the design institute for emergency relief systems … · emergency relief system design using diem...

Emergency Relief System Design Using DIEM Technology

THE DESIGN INSTITUTE FOR EMERGENCY RELIEF SYSTEMS PIERS) PROJECT MANUAL

H. G. Fisher H. S. Forrest S. S. Grossel

J. E. Huff A. R. Muller

J. A. Noronha D. A. Shaw B. J. Tilley

WILEY- 0 INTERSCIENCE A JOHN WILEY & SONS, INC., PUBLICATION

THE DESIGN INSTITUTE FOR EMERGENCY RELIEF SYSTEMS

of the American Institute of Chemical Engineers

345 East 47th Street, New York, NY 10017

dcd-wgC1.jpg

This page intentionally left blank

Emergency Relief System Design Using DIERS Technology

Emergency Relief System Design Using DIEM Technology

THE DESIGN INSTITUTE FOR EMERGENCY RELIEF SYSTEMS PIERS) PROJECT MANUAL

H. G. Fisher H. S. Forrest S. S. Grossel

J. E. Huff A. R. Muller

J. A. Noronha D. A. Shaw B. J. Tilley

WILEY- 0 INTERSCIENCE A JOHN WILEY & SONS, INC., PUBLICATION

THE DESIGN INSTITUTE FOR EMERGENCY RELIEF SYSTEMS

of the American Institute of Chemical Engineers

345 East 47th Street, New York, NY 10017

Copyright 01992 American Institute of Chemical Engineers 345 East 47th Street, New York, NY 10017

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 Ssctions 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization thmgh 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 Department. John Wiley & Sons, Inc., 11 1 River Street, Hoboken, NJ 07030, (201) 748-6011. faX (201) 748-6008.

Library of Con- Cataloging-in-PubUcation Data Emergency relief system &sign using DIERS technology : the Design

Institute for Emergency Relief Systems (DIERS) project manual / H.G. Fisher. . . [el al.].

p. an. Includes bibliographical references and index. ISBN 0-8169-0568-1 1. Chemical plantrSafety measures. 2. Relief valves.

1. Fisher, H. G. 11. Design Institute for Emergency Relief Systems. lT'155.5.E63 1992 6601 '.2814cU) 92-10226

CIP

The DIERS Project Manual was prepared by the Design Institute for Emergency Relief Systems (DIERS) of the American Institute of Chemical Engineers (AIChE). The AIChE. DIERS, its sponsor companies and contractors, the DIERS Users Group, its member companies, and the authors make no representation, warranties. or guarantees, expressed or implied, or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information. apparatus, product, or process disdased, or represent that the use of such could not lead to unsafe consequences. An extensive background and experience are required to properly understand and apply the information contained herein. Organizations are therefore urged to interpret and use the results through appropriate safety relief specialists.

~~ ~-~ ~

'Ibis book is available at a special discount when ordered in bulkquantities. For information, contact the American Institute of Chemical Engineers r at the address given above.

To the Memory of Our Colleagues, Ian Swijl and Bryan J. Tilley

Con tents

Preface

Introduction 1. Overview 2. Design Institute for Emergency Relief Systems (DIERS) 3. A Strategy for Mqjor Accidental Release Prevention 4. A Strategy for Emergency Relief System Design 5. An Appmoch to Emergency Relief System Design Assessment 6. no-Phnse VapopLiquid Flow 7. Two-Phnse Vapor-Liquid Flow Onset and Dwngngement 8. n o - P h a s e VapopLiquid €lydmdynnmio 9. DlERS BenchScale Apparatus 10. Runaway Reaction Emergency Relief System Design Computer Program 11. References Appendix A. DlERS Committees Appendix B. DlERS Sponsors Appendix C. DIEM Controctors

Chapter I. Vapor Disengagement Dynamics 1. Overview

1-1. Vapor Disengagement Dynamics 1-2. Design Considerations

2-1. Open Literature References 2-2. Projed Manual

2. Detoiled Discussion

3. References Appendix I-& The Coupling Equation and Flow Models Appendix I-B. Best Estimnte P d u r e to Calculate n*oPhase VapopLiquid

Appendix I-C. Fluid Behavior in Venting Vessels Appendix I-D. Energy and Mnteriol Bolonce Derivotions for Emergency

Annex I-D1. lnternol Energy end Venting Calculations

Flow OnseVDisengngement

Pressure Relief of Vessels

vii

xiii

xvii

xvii xviii

xi x xx

xxii M V

xxvi xxvii

xxviii mix

xxxii xxxiv

xxxvi xxxv

1

1 1 2 2 2 2 3 5

25 -XI

42 48

viii Contents

Chapter 11. Pressure Relief System Flow 1. Introduction

1-1. scope 1-2. Organization 1-3. Special Tenninology

2-1. Newtonian Flow 2-2. Complex Fluids 2-3. Useful Approximations

3-1. General Flow Equations 3-2. Nozzle Flow Models 3-3. Sharp Reductions 3-4. Pressure Recovery/Expnsionuilibrations 3-5. Pipe Flow 34. Application to Pressure Relief System Elements 3-7. Networks 3-8. Complex Fluids

4. Nomenclature 5. Acknowledgments 6. Refelrnces Appendix 11-A. Thermophyskal Property Requirements Appendix 11-B. Equilibrium Flash Calculations Appendix II-C. Model Parameters for Pipe Entrance Sections Appendix 11-D. Computer Routines in SAFIRE Program Appendix 11-E. Example Problems Appendix 11-F. Generalized Cordations and Design Charts

2. Recommended Design Metbods

3. Technology Base

Chapter 111. DIEM Phase 111 Large-Scale Integral Tests 1. Summary 2. Intduction

2-1. Program Objectivcs 2-2. Program Desaiption

3. Test Configurations 4. Test Results

4-1. Tests T1 to T?3 4-2. Tests V32-W1 to V32-W8 4-3. Tests T9, T10, T11, Tl4, and T2S 4-4. Tests T12 and T13 4-5. Test T20

51

51 51 52 52 53 53 56 57 58 58 61 77 79 81 90 95 95 97

100 100 104 105 107 111 113 131

133

133 138 138 138 141 141 141 146 147 150 152

Contents ix

4-6. Tests T17 and T18 4-7. Tests T21, "22, TZ3, and T24 4-8. ICRE Tats 32-6 to 32-11 4-9. ICRE Tests 2000-1 to 2ooo-5 4-10. ICRE Tests 32-14,32-15, and 32-18

5. Acknowledgments 6. References Appendix Ill-A. Test Configurations Appendix 111-B. Experimental Results and Model Cornprisons Appendix Ill-C. Kinetics Model for Styrene Polymerizations

Chapter IV. High Viscosity Flashing Wo-Phase Flow 1. Introduction

1-1. General Discussion of High Visaxsity Flow in Relief Systems 1-2. Why High Viscosity Systems Require Special Consideration 1-3. Necessity for Conservatism

2-1. Project Overview 2-2. Styrene Reactive Tests 2-3. Small-Scale Rubber Cement Bottom-Vented Tests 2-4. Large-Scale Rubber Cement Tests 2-5. Lrrrge-Sxle Pol ystyrene-Ethylbenzene Bottom-Vented Tests

3-1. Theory and Scaling for Highly Viscous System 3-2. General Eguations for Newtonian Fluids 3-3. Approximate Momentum Balances for Scaling Power-Law

34. Scaling Using Integrated Approximate Momentum Balance

3-5. Scaling Using Approximate Momentum Balance for Power-Law Fluids

4-1. Uncertainties

2. Summary of DIERS High Viscosity Relief Flow Tests

3. Recommended Design Practices

and Newtonian Fluids

for Newtonian Fluids

4. Unanswered Questions n h u t lligh Viscosity Flow

5. References Appendix IV-A. Simplified Theory and Sample Problems

Chapter V. Containment, Disposal, and Mechanical Design 1. Introduction 2. Blowdown Drum Design 2-1. Types of Knock-Out (Blowdown) Drums and Catchtanks 2-2. Sizing of Blowdown Drum

3. Disposal of Vapors Tram Blowdown Drums

153 154 157 161 166 169 169 171 189 288

289

289 289 290 290 291 291 292 293 293 296 297 297 299

299

313

313 314 314 319 329

X Contents

3-1. Direct Discharge to the Atmosphere 3-2. Discharge through a Saubber 3-3. Discharge theough a Vent Condenser 34. Discharge to a Flare Stack or Incinerator

4-1. Vent Piping Considerations 4-2. Catchtank Mechanical Design and Safety Considerations 4-3. Reaction Forces-GeneraI 44. Reaction Forces Equations 4-5. Reaction Forces on Safety Valve Nmles/Piping 44. Reaction Forces from Rupture Disk Discharge 4-7. Transient Effects of Reaction Forces, Rupture Disk Discharge 4-8. Thrust Restraint Design 4-9. Other Blowdown b a d Considerations

4. Mechanical Design

5. References

Chapter VI. DIEM Bench-Scale Apparatus 1. Background

1-1. DIERS Requirements for a Bench-Scale Apparatus 1-2. Limitations of Previous Test Equipment

2-1. Requests 2-2. Worst Credible Incident Scenario 2-3. Saeening Tests 24. DIERS Venting Tests and Analysis 2-5. Recommendations

3-1. Schematic Description of Apparatus 3-2. Apparatus Control and Dam Recording 3-3. Test Cell Configurations

Bench-Scnle Apparatus 4-1. Emergency Relief System (ERS) Overview 4-2, Functions of the Bench-Scale Apparatus 4-3. OnseVDisengagement Behavior Testing 4-4. Flow Rate Calculatiofliscosity Characterization 4-5. Characterization of Runaway Reaction Behavior 44. ERS Design-Analytical MethodsFAI Nomograph 4-7. ERS Design-Area : Charge Scaling (Top Vent Testmop ERS Device)

2. How the Test Methodology Fie inio the Overall Process Safety Design

3. Description of the DIE= BenchScale Apparatus

4. Emergency Relief System (EM) Sizing Using the DIERS

329 330 330 330 332 332 333 334 334 337 -349 361 362 363 363

365

-365 -765 366 -357 -767 -367 368 368 369 -369 -369 37 1 372

372 372 372 373 375 375 376 376

Contents xi

4-8. ERS Design-Area : Charge ScalingScaling Equation Method (Bottom Vent Teflop or Bottom ERS Device)

4-9. Limitations on Area : Charge Scaling for ERS Design 5. References Appendix VI-A. Experimentd ERS Siz ingSome Do and Do Not

Recommendations

Chapter VII. SAFIRE Computer Program for Emergency Relief Sizing

1. Backgmund 1-1. History 1-2. Overview

2-1. Overall Architecture 2-2. Pure-Component Physical Properties 2-3. Mixture Handling Rules 2 4 . Flash Calculations 2-5. Chemical Reactions 2-6. Vent How Calculations 2-7. Vessel Hydrodynamics 2-8. External Heat Fluxes 2-9. Mass and Energy Balances

2. Program Description

3. Dnta Input 4. Snmple Problem 5. Experience with Progroni 6. References Appendix VII-A. Input Dntn Fornis Appendix VII-B. Soniple Iiiput/Output

Index

378 378 379

-782

449

449 449 450 453 453 455 458 459 460 461 463 464 465 467 467 469

470 47 1 488

533

PREFACE

A consortium of 29 companies formed the Design Institute for Emergency Relief Systems (DIERS) in 1976 under the auspices of the AIChE to evaluate existing methods to design pressure relief systems for runaway reactions and to develop additio9al technology as needed. Approximately $1.6 million was spent acquiring test data and documenting applicable methods for the design of emergency relief systems suitable for the discharge of two-phase vapor- liquid flow. Of particular interest was the prediction of when two-phase flow would occur and the extent of vapor-liquid swell.

DIERS did not set out to add another two-phase flow computation procedure to the many that already existed. Rather, the goal was to identify methods that could be used to size safe, but not overly conservative, relief systems for two-phase vapor-liquid flow for flashing or frozen, viscous or nonviscous fluids.

Two-phase vapor-liquid flow of the type that can affect relief system size occurs as a result of vaporization/gas generation during a runaway reaction. Boiling takes place throughout the entire volume of liquid, rather than solely at the surface. Each bubble occupies volume and displaces the liquid surface upward. Individual bubbles are able to rise (slip) through the liquid with a velocity that depends on the buoyancy and surface tension and are retarded by viscosity and the foamy character of the fluid. If a sufficient volume of bubbles become trapped, the liquid surface reaches the height of the relief device and two-phase flow occurs.

The potential for runaway reactions and decompositions is much more pervasive than generally appreciated by those responsible for design and specification of emergency relief systems. During a runaway reaction, the vessel pressure is moderated by the volumetric discharge rate of the emerg-

... X l l l

xiv Preface

ency relief system and the influence of temperature, composition and mass loss on the reaction rate. If two-phase flow occurs, the rate of volumetric discharge, system mass loss and evaporative cooling will be affected by the vapor-liquid phase ratio in the vent stream. Generally, two-phase flow requires a larger relief area than all vapor or subcooled (nonflashing) liquid flow.

Techniques for sizing an emergency relief system for runaway reaction include:

- Direct empirical scaling of experimental data obtained in vessels with - Semi-theoretical graphical or analytical design methods. * Computer simulation of incidents and flow through relief systems.

a very lower thermal inertia.

Containment and mitigation of consequences also require a prediction of the vapor-liquid phase ratio of the discharged material. Ensuring that emergency relief system designs will accommodate two-phase vapor-liquid flow is of particular importance.

The DIERS research program produced approximately 50 experimental and theoretical reports, a comprehensive design computer program, and a prototype bench-scale apparatus. Many open literature articles have also appeared subsequent to the DIERS program. This material has not pre- viously been collected into a form suitable for individual or group study.

This project manual is therefore primarily intended

1. to provide a record of the DIERS research project. 2. to help organizations acquire, assimilate and implement the vast

amount of DIERS information and technology by serving as both a reference and training tool.

3. to illustrate ERS design methodology by means of selected sample problems.

4. to serve as a text for the AIChE/DIERS Continuing Education Course entitled “Emergency Relief System Design Using DIERS Technology. ”

This project manual is not a comprehensive emergency relief system design manual. Conventional emergency relief system design is adequately covered in other documents. This manual presents only the DIERS contributions to emergency relief system design technology. This technology is continually changing and expanding. The reader is therefore urged to supplement the material in this manual with open literature references as they appear.

We would like to thank all who made this manual possible. The DIERS Administrative Committee under the chairmanship of Dr H. S. Kemp provided liaison with the AIChE and the Engineering Foundation. They were

Preface xv

also responsible for contacting prospective members and obtaining funds for the research program. The Engineering Foundation provided seed money to initiate the program. The continuing support of the AIChE leadership, coun- cil and staff helped to make the DIERS program a success. The 29 DIERS sponsor organizations provided the financial contributions, technical expertise, and personnel necessary to support the work of the contractors. The research and technical expertise of the DIERS contractors, Fauske & Associ- ates, Inc; JAYCOR, Inc.; OBERT Associates, Inc.; and Professor G. B. Wallis, are gratefully acknowledged. The DIERS Technical Committees de- fined and had overall responsibility for carrying out the technical program. The expertise and suggestions of our many colleagues involved with the DIERS program and the DIERS Users Group were invaluable. Finally, we thank our many associates who contributed to our understanding of the complex phenomena discussed herein or who assisted with preparation of the many drafts of this manual.

INTRODUCTION A Perspective on Emergency Relief

System Design Practice

Harold G. Fisher Chairman, DIERS Technical Committee

1. OVERVIEW

1-1. Prevention of a Major Accidental Release

Prevention of a major accidental release of hazardous materials from a CPI/HPI facility requires proper design, construction to standard, safe operation and management surveillance. An emergency relief system design strategy, which seeks to prevent a major accidental release. should include iterative considerations of hazards and initiating event identification, assessment of risk and mitigation of consequences. Cost effective loss prevention requires an optimizing strategy to prevent, moderate (relieve) and contain a runaway reaction. This iterative three-step assessment approach to emergency relief system design will minimize the potential for unacceptable risk of a major accidental release of hazardous materials to the environment.

1-2. Emergency Relief System Design

Emergency relief system is a multifaceted problem. Of particular significance is whether the relief system must be designed for single or two-phase vapor- liquid flow. Generally, two-phase flow requires a larger relief area than all vapor or subcooled liquid flow. Containment and mitigation of consequences also depend upon a definition of the vapor-liquid phase ratio of the discharged material. Ensuring that an emergency relief system design will either avoid or accommodate two-phase vapor-liquid flow is of particular importance.

xvii

xviii Introduction

2. DESIGN INSTITUTE FOR EMERGENCY RELIEF SYSTEM (DIERS)

2-1. Introduction to DIERS

The Design Institute for Emergency Relief Systems (DIERS), a consortium of 29 companies under the auspices of AIChE (see Appendices i-A and i- B), was formed in 1976 to develop methods for the design of emergency relief systems to handle runaway reactions [l-31. Of particular interest were the prediction of when two-phase flow venting would occur and the applica- bility of various two-phase vapor-liquid flashing flow methods for sizing relief systems. DIERS spent approximately $1.6 million to investigate the two-phase vapor-liquid onset/disengagement dynamics and hydrodynamics of emergency relief systems. An overview of the DIERS research program and the significance of the recommended methodology are discussed in this Project Manual.

2-2. DIERS Research Program The DIERS program evolved over time. The initial focus involved an investi- gation of two-phase vapor-liquid

0 onset/disengagement dynamics, relief system hydrodynamics, and

0 separate effects experimental verification tests.

The second phase consisted of

0 both small- (32-liter) and large-scale (2200-liter) integral blowdown

0 computer simulation of the experimental results, and 0 technology revisions as required.

and vented runaway reaction experimental tests,

The final phase provided

0 a design computer program, 0 a bench-scale experimental apparatus, and 0 an independent review of the basic methodology.

2-3. DIERS Project Manual This project manual is a record of the DIERS research project. It will help organizations acquire, assimilate, and implement the vast amount of DIERS information and technology by serving as both a reference and training tool. An extensive background and experience are required to properly understand

Introduction xix

and apply the information contained herein. Organizations are therefore urged to interpret and use the results through appropriate safety relief specialists.

2-4. DIERS Users Group

Over 75 companies have formed a DIERS Users Group to cooperatively assimilate, implement, maintain, and upgrade the DIERS methodology. Membership is open to industrial or engineering organizations interested in the design, use or manufacture of emergency relief systems or devices [4].

2-5. Availahility of DIERS Research Results

The DIERS contractor prepared a series of comprehensive research reports and an emergency relief system design computer program, SAFIRE, which are available from the AIChE Publication Sales Department. An AIChE Continuing Education Course entitled “Emergency Relief System Design Using DIERS Technology” is periodically taught in conjunction with AIChE meetings [4].

3. A STRATEGY FOR MAJOR ACCIDENTAL RELEASE PREVENTION

3-1. Definition

A major accidental release may be described as a fire, toxic emission, or explosion resulting from uncontrolled developments in the course of an industrial activity which leads to serious effects on man or the environment inside or outside the confines of the workplace. The CPI/HPI seeks to prevent a major accidental release from its facilities by

0 proper design practice, 0 construction to standard, 0 safe operation, and 0 management surveillance.

3-2. Proper Design Practice

Proper design practices must be employed from the conceptual through the detailed engineering phases. During conceptual design, items such as site selection, plant layout and the concepts of substitution [5 ] (use less hazardous materials), intensification [ 5 ] (use less of a hazardous material) and attenu- ation [5 ] (use of a hazardous material at a lower temperature or pressure) are considered. Detailed engineering focuses on preventing potentially hazardous excursions and minimizing the consequences from conceivable releases.

xx Introduction

3-3. Construction to Standard

Construction to standard involves utilizing engineering and construction codes and standards and the practices of quality assurance to ensure a well- built facility.

3-4. Safe Operation

Safe operation considers the initial and on-going aspects of day-to-day facility operation. Pre-start-up reviews emphasize

0 physical inspection, 0 operating and maintenance procedures, 0 staff levels and training, and 0 emergency procedures and equipment.

Reviews of operating practices and maintenance procedures examine the adequacy of

0 daily instructions, 0 standard procedures and practices, 0 safety rules, and 0 training and retraining.

3-5. Management Surveillance Finally, the management surveillance system should include provisions for

plant modification approval and implementation, 0 periodic safety audits, and 0 reporting and responding to hazardous practices and incidents. -

4. A STRATEGY FOR EMERGENCY RELIEF SYSTEM DESIGN

4-1. Methodology

Identification of hazards and potential initiating events, assessment of risk and mitigation of consequences are all required to prevent a major accidental release during an emergency relief situation. The processes of identification,

lnrroducrion xxi

-

ASSESSMENT + N ACCEPTABLE 1

MITIGATION

WORST CREDIBLE SCENARIO DESIGN BASIS

FIGURE i-1. A strategy for emergency relief system design.

assessment and mitigation are iterative as applied to design of an emergency relief system (Figure i-1).

4-2. Hazard Identification

Hazard identification involves determination of the flammability, toxicity (local and acute, general and chronic), explosibility, and physical properties of materials that are important from an accidental release perspective. Poten-

xxii lntrodricrion

tial initiating events such as uncontrolled exothermic reaction, design flaws, and human error must be identified.

4-3. Assessment of Risk

Assessment includes consideration of the risk associated with design alterna- tives, determination of acceptable risk, and steps taken to minimize the potential for unacceptable risk. Differentiation between a worst case and a worst credible scenario is the critical goal. The desirable result is a worst credible scenario with a remote probability and a minor consequence.

4-4. Mitigation of Consequences

Discussion of the mitigation of a major accidental release is beyond the scope of this manual. The consequences of a minor release can usually be mitigated by preparedness as well as safety equipment and emergency procedures.

5. AN APPROACH TO EMERGENCY RELIEF SYSTEM DESIGN ASSESSMENT

5-1. Strategy The process design should attempt to arrive at an inherently safe facility; that is, one from which a worst-case event cannot cause injury to personnel, damage to equipment, or harm to the environment. This can be achieved through an iterative assessment approach which results in safety features that are intrinsic (built-in), rather than extrinsic (added-on), to the basic design. However, if the technology is not available or is cost prohibitive, a three- step iterative approach can be used to arrive at an acceptable risk at minimum cost through optimization of the measures taken to prevent, moderate (relieve) and contain a runaway reaction or decomposition (Figure i-2).

5-2. Prevention

Many factors must be considered in arriving at the best approach to deal with hazards that may accompany runaway reactions. Thermochemistry, reaction kinetics, thermal stability, process conditions/controls, abnormal operation, contaminants, equipment design, equipment and instrument failures, operating procedures and human error are all examined when evaluat- ing hazard potentials. An approach to the safe design of chemical processes which have the poreritial for a runaway reaction is to identify and analyze worst credible incident scenarios by proceeding as follows:

0 Make an exhaustive search for hazardous conditions by involving

xxiii

cv

A 4

\ f

PREVENTION

MODERATION

CONTAINMENT

FIGURE i-2. An approach to emergency relief system design assessment.

safety specialists, reaction system designers, process engineers and operating personnel.

0 Identify the sequences of events which could produce the highest pressure within a vessel and maximum flow from the emergency relief device(s).

0 Scrutinize various failure modes to arrive at the combination which produces the worst credible incident scenario.

0 Then, utilize reaction, control, process and safety engineering design technologies to prevent, moderate (relieve) and contain runaway reactions.

xxiv Introduction

Design and operating strategies that will help to prevent runaway reactions include

0 acquisition'of data to identify potential problems; 0 measurement and control of critical parameters (temperature, pres-

0 operation at conditions (temperature, pressure, concentration) that

0 installation of redundant instrumentation to increase reliability of

0 use of alarms to warn operators that a critical parameter has changed

0 training to enable operators to safely react to upset conditions; 0 automatic emergency shutdown when a critical parameter has devi-

0 prevention of contamination by proper design and operating proced-

sure, feed rate, coolant flow, catalyst level);

provide a safe margin from runaway conditions;

measurement and control of critical parameters;

from its normal condition;

ated from normal by a predetermined amount; and

ures.

These and many other steps ensure that a runaway reaction will not occur as the result of a single failure.

Finally, the analysis of the likelihood and consequences of multiple failures leads to the identification of the worst credible runaway reaction incident scenario. An emergency relief system can then be designed to handle this incident to include safe disposal of the discharged fluid.

5-3. Moderation (Relief') Techniques for sizing an emergency relief system for runaway reaction include

0 graphic or analytical design methods, 0 direct scaling of experimental data obtained in vessels with a very

0 computer simulation of incidents and flow through relief systems. low thermal inertia, and

The technique selected depends on the

0 Type and number of chemicals involved. 0 Availability of required process and experimental data. 0 Constraints imposed upon the designer.

Introduction xxv

In addition to moderation of a runaway reaction by proper sizing of emergency relief, consideration should also be given to installstion of a liquid dump system, provision for emergency blowdown of pressure, or use of a “kill” agent.

5-4. Containment

Containment can be approached in two ways. First, vessels may be designed to withstand the maximum pressure that can develop from an upset. Al- though this approach may be viable for some emergencies, such as a vapor phase deflagration, it may not be a feasible alternative for a runaway reaction or vessel fire exposure because of the extremely high pressure that can be produced.

Second, the term “containment” may also be used to describe the disposal/decontamination of the discharge from a relief system. Vent stacks, vapor-liquid separators, quench tanks, scrubbers, flares, incinerators, or combinators thereof may be used to disperse, quench, scrub, detoxify, or burn the discharged fluid. Aspects of containment are discussed in Chapter V, “Containment, Disposal. and Mechanical Design.”

6. TWO-PHASE VAPOR-LIQUID FLOW

6-1. Emergency Relief System Design

Emergency relief system design is a multifaceted problem. Of particular significance is whether the relief system must be designed for a single phase (vapor or liquid) or two-phase vapor-liquid flow. During a runaway reaction, the vessel pressure is affected by the volumetric discharge rate of the emergency relief system and the influence of temperature, composition and mass loss on the reaction rate. If two-phase flow occurs, the volumetric discharge rate, the system mass loss and evaporative cooling effects will be affected by the vapor-liquid phase ratio. Generally, two-phase flow requires a larger relief area than all vapor or subcooled (non-flashing) liquid flow.

6-2. Containment and Mitigation

Containment and mitigation requirements also depend upon a definition of the vapor-liquid phase ratio of the discharged material. Ensuring that the total emergency relief system design will accommodate two-phase vapor- liquid flow is of particular importance.

xxvi lnrroditcrion

I N I T I A L

f I N A L 107'C 6 P S I G

C H I C A G O

W A T l R

95% ( 5 5 0 G A L )

68Y. 0 9 5 G A L )

FIGURE i-3. Water blowdown experiment.

7. TWO-PHASE VAPOR-LIQUID FLOW ONSET AND DISENGAGEMENT

7-1. Liquid Swell The surface of a boiling o r gas-sparged liquid can rise to the level of a top- mounted emergency relief device if enough bubbles accumulate (i.e., are held up) in a vessel. Gas holdup can be high at low relief rates if the liquid is highly viscous or foamy. Nonviscous, nonfoamy liquids will also swell at high relief rates.

Boiling due to an exothermic or gas-generating reaction takes place throughout the volume of liquid, rather than solely at the surface. Each buble occupies volume and displaces the liquid surface upward. Individual bubbles are able to rise (slip) through the liquid with a velocity that is dependent on the buoyancy and surface tension and retarded by viscosity and the foamy character of the fluid. If a sufficient volume of bubbles become trapped, the liquid surface reaches the height of the relief device and two- phase flow occurs.

7-2. Two-Phase Blowdown Example

An example should serve to illustrate the phenomenon. A 2-inch diameter relief device (nozzle) was rapidly opened on a tank that was 95% filled with 550 gallons of city water at approximately 150°C and under its awn vapor pressure of about 58.5 psig. Approximately 28% of the tank contents vented by two-phase flow (Figure i-3). The experiment was repeated, except that

Introduction xxvii

I N I l I A L

C H I C A G O

W l D O O PPH D E l E R G E N T

107’C 6 P S I 6

F I N A L

FIGURE i-4. Foamy water blowdown experiment.

95% ( 5 5 0 G A L )

‘ tX ( 2 5 G A L )

loo0 ppm of a liquid household detergent were added. Approximately 96% of the tank contents vented by two-phase flow (Figure i-4).

Clearly, the foamy nature of the second fluid significantly influenced the character of the two-phase blowdown tests. The DIERS large-scale ex- periments are fully discussed in Chapter 111, “DIERS Phase 111 Large-Scale Integral Tests.”

7-3. DIERS Calculation Methodology for Two-Phase Flow Onset

DIERS developed and tested a method to calculate the onset (start) and disengagement (stop) of two-phase vapor-liquid flow from a vessel due to overpressure relief or depressurization [6]. A first-order lumped-parameter “drift flux” formulation [7] was utilized as a basis for a vapor holdup corre- lation. An empirical parameter Co was used to adjust the correlating relation- ships to available data. Vapor holdup in a vessel is influenced by axial and radial effects created by a two-phase boundary layer due to an external heat flux [6,8], internal circulation [6,8] and hydrostatic head. Calculation methods for two-phase flow onset and disengagement are discussed in Chapter I, “Vapor Disengagement Dynamics.”

and Disengagement

8. TWO-PHASE VAPOR-LIQUID HYDRODYNAMICS

8-1. Two-Phase Flow Models DIERS examined various two-phase vapor-liquid Row models from the open literature [9] and tested them using an overall system model against large-

xxviii Itirrodircrion

scale experimental data [ 10). Fauske [ 111 provided practical guidelines for use and Huff [ 121 included many personal insights in his discussion of the various two-phase flow models. Various models and design methods for two- phase flow are discussed in Chapter 11. "Pressure Relief System Flow." The effect of high viscosity o n two-phase flow is discussed in Chapter IV, "High Viscosity Flashing Two-Phase Flow."

8-2. Homogeneous-Equilibrium Flow

Two-phase vapor-liquid homogeneous-equilibrium flashing flow proved to be the most conservative model for estimates of flow capacity from both safety valves and rupture disks. However, this model is not sufficiently conservative for safety valve back pressure calculations or effluent containment considerations because under certain circumstances the appropriate application of other models will predict higher flow rates.

9. DIERS BENCH-SCALE APPARATUS

9-1. Experimental Data for Emergency Relief System Design

A careful experimental program that uses representative samples is required to obtain data needed as a basis for emergency relief system design. The present state of experimental development should be considered when select- ing an apparatus to acquire data.

9-2. Functions of the DIERS Bench-Scale Apparatus

DIERS sponsored the development of a bench-scale apparatus and a low thermal inertia test cell that can be used to provide thermal stability and runaway reaction kinetic data [ 13,141. The low thermal inertia essentially overcomes a limitation of other commercial devices, namely understating the magnitude of the self-heat rate and the adiabatic temperature rise. For the first time, runaway reactions in the laboratory can approximate the severity of those in industrial vessels. This behavior is extremely useful for the required validation [IS] of a computerized runaway reaction model.

This apparatus can also be used to

differentiate between materials that exhibit homogeneous versus nonfoamy behavior during emergency relief by measurement of the final void fraction in a test cell [13],

0 compare a measured to a calculated homogeneous-equilibrium flashing mass flux to determine where turbulent (nonviscous) or laminar (viscous) flow exists during a venting incident [13],

the design institute for emergency relief systems … · emergency relief system design using diem...

Documents