GUIDELINES FOR

Pressure Relief and

Effluent Hand1 i ng Systems

WILEY- 623 INTERSCIENCE A JOHN WILEY & SONS, INC., PUBLICATION

CENTER FOR CHEMICAL PROCESS SAFETY of the

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 3 Park Avenue, New York, New York 10016

dcd-wgc2.jpg

This page intentionally left blank

GUIDELINES FOR

Pressure Relief and

3Iuent Handling Systems

This is a publication of the CENTER FOR CHEMICAL PROCESS SAFETY of the AMERICAN INSTITUTE OF CHEMICAL ENGINEERS A complete list of CCPS publications appears at the end of this book.

GUIDELINES FOR

Pressure Relief and

Effluent Hand1 i ng Systems

WILEY- 623 INTERSCIENCE A JOHN WILEY & SONS, INC., PUBLICATION

CENTER FOR CHEMICAL PROCESS SAFETY of the

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 3 Park Avenue, New York, New York 10016

Copyright 0 1998 American Institute of Chemical Engineers 3 Park Avenue New York, New York 10016

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 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, 222 Rosewood Drive. Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470. Requests to the Publisher for permission should be addressed to the Permissions Deparment, John Wiley & Sons, Inc., I I 1 River Street, Hoboken, NJ 07030, (201) 748-6011. fax (201) 748-6008.

Library of Congress Cataloging-in Publication Data Guidelines for pressure relief and effluent handling systems /

prepared by the Emergency Relief EfIluent Subcommittee. P. cm.

Includes bibliography and index.

1. Chemical plants-Waste disposal. ISBN 0-8169-0476-6

2. Hazardous wastes- management. 3. Relief valves. 4. Sewage disposal. 1. American Institute of Chemical Engineers. Center for Chemical Process Safety. Emergency Relief Effluent Subcommittee TD899.C5G85 1998 97-36450 660 ' .028'6-dc21 CIP

This book is available at a special discount when ordered in bulk quantities. For information, contact the Center for Chemical Process Safety at the address shown above.

It is sincerely hoped that the informataon presented in this volume will lead to an even more impressive safety

record for the entire indusuy; however. the American Institute of Chemical Engineers, its consultants, CCPS Subcommittee members, their employers. and their employers' officers and directors disclaim making o r

giving any warranties or representations. express or implied, including with respect to fitness, intended pur- pose, use or merchantability and/or correctness or accuracy of the content of the information presented in this document and accompanying software. As between (1) American Institute of Chemical Engineers, its consult- ants, CCPS Subcommittee members, their employers, their employers' officers and directors and (2) the user of

this document and accompanying software, the user accepts any legal liability or responsibility whatsoever for the consequences of its use or misuse.

Contents

Preface Acknowledgments Acronyms and Abbreviations

'I Introduction

1.1. Objective 1.2. Scope 1.3. Design Codes and Regulations, and Sources of Information 1.4. Organization of This Book 1.5. General Pressure Relief System Design Criteria

1.5.1 Process Hazards Analysis 1.5.2 Process Safety Information 1.5.3 Problems Inherent in Pressure Relief and Effluent Handling

System Design

2 Relief Design Criteria and Strategy

2.1. Limitations of the Technology 2.2. General Pressure Relief Strategy

... Xlll

xv xvi i

9

14

14

V

vi Contents

2.2.1 Mechanism of Pressure Relief 2.2.2 Approach to Design 2.2.3 Limitations of Systems Actuated by Pressure 2.2.4 Consideration of Consequences

2.3.1 Scope of Principal USA Documents 2.3.2 General Provisions 2.3.3 Protection by System Design

2.4.1 General Terminology 2.4.2 Pressure Relief Valves 2.4.3 Rupture Disk Devices 2.4.4 Devices in Combination 2.4.5 Miscellaneous Nonreclosing Devices 2.4.6 Miscellaneous Low-Pressure Devices 2.4.7 Miscellaneous Relief System Components 2.4.8 Selection of Pressure Relief Devices

2.5.1 General Code Requirements 2.5.2 Pressure Relief Valves 2.5.3 Rupture Disk Devices 2.5.4 Low-Pressure Devices 2.5.5 Series/Parallel Devices 2.5.6 Header Systems 2.5.7 Mechanical Integrity 2.5.8 Material Selection 2.5.9 Drainage and Freeze-up Provisions 2.5.10 Noise

2.6.1 Safety Valves 2.6.2 Relief Valves 2.6.3 Low Pressure Devices 2.6.4 Rupture Disk Devices 2.6.5 Devices in Combination 2.6.6 Miscellaneous Nonreclosing Devices

2.7. Scenario Selection Considerations 2.7.1 Events Requiring Relief Due to Overpressure 2.7.2 Design Scenarios

2.8.1 Data Sou rces/Determi nation/Esti mation

2.3. Codes, Standards, and Guidelines

2.4. Relief Device Types and Operation

2.5. Relief System Layout

2.6. Design Flows and Code Provisions

2.8. Fluid Properties and System Characterization

1 4 15 1 6 1 7

1 8 1 8 21 30

30 31 31 45

49 51 52 53

58 58 60 63 64 64 70 71

71 72

72 75 77 78 79 82

83 84 85 86

4a

71

a3

a7

Contents vii

2.8.2 Pure-Component Properties 2.8.3 Mixture Properties 2.8.4 Phase Behavior 2.8.5 Chemical Reaction 2.8.6 Miscellaneous Fluid Characteristics

2.9. Fluid Behavior in Vessel 2.9.1 Accounting for Chemical Reaction 2.9.2 Two-Phase Venting Conditions and Effects

2.10. Flow of Fluids through Relief Systems 2.10.1 Conditions for Two-Phase Flow 2.10.2 Nature of Compressible Flow 2.1 0.3 Stagnation Pressure and Critical Pressure Ratio 2.10.4 Flow Rate to Effluent Handling System

2.1 1 .I Relief Device Reliability 2.1 1.2 System Reliability

2.1 1. Relief System Reliability

Appendix 2A. International Codes and Standards Appendix 2B. Property Mixing Rules Appendix 2C. Code Case: Protection by System Design

3 Relief System Design and Rating Computations 3.1. Introduction

3.1 .I Purpose and Scope 3.1.2 Required Background

3.2.1 General 3.2.2 Material and Energy Balances 3.2.3 Phase Behavior 3.2.4 Two-Phase Venting Technology 3.2.5 Methods of Solution

3.3.1 Thermal Expansion 3.3.2 Fire Exposure 3.3.3 Loss of HeatingKooling Control 3.3.4 Excess Inflow/Outflow 3.3.5 Structural Failure 3.3.6 Loss of Agitation

3.2. Vessel Venting Background

3.3. Venting Requirements for Nonreacting Cases

87 87 88 89 95 95 95 96 98 98 99

103 104 104 104 106 109 113 116

119 119 121 123 123 125 125 126 126 127 127 130 148 149 149 154

viii Contents

3.3.7 Miscellaneous

3.4 Vent Rate for Reacting Systems 3.4.1 General 3.4.2 Computer Simulations 3.4.3 Special-Case Integral Equations

3.5.1 GasNapor Flow 3.5.2 Two-Phase Flow 3.5.3 Nozzle and Piping Configuration for COMFLOW and TPHEM

3.6. Relief System Sizing and Rating 3.6.1 Pipe Runs 3.6.2 Safety Relief Valve Systems 3.6.3 Liquid Relief Valve Systems 3.6.4 Miscellaneous Low- Pressu re Devices 3.6.5 Rupture Disk Device Systems 3.6.6 Devices in Combination 3.6.7 Miscellaneous System Elements 3.6.8 Header Systems

3.7.1 Background Theory 3.7.2 Selection of Design Case 3.7.3 Design Methods

3.5. Computational Strategy and Tools for Relief Flow

3.7. Reaction Forcesnhrust

APPENDIX 3A Vessel Venting Technology and Data Acquisition 3A.1. System Schematic and Principal Parameters

3A.2. Basic Material and Energy Balances 3A.2.1 Vent Rate Criterion 3A.2.2 Energy and Material Balance

3A. 3.1 General-Case Phase Equ i li briu m 3A.3.2 Component Classification 3A.3.3 Nonequilibrium Phenomena

3A.4. Two-Phase Venting Technology 3A.4.1 Coupling Equation 3A.4.2 Holdup Correlations 3A.4.3 Tests for Two-Phase Venting

3A.5. Pressure Rise from Thermal Expansion

3A.6. Runaway Reaction Calorimeters 3A.6.1 Device Characteristics

3A.3. Phase Behavior

155 155 156 156 157

168 170 170 172

172 174 178 201 206 208 21 0 21 1 21 2 21 2 21 4 21 6 21 7

225 225 227 227 228

230 230 232 232

233 233 233 236 238 239 240

Contents ix

3A.6.2 Data Interpretation

3A.7.1 Fire Exposure 3A.7.2 Runaway Reaction

3A.7. Relief Rate by Computer Simulation

APPENDIX 3B Relief System Sizing Background

3B.1. Scope

3B.2. Fluid Flow Fundamentals 3B.2.1 Flow in Nozzles 3B.2.2 Pipe and Fittings 3B.2.3 Conditions at Maximum Flow

3B.3.1 Input to TPHEM 3B.3.2 Relationships for Analytical Integration

3 B .4.1 Nu me r ical Integration 3B.4.2 Analytical Integrals for Homogeneous Flow 38.4.3 Computer Programs

APPENDIX 3C Example System Rating Simulations 3C.1. Fire Exposure Example 3C.2 Runaway Reaction Example APPENDIX 3D Final Device Specs Required for Purchase

3B.3. Physical Property Treatment

3B.4. Computation Strategies

244

249 249 249

253

253

253 253 262 270

271 272 277

2 78 279 282 286

301

301 303

3 0 5

4 Selection of Equipment for Handling Emergency Relief Effluent

4.1. General Strategy 4.2. Basis for Selection of Equipment 4.3. Determining What May Be Discharged to the Atmosphere Safely 4.4. Factors That Influence Selection of Effluent Treatment Systems

4.4.1 Physical and Chemical Properties 4.4.2 Two-Phase Flow and Foaming 4.4.3 Passive versus Active Systems 4.4.4 Technology Status and Reliability 4.4.5 Discharging to a Common Collection System 4.4.6 Plant Geography 4.4.7 Space Availability

31 I 31 3 31 3 31 8 31 8 31 9 320 320 322 322 323

X Contents

4.4.8 Turndown 4.4.9 Need for Vapor-Liquid Separation 4.4.1 0 Possible Condensation and Steam-Water Hammer 4.4.1 1 Time Availability 4.4.1 2 Capital and Continuing Costs

4.5. Methods of Effluent Handling 4.5.1 Containment 4.5.2 Discharge to Atmosphere 4.5.3 Vapor-Liquid Separators 4.5.4 Quench Pools 4.5.5 Sccu bbers (Absorbers) 4.5.6 Flares

5 Design Methods for Handling Effluent from Emergency Relief Systems

5.1. Design Basis Selection 5.2. Total Containment Systems

5.2.1 Containment in Original Vessel 5.2.2 Containment in External Vessel (Dump Tank or Catch Tank)

5.3.1 Limitations on Combining Multiple Relief Discharges

5.3.2 Pressure Drop Guidelines 5.3.3 Discharge Piping Design Pressure 5.3.4 Materials of Construction 5.3.5 Location of Relief Devices 5.3.6 Mechanical Design 5.3.7 Separation of Headers Based on Temperature and Pressure

5.4.1 Separator Inlet Velocity Considerations 5.4.2 Horizontal Gravity-Type Separators 5.4.3 Vertical Gravity Separators 5.4.4 Separator Safety Considerations and Features 5.4.5 Separator Vessel Design and Instrumentation

5.5.1 Droplet Removal Efficiency 5.5.2 Design Procedure

5.3. Relief Devices, Discharge Piping, and Collection Headers

into Common Headers

5.4. Vapor-liquid Gravity Separators

5.5. Cyclones

323 323 324 324 324 324 324 327 328 336 342 344

354 355 355 356 357

357 358 358 358 359 360 361 362 365 366 3 74 377 378 378 3 80 3 80

Contents xi

5.5.3 Cyclone Separator Sizing Procedure

5.6.1 Design Procedure Overview 5.6.2 Design Parameter Interrelations 5.6.3 Quench Pool Liquid Selection 5.6.4 Quench Tank Operating Pressure 5.6.5 Quench Pool Heat Balance 5.6.6 Quench Pool Dimensions 5.6.7 Sparger Design 5.6.8 Handling Effluent from Multiple Relief Devices 5.6.9 Reverse Flow Problems 5.6.10 Steam/ Water Hammer 5.6.1 1 Mechanical Design Loads 5.6.1 2 Quench Tank with Effluent Scrubber

5.7.1 General Comments and Background 5.7.2 Special Requirements for Emergency Scrubbers 5.7.3 Scrubbing (Absorption) Mechanisms Considerations 5.7.4 Scrubber Design Methodology 5.7.5 Mechanical Design Considerations

5.8.1. Stack Location and Elevation 5.8.2 General Design Considerations 5.8.3 Dispersion Criteria and Stack Diameter 5.8.4 Noise and Velocity Limitations

5.9.1 General Comments and Considerations 5.9.2 EPA Requirements 5.9.3 Elevated Flare System Design Criteria 5.9.4 Safety, Blockage, and Freeze-Up Issues 5.9.5 Materials of Construction

5.6. Quench Pools

5.7. Scrubbers (Absorbers)

5.8. Release to Atmosphere

5.9. Flare Systems

APPENDIX 5A 5A.1. Example Problem 5A.2. Given Conditions 5A.3. Quench Pool Design

5A.3.1 Heat Balance 514.3.2 Sizing the Quench Pool Vessel 5A.3.3 Sizing the Sparger 5A.3.4 Size Manifold and Distributor

383

385 386 389 390 392 392 398 404 41 2 41 3 41 3 41 3 41 4

41 4 41 5 41 7 41 7 420 423

426 427 427 428 431

431 432 433 434 452 453

454

454 455 463 463 468 471 475

xii Contents

5A.4. Gravity Separator Design 5A.4.1 Piping Sizes 5A.4.2 Horizontal Separator Design 5A.4.3 Vertical Separator Design

5A.5. Cyclone Separator Design Summary

References

Glossary

Index

476 476 477 478

480

486

487

51 1

527

Computer Programs on Accompanying CD-ROM CCflow: Windows program for flow in pressure relief

and effluent handling systems

COMFLOW: DOS program for gashapor flow in pressure relief systems

TPHEM: DOS program for gashapor, liquid, and two-phase flow in pressure relief systems

Windows program for estimating Antoine coefficients, compressibility factor, and isentropic expansion exponent

Utilities:

Preface

The American Institute of Chemical Engineers (AIChE) has been involved with process safety issues in the chemical and allied industries for many years, including the establishment of the Center for Chemical Process Safety (CCPS) in 1985. The goal of CCPS was to develop and disseminate information on technical and management systems for preventing or mitigating major chemi- cal accidents. CCPS programs have focused on four main areas:

Establish and publish improved scientific and engineering practices to prevent incidents involving hazardous chemicals; Encourage the use of good process safety practices through publica- tions, seminars, symposia, and continuing education programs for engineers; Advance the state-of-the-art engineering practices and technical man- agement through research to prevent and mitigate catastrophic chemi- cal incidents; Develop and encourage the use of undergraduate education curricula to improve the safety knowledge and awareness of engineers.

Pressure relief systems have always been important components in the design of safety systems for chemical and petrochemical plants. In recent years however, with concern for possible human health effects and environ- ment harm, the chemical and petroleum industries have devoted increased attention to reducing the discharge of hazardous materials from emergency relief devices. This book was prepared in recognition of the need for guid- ance in designing emergency relief systems to minimize or contain the dis- charge of potentially harmful materials.

... Xlll

This page intentionally left blank

Acknowledgments

The American Institute of Chemical Engineers (AIChE) wishes to thank the Center for Chemical Process Safety (CCPS) and those involved in its opera- tion, including its many Sponsors whose funding made this project possible; the members of the Technical Steering Committee who conceived of and sup- ported this project; and the members of the Emergency Relief Effluent Sub- committee for their dedicated efforts and technical contributions.

Members of the Emergency Relief Effluent Subcommittee, consultants, and CCPS Staff were

Larry L. Simpson, Matthew L. Becker, Steve G. Coats, Du Pont John DiPalmu, CYTEC Stanley S. Grossel, Russell G. Hill, Howard E. Huckins, CCPS Staff; Project Coordinator James E. H u f , Alan G. Keiter, Gene K. Lee, Al R. Muller, John A. Noronha, Eastman Kodak Harvey Rosenhouse, FMC

This book was prepared by the Subcommittee members; principal authors were Jim Huff for Chapters 2 and 3, and Matt Becker, Stan Grossel, and Howard Huckins for Chapters 1 , 4 and 5. Much of the content and form

Subcommittee Chairman, Union Carbide Arc0 Chemical Company (retired consultant)

Hoffmann-LaRoche (retired consultant) CCPS Staff; Technical Editor

Dow Chemical (retired consultant) Rohm and Haas Air Products & Chemicals Goodyear Tire and Rubber

xv

xvi Acknowledgments

of the book is based on suggestions and technical information from individ- ual Subcommittee members.

The Subcommittee wishes to thank the following peer reviewers for their thoughtful and detailed comments and valued suggestions:

Richard A. Denehan and associates; Exxon. Peter D. Fletcher; Raytheon. Rudofph C. Frey; M. W. Kellogg. David J. Hesse; Battelle. Peter N. Lodaf; Eastman Chemical. Marvin F. Specht and Steve H. Gove; Hercules. A. JifI Wifday; Health and Safety Laboratory, Sheffield England.

Special appreciation is expressed to the following individuals for specific contributions:

Kent E. Gabtys, Consultant, and David G. R. Short, Du Pont, for Win- dows computer program development. Harold G. Fisher, Union Carbide, Chairman of the DIERS Users Group, for encouragement and many helpful contributions to the scope and technical content.

Joseph C. Leung, Leung Inc., for helpful comments on nonideal gas flow through nozzles. Geoqes A. Mefhem, Arthur D. Little, Inc., for performing calculations with SuperChems'" for DIERS for example problems. Ufrich Seifert and ProfessorArtur Stieffof the Institute for Environmen- tal Safety and Energy Technology and the University of Dortmund, Oberhausen Germany, for valuable suggestions on design procedures for quench pools, based on their defining experimental studies of this technology. D. Arthur Shaw, Monsanto, for performing calculations with SAFIRE for example problem.

John F. Straitz; NAO, Inc., for contributing extensive information for the flare section.

Lastly, the Subcommittee wishes to express their appreciation to CCPS management, including Thomas W. Carmody, Bob G. Perry, Jack Weaver, and Lester H. Wittenberg for their support, guidance, and patience.

AAR ACGIH AIChE AIT ANSI API ARC TM ASME BLEW CAA CAAA CCPS

CEC CEP CERCLA

CFD CFH CFR CGA

Acronyms and Abbreviations

American Association of Railroads American Conference of Government Industrial Hygienists American Institute of Chemical Engineers Autoignition temperature American National Standards Institute American Petroleum Institute Accelerating Rate CalorimeterTH American Society of Mechanical Engineers Boiling Liquid Expanding Vapor Explosion Clean Air Act Clean Air Act Amendments Center for Chemical Process Safety, American Institute of Chemical Engineers Community for Economic Cooperation Chemical Engineering Progress Comprehensive Environmental Response, Compensation, and Liability Act Computational Fluid Dynamics Cubic Feet per Hour Code of Federal Regulations Compressed Gas Association

COMFLOW Compressible Flow-Computer Program CPI Chemical Process Industry CPQRA DIERS

Chemical Process Quantitative Risk Analysis Design Institute for Emergency Relief Systems, American Institute of Chemical Engineers

xvii

xviii Acronyms and Abbreviations

DIPPR

DOT DSC EEC EEGL EOS EPA ERM ERPG E M ESD F&EI GPM HEM HNE IDLH I S 0 LEL LFL LNG LPG M A W MSDS NDE NFPA NIOSH NIST NPSH NTIS OSHA P&I P&ID PEL PERD PFD PHA PRD PRV PSE PSM PSP PSV

Design Institute for Physical Property Data, American Institute of Chemical Engineers Department of Transportation Differential Scanning Calorimeter European Economic Community Emergency Exposure Guidance Level Equation of State Environmental Protection Agency Equilibrium Rate Flow Model Emergency Response Planning Guideline Emergency Relief System Emergency Shutdown Device Fire and Explosion Index Gallons Per Minute Homogeneous Equilibrium Flow Model Homogeneous Nonequilibrium Flow Model Immediately Dangerous to Life or Health International Standards Organization Lower Explosive Limit Lower Flammable Limit Liquefied Natural Gas Liquefied Petroleum Gas Maximum Allowable Working Pressure Material Safety Data Sheet Nondestructive Examination National Fire Protection Association National Institute of Occupational Safety and Health National Institute of Standards and Technology Net Positive Suction Head National Technical Information Service Occupational Safety and Health Administration Piping and Instrumentation Diagram Piping and Instrumentation Diagram Permissible Exposure Limit Process Equipment Reliability Data Process Flow Diagram Process Hazard Analysis (preliminary hazard analysis) Pressure Relief Device Pressure Relief Valve Pressure Safety Element Process Safety Management Process Safety Progress Pressure Safety Vent

Acronyms and Abbreviations xix

PTC PVRV RCRA RD RKEOS RP RSST'" SADT SAFIRE

SCF SCFH SCFM SEM SI SIS SPEGL SRV STEL TPHEM TSCA UEL UFL UL VSP'" WEEL

Power Test Code Pressure-Vacuum Relief Valve Resource Conservation and Recovery Act Rupture Disk Device Redlich-Kwong Equation of State Recommended Practice Reactive System Screening Tool'" Self Accelerating Decomposition Temperature Systems Analysis for Integrated Relief Evaluation- DIERS Computer Program Standard Cubic Feet Standard Cubic Feet per Hour Standard Cubic Feet per Minute Slip Equilibrium Flow Model International System of Units (Le Sys th Internutionale d'llnitks) Safety Interlock System Short-term Public Emergency Guidance Level Safety Relief Valve Short Term Exposure Limits Two-Phase Homogeneous Equilibrium Flow-Computer Program Toxic Substance Control Act Upper Explosive Limit Upper Flammable Limit Underwriters Laboratory Inc. Vent Sizing Package'" Workplace Environmental Exposure Limit

This page intentionally left blank

1

Introduction

1.1. Objective

ASME, MI, and NFPA documents provide much of the information needed for design of most pressure relief systems. However, in recent years, various gov- ernmental regulations and increased industry efforts to improve safety and environmental protection practices have led to much greater focus on reduc- ing and controlling releases of materials from pressure relief systems to the atmosphere. In addition, research and studies by the Design Institute for Emergency Relief Systems (DIERS) resulted in a new body of technology on two-phase flow from relieving vessels, the effect of two-phase flow on pres- sure relief system design, and on the performance of pressure relief valves under such conditions. These developments suggested a need for a presenta- tion, from a chemical industry perspective, on treatment of the effluent from pressure relief systems, along with a more “user-friendly” coverage of two- phase flow calculation technology. Preparation of this book by CCPS was in response to this need.

The presentation is directed toward experienced process engineers and specialists with a basic proficiency in fluid dynamics and process engineering fundamentals. The objective is to present information that will guide in selecting and designing reliable emergency pressure relief and effluent han- dling systems. These systems should be designed to protect equipment from overpressure, and to either contain or safely control any hazardous materials discharged during an emergency.

This book presents information on several widely used national codes and standards, some of which have been adopted by regulatory authorities for inclusion in either federal or local regulations. These documents should

1

2 1. Introduction

be viewed by designers as representing best industry practices with proven value in providing reliable process safety systems, not just as regulations to be complied with.

1.2. Scope

General background information on pressure relief technology is presented, along with guidance for selecting relief devices and effluent handling equip- ment, and calculation procedures for designing pressure relief and selected effluent handling equipment. Numerous example problems are used to illus- trate calculation procedures. In addition, computer programs are presented for handling flow calculations for compressible gases, for evaluating complex two-phase flow situations, and for sizing effluent handling equipment. Included are

Discussions of national code and regulatory impacts on pressure relief system design and operation. Reviews of causes of overpressure events, selection of the worst case scenario and design basis for the relief system including systems involv- ing chemically reactive and highly viscous materials. Descriptions of a range of relief devices and operating performance characteristics including flow calculation methods for sizing pressure relief devices and associated piping systems. Characterization of fluid properties, including sources of property information and handling of mixtures. Methods for calculation of reaction thrust from discharge of relief sys- tems. Guidance in selecting effluent handling systems, including equipment commonly used for pressure relief system applications. This includes gravity and cyclone separators, scrubbers, quench pools, flares, and atmospheric dispersion. Calculation procedures for sizing the most widely used equipment for effluent handling, including gravity separators, cyclones, and spargers.

Maintenance, operations, and testing procedures and technology are not discussed in detail but are covered briefly in selected cases. Prevention or mitigation of overpressure incidents, essential components of a good process safety management system, are beyond the scope of this book. Such proce- dures and technology include emergency control or shutdown systems, inherent safety concepts, safety layers of protection, control of explosive deflagrations and detonations, and other measures used to reduce the fre- quency or magnitude of emergency overpressure events.

1.3. Design Codes and Regulations, and Sources of Informafion 3

If potentially hazardous materials might be discharged to the atmos- phere, specialists on the health and environmental effects should be con- sulted to determine safe levels of discharge to the air, water, and land.

1.3. Design Codes and Regulations, and Sources of Information

There are a number of organizations that provide information on pressure relief and handling of effluent from pressure relief systems. Some of these, with a brief summary of their role, are shown below (see $2.3.1, and Appen- dix 2B for a more extensive listing):

Federal and local governments. The federal government, through EPA and OSHA regulations, provides much information on requirements for process safety and environmental protection. Many states have imple- mented regulations that parallel federal regulations. Designers and operators of pressure relief systems should maintain a familiarity with these requirements. While the focus in this book is on practices, codes, and standards of U.S. origin, designers and operators of facilities in for- eign countries are urged to become familiar with any practices or regula- tions that may apply: see Appendix 2A for sources of such information. In many cases, facilities designed to meet U.S. requirements will either meet or exceed requirements based on foreign regulations.

American Society of Mechanical Engineers (ASME). The ASME publishes the Boiler and Pressure Vessel Code, which presents basic requirements for overpressure protection of vessels covered by the Code. Section VIII covers Pressure Vessels, which is applicable to the petroleum and chemi- cal process industries. Many governmental authorities have adopted the ASME Code and made it part of their regulations, so it has the force of law in many locales.

American Petroleum Institute (API). The API publishes a series of docu- ments that cover the fundamentals and application of pressure relief technology, including pressure relief of low pressure tanks, and testing and maintaining pressure relief valves. Many recommendations are pre- sented that cover various aspects of pressure relief system design, includ- ing eflluenc handling.

National Fire Protection Association (NFPA). The NFPA publishes a number of documents that present pressure relief requirements for vari- ous specific fluid services. Their standards for Combustible And Flamma- ble Liquids (NFPA 30), Liquefied Petroleum Gases (NFPA 58), and Venting Of Deflagrations (NFPA 68) are of particular interest to the petro- leum and chemical process industries.

DIERS Users Group. The Design Institute for Emergency Relief Systems (DIERS) was established in 1976 to develop a better understanding of

4 1. lntrodudion

pressure relief system technology, including vapor-liquid disengagement in vessels, and flow of two-phase fluids through pressure relief devices and piping. The effort continues through the DIERS Users Group. The result of the initial work has been published (DIERS 1992), while current develop- ments are covered in biannual meetings and associated reports where information on new research, practices and technology are discussed.

Joint Research Center (JRC) of the Community for Economic Coopera- tion (CEC) at Ispra, Italy. The JRC conducts sponsored research on vari- ous aspects of process safety for the CEC. They have been very active in developing new technology for pressure relief and effluent handling sys- tems with experimental facilities for tests on a larger scale than is avail- able elsewhere. Information on their research and technical studies is made available to the public through various conferences and the general technical literature.

Compressed Gas Association (CGA). The CGA publishes documents (e.g., CGA S-1.3) covering pressure relief devices for compressed gases and safety-related information on storage containers and cylinders.

National Board of Pressure Vessel Inspectors (NB). The National Board publishes documents related to inspection and repair of pressure relief valves, and also information on certified flow capacity of valves tested in accordance with ASME procedures.

Other sources of information that supplement the standards and codes indicated above are given as references noted within the text of each chapter in the book.

1.4. Organization of This Book

Pressure relief technology is covered in Chapters 2 and 3. The recovery, or treatment, of the effluent from pressure relief devices is covered in Chapters 4 and 5. The following is a brief summary of each chapter:

Chapter 1. Introduction

Chapter 2. Relief System Design Criteria and Strategy: Presents general information on pressure relief technology (including terminology and definitions) pressure relief design strategies, ASME Code requirements, and descriptions and layout of relief systems. Also covered are causes of overpressure, review of worst credible relief scenarios, analysis of vapor- liquid phase behavior in vessels, determination of required flow capacity, fluid properties and system characterization, flow of fluids through relief systems, and relief system reliability.

1.4. Organization of This Book 5

Chapter 3. Relief System Design and Rating Computations: Covers calcula- tion methods for sizing and rating pressure relief devices and associated piping, and for evaluating whether two-phase flow might occur. The basic equations are presented for fluid dynamics, including two-phase flow, and for sizing relief devices and piping. Methods for estimating reaction thrust from relief system discharge also are covered.

Chapter 4. Selection of Processes and Equipment for Handling Emetgency Relief Eff7uent: Presents a guide to selection of equipment and systems to treat the effluent from relief devices. The focus is on equipment and techniques that are more commonly used in pressure relief applica- tions. Information is summarized in tables that list advantages, disad- vantages, and areas of possible application for the various types of equipment.

Chapter 5 . Design Methods for Handling Effluent from Emetgency Relief Systems: Covers design methods and sizing calculation procedures for various types of equipment and processes that are commonly used to treat efluent in emergency relief situations. Methods are presented in detail for gravity separators, cyclone separators, and quench pools (including spargers for quench pools). General background informa- tion on design is presented for other items such as scrubbers, flares, and atmospheric dispersion. An example problem is presented to illus- trate the design procedures for gravity and cyclone separators, and for quench pools.

Computer Programs. Computer programs on a CD-ROM are provided to aid in making flow calculations for relief devices and piping, and for sizing selected effluent handling equipment. The family of programs, CCflow, includes the following:

TPHEM, a DOS program for two-phase flow through piping and nozzles, COMFLOW, a DOS program for gashapor flow through piping and nozzles, Windows programs for two-phase and gashapor flow through piping and nozzles, for sizing and evaluating reliefvalves, and for sizing gravity separators, cyclone separators, and spargers. Utilities program to calculate Antoine coefficients, compressibility fac- tors, and isentropic expansion coefficients. Multicomponent systems can be handled for the latter two items.

Instructions for use of all of the programs are included in the Help files provided with CCj7ow; use of TPHEM and COMFLOW are illustrated also in Chapter 3. These programs do not address determination of required reliev- ing capacity or composition of the effluent.

6 1. Introduction

1.5. General Pressure Relief System Design Criteria

Anyone with responsibility for designing, operating, and maintaining pres- sure relief systems and other process safety facilities should be familiar with: the provisions of OSHA’s process safety regulations; with the EPA risk man- agement program rules; and with local implementation of the principles embodied in the federal standards. Many states have adopted rules that roughly parallel the federal standard. The general principles of process safety management are discussed also in CCPS (1992a) and in MI 750.

While it is important to comply with all applicable regulations, it is neces- sary to keep in mind the basic objective: safety of people and preventing damage to facilities and the environment. Compliance with regulations alone may not provide an acceptable level of protection. Company standards and practices are an important source of information on design requirements for pressure relief systems. They are usually based on process safety management principles that have been developed from many years of experience, and many regulations use industry best-practices as a reference. These practices have been proven to represent good business practices as well as good process safety management, and have been incorporated into the culture of many organizations.

Some important process safety management techniques related to pres- sure relief system design, which are not covered in detail in this book, are dis- cussed briefly below. OSHA published a standard in 1992, Process Safety Management of HighIy Hazardous Chemicals (29 CFR 1910.119), to control chemical hazards in the workplace. That standard covers basic requirements for implementing a good process safety management program which involves applying “generally recognized and accepted good engineering practices” to ensure process safety in new and existing plant facilities. Two components of a process safety management program that are referred to in 29 CFR 1910.119 are particularly relevant to the design, operation, and maintenance of pres- sure relief systems; these are Process Hazards Analysis and Process Safety Information; and are discussed briefly in the following sections.

1.5.1 Process Hazards Analysis

A chemical process and plant facility should be analyzed for all possible causes of overpressure to determine the worst credible scenario. The worst credible scenario establishes the design basis for the pressure relief and for the effluent handling system. Methods for conducting such a hazards analysis and evaluation are presented in CCPS (1992b) and by Noronha et al. (1994). An equally important objective of the process hazards analysis is to reveal measures that might be taken to prevent the overpressure event from occur- ring, or reducing its magnitude if it does occur. Reducing the magnitude of

1.5. General Pressure Relief System Design Criteria 7

the event will reduce the size of the pressure relief system and simplify the design of the effluent handling system. Overpressure events can be avoided or mitigated using the following techniques as applicable:

Applying inherent safety concepts in the process design. See CCPS (1996), Englund (1995), Hendershot (1995), Kletz (1991), and Lutz (1995). This can include, for example, changing process chemistry to use less hazardous materials, avoiding extreme temperatures and pressures, and designing for total containment by increasing vessel design pressure.

Training operating and maintenance personnel. Operating and mainte- nance procedures must be written for start-up, shutdown, upset, and normal operating conditions. These written procedures must be kept up-date, and must be part of the periodic hazard review and analysis pro- gram. Proper supervisory controls must be instituted, and training and refresher courses provided for operating and maintenance personnel.

Process safety audit. A n independent audit and verification of the design can provide additional assurance that the emergency relief system will adequately protect the vessel. An audit of the initial design can include a review of overpressure events that were considered in selecting the design basis, and a check of the final mechanical design and specifications for the pressure relief system. For existing process units, a periodic audit can include a review of current process conditions, any possible mechani- cal changes in the facility since the original construction, and mainte- nance and operating records for any signs of problems, and then verification that the pressure relief system is still adequate to protect the vessel. Also see Chapter 2 , Chadwell (1999, and CCPS (1993b).

Robust and redundant process control and emergency shutdown sys- tems. In recent years, there has been increased interest in application of instrumentation to reduce either the frequency or the magnitude of over- pressure events, particularly in Europe (Parry 1994). Often, overall pro- tection system reliability can be improved by using high integrity instrumentation to supplement the mechanical pressure relief devices normally used. Instrumented systems can also be used effectively to dein- ventory and depressure a vessel to either prevent the pressure relief device from opening or to mitigate the magnitude of the release. Such instrumentation is usually independent of normal process control instru- mentation, is of high reliability, and is provided with a high degree of redundancy and diversity to avoid common cause failures. Instrument protection systems should be supported by a detailed hazard analysis to identify causes, consequences, and possible frequency of overpressure events. A periodic operational testing program under close supervisory control is also required.

8 1. Introduction

If the likelihood of a particular event can be reduced to an extremely low level, that event might be considered not credible. Under such circumstances, that event would not be considered hr ther in determining the design basis for the pressure relief system. A recent ASME Code Case (No. 2211) allows provision of overprotection of a vessel in process service by system design. Such system design is based on a detailed analysis to examine all credible sce- narios which could result in an overpressure condition. The user must ensure that the maximum allowable working pressure ( M A W ) of the vessel is greater than the highest pressure that can reasonably be expected to be achieved by the system. The system can include an instrumentation and control system which is used to limit the system pressure under all scenarios, along with a reliability evaluation of the overall safety system. Documentation must be available to regulatory and enforcement authorities where the vessel will be installed, and prior jurisdictional acceptance may be required. See Karcher et al. (1997) for further discussion on application of this concept.

I .5.2 Process Safety Information

The design basis and description of all pressure relief systems must be retained and available for review. The design basis should be kept up-to-date with current process conditions, and reviewed periodically when process hazards analyses are conducted. Such documentation should include: identi- fication and description of the design basis overpressure event and the equip- ment being protected, including required flow capacity; description and specification of relief devices; important operating parameters such as flow capacity, set pressures, materials handled; inspection, testing, and mainte- nance history. Also, see CCPS (1995). The following paragraphs summarize some of the important general requirements of 29 CFR 1910.119:

Process safety information-shall include a complete compilation of process safety information before conducting any required process hazard analysis-information pertaining to the hazards of highly haz- ardous chemicals, process technology, and process equipment. Hazards of highly hazardous chemicals-shall include data on physi- cal properties, reactivity, corrosivity, chemical and thermal stability, and hazardous effects of inadvertent mixing of different materials that could foreseeably occur. Material Safety Data Sheets meeting the requirements of 29 CFR 1910.1200(g) may be used to comply with these requirements to the extent that they contain the information needed.

+ Relief system design and design basis; 4 Design codes and standards employed;

Process Equipment-shall include documentation on the following: