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Management of Hazards Associated with Location of Process Plant Buildings API RECOMMENDED PRACTICE 752 SECOND EDITION, NOVEMBER 2003 Copyright American Petroleum Institute Provided by IHS under license with API No reproduction or networking permitted without license from IHS --`,,-`-`,,`,,`,`,,`---

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Page 1: RP 752 (2003) Plant Hazards

Management of Hazards Associated with Location of Process Plant Buildings

API RECOMMENDED PRACTICE 752SECOND EDITION, NOVEMBER 2003

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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Page 2: RP 752 (2003) Plant Hazards

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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Page 3: RP 752 (2003) Plant Hazards

Management of Hazards Associated with Location of Process Plant Buildings

Downstream Segment

API RECOMMENDED PRACTICE 752SECOND EDITION, NOVEMBER 2003

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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Page 4: RP 752 (2003) Plant Hazards

SPECIAL NOTES

API publications necessarily address problems of a general nature. With respect to particu-lar circumstances, local, state, and federal laws and regulations should be reviewed.

API is not undertaking to meet the duties of employers, manufacturers, or suppliers to warnand properly train and equip their employees, and others exposed, concerning health andsafety risks and precautions, nor undertaking their obligations under local, state, or federallaws.

Information concerning safety and health risks and proper precautions with respect to par-ticular materials and conditions should be obtained from the employer, the manufacturer orsupplier of that material, or the material safety data sheet.

Nothing contained in any API publication is to be construed as granting any right, by impli-cation or otherwise, for the manufacture, sale, or use of any method, apparatus, or productcovered by letters patent. Neither should anything contained in the publication be construed asinsuring anyone against liability for infringement of letters patent.

Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least everyfive years. Sometimes a one-time extension of up to two years will be added to this reviewcycle. This publication will no longer be in effect five years after its publication date as anoperative API standard or, where an extension has been granted, upon republication. Status ofthe publication can be ascertained from the API Standards department telephone (202) 682-8000. A catalog of API publications, programs and services is published annually and updatedbiannually by API, and available through Global Engineering Documents, 15 Inverness WayEast, M/S C303B, Englewood, CO 80112-5776.

This document was produced under API standardization procedures that ensure appropriatenotification and participation in the developmental process and is designated as an API stan-dard. Questions concerning the interpretation of the content of this standard or comments andquestions concerning the procedures under which this standard was developed should bedirected in writing to the Director of the Standards department, American Petroleum Institute,1220 L Street, N.W., Washington, D.C. 20005. Requests for permission to reproduce or trans-late all or any part of the material published herein should be addressed to the Director, Busi-ness Services.

API standards are published to facilitate the broad availability of proven, sound engineeringand operating practices. These standards are not intended to obviate the need for applyingsound engineering judgment regarding when and where these standards should be utilized.The formulation and publication of API standards is not intended in any way to inhibit anyonefrom using any other practices.

Any manufacturer marking equipment or materials in conformance with the markingrequirements of an API standard is solely responsible for complying with all the applicablerequirements of that standard. API does not represent, warrant, or guarantee that such prod-ucts do in fact conform to the applicable API standard.

All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise,

without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C. 20005.

Copyright © 2003 American Petroleum Institute

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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Page 5: RP 752 (2003) Plant Hazards

FOREWORD

This publication is intended to assist management in identifying issues related to locationof process plant buildings which might be of potential concern, understanding associatedhazards, and managing risk. Among hazards that potentially could affect occupants of pro-cess plant buildings are: fire, explosion, and toxic releases. This publication provides a meth-odology for assessing and evaluating the hazards associated with location of process plantbuildings. It is not an engineering guide for the design of blast-resistant buildings.

Serious accidental releases of toxic material or explosions that impact occupied processplant buildings are not frequent events. Preventing incidents in process plants is a bettersafety investment than providing mitigation systems or redesigning process plant buildings.The implementation of process safety management, as described in API’s historical Recom-mended Practice 750, publications of the AIChE CCPS and OSHA 1910.119 is intended toimprove industry's safety performance. Risk management involves cost-effective applica-tions of risk-reduction alternatives.

Because this publication affects many existing buildings within processing facilities, asubstantial effort may be required for full implementation of the recommended practice. Thiscould include identifying buildings of concern, conducting building evaluations, and, ifappropriate, performing building upgrades or modifications. It is recognized that a substan-tial period of time may be required for complete application of the recommended practice,due to the scope and magnitude of the endeavor.

This second edition of API RP 752

Management of Hazards Associated with Location ofProcess Plant Buildings

recognizes that available information, publications and relevant ref-erences concerning specific PSM activities constitute a growing body of knowledge. A num-ber of resource publications are specifically referenced in the body of this standard whileothers are listed in Appendix A.

API publications may be used by anyone desiring to do so. Every effort has been made bythe Institute to assure the accuracy and reliability of the data contained in them; however, theInstitute makes no representation, warranty, or guarantee in connection with this publicationand hereby expressly disclaims any liability or responsibility for loss or damage resultingfrom its use or for the violation of any federal, state, or municipal regulation with which thispublication may conflict.

Suggested revisions are invited and should be submitted to the standardization manager,American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005.

v

Copyright American Petroleum Institute Provided by IHS under license with API

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Page 6: RP 752 (2003) Plant Hazards

CONTENTS

Page

1 GENERAL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Purpose. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Referenced Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 MANAGEMENT OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1 Hazards to Occupants of Process Plant Buildings . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Process Plant Building Issues of Concern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Overview of Analysis Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 Using This Recommended Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.5 Occupancy and Emergency Role Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 EXPLOSION ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1 Stage 1—Building and Hazard Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Stage 2—Building Evaluations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Risk Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 FIRES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.1 Materials of Concern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2 Building Occupancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.3 Spacing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.4 Mitigation and Emergency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.5 Risk Reduction for Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 TOXIC MATERIALS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.1 Toxic Material of Concern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.2 Building Occupancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.3 Site Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.4 Mitigation and Emergency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.5 Risk Reduction for Toxic Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6 BUILDING CHECKLIST. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 PROCESS SAFETY MANAGEMENT DILIGENCE . . . . . . . . . . . . . . . . . . . . . . . . 18

APPENDIX A BIBLIOGRAPHY FOR ADDITIONAL READING . . . . . . . . . . . . . . . 19APPENDIX B EXPLOSION, FIRE, AND TOXIC RELEASE PHENOMENA,

AND HAZARDS TO THE OCCUPANTS OF PROCESS PLANT BUILDINGS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

APPENDIX C DAMAGE CATEGORIZATION FOR BUILDINGS ANDVULNERABILITY OF OCCUPANTS TO EXPLOSIONOVERPRESSURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

APPENDIX D PROCESS PLANT BUILDING CHECKLIST. . . . . . . . . . . . . . . . . . . . 27APPENDIX E EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figures1 Stages for Explosion Risk Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 An Analysis Process for an Explosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

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Page 7: RP 752 (2003) Plant Hazards

Page

3 Sample Risk Matrix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 An Analysis Process for a Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 An Analysis Process for a Toxic Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17C-1 Sample Overpressure Versus Vulnerability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Tables1 Examples of Process Safety Information Needs by Stage . . . . . . . . . . . . . . . . . . . . 72 Summary of Possible Explosion Effects on Buildings. . . . . . . . . . . . . . . . . . . . . . . 93 Overpressure Effects on Various Building Components . . . . . . . . . . . . . . . . . . . . 104 Overpressure on Various Building Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Typical Overpressure Effects on Unprotected People . . . . . . . . . . . . . . . . . . . . . . 116 Sample Risk-ranking Categories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13C-1 Generic Frequencies of Major Explosions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

vii

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Page 8: RP 752 (2003) Plant Hazards

Copyright American Petroleum Institute Provided by IHS under license with API

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Page 9: RP 752 (2003) Plant Hazards

1

Management of Hazards Associated with Location of Process Plant Buildings

SECTION 1—GENERAL

1.1 PURPOSE

This publication provides guidance for identifying hazardsthat may affect process plant buildings and for managingrisks related to those hazards. An analysis process set forth inthis Recommended Practice provides a structured approachthat can improve worker safety by the following:

a. Continuing to improve the understanding of identifiedhazards.b. Continuing to focus on accident prevention and addressingidentified hazards.c. Managing risk.

The methodology recommended in this document will helpprovide the user with an understanding of the relative risk ofeach building studied. This relative risk should be consideredin long-range planning and projects that involve buildingchanges (such as control building consolidation, office build-ing replacements, and so forth).

1.2 SCOPE

1.2.1 Applicability

This publication was developed for refineries, petrochemicaland chemical operations, natural gas liquids extraction plants,and other facilities covered by the OSHA Process ManagementStandard, 29

CFR

1910.119. This publication does not apply toproduction facilities surrounded by navigable waters, such asoffshore platforms or to storage tanks, wastewater tanks andsimilar facilities. Such facilities have unique siting issues whichare addressed by other recommended practices, such as RP 14Jfor off-shore facilities.

Additionally, this publication is not intended for use indesigning and locating safe refuge from the effects of fires,explosions, and toxic releases.

1.2.2 Relationship of this Recommended Practice to OSHA 29

CFR

1910.119

OSHA 29

Code of Federal Regulations

(

CFR

) 1910.119,“Process Safety Management of Highly Hazardous Chemi-cals (PSM),” includes requirements for addressing facility sit-ing as part of a process hazards analysis (PHA).

This publication is intended to assist in identifying the sit-ing issues for process plant buildings, understanding the asso-ciated hazards, and managing the risk. Hence, this publicationprovides a framework that can be used to address facility sit-ing within the PHA requirements of OSHA 29

CFR

1910.119as applied to buildings.

The PHA as required by OSHA 29

CFR

1910.119 isintended to identify scenarios that could lead to serious releaseof toxic or flammable materials or an explosion. Those parts ofthis publication intended to assist in the PHA process are iden-tified on the flowcharts (see Figures 2, 4, and 5) by a dashed-line box labeled “PHA.” The remaining parts are intended toserve as management aids in resolving issues that arise whenevaluating the location of process plant buildings.

1.3 DEFINITIONS

For the purpose of this publication, the following defini-tions apply:

1.3.1 aggregate risk:

A measure of the total risk to allpersonnel within a building(s) or within a facility, dependingupon the risks being evaluated, who are impacted by a com-mon event, taking into account the total time spent in thebuilding(s) or facility.

1.3.2 assessment:

Describes a detailed qualitative orquantitative analysis to estimate the potential likelihood andconsequences of site-specific events, and then to compare theresults with acceptance criteria.

1.3.3 confinement:

A qualitative or quantitative measureof the enclosure or partial enclosure areas where a vaporcloud may be contained.

1.3.4 congestion:

A qualitative or quantitative measureof the physical layout, spacing, and obstructions within afacility that promote development of a vapor cloud explosion.

1.3.5 evaluation-case event:

The scenario with themost severe consequences, considering all incidents and theiroutcome, that is considered plausible or reasonably believable.

1.3.6 evaluation:

Describes the application of analyticaltools to aid in making decisions about buildings.

1.3.7 hazard:

An inherent physical or chemical character-istic (flammability, toxicity, corrosivity, stored chemical ormechanical energy) or set of conditions that has the potentialfor causing harm to people, property, or the environment.

1.3.8 individual risk:

The risk to a single person inside aparticular building. Maximum individual risk is the risk to themost-exposed person and assumes that the person is exposed.

1.3.9 process plant building (also referred to inthis recommended practice as a

building

):

Any tem-porary or permanent building within a facility that could be

Copyright American Petroleum Institute Provided by IHS under license with API

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Page 10: RP 752 (2003) Plant Hazards

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impacted by a serious release from a process covered byOSHA 29

CFR

1910.119.

1.3.10 risk:

Relates to the probability of exposure to ahazard which could result in harm to personnel, the environ-ment or general public. Risk is a measure of potential forhuman injury or economic loss in terms of both the incidentlikelihood and the magnitude of the injury or loss.

1.3.11 risk assessment:

The identification and analysis,either qualitative or quantitative, of the likelihood and out-come of specific events or scenarios with judgements of prob-ability and consequences.

1.3.12 risk-based analysis:

A review of potential needsbased on a risk assessment.

1.3.13 screening:

Describes a process of comparing thesite-specific materials and process conditions, and buildingoccupancy with pre-established criteria of concern.

1.3.14 vapor cloud explosion (VCE):

An explosion inair of a flammable material cloud [see 2.1.1].

1.4 REFERENCED PUBLICATIONS

The following guidelines, standards, codes, and specifica-tions are cited in this publication; Appendix A provides addi-tional resources as a “Bibliography of Additional Reading.”

API RP 14J

Recommended Practice for Design andHazard Analysis for Offshore ProductionFacilities

RP 750

Management of Process Hazards

AIChE CCPS

1

Dow’s Fire & Explosion Index Hazard ClassificationGuide

Guidelines for Chemical Process Quantitative Risk Analy-sis,

2nd Edition

Evaluating Process Safety in the Chemical Industry: AUser’s Guide to Quantitative Risk Analysis

Guidelines for Evaluating Process Plant Buildings forExternal Explosion and Fires

Guidelines for Evaluating the Characteristics of VaporCloud Explosions, Flash Fires, andBLEVEs

Guidelines for Hazard Evaluation Procedures,

2nd Editionwith Worked Examples

Guidelines for Use of Vapor Cloud Dispersion Models

Application of the Mond Fire, Explosion and Toxicity Indexto Plant Layout and Spacing Distances,

Lewis, D. J. (1980), AIChE Loss Preven-tion No 13, p. 20

ASCE

2

Design of Blast Resistant Buildings in Pet-rochemical Facilities

Chemical Industries Association

3

Guidance for the Location and Design of Occupied Build-ings on Chemical Manufacturing Sites

Loss Prevention in the Process Industries,

Volume I, FrankP. Lees, 1980

U.S. Federal OSHA

4

29

CFR

1910.119“Process Safety Management of HighlyHazardous Chemicals”

U.S. Department of Defense

5

The Effects of Nuclear Weapons,

rev. ed.,Samuel Glasstone, Editor, published byU.S. Atomic Energy Commission [

althoughthis document is now out of print it is widelycited as an authoritative reference

]

SECTION 2—MANAGEMENT OVERVIEW

2.1 HAZARDS TO OCCUPANTS OF PROCESS PLANT BUILDINGS

2.1.1 Flammable Materials

Flammable materials, if ignited, have the potential toexpose process plant buildings to radiant heat from either aflash, jet, pool fire, or fireballs. Additionally, some flammable

materials can form vapor cloud mixtures in air, and if delayedignition occurs and certain site conditions (discussed below)exist, a vapor cloud explosion (VCE) can occur. Dependingon the magnitude of an explosion, and the location and spe-cific construction details of exposed buildings, an explosioncould result in varying degrees of risk to building occupants.

1

American Institute of Chemical Engineers, 3 Park Avenue, NewYork, New York 10016-5991. www.aiche.org/ccps/

2

American Society of Civil Engineers, 1801 Alexander Bell Drive,Reston, Virginia 20191. www.asce.org

3

Chemical Industries Association, Kings Building, Smith Square,London SW1P 3JJ, England. www.cia.org.uk

4

U.S. Department of Labor, Occupational Safety and Health Admin-istration, 200 Constitution Ave., N.W., Washington, D.C. 20210. TheOSHA portion of the

Code of Federal Regulations

is available onlineat: www.osha.gov.

5

U.S. Department of Defense. www.dod.gov

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2.1.2 Toxic Materials

Occupants of process plant buildings can be exposed totoxic materials if such material enters the building. The dis-persion properties of the toxic material involved, the releaseconditions, and the toxicity of the substance are some of thefactors to be considered when evaluating the potential fortoxic effects on process plant building occupants from intru-sion of toxic materials. The building’s ability to resist toxicmaterials ingress should also be considered.

2.1.3 Other Process Materials

Some process materials pose potential risk from runawayreactions or chemically or thermally induced decomposition.Events involving these materials have the potential to produceexplosion effects, toxic releases, fire, and projectile hazards toprocess plant buildings and their occupants.

2.1.4 Extreme Process Conditions

Extreme process conditions, such as high temperature orpressure, can pose hazards even when the process materials, atambient conditions, might otherwise be innocuous. Some ofthese conditions include, but are not limited to, the following:

a.

Compressed gases

—Failure of vessels containing high-pressure compressed gases can result in the explosive expan-sion of the gases, producing damaging blast waves.b.

Boiling liquid expanding vapor explosions

(BLEVES)—Liquids stored at temperatures well above their atmosphericboiling points will evaporate at explosive rates if depressur-ized by the failure of the containment vessel. As withcompressed gases, expansion of the resultant vapor can pro-duce blast waves. Additionally, flammable materials, ifignited, can produce fireballs.c.

Hot materials

—Rapid combination of a hot material and acold material can lead to explosive vaporization of the coldermaterial. Damaging explosions have occurred, for example,from adding water to vessels containing hot oil and from theaddition of molten metals to water.

2.1.5 Factors Influencing Events

Factors that determine the occurrence of specific types ofevent or the potential effects may include, but are not limitedto, the following:

a.

Inherent properties of the process material

—These includevariations in flammability, flammability range, vaporizationand dispersion properties, flame propagation properties (referto Appendix B for a discussion of the characteristics of explo-sions, toxicity, and chemical and thermal stability).b.

Release conditions

—These include process conditionssuch as pressures and temperatures, as well as the release rateand location of the release. For example, a pressurized releaseof a gas or vapor, if ignited immediately, can lead to a jet fire.

Alternatively, the formation of a vapor cloud before ignitionhas the potential to result in an explosion. A release of mate-rial stored under cryogenic conditions could produce aheavier-than-air vapor cloud, even if the substance is lighterthan air at ambient conditions.

c.

Other site-specific conditions

—For releases that formvapor clouds with subsequent ignition, the key factors influ-encing the development of overpressure are the degree ofpremixing with air and the acceleration of the flame front thatresults from turbulence generated during combustion. Turbu-lence can be caused by the release conditions, meteorologicalconditions, and/or the physical layout of the release area. Aflame front passing through congested areas containing obsta-cles, such as process plant equipment and structures, cangenerate turbulence in the unburned portion of the vaporcloud, leading to significant increases in flame speed andresulting overpressure. Confinement is another site-specificcondition that affects the development of overpressure.Releases occurring in open areas, with no congestion orobstructions, generally do not develop significant overpres-sure upon ignition.

d.

Process design and control

—These factors include inven-tory size and potential reduction, process controls, processalarms and interlocks, shutdown systems, redundancies andmitigation systems.

e.

Emergency response

—The response in an emergency,such as automatic shutdown and relief, operator intervention,evacuation, emergency team response, along with other fac-tors can impact event outcomes.

f.

Process safety management system

(PSM)—PSMincludes policies, programs, procedures, training, audits, andother elements intended to assure that processing plants aredesigned, constructed, operated, and maintained in a mannerthat reduces the potential for serious accidents. Examples ofPSM include equipment and piping inspection programs, per-sonnel training, safe work practices (for example, lockout/tagout and hot work permit), management of change, andoperating procedures.

2.2 PROCESS PLANT BUILDING ISSUES OF CONCERN

Process conditions vary from facility to facility and fromprocess to process. The factors indicated in Section 2.1 mayinfluence the likelihood or severity of a potential eventdepending on the products produced at the facility or the pro-cess being evaluated. At the same time, there are wide varia-tions in building design, construction, function, occupancy,and location. Therefore, evaluating the risks to building occu-pants may require assessing a wide range of situations. Somebuildings may be identified as having minimal risk for occu-pants and will require no further action. However, some eval-uations may indicate that risk-reduction measures are

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required. Depending upon the results of the evaluation, a vari-ety of options may be available, including the following:

a. Process changes to reduce inventories or eliminate materi-als of concern.b. Preventive measures intended to reduce the likelihood ofpossible releases.c. Installation of mitigation systems.d. Building structural upgrades to withstand the predictedeffects of fire, explosion, or toxic releases.

2.3 OVERVIEW OF ANALYSIS PROCESS

2.3.1 Explosions

Section 3 of this standard outlines a three-stage analysisprocess for identifying hazards and managing risk to buildingoccupants from explosions, as shown in Figures 1 and 2.

The staged approach is used to systematically identify andevaluate buildings in which occupants may be at risk. Theanalysis becomes more complex as the user progressesthrough the stages because the site-specific information isdeveloped in greater detail. The analysis process can be endedat any stage if at that point a determination is made that thereare no significant risks to occupants of the building beingreviewed. Also, users of this analysis process are not con-strained to successively follow the stages of the evaluationprocess, but may proceed directly to either Stage 2 or 3 afterconsidering the implications of Stage 1.

• Stage 1, “Building and Hazard Identification,”(Section3.1) outlines an initial identification process to selectbuildings for further investigation based on their occu-pancy level and their proximity to processes whichhave the potential for explosions.

• Stage 2, “Building Evaluation,” (Section 3.2) outlinesthree approaches to be used to evaluate potential haz-ards to building occupants. Any or all of the approachesmay be used.

• Stage 3, “Risk Management,”(Section 3.3) outlines theuse of qualitative and quantitative risk-assessment toolsto perform a more complex evaluation for buildings,coupled with proposals for reducing and controllingrisk, where warranted.

2.3.2 Fires and Toxic Releases

Sections 4 and 5 of this standard outline an approach foridentifying risks to building occupants from fires and toxicreleases. As discussed in Section 2.1, these risks depend uponvarious factors, including building design and emergency

response plans. The analysis process for fire and toxic releasesis considerably less complex than the analysis for explosions.

2.4 USING THIS RECOMMENDED PRACTICE

2.4.1 Company-specific Issues

This standard has been developed to allow users flexibilityin choosing their preferred analysis techniques. However, touse sections of this standard, a company shall consider theneed to develop the following information:

a.

Occupancy and emergency role of personnel criteria:

These are company-specific criteria that define occupancylevels within a building and the personnel role of buildingoccupants in an emergency.b.

Evaluation-case events:

These include the fire, explosion,and toxic release events that a company believes are plausibleand realistic and have the potential to impact process plantbuildings. c.

Consequence modeling/analysis programs:

These are theprograms and techniques that a company determines are to beused in evaluating the possible effects of serious releases ofhighly hazardous materials.d.

Risk acceptance criteria:

These are the criteria, eitherqualitative or quantitative, that a company establishes anduses to make decisions concerning acceptable risk.

2.4.2 Risk Reduction

If, at any time during the evaluation process, it becomesobvious that a potential hazard needs to be mitigated, then theevaluation effort should proceed directly to risk reduction.

Figure 1—Stages for Explosion Risk Analysis

Stage 1Building and Hazard Identification

Stage 2Building Evaluation

Stage 3Risk Management

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M

ANAGEMENT

OF

H

AZARDS

A

SSOCIATED

WITH

L

OCATION

OF

P

ROCESS

P

LANT

B

UILDINGS

5

2.4.3 Screening, Evaluation, and Assessment

Throughout this standard, the terms screening, evaluation,and assessment are used. For the purpose of this standard thefollowing definitions shall apply:

a.

Assessment

describes a detailed qualitative or quantitativeanalysis to estimate the potential likelihood and consequencesof site-specific events and to then compare that estimatedresult with acceptance criteria.b.

Evaluation

describes the application of analytical tools toaid in making decisions about buildings.c.

Screening

describes a process of comparing the site-spe-cific materials and process conditions and building occupancywith pre-established criteria of concern.

2.5 OCCUPANCY AND EMERGENCY ROLE CRITERIA

This section discusses occupancy and emergency role cri-teria that should be determined by each company.

2.5.1 Occupancy Strategy

Each company should define occupancy criteria accordingto its individual work environment and operating philosophy.When defining these criteria, a company should consider thefollowing:

a.

The number of people housed in buildings located in oradjacent to process units:

Direct support personnel, such asoperators, field supervisors, board operators, engineers, andmaintenance personal, may have to be located in immediateareas for logistical and response purposes. b.

Congregations of people in buildings located in closeproximity to process areas:

Activities that result in an inter-mittent or recurrent congregation of people should beconsidered. These may include activities involving severalpeople whose work or duty station is not at the process unit,training activities for personnel who do not need access toprocess equipment, and activities centered around dining,locker or facilities. c.

Personnel assigned outside the process area:

These per-sonnel (for example, analyzer technicians, process operators,and maintenance personnel) normally have job responsibili-ties that primarily are performed on unit equipment locatedoutside the process area. These personnel may temporarilyenter buildings near process areas (for example, operatorshelters) to write reports, use the phone, or seek refuge frominclement weather. d. Ability to evacuate: Buildings may provide adequatemeans of egress to permit evacuation. The potential eventsthat result in fires, BLEVEs, and in some circumstancesVCEs typically develop over a period of time. To addressthese potential situations, detection and alarms may be pro-vided, and consideration should be given to the ability to

evacuate. Process materials that have the potential for run-away reactions or chemically or thermally induceddecomposition may produce toxic, fire, or explosion effectswith little or no warning. If adequate early warning cannot beprovided, building evacuation may not be a viable option tomitigate the hazards. The potential for these circumstanceswill normally be recognized and considered during a PHA.

2.5.2 Occupancy Criteria

There are a number of alternative methods available fordefining occupancy criteria. Each method has unique merits.Each company may define its criteria based on a single methodor a combination of methods in a tiered approach. The follow-ing examples can be used singularly or in combination.

a. Occupied and Unoccupied may be defined by a companyusing qualitative criteria to designate occupied and unoccu-pied buildings. For example, occupied buildings could bedefined as those that personnel occupy while doing the majorpart of their work, such as control rooms, laboratories, andoffice buildings. An unoccupied building would then be onethat personnel visit infrequently to perform brief tasks ormonitor the process. b. Occupancy load is defined as the total integrated time forthe full- or part-time occupants in the building. The numberof personnel present in the building is a function of the nor-mal daily activities and the time of day. The occupancy loadis determined by adding all the hours that each person spendsin that building. The occupancy load is normally expressed asthe inhabited time over a specific period, based on an annualaverage. The occupancy load threshold criteria used by somecompanies ranges from 200 – 400 personnel hours per week.

Some factors to consider in determining the occupancyload are as follows:

1. The routine presence of additional personnel, such asvisitors, contractors, and trainees.2. Activities performed by personnel on a routine basis,such as calibration and maintenance of instruments.

c. Individual occupancy is defined in terms of the percent-age of an individual’s total time spent in a building. Anoccupied building may be defined as one in which at leastone person does a significant portion of his or her work. Theindividual occupancy criteria used by some companiesranges from 25% – 75%.d. Peak occupancy is defined as the number of people poten-tially exposed for a given period of time (for example, asafety or toolbox meeting where operators/crafts people meetfor a short period). The threshold peak occupancy criteriaused by some companies range from 5 – 40 persons.

Buildings with occupancy criteria above the companypre-defined threshold occupancy criteria should be evalu-ated further. Example 3 in Appendix E applies these crite-ria to hypothetical situations.

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6 API RECOMMENDED PRACTICE 752

Figure 2—An Analysis Process for an Explosion

ProcessSafety information

Are materialsbeing handledthat have thepotential forexplosion?

Do siteconditions

clearlyprecludeevents ofconcerns?

Does buildingexceed

occupancycriteria?

OccupancyCriteria

BuildingChecklist

ProcessSafety

ManagementDiligence

Stop

BUILDING AND HAZARDIDENTIFICATION(Stage 1)

3.1.1 (see note)

Select EvaluationAnalysis Method

Consequence Analysis Screening RiskAnalyze

Design or Compare toIndustry or Company

Standards

3.2

3.2.2 3.2.33.21

Is building asignificantconcern?

2.56.0

7.0

PHA

No

Yes

No

No

Yes

3.1.2

3.1.3

3.1.4

BuildingChecklist

Process SafetyManagement

Diligence

Is additionalevaluationdesired?

3.2.4

3.2.5

Yes

Yes

No

7.0 6.0 No

Risk Assessment

Is risk reduction

appropriate ?

Process SafetyManagement Diligent

Risk Reduction/Mitigation

Risk AcceptanceCriteria

Yes

No

3.3.1

BUILDINGEVALUATION(Stage 2)

RISK MANAGEMENT(Stage 3)

Note: Related paragraph numbers found within text.

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 7

2.5.3 Emergency Role of Buildings

The job duties of occupants during an emergency shouldbe considered as these can affect the role of buildings duringemergencies. Buildings should be considered for further eval-uation if the occupants are required to stay in the building orpersonnel are required to enter a building during an emer-gency. Examples of buildings that may be subject to an emer-gency role are control rooms and emergency responseshelters. Buildings that are evacuated in the event of an emer-

gency (for example, satellite control rooms) should be con-sidered for further evaluation to ensure that evacuation can beaccomplished safely.

2.5.4 Changes in Occupancy

When changes cause the building occupancy to increase orthe emergency role of the building changes, the buildingshould be reevaluated using this recommended practice or thecompany equivalent.

SECTION 3—EXPLOSION ANALYSIS

A systematic three-stage process for analyzing explosionhazards to occupants of process plant buildings is dia-grammed in Figure 2. Each box in the flowchart is numberedwith the corresponding section in this standard that discussesthe information for that box. The analysis process may beused for both new and existing buildings and is intended toprovide the user with multiple options. The user should rec-ognize that the analysis becomes more complex as oneprogresses through the flowchart. Some users may elect toperform all three steps. Others may wish to perform Stage 3,Risk Management, after completing Stage 1, Hazard Identifi-cation. This flexibility allows the users to select the bestoption to meet their specific requirements.

3.1 STAGE 1—BUILDING AND HAZARD IDENTIFICATION

This section is intended to assist teams performing processhazards analyses (PHAs) or site-specific building evaluationsto identify buildings that could potentially be impacted by anexplosion.

3.1.1 Process Safety Information

The first step for identifying and evaluating risks resultingfrom the location of process plant buildings is to gather infor-mation. The focus is on site-specific parameters with thepotential to influence the likelihood or severity of an explo-sion. Process safety information as identified by OSHA 29CFR 1910.119(d) can provide basic information on the pro-cess unit, process conditions, and materials handled. Theamount of information required increases with each evalua-tion stage of the process. Table 1 indicates some [but not all]typical process safety information needs.

3.1.2 Materials of Concern

Materials such as flammable light hydrocarbons, chemicalsand gases or heavier hydrocarbons that are processed at ele-vated temperatures and/or pressures may have the potentialfor a vapor cloud explosion (VCE) upon release. Examples of

materials that can form vapor clouds upon release includeethane, propane, butane, and related olefins (ethylene, propy-lene, and butylene). Under some release conditions, such aselevated temperatures, less volatile materials can also form avapor cloud. Overpressure can be developed from explosionsinvolving a boiling liquid expanding vapor explosion(BLEVE), vapor cloud explosion, chemical decomposition ormechanical failure of a pressure vessel.

Example 1 in Appendix E applies the preceding informa-tion to a hypothetical situation.

3.1.3 Site-specific Conditions

Although some facilities handle materials with a VCEpotential, site-specific conditions may preclude a VCE fromoccurring. Factors that influence the potential for a VCEinclude the degree of congestion and confinement, plant layout,

Table 1—Examples of Process Safety Information Needs by Stage

Stage Examples of Typical Information

1 Material Safety Data SheetsBuilding OccupancyEmergency Role of Building

2 Depending on the evaluation used, information needs may include:

Stage 1 informationInventories of MaterialsTemperatures, pressures and flow ratesBuilding construction, materials and dimensionsDistances between process units and buildingsFrequency data for explosions in similar plants

3 Depending on the analysis process, information needs may include:

Stage 1 and 2 informationSite incident historyComplete building design and construction detailsOperating proceduresDescription of passive and active mitigation systemsMaintenance and inspection standards and recordsOther pertinent information on the existing process safety management systems

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spacing between areas, sources of ignition, topography, drain-age, the amount of material released, and the release rate. Inparticular, the potential degree of congestion and confinementshould be considered, as these are often key factors contribut-ing to the magnitude of an explosion. In performing the reviewfor site-specific conditions, common equipment arrangementincluding pipe racks, columns, vessels, and other plant equip-ment in proximity to one another should be considered. Experi-ence has shown that dense arrays of equipment can providesufficient congestion for development of VCE conditions. Ifflammable materials are processed in an open plant environ-ment and they are released at sufficiently low rates, it isunlikely that a VCE will develop.

Process conditions such as operating temperatures, pres-sures, and inventory of material in equipment are also site-specific conditions that affect the potential for VCEs.

Emergency response that results in timely closing of isola-tion valves, depressuring of equipment, and shutdown of aprocess may be effective in reducing the total quantity ofmaterial released. The effectiveness of such actions in pre-venting or reducing the severity of a potential VCE willdepend upon:

• the magnitude and the initial release rate

• the time required to detect the release

• the timeliness and effectiveness of the emergencyresponse actions

• the time required for the release to subside

• the time and distance for released material to dissipate

• and the location of potential ignition sources relative tothe release.

Example 2 in Appendix E applies the preceding informa-tion to a hypothetical situation.

3.1.4 Occupancy Comparison

Once it has been determined that site-specific conditionshave the potential to contribute to an explosion impacting thebuilding, the plant buildings should be evaluated using thecompany’s predefined occupancy criteria. Buildings exceed-ing the criteria should be further evaluated. Buildings that donot exceed the occupancy criteria may be evaluated using abuilding checklist as discussed in Section 6.

3.2 STAGE 2—BUILDING EVALUATION

Stage 2, “Building Evaluation,” provides a process forevaluating the hazards to buildings. Evaluation optionsinclude (but are not limited to) the following:

a. Design or compare to industry and company standards(Section 3.2.1).

b. Consequence analysis (Section 3.2.2).

c. Screening risk analysis (Section 3.2.3).

Any combination of the evaluation options may be used,with the understanding that the methodologies becomeincreasingly complex.

The user should be aware that all the methodologies listedin this section of the recommended practice have limitationsin identifying the actual potential effects of the hazards tobuildings and their occupants. Understanding the limitationsdiscussed below, and matching them with the purpose of theevaluation, is needed before using or selecting the method-ology.

3.2.1 Design or Compare to Industry and Company Standards

Industry groups, insurance associations, the government,and many companies have developed standards and guide-lines for building design and spacing that are intended to pro-vide protection against the effects of explosions. Many ofthese standards are based on experience. Because the size andcomplexity of process units have increased, the distancesspecified in some standards may no longer apply where newunits have been installed or existing units have been rebuilt,expanded or replaced.

The user should understand the objectives of the spacingstandards. Many of the spacing standards are based on broadassumptions of process and plant conditions and not on site-specific conditions such as possible or actual operating condi-tions, confinement, and process conditions, all of which mayhave an effect on the potential for a significant overpressure.These variables should be considered when using any stan-dard, including company standards. For example, the userneeds to understand that some standards were developed spe-cifically for chemical processing units and not for petroleumrefineries. In general, insurance-industry standards histori-cally have been designed to protect property and minimizebusiness interruption in the event of an incident rather than toprotect personnel. A list of some available industry, insur-ance, and government standards is in Appendix A.

Example 4 in Appendix E applies the preceding informa-tion to a hypothetical situation.

3.2.2 Consequence Analysis

The objective of consequence analysis is to estimate thepotential magnitude of an explosion, evaluate its effects on abuilding, and relate the damage sustained by the building to thedegree of potential injury or damage to the occupants and/orequipment inside. Consequence analysis typically involves theselection of evaluation-case events that include size and dura-tion of expected release. Evaluation-case events of concernmay be identified through the PHA process. Both passive andactive independent mitigation systems should be considered.Passive mitigation systems include inherent design featuressuch as layout and spacing or design of the equipment in orderto limit the size of an explosion. Examples of active mitigation

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 9

systems include features such as automatic detection and isola-tion valves to reduce the quantity of material released, auto-matic shutdown systems, and automatic water spray systems.

There are several methods available for calculating explo-sion overpressure, such as the TNT-equivalency, Multi-Energy, and Baker-Strehlow. Additional information onexplosion calculation methods is available in Section 4-3 ofGuidelines for Evaluating the Characteristics of Vapor CloudExplosions, Flash Fires, and BLEVEs, published by the Cen-ter for Chemical Process Safety (CCPS).

If the calculated overpressure exceeds the design for thebuilding, further evaluation should be considered necessary. Ifthe building can adequately protect the occupants from theeffects of an explosion, the building can be eliminated fromfurther evaluation, and a building checklist may be completed.

Hazard evaluation tools may be used to prioritize buildingsfor consideration, based on various factors. Hazard evaluation

methods may not account for all site-specific conditions thatcould lead to significant overpressure. Techniques that takeinto account some site-specific conditions, such as the DowFire and Explosion Index and Mond Index, have also beenused. The results of these indices should be considered inconjunction with other factors, rather than as stand-alone cri-teria. These other factors should include an evaluation of theeffects of confinement and/or congestion-induced turbulenceon the potential for explosion overpressure.

As part of the consequence analysis, the building’sresponse to the overpressure should be considered. The build-ing’s response to an explosion will vary, depending on suchthings as the building’s structural design, materials of con-struction, orientation, and source of the explosion. Table 2summarizes explosions by source, type, attributes of theimpulse, and possible hazards to building occupants.

Some buildings in process plants have some inherent capac-ity to resist explosion loadings if they meet local building codesand company building standards for environmental loads suchas wind and snow or earthquakes. However, the buildings’response to explosive load within the process plant will varydepending on construction types, building styles, and sitingconsiderations over the life of the process plant. Additionally,the buildings may have been expanded or modified since theiroriginal construction. The variety of construction techniquesfor buildings makes exact determination of building responsesto explosion loads difficult. Buildings should be evaluatedbased on their specific structural design. Additional informa-

tion on building resistance to overpressures is provided in theASCE book Design of Blast Resistant Buildings in Petrochemi-cal Facilities, which notes that windows and supporting framesshould be designed for the same blast resistance as walls.

Table 3 is provided as a means to illustrate potential effectson building components. This table shows approximate val-ues; it is not intended for design purposes nor should it beused as a sole design reference.

Table 4 indicates effects of overpressure on different typesof buildings. In a consequence analysis, it may be assumedthat building occupants could incur injuries if the integrity ofthe building is exceeded. Table 4 does not include pre-engi-

Table 2—Summary of Possible Explosion Effects on Buildingsa

Source of Explosion Type of Explosion Nature of Blast WavePossible Hazards to Building

Occupants

Vapor cloud of flammable material

Deflagration Moderate to high overpressure of long duration

Building response to the explosion wave, fire and combustion products, glass shards

Condensed phase chemical explosion

Deflagration of detonation High overpressure of short duration

Building response to the explosion wave, projectiles, glass shards and ground shock

Dust cloud Deflagration Low to high overpressure of long duration

Fire, building response to the explosion wave, glass shards fire and combustion products

Release of flammable boiling liquid (BLEVE)

Physical expansion and deflagration

Moderate overpressure of long duration

Fire, combustion products, build-ing response to the explosion wave, glass shards and projectiles

Rapid loss of confinement of high-pressure gas

Physical expansion High overpressure of short duration

Building response to the explosion wave and projectiles including glass shards

Note: aThis list should not be considered all-inclusive.

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10 API RECOMMENDED PRACTICE 752

neered blast-resistant modular buildings which became avail-able since the study was published (developed primarily inresponse to security concerns). Table 5 indicates typicaleffects of overpressure on people.

A simple and conservative approach for use in screeningevaluations is shown in Table 4. The actual relationshipbetween explosion effects and building damage depends onmany parameters including overpressure and impulse of theexplosion. Additionally, building parameters play an importantpart in building evaluations; these include, but are not limitedto, things such as material of construction, configuration,

dimensions of thickness and span, details of reinforcement andconnections, type of windows and physical condition. Whenadditional evaluation is necessary, a detailed analysis of theseparameters should be considered in a structural analysis of thebuilding. Table 4 may be used for screening purposes.

3.2.3 Screening Risk Analysis

A screening risk analysis is used to determine approximateaggregated and individual risk to occupants of a process plantbuilding. This analysis method provides for coupling esti-mates of event frequency with explosion consequences to

Table 3—Overpressure Effects on Various Building Componentsa

Building Component Reflected Overpressure (PSIG) Component Response

Glass 0.2 BreakingGlass 0.5 – 1.0 Shattering with body penetrating velocitiesWooden frame 1.0 – 2.0 Structural failure and potential collapseSteel cladding 1.0 – 2.0 Internal damage to walls, ceilings and furnishingsConcrete-asbestos cladding (Transite) 1.0 – 2.0 ShatteringBrick cladding 2.0 – 3.0 Blown-inUnreinforced masonry 1.0 – 3.0 Wall collapse, possible shatteringNote: aSource: The Effects of Nuclear Weapons by Glasstone (1964).

Table 4—Overpressure on Various Building Types

Building Type Peak Side-on Overpressure (psi) Consequences

Wood-frame trailer or shack 1.0 Isolated buildings overturn. Roofs and walls collapse2.0 Complete collapse5.0 Total destruction

Steel-frame/metal siding pre-engineered building

1.5 Sheeting ripped off and internal walls damaged. Danger from falling objects

2.5 Building frame stands, but cladding and internal walls are destroyed as frame distorts

5.0 Total destruction

Unreinforced masonry bearing wall building 1.0 Partial collapse of walls that have no breakable windows

1.25 Walls and roof partially collapse1.5 Complete collapse3.0 Total destruction

Steel or concrete frame w/unreinforced masonry infill or cladding

1.5 Walls blow in2.0 Roof slab collapses2.5 Complete frame collapse5.0 Total destruction

Reinforced concrete or masonry shear wall building

4.0 Roof and wall deflect under loading. Internal walls damaged

6.0 Building has major damage and collapses12.0 Total destruction

Note: Source: The Effects of Nuclear Weapons, rev. ed., Samuel Glasstone, EditorPrepared by the U.S. Department of Defense, published by U.S. Atomic Energy Commission

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 11

determine risk to a process plant building occupant. This riskmay then be compared to company risk acceptance criteria toidentify buildings requiring further analysis. The steps for thescreening risk analysis may include the following:

a. Identify the process plant buildings and their constructionfeatures in proximity to processes where explosions mightoccur.

b. Determine the explosion loading (overpressure and dura-tion) for each event for each target building using the TNT-equivalency, Multi-Energy, Baker-Strehlow, or other method.

c. Determine the frequency of explosions, based on historicalor industry data for the type of process unit at the facility.Because the information available is often limited, the usershould carefully evaluate the applicability of any such infor-mation before using the analysis. See Appendix C for moreinformation on typical data for explosion frequency.

d. Determine the vulnerability of occupants (probability offatality) in the target building versus overpressure (overpres-sure as a function of the type of building construction). SeeAppendix C for more information on vulnerability ofoccupants.

e. Calculate the risk to an individual from a single event bymultiplying the explosion frequency, the percentage of timethat the individual occupies the building, and the vulnerabilityestimated for building occupants. To obtain the total individualrisk to an occupant, add the risks from all potential explosionevents. The total risk for the most exposed individual is themaximum individual risk. Other means of expressing individ-ual risk exist, and the user may wish to refer to the CCPS’sGuidelines for Chemical Process Quantitative Risk Analysis.

f. Calculate the aggregate risk to all building occupants. Onesimple way of expressing aggregate risk would be to deter-mine the weighted average occupancy of the building bysumming, for all individuals, the fraction of time that eachindividual occupies the building. Next, multiply the weightedaverage occupancy by the explosion frequency and the vul-nerability estimated for building occupants. To obtain thetotal aggregate risk to all occupants, sum the risks from allpotential events. There are a variety of means for expressingaggregate risk, and the user may wish to refer to the CCPS’sGuidelines for Chemical Process Quantitative Risk Analysis.

g. Compare the calculated risk with the company’s risk accep-tance criteria to determine whether additional study is required.

The screening risk-analysis method provides an additionallevel of evaluation because the frequency of the incident hasbeen included in the evaluation. By identifying risk ratherthan just the consequences, additional buildings not posingsignificant risk to the occupants may be identified. Screeningrisk analysis may require more time and effort to completethan those methods previously discussed. Additionally, theuse of industry versus site-specific data may not account forfactors such as maintenance history, process safety manage-ment (PSM) effectiveness, age and condition of the unit orconfinement/congestion.

Example 6 in Appendix E applies the preceding informa-tion to a hypothetical situation.

3.2.4 Buildings Requiring Further Evaluation

The evaluation process in Stage 2 is intended to assist inidentifying buildings with sufficient hazard to building occu-pants to justify either further evaluation or an additional riskassessment. Performing a checklist review of those buildingsthat do not need additional evaluation or assessment will ver-ify that routine risk-reduction measures have been taken. Thisis shown in Example 9 in Appendix E.

3.2.5 Additional Evaluation

If a building has been determined to require additionalevaluation, the user may wish to explore using additionalmethods in Stage 2 before concluding that a risk assessmentin Stage 3 is needed.

3.3 RISK MANAGEMENT

3.3.1 Risk Assessment

A risk assessment identifies and analyzes specific events orscenarios either qualitatively or quantitatively. The assessmentevaluates the frequency and the consequences of scenarios, anddefines an overall potential risk to the building occupants. Thetechniques for evaluating consequences found in Section 3.2.2may be useful in the risk evaluation of an explosion.

3.3.2 Identifying Events of Concern

3.3.2.1 Scenario Incident Types

A list of potential scenarios may be developed during thePHA or during the evaluation of a building. Most of the sce-narios will result from a loss of containment of a flammableliquid or gas, but other scenarios resulting in explosioneffects should be considered. A typical list of scenarios mayinclude the following:

a. Pressure failure of a vessel or piping component, resultingin a major leak.

Table 5—Typical Overpressure Effects on Unprotected People

Response Threshold Overpressure (psi)

Eardrum rupture 5Fatal head injury 8a

Serious lung damage 10a

Fatal bodily injury 11a

Note: Source: The Effects of Nuclear Weapons by Glasstone (1964). aValues are for a 165-1b individual and a 0.05-sec positive phaseexplosion duration.

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12 API RECOMMENDED PRACTICE 752

b. Corrosion perforation in piping or a vessel, or a loss of avessel connection.c. Inadvertent opening of a valve or connection to theatmosphere.d. Overfilling a vessel or tank.e. Mechanical impact or brittle failure of equipment.f. Failure of pump packing or a mechanical seal.g. Transfer operations involving loading and unloading.h. Chemical reactions that may result in runaway reaction.

3.3.2.2 Scenario Selection

Several factors are to be considered when selecting scenar-ios. These may include the following:

a. Previous incident history: A review of previous incidenthistory at a facility may indicate an increased potential for anexplosion that could impact process plant buildings.b. Industry experience: Available industry incident informa-tion for similar facilities should be considered whenidentifying possible scenarios of concern.c. Process conditions: Certain process conditions, such asextremely high or low temperatures and pressures, reactive orcorrosive process materials, and exothermic reactions, mayincrease the possibility of an explosion. Process materials(such as hydrogen and hydrogen sulfide, which may inducedegradation of materials of construction), frequent cycling ofthe process, and mechanical vibration are examples of condi-tions that have the potential to cause an explosion.d. Effectiveness of PSM systems: The effectiveness of a facil-ity’s PSM program is fundamental in reducing the frequencyof events that lead to serious incidents. e. Maintenance and inspection: The maintenance andinspection program verifies that pressure-retaining equip-ment, including piping and pressure relief valves, areinspected regularly to reduce the likelihood of an explosion.Functional testing programs verify that mitigation systems,such as protective systems and emergency controls, will oper-ate when needed.f. Other safeguards: Other safeguards that should be consid-ered in developing a list of scenarios may include, but are notlimited to the following:

1. Passive mitigation, such as spacing and equipmentplacement.2. Minimizing ignition sources through electricalclassification.3. Administrative controls, such as a hot work permitsystem.4. Monitoring systems for early detection of loss of con-tainment (such as, hydrocarbon or toxic detectors, firedetectors, and process control system).5. Mitigation systems (such as, water spray and firemonitors).

6. Emergency depressurization and deinventory systems.

7. Emergency response.

3.3.3 Qualitative Risk

Qualitative risk determination is an experience-based evalu-ation of risk. Additional information on qualitative risk deter-mination can be found in Chapter 7 of the CCPS’s Guidelinesfor Hazard Evaluation Procedures. After scenarios of concernare identified, those that have the potential to impact processplant buildings may be ranked by using a risk matrix similar tothat shown in Figure 3 and the resulting ranking shown inTable 6. Specific description of likelihood and consequencecategories need to be defined if using this risk matrix.

Companies performing qualitative risk assessments mayconsider developing a risk-ranking matrix based on company-specific risk criteria.

3.3.4 Quantitative Risk

A quantitative analysis for risks to building occupants mayuse a range of techniques, including fault tree analysis andevent tree analysis. For specific details on undertaking a quan-titative risk analysis, refer to CCPS’s Guidelines for ChemicalProcess Quantitative Risk Analysis, Chapters 2 and 3.

Figure 3—Sample Risk Matrix

IV

IV

IV

IV

IV

IV IV

III

III

III

II

II

II

I I

I

1 2 3 4Consequence category

Increasing severity

Incr

easi

ng li

kelih

ood

Fre

que

ncy

cat

egor

y

4

3

2

1

Source: Guidelines for Hazard Evaluation Procedures, 2nd edition.Copyright 1992 by the American Institute of Chemical Engineers,reproduced by permission of the Center for Chemical Process Safetyof AIChE.

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3.3.5 Deciding Whether Risk Reduction is Appropriate

In any risk assessment, after the risks are determined, adecision needs to be made as to whether risk reduction mea-sures should be taken. This usually is done by comparing theassessed risk with the company’s risk acceptance criteria.

When additional risk reduction is suggested, the riskshould be reevaluated with the risk reduction measures inplace in order to verify that risk has been sufficiently reduced.

3.3.6 Risk Acceptance Criteria

The typical risk-ranking matrix, shown in Figure 3, con-tains qualitative risk acceptance criteria, where certain combi-nations of event likelihood and severity clearly require action.In contrast, quantitative risk decision criteria assign specificnumerical values to tolerable risk.

A company should establish risk acceptance criteria beforeinitiating a risk assessment. Assistance in developing qualita-tive and quantitative risk acceptance criteria are availablefrom a number of sources6,7,8 and can be used to comparework conditions in different industries in order to develop riskacceptance criteria.

3.3.7 Risk Reduction

Reducing risk to building occupants from the effects ofexplosions generally falls into two broad categories:

a. Prevention: Activities that eliminate or reduce processincidents. Generally, prevention activities such as modifica-tions in operating and maintenance practices, equipment,maintenance, inspection, personnel training, and audits mayreduce the process incident occurrence rate.b. Mitigation: Activities that reduce the consequences associ-ated with the occurrence. Mitigation measures may includeinstallation of detection and isolation systems, fire suppres-sion systems, structural enhancements, and emergencyresponse planning.

3.3.7.1 Prevention Measures

Preventing the incident from occurring should be the firstpriority in reducing the risk. Some examples of preventiverisk-reduction measures may include but are not limited to thefollowing:

a. Reducing the inventory of hazardous material.b. Engineering controls such as process controls, emergencyshutdowns, and redundant instrumentation.c. Altering process conditions or materials to reduce thepotential for runaway reactions or corrosion.d. Increasing inspection frequencies for piping and equip-ment in corrosive service.e. Upgrading metallurgy of equipment.f. Enhancing work practices such as crane lifting or excava-tion to reduce the likelihood of incidents.g. Administrative controls such as permits for hot work,lockout/tagout, line breaking, and so on.

3.3.7.2 Typical Mitigation Measures for Buildings

Typical mitigation for process plant buildings may include,but are not limited to the following:

a. Relocating personnel or rearranging the room functionswithin the building.b. Eliminating windows.c. Modifying windows to reduce risk to occupants. Twogeneric approaches are as follows:

1. Strengthening the glazing and its framing system toresist the design explosion overpressure.2. Or designing a glazing/framing system that will fail in amanner that minimizes laceration risks to occupants (forexample, polycarbonate, tempered glass, glass with protec-tive safety film or laminated glass with internal catch bars).

d. Strengthening doorways and reducing or eliminating glaz-ing in doors.e. Providing structural reinforcement or performing modifi-cations to the building to improve flexibility and strengthenthe connections.f. Installing external walls for explosion and projectileprotection.

Table 6—Sample Risk-ranking Categories

Risk- ranking Number Category Description

I Unacceptable Should be mitigated with high priority by engineering and/or administrative controls to a risk ranking of III

II Undesirable Should be mitigated with high engineering and/or administra-tive controls to a risk ranking of III

III Acceptable with Controls

Should be verified that proce-dures or controls are in place

IV Acceptable as is No mitigation required

Source: Slightly modified from Guidelines for Hazard EvaluationProcedures, 2nd edition. Copyright 1992 by the American Instituteof Chemical Engineers; reproduced by permission of the Center forChemical Process Safety of AlChE.

6Loss Prevention in the Process Industries, Volume I, Frank P. Lees.7CCPS, Guidelines for Chemical Process Quantitative Risk Analy-sis, 1989, and Guidelines for Evaluating Process-Plant Buildings forExternal Explosion and Fire (1996).8Chemical Industries Association, Guidance for the Location andDesign of Occupied Buildings on Chemical Manufacturing Sites.

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14 API RECOMMENDED PRACTICE 752

g. Constructing a new building using explosion-resistantdesign or siting sufficiently remote for the hazard.h. Designing floor and hub drains to prevent the backflow ofexplosive vapors.

3.3.7.3 Considerations for Existing Explosion-resistant Buildings

Management of change practices should include overpres-sure considerations when revisions are made in plant opera-tions or processes that could affect buildings. Structuralevaluation of the building is to be considered when the recalcu-lated design explosion overpressure exceeds the original design

values. This review evaluation includes (but is not limited to)the loading assumptions used in the building’s structural designin view of current techniques to evaluate explosion effects, theexplosion-resistant-design philosophy at the time of construc-tion, and the as-built details relevant to explosion loadings.

3.3.7.4 Occupancy Reduction Considerations

A study of the building’s occupancy patterns may identifyopportunities to reduce the number of people at risk. Thereduction of personnel in a building is to be based on theexpected building damage and occupant vulnerability.

SECTION 4—FIRES

Uncontrolled fires in process plants can be pool fires, jetfires, flash fires, or fireballs. These fires typically result from,or could cause, explosions within process plants. Pool firesare usually the result of a spilled flammable or combustibleliquid, for example, leaks from a pipe or pump. Jet fires occurat the source of release of a pressurized flammable material,for example, a ruptured natural gas pipeline or a gasket failurein an operating pump. A jet fire impinging on a vessel may bea source of heat input that results in failure of a vessel con-taining a pressurized flammable liquid, causing a BLEVE.Flash fires usually involve flammable gases, combustibledusts, or vapors, that are mixed sufficiently with air to burnwhen exposed to an ignition source. A fireball usuallyinvolves a large flammable release that burns quickly, withradiant heat being the primary concern.

The combustion products from a fire can enter a buildingand, depending on the burning material, may create respira-tory problems, physiological hazards, fire within the building,or hazards from the lack of visibility, which could hinderegress from the building. Figure 4 provides an example of atool to evaluate fire hazards to building occupants.

4.1 MATERIALS OF CONCERN

Buildings that are located in the proximity of equipmentthat processes flammable material have the potential to beexposed to a fire. This standard applies to flammable materi-als covered under OSHA 29 CFR 1910.119.

4.2 BUILDING OCCUPANCY

Once it has been determined that site conditions have thepotential to contribute to a fire, the plant buildings should becompared with the company’s predefined occupancy criteriafor fire hazards. See Section 2.5 for additional information onoccupancy criteria.

4.3 SPACING

Unless a building is directly involved in a fire, the fire haz-ard to the structure is low for most types of construction.Radiant heat effects are significantly reduced by distance.Buildings made of noncombustible construction materialsand even some combustible materials are generally of mini-mal concern unless the building has direct flame impinge-ment. Appendix A lists, as a general reference, severalindustry and insurance standards for separation distance andspacing for fire hazards and not personnel protection. Theuser should be aware that standards are developed for variouspurposes. In general, insurance industry standards aredesigned to protect property and minimize business interrup-tion in the event of an incident. The user should determine theappropriateness of the standard to the application.

Example 7 in Appendix E applies the preceding informa-tion to a hypothetical situation.

4.4 MITIGATION AND EMERGENCY RESPONSE

The hazards for occupants in a building exposed to anexternal fire depend on the building’s materials of construc-tion, distance from the fire, and systems to prevent flammablevapor ignition. Building designs typically provide enoughtime for the occupants to leave the building, for the fire sup-pression measures to be activated, and for emergencyresponse plans to be implemented.

Generally, the emergency response plans for the buildingshould include appropriate measures for the occupants to take,such as sheltering-in-place, using personnel protective equip-ment (PPE) for escape, or depending on wind conditions evac-uating in a safe, upwind or crosswind direction. Also,procedures for ensuring a safe process shutdown should beconsidered if the process is solely controlled from the buildingthat is being evacuated.

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 15

Figure 4—An Analysis Process for a Fire

ProcessSafety Information

Are flammablematerials of

concernhandled?

Does buildingexceed

occupancycriteria?

Are applicablebuilding designand/or spacingstandards met?

Are mitigationsystems and/or

emergencyresponse

appropriate?

RiskReduction/Mitigation

Stop

BuildingChecklist

ProcessSafety

ManagementDiligence

7.0

6.0

3.1.1 (see note)

4.1

4.2

4.3

4.4

4.5

Yes

Yes

No

No

Yes

Yes

No

No

Note: Related paragraphnumbers found within text.

PHA

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16 API RECOMMENDED PRACTICE 752

4.5 RISK REDUCTION FOR FIRE

4.5.1 Prevention Measures

Generally, preventing the incident from occurring shouldbe the first priority in reducing the risk. Some examples ofpreventive risk-reduction measures may include, but are notlimited to the following:

a. Installing properly classified electrical equipment to pre-vent ignition of flammable materials.b. Implementing inspection programs for piping andequipment.c. Providing administrative control such as permits for hotwork, lockout/tagout, line breaking, etc.d. Enhancing work practices such as crane lifting and exca-vation to reduce the likelihood of incidents.e. Reducing the inventory of hazardous material.f. Altering process conditions or materials to reduce thepotential for runaway reactions or corrosion.g. Upgrading materials of construction.

4.5.2 MITIGATION MEASURES

Typical mitigation measures for reducing hazards to build-ing occupants from an external fire include, but are not lim-ited to the following:

a. Providing physical separation from the fire event by dis-tance or barriers. b. Installing isolation valves. c. Installing spill containment.d. Providing appropriate fire-resistant exterior buildingconstruction.e. Providing properly designed fire and smoke suppressionequipment inside and outside the building, which mayinclude the following:

1. Fire-fighting/suppression systems for the building andoutside monitors or water spray in appropriate locations toaddress a possible fire. 2. Building ventilation controls, which detect combustionproducts, stop fresh air intake, and stop air circulationwithin the building.3. Providing area drainage to direct spill away from thebuilding.

f. Providing escape routes in the emergency response planfor each building.

SECTION 5—TOXIC MATERIALS

Toxic materials released to the atmosphere in processplants can affect building occupants. Figure 5 may be used toevaluate toxic material effects on building occupants.

Toxic vapors may enter a building and cause impairmentand/or physiological harm to the occupants, depending on thematerial released, its concentration, and the exposure dura-tion. The combustion products from a fire may enter thebuilding have the potential to create respiratory problems,physiological hazards, or hazards due to the lack of visibilityto building occupants. The AIChE CCPS book Guidelines forUse of Vapor Cloud Dispersion Models reviews techniquesfor evaluating vapor cloud movement and dispersion.

5.1 TOXIC MATERIAL OF CONCERN

Buildings that are located in the proximity of equipmentthat process volatile, acutely toxic materials specified in 29CFR 1910.119, Appendix A should be evaluated.

5.2 BUILDING OCCUPANCY

Once it has been determined that site conditions may con-tribute to a toxic release, the plant buildings should be com-pared with the company’s predefined occupancy criteria fortoxic releases. See Section 2.5 for additional information onoccupancy criteria.

5.3 SITE CONDITIONS

The meteorological dispersion of a toxic material dependson many site-specific factors, including the physical proper-ties and release conditions of the material, the quantityreleased, the weather conditions, obstacles, and the directionand source of the release. The dispersion of these gases canbe modeled using a variety of techniques.

The amount and speed of entry of toxic material into a build-ing depends on the location and size of the entry point, exteriorconcentration of the gas, and relative pressure between the inte-rior and exterior of the building. Entry points can be open orunsealed windows or doors, vent stacks, fresh air makeupopenings, instrument cabling or conduit, or sewer connectionsthat are unsealed. An explosion that causes structural damagecould also allow toxic material to enter the building.

5.4 MITIGATION AND EMERGENCY RESPONSE

The dispersion dynamics of a gas cloud may allow sufficienttime for the occupants to appropriately respond to the release.A ventilation system that (a) controls fresh air makeup, (b) isequipped with a toxic gas detection alarm, and (c) providesautomatic or manual air intake shutdown capability may be aneffective means to control the ingress of toxic materials.

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 17

Figure 5—An Analysis Process for a Toxic Release

ProcessSafety Information

Are toxicmaterials of

concern handled?

Does buildingexceed occupancy

criteria?

Do siteconditions

preclude a toxicrelease affecting

buildingoccupants?

Are mitigationsystems and/or

emergencyresponse

appropriate?

RiskReduction/Mitigation

Stop

BuildingChecklist

ProcessSafety

ManagementDiligence

7.0

6.0

3.1.1 (see note)

5.1

5.2

5.3

5.4

5.5

Yes

Yes

No

No

Yes

Yes

No

No

PHA

Note: Related paragraphnumbers found within text.

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18 API RECOMMENDED PRACTICE 752

Generally, the emergency response plan should includeappropriate measures for building occupants to take, includ-ing, but not limited to:

• shelter in place, • use of personnel protective equipment (PPE) for

escape, or• evacuate in a safe, upwind or crosswind direction from

the source [depending on wind conditions]. A method for ensuring a safe process shutdown should be

considered and factored into the emergency response planwhen the process is controlled solely from the building that isbeing evacuated.

Example 8 in Appendix E applies the above information toa hypothetical situation.

5.5 RISK REDUCTION FOR TOXIC RELEASE

5.5.1 Prevention Measures

Generally, preventing the incident from occurring shouldbe the first priority in reducing the risk. Some examples ofpreventive risk-reduction measures may include, but are notlimited to, the following:

a. Reducing the inventory of hazardous material.b. Altering process conditions or materials to reduce thepotential for runaway reactions or corrosion.c. Providing inspection programs for piping and equipment.d. Upgrading metallurgy of equipment.e. Enhancing work practices for activities such as crane lift-ing and excavation to reduce the likelihood of incidents.

5.5.2 Mitigation Measures

5.5.2.1 The hazards to building occupants from a release oftoxic materials depend on:

a. the toxicity of the material releasedb. the quantity releasedc. factors affecting the dispersion of the toxic materiald. location of the buildinge. control of the ventilation system in the buildingf. infiltration rate into the building, and g. effectiveness of the building’s emergency response plans.

Time is usually a very important factor in reducing occu-pant vulnerability.

5.5.2.2 Typical mitigation measure for reducing the haz-ards to building occupants from a toxic material release mayinclude, but are not limited to the following:

a. ventilation system controls with appropriate detection,which stop the flow of a contaminated air supply to thebuildingb. use of elevated intake stack for potential releases ofheavier-than-air materialsc. an emergency response plan, which may include safe shel-ters in the building, controlling ventilation and infiltrationroutes and clearly identified evacuation routesd. a detection and communication or alarm system for earlynotification of a releasee. appropriate PPE for the building occupantsf. reductions in the number of people in the building.g. pressurized ventilation systems, andh. review of locations of gas infiltration into the building forpossible sealing.

SECTION 6—BUILDING CHECKLIST

Even the process plant buildings identified during the vari-ous analyses as not presenting a significant hazard to occu-pants will benefit from a checklist review to verify that

routine risk-reduction measures have been taken. Appendix Dshows an example of typical questions included on a buildingchecklist.

SECTION 7—PROCESS SAFETY MANAGEMENT DILIGENCE

The credibility techniques discussed in this standard is basedon the continuing and improving practice of process safetymanagement (PSM) within a process facility. Implementingand continuing a PSM program will help to reduce the likeli-hood of serious events. Additionally, it can assist in identifyingmeasures to mitigate the consequences to the occupants fromactual and potential hazards. Once a PSM program is estab-lished, overpressure considerations incorporated into Manage-ment of Change practices can determine if revisions in plant orprocess could affect buildings. Applicable references concern-

ing specific PSM activities are available in a growing body ofknowledge. A number of resource publications are available,including but not limited to those listed in Appendix A. Theseinclude a series of books available from the AIChE Center forChemical Process Safety (CCPS).

To maintain the currency and effectiveness of the techniquesused and validity of the assumptions discussed in this standard,it is important for a facility to assess its PSM activities by sys-tematic auditing.

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19

APPENDIX A—BIBLIOGRAPHY FOR ADDITIONAL READING

This appendix provides a list of guidelines, standards, codes,specifications, and publications that may be useful in evaluat-ing hazards to building occupants. The user should evaluateeach document for its suitability and application as described inSection 3.2.1, and use the most recent edition. Some publica-tions are considered “classics” in their field and may no longerbe in print but may be available through technical libraries.

A.1 ExplosionAIChE Center for Chemical Process Safety1(CCPS) www.aiche.org/ccps/

Plant Guidelines for Technical Management of ChemicalProcess Safety

Baker, W. E., P A. Cox, P S. Westine, J. J. Kulesz and R. A. Strahlow,

Explosive Hazards and Evaluation, Elsevier ScientificPublishing Company, 1983.

CIA (Chemical Industries Association)3

Process Plant Hazard and Control Building Design: AnApproach to the Categorization

FM Global9

FM 7-455 Process Control Houses and Other Struc-tures Subject to External ExplosionDamage

FM 7-44 Spacing of Facilities in Outdoor ChemicalPlants

GE Global Asset Protection Services10

GE GAP Guidelines GAP.2.5.2 Oil and Chemical Plant Layout and

SpacingGE GAP Guidelines GAP.2.5.2.A Hazard Classification of Process Opera-

tions for Spacing Requirements GAP.8.0.1.1 Oil & Chemical Properties Loss Potential

Estimation Guide GAP.8.0.1.1.A Vapor Cloud Explosion, Sample

Calculation GAP.8.0.1.1.B Vessel Explosion, Sample Calculation

Maynard M. Stephans Minimizing Damage to Refineries fromNuclear Attack, Natural and OtherDisasters

Nolan, R.E. and C.W.J. Bradley A Simple Technique for the Optimization ofLay-Out and Location for Chemical PlantSafety, Department of Chemical Engineer-ing, Polytechnic of the South Bank,Borough Road, London, England

Page, Robert Technical Aspects of Unconfined VaporCloud Explosions and the TNT ModelIndustrial Fire Safety, May/June 1993

U.S. Department of Defense5 www.dod.govTMS-1300 Structures to Resist the Effects of Acciden-

tal ExplosionsAMCR 385-100 U.S. Army Safety Manual MIL-STD-882C

Military Standard System Safety ProgramRequirement

A.2 FireAPI www.api.org

RP 14J Recommended Practice for Design andHazard Analysis for Offshore ProductionFacilities

RP 2001 Fire Protection in RefineriesStd 2510 Design and Construction of Liquefied

Petroleum Gas (LPG) InstallationsPubl 2510A Fire Protection Considerations for the

Design and Operation of Liquefied Petro-leum Gas (LPG) Storage Facilities

AIChE Center for Chemical Process Safety1 (CCPS)Guidelines for Engineering Design forProcess SafetyFundamentals of Fire and Explosion,AIChE Monograph Series, No. 10, Volume73. Stull, Daniel R.

FM Global9

FM 7-44 Spacing of Facilities in Outdoor ChemicalPlants

9FM Global, 1151 Boston-Providence Turnpike, P.O. Box 9102,Norwood, MA 02062-9102, USA. ww.fmglobal.com10GE Global Asset Protection Services, GE GAP Guidelines (for-merly called IRInformation) P.O. Box 5010, 85 Woodland Street,Hartford, Connecticut 06102-5010. www.gegapservices.com/prod-ucts/standard

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20 API RECOMMENDED PRACTICE 752

GE Global Asset Protection Services10

GE GAP Guideline GAP.2.5.2 Oil and Chemical Plant Layout and

SpacingGE GAP Guideline GAP.2.0.5 Protection of Buildings from Exterior Fire

Exposures

NFPA11

30 Flammable and Combustible Liquids Code58 Standard for the Storage and Handling of

Liquid Petroleum Gases59A Standard for the Production, Storage and

Handling of Liquefied Natural Gases68 Guide for Venting of Deflagrations 80A Recommend Practice for the Protection of

Buildings from Exterior Fire Exposures496 Standard for Purged and Pressurized

Enclosures for Electrical Equipment inHazardous Locations

Fire Protection HandbookSFPE Handbook of Fire Protection Engineering

U.S. Bureau of Mines12

NTIS AD701576 Flammability Characteristics of Combusti-ble Gases and Vapors

NTIS PB87-113940Investigation of Fire and Explosion Acci-dents in the Chemical, Mining and Fuel-Related Industries—A Manual

A.3 Toxic Material

AIChE Center for Chemical Process Safety1 Dow Chemical, Chemical Exposure IndexGuidelines for Consequence Analysis ofChemical ReleasesGuidelines for Post-Release Mitigation inthe Chemical Process Industry

11National Fire Protection Association, 1 Batterymarch Park, P.O.Box 9101, Quincy, Massachusetts 02269-9101. www.nfpa.org

12U.S. Bureau of Mines. Note: The Bureau of Mines was abolished bythe U.S. Congress in 1996. It is possible to purchase many USBMpublications from the National Technical Information Service (NTIS).Order Information Phone: 703-487-4650. Sales Phone: 1-800-553-NTIS. Website: http://www.ntis.gov. E-mail: [email protected].

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21

APPENDIX B—EXPLOSION, FIRE, AND TOXIC RELEASE PHENOMENA, AND HAZARDS TO THE OCCUPANTS OF PROCESS PLANT BUILDINGS

The information contained in this appendix is derived inpart from Guidelines for Evaluating the Characteristics ofVapor Cloud Explosions, Flash Fires, and BLEVES (1994),American Institute of Chemical Engineers, with permissionof its Center for Chemical Process Safety. For more detailedinformation, consult the above reference.

The primary events of concern for occupants of processplant buildings are explosion, fire, and release of toxic mate-rials.

B.1 Structural Design and Analysis for Explosion Resistance

The structural design and analysis should be performed bya structural engineer familiar with explosion design. In orderto perform this task, it is necessary to establish the pressure-versus-time relationship of the load as it applies to the differ-ent parts of the building. This determination should be per-formed with input from experts on the process and be basedon a thorough knowledge of the postulated explosion scenar-ios. However, it is important to realize that these explosiondesign criteria are only an approximation

B.2 Explosion Phenomena Explosions in process plants occur because of the loss of

containment of a pressurized gas or pressurized boiling liquid,the rapid combustion of a flammable or finely divided solidmaterial, or the uncontrolled reaction of chemical materials.

B.2.1 EXPLOSION

B.2.1.1 Explosions may occur through a variety of mecha-nisms and may take one or more of the following forms:

B.2.1.1.1 Vapor cloud explosion (VCE): A VCE is theresult of a flame front propagating through a mixture of airand flammable gas or vapor. The flame front must propagatewith sufficient velocity to create a pressure wave. For a VCEto occur, several conditions must be met. These include (a) arelease of flammable material, (b) sufficient mixing with airthat results in a flammable mixture, (c) ignition, (d) confine-ment, and (e) congestion within the flame path, which tendsto accelerate the flame front. The explosion intensity of aVCE can vary greatly and is determined by the flame speed.The flame speed in turn is affected by the turbulence createdwithin the vapor cloud. One of the key factors creating turbu-lence in a vapor cloud is the degree of congestion within therelease area. Experience has shown that normal process plantdesign and equipment spacing may create enough congestionto cause turbulence and rapid flame propagation. A high-pres-sure jet release may also produce sufficient turbulence to

increase the rate of flame propagation. The rate of release andthe elapsed time before ignition will determine the amount ofmaterial involved in a VCE, and thus affect the overpressureand duration of the explosion wave.

B.2.1.1.2 Boiling liquid expanding vapor explosion(BLEVE): A BLEVE results from the rapid release of a pres-surized liquid above its atmospheric boiling point, usuallycaused by the rapid failure of its containment vessel. The fail-ure often is caused by flame impingement on the vessel’svapor space, which weakens the metal to the point of rupture.A BLEVE may produce an explosion wave, fragmentation,and, if flammable material is involved, a fireball.

B.2.1.1.3 Physical explosion: A pressurized system mayproduce an explosion upon catastrophic rupture or failure ofthe equipment. Depending upon the failure mechanism thatleads to the release, fragmentation of the container may alsoresult. Released gas or fluid may ignite, resulting in a VCEor fire.

B.2.1.1.4 Chemical explosion: An uncontrolled chemicalreaction with sufficient energy may cause a failure of the ves-sel, resulting in overpressure and missile effects.

B.2.1.2 Damage to buildings and subsequent injury tooccupants from an explosion may be caused by the explosionwave, missile fragments, or fire.

B.2.2 EXPLOSION LOAD

The typical blast load is characterized by a rapid rise inpressure to a peak value, a period of decay to ambient pres-sure (positive phase), and a period in which the pressuredrops below ambient (negative phase).

The explosion load differs with the various parts of thebuilding. The front wall receives a larger pressure (reflectedpressure) than do the back wall, side walls, and roof. The pas-sage of the explosion wave across the building is a diffractionprocess as the wave bends and interacts with the geometry ofthe structure. Consequently, each area of the building depend-ing on its location and orientation is subject to an explosionpressure that is different from the pressure on other areas ofthe building.

The shape of the loading curve, as well as the duration ofthe load, depends on many parameters, such as the character-istics of the explosion (detonation, deflagration, pressure ves-sel rupture), material and quantity released, ignition source(location of the explosion), degree of confinement, distancefrom the explosion, possible obstructions in the path of thepressure wave, and the orientation of the loaded surface rela-tive to the direction of the explosion wave.

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22 API RECOMMENDED PRACTICE 752

The material involved in the explosion is converted into ahigh-pressure gas at high temperatures and a rapidly expand-ing shock front. The pressure behind the explosion wave isthe incident pressure. When the incident-pressure shock frontstrikes the front wall of the building, the pressure rises fromambient to the reflected pressure, which is a function of theincident pressure.

For each pressure range, there is a particle or wind velocitythat causes a dynamic pressure on objects in the path of thewave. The magnitude of the drag force that results from thiseffect is a function of the particle velocity, particle density,and object geometry. The explosion loading on an objectcomprises two components: the loading due to the overpres-sure (incident and reflected) and the loading due to dragforces induced by the particle motion. For buildings, onlyoverpressure is important—drag forces are significant, com-pared to overpressure loads, only for slender objects such aslamp posts.

The explosion wave’s potential to cause building damage istime dependent and directly related to the shape and size ofthe explosion wave’s pressure-time curve. The impulse valueis used in conjunction with the dimensions of the building,orientation of the building, and speed of the explosion waveto determine the net and instantaneous loading on a buildingfrom an explosion wave.

B.2.3 HAZARDS TO BUILDING OCCUPANTS FROM EXPLOSIONS

The response of the building to the explosion wave isrelated to the distance of the building from the explosionevent, the type of building construction, and materials of con-struction. Ground shock may also occur from an explosion;however, its effects on buildings and occupants are consid-ered insignificant compared to airborne effects.

Broken glazing shards typically cause the most injuries asbreakage of conventional annealed glass should be expectedat overpressures from 0.2 to 2.0 psi. However, collapse or par-tial collapse of the building normally poses the most severehazard to building occupants in an explosion scenario. Build-ing components such as lighting fixtures, suspended ceilings,and furnishings, which may dislodge or topple and possiblyinjure the occupants, should be evaluated. Breakable objects,primarily glass, may be a hazard to occupants. Normally openpenetrations, or the nearly instantaneous loss of window,wall, or door integrity, create openings in the building anddecrease the reflected overpressure. The explosion conditionsare transferred to the interior of the building, creating possiblehazards for the occupants, including the possibility of an indi-vidual being thrown against stationary objects and/or beinginjured by flying debris. These hazards are of concern aroundlarge openings such as windows, doors and associated entry

areas, and hallways where the explosion effects may be chan-neled. The external attachments and interior furnishings arepotential hazards to occupants if these items are displacedand impact with building components or occupants.

There is a low probability of injury to occupants from anexternal projectile impacting the building, because externalprojectiles produce extremely localized effects on buildingsin comparison to explosion loadings, which involve the entirebuilding.

B.3 Fire

Generally, process fires will be in the form of one or acombination of the following:

a. Jet fire: A liquid or gas release under pressure that burnsfrom its release point. The stability, magnitude, and distanceof the flame are a function of the release pressure, wind direc-tion, hole size, flame direction, and other site-specificvariables.b. Pool fire: A release of flammable liquid, including liquidfrom releases of a two-phase material that forms a fire inwhich the flammable material burns above the liquid surface.c. Flash fire: The combustion of a flammable gas or vaporand air mixture, in which the flame propagates through themixture such that negligible or no damaging overpressure isgenerated.d. Fireball: A burning fuel-air cloud whose energy is emittedprimarily in the form of radiant heat. The inner core of thecloud consists almost completely of fuel; whereas, the outerlayer (where ignition first occurs) consists of a flammablefuel-air mixture. As the buoyancy forces of hot gasesincrease, the burning cloud tends to rise, expand, and assumea spherical shape.

Fires expose process plant buildings to radiant heat andproducts of combustion that may enter the building. Gener-ally, fires do not impact process plant buildings as severely asdo explosions. Their effects occur over a period of time, andemergency response and mitigation can reduce the impact.

B.4 Toxic Materials

Certain materials, when released, may present a toxicexposure to personnel. In some cases, the material releasedmay contain both toxic and flammable substances and maypresent potential for fire, explosion, and/or toxic exposure.

Generally, the releases of toxic materials, unless corrosive,will not affect buildings. However, ingress into the buildingcan occur with exposure to personnel. Ingress usually occursin a time frame that allows for emergency response and miti-gation or donning of personal protective equipment.

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23

APPENDIX C—DAMAGE CATEGORIZATION FOR BUILDINGS AND VULNERABILITY OF OCCUPANTS TO EXPLOSION OVERPRESSURE

As part of the risk screening for explosion in Section 3.2.3,information is required on the vulnerability of occupants toinjury and on generic frequency of occurrence for processunits. The information presented in this appendix is one com-pany’s approach for these items and is an example only. Eachcompany should develop its own approach.

C.1 Vulnerability of Occupants

To assess the vulnerability of occupants within a givenbuilding from explosion overpressure, the following informa-tion is required:

a. Type of building.b. Overpressure relationship for each type of building.c. Damage-vulnerability relationship.

C.1.1 BUILDING TYPES

Buildings within facilities have a wide range of design andconstruction features; however, the most important is buildingstructure. The following types of major structures can befound:

B1: Wood-frame trailer or shack.B2: Steel-frame/metal siding or pre-engineered building. B3: Unreinforced masonry bearing wall building.B4: Steel or concrete framed with reinforced masonry infill

or cladding.B5: Reinforced concrete or reinforced masonry shear wall

building.

C.1.2 OVERPRESSURE DAMAGE FOR BUILDING TYPES

Damage estimates have been obtained from past incidents,effects on buildings from testing nuclear weapons, and othersources. The overpressure values shown are appropriate forthe different building types.

a. Wood-frame trailer or shack, B1: From observation ofdestruction by severe storms, it is inferred (for permanentbuildings) that an explosion loading of approximately thedesign wind pressure or about 0.2 psi would mark the onset ofserious damage to these buildings. Pressures of about 1 psicould overturn an isolated building and partially destroy tem-porary building complexes.b. Steel-frame/metal siding or pre-engineered building, B2:The frame in this type of building provides support to theroof, which is independent of the walls. Information is avail-able in the literature on explosion performance from warrecords. These records indicate that sheeting is ripped off and

internal walls are damaged at overpressures > 1.5 psi; thebuilding frame stands but cladding and walls are damaged at> 2.5 psi, and complete building collapse occurs at > 5 psi.

c. Unreinforced masonry bearing wall building, B3: Thistype of building is similar to domestic/small commercialbuildings. The walls collapse about 1.0 psi and total buildingcollapse occurs about 1.5 psi.

d. Steel or concrete framed w/reinforced masonry infill orcladding, B4: This type of building regularly suffers damagein storm conditions; however, this type of building providesrobust performance under explosion loading. Wall damagemay occur at 1.0 psi. The roof slab may collapse at 2.0 psiand total building collapse at 2.5 psi.

e. Reinforced concrete or masonry shear wall building, B5:The building is generally designed for 2.0 psi, so very minordamage will occur. The roof and walls will deflect underloading and internal wall damage at about 4.0 psi. The build-ing has major damage and possible collapse at 6.0 psi.

C.1.3 OVERPRESSURE DAMAGE AND VULNERABILITY RELATIONSHIP13

Assumptions on the relationship between damage andvulnerability of occupants (probability of fatality) werederived from several sources. The following general datawas identified:

a. Building destruction: A vulnerability of 1.0 was chosen forbuildings subject to a blast significantly greater than that lead-ing to building collapse, based on extrapolation (shown ondotted lines in Figure C-1).

b. Building collapse: A vulnerability level of 0.6 was chosenbased on building collapse data from earthquakes, war, andpast incidents.

c. Minor damage: On the basis of studies reported by Lees14,it was assumed that the vulnerability of occupants would be0.01 from minor damage, such as broken windows.

C.1.4 OVERPRESSURE VERSUS VULNERABILITY NOMOGRAM

An example summary of the relationship between levels ofoverpressure and vulnerabilities to occupants for buildingtypes is shown in Figure C-1, “Overpressure Versus Vulnera-bility.”

13For additional information, see CCPS, Guidelines for EvaluatingProcess-Plant Buildings for External Explosion and Fire.14Loss Prevention in the Process Industries, Volume I, Frank P. Lees,1980.

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24 API RECOMMENDED PRACTICE 752

Figure C-1—Sample Overpressure Versus Vulnerability

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

B5

B1, B2, B4

B3

Pea

k in

cid

en

t si

de-

on

ove

rpre

ssu

re (

psi

)

Probability of serious injury/fatality

Note: Figure taken from CCPS, Guidelines for Evaluating ProcessPlant Buildings for External Explosions and Fires. Occupant vulnerability

greater than 0.6 are inferred estimates based on limited data. This range isshown for illustrative purposes and should be used with great caution.

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 25

C.2 Generic Frequencies of ExplosionsThe process plant under consideration should be divided into

process units of isolatable inventories, with release frequenciesestimated for failures of pipes, valves, flanges, pumps, com-pressors, and other equipment. This would be supplementedwith failures from other events such as earthquakes, floods, tor-nadoes, and effects from adjacent units in order to build a com-prehensive listing of release scenarios. Knowledge of thelikelihood, release size, and duration of individual scenarioswould then be combined with vapor cloud dispersion distancesand direction to site-specific ignition sources in order to calcu-late the overall frequency of occurrence for major explosionsimpacting plant buildings.

A simple approach for risk screening was taken by onemember company. The company developed a historicalrecord of all major explosions and an estimate of the num-ber of plant years of operating experience to arrive at esti-mated frequencies of explosions for generic process units.This information is shown in Table C-1 for specific refineryunits. Companies may wish to develop similar data for theiroperations. If not, the generic data shown could be usedwhen a company is performing a screening risk-analysis.

Table C-1—Generic Frequencies of Major Explosions (see note)

Process UnitFrequency of Explosions/

Year of Operation

Alkylation 5.1 x 10 – 4 a

Cat cracking 6.5 x 10– 4

Cat reforming 2.6 x 10– 4

Crude 4.9 x 10– 4

Hydrotreating 2.0 x 10– 4

Hydrocracking 5.6 x 10– 4

All units 4.3 x 10– 4

Note: Information in this table is derived from the following sources:OSH ROM Occupational Safety and Health on CD-ROM, MajorIncident Database (MHIDAS), UK Atomic Energy Authority, July1993; Engineering Services Loss Database, Sedgwick EngineeringLimited, Sedgwick House, The Sedgwick Centre, London E1 8DX;and Worldwide Refining Report, Oil and Gas Journal, Penwell Pub-lishing Company, December 23, 1991.

a A frequency of 5.1 x 10– 4 explosions/year of operation is the sameas one explosion in two thousand years of operation.

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27

APPENDIX D—PROCESS PLANT BUILDING CHECKLIST

The following is an example of a building checklist. Some of the questions in the checklist may not apply to all work sites; like-wise, additional questions may be necessary.

Process Plant Building Checklist

Building: Date: Name:

HazardE,F,T* Questions Y N NA* Remarks

F,T 1. Is the building located up wind of the hazard?

E,F,T 2. Is the building included in an emergency response plan for fire and toxic release?Are the occupants trained on emergency response procedures? Are evacuation instructions posted?

E 3. Are large pieces of office equipment or stacks of materials within the building adequately secured?

E 4. Are the lighting fixtures, ceilings, or wall-mounted equipment well supported? Are process controls mounted on interior walls?

E 5. Is there heavy material stored on the ground floor only?

E 6. Have all the exterior windows been assessed for potential injury to occupants?

E,F,T 7. Are there doors on the sides of the building opposite from the potential explosion or fire source?

F,T 8. Are there exterior and interior fire suppression equipment available to the building?

F,T 9. Is there a detection system within the building or in the fresh air intake to detect hydrocarbons, smoke, or toxic materials?

F,T 10. Is the air intake properly located?

F,T 11. Can the ventilation system prevent air ingress or air movement within the building? Are there hydrocarbon or toxic detectors that shut down the air intake? Does the building have a pressur-ization system?

F,T 12. Are there wind socks visible from all sides of the building?

E,F,T 13. Is there a building or facility alarm or communication system to warn building occupants (of an emergency)?

T 14. Is there sufficient bottled air or fresh supplied air for the occupancy load?

E,F,T 15. Are all sewers connected to the building properly sealed to prevent ingress of vapors?

Note: *E =Explosion; F = Fire; T =Toxic; NA = Not Applicable.

This checklist may or may not be appropriate for every particular circumstance.

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29

APPENDIX E—EXAMPLES

The examples in this appendix are intended to guide the user in applying the concepts presented in this publication. Each exam-ple is designed to illustrate a specific aspect of the application of this publication, and does not attempt to convey the complexityor to make trivial the issues associated with location of buildings in process plants. The numerical criteria used as standards foroccupancy or risk acceptance in the examples are not intended to serve as numerical guides. The numbers used in these examplesare solely for illustration purposes and do not reflect recommended numerical criteria. Each company should select criteriareflecting its needs.

EXAMPLE 1—MATERIALS OF CONCERN (SECTION 3.1.2)

Problem 1:A sweet crude oil treating and storage facility has an office 35 feet from the process equipment. Because the unitcontains more than 10,000 pounds of flammable liquid, the facility is covered by OSHA process safety management regula-tions.

Solution: Due to the conditions in which the crude oil is processed at this facility, it was concluded that the risk of vaporcloud explosions (VCEs) is low, and that the primary concern is fire. Figure 4 should be used to evaluate the hazard to build-ing occupants.

EXAMPLE 2—SITE-SPECIFIC CONDITIONS (SECTION 3.1.3)

Problem 1: An occupied shipping office for a large automated chemical warehouse handling solid products extends outfrom the warehouse on the north side. The manufacturing process for this product contains flammable gases and dust. Theproduct is manufactured, compounded, and packaged in a separate building 200 feet to the south of the warehouse, andexplosion venting for the building has been designed to release to the south.

Solution: The shipping office is eliminated from further study because the site-specific condition of the process plant build-ing directs the effects of a potential explosion away from the occupied building. A building checklist may be completed.

EXAMPLE 3—OCCUPANCY CRITERIA (SECTION 2.5.2)

Problem 1:A company’s internal criterion for occupancy load is 400 hours per week. A cafeteria is located within a refinerycomplex with process units located on three sides. The closest unit containing hydrocarbon is 250 feet away. The cafeteria isopen from 6:00 – 8:00 A.M., 11:00 A.M. – 1:00 P.M., and 6:00 – 7:00 P.M. on a daily basis. Approximately 200, 600, and150 people, respectively, use the facility for each meal on weekdays. About 100 people per meal use the facility on week-ends (each individual averages 0.5 hour in the building).

Solution: The occupancy load is calculated as:Breakfast: (0.5 hour x 200 people x 5 days) + (0.5 x 100 x 2) = 600Lunch: (0.5 hour x 600 people x 5 days) + (0.5 x 100 x 2) = 1,600Evening: (0.5 hour x 150 people x 5 days) + (0.5 x 100 x 2) = 475The occupancy load is 2,675 hours per week.

Therefore, evaluation of the building using Section 3.2 should be considered necessary.

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30 API RECOMMENDED PRACTICE 752

Problem 2: The same company has two permanent trailers. Another trailer is to be added. A process unit is 200 feetfrom the trailers. A question of siting of the trailers was raised. The existing trailers house eight individuals. Theassigned personnel spend, on average, six hours per day in the trailers. In addition, two meetings are held that averagesix people for six hours per day, four days a week.

Solution: The occupancy load is calculated as:6 hours x 8 people x 5 days = 2406 hours x 6 people x 4 days x 2 meetings = 288The total occupancy load is 528 hours per week.

This exceeds the internal criterion of 400 hours per week. The company decided that the best option for reducing riskwas to not install the new trailer at this location, and to move one of the existing trailers to a different location. TheBuilding Checklist may be completed.

Problem 3: A company has tiered occupancy criteria of (1) occupancy load of 300 hours per week, (2) occupied morethan 50 percent of the time, or (3) more than 40 people in the building for one hour. A metal-clad building is used tohouse maintenance offices and serve as a warehouse. The building is approximately 350 feet from the closest processunit and 750 feet from the flammable storage area. The building is occupied by two warehouse personnel and onesupervisor 40 hours per week. Additionally, the maintenance department has 65 personnel who average 2 hours perday, five days a week, including 1 hour per week for a safety meeting in the building.

Solution: Since the first and third tier of the occupancy criteria were exceeded, further evaluation using Stage 2 or 3(Section 3.2 or 3.3) should be considered necessary. The company decided to proceed with further evaluation usingStage 2.

Problem 4: A company has established an occupancy load criteria for individuals assigned to the building more than40 percent of the time. A shelter is provided for operators’ use in the process unit. The shelter is occupied by four peo-ple less than 30 percent of the time. Employees are required to leave the building in the event of an emergency.

Solution: No further evaluation is required because the building does not meet the occupancy criteria. A buildingchecklist may be completed.

Problem 5: A small field laboratory in a chemical plant has been designated as an emergency shelter during gasreleases. The building is designed to prevent ingress of gases.

Solution: Since the building is an emergency shelter, and is designed to prevent ingress of toxic releases, no furtherevaluation is required. A building checklist may be completed. However, if there is a potential for an explosion, fur-ther evaluation should be considered necessary using Stage 2 or 3 (Section 3.2 or 3.3).

EXAMPLE 4—DESIGN AND SPACING STANDARDS (SECTION 3.2.1)

Problem 1: An engineering building at a refinery is about 500 feet from the closest process equipment. The building’soccupancy load is over 400 personnel hours and the building is of masonry/steel-frame construction.

Solution: The company’s standard states that for processing units, the distance for this building type should be more than400 feet from the hazard. A building checklist may be completed.

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MANAGEMENT OF HAZARDS ASSOCIATED WITH LOCATION OF PROCESS PLANT BUILDINGS 31

Problem 2: An existing control building is designed for 10 psi overpressure and is 150 feet from the process unit.

Solution: The applicable internal company standard on building design indicates that this building meets the current stan-dard. Since the spacing is greater than the required minimum spacing, no further explosion evaluation is required. A build-ing checklist may be completed.

EXAMPLE 5—CONSEQUENCE ANALYSIS (SECTION 3.2.2)

Problem 1: A control building designed for a 3 psi overpressure is about 100 feet from a Depropanizer. The Overhead Accu-mulator holds 4000 gallons of propane at 1100ºF and 210 psig. A process hazards analysis (PHA) concluded that a releasecould involve a severed 2-inch pipe, and would occur in a congested area.

Solution: The overpressure was calculated to be 3 psi at 150 feet, using the multi-energy method. This exceeds the design ofthe building. Based on these results, additional evaluation should be considered necessary.

EXAMPLE 6—SCREENING RISK ANALYSIS (SECTION 3.2.3)

Problem 1: A refinery has a Light Ends Unit containing propane/butane. A building of masonry construction with windowsis 50 feet from the process with a fractionator 60 feet away. The original building housed only the control room with twooperators. The building has been expanded to accommodate a change room and several offices. The control room is occu-pied by two personnel all the time on eight-hour, five-days-a-week shifts. The change room has an occupancy of ten for onehour per day, seven days a week. The offices have five people for nine hours per day, five days a week.

Solution: This solution is based on the steps outlined in Section 3.2.3 of this recommended practice as follows:

Step 1—The building meets the definition of B3, unreinforced masonry with supporting walls, as indicated in Section C.1.2.

Step 2—In calculating the overpressure, the choice of methods is up to the user. A release scenario from the fractionator,using the multi-energy method, yielded an overpressure of 5.2 psi at the control building, based on a release scenario devel-oped during the PHA.

Step 3—The frequency of explosions should be based on a company’s specific experience. If data or company-specific experi-ence is not available, then the generic information in Appendix C could be used. From Section C.2, a frequency of 4.3 x 10– 4 isused for all refinery units, since a Light Ends Unit is not listed in Appendix C.

Step 4—The vulnerability of occupants is calculated using Section C.1.3. Given the building type (B3) and overpressure(5.2 psi) from Figure C-1 in Appendix C, the vulnerability of occupants is 1.0.

Step 5—The maximum individual risk is calculated by multiplying the explosion frequency, individual occupancy fraction,and vulnerability of occupants. The maximum individual occupancy load is calculated by determining the maximum num-ber of hours per week spent inside the building by anyone (9 hours/day x 5 days/week = 45 hours/week) and dividing by168 hours/week = 0.26.

The individual risk is as follows:

4.3 x 10– 4 (explosion frequency) x 0.26 (maximum individual occupancy) x 1.0 (vulnerability of occupants) = 1.1 x 10– 4

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32 API RECOMMENDED PRACTICE 752

Based on this calculation the maximum individual risk from the Light Ends Unit is 1.1 x 10– 4. Similar calculations forthe maximum individual risk from two adjacent units is 6.3 x 10– 4 and 3.7 x 10– 3. Thus, the total maximum individualrisk for the maximum exposed individual in the building is the following:1.1 x 10– 4 + 6.3 x 10– 4 + 3.7 x 10– 3 = 4.4 x 10– 3

The total risk of 4.4 x 10– 3 calculated in Step 5 is compared to the internal company risk decision criteria. The followingcriteria will be used for this example:

The value of total risk is greater than 1.0 x 10– 3. Based on these criteria, risk mitigation or further risk assessmentshould be considered necessary using Stage 3, Risk Management.

Step 7—The risk assessment screening indicated that either risk mitigation or further risk assessment is required. Thecost of mitigation measures relative to the costs of a risk analysis should be considered. If the mitigation costs are rela-tively high, then a risk analysis may be conducted to improve the confidence level of these screening results. Possiblerisk-reduction measures to be considered include the following:

a. Modify process or install emergency shutdown system.b. Move some of the population to reduce the occupancy load.c. Modify the building by providing separate supports for the roof.d. Move the control building.

An evaluation of the most cost-effective approach for each of these options should be considered necessary.

EXAMPLE 7—FIRE (SECTION 4.0)

Problem 1: A noncombustible control room/office building is 100 feet from a pumping row in a refinery unit handlingflammable materials. The pumping row is diked, and an automatic water/foam spray system is provided. For the pur-poses of this example, the materials and site conditions preclude the potential for explosion or jet fire.

Solution: Figure 4 was used to evaluate the hazard to building occupants. The quantity of material handled exceeds10,000 pounds. The building does not meet the company’s internal standard for locations within 150 feet of flammablepumping stations. There is a building emergency response plan to vacate the building in the event of a nearby fire, andthe fire brigade will respond to control the fire. Also, the water/foam spray system on the pumps can be activated fromthe building to minimize the effects.

Based on the mitigation and emergency response capability, no additional fire evaluation should be considered. A build-ing checklist may be completed.

Total Individual Riska Action Indicated

> 1.0 x 10– 3 Risk mitigation or further risk assessment is required.1.0 x 10– 3 to 1.0 x 10– 5 Risk reduction should be considered.< 1.0 x 10– 5 Further risk or assessment reduction need not be considered.

Note: aTotal maximum individual risk for the maximum exposed individual in the building.

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EXAMPLE 8—TOXIC RELEASE (SECTION 5.0)

Problem 1: A chemical plant uses ammonia in a process. The ammonia is stored in a 15,000-gallon tank and is pumpedto the processing unit. The storage tank is 250 feet from the engineering building.

Solution: Figure 5 was used for the evaluation. Ammonia is listed in OSHA 29 CFR 1910.119, and the stored amountexceeds the specified threshold quantity. All occupied buildings on-site that could be impacted by an ammonia releaseare provided with pressurization and fresh air supply shutdown systems to preclude entry of ammonia. Additionally,ammonia detectors are located around the storage tank and pump as well as in each building air intake.

Based on the pressurization system, which prevents ingress of ammonia, no further toxic evaluation should be consid-ered necessary. A building checklist may be completed and the emergency response actions for occupants defined.

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34 API RECOMMENDED PRACTICE 752

EXAMPLE 9—BUILDING CHECKLIST (SECTION 6.0)

Problem 1: In Example 4, Problem 1, an engineering building was evaluated and it was determined that no further eval-uation was required. It may be considered for a final review using a building checklist.

Solution: See the completed Building Checklist that follows.

The following is an example of a building checklist which has been filled in. Some of the questions in the checklist maynot apply to all work sites; likewise, additional questions may be necessary.

PROCESS PLANT BUILDING CHECKLIST

Building: Engineering Office Date: July 25, 2002 Name: A. N. Reviewer

E,F,T* Questions Y N NA* Remarks

F,T 1. Is the building located up wind of the hazard? X

E,F,T 2. Is the building included in an emergency response plan for fire and toxic release? Are the occupants trained on emergency response procedures? Are evacuation instructions posted?

X

E 3. Are large pieces of office equipment or stacks of materials within the building adequately secured?

X Library and file room equipment need proper anchoring.

E 4. Are the lighting fixtures, ceilings, or wall-mounted equipment well supported? Are process controls mounted on interior walls?

X Ceiling tile and lights need better anchoring.

E 5. Is there heavy material stored on the ground floor only? X

E 6. Have all the exterior windows been assessed for potential injury to occupants? X Evaluate the need for windows on south side of building.

E,F,T 7. Are there doors on the sides of the building opposite from the potential explosion or fire source?

X

F,T 8. Are there exterior and interior fire suppression equipment available to the build-ing?

X Fire monitors in range of building.

F,T 9. Is there a detection system within the building or in the fresh air intake to detect hydrocarbons, smoke, or toxic materials?

X Smoke and toxic gas in air intake.

F,T 10. Is the air intake properly located? X Intake on process side of building

F,T 11. Can the ventilation system prevent air ingress or air movement within the build-ing? Are there hydrocarbon or toxic detectors that shut down the air intake? Does the building have a pressurization system?

X System is not capable of pressurizing the building. No automatic shutoff of air intake.

F,T 12. Are there wind socks visible from all sides of the building?

E,F,T 13. Is there a building or facility alarm or communication system to warn building occupants (of an emergency)?

X

T 14. Is there sufficient bottled air or fresh supplied air for the occupancy load? X

E,F,T 15. Are all sewers connected to the building properly sealed to prevent ingress of vapors?

X

*E =Explosion; F = Fire; T =Toxic; NA = Not Applicable. This checklist may or may not be appropriate for every particular circumstance.

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Publ 2510A, Fire Protection Considerations for the Design and Operation of Liquified Petroleum Gas (LPG) Storage Facilities

$ 75.00K2510A

RP 14J, Design and Hazards Analysis for Offshore Production Facilities $ 85.00G14J02

Std 2510, Design and Construction of LPG Installations $ 76.00C25108

RP 2001, Fire Protection in Refineries $ 75.00K20017

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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11/03

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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Page 46: RP 752 (2003) Plant Hazards

Additional copies are available through Global EngineeringDocuments at (800) 854-7179 or (303) 397-7956

Information about API Publications, Programs and Services isavailable on the World Wide Web at: http://www.api.org

Product No. K75202

Copyright American Petroleum Institute Provided by IHS under license with API

Not for ResaleNo reproduction or networking permitted without license from IHS

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