarbon gases. Consequently, the LOC for a mixture containing nitrogen will be lower than if it only contains hydrocarbon gases. Oxygen analyzers should not be used to control the addition of enrichment gas to mixtures containing nitrogen. For further information on the subject, see References 6-4/6-11 and 6-13. 6.3.4.1

Principles of Operating Fuel Rich

Figure 6-5 shows an example of a vent header system operating fuel rich. These systems are typically used in refineries or other locations such as barge unloading [Ref. 6-13] where there is a readily available source of fuel gas and commonly include a recovery system to recycle a significant portion of the stream exiting the header. Features that should be considered in the design include the following: Purge gas supply should be reliable and compatible with any materials that could be in the vent header. Purge gas should not contain materials that could condense or cause solids to accumulate in the vent header. 104

Chapter 6 - Design Approach

The design should be based on the maximum credible variation in vent gas composition. If the temperature or pressure in the vent header could differ sigruficantly from the test conditions used to determine their flammable limits, these variations should be taken into consideration when determining the flammable limits of the vent gases. Distance should be provided upstream of any potential ignition source (incinerator, carbon bed, fan, etc.) to facilitate mixing of the purge gas and vent stream entering the header.

[ T o Fuel Gas Recovery KO Tank

........... ......

;

,-*-;-

m

Drain

..I..&... .......t To Flare

:

:I--

Nalural Gas Supply

From PCV

Process Vessel 1

Figure 6-5.

- Vessel 2

~...,..................,...,......,...................*~ From PSV

Vessel 2

Example of a Rich Vent Header System 105

Safe Design and Operation of Process Vents and Emission Control Systems

6.3.4.2

Advantages and Disadvantages of Fuel Rich Vent Header Systems

Fuel rich systems contain gases that are combustible and will burn in a flare without further enrichment. By comparison, fuel lean mixtures require additional enrichment to meet EPA requirements [Ref. 6-14]. Specifically, a net heating value of 300 BTU/scf (if steam assisted) or 200 BTU/scf (if not steam assisted) is necessary to achieve a 98% destruction efficiency. Mixtures that are above the UFL can become flammable if air leaks into the system. The system may also become flammable if the temperature drops, allowing the enrichment vapors to condense. Operating above the UFL is generally considered to be inherently less safe than operating below the LFL. The phenomena referred to as cool pame combustion is possible with certain materials, such as aldehydes, ethers, ketones, and hexane. This occurs in fuel rich mixtures, and although the temperature and pressure developed is low, if sufficient air is added to bring the mixture into the flammable limits, the cool flame can transition to a normal combustion and represent a potential igrution source. Typically, the volume of fuel gas required to operate fuel rich is less than the volume of nitrogen needed to operate inerted or the volume of air required to operate fuel lean. In these instances, there is the potential to reduce capital cost by providing smaller vent headers and treatment units. The advantages and disadvantages of fuel rich vent header systems are summarized in Table 6-3. Advantages

Disadvantages

Fuel rich systems will bum in a flare without enrichment.

Potentialfor system to enter flammable region if air leaks in.

Potential lower capital cost with smaller vent headers and treatment units.

Cool flame combustion is possible with certain materials. Enrichment vapors may condense, resulting in a flammable mixture. Vent headers that operate at subatmospheric pressure are susceptible to air ingress, resulting in a flammable mixture.

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Chapter 6 - Design Approach

6.3.5

Oxidizers Other Than Oxvgen

In addition to combustions involving air or pure oxygen, many flammable gases can react with other gaseous oxidizers, such as chlorine, fluorine, and some oxides of nitrogen. In doing so, they can generate heat, light, and pressure and produce propagating flames that are very similar to those involving air or oxygen. The combustible gases have upper and lower flammable limits with these oxidizers, beyond which flames will not propagate. If the oxidizer concentration is reduced sufficiently, combustion will cease. Consequently, the same explosion prevention approaches can be applied with these gases as for oxygen or air. Table 6-4 compares the flammable limits for three flammable gases when mixed with different oxidizers. Generally, the UFLs fall between air and pure oxygen and LFLs are relatively close to the values for oxygen or air. The peak deflagration pressure for hydrogen in chlorine is higher than with air, 8.5 barg (123.28 psig), compared with 6.5 barg (94.27 psig) in air. Based on this, if a mixture of flammable gas and one of these oxidizers formed and ignited in a closed system, a potentially hazardous explosion could occur, as described in Appendix H, Case 1.1. As a result, any vent header that has the potential for a gaseous oxidizer to mix with flammable gas should be evaluated to identify potential fire and explosion hazards and to define appropriate preventive measures. Table 6-4. Flammable Limits of Various Flammable Gas and Oxidizer Mixtures, and Peak Deflagration Pressure for Hydrogen in Chlorine

Explosion Protection for Processes Involving Oxidizers Other Than Oxygen Standards, such as those developed by NFPA, primarily apply to combustions involving air and there is very little published information for combustions involving other oxidizers. Consequently, explosion protection for facilities that handle oxidizers other than air or oxygen typically requires test work to define their combustion properties. 6.3.5.1

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Safe Design and Operation of Process Vents and Emission Control Systems

Once this information is available, explosion prevention can be developed on the basis of operating fuel lean, fuel rich, or by maintaining the oxidizefs concentration below the minimum that can support combustion. When doing this, appropriate safety margins should be incorporated which, due to the lack of comparable information, will normally be greater than used by NFPA for combustions involving air or oxygen. In addition, it should be recognized that the flammability data should be based on the most severe conditions that could be present and that typically flammable limits increase with increasing temperature and pressure. 6.3.6

Explosion Protection

to The term Explosion Protection refers to measures that can be employed . . minimize the effects of a deflagration after an ignition has occurred. They are typically used in addition to designing vent systems to operate outside the explosive zone and can include: Providing explosion vents in accordance with Chapter 8 of NFPA 68 [Ref. 6-51 Installing flame arresters or other explosion isolation devices between potential igrution sources and the rest of the vent header Minimizing the length of pipe runs and avoiding turbulence inducing items, such as valves and bends between potential igrution sources and isolation devices. Providing explosion prevention systems on vessels handling flammables that are connected to the vent header Designing the header for explosion (deflagration) containment Explosion protection systems are typically used to provide a second layer of protection in normal process vent headers. Generally, they are not applicable for emergency vent headers. 6.3.6.7

Explosion (Deflagration) Venting

Explosion venting provides a method to prevent pressure piling that could otherwise generate high pressures in the header and may produce conditions favorable for a deflagration to transition to a detonation. Figure 6-6 shows examples of explosion vents [Ref. 6-15]. Explosion venting does not, by itself, extinguish the flame. Consequently, despite protecting the vent header from mechanical damage, the explosion vents will not prevent combustion propagating to any part of the header where a flammable fuel/air mixture is present. If needed, this can be addressed by providing a second layer of 108

Chapter 6 -Design Approach

protection, such as a flame arrester or an automatic fast acting valve, located as close as possible to potential ignition sources.

Figure 6-6. Explosion Vent on Vent Header (Courtesy of Fike Incorporated)

When explosion relief vents function, a mixture of unburnt and burning gases is discharged to atmosphere, creating a potentially hazardous fireball [Ref. 6-51, Consequently, explosion relief vents should be directed to locations that are unoccupied and away from equipment that could be damaged. In practice, it generally takes a relatively large number of explosion vents to protect a complete vent header system. Identifying appropriate locations for all of them can be a serious challenge. The number and location of the explosion vents depends on the initial gas velocity prior to ignition, the geometry of the header, and the combustion properties of the gas. NFPA 68: Guidefor Venting ofDef2agrations [Ref. 6-51 provides a method for determining the maximum permitted distance between explosion vents (of equal area to the cross section area of the header) for sections of the header that do not have bends or other 109

Safe Design and Operation of Process Vents and Emission Control Systems

turbulence inducing items. Additional explosion vent requirements and other design considerations include the following: Explosion relief vents should be located as close as possible to any potential ignition sources (such as a blower or a mill). Explosion relief vents should be provided on each side, and located within three pipe diameters, of any turbulence producing object, such as a 90 degree bend. The NFPA 68 method is limited to initial gas velocities up to 66 ft/sec (20 m/sec). Note: in practical terms t h ~ smay exclude the use of explosion venting in emergency relief headers. Deflagrations that begin in vessels connected to a vent header may cause powerful flame jet ignitions capable of initiating a detonation. Explosion vents can provide protection against deflagrations; however they are ineffective with detonations. As a result, vessels connected to vent headers should be provided with explosion prevention; for example, operating them inerted or controlling the vent gas composition outside the flammable limits. The weight of the explosion vent panels should not exceed 2.5 lb/ft2 (12.2 kg/m2) and their relief set pressure should be as low as practical [Ref.6-51. The structural design for the headers and their supports should take into consideration the reaction forces that could occur when explosion venting occurs. 6.3.6.2

Advantages and Disadvantages of Explosion Venting

Explosion relief vents are passive devices that do not require instrumentation to function. Ignition can continue to propagate through the header after explosion relief vents have opened. If tlus continued flame-front propagation can involve other connected vessels and equipment, additional protection in the form of flame arresters or automatic fast acting valves is required. If explosion relief vents open, a large flame can be discharged creating the potential for a secondary explosion outside the vent header system and hazards to personnel.

Explosion relief vents should be light-weight and open at low pressures. Consequently, they tend to fail if they are subjected to cyclic pressures close to their set pressure and can be vulnerable to mechanical damage, for example due to hail, ice, dropped tools, or foot traffic. 110

Chapter 6 -Design Approach

The advantages and disadvantages of explosion venting are summarized in Table 6-5. Table 6-5. Advantages and Disadvantages of Explosion Venting 0

0

0

Advantages Passive Do not require instrunlentationor utilities to function Can Provide Protection for Processes where it may not always be possible to operate outside the flammable limits, Vent panels are relatively simple and quick to replace

6.3.6.3

Disadvantages 0 0

Does not stop flame propagation Flames are discharged when explosion occurs Vulnerable to damage After an explosion has occurred in a vent header, the system will be down until the explosion vent panels have been replaced

Explosion Isolation

Explosion isolation is used to prevent ignition flashback from recognized igrution sources, such as an incinerator, and to prevent flame propagating through the vent header leading to pressure piling and detonations. Examples of isolation devices include flame arresters, fast acting valves, flame-front extinguishing systems, flame-front diverters, and seal drums. Each of these has specific characteristics that should be taken into consideration when evaluating them for a specific application. 6.3.6.3.1 Flame Arresters

For applications involving vent headers, flame arresters typically fall into one of three general classes, which can be further subdivided: 0 End-of-line In-line deflagration In-line detonation The choice of which type to install in a given application largely depends on the physical location with respect to potential ignition sources. However, it also depends on the gas classification (i.e,, the NEC Material Groups, [Ref. 6.251) and will be affected by the operating pressure and temperature. In-line flame arresters generally need instrumentation to monitor their performance and ensure they function correctly when required. If a flame stabilizes at the flame arrester, it may be damaged or become so hot that unburned gases on the other side of the unit ignite, allowing the combustion to continue to propagate through the vent header. If this is 111

Safe Design and Operation of Process Vents and Emission Control Systems

considered to be credible for the application, temperature elements should be installed to detect a flashback and an action plan developed, such as introducing an inert gas purge or shutting down production. Flame arresters are also subject to solids build-up causing a flow restriction. Consequently, their condition should be monitored. Methods for monitoring include measuring the pressure drop at a known gas flow or visual inspection. Due to the potential blockage and the resultant flow restriction this can cause, flame arresters should not be used in emergency vent headers. Further information on the use of flame arresters can be found in Deflagation and Detonation Flame Arresters [Ref.6-16]. Table 6-6 summarizes the advantages and disadvantages of flame arresters. Table 6-6. Advantages and Disadvanta s of Flame Arresters Advantages Do not require utilities to function They continue to provide a vent path permitting an orderly shut down after an explosion. They are usually undamaged by explosions and can be reused (after being inspected). Instruments are used for monitoring the flame arrester but are not needed for it to prevent a flashback.

Disadvantages Prone to plug with solids, and may be difficult or impossible to clean Have high pressure drop as compared with other isolation devices Flames may stabilize on surface of a flame arrester causing it to fail allowing gases to reignite on the upstream side of the unit. The elements are constructedfrom very thin material which does not have a corrosion allowance.

6.3.6.3.2 End-of-Line Flame Arresters As the name implies, end-of-line flame arresters are intended to prevent

flames propagating from an external ignition source and to prevent flashback into a header that is discharging directly to atmosphere. In these situations, the flame approaches the flame arrester at approximately its laminar burning velocity and ambient pressure. Thus, the performance requirements for these devices are the least challenging of any flame arrester application. Many end-of-line flame arresters only have a single flange that can be connected to the header, precluding the potential for them to be installed in-line, see Figure 6-7 [Ref. 6-17]. However, others may have an outlet flange to permit a short tail pipe to be added. In these cases, it is important to ensure this pipe does not exceed the manufacturer's recommended length and that its outlet is not obstructed. 112

Chapter 6 - Design Approach

6.3.6.3.3 In-Line Deflagration Flame Arresters Figure 6-8 illustrates an in-line flame arrester for applications where there is no potential for a detonation to occur. Depending on its design and the materials being handled, flame arresters are available that can be installed u p to a maximum of 20 to 60 feet from a potential ignition source. It is important to ensure that the manufacturefs maximum length of line and the number of bends (typically a maximum of one) are not exceeded as failure to adhere could result in the flame-front breaking through the arrester.

Figure 6-7.

Figure 6-8.

Example of End-of-Line Crimped Metal Flame Arresters

Example In-Line Crimped Metal Deflagration Flame Arrester

6.3.6.3.4 In-Line Detonation Flame Arresters Flame speed at a given location is highly dependant on the length of pipe the flame has traversed from the ignition source, the number and shape of turbulence inducing objects in that pipe, and the reactivity of the flammable 113

Safe Design and Operation of Process Vents and Emission Control Systems

gas mixture. As these increase, the flame speed can accelerate and transition to a detonation. Deflagration flames propagate by heat and mass transfer with the heat of combustion preheating the un-burned gas to its autoignition temperature. Flame arresters operating in this mode do so by cooling the flame sufficiently so that the products of combustion are unable to heat the fresh un-burnt gases to their ignition temperature. After transitioning to a detonation, the flame propagates by shock waves compressing and adiabatically heating the unburned gases to their autoiption temperatures. Flame arresters operating in this mode dissipate the shock waves as they travel through the narrow channels that make up the core of the unit so that combustion is unable to redevelop downstream of the flame arrester. Superficially, deflagration and detonation flame arresters may look similar, however, in practice detonation arresters typically have longer and narrower channels through the element and both the element and the casing of detonation arresters should be designed for sipficantly higher pressures, see Figure 6-9 [Ref. 6-19].

Figure 6-9.

Example of In-Line Detonation Arresters

During the “ m - u p ” period prior to a deflagration transitioning to a detonation, expanding gases pre-compress unburned gases ahead of the flame-front. Consequently, at the time when the transition occurs the initial detonation takes place in gas that has been pre-compressed causing an “overdriven detonation” with side on pressures in the order of 50 - 100 times the initial absolute pressure and with reflected pressures that are significantly higher. As the detonation propagates into gases that have not been precompressed, its pressure falls to the “stable” detonation pressure. 33 CFR Part 154, Marine Vapor Control Systems [Ref. 6-11], specifies vapor handling requirements for maritime installations, including the requirements 114

Chapter 6 - Design Approach

for detonation flame arresters at these applications. In the absence of comparable regulations for shore-based facilities, this regulation has been widely adopted by equipment manufacturers and plant operators. The Coast Guard regulation requires the largest and smallest size flame arrester of a given model be tested by an independent test authority to demonstrate they can function bi-directionally and withstand: Five deflagrations with an outlet pipe length equal to 10 pipe diameters Five deflagrations with an outlet pipe with a restriction 2 feet (0.6 m) from the flame arrester Five stable detonations Five overdriven detonations An endurance bum test without flame passage through the arrester of at least 15 minutes for a Class I1 detonation arrester and for at least 2 hours for a Class I detonation arrester A hydrostatic pressure test of at least 350 psig (24 barg) for at least 10 minutes Detonation arresters that meet these requirements are extremely robust, and while any application should be reviewed with the supplier, they generally can be installed without limitations on the distance between the detonation arrester and the ignition source. 6.3.6.3.5 Fast Acting Valves

Fast acting valves can contain combustion to parts of the header close to an ignition source and prevent the high pressures associated with pressure piling and detonations. These valves provide a similar function as flame arresters, however, they are less vulnerable to plugging and can be useful for systems that are prone to solids build-up or that may handle dusts. Their mode of operation is to stop all flow and, therefore, is not suitable for emergency relief systems. Externally actuated fast acting valve systems are comprised of instrumentation to detect the early phases of a deflagration, a controller, and the valve. To be effective, initiation sensors should be properly located to ensure that the valves fully close before the flame-front arrives. Depending on the line diameter, they normally can shut within 25 to 100 milliseconds. The valves are designed to withstand deflagration pressures without damage, but are not intended to withstand detonations. Consequently, they should be located within the detonation " m - u p " distance from a potential 11s

Safe Design and Operation of Procgs Vents and Emission Control Systems

ignition source, while also providing suffiaent distance from the detector to allow time for the valve to close. To achieve the very high closure rates, the valves typically rely on a blasting cap to release high pressure gas into the actuator. As a result, once the valve has closed, a factory trained technician is needed to open and reactivate it. Figure 6-10 [Ref. 6-20] shows a typical installation. The advantages and disadvantages of fast acting valves are summarized in Table 6-7. Advantages

During normal operations the unit has a low pressure drop. Can be supplied in designs that are resistant to solids buildup.

Figure 6-10.

116

Disadvantages

The unit requires complex controls and a standby battery backup. When the valve actrvates (closes)it requires a factory trained technician to reopen it. This can delay restarting the facility Inspection and maintenance must follow the manufactureis recommendations. Typically this is conducted by a factory trained technician every three months.

Fast Acting Isolation Valve (Courtesy of Fike Incorporated)

Chapter 6 - Design Approach

6.3.6.3.6 Seal Drums Seal drums consist of vessels that are partly filled with liquid (usually water) with a submerged gas distributor to introduce gases from the vent header as discrete bubbles. Seal drums are not prone to flow restrictions and can provide a reliable path for emergency vents. In addition, while it is important to maintain the correct liquid submergence of the distributor, generally their operation and maintenance is relatively straightforward and operating experience has been good [Ref.6-12]. Potential problems with seal drums include: Low liquid level resulting in a loss of the liquid seal High vent gas flow rate creating a continuous stream of bubbles displacing the liquid and allowing a path for flash-back to occur High liquid level resulting in liquid entrainment to the treatment equipment, for example causing liquid to be discharged from a flare High liquid level or sub-atmospheric header pressure causing liquid suck back into the vent header Low temperature or high dissolved solids causing the seal liquid to freeze or solidify Corrosion

For further details and a diagram of a typical seal drum, see Chapter 7. 6.3.7

Imition Sources

Vent headers should be evaluated to determine if, during normal operations or upset conditions, they could contain flammable gases. In these cases potential ignition sources should be identified and either eliminated or flash back prevention e.g. flame or detonation arresters, provided. Examples of equipment with that should be considered as potential ignition source include fans, thermal oxidizers, incinerators, flares, and carbon adsorption beds. 6.3.7.1

Carbon Adsorption Bed

When volatile organic vapors are adsorbed on carbon beds, there can be a sigruficant temperature rise that, under certain circumstances, can result in ignition leading to fires and explosions [Ref.67,641 and [Ref.6-24]. When determining if carbon beds are appropriate for a particular application, consideration should be given to the potential for heating to

117

Safe Design and Operation ofProcess Vents and Emission Control Systems

occur, potentially resulting in an ignition. If the adsorption bed is considered to be a potential ignition source: Flashback protection, such as flame arresters, should be provided. The beds should have temperature instrumentation, sometimes supplemented by C02 measurement, to detect the onset of oxidation. The carbon bed supplier should be consulted to establish recommended safety practices. For example, in some applications water is introduced into the carbon bed to condition the carbon after it has been regenerated.

6.4

Vent Header Systems Handling Toxic Gases

Typically, toxic vent gases are sent to a treatment system before discharging the treated gases to atmosphere. Vent streams with minimal concentrations of toxic compounds may be routed directly to an elevated stack for air dispersion in accordance with environmental regulations. Vent header systems frequently receive multiple sources, some of which may not be toxic. In this situation, adding a toxic stream to non-toxic streams may create the potential for toxic materials to enter vessels and equipment that would not normally be expected to contain them. Therefore, it may be beneficial to separate toxic and non-toxic streams. In addition to physiological effects of these materials on people, it should also be recognized that some toxic materials have other properties that can have a sipficant influence in the design and operation of the vent system. It is important that the Process Hazards Analysis address the entire spectrum of potential hazards, including reactivity issues and their physical properties. This can be done using an interaction matrix, such as the one presented in Chapter 5, Table 5-4. Characteristics that may be identified and which influence the design include:

118

Chapter 6 - Design Approach

6.4.1

Self-reactivity and interactions between materials that could possibly be present. For example, in addition to being toxic, hydrogen cyanide is very reactive and, if contaminated with trace amounts of basic materials, it can polymerize with explosive violence. Toxic materials, such as chlorine, hydrogen chloride, sulfur dioxide, and ammonia are typically stored under pressure as liquefied gases. If the pressure is released rapidly, the material may boil causing it to auto-refrigerate to temperatures that could cause carbon steel to lose its ductility and experience brittle fracture. Some anhydrous toxic materials, such as hydrogen chloride and chlorine, can be handled satisfactorily in carbon steel vessels. However, when vents from these vessels mix with other streams containing moisture, the combined flow may become extremely corrosive. Conversely, titanium, which has low corrosion rates with moist chlorine, may ignite spontaneously when exposed to dry chlorine. Operating- Principles for Header Svstems Handline:Toxic Gases

The following are important in the design and operation of vent headers handling toxic materials: Relief device set pressures Sub-atmospheric pressure vent header operation Separation of liquid toxics Normal practice is to specify the device set pressures at or as close to the maximum allowable working pressure of the vessel as practical in order to minimize unnecessary releases of toxic materials caused by relief devices opening prematurely. This approach is appropriate for materials that are not subject to runaway reactions. However, in cases where the toxic material is also strongly self-reactive, for example acrolein, hydrogen cyanide, and ethylene oxide, for effective overpressure protection, it may be essential to set the emergency vent opening pressure as low as practical. The potential for toxic gases to leak from process vents can be reduced by operating them at sub-atmospheric pressure. If, however, the composition is fuel rich or it is below the LOC, air leaks into the header could produce a flammable fuel/air mixture, which if ignited could cause an explosion. To guard against this, preventive measures such as oxygen analyzers with interlocks should be provided or alternatives such as 119

Safe Design and Operation ofProcess Vents and Emission Control System

operating the header fuel lean should be considered. Finally, if these approaches are not practical, the potential hazards of flammability and toxicity should be evaluated t a h g into account factors such as potential igrution sources and the odor threshold of the material, before determining if the header should operate at positive or negative pressure. Catch tanks or entrainment separators should be provided if liquids could be carried into or condense in the vent header system. In particular, measures should be taken to prevent toxic liquids being discharged from a stack as this could result in ground level gas concentrations that are considerably higher than would be predicted by vapor dispersion modeling. 6.4.2

PiDine Desim

Vent headers handling toxic materials should be designed with a minimum number of flanges and other potential leak points, while at the same time meeting the operational and maintenance requirements. Consideration should also be given to operating the headers slightly below atmospheric pressure, so that during normal operating conditions any leaks that occur will not discharge to the work place. The vent systems should be designed in accordance with ASME B32.3 - Process Piping [Ref. 6-31. Typically, the ”base code” will be adequate for most applications including many involving toxic materials. However, if the material is very toxic and difficult to contain, the system should be evaluated to determine if it meets the criteria of a Category M fluid service [Ref. 6-31. Some considerations in piping design include: Materials of construction should meet system requirements, including operating pressure, temperature, and the potential for corrosion Eliminating and minimizing piping low points, including systems for liquid removal Cleaning and inspection provisions Double block and provide a nitrogen pad for isolation (instead of double block and bleed) Access to control valves, monitoring devices, etc. Mechanical stresses and brittle fracture

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Chapter 6 -Design Approach

6.4.3

Combined Relief Valve and Rupture Disk Devices

Conventional relief valves are prone to leaks, resulting in fugitive emissions. To prevent this, common practice is to install rupture disks below relief valves in toxic service. When intact, the disk will prevent fugitive emissions. However, if it develops a pinhole leak, pressure may build-up between the rupture disk and the relief valve. This can create backpressure on the disk, increasing the pressure required for it to open. Potentially, the vent opening pressure may exceed the vessel’s MAW or in the case of reactive systems, the increased pressure and temperature may increase the reaction rate above the design basis for the vent system. Therefore, it is important to ensure that the space between the rupture disk and the relief valve is monitored for leaks, as required by Part UG-127 of the ASME Boiler Pressure Vessel Code (BPVC) [Ref. 6-21]. For a discussion on the hazards assodated with combined relief valve and rupture disk devices, see Appendix H, Past Incidents.

6.5

Reactive System

When handling reactive materials, the energy release and corresponding vent gas flows resulting from an exothermic reaction can be the overriding factor in determining the vent sizing and phase separation requirements for the vent system. In addition, the design should take into consideration the potential for contamination between vessels and reactions occurring in the header. The design of emergency relief vents for runaway reactions in the vessels is addressed in Emergency Relig System Design Using DIERS Technology [Ref. 6-21 and Guidelines for Pressure Relief and Efluent Handling Systems [Ref.6-11, 6.5.1

Reactive Svstems Desim Considerations

When considering reactive systems, the design of the vent header should not only take into consideration the vent gases from a vessel where a reaction is occurring, but should also address the implications of reactions occurring in the header itself or the potential for the vent header to act as a path for the unintended flow of reacting materials from one vessel to another. The most frequent consequence of reactions in vent headers is the formation of solids or viscous liquids restricting pressure relief from equipment. It is also possible for reactive mixtures to form in the vent header where they may subsequently explode, damaging the vent header and nearby facilities, as described in Cases 1.1and 2.2 of Appendix H. 121

Safe Design and Operation of Process Vents and Emission Control Systems

Physical design features to limit the potential for reactions in the vent header and associated equipment may include: Provide headspace in vessels to allow for entrainment separation of droplets or foams Install entrainment separators or condensers at the exit of vessels when the potential for liquid entrainment or subsequent condensation of reactive materials cannot be excluded Provide self-draining vent headers. If low points are unavoidable, provide low point drains with provisions for disposal Provide heat tracing to prevent condensation of reactive liquids Provide a continuous gas sweep to prevent monomers from accumulating and polymerizing Process related actions to minimize the potential for reactions in the vent header and associated equipment are: Identify measures to prevent unstable material being formed, e.g., peroxide formation when handling certain ethers or dienes. Establish routine decontamination procedures to prevent significant amounts of unstable materials building up in the header. Evaluate potential corrosion products from the vent header to determine if they could act as catalysts in initiating a runaway reaction in equipment connected to the vent header. Introduce a vapor phase inhibitor for applications where it is practical to do so, e.g., adding low concentrations of sulfur dioxide into headers handling hydrogen cyanide. Determine if the inhibitor added to a reactive liquid, such as a monomer, is volatile. If not, consider introducing a gaseous inhibitor to the vent header or increasing the inspection and cleaning frequency to ensure the vent is always available. See Appendix H Past Incidents, Case History €32.2, for an incident example. When selecting the materials of construction for the header, it should be noted that the combined vent stream may be more corrosive than the individual raw materials,

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Chapter 6 -Design Approach

Runaway reactions inevitably involve reaction temperatures that are significantly higher than the normal operating temperature. This can result in different reactions being favored, producing a different "spectrum" of reaction products. These materials may be more toxic than those produced by the normal process, as occurred at Seveso (SeeAppendix H Past Incidents, Case History H2.3). The design of the emergency vent and treatment system should address the volumes and composition of materials that could be produced during a process runaway reaction. 6.6

MechanicalDesign Considerations

Vent header systems play a major part in meeting environmental requirements of the facility, as well as providing emergency venting during a major process upset. The mechanical design of the vent header system warrants at least as much attention as that given to design of piping systems that handle process fluids. In addition, the problems encountered in the design of vent header piping systems are frequently more complex than those encountered in the design of a process system. In particular, emergency or combined vent header systems may be subject to a greater range of temperature, pressure, and shock caused by the wide range of conditions experienced. Additionally, the vent header system may contain any of the materials handled in the process system. The major piping stresses in a vent header system are due to thermal expansion or contraction from the entry of cold or hot materials, thrust developed by sudden vent relief flow, and dynamic loads associated with an explosion in the vent header system. In relieving systems that serve typical refinery process units, temperatures may range from well below zero to several hundred degrees. Designing for flexibility can be more complicated than for process piping systems [Ref. 6-12]. 6.6.1

Vent Header Pipe Specifications

Depending on the service, vent headers may range from heavy pipework to lightweight or non-metallic duct work. Where pressure resistant or higher integnty piping is required, the design should be in accordance with ASME B31.3 - Process Piping [Ref.6-31 for process piping. 6.6.2

Vent Header Suv~orts

Vent headers can be subjected to a variety of forces that may cause them to deform, or in extreme cases, to totally separate from connected equipment. Pipe hangers, slides, vibration dampers, etc., are available to support and 123

Safe Design and Operation of Process Vents and Emission Control Systems

restrain the headers, while still allowing for dimensional changes resulting from temperature and pressure fluctuations, etc. Causes of load on header systems include: Dead load of the header, including its normal contents and items such flame arresters, valves, insulation, etc. Additional weight of liquid present during hydro-testing or if the header becomes liquid full as a result of an upset condition Reaction forces while venting Water hammer caused by liquid slugs in the header during emergency venting Steam hammer caused by introducing steam into a header containing water that is below the saturated steam temperature Thermal expansion or contraction during start-up, shutdown, or emergency venting Thermal expansion as a result of introducing steam for cleaning/decontamination Cyclic temperature/pressure variations during normal process operations Wind, dynamic load, snow loads, earthquakes etc. Inadequately supported vent headers can lead to a variety of consequences in both the vent header system itself and surrounding process areas. These consequences can include: Header sagging between supports resulting in liquid build-up, restricting vent flows, promoting solids formation, and the potential for water hammer and mechanical shock Excessive loads on supports leading to their structural failure and collapse of the header Leaks from joints and potentially total failure of the connection between the header and the vessel Excessive bending moments on header resulting in it "pinching over", severely restricting the flow 6.6.3

The vent system should be able to withstand thrust forces caused by the relief flow, taking into considerationbending movements that may be caused by: Thermal expansion Reaction forces imposed by relief system discharges Water-hammer 124

Chapter 6 - Design Approach

Failure to provide adequate support has resulted in instances where the bending movements caused the pipe to buckle and rotate, effectively crimping it closed. Guidelinesfor Pressure Relief and €fluent Handling Systems [Ref. 6-11 discusses techniques for calculating thrust forces for single-phase and two-phase flow conditions. 6.6.4 Shock Waves Downstream of Rupture Disks When rupture disks burst, shock waves can be formed. These shock waves can cause brittle components, such as sightglasses and FRP pipework, to fail if they are installed immediately downstream of rupture disks. The potential for shock waves should be considered when selecting piping components downstream of rupture disks. Test work has demonstrated that the shock wave created by the bursting of a rupture disc can generate substantially higher local downstream pressures (1.4 - 2 times the burst pressure of the failed rupture disk) in the vent header pipework [Ref. 6-1 and Ref. 6-23, page 207-2141. 6.6.5 Corrosion Vent headers may experience increased corrosion rates when certain materials are mixed. For example, anhydrous hydrogen chloride is relatively non-corrosive; however, if it is combined with a stream containing moisture it can become highly corrosive. It may be cost effective to provide separate headers for "wet" and "dry" streams, rather than installing costly corrosion resistant alloys when handling streams that can become corrosive when mixed. Increased corrosion rates also tend to occur if liquid is present with the vent gases. Measures to address this include: Installing a condenser to cool and condense/separate the vapor in the vent gases. In some instances, it may be desirable to reheat the stream to dry the gas after the condensed liquid has separated Insulating and, if appropriate, heat tracing the vent header Installing an entrainment separator to remove entrained liquid droplets present in vent streams 6.6.6

Header OperatinP Pressure and Pressure Drop

The backpressure caused by vent gases from a relief valve through a vent header system, including any treatment equipment, will tend to hold other conventional spring loaded relief valves closed, adversely affecting their performance both in terms of their flow capacity and stability (chatter). 125

Safe Design and Operation of Process Vents and Emission Control Systems

For vent headers that can receive flows from multiple vessels, backpressure calculations should be based on the most severe credible combination of vent flows that can discharge into it simultaneously. With some exceptions, the maximum built-up backpressure for conventional pressure relief valves should not exceed 10% of the set pressure of the relief valves that could be discharging concurrently. The opening pressure for balanced pressure relief valves (using bellows, piston, or which are pilot operated with the pilot vented independently) are independent of the backpressure; however their capacity begins to decrease as the backpressures rise above approximately 30% of the set pressure. This information is generally available from the valve manufacturer and it should be used to evaluate the effects of backpressure when optimizing the vent header design. For more information, see APl 521, Guide for Pressure Relieving and Depressuring System [Ref. 6-12, Section 5.4.1.31 and ASME Boiler and Pressure Vessel Code [Ref. 6-21, Appendix M-81. 6.6.7

Thermal Stresses and Low Temperature Embrittlement

Vent headers can be subjected to temperature changes from a variety of sources, including hot and cold process flows, solar radiation, and low ambient temperatures. When temperatures change, the header will expand or contract. Unless provision is made to allow for this, the relief devices, pipe supports, and any equipment the header is connected to, may be subjected to severe stresses. These effects can be minimized by building flexibility into the header, for example by installing piping loops. Expansion bellows and similar devices have been used, but introduce potential weak points in the piping design. It is important to ensure that these features do not result in liquid accumulatingin the header Low temperatures can be experienced in header systems handling materials that are at low temperatures or as a result of auto-refrigeration when a system containing a liquefied gas is depressurized allowing it to flash. Carbon steel loses its ductility at low temperatures, at which point it becomes vulnerable to brittle fracture. For more information, see ASME Boiler and Pressure Vessel Code [Ref. 6-21]. 6.6.8

Liauid Knock-Out and Drainage -

The presence of liquid is one of the main causes of vent header system malfunctions. In general, if low points exist, liquid will accumulate. Potential sources of liquid include:

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Condensed vapor Overfilled vessels Entrained liquid in vent steams leaving equipment Closed or defective low point drains Equipment failure or operator error resulting in vacuum from a vessel operating at sub-atmospheric pressure being applied to the vent header, creating a reverse flow of liquid into the header Liquid from thermal relief valves

To address cases where liquids may enter or accumulate, knock-out tanks or entrainment separators should be provided. Headers should drain in the direction of flow toward equipment with a minimum slope of 0.06 inches/ft (5 mmim) for 1 to 6 inch (2.5- 15 cm) pipe reducing to 0.012 incheslft (1 W m ) for 16 inch (40 cm) pipe [Ref. 6-1, page 3581. Header low points should be avoided and, where unavoidable, drains provided. (When drains are provided, they must be kept clear, especially where there are solids present or where plugging is a possibility.) In situations where significant amounts of liquid could enter the vent header, consideration should be given to installing knock-out tanks at the source, as well as at the inlet to the treatment device. Further information can be found in Chapter 7. Liquid build-up in vent headers can cause extensive damage and has included: Serious fires caused by burning liquids being "rained" from flares or elevated stacks. Personnel injuries caused by hazardous materials being discharged from flares. Water hammer caused by slugs of liquid impacting bends, displacing headers off their supports and breaking connections between the vent header and individual vents from vessels, resulting in a loss of containment Incinerator firebox explosions when combustible liquids have been fed to units that were only designed to handle gas. Damaged pipe supports when headers designed for gas only operations became liquid full possibly resulting in loss of containment.

127

Safe Design and Operation ofprocess Vents and Emission Control Systems

6.6.9

Exeansion Toints and Flexible Connections

Vent headers interconnect source vessels and treatment devices that are fixed objects. Consequently, if the vent header expands or contracts or if other loads occur, stresses will develop that could cause damage. Vent headers should be designed to allow for thermal expansion or any other change in geometry that may occur during normal or upset conditions. This can be achieved by installing a series of bends or loops to allow for movement without causing excessive stress in the header or on associated equipment. This approach can take up considerable space and may adversely affect the process if there is a need to minimize the time taken for material to travel between items of equipment. Where either of these factors is unacceptable, installing expansion joints may be the only alternative. Expansion joint failures have been the cause of several severe incidents, and as a result some companies have a policy to avoid using them. When expansion joints are used it is important to ensure they are installed correctly and routinely inspected. Factors that should be considered include: The maximum out-of-line and distance between the flanges should not exceed the manufacturefs recommendations Tie-bars or external expansion joint guides provided by the manufacturer should be correctly installed and adjusted. Note: Tie-bars should also be in place during the initial installation to ensure the expansion joint is not overstressed and damaged even before chemicals are introduced Routine inspections should be conducted for cracks, e.g., using dye penetrant testing The header should be anchored to prevent excessive movement that could over-extend the expansion joint If particulates are present in the stream, they may accumulate in the bellows, restricting its movement and increasing stresses. If this occurs, it could lead to premature failure. As a general rule, expansion joints should not be used in streams containing significant levels of particulates

128

Chapter 6 - Design Approach

Figure 6-11 [Ref. 6-24] shows an expansion joint that failed due to incorrect installation techniques and corrosion.

Figure 6-11.

6.6.10

Example of Expansion Joint Failure

Valves in the Vent Header Svstem

Valves in vent header system are often required, such as for: Isolation of relief devices for maintenance Isolation of branch headers Bypassing and isolation of intermediate treatment devices These valves must be managed. The ASME Boiler and Pressure Vessel Code [Ref. 6-21, Section UG 1351 specifies that there shall be no isolation valves (stop valves) between the vessel and the relief device or in the header between the relief device and discharge to atmosphere, except asfollows: Isolation valves are permitted provided they are constructed so that there is always a path open to atmosphere, for example when there are two relief devices in parallel each of which is capable of handling the design vent flow and where the design is such that a path is always open to at least one relief device As permitted by ASME Boiler and Pressure Vessel [Ref. 6-21, Appendix M-61

129

Safe Design and Operation of Process Vents and Emission Control Systems

6.7

The isolation valve is locked or sealed open any time the vessel is in operation There is an authorized person present at the valve to operate it in the event of an emergency when the isolation valve is closed and the vessel is in operation The authorized person locks or seals the valve open before leaving the area

References

6-1.

Center for Chemical Process Safety (CCPS). 1998. Guidelines for Pressure Relief and Effluent Handling Systems. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

6-2.

The Design Institute for Emergency Venting PIERS). 1992. Emergency Relief System Design Using DIERS Technology. New York, New York. American Institute of Chemical Engineers.

6-3.

American Society of Mechanical Engineers. 2002. B31.3 - Process Piping. New York, New York.

6-4,

National Fire Protection Association (NFPA). 2002. NFPA 69: Standard on Explosion Prevention Systems. Quincy, Massachusetts.

6-5.

National Fire Protection Association (NFPA). 2002. NFPA 68: Guide

for Venting of Dejlagrations. Quincy, Massachusetts. 6-6.

Le Chatelier, H. 1891. Estimation of Firedamp by Flammability Limits, Ann. Mines.

6-7.

Woodward, J. and Lygate. 2002. Establishing Ignition Conditionsfor the Tank Manifold Fire at the Powell Duffryn Tank Terminal. Plant/Operations Progress, Volume 21, No. 3. New York, New York. American Institute of Chemical Engineers.

6-8.

Kletz, T. 2003. Still Going Wrong!: Case Histories of Process Plant Disasters and Hau They Could Have Been Avoided. Amsterdam. Elsevier.

6-9.

Clark, D.G. and Sylvester, R.W. 1996. Ensure Process Vent Collection System Safety. Chemical Engineering Progress, Volume 92, No. 1. New York, New York. American Institute of Chemical Engineers.

130

Chapter 6 - Design Approach

6-10,

Coward, H.F. and Jones, G.W. 1952. Bulletin 503, Limits of Flammability of Gases and Vapors, Washington, D.C. Bureau of Mines. United States Department of the Interior.

6-11.

United States Coast Guard. 1990. 33 CFR Part 154, Marine Vapor Control Systems. Washington, D. C. Code of Federal Regulations.

6-12.

American Petroleum Institute. 1997. API RP 521, Guidefor Pressure Relieving and Depressuring Systems. New York, New York.

6-13.

Britton, L. 1996. Operating Atmosph~ricVent Collection Headers Using Methane Gas Enrichment. Process Safety Progress, Volume 15, No. 4. New York, New York American Institute of Chemical Engineers.

6-14.

Environmental Protection Agency. 1993. 40 CFR Part 60.18, General Control Device Requiremmnts. Washington D.C., Code of Federal Regulations.

6-15.

Fike Corporation. 2002. Total Concept Explosion Protection CD, D:\2001 Fike EP Solutions, Slide 50.

6-16.

Grossel, Stanley S. 2002. Defagration and Detonation Flame Arresters. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

6-17.

Protectoseal. 2004. End-of-Line Crimped Metal Flame Arresters. http://www.protectoseal.com/27000.htm

6-18.

Protectoseal. 2004. In-Line Crimped Metal Deflagration Flame Arrester. http://www.protectoseal.com/30O~.html

6-19.

Protectoseal. 2004. In-Line http://www.protectoseal.com/2526.html

6-20.

Fike Corporation. 2002. Total Concept Explosion Protection CD, D:\2001 Fike EP Solutions, Slide 78.

6-21.

American Society of Mechanical Engineers. 2004. Boiler and Pressure Vessel Code. New York, New York ASME.

6-22.

American Petroleum Institute. 1994. API RP 520, Design and Construction of Pressure-Relieving Systems in Refineries, 4th Edition. New York, New York.

6-23.

Beveridge, H.J.R., and Jones, C.G. 1984. Shock Effects on a Bursting Disk in a Reliq Manifold. I. Chem. E. Symposium Series No. 85. Rugby, UK: Institution of Chemical Engineers.

Detonation

Arrester.

131

Safe Design and Operation ofProcess Vents and Emission Control Systems

624.

Center for Chemical Process Safety. July, 2004. Process Safety Beacon. http://www.aiche.ortr/ccvs/safetvbeacon. htrn

6-25.

National Fire Protection Association (NFPA). 2002. NFPA 70: National Electrical Code, Article 500, Quincy, Massachusetts.

132

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

7 TREATMENT AND DISPOSAL, SYSTEMS The fundamental reason vent header systems are used is to collect streams from routine process venting, emergency relief venting, or a combination of the two and route them for dispersion, treatment, disposal, or recovery in a safe, environmentally appropriate, and cost-effective manner. The vent header system and the final treatment and disposal devices should be considered one processing system whose design is integrated from the source of the vent gases to their final treatment and disposal. This chapter provides a general review of the available and commonly used equipment and systems for collecting, separating, conditioning, or treating vented streams and ultimately disposing (destroying), dispersing, or recovering the vented gases. In this chapter, these systems are collectively referred to as treatment and disposal systems. Typical components, devices, equipment, and systems are described that may be used alone or in combination. General design features are presented, but the emphasis is on the safety and operational aspects of these treatment and disposal systems and how they should be safely integrated with the design and operation of the vent header system. Th~schapter also provides a review of the selection process for treatment and disposal systems. However, this chapter is not intended to provide detailed guidance on the design of treatment and disposal systems. 7.1 Selectionof Treatment and Disposal Methods The criteria for the selection of appropriate treatment and disposal methods and systems depends on the vented stream’s characteristics, such as: Emergency or normal process discharge - Flow rate, both normal and maximum - Duration and frequency of discharge - Total quantity discharged per event 133

Safe Design and Operation ofprocess Vents and Emission Control Systems

Physical state - gas or vapor and whether containing liquids or solids Composition - materials present and their concentrations Required removal or destruction efficiency to meet environmental requirements Pressure, temperature, boiling point Hazardous properties - toxicity, ecotoxicity, flammability Nuisance properties - noise, odor, visible plume Location - relative to other process units, neighbor facilities, community Although non-hazardous and non-toxic vent gases may be discharged directly to atmosphere, even these releases may warrant treatment considerations. For example, a process that continuously or intermittently vents high pressure steam may present unacceptable issues, such as noise, odor or visible plume to a nearby community. Any process effluent stream containing liquid or solid should generally not be vented directly to the atmosphere in the vicinity of process equipment within the operating area. Even innocuous streams may present a personnel safety hazard. It is very possible that emergency vent relief streams will have two-phase flow or eject reaction mass during at least part of the emergency venting event. Even normal process vent streams may have entrained liquids or solids. Unless a vented stream is known to contain only gas or vapor, consideration should be given to routing that stream into a vent header that includes appropriate liquid/solid separation with provision for collecting and containing these for later treatment. Clearly, those streams containing sigruficant concentrations of flammable or environmentally regulated hazardous or toxic materials will require an appropriate form of treatment and disposal. Effectively meeting the necessary treatment and disposal requirements may involve the application of more than one method or system. A vent header system connects these treatment and disposal components into one continuous process. A wide range of components, equipment, and systems are used for collecting, separating, processing, treating, or disposing normal or emergency process vent streams. Those included in Table 7-1 are discussed in this chapter.

134

Chapter 7 -Treatment and Disposal Systems

Table 7-1. Methods

Treatment and Disposal Methods and Systems Systems Types or Components

Collection

Tanks

Containment Blowdown, Dump and Catch Tanks Quench Tanks and Quench Pools

Physical Separation

Vapor-Liquid Disengagement

Knock-OutTanks Cyclones Mist Eliminators

Dust and Particle Removal

Fiiters, Dust Collectors Electrostatic Precipitators Scrubbers

Absorption

Scrubbers

Packed Bed Scrubbers Venturi Scrubbers Spray Towers Tray Towers

4dsorption

Carbon Adsorbers

Beds or Drums Regeneration Systems

Recovery

Condensation

Surface Condensers, Shell-and-Tube Heal Exchangers Air-Cooled Finned-TubeCoolers

Reclaiming Processes

Fuel Gas Recovery

Flare

Elevated Flares Ground Flares Low Pressure Flares Bum Pits

Incineration

Thermal Oxidizer Catalytic Oxidizer Process Heaters and Boilers

Thermal Destruction

lispersion

Elevated Stacks

A logic diagram generalizing one possible approach to the design of vent header systems and the selection of appropriate environmental treatment and disposal options is shown in Figure 7-1. Other selection approaches and decision trees are described in The Engineering of Relief Disposal - A Reviea Paper, [Ref. 7-11, and Guidelines for Chemical Reactivity Evaluation and Application to Process Design, [Ref. 7-2, page 1711.

Safe Design and Operation o f Process Vents and Emission Control Systems

Does vented stream contain. ,

.

Yes

Liquid / solid?

Recoverable gas / vapor?

TOXICand / or flammable gas / vapor?

,

yes

c

Lfquids / solids

Separate

Condense or absorb

N~

I

b

Collect

I

Recover

*

Consider dispersion t o atmosphere

No

Treat and disperse to atmosphere

I

I

gas / vapor? I

Is flammable

concentration below

vapor?

Thermal destruction Consider dispersion to atmosphere

Figure 7-1.

136

Treatment and Disposal Method Selection

Chapter 7 -Treatment and Disposal System

7.2

Collection

One option for managing or controlling emergency venting events is to design a means to contain the event within the process equipment or within a closely connected, fully closed system. Another option is to provide a system that will collect a large part of the vented stream with a reduced gas/vapor load remaining for further treatment. Normal process vent streams may also be collected. Collection of vented streams may avoid, or at least reduce, the need for further treatment.

7.2.1

Containment

In some cases, containment options are possible and may be the first line of defense for certain process events. Containment options may eliminate the need for additional safety features or vent header systems; but, more commonly, complete containment options are not achievable. In these more common cases, the collection systems vent their gases and vapors into vent headers feeding treatment and disposal systems [Ref.7-31. 7.2.1.1

Containment in the Original Vessel

Containment in the original vessel is a safe method for controlling an overpressure incident if: An appropriate hazard analysis is used to identify credible scenarios Sufficient laboratory work and engineering analysis is performed to accurately define the maximum pressure and temperature that can be developed by the worst credible scenario Where practical, containment can simplify or even eliminate the need for a vent header system and further treatment and disposal equipment. Total containment in the orignal vessel is frequently not practical due to the nature of the process reaction or the generation of high pressures or temperatures. If an exothermic polymerization occurs in a reaction vessel containing agitators and heating or cooling coils, the entire contents could solidify. It would be extremely difficult to clean the vessel and return it to service. Reactions that generate non-condensable gases (gassy systems) may cause very high pressure in the vessel if not vented. In practice, total containment in the original vessel is infrequently an economical or technical option.

137

Safe Design and Operation of Process Vents and Emission Control Systems

7.2.1.2

External Containment

For the more typical cases where containment in the original vessel is not practical, an external containment vessel may be used in emergency situations. External total containment vessels were reported in literature as early as 1964 for the protection of batch reactors [Ref. 7-41, This concept involves capturing and retaining the entire discharge stream from the relieving vessel in a separate external vessel. In a few limited situations, external vessels have been employed to provide total containment of the received materials allowing the vessel’s collected contents to be disposed of later as appropriate. Refer also to Section 7.2.6 for a discussion of a similar concept called “quench pools”. 7.2.1.2.1

Design and Safetv Considerations

The use of a design approach based on absolute total containment in an external vessel is typically not appropriate where emergency vent streams contain a: Significant quantity of non-condensable gases Flammable atmosphere The presence of a flammable atmosphere in a containment vessel, particularly at an elevated initial pressure, such as 30 psig (2 barg) or higher, may allow a deflagration to transition to a detonation [Ref. 7-51, For reaction processes involving the potential for a runaway reaction, the external tank is often maintained with an intentional liquid level, typically water. In the event of a runaway reaction, this liquid level cools the reacting materials, quenches the reaction, and in some cases acts as a low boiler to “temper” the reaction. An external vessel containing water can temper the reaction by allowing boiling to occur so that the evaporative cooling involved can balance the heat being evolved and prevent reaction rates rising to the point where it may be impossible to provide overpressure protection with adequate vent relief area. Alternatively, the external tank may contain a chemical to stop the reaction or a neutralizing solution to render the discharged chemicals nonreactive or less hazardous. Chemicals that stop or kdl the reaction are sometimes referred to as a “short-stop” and can be added either to the external vessel or to the source vessel.

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Chapter 7 -Treatment and Disposal Systems

For either of these reaction stopping concepts to be effective, the following must be provided: Appropriate quantity of liquid or reaction stopping chemicals must be maintained in the external vessel at all times Sufficient mixing by the discharged materials with the liquid in the external tank 7.2.2

Collection with Venting

Since the design and fabrication of total containment systems is challenging and costly, such systems are infrequently used for the control of emergency venting situations. For typical emergency venting cases, an external collection vessel or tank collects the bulk of released liquids, solids, or reaction mass, while allowing the passage of gases and vapors into the vent header system. These collection vessels or tanks are often called “dump tanks” or “catch tanks”. 7.2.3

Dump and Catch Tanks

Dump and catch tanks are commonly used to receive and collect streams that may contain materials other than gases and vapors, such as combinations of liquids, slurries, and solids. Typically, dump and catch tanks are vented to a vent header system. In typical applications, the dump or catch tank receives the vented liquids or reaction mass, along with vented or evolved gases and vapors, and retains or contains the liquids. The gases and uncondensed vapors are then vented from the dump or catch tank through relief devices or an open line into a vent header system and then to an appropriate treatment and disposal device, such as a scrubber or flare. Use of dump or catch tanks can prevent the discharge of liquids or solids into a vent header system and may moderate the gas and vapor load into the vent header system. The names used for these tanks vary among industry sectors and companies. The following definitions are used in h s book:

Dump tank - A separate collection and containment vessel intended to receive an emergency discharge of liquids, liquid reaction mass, or slurries originating from the bottom of a process vessel or reactor. The “dump” is usually automatically triggered by a process safety interlock, but may also be initiated manually. An uncontrolled reaction, exothermic, or other runaway reaction can be controlled by discharging the process vessel contents to the dump tank. This may allow the process vessel to be returned to service in a shorter time.

139

Safe Design and Operation ofProcess Vents and Emission Control Systems

Catch tank - A separate containment vessel intended to receive an emergency discharge from relief devices in the process vessel's vapor space. Catch tanks are usually employed where substantial two-phase relief flow, entrained solids, or reaction mass carryover is expected to occur. The design of both dump and catch tanks must be of appropriate pressure rating and may be either vertical or horizontal vessels. The vessels should be designed, including internal features, to collect liquids and solids while allowing separation of gases and vapors. 7.2.3.1

Design and Safes Considerations

Dump and catch tanks are one means to prevent plugging a vent header system by vented liquids, solids, solidifying liquids, or polymerizing materials. Dump and catch tanks are essentially passive devices and, as such, have a high degree of reliability. Dump and catch tanks and the source vessel should be designed to contain: The maximum pressure and temperature developed in the two vessels combined Continuing chemical reaction in the receiving vessel if a reaction-stopping or neutralizing chemical is not used or is not totally effective Deflagrations, if a flammable atmosphere could be present. Design of vessels to withstand internal deflagrations is addressed by NFPA 69: Standard on Explosion Prevention Systems [Ref. 7-51 and Deflagration Containment (DPC)for Vessel Safety Design [Ref. 7-6, pages 1-61 Additional overpressure relief devices may also be needed on the source vessel and the dump or catch tank to meet ASME Boilw and Pressure Vessel Code [Ref. 7-71 or regulatory code requirements and to handle sources and causes of overpressure other than runaway reaction, such as fire exposure. Other safety and design considerations include: Shock loading effects on vessel nozzles, attachments, supports, and intemals resulting from thermal expansion, liquid slugs, or rapid gas expansion [Ref.7-81

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Chapter 7 -Treatment and Disposal System

Containment volume sized to receive the anticipated release quantity of material with sufficient head space to allow separation of gases and vapors, considering whether vessel is maintained: Empty With a minimum liquid level (instrumentation should be provided to monitor level) Location, typically very close to the source vessel Air-free or inerted where the material to be received is flammable or heated above its flash point -

-

Multiple sources may be served by one dump or catch tank. The tank should be sized to handle all sources if a common credible scenario could result in simultaneous releases. Additional considerations include: Chemical compatibility among multiple vent sources Safe continuity of process operation if the vessel is out-of-service 7.2.4

Blowdown Drums and Tanks

In some batch processes, usually involving higher pressures, the process vessel or reactor is de-pressured into a separate vessel upon completion of each batch. This is often done to: Capture any multi-phase effluent that could enter the rest of the vent header and treatment system Collect desired product Minimize pressure/flow impact on the vent header Prevent escape of liquids or droplets to atmosphere (if directly vented to atmosphere) In some applications, where these vessels collect some or all of the product stream, they may be considered more a part of the process than the vent header system. Some large scale continuous processes, such as in the oil and gas refining industry, require that certain vessels (e.g., heat exchangers) be periodically taken out-of-service for maintenance or other reasons, while the rest of the process continues operating. Removal of trapped and frequently hot process liquids is often accomplished by draining or using gas pressure to "blowdown" to receiving vessels. These vessels are typically vented to the vent header system.

141

Safe Design and Operation of Process Vents and Emission Control Systems

These receiving vessels are often called "blowdown drums'' or "blowdown tanks", but various names are used in different industry sectors and companies. The following definition is used for the purpose of this book

Blowdown drum - A separate collection vessel intended to receive a periodic or emergency discharge of liquids, liquid reaction mass, or slurries from a number of process vessels, reactors, or equipment items. Blowdowns may be initiated automatically, i.e., depressuring a batch reactor, or manually for other applications. Blowdown drums are usually maintained at a low level or essentially empty. The collected liquids are pumped elsewhere for recovery, recycle, or disposal. Gases and any uncondensed vapors are vented through relief devices or an open line into a vent header system for appropriate treatment and disposal.' 7.2.4.1

Design and Safety Considerations

The major design and safety features required of blowdown drums are similar to those for dump and catch tanks, including design for overpressures resulting from continuing reactions, external fire, internal deflagration where flammable atmospheres may be formed, and shock loadings from sudden in-flow of process liquids. Heating equipment may be required for blowdown drums if there is potential for the liquid in the drum to freeze or solidify.

A recognized good engineering practice is to design blowdown drums for a minimum design pressure of 50 psig (3.4 barg) [Ref. 7-81; some companies specify a design pressure of 125 - 150 psig (8.6- 10.3barg) [Ref. 79, pages 112-1241, Further information on blowdown drums can be found in Guide for Pressure Relieving and Depressun'ng Systems [Ref. 7-81 and Guidelines for Engineering Design for Process Safety [Ref.7-10]. 7.2.5

Ouench Drums

When gases and vapors are vented from a process at high temperature, it may be necessary to cool the hot vented stream before it can be further

Preliminary investigation of the 2005 explosion at a refinery in Texas City indicated blowdown drum sizing was an issue. 142

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handled by the vent header system. A quench drum can be used for this purpose in both normal process and emergency vent header systems. The names for vessels performing this service vary among industry sectors and companies. For the purposes of this book, the following definition is used:

Quench drum - A vessel with internal liquid sprays supplied by an external recirculation pump loop with a quenching liquid sprayed directly into the vapor space to contact, cool, and condense at least part of the hot vented gas or vapor stream before these gases enter the main vent header system. Quench drums are usually vertical cylindrical vessels with a pumped, recirculated liquid spray. The vented stream enters at a low level and exits at the top, resulting in counter-current flow. These vessels often have limited or no internal structure. Quench drums are usually sized to contain a substantial initial volume of the quenching liquid, plus headspace for condensed liquids and thermal expansion. The quenching medium, water or other suitable liquid, is sprayed directly into the drum’s vapor space to contact and cool the hot vented gas stream. For routine process operations, either continuous or batch venting, fresh cooling liquid may be supplied and condensed liquids may be purged from the system under level control.

A quench drum may also condense a significant fraction of the condensable vapors discharged from the process, thereby reducing vent header vapor loads. In some cases, a downstream condensing step may need to be added to the design of the vent header system to remove the vaporized cooling medium. Quench drums, using a separate cooling water supply, are often utilized in the oil refining and petroleum industry to cool vented process vent gases and vapors prior to sending them to a flare [Ref. 7-81, Quench drums are used similarly in other industry sectors to cool and reduce the amount of organic volatile emissions as part of an air pollution control system. Condensed liquids with cooling water (or other liquid) may be pumped to a treatment or recovery process. The uncondensed gases and vapors are then routed by the vent header for appropriate treatment or discharge to the atmosphere, if appropriate. The use of water as the quenching medium has some limitations. The system may require freeze protection in cold climates, particularly where the quench drum is used for emergency venting service. Water should not 143

Safe Design and Operation ofPmcess Vents and Emission Control Systems

be used when the vented effluent contains water-miscible organics, liquid low boilers, or fluids below 32 degrees F (0 degrees C). Further information on quench drums can be found in Guidelinesfor Engineering Design for Process Safety [Ref.7-10]. 7.2.5.1

Design and Safety Considerations

Design pressures for quench drums should be: Based on the hydraulics of the discharge system and the downstream pressure drop required to vent the residual gases and vapors through the vent header to the ultimate treatment and disposal system. Designed for deflagration containment, if a flammable atmosphere could be present, similar to the design of blowdown drums [Ref.7-51. Overpressure relief devices may also be needed to meet ASME Boiler and Pressure Vessel Code [Ref. 7-71 or regulatory code requirements and to handle other sources and causes of overpressure such as fire exposure. Some of the key design and safety considerations for quench drums

are: Selection of quench liquid, including: - Selected quench liquid must not react with the hot vented stream unless this has been considered in the thermal and pressure design of the system - Quantity of quench liquid required should be determined by heat balance calculations Containment volume sized to provide: - Sufficient headspace to allow separation of gases and vapors - Expansion of the quench liquid, collected condensate, and any anticipated liquid carryover Air-free or inerted where the material to be received is flammable or heated above its flash point Multiple sources may be served by one quench drum. Vessel design considerations include: Sized on the basis of one common credible simultaneous release scenario from the connected process vessels, such as, for example, cooling water or power failure 144

Chapter 7 -Treatment and Disposal Systems

Chemical compatibility among multiple vent sources Safe continuity of process operation if the drum or one of the process sources is out-of-service; possible need for isolation and bypass Materials of construction should be selected based on the corrosive properties of the vented stream and the quench medium at the expected operating temperature. A heating coil may be needed to prevent freezing of quench medium and any condensed material when the system is not receiving hot vented materials. Instrumentation for pressure, temperature, and level monitoring and control should be provided. 7.2.6

Quench Pools

Very hot effluent vented from emergency pressure relief systems may need to be cooled before further handling. This allows more economical design of the vent header piping and treatment and disposal systems. Additionally, emergency vent streams from some processes may contain particularly hazardous or toxic components and it may be desirable to quickly condense, collect, and remove them from the vent stream before further treatment. For these cases, a “quench pool” may be an appropriate solution. For the purposes of this book, the following definition is used: Qumck pool - A closed vessel containing a relatively large volume of liquid. The emergency vent stream is sparged subsurface through the liquid volume at high velocity, resulting in vigorous agitation and circulation of the pool contents in order to maximize cooling and condensation or reaction with the pool liquid. Most or all of the vented stream can be captured; residual vapor and non-condensable gas may be released to the vent header system for further treatment or disposal. Use of quench pools has been limited to cases where prevention of an atmospheric release was of overriding concern, for example, when dealing with toxic chemicals, radioactive materials, or where difficult to handle materials are vented. Known industrial examples of successful quench pool application do not involve significant levels of non-condensable gases mixed with the vented vapors. Quench pools have found widespread use in the nuclear power industry for handling high-pressure steam from pressure relief valve discharges [Ref. 7-12], but comparatively fewer have been employed in the process industries. “Quench drums”, a related design discussed previously 145

Safe Design and Operation ofProcess Vents and Emission Control Systems

in h s chapter, are often utilized in the petroleum refining and other industry sectors to cool vent gases and vapors prior to sending them to treatment or destruction, such as to a flare [Ref. 7-81, Cooling and condensation could also be carried out in surface heat exchangers or other types of direct contact heat exchangers, scrubbers, or absorbers, such as spray towers, tray towers, packed bed scrubbers, pipeline contactors, etc. However, these do not typically offer the potential efficiency of direct contact of the vapor submerged within a pool of quench liquid. Quench pools are passive systems that can offer a reliable and simple method for handling viscous, chemically reactive, or two-phase materials. Quench pools for handling emergency releases may be considered when dealing with the following: High temperature, hazardous, or toxic materials that cannot be directly treated, destroyed, or dispersed to atmosphere, but can be cooled, condensed, neutralized, or absorbed by the quench liquid, such as: - Runaway reactions - High vapor loads - Two-phase systems - Reactive materials - Viscous liquids Systems containing low levels (< 5 volume %) of noncondensable gases 7.2.6.1

Design and Safety Considerations

Quench pools have not been widely applied in the chemical industry as in the nuclear industry, and technical issues may need better definition and resolution. There have been many studies with steam and water mixtures developed by the nuclear industry [Ref. 7-12]. Commercial units up to about 20,000 gallons (75,700 liters) size have been reported in the chemical industry [Ref.7-13, pages 157 - 1631 and up to about 1 million gallons (3.785x 106 liters) in the nuclear industry. Where vapor condensation, reaction, or neutralization is essentially completed in the pool, and if only small quantities of non-condensables are present, the pool may be operated as an unvented total containment system. Where condensation or reaction is not complete or where considerable non-condensables are present, the pool must be vented via relief devices or an open line to a vent header system for further treatment and disposal. 146

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The source vessel must be of suitable pressure design to resist the backpressure imposed by the quench pool design during an emergency release. Conversely, the relieving pressure must be high enough to overcome: Static head due to the pool’s liquid depth Pressure drop through the sparger (at least 5 psig (0.34 barg) pressure drop for good dispersion; alternatively, a dip pipe can be used with reduced dispersion efficiency) A general procedure for determining the major design parameters for a quench pool is presented in the book Guidelines for Pressure Relief and Efluent Handling Systems [Ref. 7-31, A discussion of other design features also is presented, including the possible need for an emergency vent, vacuum breaker, mechanical design and process control instrumentation. 7.2.7

Advantaees and Disadvantages- Collection Svstems

The advantages and disadvantages of the various types of collection systems discussed in this section are briefly summarized in Table 7-2. 7.3

Physical Separation

This section discusses the physical separation of liquids or solid particulates from vent streams to prevent the accumulation of liquids or solids that may restrict flow or create reactivity hazards in the vent header system. 7.3.1

Vapor-Liauid Gravitv Seuarators

There are numerous vapor-liquid gravity separators in chemical and petroleum industry services and many technical publications covering their operation and design, including: A n Overview of Equipment for Containment and Disposal of Emergency Relief Efluents [Ref. 7-91 Emergency Pressure Relief Discharge Control by Passive Quenching- Update [Ref.7-13]

Perry’s Chemical Engineer’s Handbook [Ref.7-14] Sizing Separators and Accumulators [Ref. 7-15, pages 2542561 Equipment Design Handbook for Refineries and Chemical Plants [Ref. 7-16, pages 153-1871 Check n e s e Points When Designing Knock-Out Drums [Ref. 7-17, pages 155-1561 Design Two-Phase Separators within the Right Limits [Ref. 7-18, pages 53601 Small Scale Evaluation of Dump Tank Sizing Methods [Ref.7-19, pages 1691831 Computer Modeling Aids Separator Retrofit [Ref.7-20, pages 76-80] 147

Safe Design and Operation of Pmw Vents and Emission Control Systems

Table 7-2.

Advantaees and Disadvantages - Collection Systems Msadvantagea

Vessel

Ivithlut

Venting

Dump and CatchTanks

Prevents pluggingof emergency vent header lines; separates and collects the bulk of liquids,

Blowdown *

Moderateshlgh pressuredischarges to vent heder system Prevents pluggingof vent header lines, separatesand coileds h e muni-phaseemuent from batch or other pencdc venbng May be used for emergency venting Essentiallya passive devlce with hgh reliability May serve mre than one pcocess vent

Drum a d Tanks

*

Quench

b l s high temperaturedischargesto vent header system, reducingdesgn and matenai of construcbn constraintson vent header system May providesgnmcant condensingof condensableprocessvapors reducing vent header vapor ioad

Quench PoOlS

Passive, but must have system to venfy proper liquid level and composition reactant is used

DRm l

Requires very precrse knowledgeof emergency scenanos,r e a m chemistry,etc Requiresdetail& engineering analysis and iab wok to define maximum pressure and temperature Not recommendedfor reacbons prcducing a signkant amount of non-condensabiegases ~igherCOSI for vessel Must be locatedin cbse proximityto process vessel sewed If receivingflammablesor matends heated above h i r Rash p i n t tank needs to be maintrined air-freeor inerted May require instrumentationto ensure hat it IS empty or to maintain a desired iow level Usually locatedrelativelyclose to process vessel served May require 8s own pressurecontrol or pressurerelief system to let down high discharge pressuresinto the vent header system Usually requires instrumentationfor level, temperatureor pressure and pump out system May vaporize some of quench lquid and may require a downstreamcondensingstep to remove or recover quench liquid An x W e treabmnt system requiringa functioning quench iiquid spray system typicaily pumped recirculatbnwith quench iiquid makeup and sump level conbol When used in emergercy venting sewia, a high level of reliabilityis required of quench liquid system Quench liquid may become i d e n with dissolved or entrained process material and may itseii require treatment If water is used in wid climates,freeze protecCon may be required Pilot plant demnstrabon or scalwp test may be needed For reactive matenals:may not be suitablefor matenalswm slow to M e r a t e reacbn rates when used as an absobr Large pool volume of quench liquid Dispositionmay present environmental problem - Lower boiling materials requireeven larger pool liquid volume Noncordensables in the vented stream lower recovery etiency and require relief venting to a vent header system

-

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The basic principles and equations for design are well established, but somewhat empirical, and selecting appropriate design parameters for a specific service involves some judgment. Sizing procedures for hock-out tanks are presented in APl RP 521, Guide for Pressure Relieving and Depressuring Systems [Ref. 7-81 and Guidelinesfor Pressure Relief and Efluent Handling Systems [Ref. 7-31, which includes a CD containing helpful calculation software. 7.3.2

Knock-Out Tanks and Drums

Liquids, including water, may collect in vent header systems due to condensation, leakage, or carry-over from a vent release into the system. The presence of liquid in the vent header system can result in serious operational problems and equipment failures, including: Restriction or blockage of vent gas flows due to: - Liquid pooling - Solids build-up, solidification of high melt point material, freezing, or polymerization Discharge of burning material from flares Damage to treatment equipment not designed to handle liquid feeds Accelerated corrosion Mechanical damage to vent header lines as a result of liquid slug flow causing water-hammer A particular concern with blocked vent gas flows is the potential for overpressure or vacuum collapse of atmospheric storage tanks or other vessels connected to the header. Knock-out drums are typically installed immediately upstream of flashback prevention devices and final treatment and disposal equipment, such as oxidizers and flares. Their installation should also be considered at points where branch or sub-headers join main headers and at potential liquid sources. Depending on composition, separated liquid may be: Recovered and returned to the process Sent for recycling Used for fuel Re-vaporized and sent to a flare or other incineration device

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More than one knock-out drum may be required in a vent header system due to: Liquid carryover from certain vessels or units

Cooling and condensation in the vent header system A generalized knock-out drum design for flare service is shown in Figure 7-2 [modified from Ref. 7-81,

To Flare

b

From Vent Header Drain Options

............,

Automatic shown or Manual w t h High Level Alarm

From Drains or Other Connections Optional to Maintain Minimum Liquid Level

Figure 7-2.

Flare Knock-Out Drum

An overview of methods of sizing knock-out drums and various other types of vapor-liquid separators used in the chemical industry is given in the article An Overviav of Equipment for Containment and Disposal of Emergency Relief Eflumts [Ref. 7-91. A sizing method for knock-out drums in flare service is outlined in API RP 521, Guide for Pressure Relieving and Depressuring Systems [Ref. 7-81.

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Considerations in the design of a knock-out drum include: Sizing the knock-out tank for the maximum flow rates that could be encountered, including: - Sufficient vapor space volume to allow vapor-liquid separation - Adequate total tank volume to prevent liquid carryover, considering quantity of liquids anticipated Providing a heating system to: - Prevent high viscosity liquids from becoming excessively viscous - Prevent solidification of high melt-point materials - Provide freeze protection in cold climates Evaluating reactivity of chemicals that might be collected in the knock-out drum, especially when external heating is applied 7.3.3

Mist Eliminators

In wet scrubbers, the process of contacting gas and liquid streams results in entrained droplets. In the downstream vent header system, these droplets may coalesce or accumulate and pool in low spots. Mist elimination or entrainment separation should be provided to remove these droplets before the gas stream exits the scrubber. Common mist eliminator devices use multiple rows of chevron shaped deflectors or mesh pads. Chevrons are simply zigzag baffles that cause the gas stream to turn several times as it passes through the mist eliminator. The liquid droplets are collected on the blades of the chevron and drain back into the scrubber. Mesh pads are made from woven or interlaced non-woven synthetic or metallic fibers that coalesce fine droplets as the gas stream flows through. Mist eliminators are simple devices. However, they may become coated or plugged and increase flow resistance and pressure drop. A sigruficant increase in backpressure on a vent header system can be a potential safety problem, particularly if the system is serving low-pressure relief sources. 7.3.4

Cvclones

Cyclone separators may be used for separation of droplets or particulates from a vented gas stream. The gas stream enters tangentially and relies on centrifugal force for separation.

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Advantages of cyclone separators include that they can be designed to withstand deflagration pressures and do not involve moving parts. Disadvantages include a limited range of efficient operation with respect to particle size and flow rates. Another disadvantage is the auxiliary equipment required for liquid accumulation or solids removal. 7.3.5

Advantapes and Disadvantages - Phvsical Seuarators

The advantages and disadvantages of the various types of physical separation systems discussed in this section are briefly summarized in Table 7-3. Table 7-3.

System KnKk-Out Tanks and Drum

Advantages - and Disadvantages - - Physical Separators Advantages Simple means of separating liquid from vapor May be used immediately downstream of process vents likely to release two-phase flow May be used before treatment devices, such as flares or carbon adsorption beds, to collect liquids condensed within the vent header piping Recovery of separated liquid

Mist Eliminators

Usually a component of other devices, such wet scrubbers Simple means of separating liquid droplets or mist from vent stream

Cyclones

Simple means of separating liquid droplets and solid particulates from vent stream Can be designed to withstand an internal deflagration

Disadvantages Require provision for removing collected liquids that may include level controls, alarms and manual or automatic drains or pump out If allowed to become liquid full, this can effechely restrict or block the vent path resulting in high back -pressure on all connected equipment

Can collect solids and become plugged, causing flow restriction and back-pressure on all connected equipment Efficiency limitation on range of flowrates and particle / droplet size Requires additional equipment for the safe removal of separated solids / liquids Not usualiy suitable for the wide flowrate range in combined normal and emergency vent header systems

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AbSQrphn Absorption involves the process of contacting a vapor and gas stream with an absorbing liquid to remove specific materials from the gas stream. This may be by physical absorption or may also be accompanied by chemical reaction such as the use of caustic solutions to remove chlorine for vent gases. Typically, this is conducted in equipment commonly referred to as scrubbers or absorbers. Absorbers can be designed to operate over a wide-range of conditions to meet specific emission control objectives. Typically, absorption systems can operate with collection efficiencies of 70 to over 99 percent. Absorbers may be used as a final treatment system or an intermediate treatment step. Intermediate treatment can remove potentially corrosive materials from the vent gas stream and allow for the use of more economical materials of construction in the downstream vent header system. Absorbers are typically followed by entrainment separators, such as mist eliminators or cyclones, to remove droplets from the exit stream. Other design intentions for absorbers may include cooling and condensing gases and vapors, capture or removal of liquid droplets or solid particles from the gas/vapor stream, and chemical reaction. This technology is well described in literature, including information on acquiring needed vapor-liquid equilibrium data, and designing and scaling-up equipment. See Perry ‘s Chrmical Engineer’s Handbook [Ref. 7-14], Mass Transfw in Engineering Practice, [Ref. 7-21], and Handbook of Separation Process Technology, [Ref. 7-22]for background information and descriptions of the basic technology of absorbers. Emergency relief systems handle hazardous materials and must be highly reliable. When used for emergency relief systems, absorbers and the support systems they require to operate must be equally reliable. Packed towers are the most commonly used absorbers in emergency relief applications; however, other types of intemals may be used. Problems encountered with absorbers include solids build-up restricting flow through the absorber or poor gas-liquid contact reducing absorbing efficiency. The performance of absorption systems can be monitored directly or indirectly by: Temperature differential between inlet and outlet liquid flows Pressure differential between inlet and outlet gas flows Liquid flow rate into the scrubber Concentration of absorbed material in outlet flows 7.4

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Safe Design and Operation ofProcess Vents and Emission Control Systems

There are several equipment designs for contacting the absorbing liquid with the vent gas stream. The most common ones are: Spray towers Tray towers Packed-bed scrubbers Venturi scrubbers 7.4.1

Smav Towers

A spray tower is the simplest device used for gas absorption. It consists of an open vessel with one or more sets of spray nozzles to distribute the scrubbing liquid. Typically, the gas stream enters at the bottom and passes countercurrent upward through the sprays. In general, spray towers are not as efficient as those absorbers with internal components. As part of a vent header system, spray towers are commonly used in fouling service that may plug other types of absorbers. Spray towers are frequently used as an intermediate treatment step in vent header systems. 7.4.2

Trav Towers

Tray towers are vertical columns with one or more horizontal trays provided to enhance gas-liquid contact. Tray towers can have several different types of intemals, including: Sieue truys - use perforated plates. These are the simplest type of trays Impingement trays - have small impingement targets above each perforation to enhance gas-liquid contact Bubble cup trays - have risers covered with caps. These can operate over a wide range of gas and liquid flow rates without adversely affecting efficiency Valve trays - have liftable valves or caps that improve gas-liquid contact when the gas flow rate vanes Tray towers are infrequently used as absorbers in vent header treatment and disposal service due largely to their relatively higher initial cost and complexity when compared to other absorption treatment options. 7.4.3

Packed-Bed Scrubber

Packed-bed scrubbers are vertical columns containing packing to provide a large surface area for gas-liquid contact. Absorbing liquid is introduced at the top of the column and flows down over the packing material. The vent 154

Chapter 7 -Treatment and Disposal Systems

gas stream enters below the packing and flows up through the column, absorbing soluble gases or vapors. There are two general categories of paclang; structured and randomfill. Structured packing is manufactured in prefabricated sections with labyrinth flow passages often chevron-shaped which are fitted into the scrubber vessel. Random-fill packing consists of specially designed shapes that are loaded loosely into the scrubber and supported at the bottom and restrained at the top typically by a perforated plate or screened supports. There are many types of each kind. Each provide large surface areas while maintaining open space for gas flow. Common materials of construction include plastic, metal, and ceramic.

In vent header systems, packed-bed scrubbers are commonly used as intermediate components within the systems as well as end-of-pipe treatment devices. They typically permit a wide range of vent gas flowrates that makes them suitable in some cases for both normal and emergency vent service. The scrubber liquid-pumped circulation, spray and makeup system may require suitable instrumentation and alarms to ensure its performance and quality. The packed-beds of these scrubbers can become plugged with reaction products, salts or other solids from the vent streams. Therefore, instrumentation is often required to monitor pressure drop across the packed-bed. 7.4.4

Venturi Scrubbers

Venturi scrubbers use high velocity liquid jets directed into a venturi nozzle to contact an absorbing liquid with the vent gas stream. Venturi scrubbers require substantial pumping power and can be energy intensive. The venturi effect also creates a low inlet suction pressure that can provide a motive force that may be useful in the design of low pressure vent header systems. Venturi scrubbers are effective, but may require multiple stages in series to acheve acceptable performance. 7.4.5

Advantazes and Disadvantapes - Absorution Systems

The advantages and disadvantages of the various types of physical separation systems discussed in this section are briefly summarized in Table 7-4.

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Safe Design and Operation of Process Vents and Emission Control Systems

Table 7-4.

Advantages and Disadvantages - Absorption Systems Advantages

Requires continuous circulating pumped system with sump and liquid makeup that can be energy

7.5

Adsorption

Adsorption involves the process of contacting a vapor and gas stream with the surface of a solid adsorbent material. In adsorption, the gas molecule is lightly held to the surface of the adsorbent by weak electrostatic forces. These adsorbent materials include silica gel, activated alumina, molecular sieves, polymers, and activated carbon. Adsorbers typically have a relatively low capacity and are used for vents that release small quantities of volatile organics or odoriferous materials. Within this limitation, they have the advantage that they may not require utilities. The most common material for the adsorption of organics is activated carbon. A properly designed activated carbon system is generally capable of removing 95 to over 98 percent of an organic contaminant.

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Activated carbon beds can be used for both intermediate and end-ofline treatment to remove organic vapors. There are two primary types of carbon beds: Carbon drums or beds - These are replaced with fresh drums or beds when they reach their working capacity and may be regenerated offsite by a contract service. Regeneration systems - Adsorption swing regeneration systems provide automatic or manual valve switching to put fresh regenerated drum(s) on-line and then regenerate the spent drum(s). The principal safety consideration for both types of carbon bed systems is the potential for fire. Temperature detection and alarm to an attended location should be considered. Flashback prevention, such as flame arresters, should also be provided. The amount of material adsorbed by activated carbon is termed its retention or capacity. There are several types of capacities and they all depend on the operating conditions and on the particular organic contaminant being collected: Saturation capacity is the maximum amount of organic material the carbon can hold. Breakthrough capacity is the amount of organic material the carbon can hold before significant organic concentration begins to exit or break through the carbon bed. Heel c'spacity is the amount of organic material remaining in the carbon bed after it has been regenerated. Working capacify is the difference between breakthrough capacity and heel capacity and represents the amount of organic material that can be adsorbed in each operating cycle. A typical working capacity is 10 - 20 pounds (4.5 - 9 kilograms) of contaminant per 100 pounds (45.4 kilograms) of carbon. Once saturation capacity is reached, the vent gas stream will flow through the carbon bed with no reduction in its organic content. After breakthrough capacity is reached, the outlet stream will have an increasing level of organic vapors.

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Safe Design and Operation of Process Vents and Emission Control Systems

Since the capture and retention of organic contaminants is higher at lower temperatures, carbon adsorbers are usually operated at temperatures less than about 125°F (52°C). Retention tends to increase with higher concentration organic streams. Retention can be affected by: Temperature Pressure Organic concentration Contaminant molecular weight Moisture Particulate matter in the system 7.5.1

Advantapes and Disadvantages - Carbon Adsomtion

The advantages and disadvantages of the activated carbon adsorption systems discussed in h s section are briefly summarized in Table 7-5.

-

Table 7-5. Advantages and Disadvantages Carbon A d s o m t i o n Disadvantages System Advantages I

I

Carbon Beds

Simple Low initial cost Effective on low flowrate, low temperature vent streams

Regeneration Systems

Simple Lower operating cost Effective on low flowrate, low temperature vent streams

7.6

Use typically limited to volatile organics Low removal capacity Not suitable for use on emergency vent streams Potential for fire at the bed Higher operating cost Aldehydes and ketones (and other similar solvents) present problems initiating a chemical reaction Use typically limited to volatile organics Low removal capacity Not suitable for use on emergency vent streams Potential for fire at the bed

Recovery

In many cases, it may be advantageous to recover or reclaim some part of the vented process streams. The incentive may be purely economic, such as reclaiming flammable gas from the vent header system for its heating value as a fuel for process heaters and furnaces. In a few cases, the reclaimed gas or vapor or its condensed liquid may be reused in the process itself. Regardless of the economic incentives, recovery of any material that would otherwise 158

Chapter 7 -Treatment and Disposal Systems

require treatment or disposal results in less pollutant emissions. It is important to recognize the potential for trace component build-up when considering recovery or recycle options. 7.6.1

Condensing Svstems

In condensation processes, vapor phase materials can be recovered from a stream by cooling or compression, causing the material to condense. Economic factors favor the use of cooling rather than compression. Indicators of condenser system performance are: Outlet gas stream temperature Coolant flow rate Coolant inlet and outlet temperatures Concentration of volatile organics in outlet stream There are two broad categories of condenser systems in use: Direct contact condensers - These typically involve the intimate mixing of the coolant and process stream. Direct contact condensers are types of absorbers or scrubbers as discussed in the previous section. Surface condensers - These are heat exchangers that typically use air or cooling water to reduce the gas stream temperature to as low as 40 degrees F. Temperatures as low as 0 degrees F can be achieved using brine coolants.

Two types of surface condensers commonly in use in vent header systems are: Shell-and-tube heat exchanger - Shell-and-tube heat exchangers are simple, reliable and reasonably efficient devices provided the coolant supply is functioning. Coolant leaks in shell-and-tube heat exchangers can flow into the vent header system. Pluggage or freezing of material in the condensate drain lines can occur, resulting in flooding of the heat exchanger with liquid flowing into the vent header system. Coolants used can include: - Cooling tower water - Chilled water - Refrigerants

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Air-cooled heat exchangers - Air-cooled heat exchangers can be used to cool and condense vapor in vent streams. These heat exchangers may use natural or fan-forced draft and may include shutters for cold weather operation. For forced draft units, fan failure or power loss can significantly and rapidly reduce their condensing and cooling efficiency; industry experience suggests a 65-75'/0 loss in condensing efficiency. 7.6.2

Gas Recoverv Svstems

Environmental and economic considerations have increased the use of gas recovery systems to reclaim gases from vent header systems for other uses. Typically, the gas is recovered from a vent header feeding a flare. Depending on vent gas composition, the recovered gas may be recycled back to the process for its material value or used as fuel gas. Vent gas recovery systems are commonly used in refineries to recover flammable gas for re-use as fuel for process heaters. Their use is less common but increasing in other industry sectors. Some vent gas recovery systems have been installed to reduce flare operating rates or time duration in order to meet local regulatory limits. A conceptual design for a vent gas recovery system is shown in Figure 7-3 [Ref. 7-81, These systems usually consist of one or more compressors with suction taken from the main vent header downstream of any branch header connections. The compressed vent gas is treated appropriately for its composition and used as a fuel gas for plant heating needs or the recovered gas may be recycled back to the process. Refinery vent gas recovery systems commonly use positive displacement compressors with load controls to compress the gas to a pressure sufficient to deliver it to whatever cleanup treatment process is needed. Such compressors may require a somewhat higher vent header back-pressure in order to achieve stable compressor operation.

Vent streams not suitable for recovery could be routed to another vent header system. Low flowrate vent streams unsuitable for recovery may be connected downstream of the recovery system compressor suction connection provided there is sufficient positive flow of the other streams to ensure that the unsuitable stream is not drawn into the recovery system.

160

To Flare

Figure 7-3.

Conceptual Gas Recovery System

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Safe Design and Operation of Process Vents and Emission Control Systems

7.6.2.2

Design and Safety Considerations

Recovery systems can be used on combined normal and emergency vent header systems, However, the capacity of recovery systems is usually sized for some percentage of the normal vent flowrate with the balance of the vent gas flowing to the end-of-pipe treatment device. They are not sized for emergency venting loads due to the safety implications of availability on demand and back-pressure on the vent header caused by the large difference and sudden change from normal venting flowrate to emergency venting flowrate. The capacity of vent gas recovery systems is often limited by economic cost and the difficulty of designing compressor systems and their controls to operate at the relatively low suction pressures and often wide range of flowrates presented by vent header systems. Since the compressors of vent gas recovery systems require a relatively stable positive pressure at their suction intakes, the means to safely achieve th~spack-pressure control on the vent header becomes a key safety consideration. One proven means to safely provide back-pressure is the liquid seal drum. The seal d m provides a relatively constant low back pressure on the vent header and provides a narrow, but usually adequate control range for the vent gas recovery control system. The liquid seal drum should be designed to function over the operating pressure range of the recovery system. At higher vent release rates, the vent gas flows through the seal drum and out to the flare or other end-of-pipe treatment device. However, the narrow operating ranges afforded by liquid seals may not be acceptable. An alternate method is to use a high reliability fail-open control valve to regulate the vent header back-pressure to the suction of the recovery system. When using a pressure control valve, a positive exit path to the endof-pipe treatment device must be provided. Two approaches are suggested in APl RP 521, Guide for Pressure Relieving and Depressuring Systems [Ref. 7-81 to provide a flow path around the back-pressure control valve: A low-pressure, high-capacity pilot-operated pressure relief valve can be used. The sensing line for the pressure relief valve pilot should be provided with a clean gas purge and a backflow preventer A rupture disk can also be used

162

Chapter 7 -Treatment and Disposal Systems

If a back-pressure control valve must be used in the vent header line to regulate recovery system suction pressure, the control valve should be of a fail-open design and be interlocked to go fully open upon a higher-thannormal header pressure, high-oxygen content, or when the compressors are unloaded or shutdown. These interlocks are not a substitute for a positive path around the control valve. Figure 7-4 [Ref. 7-81 shows the preferred and alternate methods to preserving a reliable, open exit path for the vent gases. One major safety consideration in the application of vent gas recovery systems is the maintenance of a flow path through the vent header to the flare or other treatment device, especially for combined emergency and normal vent header systems. The design of recovery systems should not compromise this path. The recovery system must be designed as a side stream from the main vent header. Main vent flow should not be through any compressor knock-out vessel or suction piping. The tie-in line to the recovery system should come off the top of the vent header line to minimize the possibility of liquid entrance. The second major safety consideration is the prevention of back flow of air from the end-of-pipe treatment device into the vent header and the compressors at low process vent gas loads. The oxygen content of the vent gas stream should be measured and provisions made to automatically and reliably shut down the compressors if potentially dangerous conditions exist. All compressors should be equipped with highly reliable low suction pressure shutdown controls. Additional instrumentation may also be appropriate to detect reverse flow in the section of header between the treatment device and the compressor suction take-off and automatically shut down the recovery system. The vent header system must also be studied to verify that the backpressure imposed by the pressure relief device (assuming the control valve is closed) at hul header load will not impose unacceptable back-pressures on relief devices and control valves releasing into the vent header at the processing units. In the alternative pressure control design configurations, nonreclosing devices such as mpture disks or breaking-pin devices may be used instead of a pressure relief valve. These installations must also be carefully reviewed to ensure that the devices operate at as low a pressure as possible and that they do not cause unacceptable back-pressure on the vent header. 163

I

A

i~

Flare KO DNm

Fuel Gas Recovery

To Flare

Seal Pot

Preferred System -Water Seal

To Flare

Fuel gas purge

From Process Unil Flare KO DNmS

............................. T

Flare KO Drum

.......................................................... Fuel Gas Recovery

Open valve on high pressure or compressor

Alternate System .1

Rupture disk or other non-re-closingpressure relief device

,..........................

..............................................................

Figure 7-4.

164

T

PI

,

Vent Gas Recovery Pressure Control

To Flare

1 1

Flare KO Drum

Chapter 7 -Treatment and Disposal Systems

7.6.3

Advantages and Disadvantapes - Recoverv Svstems

The advantages and disadvantages of the condensing systems and gas recovery systems discussed in this section are briefly summarized in Table 76. 7.7

Thermal Destruction

Thermal destruction processes are commonly used as end-of-pipe treatment devices. In these processes, organic compounds may be oxidized and inorganics thermally altered to produce other less objectionable materials. For example, hydrocarbons oxidize to form carbon dioxide and water vapor. By-products of thermal destruction processes are not necessarily harmless. For example, organic materials containing chlorine, fluorine, or sulfur may form hydrochloric acid vapor, hydrofluoric acid vapor, sulfur dioxide, or other compounds. There are three basic types of thermal destruction processes: Flares Thermal oxidizers Catalytic oxidizers 7.7.1

Flares

Flares are used primarily in hydrocarbon service, but generally not applied to odorous or toxic materials due to the poor destruction efficiency. Flares are usually used for gas streams that have an organic vapor concentration greater than 2 to 3 times the lower explosive limit. If the vent gases do not have sufficient heat content, fuel must be added to the gas stream. This is referred to as a fired or endothermic flare and these devices should not be used for emergency relief vents [Ref.7-3, page 3501. Flares used as environmental control devices for volatile organic compound (VOC) emissions control are governed by federal regulations [Ref. 7-23]. Environmental regulations have generally provided exceptions for emergency venting. In some parts of the US., the federal requirements have been extended to cover emergency conditions. State and/or local permits are required to construct and operate flares. Regulatory authorities may also require smokeless (zero visible emissions) operation up to a prescribed percentage of the flare’s maximum design flow rate.

165

Safe Design and Operation of Process Vents and Emission Control Systems

Table 7-6.

Advantages and Disadvantages - Recoverv Systems

System Condensing Systems Shell-and-Tube Heat Exchangers

Condensing Systems Air-Cooled Heat Exchangers

Gas Recovery Systems

,

Advantages

Simple Reliable operation Efficient recovely of condensable vapors for reuse or disposal Effective cooling of vent stream Reduces vapor load on vent header system

Simple Reliable operation Economical recovery of condensable vapors Cooling of vent stream Reduces vapor load on vent header system Relatively economical means of recovering large amount of gas or vapor that are not easily condensed Reduces vapor load on vent header system Can significantly reduce the operating rate of end-of-pipe treatment devices, such as a flare

I

Disadvantages Pluggage or freezing can occur In the process flow tubes, causing back-pressureon vent header system In the condensate drain lines, causing liquid flooding with liquid running into the vent header system Potential for cooling water I liquid to leak into vent header stream If fan-forced drafl, then loss of electrical power results in a sudden major loss of cooling capacity Potential to freeze up and plug in cold weather conditions, causing back-pressureon vent header system Requires vent header system to operate at a higher positive pressure; the back-pressuredesign may provide potential for blocking in the header due to pluggage or back-pressurevalve malfunction (see preferred and alternate designs) May present a problem I any of the collected vent streams have a low discharge pressure Requires compressors and a relatively complicated control system Not suitable for emergency venting; requires control system to shut it down in an emergency

-

An advantage of flares is their capability to handle almost any combustible gas or vapor and tolerate variations in concentration, flow rate, heating value, and inert gas content. Flares can continue to bum in the event of a utility failure. In the event of flame failure, elevated flares provide dispersion of the vented stream due to the elevated discharge point.

166

Chapter 7 - Treatment and Disposal Systems

A typical flare system consists of some or all of the following components: Liquid separation - Knock-out and quench drums (pots) and related equipment Flashback prevention - Flame arresters and seal drums Flare assembly - Flare tip, burners, pilots, and ignition systems - Auxiliary piping for utilities, for example, steam, fuel gas, instrument air Auxiliary equipment and instrumentation - Flame scanners and monitors - Smoke suppression control system - Instrumentation, analyzers, and alarms The following common types of flares are used across a number of the process industry sectors: Elevated flare Ground flare Low pressure flare Bumpit Guidance for design of flares can be found in API Std. 537, Flare Details fmGeneral Refinery and Petrochemical S m i c e [Ref.7-24]. 7.7.1.1

Elevated Flares

Elevated flares provide the best dispersion of materials. For many applications, the elevated flare may be the only acceptable means of flaring "dirty gases" that may result in evolution of particulates or corrosive compounds. Disadvantages of an elevated flare are poor destruction efficiency, exposure of plant personnel and facilities to radiant heat during a major release, noise, and annoyance of the public due to the visible flame. Despite its disadvantages, an elevated flare is the most common choice for high flare loads or for handling over-capacity releases in conjunction with a ground flare.

167

Safe Design and Operation of Process Vents and Emission Control System

Design of elevated flares is dictated by radiation at grade level and the possibility of falling sparks. Sizing criteria and calculations for elevated flares are detailed in Section 5 and Appendix C of APl RP 521, Guide for Pressure Relieving and Depressuring Systems, [Ref.7-81. 7.7.1.2

Ground Flares

Ground flares are often used where it is desired to reduce the possible adverse impact of flare luminosity or noise levels and where there are structure height limitations that would result in an elevated flare stack producing unacceptable ground level thermal radiation loads. Ground flares are of two types: enclosed and open field (matrix) type. Ground flares may have multiple burners witlun a refractory lined structure. Because of their proximity to grade, a ground flare’s combustion process must not produce toxic pollutant by-products. A significant advantage of ground flares is that multiple vent header systems can be fed to separate burners within the ground flare shroud for combustion in a single device, avoiding the need to combine vent systems. An inherent hazard with ground level flares is that flameout will result in ground level release of vented materials.

Some facilities use a ground flare for normal process vent operation and an elevated flare for emergency releases. 7.7.1.3

Flaresfor Low Pressure Vents

Vent streams from storage tanks, separators, wastewater treatment units, and other equipment operating at or near atmospheric pressure can be treated in appropriately designed flare systems. In these cases, the low pressure in the source vessels provides little or no driving force for gas flow through the header and the flashback prevention devices. Large diameter vent headers may be required to minimize pressure drop. In addition, blowers may be needed where the pressure is inadequate. However, the use of blowers in flammable service introduces safety hazards and may: Provide a source of ignition Draw air in the header Reduce system reliability

168

Chapter 7 -Treatment and Disposal System

Other design and safety considerations due to the low operating pressure are the increased potential for: High winds that blow out the flare flame - special flare designs may be required Air leakage - oxygen monitoring may be required 7.7.1.4

Burn Pits

A burn pit is an excavated pit used to bum either gas or liquid vent streams, usually with simple burners. The typical arrangement provides minimal fuel/air mixing, produces large amounts of smoke, and must be located remotely from process units, personnel, and community. Under current United States environmental regulations, it is unlikely that an operating permit would be granted for a bum pit. 7.7.1.5

Seal Drums and Knock-Out Drums

Seal drums use an inlet dip pipe submerged a specific depth in a liquid to provide the following functions for a vent header system: Prevent reverse flow of air into the header from the flare stack or other end of pipe treatment device Provide flashback prevention Establish the minimum internal pressure at which the vent header will operate Sub-atmospheric pressure may develop in a vent header due to cooling and condensation of header gases. A substantial vacuum capable of breaking the liquid seal of a seal drum can be developed by some events, such as after a large venting of hot, condensable vapors or during a rainstorm that causes rapid cooling of un-insulated header pipework.

To prevent air entry through the seal drum, the height of the dip leg header inlet, the dip leg submergence, and the density and quantity of seal liquid in the drum must be sufficient to prevent the seal from being broken. When possible, a minimum dip leg header inlet height of 10 feet (3 meters) should be considered. A low-pressure activated system that automatically adds extra purge or inert gas into the header system in the event of vacuum can also be considered in addition to a seal drum.

169

Safe Design and Operation of Process Vents and Emission Control Systems

Seal drums share some common design features with hock-out drums, including vapor-liquid disentrainment in the vapor space to prevent flare surges. The liquid capacity of a seal drum should be sufficient to prevent back flow regardless of the circumstances. Proprietary seal drums (also called hydraulic flame arresters) use alternative design guidelines, and often offer operation and economic advantages. These designs are discussed in Ref 7-29. Seal drums are usually designed for at least 50 psig (3.4barg) to withstand internal explosion. APl RP 521, Guide fw Pressure Reliming and Depressuring Systems [Ref. 7-81 presents a method for seal drum sizing. A typical seal drum in flare service is shown in Figure 7-5.

Figure 7-5.

170

Seal Drum

Chapter 7 - Treatment and Disposal Systems

Knock-out drums collect and separate liquid from the vent gas stream. Provisions are needed to control level below the design maximum and dispose of the separated liquid. Knock-out drums have been previously discussed in Section 7.3.2. In some instances, it is possible to incorporate the functions of both seal drum and knock-out drum in one vessel. 7.7.1.6

Design and Safety Considerations

When designing flare systems, the issues and considerations that may affect the vent header system include: Material to be flared - flow rate and properties, particularly heating value Location of flare may require: - Lengthy piping runs may be needed to achieve acceptable flare separation distances from plant operations and community - Noise and flame luminosity consideration due to state or local regulatory requirements and result in increased vent header run distances * Installation of separate flare systems for: - Normal and emergency vent header systems which provides an opportunity for back-up capability for the normal vent header system into the emergency flare - Streams containing air in order to avoid flammable conditions in the vent header system, which may require the addition of a fuel gas to the vent stream Materials of construction, especially with regard to low temperatures or corrosive or reactive chemicals Measures to prevent discharging large burning droplets, such as cooling and condensing the vent stream [Ref.7-8 and 7-14] Smoke-free operation may be required due to federal regulations. This can be achieved by various methods; the more common is steam injection [Ref.7-81 Safety concerns in flare design involve the risk of explosion or fire due to improper flare design or operation. Routine scenarios encountered during maintenance and operation should be carefully considered to avoid contamination of relief systems with oxygen or reactive materials.

171

Safe Design and Operation of Process Vents and Emission Control Systems

Some of the safety concerns are listed below: Air ingress (backflow) from the top of the flare into the vent header can form a flammable atmosphere in the vent header system Ignition source by the flare flame or hot surfaces can cause flashback into the flare header Flashback prevention provided by seal drums, header purging, molecular seals, and separate headers and flare risers Liquid accumulation during high vent gas flow rates or overfill can result in: - Burning liquid ejected from the flare tip - Blockage of the vent header flow path by liquid filling of seal and knock-out Loss of flame may allow untreated vent gases to be discharged through the flare 7.7.2

Thermal and Catalvtic Oxidizers

Thermal oxidizers are used to treat vent streams containing organic vapors with a wide range of concentrations. A thermal oxidizer consists of a refractory lined chamber that has one or more gas- or oil-fired burners located at one end. The burners are used to heat the gas stream to the necessary temperature to oxidize the materials, typically 1,300 to 1,800"F (704 to 982°C). The vent gas stream does not usually pass through the burner itself, unless a portion of the gas stream is used to provide the oxygen or fuel value needed to support combustion, The combustion chamber and downstream sections are sized to provide sufficient residence time to complete the oxidation reactions, typically 0.3 to 0.5 seconds and occasionally longer than 1second. Catalytic oxidizers are used for vent streams that have an organic vapor concentration that is less than 25 percent of the LFL. In catalytic oxidation, the gas stream is passed through a catalyst bed heated to 600 to 850°F (316 to 454°C) with sufficient dwell time to complete the low temperature catalyzed oxidation process. Typical catalysts are noble metal oxides deposited on a substrate. A fired burner is used to heat-up and maintain bed operating temperature. In some cases, bed temperature may be maintained by the catalytic oxidation process itself without burning supplemental fuel, except for start-up.

112

Chapter 7 -Treatment and Disposal Systems

Thermal oxidizers may incorporate vent gas stream preheating or auxiliary steam generation as part of the equipment design. Either type of oxidizer may be followed by either or both a quench tank to cool the combustion products and a scrubber to provide final treatment. Oxidizers are usually located closer to the process facilities they serve since most authorities, insurers, and companies allow less separation distance for oxidizers compared to flares, particularly elevated flares. This results in shorter vent header lines. An advantage of oxidizers is higher destruction efficiency with controlled temperature and residence times to address destruction of specific troublesome compounds. 7.7.2.1

Design and Safety Considerations

The most important operating parameter used to evaluate the performance of a thermal oxidizer is the outlet gas temperature. This is always monitored, since it is used to control the fuel flow to the burners. Lower temperature will result in decreased destruction because of the lower reaction rates. Also, as the temperature decreases below 1,300"F (704T), equilibrium conditions cause the carbon to be increasingly converted to carbon monoxide or CO, rather than C02. Increasing vent stream concentration raises catalyst bed temperature. Vent stream concentrations in the flammable range can be ignited on contact with the hot catalyst bed and may result in an explosion. Catalyst may also be damaged by higher temperatures. Catalytic oxidizers should be equipped with flash back prevention on each inlet line [Ref.7-51. Flame or detonation arresters should be located in accordance with oxidizer manufacturer's recommendations regarding the distance of the arrestor device to the oxidizer fire box and heating of the arrestor face by radiation as well as conduction. Catalytic oxidizers have lower operating temperatures, generally resulting in lower operating fuel costs and, usually, lower initial cost. However, the catalysts are relatively easy to poison or foul resulting in premature and costly replacement. Factors that contribute to loss of performance in thermal and catalytic oxidizers: Low combustion temperature Inadequate residence time Inadequate mixing of the gases 173

Safe Design and Operation ofProces Vents and Emission Control Systems

Burner combustion problems (thermal oxidizers) Fouling or plugging of the pre-heat exchangers or steam generators Fouling or loss of catalyst activity (catalytic oxidizers) 7.7.3

Process Heaters Used for Thermal Destruction

Process heaters and boilers have been used as an economic alternative for the destruction of lower flow rate vent streams instead of routing these via main vent headers to flares, incinerators, or oxidizers. Process heaters and boilers usually operate with combustion chamber temperatures of 1,800"F (982°C) or higher and may have flue gas residence times of 1 to 2 seconds or longer. Assuming adequate mixing of gases in the fire box, these conditions may provide appropriate destruction of volatile organics similar to thermal oxidizers. The vent streams suitable for feeding to process heaters are often of low flow rate from low pressure source equipment that cannot be effectively vented into main vent header systems. These streams may be vents collected from sources such as main vent header knock-out drums, process separators, or liquid drain tanks. The vent gas stream is typically piped close to the main burners or may be drawn in as part of the combustion air supplied to the burners. A final liquid knock-out should be provided for these steams before they are routed to the process heater or boiler. A flame arrestor or other flashback prevention should also be provided. 7.7.3.1

Design and Safety Considerations

Although the concept is simple, there are design considerations in the use of a process heater or boiler to incinerate vent gas streams. The heater unit must operate continuously at a reasonable load, typically 40 to 100 percent of design load, in order to have the necessary internal temperatures for organic vapor destruction. The vent gas stream should be only a small portion of the total fuel flow to the unit. If the volume of the added vent gas stream is large, the increased combustion gas velocities through the heater would lower the residence time resulting in reduced and, possibly unacceptable, destruction efficiency. Safety issues include the potential for flashback into the source equipment causing an explosion and liquid overflow from the source equipment through the vent lines to the burner, resulting in a pool fire underneath the process heater. Provisions for flashback prevention and liquid knock-out should be considered. 174

Chapter 7 -Treatment and Disposal Systems

7.7.4

Advantaces and Disadvantages -Thermal Destruction Svstems

The advantages and disadvantages of the principal thermal destruction systems discussed in h s section are briefly summarized in Table 7-7. Table 7-7. System

Fbres

Oxidizers

Advantages and Disadvantages - Thermal Destruction Systems Advantages Able to handlewde vaMtions in flow rates and matel types Elevated Rares providedispersionof Rare gas mbustion produck and some dispersion of vent gas if the flare flames wt Groundflares m acawnmodatea number of diffwentvent s t e m gaseous or liquid

Thermal oxidKers can handlea rangeof

concentrafiw

Maybelocateddosertoprocessfacilii (shorter vent header pipe NIIS) PrWidegreater desbuctoneffdenaes than flares Can handle multiple separale vent headermrceslines avoiding intemnedion issues

Disadvantages Requ~resadequate separabondistances to othw Wltles and property Inesthat may signihlly naease vent header PlPeMlengh Requm a mmbusbbleconmlrabon vent gas or may needlo sgnlcanUy add fuel to ensue adequate ambush flame Flaresdonotoffertheoppoltumtyfor past-saubbiy necessary to address the by-pmdudsfran combusbcm LowwdWcboneRaency Potenbaliy law rellabikydue to dependence on utility decbic p e r , immentation, and mtrols Requires longer Sme to restadand bring mCne aiter a shutdown Shutdm or failure of an oxidizer may allow the releaseof toxic OT flammable vent gas hwnthe relative low height of the unit's vent stack. resubng in near ground lev4 re!eases LimRed operating flow rate range mmparedto flares; catdytictypes can have even n m e r operatingrange Requires more inslnrmntab tfian flares, i.e., ombush controls, intekcks,blwrs, a 7 t W catalybctypescan experience bed poisoniq, fwCng. or plugging Increasedpotenbdfor heater fires Potentialfw flashbadcto S O U ~ equipment with possbleequipment

mst

Process Heaters

A possible econamical alternativeto treat limitedvent gas volumes at low flowrates and at low vent dischargepressure A means of treating vent sbeam~that are too low in pressureor otheMlise impatible with main vent header

0

explosion

systems May allow very shortvent header piping wn lengths for appropriatevent steams

since p r m heaters are usually located within unit b a l t limb ~~

I 75

Safe Design and Operation of Process Vents and Emission Control Systems

7.8

Dispersion of Vent Gas

Discharge of process effluent directly to the atmosphere may only be done if the materials released are non-hazardous or do not meet the regulatory thresholds requiring treatment. However, even for non-hazardous vent materials, periodic normal process venting or emergency venting could abmptly eject debris, dust, liquid slugs, or droplets, possibly at elevated temperature that could cause harm to personnel in the area. Some materials considered non-hazardous may have odor or cause noise when released to the atmosphere and can be objectionable to site personnel and a nuisance to neighbors. Consequently, it may not be acceptable to release even relatively innocuous materials. Hazardous vapors and gases may sometimes be dispersed to the atmosphere using an elevated stack where the quantity is not large and the concentration is not high. Air pollution control regulations may require that large quantities or high concentrations of hazardous materials be treated or disposed of by other methods. Flammable process effluents can often be safely dispersed using an elevated stack. The elevated stack design must provide sufficient dispersion and mixing in the air at the appropriate height above ground to ensure that concentrations at ground level are below acceptable levels for toxicity and below the lower flammable limit both locally and downwind. Dispersion of process vent streams into the atmosphere from elevated stacks relies on effective mixing and dilution of the discharged materials to ensure that they are below safe threshold levels for toxicity and flammability. Elevated dispersion stacks are probably the lowest cost method of process gas/vapor effluent disposal and are also the simplest in terms of hardware. They are essentially passive in operation and, therefore, probably have the highest on-demand reliability for normal as well as emergency process venting. 7.8.1

Design and Safe& Considerations

Consideration of dispersion as a disposal method for hazardous or toxic gases and vapors requires a detailed and well-documented engineering and environmental review, including a dispersion analysis and, possibly, a consequence analysis. These analyses are necessary to confirm that the proposed stack design can handle the expected process vent stream (material characteristics, flowrates, etc.) and will prevent harmful concentrations affecting personnel at grade or in elevated working areas or platforms within the plant, as well as any community or environment receptors. 176

Chapter 7 -Treatment and Disposal Systems

Dispersion modeling and consequence analysis for selected hazardous materials for all credible scenarios must usually also satisfy federal, state, and local regulating agencies. When required, the dispersion analysis must use techniques that are technically capable of handling the given effluent discharge conditions and that are approved by any involved regulatory agencies. These books should be consulted for more information on dispersion technology and calculation procedures: Understanding Atmospheric Dispersion of Accidental Releases, [Ref. 7-25],Guidelines for Use of Vapor Cloud Dispersion and Source Emission for Accidmtal Releases, [Ref. 7-26], and Chemical Process Safety Fundamentals with Applications, [Ref.7-27]. 7.8.2

Atmospheric Dispersion Desim

The fundamental design considerations for atmospheric dispersion are: Stack height to allow adequate dispersion and dilution before reaching ground level or nearby work locations Stack diameter and discharge velocity to facilitate mixing of the discharge with air Location of stack and its discharge relative to nearby structures, work locations, and neighbors Measures required to control or eliminate liquid in the discharge stream Historical weather data for the location A broad approach to the design of elevated stacks for dispersion to the atmosphere, including simplified calculation methods for stack diameter and height and further references can be found in the book Guidelines for Pressure Relief and Efluent Handling Systems [Ref.7-31. The concepts presented there were intended for emergency venting; however, they can be useful for normal process venting as well. Stack outlet discharge diameter must be designed to provide sufficient effluent discharge velocity throughout the system’s flowrate range to provide adequate initial mixing and upward momentum, particularly at the low end of the design flowrate range. Dense gases (heavier-than-air)are more difficult to disperse than lighter-than-air gases; however, a properly designed stack can satisfactorily disperse even dense gases. Some companies use the following guide for dispersion stack discharge velocities to achieve acceptable dispersion: Lighter than air gases - greater than 100 feethecond Heavier than air gases - greater than 500 feethecond 177

Safe Design and Operation of Process Vents and Emission Control System

Dispersion stack height and discharge velocity do allow a range of trade-off opportunities since a taller stack may compensate lower discharge velocity. In some cases, air blowers have been designed into stack systems to compensate for low discharge velocity and to aid in dispersion. The height of a stack may present another safety concern. Tall stacks should be designed to withstand both the wind loads characteristic of the geographic area and any flow-induced vibration during a high flow rate venting event. Liquid entrained in the vented vapors and gases can present the hazard of fall-out or rain-out below and downwind of the stack. Liquid entrainment is more likely to occur with emergency venting streams; but, the accumulation of liquid in vent header lines leading to the stack should be anticipated in nearly all cases. Finely dispersed small droplets typically are not a problem. Flammable liquid droplets larger than about 150-600 pm could drop out as flaming rain if ignited. Droplets of corrosive or toxic liquids of even smaller size could present unacceptable hazards. Releases of large quantities of hazardous or toxic volatile liquids, vapors or gases could form a toxic atmosphere when atmospheric conditions prevent adequate dispersion. In general, dispersion stack designs should include the capability to disengage and drain any entrained liquid. In most cases, this means that a knock-out tank or pot should be provided. Removed liquids are typically pumped from the knock-out tank to an appropriate location. Alternatively, some installations have used a drain leg with a liquid seal instead of pumped removal. If a liquid sealed drain leg is used, the height of the seal should be specified to provide a head equivalent to at least 1% times the stack's backpressure at maximum relief load to prevent release through the liquid sealed drain. 7.8.3

Advantages and Disadvantages - Dispersion to AtmosDhere

Table 7-8 presents a summary of the advantages and disadvantages of the atmospheric dispersion method of disposal.

178

Chapter 7 -Treatment and Disposal Systems

Table 7-8.

Advantages and Disadvantages - Dispersion to Atmomhere Advantages Lowestcost Passive, highest reliability Minimum productionintemption

Disadvantages Safe quantity or concentration handled may be small for toxichazardous materials Tall stack may be required Remote stack location may be required increasing length of vent header system Dispersioneffectiveness reduced at lower discharge velocity (low flowrate) Liquid droplet discharge possible Discharge may be objectionable to neighbors -odor, noise, rain-out of liquids or solids

L 7.9

References

7-1.

Kneal, M. 1984. The Engineering of Relief Disposal - A Review Paper. Institute of Chemical Engineers Symposium Series, 85,183. London, England.

7-2.

Center for Chemical Process Safety (CCPS). 1995. Guidelines for Chemical Reactivity Evaluation and Application to Process Design. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York.

7-3.

Center for Chemical Process Safety (CCPS). 1998. Guidelines for Pressure Relief and Effluent Handling Systems. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York.

7-4.

Welding, T. V. 1984. Operational Experience with Total Containment Systems. Institution of Chemical Engineers Symposium Series, No. 85. Pages 251 - 263. London, England.

7-5.

National Fire Protection Association (NFPA). 2002. NFPA 69: Standard on Explosion Prevention Systems. Quincy, Massachusetts.

7-6.

Noronha, J. A., Merry, J. T., and Reid, W. C. 1982. Deflagration containment (DPC) for Vessel Safety Design. Plant/Operations Progress, l(1). New York

7-7.

American Society of Mechanical Engineers. 2004. Boiler and Pressure Vessel Code. ASME. New York. 179

Safe Design and Operation ofprocess Vents and Emission Control Systems

7-8.

American Petroleum Institute. 1997. APl RP 521, Guidefor Pressure Relieving and Depressuring Systems. New York.

7-9.

Grossel, S. S. 1990. A n Ovewim of Equipment foy Containment and Disposal of Emergency Relief Efluents. Journal of Loss Prevention. Process Industries, 3. New York

7-10.

Center for Chemical Process Safety (CCPS). 1993. Guidelines for Engineering Design for Process Safety. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York.

7-11.

National Fire Protection Association (NFPA). 2002. NFPA 68: Guide

for Venting of Deflagrations. Quincy, Massachusetts. 7-12.

Kanvat H., Lewis, M.J., Mizzen, M, Sandarac, 0. 1986. Pressure Suppression System Containments. A State-of-the-Art Report by a Group of Exerts of the NEA Committee on the Safety of Nuclear Installations, CSNI Report No. 126, OECD Nuclear Energy Agency, Paris.

7-13.

Keiter, A. G. 1992. Emergency Pressure Relief Discharge Control by Passive Quenching- Update. Plant/Operations Progress. ll(3). New York.

7-14.

Perry. 1997. Perry’s Chemical Engineer’s Handbook. 6th edition. (D. W. Green, ed.) McGraw-Hill. New York.

7-15.

Watkins, R. N. 1967. Sizing Separators and Accumulators. Hydrocarbon Processing. 46(11). Gulf Publishing. Houston.

7-16.

Evans, F. L. 1980. Equipment Design Handbook for R8nwies and Chemical Plants, 2nd ed., Vol. 2 Chapter 5, Separators and Accumulators and Chapter 6, Flare Stacks. Gulf Publishing. Houston.

7-17.

Niemeyer, E. R. 1964. Check These Points When Designing Knock-Out Drums. Hydrocarbon Processing and Petroleum Refiner (now Hydrocarbon Processing). June. Gulf Publishing. Houston.

7-18.

Svrcek, W. Y. and Monnery, W. D. 1993 Design Two-Phase Separators within the Right Limits. Chemical Engineering Progress. October. New York

7-19.

Mchtosh, R.D., Nolan, P. F., Rogers, R. L. and Lindsay, D. 1995. Small Scale Evaluation of Dump Tank Sizing Methods. Journal of Loss Prevention in the Process Industries. 8(3). Amsterdam, Netherlands.

180

Chapter 7 -Treatment and Disposal Systems

7-20.

Fewel, FK. J. and Kean, A. J. 1992. Computer Modeling Aids Separator Retrofit. Oil and Gas Journal, 90(27). Pennwell. Tulsa, Oklahoma

7-21.

Lydersen, A.L. 1983. Mass Transfer in Engineering Practice. John Wiley & Sons. New York.

7-22.

Rousseau, R. W. 1987. Handbook of Separation Process Technology. Chapter 6, Absorption and Stripping. John Wiley & Sons. New York.

7-23.

Code of Federal Regulations. 40 CFR Part 60.18. General Control Device Requirements (Flares). Washngton, D.C.

7-24.

American Petroleum Institute, 2003. API Std. 537, Flare Details for General Refinery and Petrochemical Service. First Edition. New York.

7-25.

Center for Chemical Process Safety (CCPS). 1995. Understanding Atmospheric Dispersion of Accidental Releases. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York.

7-26.

Center for Chemical Process Safety (CCPS). 1996. Guidelinesfor Use of Vapor Cloud Dispersion and Source Emission for Accidental Releases. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

7-27.

Crowl, D. and J. Louvar. 2002. Chemical Process Safety Fundamentals with Applications. 2nd Edition. Englewood Cliffs, New Jersey: Prentice-Hall.

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by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

HAZARD ANALYSIS AND CONSEQUENCE ASSESSMENT Support and utility systems often receive a minimal or low level of safety, health, and environmental consideration and assessment. Vent headers and their associated treatment and disposal systems are often grouped with these perceived lower hazard support systems. However, vent header systems are very frequently handling materials that present significant hazards with severe consequences. Vent header systems should be given the same level of hazard consideration as any other section of the process system. Conducting hazard assessments is of major importance for the safe design and operation of vent header systems, including their treatment/disposal systems. Additionally, consequence assessments, including dispersion modeling, are often needed to evaluate the effluent from a vent header system’s final treatment/disposal device. Such assessments can help define new design parameters or confirm the adequacy of the system‘s capability to provide the required protection for plant personnel, community, the environment, and plant assets. Support for the importance of performing formal, detailed hazard analysis of vent header systems can also be found in safety and environmental laws and regulations. The Clean Air Act of 1990, in the section titled Prevention of Accidental Releases, describes a “general duty to identify hazards that result from releases (of hazardous materials) using appropriate hazard assessment techniques”. The EPA regulations pertaining to the Risk Management Plan program, 40 CFR Part 68, Chemical Accident Prevention Provisions [Ref. 8-11, specifically lists relief vent systems as process components that are included in the Process Safety Information used as a basis for conducting hazard reviews and analyses. Facilities that are covered by the OSHA Process Safety Management Program (PSM) are required to perform hazard analyses of covered processes, including vent header systems [Ref.8-21. 183

Safe Design and Operation of Process Vents and Emission Control Systems

The guidance provided in law, regulation, and in t h ~ book s does not attempt to specify or define an "acceptable" level of risk. The focus is placed on identification, analysis, and assessment of hazards and consequences, in this case, associated with vent header systems and their effluent to ensure that appropriate safeguards and mitigating features are in place.

8.1

Hazard Analysis Methods

There are a variety of hazard identification and analysis tools that are widely used to identify and assess hazards. For information on how to conduct hazard analyses, refer to Guidelines for Hazard Evaluation Procedures [Ref. 8-31, Guidelines for Chemical Process Quantitative Risk Assessment [Ref.8-41, and Layer of Protection Analysis, [Ref.8-51, Commonly used analysis methods include: Hazard Identification (HAZID) What-If? What-If?/Checklist Hazard and Operability (HAZOP) study Failure Mode and Effects Analysis (FMEA) Fault Tree Analysis (FTA) Layer of Protection Analysis (LOPA) Quantitative Risk Assessment (QRA) More than one hazard analysis may be required to identify and assess hazards and, where needed, perfom further semi-quantitative or quantitative analysis. Figure 8-1 illustrates different types of hazard assessments, their complexity, and the time and resources required. The semi-quantitative tools, such as Layer of Protection Analysis (LOPA), Fault Tree Analysis (FTA), and Quantified Failure Mode and Effect Analysis (FMEA) are particularly effective in evaluating the effectiveness of existing and proposed protective features. They do require more effort to evaluate each scenario, and thus should be targeted at selected scenarios. In general, these semi-quantitative methods provide order-of-magnitude estimates of likelihood for individual hazard scenarios.

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Figure 8-1.

Hazard Assessment Methods

Layer of Protection Analysis or LOPA applied to vent header systems may be used to: Assess whether existing or planned features of a vent header system provide adequate protection against specific causeconsequence scenarios Determine the appropriate design integrity level of protective safety instrumented systems (Safety Integrity Level or SIL) associated with a vent header system including its treatment/disposal systems Select among alternative vent header design options and to identify possible risk or cost reduction opportunities [Ref.8-51 Quantitative Risk Assessment (QRA) methods may be indicated for scenarios involving complex interactions between external events, equipment failure or human performance. 8.2

Hazard Analysis Process

The process of conducting a hazard analysis for a vent header system is the same as for a process unit, that is: Identify hazard scenarios (causes and their credible consequences ignoring the existing or planned safeguards) Identify safeguards 185

Safe Design and Operation of Process Vents and Emission Control Systems

Conduct risk ranking - Determine the severity of the consequences without safeguards - Determine the likelihood of the scenario using safeguards 0

8.2.1

- Determine the risk Based on the risk ranking, propose additional safeguards to reduce risk to an acceptable level

Identification of Causes

A broad range of causes of hazards may threaten the intended safe operation of a vent header system and its treatment/disposal equipment. These causes may originate from the process or the vent header system.

All potential causes of hazards from the process into the vent header system should be identified, including both normal and abnormal events. Some examples of potential process causes of hazards in vent header systems include: Changes in process venting conditions much higher or lower than normal including: - Flowrate - Temperature - Pressure - Composition 0

Operation of automatic process and venting controls in a manual mode Relief valve lifting or rupture disk relieving (emergency process venting)

0

Common mode failures resulting in simultaneous vent releases where the header serves more than one process unit or operation Start-up and shutdown transients resulting in: - Much higher or lower flowrate, temperature, or pressure than normal venting conditions - Potential for receiving different materials, such as air, inert gas, or water into a header system not designed for these materials - Controlled blowdown of process equipment or vessels that may include process liquids

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External fire exposure to the process resulting in overpressure venting Abnormal heat input to process Process changes affecting chemical reactions or vented materials Mechanical failures such as heat exchanger tube failure, valve failure, etc. Local or plant-wide loss of support utilities to process (electric power, cooling water, instrument air, steam, etc.) Similarly, causes that originate in the vent header system should be considered. These may include: Liquids pooling in low points of header Build-up of solids, dust, condensed viscous material, polymerization Mixing of incompatible or reactive materials in the vent header system Loss of purge gas for "rich" vent header systems (inert or other gas) Loss of dilution air for "lean" vent header systems External fire exposure to the vent header system piping and components Mechanical failures in the vent header system, such as heat exchanger tube failure, flange gasket failure, valve failure, etc. Local or plant-wide loss of support utilities to vent header equipment such as knock-out tank level instruments, pumps, automatic drain valves, scrubber blowers, flare or thermal oxidizer utilities (electric power, cooling water, instrument air, steam, fuel gas, etc.) One source of information on possible causes of malfunction of piping and other components of vent header systems may be found in the book Guidelinesfor Design Solutionsfor Process Equipment Failures [Ref.8-81. Checklists can also be developed and used to identify hazards associated with vent header systems. Such tools could be built around the safety and design issues presented in Chapters 3 through 7 of this book. Separate checklists could be developed for the following: 0 Leansystems Richsystems 187

Safe Design and Operation of Process Vents and Emission Control Systems

Inert systems Normalventing Emergency venting 8.2.2

Development of Conseauences

The causes originating in either the process or in the vent header system may suggest potentially hazardous consequences. As in any process PHA, these consequences should be fully developed to their worst credible severity disregarding any safeguards. These worst credible consequences may occur in either or both the process unit or the vent header system. Some examples include: Overpressure or explosion in piping or equipment in the vent header system, its treatment/disposal systems, or in connected process equipment Hazardous discharge to atmosphere within the operating area or offsite Fire either in equipment or vented into operating area Smoke vented into operating area High thermal radiation, such as from excessive flowrate to a flare 0

Reaction between incompatible materials causing vent header system damage Flow restriction causing back-pressure on the process or damage to the vent header system Escalation into process problems causing shutdown Polymerization causing flow restrictions or system damage Corrosion Reaction forces on piping and equipment from venting Water hammer (velocity hammer) during venting Reverse flow via the header from one vessel into another Pressure reversal resulting in back-flow to process or connected vessels Noise or nuisance release of non-toxic odorous materials

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Adequate consideration should be given to identification of potential common-mode failure initiating events, such as utilities upsets, particularly for vent header systems with multiple components and for those systems serving more than one process unit. 8.2.3

Estimation of Hazard Scenario Risk

Once identified, each cause-consequence scenario should be evaluated with respect to the sevwity of consequence in the absence of protective features and the likelihood ofoccuwence in order to characterize the relative risk posed. The likelihood can be estimated from a number of sources, including historical data, operational experience, knowledge/judgment, or analytical methods. The consequence and likelihood numbers are then combined using a risk ranking matrix to determine the overall risk. Where the risk is determined to be unacceptable, recommendations are made for additional safeguards. 8.3

Consequence AssessmentTechniques

Consequence assessment often involves conducting air dispersion modeling of a released hazardous gas or vapor, followed by an assessment of the impact on the affected workplace, community, or environment. Such assessment is often required by regulation or by the conditions of construction and operation permits. Generally, dispersion models are used to predict regions of toxicity or flammability. Release rate calculation examples applicable to vent header system discharges can be found in Guidelines for Cotzsequmce Analysis of Chemical Releases [Ref.8-61. Consequence assessment and related air dispersion modeling can also be used to confirm the performance of existing or proposed vent stream treatment and disposal equipment under design operating conditions. The methodology can similarly be used to assess onsite and offsite consequences during a treatment system malfunction. Consequence assessments can be performed to characterize: Toxic gas/vapor release impact zones Effects of combustion products from flares, incinerators, or thermal oxidizers under normal or malfunction conditions

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Thermal radiation zones for flares and simple atmospheric dispersion from vent stacks containing flammable streams that may ignite Liquid fallout zones for flares and stacks, including vent stacks for scrubbers and other treatment equipment Vapor cloud explosion overpressure zones Upper and lower vapor cloud explosion limit zones Quantitative consequence assessment is frequently performed as an integral part of the design of a process and its vent header system and associated treatmentidisposal systems. The results of a consequence assessment are often crucial when: Designing a new unit or plant Considering upgrades to an existing unit or plant Evaluating the necessity of adjacent buffer zones around a plant The type of project, e.g., new plant or changes to an existing facility, influences the type, level, timing, and amount of consequence assessments required. For new projects, it may be necessary to conduct consequence assessments several times iteratively as the design progresses. In the concept phase, it maybe helpful to know the thermal radiation zones, toxic impact zones, and liquid fallout zones (from dispersion stacks, flares, or scrubbers) to assist in evaluating and ranking potential sites. While there may not be precise process data available at this stage of the design, a high level consequence analysis can provide the project management team with relevant input that may exclude particular sites from further consideration or assist in the selection among competing sites. As the project progresses further into the design stage, the consequence assessment should be refined to properly reflect the details of the chosen equipment configuration. The refined analysis may be used to evaluate public impact as well as provide direct feedback for design improvement, such as the optimal location, height, etc. of the point of discharge to atmosphere of the vent header system and its treatment/disposal system.

In the final stages of design, air dispersion modeling and consequence assessment should be refined to reflect the final process details. Analyses are often necessary to confirm final design details, such as height and location of a flare, atmospheric dispersion stack, or discharge points from final treatment devices and systems. 190

Chapter 8 -Hazard Analysk and Consequence Assessment

For an existing plant, consequence analysis of the vent header system serves many of the same uses. Consequence assessment for an existing operation can be of great use in emergency planning. Consequence assessments and modeling results provide an excellent source of data for preparing emergency response pre-incident plans in conjunction with Local Emergency Planning Committees (LEPC) and associated response organizations. Consequence assessments can be very useful to Process Hazards Analysis (PHA) teams analyzing vent header systems. For example, an explosion analysis could help determine if the vent system piping, drums, seal pots, and treatment equipment, such as carbon absorption beds, have sufficient structural strength to resist major damage and further escalation in the event of an explosion. Guidelines for Consequence Analysis of Chemical Releases [Ref. 8-61 contains a wealth of information directly applicable to analysis of gas and vapor releases from vent header systems' end-of-pipe treabnent devices. Dispersion modeling can be performed to predict the concentrations of interest, such as zones within the flammable region or above threshold toxic concentrations. Flare height, wind speed, and release temperature of the material affect dispersion from an unlit flare. In modeling, the extent of a flammable region is generally '/2 LFL [Ref. 8-61. Guidelines for Consequence Analysis of Chemical Releases [Ref. 8-61 provides a comprehensive review of criteria and methods used to evaluate toxic effects. There are a number of airborne toxic exposure criteria used in consequence assessments of toxic gaslvapor releases including: Emergency Response Planning Guidelines (ERPG) - provides three concentration ranges defining a range of adverse effects based on up to 1 hr of exposure: ERPG-I, transient effects; ERPG-2, impair ability to take emergency action; ERPG-3, serious but not life threatening. Acute Exposure Guideline Leuels (AEGLs) - provide threshold exposure limits for the general public applicable to emergency periods of 10 and 30 min, 1 hr, 4 hr, and 8 hr: AEGL1, notable discomfort but transient; AEGL2, irreversible or long-lasting effects, impaired ability to escape; AEGW, life threatening effects or death.

191

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Immediately Dangerous to Life and Health (IDLHs) - must be adjusted for sensitive population, such as elderly, disabled or ill. For flammable vapors, the IDLH is defined as 1/10 of the LFL. Emergency Exposure Guidance Levels (EEGLISPEGL) - for 44 chemicals intended to promote healthy military personnel. Short Term Public Emergency Guidance Level (SPEGL) is defined as acceptable exposure to the general public. SPEGL is usually set at 10-50% of the EEGL. Threshold Limit Values - Short Term Exposure Limits (TLV-STEL) the maximum concentration to which workers can be exposed for a period of up to 15 minutes without suffering 1) intolerable irritation, 2) chronic or irreversible tissue change, 3) narcosis of sufficient degree to increase accident proneness, impair self rescue, or materially reduce worker efficiency, provided that no more than 4 excursions per day are permitted. Where flares are used, the impact of thermal radiation under high flare rate scenarios should be considered. Common assessment techniques and exposure intensity-time thresholds associated with thermal radiation can be found in a number of sources [Ref.8-6 and 8-71,

8.4

References

8-1.

Environmental Protection Agency. 2004. Accidental Release Prevention Requirements: Risk Management Program Requirements Under Clean Air Act Section ZIZ(rN7). 40 CRF Part 68. Washington D.C. Code of Federal Regulations Occupational Safety and Health Administration. 1992. Process Safety Management of Highly Hazardous Chemicals. 29 CFR 1910.119. Washngton D.C. Code of Federal Regulations Center for Chemical Process Safety (CCPS). 1992. Guidelines for Hazard Evaluation Procedures, 2 n d Edition. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers. Center for Chemical Process Safety (CCPS). 2000. Guidelines for Chemical Process Quantitative Risk Assessment. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers.

8-2.

8-3.

8-4.

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8-5.

8-6.1

8-7. 8-8.

Center for Chemical Process Safety (CCPS). 2001. Layer of Protection Analysis - Simplified Process Risk Assessment. Znd Edition. A CCPS Concept Book. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers. Center for Chemical Process Safety (CCPS). 1999. Guidelines for Consequence Analysis of Chemical Releases. American Institute of Chemical Engineers, New York. American Petroleum Institute. 1997. API R P 521, Guidefor Pressure Relieving and Depressuring Systems. New York, New York. Center for Chemical Process Safety (CCPS). 1998. Guidelines for Design Solutions for Process Equipment Failures. American Institute of Chemical Engineers, New York.

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by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

OPERATIONS AND MAINTENANCE Vent header systems collect vent streams from multiple vessels and direct the combined stream to treatment and final disposal devices. Depending on the process, vent header systems may include condensers, knock-out tanks, flame arresters and a variety of instrumentation. At many facilities, safety and environmental considerations require vent header systems to be operational as a prerequisite for operating the facility. Consequently, monitoring the status and ensuring maintenance is conducted on a scheduled basis is just as important for vent header systems as for the process equipment they serve. Factors that adversely affect vent header systems include flow restrictions caused by build-up of liquids or solids, defective pressure relief devices, corrosion, and malfunctioning instrumentation. Effective inspection and maintenance programs should be in place to ensure vent header systems are operable. Normal process vent header systems handle the vent gases and vapors generated during day-to-day operations. If they become defective, there will normally be deterioration in the facility’s performance. This typically can be seen as increased pressure drop in the vent headers and less effective control of emissions. Although these effects can adversely impact performance from the standpoint of environmental and industrial hygiene, they may not constitute an immediate hazard to personnel or the equipment. Emergency vent systems typically are only required to operate during upset conditions and may not receive vent flows for long periods. During this time, defects could develop that may not be indicated by the facility’s performance. Unless they are tested and inspected, operating personnel may be unaware that there is a defect. Failure on demand of an emergency vent system has the potential to cause a catastrophic incident, potentially exposing personnel and the facility to severe hazards. 195

Safe Design and Operation of Process Vents and Emission Control Systems

This chapter provides a general review of issues that should be considered as part of the inspection and maintenance program and design considerations to assist in implementing these measures. 9.1

DailyInspections

Operating personnel should conduct daily inspections of vent header systems. Procedures should be developed that specify operating conditions and provide guidance for troubleshooting problems that may occur with the vent header system. As part of the facility's normal operations, vent header systems should be monitored by the operator to confirm they are operating as intended. This is typically conducted by performing a vent header walkthrough that includes visually inspecting for signs of corrosion, piping properly on supports, and signs of heat effects or warping. The following is a typical list of items that should be monitored: Purge gas flows for explosion protection Instrument readings, e.g., flame arrester differential pressures, knock-out tank levels, etc. Low point drains are open and draining Inhibitor addition systems are functioning Status of locked or tagged open valves in vent header, see Chapter 6, Section 6.6.10 Status of "burst d i s k indicators on rupture disks Heat tracing to prevent condensation or solidification in headers 9.2 Scheduled Inspections and Maintenance lnspedion and maintenance of vent header systems should be scheduled and managed through a facility's preventative maintenance program.

9.2.1

Materials Build-Up

Solids or liquids build-up in a vent header system will reduce its capacity and can lead to the following incident scenarios: Reduced emergency vent header capacity may result in vessel failure in the event of an incident, such as a runaway reaction or external fire. When emptying a vessel, air or some other gas must enter to replace the liquid being removed. If the vent header is restricted, the gas flow may be insufficient to prevent the pressure falling below atmospheric, causing the vessel to collapse. Note: most large storage tanks can only withstand a few inches of water gauge "vacuum" before failure occurs. 196

Chapter 9 - Operations and Maintenance

If gas does not flow into a vessel when it cools, its pressure will drop. A tank exposed to solar radiation will heat up. Subsequently, if it is cooled, for example by a sudden rain storm, its pressure will fall. If the vent header is blocked, the tank pressure may drop below atmospheric causing it to collapse. Table 9-1 describes several conditions that can result in vent header systems becoming plugged. Industrial experience indicates that liquid buildup has been a common factor in many cases where vent systems have plugged. Self-reactive materials, such as monomers, frequently have inhibitors added to prevent polymerization or decomposition while they are being stored. Many of these mhibitors have low volatility. If monomer in a tank vaporizes and then condenses in the header, the liquid formed will contain virtually no inhibitor and polymerization is likely to occur (See Appendix H, Past Incidents). To minimize the potential for liquid accumulation, vent headers should slope continuously in the direction of flow, towards equipment. If this is not practical, drains should be provided at low points. These low point drains should be checked on a routine basis to confirm they are open and operable. The design of vent header systems should make provisions for inspection. These provisions can include blind flanges, removable spool pieces, cleanout/rod-out connections, and selection of inspection points during design and layout. Inspection and maintenance work plans and schedules should be developed that include the following steps: Establish frequency - Schedule frequency suffiaent to identify solids build-up or other problems for early correction before hazards are created, In many applications, annual inspections have been found to be adequate. Address personnel exposure - There is an increased potential for personnel exposure due to reactions occurring in combined vent streams, produang streams that may be more toxic than the individual materials. Early detection of corrosion - There is an increased potential for increased corrosion due to combining vent streams. Evidence of undetected offires - Inspection should include flame arrestors and vent header system to determine if there is evidence of an undetected fire

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Table 9-1. Potential Causes of V e n t Restriction Condition

Cause

..

I

from condensation or entkinmeniinto header. 1.2 Liquid phase reaction forming solid build-up, e.g. polymeiization of condensed or entrained liquid in header.

1.3 Vapor phase polymerization causing solid build-up. 1

1.4 Vapor phase reaction, e.g., solid ammonium carbonate formed by the reaction between ammonia and carbon dioxide.

1.5 Solid build-up as a result of sdidification of a hgh boiling vapor. 2. Relief valve or

rupture disk fails to open at specified pressure

2.1 Relief valve is set incorrectly or the wrong rupture disk has been installed. 2.2 Inlet line between the header and the relief device is plugged. 2.3 Rupture disk below a relief valve develops pinhole, delaying the vent opening until approximatelytwice the set pressure.

2.4 Rupture disk reinstalled after

3. Hgh pressure drop across Rame arrester

having been removed for inspection, 3.1 Flame arester element blocked with process material or cornsion products.

Potential Preventive Action

1.1.1 Slope header to eguipment and

provide low point liquid coll&tiin and removal. 1.2.1 Sarneas 1.1.1 and,Ithesolidis formed by a reaction involving materials from dfferent equipment, consider sending their vents to separate headers. 1.3.1 Investigate introducing gas phase inhibitor, e.g., SO2 inhibitor with HCN. 1.4.1 If solid is formed by a reaction involving materials from different equipment, consider sending their vents to separate headers. Otherwise, reevaluate inspectionfrequency and, if appropriate, reduce the time between inspections. Consider at-source scnrbbing to remove reactive mrnoonents . _ 1 5 1 Provide insulation and heat tracing to header system, including PSVs and name amsters. 2.1.IAdministrative controls to check set pressure on rupture disk and relief valve tags against MAWP of vessels. 2.2.1 Inspect this during scheduled inspections. Reduce time between inspections if plugging is encountered. Add purge gas to keep line clear. 2.3.1 Install pressure gauge or pressure detector wth alarm in zone between rupture disk and relief valve. Consider setting combination or iupturc disk and relief valve set pressures to vessei MAWP.

2.4.1 Do not reuse rupturedisks

unless they are installed in holders.

3.1.1 EstaMish routine inspection

program for Rame arresters to detect build-up. Reduce time between inspections if plugging is encountered. Also provide redundant Rame arresters and cleaninq connections.

Procedures should be developed for the following: Isolating the vent header from equipment in preparation for line entry 198

Chapter 9 -Operations and Maintenance

Vent header decontamination Procedures for removing solids from inside the header Returning the header to service

To prevent damage, the cleaning or decontamination method must be compatible with the header design. For example, when cleaning with steam, the design must make allowances for thermal expansion that will occur when the temperature in the header is increased. Materials of construction, including items such as gaskets, also must be suitable for the steam temperature. If materials such as acids, caustics, or solvents are used for cleaning, the materials of construction used in the vent header must be compatible with them. 9.2.2

Pressure Relief Valves and Rupture Disks

Pressure relief valves (PSVs) and rupture disks must open reliably at their set pressure and not leak. Leaking may allow materials to enter the vent header that could: Cause a build-up Create a flammable atmosphere Lead to corrosion in the vent headers PSVs should be on an effective pressure relief valve testing program that includes: Inspection of the inlet and outlet pipes for solids build-up after removing the pressure relief valve. Confirmation that the pressure relief valve will open at its set point. This is accomplished by “popping” the valve before it is decontaminated, Assurance that further investigation is conducted if the valve ”popped” outside the acceptable limits. Rupture disks should be on an inspection program to confirm they have not deteriorated during the time they have been in service. Rupture disks should not be returned to service after being removed for inspection unless recommended by the manufacturer. In general, rupture disks should only be reused if they are mounted in holders that can be removed from the header as a unit and then inspected and returned without the disk being dismantled from the holder. If the rupture disk has cracks, holes, or if it appears corroded, inspections and replacement should be more frequent. In tlus situation, it may also be appropriate to investigate alternative materials of construction or a different type of disk. 199

Safe Design and Operation of Process Vents and Emission Control Systems

If the rupture disk has a "burst disk indicatof', it should be on a routine inspection program and operating personnel provided training on its sigruficance and the actions required if a "burst" is detected. 9.2.3

Conservation Vents

Conservation vents on vent header systems should be on routine inspection programs to confirm their parts operate freely and that they have not experienced physical damage or corrosion. The inlet nozzles and, if installed, tail pipes should be checked to confirm solids build-up has not occurred. 9.2.4

Exdosion Prevention Svstems

Flame arresters have narrow flow paths that are prone to plugging. In addition, the elements are constructed from thin metal sections with no corrosion allowance. Depending on the application there may be instrumentation to monitor differential pressure across the element and to detect flames that have flashed back to it. Flame arresters, and their associated instrumentation, should be on routine inspection programs. For new facilities, it may be appropriate to inspect a flame arrester element within 3 months of being put in service. Provided there is no solids build-up or visible corrosion, subsequent inspections could be conducted annually. Further increases in the period between inspections may be appropriate provided it is consistent with the manufacturer's guidelines and there have been no indications of build-up or corrosion. Flame arresters should be located so they are accessible and can be removed for inspection. For further information on flame arresters, see Deflagration and Detonation Flame Arrestors, [Ref.9-11, 9.2.5

Fast Acting Valves and Chemical Isolation Svstems

Automatic fast acting valves and chemical isolation systems operate by detecting and responding to an incipient explosion before the flame front has accelerated to a hazardous level, or propagated into interconnected equipment. To achieve this, the systems must detect and respond extremely rapidly requiring the use of specialized detectors, controllers, and final equipment. NFPA 69 specifies that inspection and maintenance for these systems be conducted in accordance with the manufacturers recommended practices [Ref. 9-21. In practice, this generally requires the inspections to be conducted every three months by personnel trained by the system's manufacturer.

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9.2.6

Explosion Relief Panels

Explosion relief panels have no moving parts and do not require maintenance, however, they are relatively fragile and do not have a corrosion allowance. As a result, they should be on a routine inspection program to check for mechanical damage or corrosion. In applications where significant fouling can occur, they should be checked for solids build-up. 9.2.7

Inerting Svstems

Inerting systems should be on a routine inspection program to ensure instrumentation, such as oxygen sensors and flow monitors in the vent header, are calibrated correctly. When instrumentation is provided to verify the quality of the inert gas supply, it should be on a routine maintenance program. 9.2.8

Instrument and Controls

Vent header system instrumentation and controls are equally important as process instrumentation and should be inspected and recalibrated on a routine basis. Instrument inspections should include: Pressure gauges for measuring the differential pressure across flame arresters Safety Instrumented Systems (SIS) Purge gas instrumentation Level indicators on knock-out tanks and seal drums Flammable and toxic gas analyzers or detectors Dilution air controls 9.2.9

Low Point Drains

Drains in vent header systems should be checked on a routine basis to confirm that they are not blocked and that they are physically in satisfactory condition. In many cases, the low point drains will not accumulate anything until some upset or facility revision changes the situation. Continuous monitoring (level alarm) and automatic draining to catch pot should be considered. 9.2.10

Corrosion and Erosion

Vent headers should be routinely inspected for corrosion and erosion. Records should be retained to provide documentation of the corrosion rate.

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9.2.11 Structural Suo~ortsfor Vent Headers Strudural supports for the vent header and ancillary equipment, such as flame arresters and expansion joints, should be on a routine inspection program to confirm they are mechanically sound and have not experienced significant corrosion. The supports must be able to withstand the maximum credible forces they could experience, including the reaction forces during emergency venting and, if credible, the header being liquid full. If the supports require modification or replacement, steps should be taken to ensure the headers are sloped correctly and liquid will not accumulate in low points. 9.2.12 Insulation and Heat Tracing Serious incidents have occurred when high melting point materials have solidified in vent header systems and relief valves, preventing them from opening. Vent systems handling materials with melting points above the minimum ambient temperature should be evaluated to determine if solids could accumulate and restrict the vent flow. Requirements for heat tracing and insulation should be identified during the design stage [Ref. 9-3, Section 4.6.31. Requirements for maintaining temperature should be included in operating procedures and training. The inspection program should confirm insulation is in place and heat tracing is operable. 9.3 Management of Change When changes are made to the vent header system, the facility management of change program should be used to ensure that: Changes are reviewed for their safety, health, and environmental impact and approved and authorized All appropriate process safety information is updated Design does not present any new hazards Maintenance and inspection procedures for the vent header system are reviewed and updated appropriately References 9.4 9-1. Grossel. S.S. Center for Chemical Process Safety (CCPS). 2002. Def7agration and Detonation Flame Arresters. New York, New York Center for Chemical Process Safety of the American Institute of Chemical Engineers. 9-2. National Fire Protection Association (NFPA). 2002. NFPA 69, Standard on Explosion Prevention Systems. Quincy, Massachusetts. 9-3. American Petroleum Institute. 1997. API RP 521, Guide for Pressure Relieving and Depressuring Systems. New York, New York. 202

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by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

A ACRONYMS AND ABBREVIATIONS AEGL AIChE AIHA ANSI API ASME ASTM BPVC CCPS DDT DIERS DOT DPC EEGL EPA ERPGs FMEA FRP FTA HAPS HAZOP

Acute Exposure Guideline Levels American Institute of Chemical Engineers American Industrial Hygiene Association American National Standards Institute American Petroleum Institute American Society of Mechanical Engineers American Society for Testing and Materials Boiler and Pressure Vessel Code Center for Chemical Process Safety Deflagration to Detonation Transition Design Institute for Emergency Relief Systems Department of Transportation Deflagration Pressure Containment Emergency Exposure Guidance Levels Environmental Protection Agency Emergency Response Planning Guidelines Failure Mode and Effects Analysis Fiber Reinforced Plastic Fault Tree Analysis Hazardous Air Pollutants Hazard and Operability 203

Safe Design and Operation of Process Vents and Emission Control Systems

HCI HSE IDLHs ISA

L P LEL LEPC LFL LOC LOPA MACT MAWP MSDS MOC NFPA NESHAP NIOSH NOAA NSPS 0 2

OSHA PHA PRV PVC QRA RMP RP SIC SIL SIS 204

Hydrogen Chloride Health and Safety Executive Immediately Dangerous to Life and Health Instrumentation, Systems, and Automation Society Length-to-Diameter Ratios Lower Explosive Limit Local Emergency Planning Committees Lower Flammable Limit Limiting Oxidant Concentration Layer of Protection Analysis Maximum Achievable Control Technology Maximum Allow able Working Pressure Material Safety Data Sheet Minimum Oxygen Concentration National Fire Protection Association National Emission Standards for Hazardous Air Pollutants National Institute for Occupational Safety and Health National Oceanic and Atmospheric Administration New Source Performance Standards Oxygen Occupational Safety and Health Administration Process Hazards Analysis Pressure Relief Valve Poly Vinyl Chloride Quantitative Risk Assessment Risk Management Program Recommended Practice Standard Industrial Classifications Safety Integrity Level Safety Instrumented Systems

Appendix A -Acronyms and Abbreviations

SOCMA SPEGL STEL TLV

us.

UEL UFL

voc

Synthetic Organic Chemical Manufacturers Associa tion Short Term Public Emergency Guidance Level Short Term Exposure Limit Threshold Limit Value United States Upper Explosive Limit Upper Flammable Limit Volatile Organic Compound

205

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

B GLOSSARY Absorption - The process of contacting a vapor and gas stream with an absorbing liquid to remove specific materials from the gas stream. Adsorption - The process of contacting a vapor and gas stream with the surface of a solid adsorbent material. Adiabatic Flame Temperature - The temperature developed by the combustion of a fuel and oxidizer mixture in conditions where there are no heat losses. In practice this value is difficult to measure experimentally and most published figures are the results of theoretical calculations. Atmospheric Storage Tank - A storage tank designed to operate a t any pressure between ambient pressure and 0.5 psig (3.45kPa gage). Autoignition Temperature - The minimum temperature at which combustion can be initiated without an external ignition source in a standardized test apparatus. Blowdown Drum - A separate collection vessel intended to receive a periodic or emergency discharge of liquids, liquid reaction mass, or slurries from a number of process vessels, reactors, or equipment items. Blowdowns may be initiated automatically, i.e., depressuring a batch reactor, or manually for other applications. Blowdown drums are usually maintained at a low level or essentially empty. The collected liquids are pumped elsewhere for recovery, recycle, or disposal. Gases and any uncondensed vapors are vented through relief devices or an open line into a vent header system for appropriate treatment and disposal.

207

Safe Design and Operation of Process Vents and Emission Control Systems

Catch Tank - A separate containment vessel intended to receive an emergency discharge from relief devices in the process vessel's vapor space. Catch tanks are usually employed where substantial two-phase relief flow, entrained solids, or reaction mass carryover is expected to occur. Combustion - An exothermic reaction between a fuel and an oxidizer (usually but not necessarily oxygen) that results in a significant temperature rise and usually a visible flame or glow. Corrosivity - A complex series of reactions between water and metal surfaces and materials in which the water is stored or transported. Combustible Liquid - A liquid having a flash point at or above 100'F (37°C). Combustible liquids are subdivided as follows: Class I1 liquids include those having flash point at or above 100°F (37°C) and below 140°F (60°C). Class IIIA liquids include those having flash points at or above 140°F (60°C) and below 200°F (93°C). Class IIIB liquids include those having flash point at or above 200°F (93°C). Deflagration - A combustion that propagates by heat and mass transfer through the un-reacted medium at a velocity less than the speed of sound. Design Case - Conditions to be used for the design of a vent system to ensure it will meet safety, health, environmental, and commercial objectives. For emergency vent headers, this will normally be based on the "worst credible case" scenario. For normal process vents, the design should take into consideration all non-emergency situations unless they will be addressed by some other means, such as by providing a temporary vent system for maintenance operations. Detonation - A combustion that propagates by adiabatic compression heating caused by a shock wave, and which travels through the unreacted medium at a velocity equal to or greater than the speed of sound.

208

Appendix B - Glossary

Dispersion Modeling - A technique that can be used to confirm that the residual or converted products from a normally operating end-of-pipe treatment system do not present any residual hazards to plant personnel, community, or environment. In addition, modeling could be employed to assess the impact on plant personnel, community, or environment if the treatment system malfunctions and releases the collected vent streams untreated. Dump Tank - A separate collection and containment vessel intended to receive an emergency discharge of liquids, liquid reaction mass, or slurries originating from the bottom of a process vessel or reactor. The "dump" is usually automatically triggered by a process safety interlock, but may also be initiated manually. An uncontrolled, exothermic, or other runaway reaction can be controlled by discharging the process vessel contents to the dump tank. This may allow the process vessel to be returned to service in a shorter time. Entrainment - To draw in and transport (as solid particles, liquid droplets, or gas) by the flow of a fluid. Equipment - A term used in this book to describe any form of process vessel, storage tank, or any other items that may be connected to a vent system. Explosion - A sudden increase in the atmospheric pressure perceptible to an observer as a bang or boom, Explosions can be caused by several scenarios, including: A deflagration or detonation inside equipment causing it to rupture. Detonations involving high explosives. Physical explosions, such as a vessel failing as a result of it being subjected to pressures above its ultimate strength. Explosion Prevention - Where the gas composition is maintained outside the flammable region. Explosion Protection - Where devices are provided to minimize pressure development if ignition occurs. Fire Triangle - Three basic conditions are required for fire to take place. These are fuel, oxygen, and heat. Fuel - the reducer; any combustible material, solid, liquid or gas. Most solids and liquids must vaporize before they will burn. Oxygen - the oxidizer; sufficient oxygen must be present in the atmosphere surrounding the fuel for fire to bum. 209

Safe Design and Operation of Process Vents and Emission Control Systems

Heat - sufficient energy must be applied to raise the fuel to its ignition temperature. Fire can only occur when all three of the above elements are present and in the proper conditions and proportions. These three basic conditions are often represented as a fire triangle shown in 0. The combustion reaction itself is often included as a fourth central element of the fire triangle.

Figure B-1. The Fire Triangle If one of the sides of the fire triangle is missing, the fire will not start. If one side is removed, the fire will be extinguished. The fire triangle forms the foundation for all methods of fire prevention and firefighting (NFPA, 1997). Flame Arrester - A device fitted to the opening of an enclosure or to the connecting piping of a system of enclosures and whose intended function is to allow flow but prevent the transmission of flame from either a deflagration or detonation. Flame Speed - The speed of a flame relative to a fixed reference point. Flammable Gas - A gas that, if mixed with a gaseous oxidizer such as air or chlorine and ignited, can burn with a flame. The term flammable gas includes vapors from flammable or combustible liquids above their flash points.

In this book the term "flammable gas" generally includes vapors from liquids that are above their flash points, as well as combustible materials that are gases a t normal temperature and pressure. Flammable Limits - The minimum (LFL) and maximum (UFL) concentration of combustible gas, vapor, mist, or dust, (or a combination of these materials), mixed with a gaseous oxidizer, that if ignited can burn. These limits are equipment specific and different test methods can produce significantly different values. The terms upper or lower flammable limits are synonymous with the terms "upper or lower explosive limits". 210

Appendix

B - Glossary

Flammable Liquids - A liquid having a flash point below 100°F (37°C) and a vapor pressure not exceeding 40 psia. NFPA further classifies flammable liquids as follows: Class 1A liquids have a Reid vapor pressure that does not exceed 4Opsi (2068.6mm Hg) at 100°F (373°C) and have a closed-cup flash point below 73°F (223°C) and a boiling point below 100°F (373°C). Class 1B liquids have a closed-cup flash point below 73°F (223°C) and a boiling point at or above 100'F (37.8"C). Class 1C liquids have a closed-cup flash point at or above 73°F (223°C) but below 100°F (37.8"C). Flammable Vapor - A vapor that is above its lower flammable limit (LFL) concentration. A zone of flammable vapor will exist in equilibrium with a flammable or combustible liquid any time it is above its flash point. In this book the term "flammable gas" generally includes vapors from liquids that are above their flash points, as well as combustible materials that are gases at normal temperature and pressure. Flashback Arrester - A device to limit damage from a flashback by preventing propagation of the flame front beyond the location of the arrester. Flashback Prevention - Prevention of a recession of the flame into or back of the mixing chamber. Flashpoint - The temperature at which a liquid develops sufficient vapor pressure to form a vapor/air mixture capable of undergoing combustion after ignition from an external energy source. (Fire point is the temperature at which the reaction will be sustained). Frequency - The number of occurrences of an event per unit of time. Hazard and Operability Study (HAZOP) - A technique to identify process hazards and potential operating problems using a series of guidewords to study process deviations. Hybrid Mixture - A mixture of flammable gas and combustible dust or mist. Incident - An unplanned event with the potential for undesirable consequences. Inherently Safer - A term applied to a component, system, or facility in which potential dangers have been removed or reduced. Inherent safety is incorporated during development, design, or engineering. 21 1

Safe Design and Operation of Process Vents and Emission Control Systems

Limiting Oxidant Concentration (LOC) - Is the concentration of oxidant, e.g. oxygen, air, chlorine, etc. below which the combustion reaction can no longer generate sufficient energy to produce a propagating flame. The LOC is synonymous with the term Minimum Oxygen Concentration (MOC). Minimum Oxygen Concentration (MOC) - See Limiting Oxidant Concentration. Plume - A visible or measurable discharge of a contaminant from a given point of origin that can be measured according to the Ringelmann scale. Quench Drum - A vessel with internal liquid sprays supplied by an external recirculation pump loop with a quenching liquid sprayed directly into the vapor space to contact, cool, and condense at least part of the hot vented gas or vapor stream before these gases enter the main vent header system. Quench Pool - A closed vessel containing a relatively large volume of liquid. The emergency vent stream is sparged subsurface through the liquid volume at high velocity, resulting in vigorous agitation and circulation of the pool contents in order to maximize cooling and condensation or reaction with the pool liquid. Most or all of the vented stream can be captured; residual vapor and non-condensable gas may be released to the vent header system for further treatment or disposal. Risk - A measure of economic loss or human injury in terms of both likelihood and the magnitude of the loss or injury. Risk Analysis - The development of a quantitative estimate of risk-based engineering evaluation and mathematical techniques for combining estimates of incident consequences and frequencies. Risk Assessment - The process by which the results of a risk analysis are used to make decisions either through a relative ranking of risk reduction strategies or rhrough comparison with risk targets. Risk assessment is often defined as the qualitative estimation of probability and consequence of an incident or incidents. Runaway Reaction - An exothermic reaction where the heat evolved from reacting materials exceeds the heat being removed from the equipment. The imbalance between the heat of reaction and the heat removal causes the temperature to rise, which in turn greatly increases the reaction rate. This can create a positive feedback loop that results in rapidly accelerating reaction rates referred to as a “runaway reaction”.

212

Appendix B - Glossary

Safeguards or Protective Features - Design features, equipment, procedures, etc. in place to decrease the probability or mitigate the severity of a causeconsequence scenario. Severity - The maximum credible consequences or effects, assuming no safeguards are in place. Stoichiometric Mixture - A balanced mixture of fuel and oxidizer such that no excess of either remains after combustion. Stoichiometry - Calculations about masses or volumes of reactants and products involved in a chemical reaction. Tempered - A term that describes an exothermic reaction involving a volatile reaction mass where the heat losses due to the latent heat of vaporization balances the heat of reaction. In this situation, the temperature and reaction rates do not increase, and hence the reaction does not “runaway”. Toxic Gases - Materials that can cause physiological harm other than asphyxiation and that are immediately dangerous to life and health and can be fatal at relatively low concentrations, such as phosgene or hydrogen sulfide. Worst Credible Case - The most severe consequences, considering all scenarios and their outcomes, that is considered plausible or reasonably believable. Vapor - The gaseous phase formed by a material that is liquid at ambient temperature and pressure.

213

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

C SELECTED US ENVIRONMENTAL AIRPOLLUTION CONTROL REGULATIONS A number of US federal, state and even local air pollution control regulations require plant owners to reduce and control their emissions to the atmosphere. Since vent header systems that collect multiple emission sources are often part of an effective and economical solution, the regulations may impact the selection, design and operation of vent header systems and their end-of-pipe control devices, The following lists focuses only on the US federal air pollution control regulations that may have the broadest impact on vent header systems. The reader should consult with authoritative environmental resources for specific applications to ensure that other federal as well as state and local air pollution control regulations are appropriately understood and considered. The following regulation lists are separated into those specifying which processes and industry sectors are covered and those that pertain to specific equipment items of the affected processes. PROCESS RELATEDREGULATIONS The US. Environmental Protection Agency (EPA) has developed and issued a number of National Emission Standards (NES) for designated Hazardous Air Pollutants (HAPS). These regulations titled National Emission Standards for Hazardous Air Pollutants for Source Categories derive from the Clean Air Act of 1993, Sections 112(g) and 1126). These regulations are collectively known by the acronym NESHAPs. These regulatory standards have been and continue to be developed and applied based on a specific process within an industry sector or a group of similar processes within related industry sectors. These regulations will continue to have an impact on the design and operation of vent header systems. 215

Safe Design and Operation of Process Vents and Emission Control Systems

These process and industry specific regulations can be found a t Title 40 Code of Federal Regulations Chapter I, Environmental Protection Agency Subchapter 3, Air Programs Part 63, National Emission Standards for Hazardous Air Pollutants for Source Categories Subparts A to TTTIT (or more simply: 40CFR63.1 to 40CFR63.9880) These standards require that all "major sources" within the designated industry sector implement a Maximum Achievable Control Technology (MACT) to reduce their emissions of the designated hazardous air pollutants. The EPA defines these major sources as "stationary sources or groups of stationary sources located within a contiguous area and under common control that emit or have potential to emit considering controls, in the aggregate, 9.07 Mg/yr [ l o tpy] or more of any one HAP or 22.68 Mdyr [25 tpy] or more of any combination of HAP." The MACT regulations in general state a reduction level that is to be achieved without specifically requiring the use of a particular method. However, the MACT regulations do contain certain requirements and refer to other regulations that specify certain details of design, operation, maintenance and documentation for specific treatment devices that a user may choose to employ. The purpose of this reference is only to indicate in general the principal US federal environmental regulations that may apply to the covered processes and may therefore impact the design and application of vent header systems. Tables C-1 and C-2 in this appendix lists the industry sectors for which MACT standards have been issued through July 2005, the subpart section (alphabetical and numerical description) in the Code of Federal Regulations (CFR) where they can be found, the date promulgated in the Federal Register and the effective compliance dates.

216

Appendix C - Selected US Environmental Air Pollution Control Regulations

Table (2-1.

Processes Affected by Current Regulations - 2005

Source CategoriesAffected by MACT Standards GeneralProvisions National Emmion Sfds for Organic HazardousAir Pollutants JNESHAPJ from fhe Synfhefic Organic Chemical Manufacturinglndusfry General (mcludmg rdentrficabon of process vents affected) Process vents, storage vessels, transfer operations and wastewater Equipment leaks For Certain Other Processes subject to negotiated regulation for equipment leaks Aerospace Asphatt Processing and.Asphatt Roofing Manufacturing Auto and light Duty Trucks (Surface coating) Boat Manufacturing Brick 8 Sfructural Clay Products Manufacturing Clay Ceramics Manufacturing Cellulose Products Manufacturing MiscellaneousViscose Processes - Cellulose Food Casing - Rayon - Cellulosic Sponge - Cellophane Cellulose Ethers Production - CaroxymethylCellulose - Methyl Cellulose - Cellulose Ethers Coke Ovens Pushins, Quenchina, and 9attery Stacks Chromium Elecfroplafing Chromic Acid Anodizing Decorative Chromium Electroplating Hard Chromium Electroplating 9egreasing Organic Cleaners HalogenatedSolvent Cleaners 9ry Cleaning, Commercialand 'ndusfrial Drycleaning dry-to-dry Drycleaning transfer machines

40CFR63 Sub Part 8. No. A 63.1

Date 8. Cltation

F, 63.100-107 G, 63.110-153

04122194 (59FR19402)

FIG-05114200 1 H-05/12/99 New Sources 0511298

H, 63.160-183 I, 63.190-193 GG 63 741 LLLLL 63.8680 1111 63.3080

ww

63.5680 JJJJJ 63.8380

KKKKK

63.8530

(60FR45948) 04/29/2003 (68FR22975) 04/26/04 69FR22601

[ 1

51112006

I

04126107

1

66FR44217 (68FR26689) 05/16/2003 68FR26689

--7-

1

I

05116/2006

uuuu

0611112002 (67FR40043)

06/1112005

63.7280

ccccc

4/14/2003 (68FR18007)

411412006

N 63.340

01/25/95 (60FR4948)

Decorative 01/25/96 Others 01/25/97

T 63.460

12/02/94 (59FR61801) 0912293 (58FR49354)

12/02197

63.5480

M

63.320

09/23/96

217

Safe Design and Operation of Process Vents and Emission Control Systems

Source Categories Affected by MACT Standards

Flexible Polyurethane Foam Fabrication Operation Friction Products Manufacturing

Large Apphances (Surface coating) Leather Finishing Operations Lime Manufacturing Magnetic Tape (Surface coating)

Marine Vessel Loading Operations

218

PPPPP 63 9280

05/27/2003 (68FR28774)

05/27/2006

0 63 360 0000 63 4280

12/06/94 (59FR62585) 05/29/2003 (68FR32171) 05120199 (64FR27450) 10107198 (63FR53980) 04/14/2003 (68FR18061) 1011812002 (67FR64497) 12/14/94 (59FR64303)

12/06/98

63 1650 111 63 1290 MMMMM 63 8780

Flexible Polyurethane Foam Production

Iron and Steel Foundries

Compliance Date

xxx

Ferroalloys Production

(Chromium water treatment compounds) infeurated Iron and Steei

Federal Register Date &Citation

a NO.

Engine Test CellslStands (Combined with Rocket Testing Facilities) (Commercial) Ethylene Oxide Sterilization F acilities Fabnc Printing Coating & Dyeing

Gasoline Distribution (Bulk Gasoline Terminals & Pipehe Breakout Stations) Generic MACT for Acetal Resins Hydrogen Fluoride Polycarbonates Produchon AcrylidModacrylic Fibers Generic MACT for Carbon black production Cyanide chemicals mfg Ethylene processes Spandex production Hvdrochloric Acid Production Fumed Silica Production industrial, Commercial and institutional Boilers and Process Heaters

40CFR63 Sub Part

QQQQQ 63 9480 R 63 420

I

I

05/29/2006 05/20/2001 10/08/2001 04/14/2006 10/18/2005 12115/97

63 1100

w

6/29/99 (64FR34853)

06/29/2002

YY 63 1100

7/12/2002 (67FR46257)

7112/2005

NNNNN 63 8980 DDDDD 63.7480

I

I

04/17/2003 (68FR19075) 09113/04 (69FR55217)

63.400

(59FR46339)

FFFFF 63 7780 EEEEE 63 7680

63 5280 AAAAA 63.7080 EE 63.701

5/20/2003 68FR27645 04/22/04 69FR21905 7/23/2002 67FR48253 02/27/2002 67FR915510 01/05/04 (69FR393) 12115/94 (59FR64580)

Y 63.560

09/19/95 (60FR48388)

63 4080

1

04/17/2006

I

09/13/07

5/20/2006 04/22/07 07/23/2005 02/27/2005 01/05/07 Without new control devices 12115/96 With new control devices 12/15/97 MACT-09/19/99 RACT-09119198

Appendix C - Selected US Environmental Air Pollution Control Regulations Source CategoriesAffected by MACT Standards

Mercury Cell Chlor-Alkali Plants Jformerly Chlorine Production) Mefal Can (Surface Coafingj Metal Coil (Surface Coatingj Metal Furndure (Surface Coafingj (Misc.) Mefal Parts and Products (Surface Coafingj AsphalVCoal Tar Application to Metal Pipes Mineral Woo1Production Misc. Coating Manufacturing Municipal Solid Wasfe Landfills Nafural Gas Transmission and Storage Nufrifional Yeasf Manufacturing /formerly Bakers Yeasf) Off-Site Wasfe & Recovery Operafions Oil & Natural Gas Production (Misc.) Organic Chemical Production & Processes (MON) Alkyd Resins Production Ammonium Sulfate Production Benzyltrimethylammonium Chloride Prod. Carbonyl Sulfide Production Chelating Agents Production Chlorinated Paraffins Production Ethyllidene Norbomene Production Explosives Production Hydrazine Production Maleic Anhydride Copolymers Production Manufacture of Paints, Coatings, 8 Adhesives OBPNl, 3-diisocyanate Production Photographic Chemicals Production Phthalate Plasticizers Production Polyester Resins Production Polymerized Vinylidene Chloride Prod.

40CFR63 Sub Part

8 No. 11111 63.8180 KKKK 63.3480

Federal Register Date 8 Citation

Compliance Date

12/19/2003 (68FR70903) 11113/03

12/19/2006

ssss

63.5080 RRRR 63 4880 MMMM 63.2130 DDD 63.1175 HHHHH 63.7980 AAAA

63 1930 HHH 63.1270

cccc

63.2130 DD 63.680 HH 63.760

FFFF 63.2430

11/13/2006

6110/2005 (68FR28605) OllO2104 (69FR129) 06/01/99 (64FR29489) 12/11/03 (68FR69163) Oil1612003 (68FR22270) 06/17/99 (64FR32610) 5/21/2001 (66FR27876) 07/01/96

0512312006 01/02/07

06/01/2002 12111/06

___ 06/17/2002 512 112004 02/01/2000 06/17/2002

1111012003 (68FR63851)

1111012006

219

Safe Design and Operation of Process Vents and Emission Control Systems Source Categories Affected by MACT Standards Polymethyl Methacrylate Resins Prod Polyvinyl Acetate Emulsions Prod. Polyvinyl Alcohol Production Polyvinyl Butyral Production Quaternary Ammonium Comp. Prod. Rubber Chemicals Production Symmebical Tetrachloropyridine Production Organic Liquids Dafribution (Nongasoiine) Paper and Other Web (Surface Coating) Pesticide Active ingredient Production 4-Chlror-2-Methyl Acid Production 2,4 Salts & Esters Production 4,6-dinitro-c-cresol Production Butadiene Furfural Cobimer Captafol Production Captan Production Chloroneb Production Chlorothalonil Production Dacthal (tm)production Sodium Pentachlorophenate Production Tordon (bn)Acid Production Petroleum Refineries Pefroieum Refineries Catalytic Cracking Catalytic Reforming Sulfur Recovery Units PharmaceuticalsProduction Phosphoric Acid Phosphafe Fertilizers Piasbc Parts (Surface Coating) Piywood and Composife Wood Products (formerly Plywood and Particle Board Manufacturing) Poiyether Poiyois Producfion

220

40CFR63 Sub Part 8 No.

Federal Register Date 8 Citation

Compliance Date

EEEE 63.2330 JJJJ 63.3280

02/03/04 (69FR5038) 12/04/2002 (67FR72329)

02/03/2007

06/23/99 (64FR33549)

12i2312003

08/18/95 (60FR43244) 0411 112002 (67FR17761)

08118198

MMM 63.1360

cc

63.640

uuu

63.1560

12/04/2005

0411112005

GGG 63.1250 AA 63.600 BB 63.620 PPPP 63.4480 DDDD 63.2230

09/21/98 (63FR50280) 06/10199 (64FR31358)

09/21/2001

4/19/2004 (69FR20968) 7/30/04 (69FR45943)

411912007

PPP 63.1420

06/01/99 (64FR29419)

06/0112002

0611012002

xxxx

Appendix C - Selected US Environmental Air Pollution Control Regulations

Source Categories Affected by MACT Standards Poiymers and Resins I ButylRubber EpichlorohydrinElastomers Ethylene Propylene Rubber Hypalon (TM) Production Neoprene Production Nitrile Butadiene Rubber PolybutadieneRubber Polysulfide Rubber Styrene-Butadiene Rubber 8 Latex Polymers and Resins /I Epoxy Resins Production Non-Nylon Polyamides Production Polymers and Resins ill Amino Resins Phenolic Resins Polymers and Resins iV AcrylonitrileButadiene-Styrene Methyl Methacrylate-Acrylonitrile+ Methyl Methacrylate-Butadiene* Polystyrene Styrene Acrylonitrlle PolyethyleneTerephthalate Nitrile Resins Polynnyi Chloride and Copoiymers Production Portland Cement Manufacturing Primary Aluminum Primary Copper Primary Lead Smelting Primary Magnesium Refining Printing and Pubhshing (Surface coabng) Publicly Owned Treatment Works JPOTW) Pulp & Paper (Non-combustion) MACT I Puip 8 Paper Combustion Sources at Kraff Soda and Sulfite Mills - P u/o and Paper MACT II Pulp & Paper (Non-chem) MACT 111

4OCFR63 Sub Part

Federal Register Date &Citation

Compliance Date

U 63.480

09/05/96 (61FR46906)

07/31/97

63.520

W

03/08/95 (60FR12670)

03/03/98

000 63.1400

01/20/2000 (65FR3275)

01/20/2003

JJJ

09/12/96 (61FR48208)

0713 1/97

711012002 (67FR45885) 06114199 (64FR31898) 10107197

711012005

&No.

63.1310

J

63.210 LLL 63 1340 LL

63840

wv

63.1580 S 63.440

MM

63.860

S

63.440

10107199 06/12/2005

QQQ

63.1440 TTT 63.1541 TTTTT 63 9880 KK 63 820

0611012002

(64FR30194) 10/10/2003 (68FR58615) 05130196 (61FR27132) 10/26/99 (64FR57572) 04115/98 (63FR18504) 01/12/01 (66FR3180) 03/08/96 (61FR9383)

05/04/2001 1011012004 05/30/99

0411612001

221

Safe Design and Operation of Process Vents and Emission Control Systems

222

Appendix C -Selected US Environmental Air Pollution Control Regulations

EQUIPMENT SPECIFIC REGULATIONS The following regulations pertain to speafic process equipment items or process functions and are generally applicable only where referenced by other regulations; these regulations can be found under: Title 40 Code of Federal Regulations Chapter I, Environmental Protection Agency Subchapter 3, Air Programs Part 65 Subparts A to G (or more simply: 40CFR65.1to 40CFR65.167) Table C-2. Process, Equipment & Operation Specific Regulations - 2005

A

1

Part/ SubPartNo. 65.1

1

TZle Applicability

65.2

Definitions

A

65.6

Startup, Shutdown, Malfunction Plan 8 Procedures

C

65 40-48

Storacte Vessels

D

65.6C-67

Process Vents

E

65.80-87

Transfer Racks

F

65.100-1’20

Equipment Leaks

65,140-,67

Closed Vent Systems, Control Devices 8 Routing to Fuel Gas or Process (recwerv)

A

G

223

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX VENTHEADER DESIGNCHECKLIST This checklist provides a tool for identifying and collecting the basic information needed for the design of both normal process and emergency vent headers. In some instances the required information, process definition, or material properties may not be readily available. In these cases, the checklist can be useful in identifying where laboratory testing or pilot plant studies will be needed, for example, to obtain information on reactivity, flammability, and corrosion. In many cases the vent system design takes place while the process chemistry and equipment design are optimized, and/or the facility layout is being established. As a result the entire design becomes an iterative process, in these cases the checklist can assist by providing a guide to prevent issues being overlooked.

225

Consider d the following add to the identfied normal process vent flows Vent gases developed by the process, e g , reachon offqases Vent gases displaced when liquid levels change in equipment Vent flows to allow for vapor expansion or contractionresulting from solar radiation and changes in ambient conditions Determine the realistic flows from all of the sources and perform a statisticalanalysis

Identi temperatures of each vent stream or in the source equipment during normal process ventino

Identifypressures in the source equipment during normal process venting

Matenals present Concentrations Determine the anticipatedflow rates dunng normal process venting, SpeufiCally. Maximum imtantaneous flow rate Average flow rate Duration, d batch or intermittentoperation

Far each m m a l process vent stream, list the following:

---

Idenbfythe materials and conditions for each normal p m e s s vent stream.

Determine how the m a 1 process venhng occurs in time: Continuously as a conbnuous process Intermittently as a batch pocess;if intermittently. determine the cycle time and venting duration Identify worst credible scenarios that could result in emergency vent rdeasesfrom each affected source equipment (utilize hazard analysis studies).

-

Identify source equipment for each vent stream.

Collect available informahon. such as pmess descripbon. prccess or engineenng flow diwrams, enerav and matenal balances. black flow diwrams. etc

header svsiem.

Identifynormal process and emergency vent streams that may be included in the vent

Vent Header Design Information Checklist

I

Source 1

Vent Header Design Infomation Checklist Identify any additional materials present in the source equipment which are not routinely expected to be In the vent stream BUT couM be in an emergency vent stream. such as. raw matenab. products, intermediates, and byproducts. for hazardous materials, idenbfy the f0llowlng Material Concentration in the source equipment Toxicity. flammability or other properties that may create additlonal hazards if the materialenters the vent header Identfv maximum pressures in the source equipment durinq or at start of emerqency venbnq ldentfy maximum temperatures in the source equipment during or at start of emergency venting Estimate anticipated flow rates during emergency venting. speclfically Maximum instantaneous flow rate Total quantity released Durationof venting Normal process Anticipated non-standard operations, such as steam cleaning Solar radialion

Identify the upper and lower temperature that wuM be expertend, due lo

.-

Identify the materials and conditions that could occur in the vent header system hell, including: Contaminants that couM reasonably be expected to occur over the lifetimeof the facility Such as ambient air, ovefflow of lquid from a scrubber, or a flare seal drum, cooling water and Steam utiliks, corrosion producls, solids buildup, cleaning materials.and contaminants in purge oases

Determine ifcondensalion of vapors could aaur in the header identfy the Dew Point temoeratures of each vent stream (where aodrable) Determine 1solidfication of condensed or entrained liqud could aaur in the vent header identfy Freezing or SoliddimtionPoints of liquids that could condense out of a vent stream or be entrained into it dunng venting (where applicable) Estimate the minimum and maximum temperatures in the vent header based on sbeams it could rereive

Data and Comments

Source

___

~

2h

3a

3 -

3b

-

3c

-

Flashpints

wer flammable limits

Concentration in the vent stream

M

4h

Item

Oxides of nitragen Chlorine. etc Identify reactive matenals such as nitrates. peroxides monomen, etc that may react. Dolvmerizeor decumwse in the vent header

IdenWy potentialsources of oxidizers in the vent header, such as

Estimate Re minimum and maximum pressures in R e vent header based on streams it could receive

Vent Header Design InformationChecklist

I

Compatibility of the vent stream pressures

Toxicity Corrosivi

Compabbility of the vent stream materials properties.such as

Physical locatim of the source equipment

Identifythe vent streams or source equipment that could be combined into a common vent headerbased on:

.

Determine if emwgency scenarios could subject the vent header and connected equipment to pressure or vacuum due to External hre Runaway teadons Sudden moling causing signrkant volume contraction in the header, such as - After steam cleaning - Dunng a thunder shower on a hot day - Following a hgh temperature short duration venting into a cold vent header

by products

I Determine the toxicity of vented materials. including raw matenals, products, intermediates,and

I

Flares Incinerators Scrubken Carbon adsorpbon

If self reacbve, consider introducing inhibtlor and eliminating potentialcatalysts or initiators. e g , by selecting materials of construclwnlhat do not have a catalytic effect on the reaction If reacbve with other materials, consider separabng the materials to different vent header systems

For systems that may build-up solids, VISCOUS or othw materials in the header, consider Installing means lo prevent or remove buildilp Define appropriate administrative mntrols, such as, inspectionlcleaningfrEqJenCy

. .

For systems handling toxic materials, determine whether the following are appropriate baslc deslgn features Identifypipe speclficatlonin accordance with ASME. 631.3. and, if appropnate, spec9 Category M piping sewice If practical,operate header at subatmspheric pressure

. .

For systems handling reactive materials, determine whether the following are appropnate base design features (Reference D-1, D- 2, D-3)

.-

.

source 2

'

For systems handling flammables, determine whether the following are appropnate basic desgn features. hefting or operating above or below the flammable limns Explosion venting andlor isolation Installabon of flash back prevention devices, e g , flame arresten and seal pols In addition to all of lhe above, minimizing ignitionsources by administrative controls of hot work and addressing electrostatic hazards

Identifythe basic design approachfor the vent header.

-.

Condensers Scrubbers ~- Recovery syslems Common enduf-line devices, for example

Common intermediate treaknent devices. for example:

..-

Determine form@)of treatment devices that will be needed, for example:

Vent Header Design Informatiin Checklist

Data and Comments

I

For systems lhat may receive liquids from source vessel overtlow or vent stream liquid entrainment, consider whether specific proteaon SI needed in the basic desgn: Provide adequate head space. wdh instrumentation. alarms, and interlacksto prevent lquid entrainment Provide instrumentation. alarms, and interlocks and, 1 critical, install normally empty knockout tanks to prevent materialovemowing into a vent header

For systems handling high melting point materials.consider whefhw heal tracing and insulatwnmay be required for header and drains.

For systems handling corrosive materials. review materialcorrosion data and consider the following. Separate corrosive vent stream from others, consider scrubbing or other intwmedtafe treatment to make them mmpatiblowith other streams (where applicable) For normal process vent headers, select materials of constructionsuitable for lorlgterm reliableservice For emergency vent headers. select materials of construclionsuitable for the frequency and durationof exposures for the header with considerations for clean out and long term reliability

7e

71

7s

-

-

.

.

Vent Header Design Information Checklist

Item Source1

Source2 Source3

Swm4

SOUm...

Safe Design and Operation of Process Vents and Emission Control Systems

D1 D-1. D-2.

D-3.

132

REFERENCES Bretherick L.1990. Reactive Chemical Hazards. 4*h edition. Butterworths, London Pohanish R.P. & Green S.A. 1997. Rapid Guide to Chemical Instabilities. Van Nostrand Reinhold, New York National Fire Protection Association. NFPA 49, Hazardous Chemical Data. Quincy, Massachusetts

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

E

NORMAL VENT HEADER SOURCE CONTROL AND CONFIGURATION EXAMPLES The composition and conditions of vent gases feeding into normal vent header systems play an important part in determining the requirements for these systems. This appendix provides some examples of applications illustrating typical configurations and controls for different types of source vessels and potential hazards. The process requirements and interactions with the vent header system play an important role in determining the vent header configuration, particularly with respect to the interface between the process and the vent header. Practices vary from industry to industry and company to company regarding the specific controls and valve/piping arrangements used. These examples are included to conceptually demonstrate the need to consider interactions between the process and the vent header system, as well as potential problems that may be encountered. The examples illustrated are: Example 1 - Vent header system suitable for applications where vessels contain materials that do not need to be stored under a controlled atmosphere and where the materials are compatible with each other. Example 2 - Vent header system for a simple inerted vessel. This arrangement minimizes the initial installation and maintenance costs; however, it uses the purge gas inefficiently. Consequently, the approach is typically used for applications that only require low purge gas rates or where operating at the maximum purge rates continuously would not result in an excessive increase in purge gas costs compared with controlling flows to meet actual requirements. 233

Safe Design and Operation of Process Vents and Emission Control Systems

Example 3 - Vent header system for vessels that for safety or quality reasons are operated with an inert or some other blanketing gas. This and similar systems are used extensively in facilities handling flammable liquids. Nitrogen is commonly used in these applications, although other inert purge gases such as carbon dioxide can be used provided they are compatible with the materials being handled and reliable sources are available. Example 4 - Vent header system for vessels that are operated under a blanketing gas using solenoid valves. This system can cause control difficulties due to pressure swings when the valves are opened or closed. It is not a recommended basis for design. Example 5 - Vent header system for a flammable monomer storage tank. In addition to maintaining an inert atmosphere in the tank for explosion protection, nitrogen purges are directed to zones that could become stagnant, to reduce the potential for polymer build-up. Example 6 - Vent header system for a reactor handling a mixture of a high boiling thermally unstable material and a volatile solvent. The reactor is provided with an emergency relief vent, which in the event of an unintended exothermic reaction, allows the volatile solvent to vaporize and cool (temper) the reaction mass. For tempering to be effective, there must be sufficient volatile solvent present to vaporize and cool the reactants to below the temperature where heat losses exceed the heat generation rate. The design includes an isolation valve in the normal process vent header that can close to prevent solvent loss during a prolonged production outage. Also, a control valve is provided that can limit the vent gas flow and prevent the treatment device being overloaded by the vent gas flows generated by the exothermic reaction. Example 7 - Vent header system for a distillation column with pressure control. The system enables the column to operate at a constant pressure irrespective of rates.

234

Appendix E - Normal Vent Header Source Control and Configuration Examples

Example 1: Vent System for Non-Hazardous Materials

2

To Slack

Description: This example represents the most basic configuration for a vent header system, comprising several vessels connected directly to a header wrthout controls, purge gases, or other "safev features. Advantages: Can be the least expensive to install Easily understood, minimizing the likelihood of human error Pressure on the process is kept to a minimum No instrumentatiodcontrol valves on vent header where they could be subject to phging

Disadvantages: The vessels share the same vapor space this can resuk in cross contamination bebeen vessels Materials in the vessels must be compatible. This may limit the ability to make changes at a later time Not suitable for siuations where flammable gaslair mixtures are present, due to potential for air ingress from stack Not suitable for handling materials that are incompatible Pressure fluctuations in the vent header due to changing flows/ fouling, etc. directly impact the process vessels

Comments: This arrangement is appropriate for facilities handling materials that can tolerate pressure fluctuations, are not flammable, and are chemically compatible. Examples include: Aqueous solutions and other materials with flash points that are sufficiently high to exclude flammability hazards and that are compatible one with another Vents systems for minimizing odors or other "nuisance" issues, rather than addressing safety concerns Simple mixing operations

235

Safe Design and Operation of Process Ven$ and Emission Control Systems

Example 2: Simple lnerted System Purge Gas

,

1.1

I n

V I

I

Vent Header

t

Description: This example illustrates a purge system for controlling the atmosphere in a vessel, for example to provide explosion protection for a vessel containing a flammable liquid. To ensure the purge rate is adequate, it must be equal to or greater than the peak demand. As a result, this approach is maink used for small vessels and where variations between the average purge gas requirements and peak purge gas rates is small or can be predicted and additional action taken. Advantages: Simple and inexpensive to install 9 Minimal pressure maintained on process vessel May be suitable for systems where purging is for quality rather than safety reasons and where a short loss of purging would not result in a significant loss

Disadvantages: 9 Continuous purge gas requiredat a rate equivalent to the peak demand, e.g., equal to the maximum oufiow from the vessel or sudden cooling of the vapor space 9 Vessel subject to pressure fluctuations from vent header

Comments: This arrangement is typically used for applications that only require small purge gas flows where the low initial cost and minimal maintenance requirements offset the inefficient use of purge gas.

236

Appendix E - Normal Vent Header Source Control and Configuration Examples

Example 3: Vent System Handling Flammable Materials (with Control Valves)

-f

Description: In this example, air is prevented from entering through any defects that may exist in the dent header by operating the vent header system and the source vessels at a small w a i v e pressure. This ensures any leaks will be away from the system and that air will be unable to enter by reverse flow through the vent stack. Advantages: Positive pressure minimize the potential for air to enter Purge gas usage and vapor losses can be reduced by vapor balancing if there is a need to transfer liquids between vessels

Disadvantages: Shared vapor space can result in cross contamination between vessels If leaks exist, vent gases will discharge to atmosphere The liquids in the vessels milst be compatible. This may limit the ability to make changes at a later time Process vessels are subject to pressure changes between regulators set points

Comments: 3perating the vessels and the header at a small positive pressure provides a reliable system for Preventing air leaking into vent systems. This approach may not prevent air being present from other msible sources such as: Air remaining in vessels afler being opened for maintenance Leaking pump seals allowing air to be draw into a liquid being fed to a vessel A failure of the purge gas supply or an insufficientsupply of purge gas to meet the maximum demand, for exemple when emptying a vessel after being washed Air entering a vessel if it is opened to receive a solids feed or for taking samples To minimize the potential for air to accumulate due to these or other circumstances, common practice is to provide a small continuous feed of purge gas to the vessels. Also, in some instances flame arresters are installed in the lines connecting source vessels to the vent header. Procedures for returning a vessel to sewice after maintenance activities should include measures to re-establishan inert atmosphere in the vessel(s) before introducing flammable liquids.

237

Safe Design and Operation ofProcess Vents and Emission Control Systems

Example 4: Vent System Handling Flammable Materials (with XV Valves)

Description: This figure illustrates a vent system operating at a small positive pressure using solenoid valves. it is similar in concept to Example (2); however, this approach can result in difficulties due to the sudden pressure swings encountered when the blocks valves openlclose. Advantages: Positive pressure minimizes the potential for air to enter Purge gas usage, and vapor losses can be reduced by vapor balancing if there is a need to transfer liquids between vessels

Disadvantages: When the solenoid valves operate pressure cycling may occur. This can results in high instantaneous flow rates and unnecessaly vapor releases to the vent stack Shared vapor space results in cross contamination between vessels The liquids in the vessels must be compatible. This may limit the ability to make changes at a later time Process vessels are subject to sudden pressure fluctuations

Comments: Experience with a system using solenoid valves is reported to have been problematic. Consequently, this approach is not recommended.

238

Appendix E - Normal Vent Header Source Control and Configuration Examples

Example 5: Vent System for a Storage Tank Containing a Volatile Organic Monomer

15pl Fa

Closed

5 0 PSI Purge Gas

To Flare

0 5 DSW

Description: A volatile, flammable, monomer is stored in pressure vessel that is operated at between 3.5 and 1.5 psig. Explosion prevention is provided by inerting the vessel and headers with nitrogen. In addition, all stagnant zones are continuously purged with nitrogen to minimize the formation of solid solymer. 4dvantages: Purging potentially stagnant areas continuously with nitrogen reduces polymer build-up to a tolerable level Maintaining a small positive pressure in the tank assures air is not able to enter and form a flammable mixture The vessel is maintained at a stable pressure

Disadvantages: Operating the tank at a positive pressure increases the potential for the material to leak into the operating area Potentialfor polymer formation in vent header and associated controls

:omments:

3perating at a small positive pressure is a reliable method for preventing air leaks into the tank and vent leader. In addition, nitrogen purges reduce the polymer build-up to a level that does not adversely affect xrfomance between scheduled maintenance.

239

Safe Design and Operation of Process Vents and Emission Control Systems

Example 6: Vent System for a Reactor Handling a High Boiling Thermally Unstable Material and Low Boiling Solvent --@

w

.-.-...-

To V e n t Scrubber

valve

e

Description: The reactor handles a high boiling, thermally unstable compound with a low boiling solvent. In the event of an exothermic reaction, the solvent will boil and temper the reaction. Premature loss of the sokent, e.g., during a prolonged unscheduled production stoppage or a failure of the reactor's 'leating/mling system, could result in insufficient sobent being present. In addition, during an exothermic reaction, the flow in the normal process vent header will increase and may exceed the capacity of its treatment device. Features in the design include: An XV valve in the normal process vent header that will be closed if there if a production stoppage during a reaction batch A control system to limt the maximum Row of normal process vent gases during a runaway reaction to prevent it exceeding the capacity of the treatment device The Dressure control vake is set to own above the maximum anticipated pressure in the normal . . prockss vent header. If considered necessar I checkvalve can also be installed to prevent reverse Row caused by out-of-specificationhi Dressure in the Drocess vent header. Advantages: Vessel is maintained under an inert atmosphere mitigating flammability issues Vessel can be isolated from the vent header under emergency conditions preventing solvent boil-off at low pressure Vessel normally operates under stable pressure conditions

Disadvantages: Operating the vessel at a positive pressure increases the potentialfor the material to leak into the operating area Must rely on the emergency relief vent when the solenoid valve or the control valve in the normal process vent header is shut

Comments: Closing the solenoid valve in the normal process vent during a production outage can prevent loss of the volatile solvent needed to temper an exothermic reaction. This example shows the valve being manually operated. Practical consideration should be given to making this an automated system. 240

Appendix E - Nonnal Vent Header Source Control and Configuration Examples

Example 7: Distillation Column with Pressure Control

Description: Distillationcolumn with constant overhead pressure achieved by controlling the purge gas and the vent qases flows. Advantages: Provides stable pressure on distillation column Enables column to be shut down and started up without passing through the flammable region

Disadvantages: Added complexity of control system

241

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

F PHA HAZOP DEVIATION TABLE

243

I

I

equipneni

Hgh wncentrabon of Rmmabie, wrrosre orioxic High wncenisaton of airloxygen in

eic

More of one mmponent Hgh mole ut ,

High kvel in KO Tank, Scrubber. Seal Potftankor Separator

High temperature in header kne or

MATERIAL CORROSION I EROWN BATCH PROCESS

diluenl

ADDITION OF GAS lneR e n r i c h n t

No addlton

-

Condensahon orsolidfmtnn occurs

pipng or equipment More addlton

n in header .w l.~ m e ~ a t i oreaclmn

Continued pmcess react~onor

nch system

I

-t

COMPOSITION

TEMPERANRE

Hgh pressure in header line or equipneni

-

-

Reverse Row of Vent qaslvawr into connected . . vessek Air backtbws into header

REVERSE

GUIDEWORDS

-

I

I

I

I

Phase imrsmn in Separator or KO Tank Scaling. Acwmulabon

-

flammable or to^^

Low wncenlralion

Pressure in header line or

Less or reduced Rovr of vent g&w

LESS

HAZOP Deviations for Vent Header Systems

Empty, nu level in Seal PoMank or Separator No rented malenal present

r

I

Table F-1.

Typical DESIGN INTENTION Parameters

I

I

I

I

-

-

-

atmosphere

Leak. release of vent uas/vawr into wolk are& or

PARTOF

I

I

I

I

Sunultaneous emergency and normal ventings

-

Two-phase Vewng OCCUR

Leak, release of vent uaslvawr

ASWELLAS

I

I

I

I

-

Emulslfkalfon in Separator. Seal Potftank.etc Scaling. Accumuhlon

..

Complete failure of header line or eauiment

OTHERl"

II

Appendix F - PHA HAZOP Deviation Table

Table F-2.

Additional Deviations

Possible CategorieslSources Utility Service Failures

Maintenance Hazards

Operation Hazards

External Events Hazards

Sampling Hazards

Human Factors

Additional Deviations Electric power Compressed air Steam Water Inert gas, nitrogen Equipment under pressure Equipment too hot Equipment contaminated Mechanical seal damage Exchanger tube leak Maintenance vehicles contacting equipment Start-up Shutdown Abnormal (outside normal design range) Emergency High wind Subfreezing temperature, ice Extreme heat Heavy rain, flood Wind-blown dust Earthquake Lightening Sampling in difficuit locations Non-standard sample stations Contact with material (hot, toxic, corrosive, flammable) Equipment access Visibility of instrumentation Valve height and access Procedural issues Control panel layout

245

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

G WORKED EXAMPLES This appendix provides examples of vent systems for facilities where vent gases are collected and routed to a treatment unit before being discharged to atmosphere. For new projects, or locations undergoing sigruficant changes, the design should be evaluated to ensure it is as inherently safe as practical; for example, by minimizing inventories of hazardous or environmentally sensitive materials. This is generally most successful if it begins during the preliminary design phase of the project when there is still the opportunity to re-evaluate the process and to make changes without incurring excessive cost. As the project becomes more defined, the specific requirements for the normal process and emergency venting should be identified and addressed:

Normal process vent requirements should be developed as part of the process design and take into consideration all non-emergency situations, such as vent flows from: The process chemistry and other process requirements Filling or emptying (vacuum relief) of tanks and vessels Solar radiation and rapid cooling due to rain, etc. Maintenance activities, such as steaming out vessels in preparation for vessel entry, and emptying wash water from storage tanks

241

Safe Design and Operation of Process Vents and Emission Control Systems

Emergency vent requirements should be based on worst credible upset conditions, typically assuming multiple failures. For example it could involve a combination of equipment failure, loss of utilities, and human errors. Emergency venting requirements should consider, but not be limited to the following: External fire exposure Runaway reactions Explosion protection

As the specific requirements for the vent headers are identified it can be helpful to incorporate them into a "design basis". The design basis provides a useful reference document for the design team, and subsequently it can be a valuable source of information for the plant's operating personnel. The design basis will typically include information such as; the worst case incident scenario(s), normal process vent flows and conditions, deflagration and reactivity testing information. Figure G-1 provides a flow path for developing the vent headers for a facility.

In many instances this process is iterative in nature. For example adding an This appendix provides illustrations of typical vent header systems for the following: Flammable liquid handling processes with separate examples of systems that operate, inerted, fuel lean, and fuel rich Loading road tankers with flammable liquids Refinery vent system (including combined process and emergency vents) A facility handling reactive chemicals

G1.

INERTED FLAMMABLE LIQUID STORAGE

When mixed with air the vapor from a flammable liquid can form an explosive mixture. If this mixture ignites the resulting explosion may propagate throughout a facility's vent system potentially damaging both the vent header and the equipment connected to it. Explosion prevention methods include operating, inerted, fuel rich, or fuel lean.

248

Appendix G - Worked Examples

Section 3.1.1 Chapter 5

3".2

Understand Normal Process Vent Requirements

Chapter

Develop Emergency Venting Scenarios

Chapter 5

Investigate

Section 3.2.2 Chapter

t----.

Preliminary Hazard Assessments and Design Reviews

Determine if Vent Headers Should be Combined

H

Chapter 5 Treatment is Required

Chapter 6

Define Vent Header System Preliminary Design

Chapter 8

Implement Improvements Identified in Hazard Reviews

Chapter

Finalize Design of Vent

I

Figure G-1.

'

-

Process Hazards Analysis and Final Design Reviews

Chapter 8

Steps in Vent Header System Design

249

Safe Design and Operation of Process Vents and Emission Control Systems

This example illustrates a flammable liquid storage tank operating ”inerted”, i.e., the tank operates with an inert gas purge that maintains the oxygen concentration below the limiting oxygen concentration for combustion. The section describes the steps for defining the design requirements for this approach. Included is an assessment of methods by which air could enter the system, potential ignition sources, and selects treatment options. G1.l Facility Description Figure G-2 illustrates the vent system for a low pressure feed tank handling a Class 1B flammable liquid. The tank receives batches from mix tanks and provides a continuous feed of this material to reactors in an adjacent production facility. G1.2 Identify Normal Vent Process Requirements Flammable vent gases are displaced from the feed tank when batches are transferred to it from the mix tanks. Requirements for the vent header system include: Addressing potential flammability hazards Treatment of the vent gases to meet environmental requirements Providing a means to recover raw materials from the process vent gases

G1.2.1 Flammabilitv Considerations Flammable liquids storage tanks typically contain vapor above the LFL and if air is also present the mixture may bum explosively. During normal operations, when the liquid level drops, or the head space cools, gas must enter to prevent the pressure going sub-atmospheric and the tank collapsing. If air is used for this purpose a flammable mixture may form creating a potential explosion hazard. This can be avoided by the use of an inert or fuel gas purge to exclude air.

G1.2.2 Liquid Build-up Considerations The tank is an atmospheric pressure tank designed to operate up to a maximum of 8” W.G. positive pressure, or 4”W.G. sub atmospheric pressure. If liquid collects in the vent header it may restrict vent gas flow, increasing the pressure or vacuum that occurs as liquid is transferred into or out of the tank. Due to the tanks low pressure rating a relatively small flow restriction in the header could cause it to fail. 250

Appendix G - Worked Examples

G1.3

Develop Emergency Venting Scenarios

G1.3.1 Emeraencv Overpressure Protection External fire exposure is the worst credible case scenario identified for emergency overpressure in the feed tank. G1.3.2 Emeraencv Vacuum Protection Sub-atmospheric pressure can develop in the tank by the following events, both of which could occur at the same time:

0

G1.4

A failure of the nitrogen blanketing system while the tank is being emptied Sudden cooling by a thundershower on a hot day

Specify Vent System Design and Treatment Options

Explosion protection for the normal process vent header should include, minimizing potential ignition sources, and operating outside the flammable limits. Measures to minimize potential ignition sources include: Installing a dip pipe for liquid entering the feed tank to minimize electrostatic charge generation Grounding and bonding vent pipework to prevent electrostatic charge accumulation The feed tank has a nitrogen blanketing system that maintains the tank pressure slightly positive (approximately 2 - 4" WG) while the tank is emptying. A conservation vent is provided set to open at 6" WG, to relieve pressure as the tank is filled.

A small continuous nitrogen purge is provided at the closed end of the normal process vent header to prevent air migrating in from the "open" end, or through any leaks that may exist. The normal process vent gas is scrubbed with water before it is released to atmosphere. Gases leaving the scrubber meet environmental requirements, and are too lean to bum. Rich water exiting the scrubber is sent to recovery to separate and recycle the raw materials.

A scrubber failure could result in vapor, above its LFL being vented to atmosphere. A flame arrester is installed at the open end of the vent header to protect against flash back if air is also present in with the vapors.

25 1

k-; 4

@-

8

B

L

J

2

U

I

Appendix G - Worked Examples

Controls and alarms associated with the vent system include: Low flow alarm on water flow to the scrubber Low pressure alarm in the nitrogen line supplying the feed tank High liquid level alarms in the vent scrubber sump Liquid level instrumentation on the feed tank, with high alarm, and high level interlock to stop liquid transfers from the blend tanks The design basis for the emergency pressure relief scenario is an external fire. The facility has good drainage and is provided with deluge fire protection. External fire is considered to be a low probability event; further, vapor from the emergency vent would not constitute a hazard to personnel either on or off site. As a result the emergency vent discharges to atmosphere without treatment. The design basis for vacuum protection is the combined effects of a failure of the nitrogen blanketing system and sudden cooling of the tank by a thundershower on a hot day. To protect against ths the tank’s conservation vent has a vacuum breaker to allow air to enter if the pressure drops below 3” WG vacuum.

G1.5

Determine if Process and Emergency Vent Headers Should be Combined

The emergency vent header discharges directly to atmosphere without treatment. In addition the emergency vent flow rate is considerably hgher than the normal process vent flow rate. Combining both systems would require a significantly larger vent scrubber to handle the total flow, and would not provide measurable benefits. As a result there is no merit in combining the systems.

G1.6

Determine if Intermediate Treatment Is Required

Inerting and scrubbing the normal process vents provides an effective system for handling the vent gases. No benefits were identified that would justify installing intermediate treatment. G1.7

Specify Vent Header System Preliminary Design

The preliminary design should be provided for the hazard review and include the most up to date information on items such as:

253

Safe Design and Operation of Process Vents and Emission Control Systems

The design basis including the "design case" scenarios for vent flow rates and compositions in the normal process vent header Header layout drawings Specifications for the vent header equipment, such as the vent scrubber specifications Materials of construction requirements

G1.8

Implement Improvements Identified in Hazard Reviews

Changes to the facility that arise from concerns identified during hazard reviews or as a result of new information as the project design develops should be addressed during the design of the vent system. f i s may require the design basis for the vent system to be modified as the project progresses.

G2.

FLAMMABLE LIQUID PROCESS OPERATING FUEL LEAN

As indicated in Section G1.O, the vapor from a flammable liquid can produce an explosive mixture when mixed with air. Explosion prevention methods include operating, inerted, fuel rich, or fuel lean. This example illustrates a vent system that operates fuel lean, i.e., the composition of flammable vapor is maintained below the concentration required for a deflagration to propagate. Figure G-3 shows a multi tank flammable liquid storage facility in which individual tanks operate air blanketed. As a result from time to time they will contain flammable vapor/air mixtures. Explosion protection for the vent header is provided by maintaining a continuous air flow that rapidly dilutes the vent streams from the source vessels to substantially below the LFL. Features that should be considered in the design include minimizing potential igrution sources in the tanks, installing detonation arresters in the lines between the tanks and the header, and monitoring the air flow in the vent header.

G2.1

Facility Description

A flammable liquid, flash point 87'F (31°C) is stored in air blanketed atmospheric storage tanks. The tanks are equipped with pressure - vacuum conservation vents for normal venting, and have reliving manways for emergency over pressure protection (see Figure G-3).

254

Appendix G - Worked Examples

G2.2

Identify Normal Vent Process Requirements

Vent gases from the tanks are a mixture of air and organic vapor from the product. Requirements for the vent header system include: Addressing potential flammability hazards Routing the vent gases to a treatment system to remove, or reduce the concentration of organic vapors to an acceptable level for release to atmosphere G2.2.1 Flammabilitv Considerations The liquid has a flash point of 87°F (31°C). If the tank's head space temperature is lower than 87°F (31"C),it will not contain sufficient organics to support combustion. When exposed to solar radiation the head space temperature can reach 140°F (60°C). Consequently during normal weather patterns the flash point can readily be exceeded. Explosion protection, e.g., inerting or operating fuel rich is employed extensively in the petrochemical industry; however, many flammable storage tanks are operated air blanketed. In these cases it is important to ensure potential i p t i o n sources at one point in a vent header system will not become a cause of i p t i o n elsewhere. G2.2.2 Environmental Considerations

To meet environmental control requirements the normal process vents must be treated before they are released to atmosphere; in addition, even at very low concentrations the vapors have an unpleasant odor that would be unacceptable to the local community. G2.3

Develop Emergency Venting Scenarios

G2.3.1 Emergencv Overpressure Protection The design case for emergency overpressure protection is external fire exposure. G2.3.2 Emergencv Vacuum Relief The design case for emergency vacuum relief is a failure of the pressure vacuum conservation vent to open during conditions that cause subatmospheric pressure in the tank. For example when the liquid level is being lowered, or the tank cools as a result of a thunderstorm on a hot day.

255

Safe Design and Operation of Process Vents and Emission Control Systems

G2.4

Specify Vent System Design and Treatment Options

Explosion protection for the normal process vent header should include, minimizing potential ignition sources, and operating outside the flammable limits. Measures to minimize potential igrution sources include: Installing dip pipes in tanks to minimize electrostatic charge generation Grounding and bonding vent pipework to prevent electrostatic charge accumulation Installing a flame arrester at the thermal oxidizer The tanks are air blanketed and will, at times, contain flammable vaporiair mixtures. To provide an additional layer of protection beyond minimizing ignition sources the vent gases are diluted with air forming a non-igrutable, lean mixture. The facility has a thermal oxidizer to dispose of liquid waste from the manufacturing process. The combustion air for the thermal oxidizer is also used as a source of air to: Dilute the vent gases, to significantly less than 25% of the LFL Dispose of the resulting lean air/vapor mixture Design features for this system include: Ensuring the tank vents enter the combustion air header at a point where the air is turbulent Providing adequate distance for mixing between the addition point and any potential ignition source (such as the blower) Instrumentation to monitor the combustion air flow with an interlock to stop the transfer pump feeding the tanks if the air flow is below the required value An interlock to close the combustion air exhaust damper, and stop the product transfer pump if the combustion air blower stops Flame arresters below the conservation vents on the storage tanks

256

I

I

I"..)

Safe Design and Operation of Process Vents and Emission Control Systems

Locating the combustion air inlet away from potential sources of flammable vapors, and ignition points Confirming the thermal oxidizer burner can operate satisfactorily over the maximum range of flammable concentrations it could be exposed to when liquid is transferred to the storage tanks When operating fuel lean it is particularly important to ensure tanks do not overflow flammable liquid into the vent header. This is prevented by the follows: Each storage tank has a high level sensor separate from the basic process controls interlocked to closes an XV valve in the tank's inlet line The vent header is located several feet above the storage tanks. If a tank is overfilled liquid will preferentially overflow through its pressure relieving manway rather than overcoming the vertical height required to reach the vent header The tanks have pressure - vacuum relieving manways for emergency pressure and va,cuum protection. These manways vent directly to atmosphere and do not have vent headers G2.5

Determine if Process and Emergency Vent Headers Should be Combined

In this application the emergency vents discharge directly to atmosphere. It would not be practical or beneficial to combine them with the normal process vent. G2.6

Determine if Intermediate Treatment Is Required

Intermediate treatment is not applicable. G2.7

Specify Vent Header System Preliminary Design

The preliminary design should be provided for the hazard review and should include the most up to date information on items such as the following: The design basis used for determining the vent flows from the tanks

A review of the thermal oxidizer vendor information on the effects of introducing low concentrations of flammable vapor into the combustion air 258

Appendix G - Worked Examples

Details of proposed interlocks Header layout drawings Specifications for the vent header equipment, e.g., the blower, the flame/detonation arrester requirements

G2.8

Implement Improvements Identified in Hazard Reviews

Changes to the facility that arise from concerns identified during hazard reviews or as a result of new information as the project design develops should be addressed during the design of the vent system. This may require the design basis for the vent system to be modified as the project progresses.

G3.

FLAMMABLE LIQUID PROCESS OPERATING FUEL RICH

The explosion hazards from flammable liquids can be addressed by maintaining the vapor composition, below its lower oxygen concentration, fuel lean, or fuel rich. This example illustrates a reactor and its associated headers that operate fuel rich, .i.e., the vapor concentration is maintained above the upper flammable limit.

G3.1

Facility Description

Figure G-4 illustrates the vent system for a batch reaction process involving two raw materials. The first step for each batch is to rapidly feed a complete charge for one of the raw materials to the reactor. Following this the second raw material is fed at a predetermined rate, based on maintaining the reaction temperature in specification. Both materials are flammable, and can form solid polymers The process is part of a larger facility with an emergency vent header that operates fuel rich and leads to a flare.

G3.2

Identify Normal Vent Process Requirements

The maximum process vent gas flow from the reactor occurs while the initial raw material is being charged. The maximum blanketing gas requirements occur while the completed batch is being transferred to a hold tank. Requirements for the normal process vent header system include: Addressing potential flammability hazards To route the vent gases to treatment before they are discharged to atmosphere

259

Safe Design and Operation of Process Vents and Emission Control Systems

G3.2.1 Flammabilitv Considerations The reactor feeds are Class 1B flammable liquids. At the end of each batch the product is transferred to storage. As tlus transfer is made gas must enter to replace the liquid product. If air enters, either intentionally or as a result of leaks, a flammable air/fuel mixture could form. G3.2.2 Liauid Build-up Considerations The liquid raw materials can polymerize. Consequently if they collect in the header solids may form restricting the flow of vent gases through it. Measures should be taken to minimize condensation in the vent header, to prevent entrainment or liquid overflowing from the reactor, and to provide adequate drainage from the header. G3.3

Develop Emergency Venting Scenarios

G3.3.1 Emeraencv Overtxessure Protection The design case for emergency venting is a runaway reaction initiated by external fire exposure. G3.3.2 Emeraencv Vacuum Protection The reactor is rated for full vacuum and consequently does not require emergency vacuum protection. G3.4

Specify Vent System Design and Treatment Options

Explosion protection for the headers includes, minimizing potential ignition sources, and operating outside the flammable limits. Potential ignition sources are controlled by: Installing dip pipes in the reactor to prevent electrostatic charge generation by free falling liquid Grounding and bonding the vent pipework Installing a flame arrester at the thermal oxidizer Providing a flare drum, designed to prevent flashback, at the base of the flare Maintaining the headers fuel rich by providing continuous fuel gas purges at the end of each header, furthest away from the open end

260

Safe Design and Operation of Process Vents and Emission Control Systems

The reactor and vent headers operate fuel rich. Fuel gas is supplied to the reactor from a pressure regulator that controls at 4 WG and has a conservation vent which opens at 8 WG”. Vent gases from the conservation vent discharge into the normal process vent header which is routed to the thermal oxidizer. The emergency vent is equipped with a rupture disk. It discharges to a knockout tank designed to handle two phase venting resulting from a runaway reaction initiated by an external fire. Controls and alarms on the vent system include: Low pressure alarm on fuel gas blanketing the reactor Local flow indication for the fuel gas purges at ends of header branches High liquid level alarms in thermal oxidizer knockout pot and the emergency header knockout tank High differential pressure indicator and high temperature alarm for thermal oxidizer’s flame arrester High and low level alarms on the flare drum G3.5

Determine if Process and Emergency Vent Headers Should be Combined

The flare is designed to provide a reliable treatment system for handling the h g h vent gas flows associated with very infrequent emergency incidents. Additional fuel gas is not required to augment the calorific value of these gases. The normal process vent flow rates are low and intermittent. If they are fed to the flare additional fuel gas will be needed to acheve a stable flame. 17us would add sigruficant operating cost malung the approach uneconomical. Conversely the thermal oxidizer is designed to operate efficiently with the low and variable process vent gas flows. They are, however, vulnerable to flame failures, which can occur as a result of a sudden changes in the feed rate or a loss of utilities during an emergency. Further it can take a considerable time to purge and relight the thermal oxidizer. Based on the above it is not practical to combine the normal and emergency vents. G3.6

Determine if Intermediate Treatment Is Required

The design case emergency venting scenario for the reactor predicts a two phase gas/liquid flow. Two phase flow could result in an unacceptably high pressure drop in the emergency vent header, which could potentially 262

Appendix C - Worked Examples

overpressure the reactor. To address this, the emergency vent is directed to a large knockout tank where liquid can separate and provide a vent stream that is substantially 100% gas. G3.7

Specify Vent Header System Preliminary Design

The preliminary design should be provided for the hazard review, including the most up to date information on items including: The design basis including the reactivity and flammability "design case" used to specify the vent system requirements Header layout drawings Specifications for the vent header equipment, e.g., detonation or deflagration arrester requirements, and the design basis for the emergency knockout tank Materials of construction requirements

G3.8

Implement Improvements Identified in Hazard Reviews

Changes to the facility that arise from concerns identified during hazard reviews or as a result of new information as the project design develops should be addressed during the design of the vent system. This may require the design basis for the vent system to be modified as the project progresses.

G4.

ROAD TANKERS, FLAMMABLE LIQUID LOADING

Terminals for loading road tankers typically receive "empty" tankers filled with vapor from the previous shipment. There may, however, be occasions when the returned tankers contain air with little or no volatile organic materials present. Explosion protection for these facilities typically involves eliminating ignition sources, and installing flame arresters or detonation arresters, as appropriate.

G4.1

Facility Description

A terminal operation loads several different flammable liquids into road tankers (see Figure G-5). These liquids are volatile with equilibrium vapor concentrations above their upper flammable limits. Empty road tankers return to the site air blanketed and typically are not purged or cleaned before being refilled. As a result the vent gases displaced by the incoming liquid can be saturated with flammable vapors. In order to meet environmental requirements these vent gases must be treated before they can be discharged to atmosphere during the loading operation.

263

Safe Design and Operation of Process Vents and Emission Control Systems

G4.2

Normal Process Vent Requirements

Vent gases from several loading spots feed to a vent header and are then sent to a thermal oxidizer. The gases are flammable; consequently, the vent system design must take into consideration the potential for vapor/air mixtures to form that could burn explosively. G4.2.1 Flammabilitv Considerations Road tankers arrive on site containing air saturated with vapor from the previous cargo. In most cases the tankers are refilled without being purged or cleaned since the liquids loaded have similar properties. As a result vapors entering the vent header are typically fuel rich and are not an immediate deflagration (explosion) hazard. A variety of scenarios could result in air entering the header, potentially creating a flammable airhapor mixture. Explosion protection is achieved by taking measures to minimize the time the headers contain flammable air/vapor mixtures, and eliminating ignition sources. Specifically this includes: 0

0

Electrically grounding the road tanker Ensuring all conductive parts of the vent header are grounded and bonded Installing a detonation arrester in the header close to the thermal oxidizer Installing flame arresters at connections to road tankers Provide lightning protection and cease operations during thunder storms Establishing administrative procedures to control hot work, and other activities that could create an ignition source

These measures, along with operating to minimize instances when the header contains flammable mixtures have historically provided effective explosion protection for tank car loading facilities. G4.2.2 Liauid Build-up Considerations The following measures are incorporated to prevent liquid buildup in the header: The header is sloped towards the knockout tank at the thermal oxidizer Liquid transfers to the tankers are controlled by flow totalizers 264

Appendix G - Worked Examples

Redundant high level detectors are provided in the vent lines exiting the tankers to prevent liquid overflowing the tanker if a flow totalizer malfunctions or if it is improperly set The knockout tank at the inlet to the thermal oxidizer has a high level alarm

G4.3

Develop Emergency Venting Scenarios

Road tankers have emergency pressure and vacuum vents installed on the tanks. These vents are designed to discharge directly to atmosphere. There is no emergency vent header provided. G4.4

Specify Vent System Design and Treatment Options

The vent gases are treated in a thermal oxidizer. Alternative end-of-line treatment systems considered include: a vapor recovery unit, carbon beds, and a flare. The thermal oxidizer was selected as a compromise considering environmental, economic and community nuisance issues. The header typically operates fuel rich; however, the potential exists for air to enter creating a flammable mixture. To protect against tlus a detonation arrester is provided between the thermal oxidizer and its knockout pot.

If the facility is to routinely handle liquids with vapor compositions within their flammable limits at ambient temperatures, e.g., ethanol, additional protection such as inerting may be appropriate. Combining vents from loading spots that are fuel rich, with vents that are fuel lean may create a flammable air/vapor mixture. In this case it may be inherently safer to provide separate vent headers to transfer the fuel rich and fuel lean streams to the thermal oxidizer. Controls and alarms for the vent system include: High temperature alarms to detect flashback to flame and detonation arresters Interlocks to stop feed to the tanker if liquid level is detected in the vent line exiting the tanker, or in the knockout tank at the thermal oxidizer

265

Appendix G - Worked Examples

G4.5

Determine if Process and Emergency Vent Headers Should be Combined

Not applicable. Emergency vents on the tankers are not designed to be directed to a header, see Item G4.3. G4.6

Determine if Intermediate Treatment Is Required

A knockout tank is provided immediately upstream of the thermal oxidizer’s flame arrester. G4.7

Specify Vent Header System Preliminary Design

The preliminary design should be provided for the hazard review and should include the most up to date information on items such as the following: 0

0 0

0

G4.8

The design basis including the maximum unloading rates, and the external fire ”design case” Header layout drawings Requirements for the vent header equipment, e.g., detonation or deflagration arrester requirements, and knockout tank specifications Materials of construction and flexible hose requirements

Implement Improvements Identified in Hazard Reviews

Hazards identified during hazard analysis reviews or as a result of new information as the project design develops should be documented and resolved before the design is considered final. G5.

REFINERY EXAMPLE: CRUDE AND VACUUM UNITS

Tlus example presents a case where all of the normal process and emergency vent streams from the two involved columns contain reasonably similar flammable vapors and would appear candidates to be combined. However, the vacuum column presents a problem in that it requires a very low normal process vent discharge pressure below that of a typical vent header system. The case illustrates the need to examine all characteristics of a vent stream to determine an appropriate disposition.

267

Safe Design and Operation of Process Vents and Emission Control Systems

G5.1

Facility Description

An expansion is planned for a refinery that will add the following additional units:

0

A crude oil process unit with a low pressure (atmospheric) distillation column A vacuum unit with its vacuum distillation column

The Crude Unit will process a heavy crude oil feedstock by heating, removing sediments and water-soluble components, distilling and collecting usable products including diesel, kerosine and naphtha. The atmospheric column operates at about 35 psig (2.4 bar) and 270°F (132C) with overhead gases compressed and fed to a fuel gas system and the bottom liquids fed to the vacuum unit. The Vacuum Unit further separates heavier liquid gasoil products. The vacuum column operates at about 20 mmHg and 2200F (1040C) with 3stage steam ejectors with inter-stage condensing to collect additional hydrocarbons and separate sour water (containing hydrogen sulfide). G5.2

Normal Process Vent Requirements

All of the normal process vent streams from these unit operations contain flammable gases and vapors. Most of the normal vent streams will also contain hydrogen sulfide (HzS) and water vapor. The Crude Unit’s major normal process vent stream is from the atmospheric column overheads pressure control vent which operates at about 35 psig (2.4 bar) and 2700F (132C). The Vacuum Unit’s noimal vent stream is from the discharge of the column vacuum system. To achieve the desired vacuum column operating pressure, it has been determined that the back-pressure on the vacuum system from its liquid receiver or hot-well must be less than 1 psig (0.07bar). G5.3

Emergency Venting Scenarios

The vented materials for all identified emergency venting scenarios are expected to be flammable and may contain significant concentrations of H2S and water vapor.

268

Appendix G - Worked Examples

For the Crude Unit, loss of cooling of the overhead stream from the atmospheric column can cause catastrophic overpressure and is considered to be the primary emergency venting scenario. The atmospheric column is provided with multiple pressure safety valves (PSVs) set at 50 psig (3.45 bar). For the Vacuum Unit, overpressure of the vacuum column is the primary concern. Further, these columns are typically designed with lower maximum allowable design pressure ratings, in this case 25 psig (1.72 bar). Primary overpressure protection is a PSV venting directly to atmosphere to eliminate back-pressure and ensure maximum venting capacity. The overpressure scenario occurs as follows: loss of cooling and condensing capability on the column overhead vapor stream in the vacuum jet results in loss of vacuum and with column liquid temperatures and continued heating from the column reboiler there is a rapid boil-up of material with a consequent rapid pressure rise. G5.4

Investigate Vent System Design and Treatment Options

Essentially all of the process vent streams that can be collected will be "rich; a few minor streams may be inerted. The large total quantity of vapors and gases anticipated to be released and the environmental considerations indicate the need for end-of-pipe treatment by a flare system to combust the hydrocarbons and destroy the H2S. The likely wide range in flowrate within the vent header system favors the use of a flare over a thermal oxidizer, particularly, if a combined normal and emergency vent header system is considered. The basic flare system design must include: Knockout Tank and Seal Drum or similar device(s) to: -

Prevent reverse flow of air into the header from the flare stack

-

Provide flashback prevention

-

Establish the minimum internal vent header operating pressure

The vent header design must include at the most remote upstream end(s) of the header an uninterruptible flow of a flammable purge gas to ensure that the header remains under positive pressure and free of air.

269

Safe Design and Operation of Process Vents and Emission Control Systems

G5.5

Determine If Normal and Emergency Vent Headers Should Be Combined

Based on the common materials vented and the reasonably compatible range of vent pressures and temperatures, it would be practical to combine most of the above described normal process and emergency vent streams. However, some streams will require intermediate treatment.

G5.6

Determine If Intermediate Treatment Is Required

The Crude Unit’s normal vent is from the atmospheric column overhead stream which is cooled and condensed and therefore requires liquid knockout. This knockout receiver requires level control to prevent liquid over flow into the header. For the emergency vent, due to the need to ensure an open vent path and the anticipated infrequency of emergency venting, no liquid knockout is provided on the emergency vent stream. The Vacuum Unit presents a different treatment situation for the normal process vent. The required low (< 1 psig, 4.07 bar) venting backpressure makes it impossible to put this stream into the vent header which will most probably operate at 2 to 4 psig (0.138 to 0.276 bar). However, a process heater is located within the unit that can (with necessary air permits) be used to achieve the environmentally required combustion efficiency. The vent stream from the vacuum column hot-well should be routed to a knockout tank then through a flame/detonation arrester to an eductor burner installed in the process heater. Alternatively, a small thermal oxidizer could be used to treat t h s normal vent stream. The vacuum column emergency vent is directly to atmosphere; the quantity of flammable vapors and steam released and the height of the relief valves on top of the column will be verified to provide adequate dispersion.

G5.7

Finalize Vent Header System Preliminary Design

The preliminary design including layout, intermediate treatment requirements, line sizing and specification, materials of construction and the selected treatment system should be documented. This becomes the basis for a formal hazard analysis. See Figure G-5 for the basic design of a vent header system for t h s example.

G5.8

Implement Improvements Identified in Hazard Analysis

The formal hazard analysis should be documented and the recommendations arising from it should be incorporated into the final vent header design for construction.

270

Appendix G - Worked Examples

G6.

REFINERY EXAMPLE: COKER UNIT AND GAS PROCESSING PLANT

This example presents a simple refinery heavy oil coker and a related gas plant and illustrates the need for careful analysis of vent streams and their intermediate treatment steps that may be required. In this case all of the normal process and the emergency vent streams can ultimately be combined for final end-of-line treatment but not before extensive intermediate separation and cooling steps are taken to make one group of streams compatible with the other. G6.1

Facility Description An expansion is planned for a refinery that will add the following additional units: A coker unit consisting of coke drums and a fractionator column to produce coke from part of the process stream from the crude/vacuum units A gas plant consisting of compressors, de-pentanizer, deethanizer and unifiner to further process gases from the coker unit to usable products and fuel gas for the process heaters

The Coker Unit receives vacuum column bottoms liquids that are fed to the Fractionator Column, heated and fed to each of the coke drums sequentially in a batch-wise process. The l g h l y heated heavy oil rapidly solidifies within the drums to coke with the off-vapors returning to the fractionator. The drums are cooled, opened and the coke is cut out using high pressure water jets and the drums are then closed and warmed up to prepare for the next batch cycle. The coke drums experience temperature up to 850°F (454°C) and pressure to 175 psig (12.1 bar) at various points in the batch cycle; the fractionator operates at about 35 psig (2.41 bar) and 520°F (271°C). Coke is produced in each of two drums sequentially in a batch mode operation. Additional heavy gasoil liquid products are distilled in the fractionator and the overhead gases are compressed and fed to the gas plant. The Gus Plant processes the compressed coker gases to separate residual sour water and collect additional light hydrocarbon products.

271

Safe Design and Operation of Process Vents and Emission Control Systems

G6.2

Normal Process Vent Requirements

All of the normal process vent streams from these unit operations contain flammable gases and vapors. Most of the normal vent streams will also contain hydrogen sulfide (HzS) and water vapor.

The Coker Unit has two major normal process vent streams: the fractionator column overheads pressure control vent and an open vent from the coker blowdown system’s receiver that can operate against a moderate back pressure. The Gas Plant has a number of normal vent streams from process pressure control vents.

212

Figure G-6.

CRUDE UNIT

Refinery Example - Crude and Vacuum Unit

VACUUM UNIT

I

Safe Design and Operation of Process Vents and Emission Control Systems

G6.3

Emergency Venting Scenarios

The vented materials for all identified emergency venting scenarios are expected to be flammable and may contain significant concentrations of H2S and water vapor. The Coker Unit may require emergency venting of streams from either the coke drums or the fractionator. Emergency vent streams from both sources may contain a significant amount of solid coke, from particles to large pieces, in addition to flammable gases and hot hydrocarbon liquids. Normal and emergency vent streams may range from 500 to 800°F (260 to 427°C). The Gas Plant has a number of vessels that may experience overpressure due to excessive heating, loss of pump circulation or loss of overhead stream cooling. Overpressure protection by PRVs is provided for the affected vessels. G6.4

Investigate Vent System Design and Treatment Options

Essentially all of the process vent streams that can be collected will be "rich"; a few minor streams may be inerted. The large total quantity of vapors and gases anticipated to be released and the environmental considerations indicate the need for end-of-pipe treatment by a flare system to combust the hydrocarbons and destroy the H2S. The likely wide range in flowrate witlun the vent header system tends to favor a flare over a thermal oxidizer, particularly, if a combined normal and emergency vent header system is considered. The basic Flare System design must include: Knockout Tank and Seal Drum or similar device(s) to

G6.5

-

Prevent reverse flow of air into the header from the flare stack

-

Provide flashback prevention

-

Establish the minimum internal vent header operating pressure

Determine If Normal and Emergency Vent Headers Should Be Combined

Based on the common materials vented and the reasonably compatible range of vent pressures and temperatures, it would be practical to combine 274

Appendix G - Worked Examples

most of the above described normal process and emergency vent streams. However, some streams will require intermediate treatment. G6.6

Determine If Intermediate Treatment Is Required

The Coker Unit also presents a venting problem. Normal batch venting from the coke drums as well as emergency venting from the drums and the fractionator column requires cooling of these high temperature streams and separation of the solid coke that they may entrain. This is done in a separate quench drum system with recirculation and by cooling, condensing and liquid knockout of the gases from the quench drum before they can be vented into the header system. The gas plant includes several columns and vessels that require emergency venting through PSVs into a header system. These columns and vessels also have normal process pressure control vents that also must be collected into a header system and will require liquid knockout at the process prior to venting into the header system. G6.7

Finalize Vent Header System Preliminary Design

The preliminary design including layout, intermediate treatment requirements, line sizing and specification, materials of construction and the selected treatment system should be documented. This becomes the basis for a formal hazard analysis. See Figure G-6 for the basic design of a vent header system for this example.

G6.8

Implement Improvements Identified in Hazard Analysis

The formal hazard analysis should be documented and the recommendations arising from it should be incorporated into the final vent header design for construction.

G7.

REACTIVE SYSTEM

Th~sexample illustrates the importance of considering the combined affects of reaction hazards and the equipment characteristics. The worst credible case was shown to be the reactor agitator stopping and then restarted several minutes later. While the agitator is stopped raw materials collect in the reactor and subsequently will react violently when the agitator is restarted. Th~scan cause two phase venting and substantially increase the volume required for the emergency vent catch tank.

215

Safe Design and Operation of Process Vents and Emission Control Systems

G7.1

Facility Description

A chlorinated inorganic liquid is hydrolyzed by reacting it with water in a glass lined, agitated reactor. The reaction is strongly exothermic, however, the liquids are immiscible. Reaction occurs at the interface between the liquids. In the absence of agitation the liquids form separate layers resulting in a low overall reaction rate. During normal operations the agitator disperses the two liquids, and the reaction takes place almost instantaneously. The reactor operates at close to atmospheric pressure and the heat of reaction in removed by allowing the reaction mass to boil (See Figure G-7). G7.2

Identify Normal Process Vent Requirements

Vent gases from the reactor consist primarily of water vapor and hydrogen chloride (HCl), which are condensed to form a saleable biproduct acid. The vapors are not flammable and therefore explosion protection is not needed.

G7.2.1 Liquid Build-up Considerations Measures to prevent liquid buildup in the header include: Eliminating low points in the header where liquid could collect Minimizing the distance between the reactor and the condenser Designing and operating the hydrolyzer to minimize liquid entrainment in the vent gases

G7.2.2 Materials of Construction Vent gases from the hydrolyzer are corrosive to carbon and stainless steels. The normal process vent system should be constructed from materials that are compatible with these conditions, such as titanium, and certain reinforced plastics, and lined steel. During normal operations a rupture disk prevents the process gases entering the emergency vent header. As a result it may be acceptable to use less costly materials that are only able to withstand the process gases for short periods of time.

276

Figure G-7.

Refinery Example - Coker Unit

Q........ .:

Figure G-8.

Reactive System

2

Lu>

P

E5

Appendix G - Worked Examples

G7.3

Develop Emergency Venting Scenarios

The worst case scenario was determined to be an undetected malfunction of the hydrolyzer agitator allowing a layer of the chlorinated feed material to accumulate in the reactor, followed by the agitator starting. This could result in a violent reaction developing high pressure in the reactor, the vent header, and equipment connected to it.

No combustible materials are handled in the area; as a result the vent design does not need to address external fire and there are no deflagration (explosion) hazards inside the equipment. G7.4

Specify Vent System Design and Treatment Options

The normal process vent stream from the hydrolyzer is fed to the condenser where most of the water vapor and HCl are condensed. Vent gases leaving the condenser are fed to a knockout pot and then treated in a caustic scrubber to remove the remaining HCl. Controls and alarms for the normal process vent header include: Hydrolyzer high level alarm and feed interlock Hydrolyzer condenser vapor outlet high temperature alarm Tail gas scrubber, low caustic feed flow alarm Tail gas scrubber sump high level alarm Test work demonstrated that the design case emergency venting scenario could result in two phase venting, carrying approximately 70% of the reaction mass into the vent header. To handle this large volume of liquid, and to separate the phases, the emergency vent is discharge into a knockout tank. Vent gases from the knockout tank are fed to a water scrubber and discharged to atmosphere. The emergency scrubber must be available at very short notice any time the hydrolyzer is operating. To satisfy this requirement water is continuously fed to the scrubber and re-circulated back to it through a storage tank. Controls and alarms for the emergency vent system include: Rupture disk with indicator/alarm at entry to emergency vent header Liquid detector in emergency knockout tank with alarm and interlock to stop hydrolyzer feeds Low flow alarm on the water feed to the emergency scrubber High level alarm in the emergency scrubber

279

Safe Design and Operation of Process Vents and Emission Control Systems

During emergency venting a two phase flow may be discharged to the normal process vent system which may become substantially liquid full. To ensure it is not damaged when emergency venting occurs it must be designed to withstand: Its weight liquid full The maximum pressure developed during a worst case venting scenario The maximum temperature that could occur, including the temperature rise caused when the highly acidic reaction mixture mixes with the caustic scrubbing liquid G7.5

Determine if Process and Emergency Vent Headers Should be Combined

The design case for the emergency vent system predicts two phase venting, and very high gas flow rates. It would be costly to provide a hydrolyzer condenser to handle these conditions. In addition an emergency vent system with a rupture disk that only opens when an emergency occurs would be exposed to corrosive conditions very infrequently. As a result it could be constructed from less expensive materials of construction than are required for the normal process vent system. Consequently, it is not cost effective to combine the normal and emergency vents. G7.6

Determine if Intermediate Treatment Is Required

Intermediate treatment includes:

A condenser and a knockout pot in the normal process vent header system An emergency catch tank to separate the gas and liquid phases in the emergency vent header G7.7

Specify Vent Header System Preliminary Design

The preliminary design should be provided for the hazard review, including the most up to date information on items such as:

0

The design basis including the emergency venting requirements caused by a delay in starting the reactor agitator Header layout drawings Proposed materials of construction for both header systems Instrumentation and interlock requirements

280

Appendix G - Worked Examples

G7.8

Implement Improvements Identified in Hazard Reviews

Changes to the facility resulting from concerns identifies during hazard reviews, or as a result of new information as the project design develops, should be evaluated to determine if they will affect the requirements for the vent headers.

281

Sufe Design and Optvation ofPI-oc.ess Vents and Emission C o n t i d $wteins

by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

H CASEHISTORIES This appendix discusses incidents that have involved vent header systems. Each case study contains a brief description, a list of lessons learned and preventive measures that could prevent a reoccurrence. The case histories have been grouped by the principal mechanism involved in the incident: Combustions in headers and vessels when fuel and oxidizer streams mix and ignite. Chemical reactions either damaging or restricting flow through the vent header system. Collapse of low-pressure tanks when solids or liquids build-up in the vent header causing sub-atmospheric pressure in the tank, e.g., when liquid is pumped out or the vapor space cools.

H1.

COMBUSTION INCIDENTS

Case History H1.l Facility

Vent Header Explosion in Hydrocarbon Chlorination

An explosion occurred in the vent header at a facility producing a chlorinated solvent by reacting chlorine and a hydrocarbon gas. The facility has several reactors that vent into a common process vent header. The gases are routed to an incinerator and then to a scrubber. The scrubbed gases are finally discharged to atmosphere from an elevated stack.

283

Safe Design and Operation of Process Vents and Emission Control Systems

The reaction process operates with a small excess of hydrocarbon gas producing a vent stream consisting of the excess hydrocarbon gas, hydrogen chloride, and solvent vapor. This stream is corrosive to most metals, to overcome this, the vent header is constructed from PVC pipe. The incident occurred when one of the reactors experienced a process upset that stopped the hydrocarbon feed while allowing the chlorine feed to continue. Chlorine flowed through the reactor into the header where it mixed with the excess hydrocarbon gas from the other reactors. Th~sformed a flammable mixture that igruted causing an explosion that shattered the PVC vent header. The ignition source could not be identified with total certainty; however, it may have occurred at the incinerator. Lessons Learned

This incident demonstrates that explosions and fires can occur with oxidizers other than air, it also highlights the importance of evaluating the implications of all combinations of feed interruptions: Chlorine can behave as an oxidizer capable of supporting combustions and may develop deflagration pressures similar to those produced by hydrocarbodair mixtures. Interlocks (safety instrumented systems) should be provided to monitor the feed ratio and take appropriate action. Explosion protection should be provided in systems if there is a credible scenario that could result in a flammable fuel/oxidizer mixture. Preventive Measures Interlocks should be provided, including automated valves that are independent of the process control system that will stop both chlorine and hydrocarbon flows if the ratio is outside the control limits (note: a bypass will be needed for reactor start-up). Flame arresters should be provided in the header located near the incinerator (note: flame arresters designed for air/hydrocarbon applications may not be effective in chlorine service and testing may be needed to confirm a specific model is effective in this service).

284

Appendix H - Case Histories

Case History H1.2 Header Explosion

Low Catalyst Activity and Process Upset Causing Vent

A production facility has several reactors where a hydrocarbon gas is reacted with air. The process flows from these reactors are combined and sent to an absorber to obtain the crude product. Vent gases from the absorber are fed to a thermal oxidizer, and the exhaust from it is discharged to atmosphere. Due to an unrecopzed condition the catalyst in one of the reactors had deactivated resulting in un-reacted hydrocarbon gas in its outlet gas stream. The hydrocarbon gas is not soluble in the absorber liquid and remained in the vent streams being fed to the incinerator. A process upset in one of the other reactors caused it to shut down, after which air flows were automatically continued to purge hydrocarbons from the reactor. When the air mixed with the hydrocarbons, a flammable mixture was formed. This mixture igruted and flashed back into the absorber, causing significant damage. It is believed the incinerator was the source of ignition. Lessons Learned Vent headers receiving flows from multiple sources may at some time receive any combination of compositions that could exist in the individual source vessels. The safety design basis should therefore include all combinations of compositions, and conditions that could occur even if they only occur very infrequently. Preventive Measures Preventive measures that were implemented include: 0 The control system was revised to monitor conditions in the reactor and to alarm if the conditions could result in low conversion. Additional training was provided for operating personnel highlighting conditions that indicate a potentially hazardous condition and the corrective measures that should be taken.

285

Safe Design and Operation of Process Vents and Emission Control Systems

Case History H13 Vent Header Fire and Tank Explosions Initiated by Carbon Adsorption Shortly after three storage tanks had been put into service handling crude sulfate turpentine (CST) (Class lC, flammable liquid), nearby residents began to complain of the strong unpleasant odor from the facility. In response, the company installed a vent header system and a carbon adsorption system to collect the odiferous materials. The carbon adsorption system consisting of two 55 gallon drums containing activated carbon. In addition, the design included flame arresters at the outlets from the storage tanks, however, at the time of the incident they had not been installed [Ref. 1, paper 6a]. The incident was witnesses by an Air National Guard pilot wearing infrared night vision goggles. Initially, he reported seeing part of the header and the carbon beds were hot. A short time later, fire erupted from the end of the header followed by a tank explosion. Approximately 10 minutes later, the other two tanks caught fire. Lessons Learned Woodward and Lygate [Ref. 21 conducted a study of the incident revealing: Diurnal temperature variations in the CST tanks forced fuel rich vapors through the activated carbon canisters in the afternoon hours of the day loading vapors onto the activated carbon. These same temperature variations lead to cooling in the evening hours and pulled air through the carbon canisters. The temperature and flow conditions were such that the composition of hydrocarbons in the vapor space, and in the PVC piping to the carbon canisters, was above the flammable LFL during the afternoon hours and early evening. The inflow of air in the early evening brought the concentration of oxygen above the minimum combustion limits. The inflow of air through the activated carbon canisters cause oxidation of a portion of the adsorbed hydrocarbon and the heat generated by oxidation raised the temperature in the canister and the vapor flowing into the PVC piping.

286

Appendix H - Case Histories

The study [Ref. 21 also concluded that the dimensions of the header and the conditions in it were consistent with flame front acceleration that could have resulted in the deflagration transitioning to a detonation. However, the damage sustained was consistent with a deflagration. Discussion When considering the use of carbon adsorption systems consideration should be given to the potential for them to act as ignition sources. Typically large beds will be provided with temperature sensors to assist detecting hot spots where oxidation is occurring. Hot spots tend to form in localized zones; consequently a large number of temperature detectors may be needed to cover the entire volume. Alternatively, a more reliable method can be to monitor for combustion products such as COZ and CO [Ref. 31. The carbon beds involved in this incident did not have temperature sensors or any other form of monitoring.

In addition to the implications for systems with carbon adsorption systems, the Woodward and Lygate study [Ref. 21 illustrates how diurnal temperature cycling can create flammable atmospheres in the vent headers on air blanketed storage tanks (and also in the tanks). Consequently, appropriate protective measures such as flame arresters, or fast acting isolation valves, should be considered for vent headers on air blanketed tanks handling flammable liquids. Case History H1.4

Explosion in Lean Header

Exhaust air from a drier at a facility manufacturing a particulate solid contained volatile organic compounds (VOCs) and some combustible dust [Ref.4, page 231. As originally designed, the exhaust air was discharged directly to atmosphere. However, following the passage of the 1990 Clean Air Act, a thermal oxidizer was installed to eliminate VOCs from the exhaust air. Due to space limitations, the thermal oxidizer was located approximately 300 feet from the drier and a vent header was installed connecting it to the drier. Dust tended to accumulate in the header. To address this, the header was cleaned out on a semi-annual basis. After the system had operated satisfactorily for approximately five years, it was decided to conduct maintenance on the drier, keeping the thermal oxidizer running. While restarting the drier after the maintenance was complete, an explosion occurred in the vent header. The vent header and the thermal oxidizer were damaged and a fireball was discharged into 281

Safe Design and Operation of Process Vents and Emission Control Systems

the operating area. The accident investigation concluded that the most likely cause was that starting the drier caused a pressure pulse that disturbed the dust layer, forming a combustible dust cloud. This cloud then was ignited at the thermal oxidizer, causing a more violent secondary dust explosion in the header. Lessons Learned This incident demonstrates that explosion prevention by operating fuel lean has potential failure modes. Further it illustrates that even low concentrations of dust can create a hazard: 0 Operating fuel lean is an effective explosion protection method only as long as the system maintained below the combined lower flammable limit for all materials present (note: the combustion energy of dusts and gases are additive, see Section 4.18, Hybrid Mixtures). Dust present in concentrations well below the lower flammable limit can create an explosion hazard if they fall out of suspension and form a dust layer that could be re-entrained by a mechanical shock or a change in the air flow. Preventive Measures Potential preventive measures include [Ref. 41: 0 Installation of a particulate removal system between the drier and the thermal oxidizer. Design of an exhaust system with sufficient velocity to minimize dust accumulation in the ductwork. 0 0

More frequent cleaning of the ductwork. Explosion detection and suppression systems in the ductwork. Explosion vents in the ductwork.

Case History H1.5 Australia

Explosions at Coode Island Terminal, Mevaporourne

An explosion and fire occurred at a terminal comprising of 45 tanks with a total capacity of 12x106 gallons (45,000 m3). Fourteen of the tanks were totally destroyed and 18 others were damaged. It is believed that the ignition source was a lightning strike and that the combustion between tanks, propagated through the vent headers. Figures H-1 and H-2 show the facility before and after the incident.

288

Appendix H - Case Histories

Lessons Learned Vent headers can provide a path for fires and explosions to propagate between vessels. When large numbers of vessels are located in close proximity the potential for a major loss is increased. Preventive Measures Provide explosion prevention, e.g., operate with nitrogen inerting, or fuel rich. Consider providing fire breaks between multiple tanks.

Figure H-1:

Site ‘A’ Plan View Coode Island, Melbourne Before Incident

(Reproducedby permission of Terminals Pty. Ltd.)

289

Safe Design and Operation of Process Vents and Emission Control Systems

Figure H-2.

Coode Island - Explosion Damage

(Reproduced by permission of Terminals Pty. Ltd.) H2.

REACTIVE CHEMICAL INCIDENTS

Case History H2.1

Monomer Storage Tank Overpressure Event

A monomer storage tank had been in service for several years during which time normal practice had been to maintain its contents below a specified temperature. The vent header had a small nitrogen purge and by operating the tank at or below the specified temperature monomers did not condense and build-up inside the vent header. This information was lost over time, and for other operational reasons the temperature was increased. The inhibitor in the stored monomer was non-volatile; as a result material that condensed in the header did not contain any inhibitor. Several months after the tank temperature had been increased the header blocked. Pressure built up in the tank causing it to deform and crack at the bottomto-side weld, allowing liquid monomer to leak into the dike.

290

Appendix H - Case Histories

Lessons Learned The incident demonstrates the need to effectively managing vent header systems’ and the importance of recognizing the implications of phase changes on the availability of inhibitors, specifically: It is important to document the design basis for vent systems rather than relying on ”corporate memory”, or assuming the design basis will be obvious to future operating personnel. Phase changes, such as evaporation and condensation of a liquid, can result in an inhibitor being “lost”. Preventive Measures Operating documents were updated explaining the reason for operating the tank at the specified temperature. The nitrogen purge rate was increased to provide an additional safety margin. Case History H2.2 Header

Explosion Involving Reactive Materials Formed in a

A plant header system received vents from several sources, including units handling nitrobenzene, oxides of nitrogen, and ammonia. The header had been designed to drain; however, as a result of modifications an unrecognized low point had been created where liquid could collect. During normal operations, materials in the header are acidic. As a result of operating problems the header became alkaline. It is believed that under these conditions an unstable nitrite compound formed and collected in the low point. After the operating problems had been resolved, pH in the header returned to being acid. Ammonium nitrite is not stable in acid conditions as a result it decomposed violently, shattering the pipework with pieces being discharged over 400 feet. Lessons Learned The incident demonstrates the need to have strict management of change when modifying vent headers, and it is important to ensure potentially hazardous reactions have been identified, specifically:

’

Corporate memory is ten years long - T. Kletz. 29 1

Safe Design and Operation of Process Vents and Emission Control Systems

0

When modifying vent headers the work order should identify the important requirements, such as the need for the header to slope to drain, insulation requirements, etc. After completing the modification the installation should be inspected to confirm it was installed as specified. Appropriate personnel are needed when process hazards analyses are being conducted, for example research chemists may be required to evaluate reactivity hazards. When hazards, such as the potential to form unstable materials, are identified the information should be documented and made available to operating personnel.

Case History H 2 3 Runaway Reaction Caused Dioxin Contamination at Seveso, Near Milan, Italy

A runaway reaction in the trichlorophenol reactor at Icmesa Chemical Company, Seveso, caused the rupture disk to fail discharging a white aerosol cloud into the atmosphere. The incident occurred on Saturday 10* July 1976, approximately 7 ?hhours after the plant had been shut down for the week end. Included in the release was approximately 2 kg of the extremely toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin(dioxin). The immediate cause of the incident was a failure of operating personnel to follow procedures. Specifically, 50% of the ethylene glycol solvent should have been distilled off the batch, after which quench water should have been added and the agitator left running. In practice only 15% of the ethylene glycol was distilled off, no quench water was added, the agitator was stopped and the temperature recorder was shut off. At the time the temperature recorder was shut down it had been reading 158°C. Following the incident, the temperature was found to be 230°C. The temperature rise appears to have been caused by several factors including a previously undetected exothermic reaction at about 180°C and unexpected heat transfer from parts of the reactor heated by superheated steam. The rupture disk discharged directly to atmosphere without treatment, exposing personnel and nearby livestock to the dioxin. The normal reaction mass includes sufficient dioxin to be a concern. If it is exposed to high temperatures for a prolonged period, the amount of dioxin increases greatly.

292

Appendix H - Case Histories

During the days following the incident, there were extensive discussions between company personnel and local officials. Ultimately, after a period of approximately two weeks, personnel close to the plant were evacuated. Subsequently, as the severity of the incident became apparent, personnel were evacuated from a larger area. There were many wild animals and livestock deaths and although some people living nearby experienced severe chloracne and similar conditions, there were no human fatalities. The incident received extensive coverage throughout Europe and initiated a wide ranging Commission into the European Chemical industry. Subsequently, the EEC implemented the Seveso Directive which became the first regulation affecting process safety and the starting point for similar regulations elsewhere in the world. Lessons Learned 0

It is important to establish lines of communications and action plans with the local authorities and community before an incident occurs. The rupture disk should have been piped to a vent header and a knockout tank or quench pool. Vent headers should have treatment devices based on the worst case scenario, which should take into consideration the prolonged heating or conditions that could have been present prior to the release.

0

0

Testing for exothermic reactions must be thorough and capable of detecting mild exothermic reactions. Training and supervision must be in place to assure that operating personnel adhere to the operating procedures.

Preventive Measures 0

0

0

0

Establish contacts with local authorities and community, conduct "dry run" accident training. Install treatment device to ensure that in a worst case hazardous concentrations of extremely toxic materials will not be released to the environment. Ensure process materials have been adequately evaluated for exothermic reaction hazards. Establish training to ensure operating procedures are understood and adhered to. 293

Safe Design and Operation of Process Vents and Emission Control Systems

H3.

VACUUM FAILURES

Case History H3.1 Roof to Fail

Liquid in Vent Restricts Vacuum Relief Causing Tank

A facility pumped product to batch receiving tanks for analysis before it was transferred to the storage tank. To minimize vapor releases and the associated odor when transferring between the tanks, it was decided to install a new vent header that would allow vapors to "balance" between the tanks rather than vent to atmosphere. Each tank had its own emergency relief valve for overpressure relief. A single conservation valve was mounted on the vent header to provide vacuum relief. Existing equipment obstructed the proposed new vent header and to address this, the header was rerouted creating a low point between the storage tank and the vacuum relief. The tanks operated satisfactorily with this arrangement until one of the receiving tanks was overfilled. Liquid from this tank flowed into the vent header filling the low point. Several hours later while liquid was being transferred out of the storage tank its conical roof collapsed. On investigation, it was shown that the low point in the vent header had created a 12-inch liquid seal. The pressure drop caused by this seal exceeding the "vacuum" rating of the tank causing the roof to fail. Lessons Learned It is important to ensure vent headers have adequate drainage. When a new header is installed, or an existing one is modified, it should be closely examined to confirm it has been installed in accordance with the design. Preventive Measures The vent header was rerun to provide a slope for draining and to eliminate low points. Emergency vacuum relief valves devices were installed on each tank to protect against the vent header being restricted.

294

Appendix H - Case Histories

H4.

REFERENCES

H1. Chung, D. Explosion and Fire at Powell Duffryn Terminals, Savannah, Georgia. LPS 2000, AIChE Proceedings of 34thAnnual Loss Prevention Symposium. American Institute of Chemical Engineers, New York, New York. H2. Woodward, J. and Lygate, J. 2002. Establishing Ignition Conditions for the Tank Manifold Fire at the Powell Duffryn Tank Terminal. Progress Safety Progress, Volume 21, No. 3. New York, New York. American Institute of Chemical Engineers.

A. A. Naujokas Spontaneous Combustion of Carbon Beds. Page 126, H3. Plant/Operations Progress, Volume 4, No. 2. New York, New York. American Institute of Chemical Engineers.

T. J. Meyers, et. al. Fires and Explosions in Vapor Control Systems: A H4. Lessons Learned Anthology. LPS 2002. AIChE Proceedings of 34thAnnual Loss Prevention Symposium. American Institute of Chemical Engineers, New York, New York.

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by Center for Chemical Process Safety Copyright 02006 John Wiley & Sons, Tnc.

APPENDIX

I HISTORICAL PERSPECTIVE ON AIRPOLLUTION CONTROL HISTORICAL BACKGROUND ON AIR POLLUTION From the beginning of the Industrial Revolution through the mid-l900s, it is a fair criticism to note that industry in general did not have protection of the environment or public health as a sigruficant philosophical or operating concern. In more recent times, the refining, chemical processing, and related industries have been required to meet higher societal expectations in the form of new laws and regulations. The business and industrial community has become increasingly aware of the need to ensure their right-to-operate in the eyes of the general public and the specific communities in which they operate. Air pollution from human activities began long before the Industrial Revolution. To be sure, the advent of the Industrial Revolution resulted in increasingly larger scale manufacturing and industrial operations that sigruficantly increased the air pollution levels. These operations required increasingly larger sources of energy, usually from the burning of fossil fuels. They released gases, vapors, and particulates to the atmosphere. Process facilities were frequently concentrated geographically, thus creating new or exacerbating existing local and regional air pollution problems. In time, power generation from the burning of coal and the emissions from ore processing, metal foundries and mills, cement plants, glass manufacturing, chemical plant processes and other industries became the dominant sources of air pollution. Current United States environmental laws and regulations to improve air quality have resulted from a lengthy list of historical and more recent air pollution events. 11.

297

Safe Design and Operation of Process Vents and Emission Control Systems

In London, England, air pollution was a serious problem beginning as early as the 1300s as low grade coal replaced wood for heating and cooking. Major air pollution events occurred through the 1950s. A few of the milestone events are: 0 In 1306, major smoke and soot pollution prompted King Edward I to proclaim a ban on burning sea coal in London. 0 In 1873, a particularly dense coal-smoke saturated fog in London resulted in an estimated 268 deaths. 0 In 1909, winter inversions and smoke accumulations in Glasgow, Scotland lulled over 1,000 persons. In a report about the incidents, Dr. Harold Antoine Des Voeux coined the term "smog" as a contraction for smoke-fog. 0 In 1952, a severe sulfur-laden fog killed an estimated 4,000 Londoners and spurred Parliament to enact the 1956 Clean Air Act to reduce coal burning and begin serious air-pollution reform in England. In the United States, concern for the air quality in and around large cities was increasing during the latter 1800s and resulted in local laws and regulations followed ultimately by federal air pollution control regulations. Some of the noteworthy events included [Ref. 1 and 21: 0 By 1881, a few cities, such as Chicago and Cincinnati, enacted limited municipal smoke abatement laws and regulations to reduce smoke and ash from factories, railroads, and shps. In 1928, the United States Public Health Service began checking air pollution in eastern cities and reported that sunlight was reduced by 20 to 50 percent in New York City. In November 1939, the city of St. Louis experienced nine days of extreme smoke air pollution with near zero visibility at midday even with street lights on. City officials and community, business, and industry leaders developed and implemented controls and regulations; St. Louis was the first major U. S. city to limit the use of soft, low quality coal. 0 During the late 1940s, serious smog incidents in Los Angeles further heightened public awareness and concern about this issue.

Appendix I - Historical Perspective on Air Pollution Control

In 1948, an air pollution inversion event in Donora, Pennsylvania, killed 20 people and sickened about 40 percent of the town's 14,000 inhabitants. In November 1953, a smog incident in New York City resulted in the death of between 170 and 260 people. In 1963 and 1966, regional weather patterns resulted in air inversions that trapped local air pollutants in the New York City area, resulting in 405 and 168 deaths, respectively. More recently, international signal events involving toxic chemical releases at Bhopal, India and Seveso, Italy brought an even sharper focus on prevention of catastrophic releases and their impact on people and the environment. BRIEF REVIEW OF LAWS AND REGULATIONS I2. These and other air pollution events led the U. S. Congress to pass the Air Pollution Control Act of 1955 that established the federal government as having preeminent control over air pollution control matters. More important for the subject matter of t h s book, the Clean Air Act amendments in 1967 (also called the Air Quality Control Act) required the setting of national emission standards for pollutants. These emission standards were applied across the country to all stationary sources and recommended some control technologies. The setting of one common standard for each listed pollutant triggered decades of debate between those insisting on a monolithic singular approach to regulating air pollutants and those favoring the more pragmatic approach of regulating on an industry-byindustry basis. In 1970, Congress re-wrote the original Clean Air Act adding these major features: Established National Ambient Air Quality Standards for the most hazardous high volume pollutants, called "criteria" pollutants: - Airborne particulates (PM) Sulfur oxides (SO) Carbon monoxide (CO) - Nitrogen oxides (NO) - Ozone(0) - Lead(Pb)

299

Safe Design and Operation of Process Vents and Emission Control Systems

Established New Source Performance Standards (NSPS) to regulate emissions from new facilities. Required identification of "other" hazardous air pollutants (HAPs) and development of standards to reduce their emissions Empowered the newly created Environmental Protection Agency (EPA)to set these standards. These latter two points are noteworthy since they required the EPA to significantly reduce day-to-day "routine" emissions of those air pollutants known or suspected to cause serious health problems.

In the 1970s and 1980s, the EPA attempted to regulate air pollutants using the mandated chemical-by-chemical approach based on health risk. There were numerous legal, scientific, and policy debates over w h c h pollutants to regulate and how stringently to regulate them. Debates focused on risk assessment methods and assumptions, the amount of health data needed to justify regulation, analyses of costs to industry, and benefits to human health and environment. This risk-based decision process ran into the inevitable risk quandary question - what level of risk is acceptable or "how safe is safe". The regulatory process proved difficult and minimally effective at reducing emissions. During the 20 years preceding 1990, the EPA was only able to implement regulations for seven hazardous air pollutants: asbestos, benzene, beryllium, inorganic arsenic, mercury, radionuclides, and vinyl chloride. Collectively, the EPA estimates that these seven standards cut annual air toxics emissions by an estimated 125,000 tons. A new strategy was adopted by Congress with the passage of the Clean Air Act of 1990. EPA was directed to use a "technology-based'' and performance oriented approach to significantly reduce emissions from major sources of air pollution. Section 112(b)of this act established a list of hazardous air pollutants (HAPs). The current list of these Hazardous Air Pollutants (HAPs) contains 188 chemicals or groups of chemicals that are identified in Table 1-1. The 1990 act required EPA to develop regulations termed National Emission Standards for Hazardous Air Pollutants (NESHAP). EPA was directed to identify the principal source industry sectors and develop regulations for each, called Maximum Achievable Control Technology (MACT) standards. These standards require the covered facilities to meet specific emission limits based on levels already being achieved by similar emitting sources in that industry sector. 300

Appendix I - Historical Perspective on Air Pollution Control

The 1990 act also further strengthened the National Ambient Air Quality Standards for the "criteria" pollutants established in 1970 particularly regarding the ozone precursors, NOx and Volatile Organic Compounds (VOCs). Much of this authority was delegated to the states to allow regulatory control specific to the local and regional needs for "criteria" pollutant reductions. 13. IMPROVED AIR QUALITY Air pollution data collected by EPA indicates that t h s new "technologybased" approach has produced real, measurable reductions. EPA periodically reports the levels of the criteria pollutants in the air and the amounts of emissions from various sources to see how both have changed over time and to summarize the current status of air quality. These air quality trends are generated using measurements from monitors located across the country. Table 1-2 shows that the air quality based on measured concentrations of the principal air pollutants has improved and that reported emissions for these pollutants have been significantly reduced nationally over the 20-year period 1983 - 2002 [Ref. 31. Based on the 1996 National Toxics Inventory data, those industry sectors defined as Major Sources accounted for about 26 percent of air toxics emissions, smaller Area Sources and other sources (such as forest fires) for 24 percent, and Mobile Sources for 50 percent. Accidental releases, which obviously contribute air toxics to the atmosphere, are not included in these estimates. Clearly, the processing and related industries for which this book is intended are major contributors to airborne pollution in the U. S., although they are not the largest source.

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Table 1-1. Current Hazardous Air Pollutants - HAPS as defined in Clean Air Act of 1990, Section 112(b)

15 16 17 18 19

20 21

22 23 24 25

26 27 28

29 30 31

302

I

1746016 95954

1

88062 94757 51285 121142 95807 584849 53963 532274 79469 91941 119934 119337 101779 101144 534521

I

2.4.5TkhhbroDheWl 2 4 GTnchbrophenol 2 4 Q sdts and esters 2,4-[)lnibophend 2,4-[)lnibutoluene 24-Toluene dimine 7 2,CToluenediisocymale 2-Acetylminoflwrene 2-ChbmaQlopheme Z-Nitopmpae 3,8Mbroter&ene 3,SDimMxybedine 3,J-Dimethylbewkline 4,4--Mebpwdimiline 4,CMethpne bis(2chioromiline)

Appendix I - Historical Perspective on Air Pollution Control

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Safe Design and Operation of Process Vents and Emission Control Systems

109 110 111

125 126 127 128

304

I

I

76148 118741 a7683

0 108394 0 67561

I I

I

I I

Heptachbr Hexachbrobenzene

mCml ~rurryCornpourck Methanol

Appendix I -Historical Perspective on Air Pollution Control item No.

CAS No. 62759

ChemicalName N-NitrosodimeVlylarnine N-Nitrosomrpholine N NitrosoN-methylurea

90340

oAiisldine

95487 152

95534 95476 56382

153 154

I

169

I

oToludiw

I

Paiathlon

1

593602

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Safe Design and Operation of Process Vents and Emission Control Systems

Notes: For all listings above which contain the word "compounds" and for glycol ethers, the following applies: Unless otherwise specified, these listings are defined as including any unique chemical substance that contains the named chemical (i.e., antimony, arsenic, etc.)as part of that chemical'sinfrastructure. 1. XCN where X = H' or any other group where a formal dissociationmay occur. For example KCN or Ca(CN)2 2. Includes mono- and di- ethers of ethylene glycol, diethylene glycol, and triethylene glycol R-(OCH2CH2)n-OR where: n = 1,2, or 3 R = alkyl or aryl groups R = R, H or groups which, when removed, yield glycol ethers with the structure:R-(OCH2CH)n-OH.Polymers are excluded from the glycol category. 3. Includes mineral fiber emissions from facilities manufacturing or processing glass, rock, or slag fibers (or other mineral derived fibers) of average diameter 1micrometer or less. 4. Includes organic compounds with more than one benzene ring, and which have a boiling point greater than or equal to 100 C. 5. A type of atom which spontaneouslyundergoes radioactive decay.

Table 1-2. Improvement in Air Quality and Reduced Emissions 1983-2002

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Appendix I -Historical Perspective on Air Pollution Control

Note: Negative numbers indicate improved air quality or reduced emissions. Positive numbers indicate worsened air quality . . or increased emissions. NA: Trend data not available. Not statistically significant. a: Based on percentage change from 1999. b: Includes only directly emitted particles. C: Based on percentage change from 1985, prior estimates uncertain. d: Lead emissions are for 1982-2001. e:

14. 11. 12.

13.

REFERENCES Urbinato, D. Summer 1994. Taking Tomics Out of the Air. Washngton D.C. EPA Journal. Fleming, J. R., Knorr, B. R. 2003. History of the Clean Air Act. American Meteorological Society website (wwn;.aiiictsoc.org) ; EPA Office of Ecosystems and Communities website (www.eDa.gov/ecocommunity) U. S. EPA Office of Air Quality, Planning and Standards. August 2003. Latest Findings on National Air Quality - 2002 Status and Trends. EPA DOC.454/K-03-01.

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Abbreviations, 203-205 Absolute pressure, 46, 65 Absolute temperature, 46, 5 I Absorbers, 146 Absorption, treatment and disposal system: advantages and disadvantages of, 155-156 characterized, 135-136, 153-154 defined, 207 monitoring, 153 packed bed scrubber, 1 5 6 155 spray towers, 154 tray towers, 154 Venturi scrubbers, 154-1 55 Accidental releases, 79, 301 Acetaldehyde, 42 Acetylene, 36, 55, 103 Acrolein, 119 Acronyms, 203-205 Action plan, 1 12 Activated carbon, 156-157 Acute Exposure Guideline Levels (AEGLs), 66, 191 Adhesives, 5 Adiabatic flame temperature, 207 Administrative control, boundaries of, 13 Adsorption, in treatment and disposal system: advantagesidisadvantages of, 157 characterized, 135-136, 156-157 defined, 207 Aerosols, 43-44 Agitatiodagitators, 28, 63, 137, 145 Agricultural chemicals, 5 Agron, 97 Air-cooled: finned-tube coolers, 135

308

heat exchangers, 160, 166 Air dilution flow rate, 91 Air dispersion modeling, 190 Air emissions, HON rule applications, 3 1-32 Air leakage, 106, 169 Air links, I 19 Admethane mixtures, 97 Air pollution:: control systems, 143 federal regulation of, 9-10, 30, 75 historical perspectives, 74-75, 297-299 improved air quality, 301,306 laws and regulations, 299-301 Air Pollution Control Act (1955), 76, 299 Air purging, principles of, 92-95 Air quality, 75,78-79,30 1,306 Air Quality Control Act, 76, 299 Air toxics, 30, 79 Alarm systems, 67 Alumina, activated, 156 Ambient air, 90, 96 Ambient pressure, 37,46, 65 Ambient temperature, 39 American Industrial Hygiene Association (AIHA), 66-67 American Institute of Chemical Engineers (AIChE): accomplishments of, 1 Center for Chemical Process Safety (CCPS), 1 , 3 Design Institute for Emergency Systems (DIERS), 3 4 , 6 4 Process Piping, 120 Ammonia, 38, 68, 119 Anchored headers, 128 Anhydrous toxic materials, 119

INDEX

An Overvieit, qf Eqiripment f o r Containment and Disposal of Emergency Relief Effluents, 150 API Standards: 52 I , Guidefor Pressure Relieving and Depressuring System, 126, 149- 150, 162. 168, 170 537, Flare Detailsfor General Refiner), and Petrochemical Service, 167 620, 15 650, 15 Aqueous-based solvents, 23 Aqueous solutions, fuel lean systems, 91 Arrester(s), see specific types of arresters Arsenic, inorganic, 77 Asbestos, 77 ASME Boiler and Pressure Vessel Code (BPVC): Appendix M-6, 129-1 30 B31.3, 3,120, 123 overpressure relief devices, 140 Parts UG 125-137, 16 Part UG- 127, 12 1 Pipingfor Categovy M Fluid, 26 Process Piping, 120, 123 Section UG 135, 129 Section VIII, 15 Asphyxiation, 25 ASTM E68 1,39 Atmosphere, flammable, 138, 142 Atmospheric dispersion, 177-178, 190 Atmospheric pressure, 46, 96, 120, 168 Atmospheric release, 145 Atmospheric storage tank, 61, 207 At-source treatment, 79-82, 86 Autoignition temperature, 207 Auto-refrigerated materials, 126 Back-flow, 102, 162, 172, 188 Back-pressure, implications of, 22, 88, 125-126, 147, 152, 162-163, 178, 188 Back-up electrical power, 93 Batch proccsses, 23 Batch reactors, 138, 142 Batch venting, 143 Beds: carbon, 157 in treatment and disposal systems, 135

309

Benzene, 38, 77 Beryllium, 77 Blanketing gas, 234 Bleach, 23 Blowdown, generally: drums. 141-142,207 systems, 2 tanks, 80-81, 135, 141-142 Blowers, in flammable service, 168 Boilers, 135, 144, 174 Boiling point, treatment and disposal process, 134 Branch headers, 129 Breakthrough capacity, I57 Brine coolants, 159 Brittle fracture, 120 Bubble cap trays, 154 Building codes, 89 Build-up solids, 14 Bulk mixing, 28 Bureau of Mines, 36, 39 Burgess-Wheeler Law, modified, 3 9 4 0 Burning material, 149 Bum pits, 135, 169 Burnt gases, 109 Burst disk indicator, 200 Butane, explosion prevention, 107 By-products, 28, 168 Capacity, adsorption process, 157 Carbon: activated, see Activated carbon adsorbers, 135, 158 adsorption, 2 8 6 2 8 7 adsorption bed, 94. 1 17-1 18, 152 beds, 105, 157 characterized, 1 17 drums, 157 Carbon dioxide, 36,44, 165 Carbon monoxide, 26, 40-41, 173, 299 Carbon steel equipment, 103 Case histories: combustion incidents, 283-289 reactive chemical incidents, 29Ck293 vacuum failures, 294 Catalyst bed temperature, 172- 173 Catalytic oxidizers, 135, 172-175 Catch tanks, 120, 135, 139-140, 142,208 Category M fluid service criteria, 120

310

INDEX

Change management, 202 Checklists: Hazard Analysis, 187--188 vent header design, 225-23 1 What-If, I84 Chemical compatibility, 141. 145 Chemical industty, vapor-liquid gravity, 146 Chemical interactions, in reactive systems, 6 9 74 ~ Chemical isolation, 25, 2 0 6 2 0 1 Chemical plants, air pollution regulations, 30 Chemical Process Safety Fundamentals with Applications, 177 Chemical properties, merging streams and, 87 Chemical reactions, 187 Chemical Reactivity Worksheet, 73 Chemical suppressant system, 66, 89 Chlorine, 23, 68, 153, 165 Clean Air Act (1990), 29-30,75-77, 183, 296,300 306 Clean Air Act ( 1 993), 2 15 Cleaning materials/methods, 28, 63, 198-1 99 Cleaning requirements, 88, 120 Closed low point drains, 127 Coal, as energy source, 75, 297 Coal mines, 35 Coated mist eliminators, 15 1 Coatings, 5 Code for Process Piping, 86 Code of Federal Regulations (CFR), 2 16 Code requirements, 19 Cold liquids, discharge of, 86 Cold process flow, I26 Collection, in treatment and disposal system: advantages of, 147-148 blowdown dnims and tanks, 141-142 catch tanks, 135, 139-140 containment, 137-139 defined, 135 disadvantages of, 147-148 dump tanks, 135, 139-140 importance of, 136 quench dnims, 142-145 quench pools, 138, 145-147

with venting, 139 Combined vent header systems, characteristics of. 16 Combining vent streams, hazards associated with, 21L22 Combustibility, 23- 25 Combustible ducts, 43 Combustible liquid, 65, 208 Combustion: by-products of, 175 chamber, dimensions of, 38 cool flame, I06 defined, 208 effects of, 189 explosion prevention strategies, 89 flammable materials, 97 ground flares. 168, 175 oxidation reactions, 172 propagation of, 39 technology, see Combustion technology thermal oxidizers, 174 Combustion technology: characteristics of, 35 flammable limits, 36-44 Common mode failures, in Hazard Analysis, 186, 189 Community proximity, emergency vent header systems, 15,66 Compatibility issues. 2 1 Compatible inert gases, 99 Compatible vent header systems, 87 Complex vent header systems, 10-1 1,86 Composition: combining vent streams, 80 emergency vent header system, 15 in Hazard Analysis, 186 normal process vent header system, 14 in treatment and disposal process, 134 Compression, 159 Compression heating, 47 Compressors, gas recovery systems, 160, 162-163 Concentration, carbon adsorption and, 158 Condensation: characterized, 22, 62-63 containment methods and, 144 knock-out drums and, 150 prevention of, 122 in treatment and disposal systems, 135

INDEX

Condensers, 80. 86, 125 Condensing systems, 159-160 Confinement, toxic gas releases, 26 Connections, flexible, 128-129 Consequence analysis: importance of, 176-1 77, 183-1 84 techniques, 189-192 Conservation vents, inspection and maintenance of, 200 Construction materials: containment methods and, 145 flare system design, 17 1 influential factors, 86, 120 inspection and maintenance, 199 packed-bed scrubbers, 155 Containment, in treatment and disposal systems: defined, 135 design and safety considerations, 138-139 external, 138 importance of, 127, 137 in original vessel, 137 Contamination, 62 Continuing reactions, 142 Continuous processes, 23 Continuous venting, 143 Controls: in Hazard Analysis, 186 inspection and maintenance of, 201 Control valves, access to, 120 Coode Island Terminal (Mevaporourne, Australia) explosions, 288-290 Coolants, in condensing systems, 159 Cool flame combustion, 36,42, 106 Cooling: coils, 137 containment methods and, 144 evaporative, 138 knock-out drums and, 150 process, 159 system, failure of, 28 tower plumes, 94 water, 69, 12 Corrosion: accelerated, 149 implications of, 69, 125, 195, 197, 199-200 inspection and maintenance, 201

311

mechanical design and, 125 reactive systems, 122 Corrosive chemicals, 17 1 Corrosive gases, 16,22 Corrosive liquids, 178 Corrosive materials, merging streams and, 88 Corrosivity: design basis, 85-86 implications of, 208 merging streams and, 88 Credible scenarios, 137 Cyclic pressure, explosion preventiodprotection, 1 10 Cyclones, 135 Cyclone separators, 15 1-1 53 Damaged expansion joints, 128 Dead load, 124 Decision trees, 135 Decomposition, 22, 36, 68-69, 197 Decontamination methods, 122, 199 Defective low point drains, 127 Deflagration: cell size and, 52 characterized, 4 , 4 5 4 6 , 65, 89 containment methods and, 138, 140 defined, 208 explosion protection, 108-1 10 explosion venting, 1 10 flames, 4&47,51, 113-1 I4 internal, 142, 152 Deflagration and Detonation Flame Arresters, 3, 112,200 Dejlagration Containment (DPC) for Vessel Safe@ Design, 140 Deflagration to detonation transition (DDT), 4 8 , 5 1,53,65 Depressurizing, two-phase venting, 64 Design approach: case illustration, 208 design basis, 85-86 flammable materials, 88-1 18 mechanical design, 123-130 merging vent streams, 86-88 reactive systems, 121-123 toxic gases, 118-121 Design Institute for Emergency Relief Systems (DIERS), 3 4 6 4

312

INDEX

Design philosophy: combining vent streams, hazards associated with, 21-22 combustibility, 23-25 design sequence, 19-21 flammability, 23-25 inherent safety, 2 1, 23 reactivity, 27-29 regulatory issues, 29-32 toxicity, 25-27 Design vent flow, 129 Destruction efficiency, environmental requirements, 134 Des Vouex, Dr. Harold Antoine, 75 Detectors, flammable gas, 91, 95-96 Detonation: arresters, 25, 45. 66, 93; 89, 115 cell size, 51 cell widths, 55--56 characterized, 4, 46-48 compared with deflagration, 52 containment methods and, 138 defined, 208 deflagration to detonation transition (DDT), 4 8 , 5 I , 53 flame arresters, 1 15 in-line arresters, 114-1 15 limits, 49 run-up distance, 48, 5@5 I , 54-55 Dew point, 99 Dienes, 122 Dilution air, 93-94, 201 Dioxin, 282-283 Dip leg header inlet, minimum height, 169 Dip pipe, 147 Direct contact condensers, 159 Dirty gases, 167 Discharge, generally: duration and frequency of, 133 stream, external containment of, 138 velocity, atmospheric dispersion, 177-179 Dispersion: analysis, 176 containment methods and, 147 hazardous conditions, 66 influential factors, I5 modeling, 183, 189, 191, 209 toxic gases, 26-27

of vent gas, see Vent gas dispersion Disposal, emergency relief vent discharge, 15

Distillation column, with pressure control, 234,241 Distillation systems, 13 Double block, 120 Double pump seals, 32 Downstream, generally: condensing, 143 equipment, 87 vent header systems, 15 1, 153 Drain leg, dispersion tank design, 178 Drains, 127, 197, 201 Drums: blowdown, 141-142,207 carbon, 157 knock-out, 149-151, 169, 171-172, 174 quench, 142-145,212 seal, 162, 169-172, 201 in treatment and disposal systems, 135 Dump tanks. 135, 139-140, 142,209 Dust, 43,62-63, 135 Dye penetrant testing, 128 Economic considerations: atmospheric dispersion, 179 treatment and disposal systems, 158~-160, 162 Ecotoxicity, significance of, 134 Effluent, 23, 26,76, 145, 177 Electrical power, back-up, 93 Electric power plants, air pollution regulations, 30 Electrostatic forces, 156 Electrostatic precipitators, 135 Elevated flares, 135, 167-1 68 Elevated stacks, in treatment and disposal systems, 27, 135 Embrittlement, 126 Emergency discharge, 133 Emergency Exposure Guidance Levels (EEGL), 192 Emergency operations, 2-3 Emergency planning, 19 1 Emergency pressure relief systems, 145 Emergency releases, 30, 1 4 6 1 4 7 , 168 Emergency relief: device, 74

INDEX

headers. 110 systems, 3, 153 Emergency Relief System Design Using DIERS Technologv, 64, 86, 12 I Emergency response training, 67 Emergency Response Planning Guidelines (ERPGs), 66-67, 191 Emergency vent header systems: atmospheric dispersion, 177-1 78 characteristics of, 15, 60, 124, 143, 147, 153, 162, 176 collection methods, 139 combined with normal vent header systems, 16, 18 defined, 13 design considerations, 19 equipment with, 17 flows, 95 flare system design, 17 1 hazard scenarios, 60-62 interconnections, 88 opening pressure, 1 19 operations, 195 relief streams, 134 safety requirements, 248 Emissions, 2, 1 1 , 30, 32, 77, 299-300, 306 Enclosed ground flares, 168 End-of-line, generally: crimped metal flame arresters, 1 13 flame arresters, 1 1 1- 1 12 treatment, 2,82: 88, 157 End-of-pipe post treatment releases, 9 End-of-pipe treatment, 87, 162-163, 169 Endurance bum test, 115 Engineering: analysis, 137 design, 19 reviews, 176 Engineering of ReliefDisposal-A Review Paper, 135 Entrained liquid, 125, 127, 178 Entrainment, defined, 209 Entrainment separation/separator, 80,86, 120, 122, 125, 127, 151, 153 Environmental air pollution control regulations, 9 Environmental considerations, operations requirements, 195

313

Environmental laws and regulations, 30, 75, 165, 169, 183 Environmental Protection Agency (EPA): air pollution regulation, 77 Chemical Reactivity Worksheet, 73 Clean Air Act, 29, 77778 fuel rich vent header systems, 106 functions of, 300-301 Hazardous Organic National Emission Standards for Hazardous Air Pollutants, 3 1 National Emission Standards for Hazardous Air Pollutants (NESHAP), 7 8 , 2 15 Risk Management Program Rule (40 CFR Part 68), 29 Environmental reviews, 176 Equipment: auxiliary, 167 design basis, 86 explosion prevention, 102 explosion protection, 109 failure, 15, 127 leaks, 32 malfunctions, 2 1 potential ignition source, 1 I7 pressure/vacuum in, 69 reactive systems, 122 reliability, 21 significance of, 13, 17, 142 thermal and catalytic oxidizers, 173 Erosion, inspection and maintenance, 201 ERPG-lIERPG-2,67 Ethane, 55 Ethene, 53 Ethers, 122 Ethylene, 38, 55 Ethylene oxide, 36, 103, 119 Existing operations, consequence analysis, 191 Exothermic polymerization, 137 Exothermic reactions, 139, 234 Expansion: bellows, 126, 128 joints, 128-129, 202 Explosion: causes of, 10,61, 127, 174-175, 188, 190 containment, 65-66 defined, 209

314

INDEX

Explosion (continued) internal? 170 isolation, 1 1 1-1 17 prevention, 2, 25, 65-66, 88-90, 200, 209,248 protection, 25, 65-66, 88, 08-1 17, 209, 234 reduction strategies, 23 relief panels, 201 reliefvents, 66, 24: 109-1 10 suppression, 25 vent header, 2 1 Exposures, toxic, 61 External expansion joint guides, 128 External fire, 142 External total containment. 138 Failure: common modes of, 2 1 emergency vent header system, 15 expansion joints, 128-129 Failure Mode and Effects Analysis (FMEA), 184 Fans, 105, 1 17 Fast acting valves, inspection and maintenance of, 200-20 1 Fault Tree Analysis (FTA), 184 Federal Register, 2 16 Federal regulations, 165, 171, 177 Filters, 135 Fire(s): characterized, 13, 142 codes, 89 combustion sources, 24 damp, 35 exposure to, 140, 144, 187 external, 15 prevention, 2 reduction strategies, 23 sources of, 10, 61, 127, 157, 175, 188 two-phase venting, 64 undetected, 197 Fire Protection in the Chemical, Petrochemical, and Hydrocarbon Processing Industries, 62 Fire Risk Assessment, 62 Fire Triangle, 209-2 10 Flame, generally:

acceleration, 65 arresters, see Flame arresters propagation, 38,40-41,45, 97 radiation heat, 45 speed, 2 10 temperature, 39 Flame arresters: advantages of, 1 12 characterized, 25,45, 63, 66, 89, 157, 174, 198,200-201 defined, 2 10 disadvantages of, I I2 end-of-line, 11 1-1 12 explosion prevention, 117-1 18 explosion protection, 94, 108-1 09 in-line, 11 1-1 13 in-line deflagration, 1 I 1, 1 13 in-line detonation, 1 1 1, 1 13- I 15 Flame-front propagation, 1 10 Flammability: analyzers, 91 deflagrations, 45-46 design basis, 86 detonation phenomena, 47-56 diagram of, 89 explosion prevention, 88-90 explosion protection, 91 flammable limits, 36-44 implications of, 22-25, 134, 176 limiting oxidant concentration (LOC), 44-45 pressure piling, 4 6 4 7 Flammable gas analyzersidetectors, 201 Flammable gases, 16, 24-25, 43,95-96, 210 Flammable limits: combined gas streams, 42 cool flame, 42 defined, 2 10 hybrid mixtures, 43-44 influential factors, 38 mixture stoichiometry, 36-38 pressure, effects on, 40-42 temperature, effects on, 3 9 4 0 types of, 3&44 variability, 39 Flammable liquids: characterized, 24-25, 178, 21 1 truck loading example, 262-268

INDEX

Flammable materials: combustion technology, 35 handling with valves, 237-238 in streams, 134 Flammable mixtures, 14, 36, 38 Flammable vapor: defined, 2 11 operating fuel lean example, 253-260 operating fuel rich example, 259-264 Flange gasket failure, 187 Flare knock-out drum, 149-150 Flares: assembly, 167 characterized, 2,45, 94, 117, 127, 146, 152 combustion products, 189 thermal radiation, 192 in treatment and disposal systems, see Flares in treatment and disposal systems Flares in treatment and disposal systems: advantages of, 1 6 6 167 characterized, 135, 165--166 design considerations, 168-169, 171-172 flame failure, 166 safety considerations, 171-172 system components, 167 types of, 167-171 Flashback, generally: arrester, 2 1 1 prevention, 45, 93-94, 99, 157, 167-168, 172, 174-175,211 Flashpoints, 35, 91, 141, 21 1 Flow, generally: capacity, 10, 125 design basis, 85 merging streams, 88 path, gas recovery systems, 163 rate of, see Flow rate resistance, 151-152 restrictions, 1 12, 153, 188 Flow rate: atmospheric dispersion, 177-1 78 coolant, 159 explosion prevention, 102 flare system design, 17 1-1 72 gas recovery systems, 162 in Hazard Analysis, 186

315

significance of, 14-15,22,72, 133, 152, 155 thermal destruction, 174-1 75 Flue gas, 174 Fluorine, 165 Foamy liquids, two-way venting, 63 Formaldehyde, 68 Fouling, oxidizers, 174 Freezing, 149, 151, 159, 166 Frequency, defined, 2 1 1 Frictional heating, 65 Fuel, generally: combustion properties, 35,46 flammable range, 36 Fueliair mixture, 101, 108, 119, 169 Fuel gas(es), 25, 106, 135, 160, 171 Fuel lean vent header systems: advantages of, 96-97, 106 defined, 89 disadvantages, 9 6 9 7 explosion prevention, 90 toxic gases, handling strategies, 120 Fuelioxidizer mixture, detonation limits, 48 Fuel rich vent header systems: advantages of, 106 disadvantages of, 106 operation principles, 89, 104105 toxic gas leaks, 1 19 Fugitive emissions, 30, 32 Gasiair mixture, 92 Gas(es), generally: bubbles, 63 collection methods, 139 combustion property, 24 cooling, 146 constant, 46 containment methods, 14 1 evolution, 63 expansion, 46, 140 flammable, 22,43-44,64-66, 2 10 hazardous, 176, 178 liquefied, 126 non-condensable, 138, 145-146 recovery of, see Gas recovery systems releases, normal routine, 5 specific heat, 39 toxic, 213 in vented streams, 134

316

INDEX

Gas-liquid contact, absorption systems, 153-154 Gas phase detonations, 47 Gas recovery systems: characterized, 135, 160 conceptual design, 16 I design considerations, 162- 163 safety considerations, 162-1 63 Gas streams, incompatible, 87 Gas supply, inert, 99 Gaskets, 199 Geographic separation, 88 Glossary, 207-21 3 Ground flares, 135, 167-168 Guide for Pressure Relieving and Depressuring Systems, 142 Guidelines for Chemical Process Quantitative Risk Assessment? 184 Guidelines for Chemical Reactivity Evaluation and Application to Process Design, 135 Guidelines f o r Consequence Analysis of Chemical Releases, 189, 19 1 Guidelines f o r Design Solutions for Process Equipment Failures, 187 Guidelines for Engineering Design for Process Safety, 142, 144 Guidelines for Hazard Evaluation Procedures, 184 Guidelines for Post-Release Mitigation Technology in the Chemical Process Industry. 3 Guidelines for Pressure Relief and EfJuent Handling Systems, 3, 86, 12 1, 124, 147, 177 Guidelines f o r Use of Vapor Cloud Dispersion and Source Emission For Accidental Releases, 177 Guidelines f o r Vapor Release Mitigation, 3 Handbook of Separation Process Technology. 153 Hazard analysis: importance of, 137, 183-184 methods of, 184-1 85 process, see Hazard Analysis Process (HAP) Hazard Analysis Process (HAP):

consequences, development of, 188-1 89 hazard scenario risk, 189 identification of causes, 186-1 88 overview of, 185-186 Hazard and Operability (HAZOP) Study, 184,211 Hazard Identification (HAZID), 27-28, 184 Hazardous Air Pollutants (HAPS): current listing, 292-296 emissions standard, 77 establishment of, 30 Hazardous Organic National Emission Standards, 31-32 identification of, 290 major sources, 30-3 1 , 79 minor sources, 30-3 1 mobile sources, 30, 79 non-point sources, 30 point sources, 30 regulations, 2 15-2 16 Hazardous discharge, sources of, 124, 188 Hazardous liquids, 178-179 Hazardous materials, I , 29-30, 87, 127, 134, 146, 153 Hazardous Organic National Emission Standards (NESHAP), 3 1-32 Hazardous properties, treatment and disposal process, 134 Hazards, 10, 68. See also Hazard Analysis; specific types of hazards Hazard scenarios, identification of, 60 Heat, generally: exchangers, see Heat exchangers generation, 234 input rate, 63 shield, 45 tracing, 122, 125,202 transfer fluids, 69 Heaters, 135, 158, 160, 174-175 Heat exchangers, 146, 159-1 60, 166, 187 Heating coils, 137 Heating equipment, 142 Heating system, in physical separation process, 15 1 Heel capacity, 157 Helium, 44,97 High melt point material, 149, 15 1 High pressure: hold tank, 61

INDEX

vessels, 22 High vapor loads, 146 HON rule, 3 1-32 Hot process flow, 126 Human error, impact of, 1 5 , 2 I , 65 Hybrid mixture, 2 1 1 Hydraulic flame arresters, 170 Hydrocarbons, 38,4 1,65, 104 Hydrochloric acid vapor, I65 Hydrofluoric acid vapor, 165 Hydrogen, generally: characterized, 40, 44, 53, 55 chloride, 119 cyanide, 68, 119, 122, 125 dioxide, 36 explosion prevention, 107 sulfide, 25, 38 Hydrostatic pressure test, 1 15 Hydrostatic pressure test, 1 15 Hydro-testing, 124 Ignitable mixtures, 16, 22 Ignition: energy, minimum, 3 5 sources, 24-25,45, 65, 89, 104, 108, 115-1 18, 172 Immediately Dangerous to Life and Health (IDLHs), 66, I92 Impingement trays, 154 Incident, defined, 21 1 Incineration, in treatment and disposal systems, 135 Incinerators, 45, 105, 1 17, 127, 189 Incompatibility: gasesiliquids, 22 materials, 22, 188 vent streams, 10, 87 Industrial Revolution, 74 Industry initiatives, 66 Inertediinerting vent header systems: advantages of, 103 characterized, 89, 97--99,233, 236 disadvantages of, 103 inspection of, 20 1 lean fuel, 92 maintenance, 20 1 principles of,99- I02 Inerted flammable liquid storage example, 248-254

317

Inert gas, 14, 36, 44, 89, 95-96. 100, 169 Inherently safer, defined, 2 1 1 Inhibitor depletion, 27 Injury, sources of, 127 In-line crimped metal deflagration flame arrester, 113-1 14 In-line flame arresters: characteristics of, 1 1 1- 1 12 in-line deflagration, 1 13 in-line detonation, 1 13-1 14 Inorganic chemicals, 5 Inorganic compounds, thermal destruction of, 165 Inspection process, 63, 112, 120, 128 Instrumentation: auxiliary, 167 explosion prevention, 1 18 explosion protection, 94 failure, 15 fuel lean systems, 9 1 fuel rich vent header system, 104 gas recovery systems, 163 inspection and maintenance of, 201 malfunctioning, 195 normal process vent systems, 60 process control, 147 Insulation, inspection and maintenance of, 202 Intentional routine controlled venting, 2 Interaction matrices, 21, 69: 73 Interconnected vent headers, 88 Interconnecting emergency vents. merging streams, 88 Interface requirements, 60 Intermediate treatment, 16, 129, 153, 157 International laws and regulations, 29 Iron sulfides, 69 Isolation: deflagration protection, 89 toxic gas leaks, 120 valves, 45, 66, 129-130 Kletz, Dr. T., 2 I Knock-out drums, 149-152, 169, 171-172, 174 Knock-out tanks, 127, 135, 149-152, 178, 187,201 Laboratory work, 137

318

INDEX

Large scale continuous processes, 141 Layer of Protection Analysis (LOPA), 184-185 Lead, 299 Leak(s): coolant, 159 detection systems, 32 potential of, 96 Lean header, explosion in, 287-288 Lean vent header systems, 44, 65, 187 Le Chatelier method, 42. 93 Legislation: Air Pollution Control Act (1955) 76 Clean Air Act, 76 77, I83 review of, 289-29 I Length-to-diameter (L,ID) ratios, 47, 50 Limiting oxidant concentration (LOC), 44-45,92-93,97,99, 103, 119,212 Liquefied gas. 126 Liquid(s): accumulation of, 126-128, 197--198 build-up, 62, 102 burning. 127 carryover, 150- I5 1 cold, 86 combustible, 43,65, 127 condensed, 143 entrainedientrainment, 62 -63, 86, 122, 127, 134, 178 fallout zones, 190 feeds, 149 flammabile, 2 1 1 knock-out and drainage, I26 merging streams, 87 quench, 144 reactive, 122 reverse flow of, 127 separated, 149-150, 167 slug flow, 149 solidifying, 140 toxics, separation of, 1 19 Load, implications of, 124 Local Emergency Planning Committee (LEPC), 191 Local regulations, 17 1, 177 Location: of flares, 171 significance of, 134 stack design, 177, 179

Locked vahes, 16 Lower explosive limit (LEL), 36 Lower flammable limit (LFL), 36, 38-40, 4 2 4 4 , 65, 89, 91 92, 95, 106, 172, 210,251 Low pressure. generally flares, 135, 167-169 hold tank, 6 1 implications of. 42, 88 vessels. 22 Low temperature brittle fracture, 14 combustion effects. 42 Luminosity, flare system design, 171 Maintenance: costs, 82, 91 explosions during, 21 flare systems, 171 importance of, 10, 129 normal process vent header systems, 14 personnel exposures, 22 scheduled inspections, 196-202 Malfunctions, 126-1 28 Marine Vapor Control Systems, 114-1 15 Maritime, vapor handling requirements, 114 Mass Transfer in Engineering Practice, 153 Material combinations, reactive systems, 69 Material Safety Data Sheets (MSDS), 36, 73 Materials build-up, inspections and maintenance of, 196-200 Maximum Achievable Control Technology (MACT) standards, 3 1 , 216-222, 300 Mechanical damage, 110, 149 Mechanical design considerations: connections, flexible, 128-129 corrosion, 125 expansion joints, 128-1 29 header operating pressure and pressure drop, 125-126 liquid hock-out and drainage, 126-127 overview of, 123 pipe specifications, 123 shockwaves, downstream of rupture disks, 125 stresses on vent header piping, 124-125

INDEX

thermal stresses, low temperature enibrittlement, 126 valves, 129--130 vent header pipe specifications, 123 vent header supports, 123- 124 Mechanical failures, 187 Mechanical stresses, 120 Mercaptans, 66 Mercury, 77 Merging streams, 86-88 Mesh pads, 15 1 Metal oxides, 172 Methane, 35-37,45, 53, I07 Methaneiair mixture, 4 6 4 7 Minimum oxygen concentration (MOC), 44,97,212 Mist eliminators, 135, 151-153 Mixing implications, 28 Mixing vent streams, 2 1 Modeling, dispersion, I77 Moisture, carbon adsorption and, 158 Molecular sieves, 156 Molecular structures, high energy, 70-7 1 Molecular weight, carbon adsorption and, 158 Moles, 46 Monitoring system, 10, 93, 112, 120, 145 Monomers, 122 Multiple vent header systems, 16, 168, 175, 189 Multiple vent streams, 10 n-Butane, 55 National Ambient Air Quality Standards, 78,289,291 National Electrical Code requirements, 96 National Emission Standards for Hazardous Air Pollutants (NESHAP): characterized, 215, 300 Maximum Achievable Control Technology (MACT) rules, 3 1,78 National Fire Protection Association (NFPA) standards: NFPA 68, Guide f o r Venting of DeJagrations, 89, 109-1 10 NFPA 69, Standard on Explosion Prevention Systems, 45, 89,99, 140, 200 explosion prevention, 107-108

319

National Toxics Inventory, 79, 301 Nearby communities, environmental influences, 134 Negotiated regulations, 3 1 Neutralization, containment methods and, 140, 146 New Source Performance Standards (NSPS), 77,290 Nitrogen, 36-37,4445, 104, 120,234,251 Nitrogen oxide (NOx), 30, 78,299,301 NOAA, Chemical Reactivity Worksheet, 73 Noise levels, impact of, 15, 134, 167-1 68, 171, 179, 188 Non-condensable gases, fuel lean systems, 92 Non-corrosive materials, merging streams and, 88 Non-hazardous materials, 1, 176, 233,235 Non-hydrocarbon fuels, 42 Non-routine operations, explosions during, 21 Non-toxic materials, 22, 188 Normal process, see Normal process vent header systems; Normal process vent streams discharge, 133 treatment systems, 66 vent operation, 168 vent requirements, 247 Normal process vent header systems: atmospheric dispersion, 177 bum as fuel, 16 characteristics of, 14, 143, 152, 176 combined with emergency vent header systems, 16, 18 defined, 13 equipment with, 17 flare system design, 171 flow rates, 15 off-gas emissions, 16 recover for material use, 19 source control and configuration examples, 233-241 treat and release, 19 Normal process vent streams: condensation, 62-63 design case scenario, 60 identification of, 60-61, 137 liquid entrainment, 62-63

320

INDEX

Normal process vent streams (continued) two-phase venting, 63-64 Noxious materials, 66 Nuclear power industry, 145-146 Nuisance properties, treatment and disposal systems, 134 Nuisance release, I88 Occupational Safety and Health Administration (OSHA): combustibility issues, 24 Process Safety Management Program (PSM), 185 Process Safety Management Standard (29 CFR 1910.119), 29 Odor, impact of, 15,26,66-67, 134, 179 Odoriferous materials, 156 Off-gases, 14, 16,27, 29 Oil and gas: production and processing applications, 4 refining, 141, 143 Onsite concentrations, 67 Open ground flares, 168 Operating costs, 82 Operations: change management, 202 daily inspections, 196 flare systems, 171 scheduled inspections, 196-202 types of, 195-196 Operator error, 127 Operator response, as safety issue, 21 Organic(s), generally: chemicals, synthetic, 4 compounds, oxidized, 165 contaminants, 157-158 gases, flammability of, 38 solvents, 23 volatile emissions, 143 water-miscible, 144 Outlet gas temperature, 173 Overdriven detonation, 47,50-5 1, 65, I 14 Overfilled vessels, 127 Overpressure: detonation process and, 50-5 1 , 55 explosion, 65 implications of, 15, 6 1, 142 incident of, 290-29 1 potential for, 149

protection, 74, 137-138, 251 relief, 2-3, 5 , 15, 28, 140, 144 sources of, 188 venting, 187 zones, 190 Overstressed expansion joints, 128 Ownership issues, 10 Oxidants, 42, 45-46 Oxidizer(s): characterized, 24, 107, 149, 174 combustion process, 35,65 explosion prevention, 65 explosion protection, 107-108 flammable range, 36 other than oxygen, 107-1 08 0xy gen : analyzers, functions of, 1 19- 120 combustion and, 25 concentration, limitation of, 35 explosion prevention, 99, 103 flammability, 36,45 flammable gas detection, 96, 99 fuel lean systems, 90 gas recovery systems and, 163 monitoring of, 169 Ozone, 299 Packed bed scrubbers, 135, 146, 154-155 Packed towers, 153 Packing material, 154-1 55 Paints, 5 Paraffin hydrocarbons, 4 1 Parallel relief devices, 129 Particle size, significance of, 152 Particulates, 158, 299 Past incidents, case history, 122-123 Peak explosion pressure, 46 Permits. 10, 66, 165, 169 Peroxides, 69, 122 Perry's Chemical Engineer's Handbook, 153 Personnel: emergency response training. 67 exposure, 22, 167, 176, 197 hazards, 26, 99, 102, 1 10 injuries, 127 training for, 67, 200 Petrochemicals, 4 Petroleum industry, 143, 146

INDEX

Petroleum refining, 4 pH, reactivity and, 27,80 Pharmaceuticals, 5 Phosgene, 25 Phosphine, 69 Physical changes, 22 Physical design, 13 Physical properties, merging streams and, 87 Physical separation, in treatment and disposal system: advantages of, I52 components of, generally, i35-I 36 cyclones, 15 1- 153 disadvantages of, 152 knock-out tanks and drums, 149-152 mist eliminators, 151-153 vapor-liquid gravity separators, 147-149,

152 Physical state, significance of, 134 Pipeline contactors, 146 Pipeslpiping: flare system design, 171 loops, 126 mechanical design considerations,

123.-125 pipe diameter, 50-51 pipe geometry, 46 pipe supports, damage to, 127 toxic gas leaks, I20 Pipingfor Category M Fluid, 26 Plant air, 72 Plastics, 5 Pluggagdplugging: absorption systems and, 154-156 in condensing systems, 159,166 merging streams and, 87 oxidizers, 174 prevention of, 140 sources of, 10,115, 155 Plugged mist eliminators, 15 1 Plugged vent header systems, inspection and maintenance of, 197-198 Plume, 134,212 Polymerization, 68-69,86-87,122,137,

140,149,188,197-198 Polymers, 5 , I56 Polymers and plastics, 5 Power loss, impact of, 160,I66

321

Pre-compressed gas, detonation of, 50-5 1 Pre-heat exchangers, 174 Pressure, impact of: absorption systems, 153 carbon adsorption and, 158 combining vent streams, 80 containment methods and, 137,140,

144-145 deflagrations, 4-7 design process, 85, 123 emergency vent header system, 15 explosion prevention, 89,99,114 explosion reliefvents, 110 flame temperature, 46 flammable limits and, 38,4042 fuel lean systems, 91 fuel rich vent header system, 104-106 gas recovery systems, 163-164 gauges, 20 in Hazard Analysis, 186 limiting oxidant concentration (LOC), 44 mechanical design, 125-1 26 merging streams, 88 normal process vent header system, 14 physical separation process, 15 1 piling, 46~-47, 65,1 1 1 relief devices, 16,I95 relief systems, regulation of, 15 relief valves (PRVs), 162--163, 199 treatment and disposal process, 134 Process control failures, 28 Process effluent streams, I34 Process Hazards Analysis (PHA): consequence assessments, 191 HAZOP Deviation Table, 243-245 importance of, 19-2I , 1 1 8 Process Piping (ASME B3 l.3), 3, 120 Process Safety Information, 183 Process start-up schedules, 10 Process upsets, 21 Process vents, HON rule applications, 32 Propagation velocity, detonation and, 50 Propane, 53 Proprietary seal drums, 170 Protective features, defined, 213 Protective measures, reactivity, 29 Public health issues, 74 Public image, 1 1 Public nuisances, 26,66

322

INDEX

Purge gas, 104, 105, 169, 187, 198, 201, 233-234 Purging, 14, 86, 92, 112, 162, 172 Pyrophoric materials, formation of, 69 Quantitative Risk Assessment (QRA), 184-185 Quench drums, 142-146,212 Quench pools, 138, 145-147,212 Radiant heat exposure, 167 Radiation levels, 168 Radioactive materials, 145 Radionucleotides, 77 Rain-out of liquidsisolids, 179 Rainstorms, impact of, 169, 197 Random-fill packing, 155 Rapid mixing, 92 Reaction, generally: forces, 124 mass, 134, 140, 142 products, hazardous, 22 rate, 74, 138 stopping, 138-140 vessels, 137 Reactive chemicals, 140, 171 Reactive gases, 16 Reactive hazards, protective measures, 29 Reactive materials, 16, 19, 63, 146, 29 1-292 Reactive response, 80 Reactive systems: characterized, 68-69, 121 chemical interactions, 69-72 design considerations, 121-123 relief device set pressure, effects of, 74 Reactive with combinations of materials incidents, 27 Reactivity: cell size and, 5 1 hazard identification, 27-28 implications of, 22 mixing implications, 28 potential, 87-88 runaway reactions, 28 reactive hazards, protective measures, 29 reactive systems, 68-74 Reactor, merging streams, 87. See also Batch reactor

Reclaiming processes, in treatment and disposal systems, 135 Recovery, in treatment and disposal system: advantages of, 165-1 66 characterized of, 135-136, 158-159 condensing systems, 159--160, 166 disadvantages of, 165-1 66 gas recovery, 160-165, 166 Recycling, 14, 160 Refinery examples: coker unit and gas processing plant, 270, 27 1,272-276 crude and vacuum units, 267-272 reactive system, 275-28 1 Refinery fuel gas, 14 Refinery industry, 14 1. See also Oil and gas, refining Refinery vent gas recovery systems, 160 Refrigerants, 159 Regeneration systems, 135, 157 Regulations: equipment specific, 223 historical background, 74-76 impact of, generally, 10- 1 1, 13 improved air quality, 78-79 overview of, 29-32 process-related, 2 15-222 review of, 7678,289-291 toxic and noxious materials, 66 types of, 9 Release impact zones, 189. See also Accident releases Reliefdevices, 139, 142, 146 Relief valves, 202 Relieving vessels, containment systems, 138 Removal, environmental requirements, 134 Residual gases, 27 Resins, 5 Responsible Care, 66 Retention systems, carbon adsorption, 158 Right-to-operate, 1 1, 74 RisWrisk analysisirisk assessment, defined, 212 Run-up distance, 48, 50-5 1 , 54-55, 115 Run-up period, explosion prevention, 1 14 Runaway polymerizations, 22 Runaway reaction(s), 13, 15, 28, 61-62,

INDEX

68 69. 73. 119. 122-123, 138-140, 146.2 12.292-293 Rupture disks, 121, 125, 162, 198-200

Safeguards, defined, 2 13 Safety considerations, see Consequence assessment, Hazard analysis adsorption process, I57 design and, 2 I , 23 equipment manufacturer, guidehnes/recommendations, 128 flares, 171-172 operations requirements, 195 oxidizers, 173 personnel injuries, 127 process effluent streams, 134 Safety Instrumented Systems (SIS), 201 Safety Integrity Level (SIL), 185 Safety margin, 42, 45, 60 Safety requirements. 1-2 Safety reviews, 19 Saturation capacity, 157 Scrubbers characterized, 2, 80, 86, 135, 146, 190. 25 1 packed bed, 154-155 Venturi, 154 - 155 wet, 151-152 Seal drums, 94, 117, 162, 169-172. 201 Sealed valves, pressure relief devices, 16 Seal tanks, 94, 102 Secondary explosion, 1 I0 Self-draining vent headers, 122 Self-reaction scenario, 68 Self-reactive chemical incidents, 27 Self-reactive materials, 197 Self-reactivity, 1 19 Separators, 168 Severity of consequence, 189 defined, 2 I3 Seveso (Italy) plant, 29,292-293 Shell-and-tube heat exchangers, 135, 159, 166 Shock design process, 123 loading. effects of, 140, 142 mechanical, 124 waves, 125

323

Short-stop chemicals, 138 Short Term Public Emergency Guidance Level (SPEGL), 192 Shutdown: causc of, 188 explosion prevention, 99 explosions during, 2 1 fuel lean systems, 9 1 Hazard Analysis, 186 normal process vent header systems, 14 schedule. 10 Sieve trays. 154 Silica gel, 156 Simultaneous release scenario, 144 Slugs, 127. 140 Slurries, 142 Smog, 75-76 Smoke, sources of, 188 Smoke-free operations, flare system design, 165, 171 Sodium hypochlorite, 23 Solidification, in physical separation process, 149, 15 1 Solids: accumulation, 62-63 build-up, 63, 69 entrained, 86, 134, 140 merging streams, 87 Solvents: flammable, 23 volatile, 234 Sonic velocity, 50 Source vessels, 60 Sparger, 147 Specialty chemicals, 5 Specific heat, 3 9 , 4 4 Splashing, two-way venting, 63 Spray towers, 135, 146, 154 Stable detonation, 47, 50-5 I , 1 I5 Stack( s): atmospheric dispersion, 177-1 78 design considerations, 190 elevated, 176 Standard Industrial Classifications (SIC), 3 1 Start-up: continuous reactions, 23 explosion prevention, 99 explosions during, 2 1 fuel lean systems, 9 1

324

INDEX

Hazard Analysis, 186 normal process vent header systems, 14 State regulations, 177 Static electricity, 65 Steam, 61,72, 127, 134, 145, 173-174 Steam vents, 93 Stoichiometric mixtures. 3 6 , 2 13 Stop valves, 129-130 Storage tank(s): atmospheric, 61, 149, 207 characterized, 168, 234,239 inspection and maintenance of, 196 monomer overpressure event, 290-291 Storage vessels, HON rule applications, 32 Stream(s): chemical and physical properties, 16 effluent, 134 emergency vent, 145 flare system design and, 17 1 gas recovery systems, 162-163 particulates in, 128 vapor phase materials, 159 ventivented, 160, 166 Stress, piping, 124- I25 Structural supports, inspection and maintenance of, 202 Structured packing, 155 Sub-atmospheric pressure, 61, 119, 127, 169 Sulfides, 103 Sulfur, 103, 165 Sulfur dioxide, 68, 122, I65 Sulfur oxides, 299 Support utilities, loss of, 187 Surface, generally: condensers. 135, 159 detonation process and, 50 roughness, 46 tension, two-phase venting, 64 Synthetic fibers, 5 Synthetic organic chemicals, 4 Synthetic Organic Chemical Manufacturing Industry (SOCMI), 3 1 Tanks: catch, 137-1 39, 142,208 blowdown, 141-142 dump, 139--140, 142, 209 knock-out, 178, 187, 201

storage. 196, 207, 234, 239, 290-291 in treatment and disposal systems, 135 vacuum ratings, 15 Temperature: absorption systems, 153 adiabatic flame, 207 adsorption process, 157 autoignition, 207 combining vent streams, 80 containment methods and, 137, 140, 142, 144-146 coolant inlet and outlet, 159 design basis and, 85 design process, 123 emergency vent header system, 15 explosion prevention, 89,99, 112 flammable limits and, 3 8 4 0 flare system design, 17 1 fuel lean systems, 91 fuel rich vent header systems, 104, 106 in Hazard Analysis, 186 ignition sources and, 117 inert gases, 101 limiting oxidant concentration (LOC), 44 low, 126 merging streams, 88 normal process vent header system, 14 organic contaminants and, 158 outlet gas stream, 159 reactivity and, 27 thermal expansion, 199 thermal oxidizers and, 172-1 73 treatment and disposal process, 134 vapor pressure and, 24 Tempered; defined, 2 13 Tempering process, 63 Thermal conductivity, 97 Thermal destruction, in treatment and disposal systems: advantages of, 175 by-products of, 165 characterized, 135-136, 165 disadvantages of, 175 flares, 165-172, 175 oxidizers, thermal and catalytic, 172-1 75 process heaters, 158, 174-175 Thermal expansion, 124-125, 128, 140, 199 Thermally unstable materials, 88, 234, 240

INDEX

Thermal oxidizers, 2, 94, 117, 135, 172-175, 189 Thermal radiation, 168, 175, 190, 192 Thermal relief valves, 127 Thermal stresses, 126 Thermally unstable materials, merging streams and, 88 Threshold concentrations, 66 Threshold exposure limits, 191--192 Threshold Limit Values-Short Term Exposure Limits (TLV-STEL). 192 Tie-bars, 128 Titanium, 1 19 Topography, emergency vent header systems and, 15 Total containment, 137, 146 Toxic chemicals, 145 Toxic effects, evaluation of, 191 Toxic exposure, 6 1, 19 1 Toxic gas(es): analyzersidetectors, 20 1 characterized, 16 defined. 2 13 design basis, 86 handling strategies, 118-121 release, warning systems, 67 Toxicity: design basis, 86 implications of, 22, 134, 176 protective measures, 26-27 toxic gases defined, 25 toxic hazard assessment, 26 Toxic impact zones, 190 Toxic liquids, 178-1 79 Toxic materials, 66, 68, 88, 134, 146 Toxic volatile liquids, 178 Transfer operations, HON rule applications, 32 Tray towers, 135, 146, 154 Treatment and disposal systems: absorption, 135-136, 153-156 adsorption, 135-136, 156-158 collection, 135-147 defined, 133 dispersion ofvent gas, 135-136. 176-179 flowchart, 136 historical perspectives, 10 physical separation, 135-136, 147- 152 recovery, 135-136, 158-165

325

selection factors, 133-~136 thermal destruction, 135-136, 165-175 Turbulence, 5&5 I , 93, 1 10 Two-phase flowirelief flow, 134, 140 Two-phase systemsiventing, 63-64, 146 Unburnediunburnt gases, 3 9 , 4 6 4 7 , 50, 109, 114 Unburned mixtures, flammability, 45 Uncontrolled reactions, 139 Understanding Atmospheric Dispersion of Accidental Releases, 177 Understanding Explosions, 3 United States: air quality issues, 75 environmental air pollution control, see U S . environmental air pollution control regulations explosion prevention regulation, 89 pressure relief systems, 15 U.S. environmental air pollution control regulations: equipment-specific, 223 process-related, 2 15-222 United States Public Health Service, 75 Unstable materials, formation of, 69 Upper explosive limit (UEL), 36 Upper flammable limit (UFL), 36. 39-40, 42,65, 89, 104, 106-107,210 Utilities, 10, 28, 62, 95, 189 Vacuum, generally: breaker, 147 collapse, 6 I , 149 failures, case illustrations, 284 normal process vent header system, 14 protection, 25 1 , 253 relieving devices, 15 systems, 101-102 Valve(s): automatic fast-acting, 1 10, 1 1 5-1 16 back-pressure control, 162- 163 combined relief with rupture disk devices, 121 control, 120, 234, 237 fail-open control, 162 failure of, I87 fast-acting, 115-1 17 high speed isolation, 89

326

INDEX

Valve( s ) (cotitinlied) isolation, 25, 45, 66, 234 pressure control, 101 relief, 121, 198-199,202 solenoid, 234 switching methods. 157 thermal relief, 127 trays, 154 vent header system, 129-130 Vapor(s): cloud explosion, 190 collection methods, 139 combustion properties, 24 condensable, 143 condensed, 127 containment methods, 141, 146 control systems, 2 defined, 2 13 flammable, 64-66,211 hiel lean systems, 91 handling requirements, maritime installations, 1 14-1 15 hazardous, 3, 176, 178 normal routine releases, SO phase inhibitor, 122 pressure, 24 space, 16 uncondensed, 139 in vented streams, 134 volatile organic, 23, 11 7 Vaporization, 74 Vapor-liquid, generally: disengagement, 135 gravity separators, 147-149, 152 Velocity; gases in air purging, 92 Vent(s), generally: collection system, 2, 13 design basis, 2 1 header systems, see Vent header systems manifolds, 2 restriction, causes of, 197-198 sources, identification of, 60 streams, 10, 30, 86-88, 137, 173 Vent gas: composition of, 89, 104, 160 compressed, 160 dispersion of, see Vent gas dispersion emergency releases of, 30 emergency vent header system, 15

flows, in physical separation process, 149 Vent gas dispersion: advantages of, 178--179 atmospheric dispersion design, 177--178 characterizedl3S-136, 176 design considerations, 176-1 77 disadvantages of, 178 -I 79 safety considerations, 176-177 Vent header systems: air pollution controls, 9-10 collection systems, 2 cost-effective, 10, 21, 82 defined, 2 design information checklist, 60, 225-23 1 design of, 2--3. 249 design philosophy, 19-32 gross liquid discharge, 61 operations, overview of, 2 ownership issues, 10 pressure fluctuations, 13 purpose of, 133 types of, 13- 19 Venturi scrubbers, 135, 154-155 Vessel(s), generally: breathing, 14 collapse, 14 volume, 46 Vinyl chloride, 77 Violent reaction, 10 Viscosity, two-phase venting, 64 Viscous liquids, 63, 146 Volatile liquids, 28 Volatile organic compounds (VOCs), 30, 78, 156, 159, 165, 174,301 Volatile organic monomers, 239 Volatile organic vapors, 23, 117 Warning systems, 67 Wash water, “dumping,” 61 Wastewater streamslcollectionitreatment operations, air emissions from, 32 Wastewater treatment units, 168 Water: chilled, 159 containment methods and, 143-144 cooling tower, 159 hammer, 62, 124, 127, 149, 188

INDEX

two-phase venting, 64,69 vapor, 165 Weather: emergency vent header systems and. 15 historical data, 177 impact of, 166 Wet scrubbers, 151-152 What-If?, 184

327

Wind directors, 67 Working capacity, 157 Worst-case emergency venting scenarios, 61 Worst-case events, 2 1 Worst-case release, 67 Worst credible caseieventslscenarios, 29, 137,213