Recommended Practice for Safe Operation of Hydrofluoric Acid Alkylation Units

API RECOMMENDED PRACTICE 751SECOND EDITION, FEBRUARY 1999

API ENVIRONMENTAL, HEALTH AND SAFETY MISSIONAND GUIDING PRINCIPLES

The members of the American Petroleum Institute are dedicated to continuous efforts toimprove the compatibility of our operations with the environment while economicallydeveloping energy resources and supplying high quality products and services to consum-ers. We recognize our responsibility to work with the public, the government, and others todevelop and to use natural resources in an environmentally sound manner while protectingthe health and safety of our employees and the public. To meet these responsibilities, APImembers pledge to manage our businesses according to the following principles usingsound science to prioritize risks and to implement cost-effective management practices:

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To recognize and to respond to community concerns about our raw materials, prod-ucts and operations.

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To operate our plants and facilities, and to handle our raw materials and products in amanner that protects the environment, and the safety and health of our employeesand the public.

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To make safety, health and environmental considerations a priority in our planning,and our development of new products and processes.

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To advise promptly, appropriate ofÞcials, employees, customers and the public ofinformation on signiÞcant industry-related safety, health and environmental hazards,and to recommend protective measures.

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To counsel customers, transporters and others in the safe use, transportation and dis-posal of our raw materials, products and waste materials.

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To economically develop and produce natural resources and to conserve thoseresources by using energy efÞciently.

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To extend knowledge by conducting or supporting research on the safety, health andenvironmental effects of our raw materials, products, processes and waste materials.

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To commit to reduce overall emissions and waste generation.

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To work with others to resolve problems created by handling and disposal of hazard-ous substances from our operations.

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To participate with government and others in creating responsible laws, regulationsand standards to safeguard the community, workplace and environment.

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To promote these principles and practices by sharing experiences and offering assis-tance to others who produce, handle, use, transport or dispose of similar raw materi-als, petroleum products and wastes.

Recommended Practice for Safe Operation of Hydrofluoric Acid Alkylation Units

Refining Department

API RECOMMENDED PRACTICE 751SECOND EDITION, FEBRUARY 1999

SPECIAL NOTES

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

API is not undertaking to meet the duties of employers, manufacturers, or suppliers towarn and properly train and equip their employees, and others exposed, concerning healthand safety risks and precautions, nor undertaking their obligations under local, state, or fed-eral laws.

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, byimplication or otherwise, for the manufacture, sale, or use of any method, apparatus, or prod-uct covered by letters patent. Neither should anything contained in the publication be con-strued as insuring anyone against liability for infringement of letters patent.

Generally, API standards are reviewed and revised, reafÞrmed, or withdrawn at least everyÞve 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 Þve years after its publication date as anoperative API standard or, where an extension has been granted, upon republication. Statusof the publication can be ascertained from the API ReÞning Department [telephone (202)682-8000]. A catalog of API publications and materials is published annually and updatedquarterly by API, 1220 L Street, N.W., Washington, D.C. 20005.

This document was produced under API standardization procedures that ensure appropri-ate notiÞcation and participation in the developmental process and is designated as an APIstandard. Questions concerning the interpretation of the content of this standard or com-ments and questions concerning the procedures under which this standard was developedshould be directed in writing to the coordinator of the ReÞning Department, American Petro-leum Institute, 1220 L Street, N.W., Washington, D.C. 20005. Requests for permission toreproduce or translate all or any part of the material published herein should also beaddressed to the director.

API standards are published to facilitate the broad availability of proven, sound engineer-ing and operating practices. These standards are not intended to obviate the need for apply-ing sound engineering judgment regarding when and where these standards should beutilized. The formulation and publication of API standards is not intended in any way toinhibit anyone from 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 © 1999 American Petroleum Institute

FOREWORD

In January 1990, API issued Recommended Practice 750,

Management of Process Haz-ards

, which outlines the key elements of a comprehensive program for managing all poten-tially hazardous processes. In April 1991, API issued a background paper entitled ÒThe Useof Hydroßuoric Acid in the Petroleum ReÞning Alkylation Process.Ó [1] The paper outlinesfour systems that, if properly installed and maintained, will minimize the risks associatedwith this process. Although these documents will have a beneÞcial impact on hydroßuoricacid (HF) alkylation safety, a supplemental document that expands on concerns speciÞc toHF alkylation can have an even larger effect. This recommended practice, developed by anAPI committee concerned with further improving the industryÕs good safety record, isintended to serve that purpose. It is an outline of many of the practices used effectively in theindustry to minimize the process hazards of HF alkylation. Throughout this recommendedpractice, it is assumed that the reader is familiar with API Recommended Practice 750.

HF alkylation is a widely used reÞnery process important in producing a signiÞcant shareof the nationÕs high-quality motor gasoline. The acid used in these units is a hazardous andcorrosive liquid which, if accidentally released, can form a vapor cloud. Contact with HF liq-uid or vapor can result in serious, painful chemical burns and adverse health effects, some-times with delayed onset.

However, with proper design of alkylation units and careful process management, theacid in these units does not present a signiÞcant risk to the community or the environment.This process has been operated for over 50 years, with only a small number of incidentsaffecting the surrounding communities. Moreover, improvements in process design andmanagement are continually being made to further reduce the risks to workers and surround-ing communities. The petroleum and chemical industries have conducted extensive researchon HF alkylation safety; the results are being used to prevent incidents and to mitigate theeffects of an incident if one occurs.

When the engineering systems and procedures described in this recommended practiceare properly implemented, they will further reduce the potential for an HF release, mitigatethe effects of a release in the unlikely event that one occurs, and provide for oversight of theentire process.

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 conßict.

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

iii

CONTENTS

Page

1 HAZARDS MANAGEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Process Hazards Management Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Environmental Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Incident Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 OPERATING PROCEDURES AND WORKER PROTECTION . . . . . . . . . . . . . . . . 32.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Health Hazard Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Operating Manuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4 Training. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.5 Protective Equipment and Clothing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.6 Controlled Access to the HF Alkylation Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.7 Medical Response to HF Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.8 HF Sampling and Handling of HF Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 MATERIALS, NEW CONSTRUCTION, INSPECTION AND MAINTENANCE . . 73.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Materials Performance in HF Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.3 New Construction Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.4 Inspection of Commissioned HF Unit Equipment. . . . . . . . . . . . . . . . . . . . . . . . . 93.5 Equipment Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 TRANSPORTATION AND INVENTORY CONTROL . . . . . . . . . . . . . . . . . . . . . . 134.1 ReÞner-Shipper Cooperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2 HF Unloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.3 Inventory Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 RELIEF, UTILITY, AND MITIGATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . 145.1 Relief and Neutralization Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.2 Utility Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.3 Mitigation Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176.1 Standards, Codes, and SpeciÞcations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176.2 Books, Articles, and Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

APPENDIX A ELEMENTS OF A COMPREHENSIVE AUDIT . . . . . . . . . . . . . . . . . 19APPENDIX B HF EXPOSURE LIMITS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21APPENDIX C PROCEDURES FOR UNLOADING ACID. . . . . . . . . . . . . . . . . . . . . . 23APPENDIX D EXAMPLES OF TASKS FOR EACH CLOTHING CLASS . . . . . . . . 25APPENDIX E DESIGN FEATURES OF AN ACID-TRUCK

UNLOADING STATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27APPENDIX F MONITORING AND DETECTION SYSTEMS . . . . . . . . . . . . . . . . . . 29APPENDIX G WATER MITIGATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33APPENDIX H EMERGENCY ISOLATION OF AN HF RELEASE . . . . . . . . . . . . . . 35APPENDIX I BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

TablesB-1 Exposure Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

v

1

Safe Operation of Hydrofluoric Acid Alkylation Units

1 Hazards Management

1.1 PROCESS HAZARDS MANAGEMENT PLAN

1.1.1 General

API Recommended Practice RP 750 outlines an 11-stepprocedure for managing the hazards found in reÞning processunits. The OSHA regulation ÒProcess Safety Management ofHighly Hazardous ChemicalsÓ [29

Code of Federal Regula-tions

(CFR) Part 1910.119] and the EPAÕs ÒRisk ManagementProgramÓ (RMP) rule (40

CFR

Part 68) also have a signiÞ-cant impact on hazards management for facilities in theUnited States.

Process hazards management has excellent application tohydroßuoric acid (HF) alkylation units. These units handleliqueÞed petroleum gas (LPG) and hydroßuoric acid (HF)which, if released in quantity, may cause signiÞcant Þre andtoxic hazards. Each operating HF alkylation unit should havea process hazards management plan.

Process hazards management plans used for HF alkylationunits must comply with federal, state, and local regulations.These regulations may have requirements that differ fromthose of API RP 750.

1.1.2 Process Hazards Analysis

1.1.2.1 Priority

A process hazards analysis is a logical Þrst step in a pro-cess hazards management program and should be conductedin all HF alkylation facilities. This analysis will help in identi-fying and evaluating events that could lead to releases of HFor LPG. Alkylation units should be high on the priority list ofprocess units to be analyzed because of the dual hazards pre-sented by HF and LPG. For the same reason, the maximuminterval between analyses should be 5 years. Applicable regu-lations should be reviewed regarding the permissible interval.

1.1.2.2 Methods

Many analytic techniques are available to the reÞner forevaluating process hazards.

Guidelines for Hazard Evalua-tion Procedures

[2]

and ÒProcess Safety Management ofHighly Hazardous Chemicals

9

Ó summarize some advantagesand disadvantages of hazard analysis systems and provideguidance in selecting appropriate tools for process hazardsanalysis. A hazard and operability (HAZOP) study is onemethod of process hazards analysis appropriate to an HFalkylation unit. The Þnal choice of analytic technique willdepend on a number of site-speciÞc criteria. There may be abeneÞt in varying the technique from one analysis to another.

Regardless of the method selected, the following situa-tions should be included in the analysis:

a. Routine operations, including acid unloading andsampling.b. Start-up.c. Shutdown.d. Upset conditions.e. Emergencies.

Vapor cloud dispersion modeling can be used in conjunc-tion with process hazards analysis to help judge the conse-quences of a speciÞc hypothetical release scenario.

1.1.3 Management of Change

In addition to the procedures suggested by API Recom-mended Practice 750, signiÞcant changes in controls/criticalalarms/instrumentation, equipment/piping, operating limits,operating procedures, relief/safety systems, technology, orfacilities in an HF alkylation unit should be subject to someform of process hazards analysis. Particular attention shouldbe paid to the potential for loss of containment integrity thatmay result from the changes.

1.1.4 Emergency Response and Control

1.1.4.1 Content

An emergency response and control plan should be estab-lished for each HF alkylation unit. This plan must be in accor-dance with existing federal, state, and local regulations andindustry guidelines. (See, for example, 29 CFR Parts 1910.38and 1910.120, as well as other OSHA references listed in6.1.) In addition to the items covered by regulations, the planshould evaluate and address the following items:

a. The consequences of a potential HF release in addition toan LPG release.b. The need for, and sources of off-site emergency responseequipment and personal protective equipment suitable for HFexposure.c. The need for, and location of emergency medical treat-ment for HF exposure, including the location of clinics andhospitals that are familiar with HF burn care.d. The possible contamination of runoff water with HF.e. The need for off-site emergency response personnel whoare trained in handling both HF and LPG emergencies.f. The mechanism for communicating to the rest of the reÞn-ery and surrounding community a response appropriate to thesituation; for example, evacuation or shelter in place [7].

1.1.4.2 Emergency Response Team

An emergency response team is desirable to help establishon-site control of an HF alkylation emergency. Such a team is

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normally part of the reÞneryÕs general emergency responseorganization. The team should be trained in all areas of theemergency response plan in accordance with applicable fed-eral, state, and local regulations.

Information gained from real and simulated HF and LPGincidents should be reviewed to determine what improve-ments need to be made to the HF alkylation unit and the reÞn-eryÕs emergency response and control plan.

1.1.5 Audit Programs

1.1.5.1 Frequency of Audits

Auditing allows an organization to systematically reviewits success in satisfying the provisions of API RP 750 thataddress process hazards management and to assess compli-ance with the organizationÕs internal policies and procedures.Auditing should also verify that federal, state, and localrequirements are being met. Each operating unit should havea comprehensive audit plan and should be audited at leastevery 3 years.

1.1.5.2 Plan Content

The audit plan should cover safety, hazard, and operabilityaspects of the HF alkylation unit. The plan should identify bytitle the individuals responsible for carrying out the audit andshould specify the appropriate training for these individuals.It should provide a checklist speciÞc to the HF alkylationunit, including items such as those listed below (see Appen-dix A for further details).

Auditors should review a representative sample of each ofthe following items, concentrating on the time period sincethe last audit:

a. HF-related incident reports and industry experiences.

b. Unit records, including operating procedures, logs, check-lists, and operator training records.

c. Inspection and maintenance records and training recordsfor personnel in this area.

d. Mechanical and procedural changes in the unit.

e. Testing and maintenance of detection, monitoring, andautomatic control systems used to minimize the conse-quences of an HF-related incident.

f. Testing and maintenance of systems used to mitigate theaccidental release of HF to the atmosphere.

g. Evidence of compliance with and understanding of estab-lished procedures, obtained from observing and interviewingunit and plant personnel.

h. Mechanisms for investigating and implementing technol-ogy changes that reduce the risk of an accident.

The balance of this recommended practice primarilyaddresses the HF aspects of an alkylation unit.

1.2 ENVIRONMENTAL IMPACT

1.2.1 General

Operation of an HF alkylation unit generates waste mate-rial and by-products that, because of their physical or toxico-logical properties, may require on-site processing prior toÞnal disposition. To achieve this, procedures and facilitiesshould be in place for the safe handling of these materialsboth on and off-site. Handling methods must be in compli-ance with applicable environmental regulations.

1.2.2 Process By-products

Process by-products that may require further treatmentinclude the following:

a. Constant-boiling mixtures (CBMs) or HF-water mixtures.b. Acid-soluble oils (ASOs) or polymers.c. Neutralization pit and caustic regeneration solids.d. Deßuorinator solids.e. Acid-area surface water drainings.f. KF, KOH, NaF or NaOH drainings from treater operation.g. Neutralizing and cleaning chemicals from turnarounds.h. Runoff from water mitigation systems.

1.2.3 Vent Gas Scrubbing

In HF alkylation, the main potential air emission from rou-tine operation is the acid itself. There should be a scrubber toremove HF from acidic vent gas before the gas is directed to aßare. This control mechanism should result in insigniÞcantHF emissions.

1.2.4 Leaks

Small acid leaks occasionally develop in HF alkylationunit equipment and may become large leaks if not handledpromptly. A regular monitoring program should help to iden-tify this type of leak so that repairs can be made before alarger leak develops. Regular operator inspections are recom-mended to identify very small leaks.

1.3 INCIDENT REVIEW

If an HF release is large enough, there will be regulatoryreporting requirements under the

Emergency Planning andCommunity Right to Know Act

(Superfund Amendments andReauthorization Act, Title III), the

Comprehensive Environ-mental Response, Compensation and Liability Act

, and otherlaws. Moreover, all HF incidents, including releases of less-than-reportable quantities and potentially serious incidents(near misses), should be investigated and root causes identiÞed.Procedures should clearly specify the level of investigation tobe conducted based on the severity of the event, the format anddistribution of the investigation report, and the parties responsi-ble for taking corrective action to prevent recurrences.

S

AFE

O

PERATION

OF

H

YDROFLUORIC

A

CID

A

LKYLATION

U

NITS

3

2 Operating Procedures And Worker Protection

2.1 GENERAL

API RP 750 clearly spells out the need for careful commu-nication of the design intent, capability, and limitations ofaffected process units to personnel working on these units.HF alkylation units need speciÞc written operating proce-dures that address the toxic and corrosive nature of the acidcatalyst. Because HF is not usually found in other operatingparts of the reÞnery, some of the procedures may be unique tothe HF alkylation unit, and special training may be warranted.

2.2 HEALTH HAZARD INFORMATION

Pure hydrogen ßuoride is a clear, colorless, corrosive liquidthat boils at 67¡F. Depending on conditions, hydrogen ßuo-ride can form a vapor cloud if released to the atmosphere. Ithas a sharp, penetrating odor that humans can detect at verylow concentrations in the air. It is completely soluble in water,forming hydroßuoric acid. In concentrated solutions, the acidfumes when exposed to moist air.

Even brief contact with hydroßuoric acid liquid or vaporcan produce serious, painful chemical burns, sometimes withdelayed onset. The vapor can be extremely irritating to theeyes, skin, and respiratory tract. Short-term exposure athigher concentrations can lead to serious health effects ordeath as a result of extensive respiratory damage. There maybe chronic health effects, such as ßuorosis, from repeatedexposure.

Acid Soluble Oil (ASO) is a light-to-dark colored liquidwhich can contain varying concentrations of HF (hydroßuo-ric) Acid. Depending on its HF Acid content, it may have asharp, pungent, irritating odor. It is relatively insoluble inwater. Contact with unneutralized ASO can produce serious,painful HF Acid burns, sometimes with delayed onset.

FOR ADDITIONAL INFORMATION, CONSULTYOUR EMPLOYER, THE MANUFACTURER OR SUP-PLIER OF THE MATERIAL, OR THE MATERIALSAFETY DATA SHEET.

Facilities must comply with the requirements of workerprotection health standards and regulations applicable to thefacilityÕs location. For example, 29

CFR

Part 1910.1000

9

cur-rently sets routine workplace exposure limits for HF (as ßuo-rine) at 3 parts per million for an 8-hour time-weightedaverage. The same rule sets the routine short-term exposurelimit at 6 parts per million for a 15-minute time-weighted aver-age. Appendix B contains other sources of exposure guidelines(refer to the most recent editions of the applicable sources).

Facilities must also comply with applicable hazard com-munication (right-to-know) regulations. For example, 29

CFR

Part 1910.1200

9

addresses labeling, material safety datasheets, worker training, and record-keeping requirements.

The applicability of 29

CFR

Part 1910.120

9

(for U. S.facilities) must be considered in light of speciÞc operationsand emergency response plans at the site.

2.3 OPERATING MANUALS

2.3.1 Content

Manuals for operating and other procedures (such as Þrstaid) should be developed and made available to all assignedoperating personnel of an HF alkylation unit. These manualsshould be unit and site speciÞc, and they should include thedescriptive materials called for in API RP 750, as well as pip-ing and equipment limitations, steps to follow in the event ofan HF release, and detailed Þrst-aid procedures. Routineoperations that are unique to HF alkylation require very spe-ciÞc procedures. Such operations include acid sampling, unitneutralization and dry-out, and unloading of fresh acid ship-ments. Other procedures are mentioned in the applicable sec-tions of this recommended practice.

An example of a procedure for unloading fresh acid is pro-vided for reference in Appendix C.

2.3.2 Response to an HF Release

The manuals should include a section on detection of HFreleases and appropriate response procedures. This sectionshould guide the operator in the steps to be taken if an HFrelease occurs, including the following information:

a. Accounting for all personnel in the unit. b. Criteria for initiation and operation of the following emer-gency systems, where available:

1. A unit evacuation horn or other notiÞcationmechanism. 2. Remote-operated isolation valves.3. Water mitigation.4. Rapid deinventory and emergency acid movements.

c. Initiation of emergency response or other contingencyplans (see 1.1.4).

2.3.3 Temporary Shelter-in-Place Facilities

Where on-site facilities are used for temporary shelter inplace, written procedures should guide operating and otherpersonnel in entering and securing the shelter rooms againstHF intrusion and in testing the atmosphere in the rooms. Theprocedures should also provide criteria for leaving the tempo-rary shelters. The appropriate personal protective equipmentand escape breathing apparatus to be kept in inventory at suchlocations should be considered.

2.3.4 Testing of Critical Systems

A written procedure should be in effect for identifying andperiodically testing critical alarms and HF detection, isola-tion, and mitigation systems. The procedure should includevalve stroking, isolation and testing of primary elements and

4 API R

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controls, testing of critical pump autostart/stop systems, test-ing of water-spray equipment, and calibration and testing ofdetection and shutdown systems.

2.3.5 Changes In Procedures

Written operating procedures should specify the job titleof the person responsible for authorizing changes to proce-dures, for ensuring that such changes receive appropriatereview and documentation, and for ensuring that appropriatetraining has taken place.

2.4 TRAINING

2.4.1 General

Many standards and regulations are relevant to trainingrequirements in process units. Examples include 29

CFR

Part1910.1200

9

and API RP 750, Section 7. Some matters speciÞcto HF alkylation that should be considered in alkylation unittraining programs are presented in 2.4.2 through 2.4.6. Proce-dures should be developed to ensure that persons assigned tothe facility possess the required knowledge and skills to carryout their duties, including start-up, shutdown, and safe off pro-cedures.

Response training for operators in nearby areas should beconsidered so that these operators can help effect safe shut-down or safe continued operation of their units in the event ofan HF release.

2.4.2 Training of Operators

Because of the potential hazards of HF, training of operat-ing personnel in the process and mechanical limitations of theHF alkylation unit is especially important. The systemsinstalled for prevention, detection, and mitigation of HFreleases may be complex and unique to the HF alkylationunit. Training should emphasize the importance and properoperation of these systems.

2.4.3 Training of Maintenance Personnel

Employees involved in maintaining the mechanical integ-rity of equipment in the HF alkylation unit should be trainedin the mechanical and materials limitations, procedures, andsafe work practices applicable to their jobs, including the haz-ards of HF.

2.4.4 Training of Other Personnel

Appropriate written procedures for personnel other thanoperators and maintenance workers should be developed andmade available to all persons who work on or enter an HFalkylation unit routinely, such as supervisors and technicaland contractor personnel. These procedures should addressthe hazardous nature of HF, appropriate Þrst-aid procedures,steps to be taken in the event of an HF release, and otherinformation relevant to the speciÞc work assignment. A vari-

ety of presentation methods may enhance communication tothe targeted personnel. These procedures should be includedin a reference manual.

2.4.5 Emergency Response Training

The emergency response plan outlined in 1.1.4 should beincluded in the training of employees and off-site support per-sonnel who are designated to respond to an emergency in theHF alkylation unit. To aid proÞciency, periodic drills and sim-ulations are recommended.

2.4.6 Personal Protective Equipment Training

Training must be provided for all personnel who enter orwork in the HF alkylation unit, or who are designated torespond to emergencies in the unit, in the use of applicablepersonal protective equipment and clothing.

2.5 PROTECTIVE EQUIPMENT AND CLOTHING

2.5.1 Personal Protective Equipment and Clothing

2.5.1.1 Availability and Written Policy

Proper protective equipment must be available for all per-sonnel who work in or enter an HF alkylation unit for any rea-son. Each HF alkylation unit should have a written policy thatoutlines requirements for use and training on protectiveequipment and clothing, including the potential health impactof using protective equipment under extreme ambient condi-tions. Heating and cooling systems for personnel who wearpersonal protective equipment in extreme working conditionsare commercially available and should be considered.

2.5.1.2 Classes of Personal Protective Equipment and Clothing

In selecting personal protective equipment, a combinationof clothing and equipment should be chosen to conform toapplicable regulations and to provide an appropriate level ofprotection without signiÞcantly impairing work performance.As conditions change, the level of protection should changein a way that is appropriate to the situation.

As a guideline, levels of protection may be divided intofour classes denoted as A, B, C, and D, where A is the lowestlevel of protection and D is the highest. This classiÞcationsystem is widely used in the reÞning industry and is outlinedbelow. Note, however, that EPA and OSHA have a letteringsystem for classiÞcation of personal protective equipmentthat is the opposite of the one used in this recommended prac-tice (that is, A is the highest level of protection and D is thelowest). The substantive criteria within each letter level maynot be completely comparable between the industry and gov-ernment schemes. Therefore, the list below should be used inthe context of OSHA regulations on personal protectiveequipment and respiratory protection. (See 29

CFR

Part

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OF

H

YDROFLUORIC

A

CID

A

LKYLATION

U

NITS

5

1910.132-136

9

and, if applicable, Part 1910.120

9

.) Distinc-tions between the list below and EPA/OSHA levels should benoted in training and should be explained to any municipal orother external hazardous materials teams who are employed.

a. Class A clothing consists of a face shield or goggles, acid-resistant gauntlets or gloves, acid-resistant rubbers or boots,and an optional acid-resistant jacket. This class is used when nophysical contact with acid-containing equipment is to be made.

b. Class B clothing consists of a face shield, optional gog-gles, acid-resistant gauntlets, acid-resistant rubbers or boots,an acid-resistant jacket, and acid-resistant overalls. This classis used for routine work on acid-containing equipment whenno acid exposure is expected.

c. Class C clothing consists of an air-supplied HF hood, acid-resistant gauntlets, acid-resistant rubbers or boots, an acid-resistant jacket, and acid-resistant overalls. If a higher level ofrespiratory protection is needed, based on site-speciÞc con-siderations and the type of work being performed, positive-pressure self-contained breathing apparatus (SCBA) or an air-line-fed respirator with escape pack should be used under anacid hood. This class is used when low-level HF exposure isanticipated.

d. Class D clothing consists of a totally enclosed acid-resis-tant pressurized suit with SCBA or with air-line-fed respiratorwith escape pack. The SCBA for class D clothing should con-tain a nominal 30-minute air supply. Air-supplied suits shouldbe equipped with an escape air supply. These suits shouldinclude provisions for positive remote communications, suchas a radio inside the suit. This class is used where exposure toHF vapor is expected and where there is potential for expo-sure to liquid HF. Direct contact with liquid HF may result inpremature failure of protective equipment and should beavoided whenever possible.

Appendix D outlines typical tasks that help deÞne the needfor clothing in each of the classes listed above.

2.5.1.3 Backup Personnel

When work is performed that requires Class C or D equip-ment and clothing, as described in 2.5.1.2, standby personnelshould be present, dressed in appropriate equipment andclothing , to assist the work party with egress from the workarea should it become necessary (see 29

CFR

Part1910.134

9

).

2.5.1.4 Multiple Storage Areas

Provision of an inventory of protective clothing for emer-gency response in more than one location should be consid-ered in the event that one location becomes inaccessibleduring an incident. In cold climates, heated storage areasshould be considered for personal protective equipment.

2.5.1.5 Care and Cleaning

Designated areas and facilities should be provided for neu-tralization, cleaning, and storage of all protective clothing.

Protective clothing should be neutralized and cleaned aftereach use. Since protective clothing is acid resistant, not acid-proof, it should be washed and neutralized immediately afterany contact with HF. Procedures should be established tokeep potentially contaminated clothing and equipment awayfrom both the clean change area and the control room.

2.5.1.6 Inspection

All protective equipment, including new clothing, shouldbe inspected before use. Procedures should be developed forinspection, testing, and replacement of protective clothingand equipment. Testing of gloves and inspection of bootsafter each use should be considered.

2.5.2 Safety Showers and Eyewash Stations

Safety showers and eyewash stations should be providedin the HF alkylation unit. These showers should be located toprovide timely and unrestricted access by personnel from allacid-containing locations in the unit. Control room and/orlocal alarms should be provided to alert unit operators when asafety shower is activated. Each shower should be tested at adeÞned frequency, and the results of the tests should be docu-mented (see ANSI Z358.1

3

).

2.6 CONTROLLED ACCESS TO THE HF ALKYLATION UNIT

2.6.1 Unit Demarcation

The HF alkylation unit should be distinctively marked atall points of entry. Such markings should warn people that HFis present, that access is strictly limited, and that protectiveclothing is required.

2.6.2 Entry By Maintenance Personnel

A sign-in/sign-out procedure or another comparablemeans of accounting for personnel on the unit, administeredby the operator in charge of the unit, is recommended. Allmaintenance personnel should receive a safety orientationbefore entering the alkylation unit. A work permit system isalso recommended as a prerequisite for maintenance work onthe unit. The permit should describe the equipment to beworked on and the protective clothing required and afÞrm thatthe equipment is properly prepared for work.

2.6.3 Entry By Visitors

Access into the HF alkylation unit by visitors and other per-sonnel not normally assigned to work in that area should becontrolled and documented. A sign-in/sign-out procedure oranother comparable procedure, administered by the operator

6 API R

ECOMMENDED

P

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751

in charge of the unit, is recommended. All visitors shouldreceive a safety orientation before entering the alkylation unitand should wear the proper protective clothing (see 2.4.6 and2.5.1.2).

2.7 MEDICAL RESPONSE TO HF EXPOSURE

2.7.1 General

Written procedures should outline an appropriate responsewhen people are exposed to HF vapor or liquid, includinginhalation. Symptoms of exposure should be included. Sincespeed of response is a primary means of minimizing theimpact of HF exposure, proper training in this area is ofutmost importance.

Operating procedures and training should help the Þrst-aidprovider determine whether immediate medical assistance isneeded. Procedures should include instructions on transporta-tion to a medical facility and appropriate communication withthat facility. In emergency cases, a knowledgeable employeeshould accompany the affected person to the medical facility.This employee can ensure that attending medical personnelare aware of the HF involvement and can furnish them withcopies of any prearranged treatment plans.

2.7.2 First Aid

2.7.2.1 First-Aid Kits

Suitably equipped Þrst-aid kits should be readily availablein HF alkylation units. Operating procedures should indicatethe number, location, content, and replenishment schedulesfor Þrst-aid kits. Refrigerated storage of kits will extend theshelf life of certain of their key components.

2.7.2.2 Portable First-Aid Kits

Placement of additional portable Þrst-aid kits at the site ofwork in enclosed or difÞcult access areas should be considered.

2.7.2.3 Trained Personnel

Personnel trained in HF Þrst aid should be available on allshifts.

2.7.3 Follow-Up Medical Treatment

Each facility should develop a prearranged plan for fol-low-up medical examination and treatment, as needed afterinitial Þrst aid, at one or more nearby medical facilities. Theplan should include written protocols for treatment of HFexposures, provision of protective equipment for facility per-sonnel as needed, stocking of supplies for HF treatment,appropriate training, communication with reÞnery medicalpersonnel, and procedures for hospital admissions which maybe required after emergency treatment.

2.8 HF SAMPLING AND HANDLING OF HF SAMPLES

2.8.1 Training for Acid Sampling

Sampling of streams that contain potentially harmful quan-tities of acid requires special precautions. Procedures shouldbe established for HF sampling techniques, design of connec-tions and equipment, and communication of hazards. Appro-priate training should be provided for all operating andlaboratory personnel who may collect or handle samples thatcontain HF, or may conatin HF.

2.8.2 Design of Sampling Stations

2.8.2.1 Location

HF sampling connections should be located at grade or onan unobstructed structure that permits easy egress for personsin protective clothing.

2.8.2.2 Minimizing Exposure

Sampling systems for streams that contain HF should bedesigned to minimize exposure of personnel to acid. The useof a closed acid-sampling system should be considered.

2.8.2.3 Valves

Sample connections should have two block valves perconnection. When the connection is not in use, both valvesshould be closed and the open end of the sample connectionshould be sealed or plugged.

2.8.2.4 Marking Sample Points

Sample points should be permanently connected andclearly marked as acid-sampling points.

2.8.3 Sample Containers

Written procedures should be established for HF samplecontainers. Procedures should cover materials of constructionand procedures for neutralizing, cleaning, storing, and period-ically inspecting and testing HF sample containers. Samplecontainers should be clearly identiÞed as containing HF.These containers should only be used for HF.

2.8.4 Laboratory Safety

2.8.4.1 Designated Area

Laboratories should provide a designated area for storing,handling, and analyzing HF-containing samples.

2.8.4.2 Fume Hood

Laboratories should be equipped with a fume hood (withappropriate neutralizing facilities) for handling HF.

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OF HYDROFLUORIC ACID ALKYLATION UNITS 7

3 Materials, New Construction, Inspection And Maintenance

3.1 GENERAL

Materials of construction and component fabrication guid-ance provided in this recommended practice is based on test-ing and successful Þeld experience since thecommercialization of HF alkylation units in the early 1940s.The materials speciÞcations and fabrication requirements forpressure equipment components intended for HF service andthe process conditions at which they should be applied havebeen well deÞned by licensors of the HF alkylation process.Unexpected corrosion or deterioration can be avoided by fol-lowing these materials speciÞcations and fabrication require-ments and by maintaining the unit operating conditionswithin the speciÞed process limits.

Similarly, the inspection and maintenance guidance pro-vided in this recommended practice is based on many yearsof owner/operator experience. The owner/operator of eachHF alkylation unit should develop and maintain comprehen-sive written inspection and maintenance procedures aimed atpreserving the unitÕs mechanical integrity. Adherence to theseprocedures is one of the most important measures for prevent-ing a release of HF.

The terms ÒMain AcidÓ and ÒTrace AcidÓ are used in sev-eral sections throughout this chapter. Trace Acid service isdeÞned to include only those streams that contain primarilyliquid hydrocarbon streams with small amounts of solubleacid as deÞned by the owner-operator. All other services withfree acid present shall be considered in Main Acid service. Inaddition, services which are designed to operate in Trace Acidservice but which are regularly subjected to free acid break-through shall be considered in Main Acid service for inspec-tion purposes.

3.2 MATERIALS PERFORMANCE IN HF SERVICE

The principal materials of construction for HF alkylationunits are carbon steel and the nickel-copper Alloy 400 (e.g.,Monel 400). Other materials, such as 70-30 copper-nickeland nickel-base Alloys C-276 and B-2 (e.g., Hastelloy C-276and B-2), have been used in selected applications. All aspectsof corrosion of these materials cannot be adequatelyaddressed in this recommended practice. NACE Publication5A171, Materials for Receiving, Handling, and StoringHydroßuoric Acid7, provides a good overview of materialsperformance in HF service.

3.2.1 Carbon Steel

Corrosion of carbon steel by HF is a function of water con-tent, temperature, velocity, and contaminants. Carbon steel hasdemonstrated satisfactory resistance to concentrated HF up toapproximately 150¡F. This temperature should not be consid-ered as a limit. Actual experience in alkylation units varies,

some indicating unsuitable performance at lower temperaturesand some indicating suitable performance up to 180Ð200¡F,depending on the speciÞc exposure conditions. Carbon steelrelies to a large extent on the presence of a protective iron ßu-oride Þlm for its HF corrosion resistance. Loss of this protec-tive Þlm can result in accelerated corrosion rates. Welding slagis rapidly attacked by HF. It has also been reported that certainresidual elements (Cr, Ni, Cu) in carbon steel may contributeto accelerated HF corrosion (See NACE Publication 5A1717

for further details). A byproduct of the HF corrosion reaction with carbon

steel is atomic hydrogen. This atomic hydrogen can enter thesteel and cause hydrogen blistering, hydrogen embrittlement,and various forms of environmental cracking such as hydro-gen stress cracking (HSC), hydrogen-induced cracking(HIC), and stress-oriented hydrogen-induced cracking(SOHIC). Dirty steels, those with high inclusion content, aremost susceptible to hydrogen blistering and HIC damage.Cleaner steels manufactured with special processing andchemistry controls to provide lower inclusion content, such asthose referred to as HIC-resistant steels, can provide resis-tance to hydrogen blistering and HIC damage in HF service.Base metal, weld deposits, and weld heat-affected zones withhigh hardness are susceptible to HSC. Susceptibility to HSCcan be reduced by limiting the maximum hardness and bypostweld heat treatment as required. Areas of high residualstress adjacent to welds are susceptible to SOHIC, which canbe reduced by use of clean steels and postweld heat treatment.

Arsenic can play a role in promoting hydrogen blisteringand/or cracking of carbon steel. The arsenic content of thefresh HF acid should be minimized and limited to not morethan 25 parts per million by weight.

3.2.2 Alloy 400

Alloy 400 is more resistant to HF than carbon steel, and itis commonly used in areas where the corrosion resistance ofcarbon steel is unsuitable, or where scale buildup is detrimen-tal to the process. Alloy 400 has good resistance to HF up toabout 300Ð350¡F, depending on the speciÞc exposure condi-tions. Oxygen contamination in the HF can increase the cor-rosion of Alloy 400, and rapid pitting attack has beenexperienced. The presence of oxygen also increases the likeli-hood of cracking Alloy 400 containing high tensile stressesresulting from cold work or weld fabrication, particularlywhen exposed to HF vapor. A stress relieving heat treatmentcan be used to provide resistance to cracking of Alloy 400 inHF service.

3.3 NEW CONSTRUCTION GUIDELINES

3.3.1 Pressure Vessels

The hardness of carbon steel weldments on new pressurevessels for HF service should be controlled in accordance

8 API RECOMMENDED PRACTICE 751

with NACE Standard RP0472, Methods and Controls to Pre-vent In-Service Environmental Cracking of Carbon SteelWeldments in Corrosive Petroleum ReÞning Environments7.Methods to achieve the necessary hardness control mayinclude one or more of the following:

a. Control of base metal chemistry, such as limiting the car-bon equivalent (CE) and the residual element (V, Cb) contentof the steel. b. Selection of weld process and Þller metal. c. Use of special welding procedures regarding preheating.heat input, etc. d. Postweld heat treatment.

The procedure for postweld heat treatment should be inaccordance with Section VIII, Division 1, Paragraphs UW-40,UW-49, and UCS-56, of the ASME Boiler and Pressure VesselCode4 except as follows. The postweld heat treatment temper-ature should be 1150¡F held for one hour per inch of thick-ness, with a one hour minimum. Postweld heat treatment at alower temperature with a longer holding time, as permitted bythe ASME Code, should not be used. Industry experience indi-cates that PWHT at lower temperatures is not as effective inreducing heat-affected zone hardness.

Pressure-containing welds in new vessels should beinspected per applicable codes. Slag inclusions are rapidlyattacked by HF and should be minimized. Inspection of theinternal vessel surfaces in the vicinity of welds using wet mag-netic particle testing (WMT) or wet ßuorescent magnetic par-ticle testing (WFMT) should also be considered to identifyany fabrication ßaws that could promote in-service corrosionor cracking.

Additional nondestructive examination may be valuable toserve as a more comprehensive baseline for subsequentinspections after commissioning of the vessel to help interpretvessel defects and changes.

3.3.2 Piping

Each operating HF alkylation unit should have a completewritten speciÞcation for piping systems in HF service. Thelicensor initially provides this speciÞcation for new or licen-sor-revamped units. This speciÞcation should be kept currentby the owner-operator. The speciÞcation should cover con-struction materials, minimum pipe sizes, minimum wallthicknesses, minimum corrosion allowances, welding hard-ness limits, postweld heat treatment requirements, ßange rat-ings, bolting, gasketing, valves and packing. Any deviationfrom the speciÞcation should receive safety and process haz-ards management review.

The hardness of carbon steel weldments on new piping forHF service should be controlled in accordance with NACEStandard RP0472, as discussed in 3.3.1 for pressure vessels.The procedure for postweld heat treatment should be inaccordance with the ASME B31.3 Boiler and Pressure Vessel

Code4. The postweld heat treatment temperature should be1150¡F held for one hour per inch of thickness, with a onehour minimum. Postweld heat treatment at a lower tempera-ture with a longer holding time should not be used. Industryexperience indicates that PWHT at lower temperatures is notas effective in reducing heat-affected zone hardness.

Pressure-containing welds in new piping should beinspected per applicable codes. Slag inclusions are rapidlyattacked by HF and should be minimized. Stagnant ßow pip-ing and connections should be eliminated where possible.

3.3.3 Pumps

Pumps in HF service should preferably have dual seals orshould be of sealless design. If pumps equipped with singleseals are used, they should have auxiliary mechanical systemsto limit potential leak rates to the atmosphere from failure ofthe seal. Provision should be made for monitoring large pumpand driver vibration and bearing-housing temperatures so thatfailures that could result in emissions can be prevented. Alter-native lubrication systems, such as oil mist, may be beneÞcialin reducing bearing failures that could cause seal failures.

All pumps in HF service should be constructed of materi-als resistant to HF corrosion, such as carbon steel, Alloy 400,and Alloys C-276 and B-2. SpeciÞc parts of carbon steelpumps for which HF-resistant alloy materials should be con-sidered include the shaft, impeller, wear rings, throat bushing,and seal components.

3.3.4 Flanged Joints

For raised face type ßanges, spiral-wound Alloy 400 gas-kets with a virgin-polytetraßuorethylene(PTFE) or ßexible-graphite-Þller should normally be used in HF service. Gasketdimensions should be in accordance with ASME B16.204.Mating ßanges with face proÞles meeting ANSI B16.53

dimensions should be joined with spiral wound gaskets. Thegaskets should have a carbon steel outside/centering ring andan inner ring of PTFE or metal matching the ßange materialor better (i.e. Alloy 400). Addition of the inner ring is to Þllthe void area and to minimize acid intrusion between theßange faces, thus reducing ßange corrosion.

If ring joint type gaskets are used, the rings should beANSI/ASME soft iron (steel) rings with a maximum hard-ness of 90 HB. Fully annealed Alloy 400 rings are an accept-able alternate. Mating surfaces of ßanges should be carefullycleaned and dried before being assembled, because rust anddirt are rapidly attacked by HF and can result in a leak.

Care should be taken not to overtorque ßange bolts. If TypeA193-B7M or Alloy K-500 bolts are overtorqued, they maybecome work hardened and subject to cracking if exposed toHF. Alloy K-500 bolting should never be used in pressureboundary closures for this reason. If B7 bolts are overtorqued,they may bend or warp the ßanges and cause leaks. If B7 boltsare used in ßanges, HF leaks must be promptly stopped and

SAFE OPERATION OF HYDROFLUORIC ACID ALKYLATION UNITS 9

the exposed bolts replaced. If B7M bolts are used, a represen-tative sample should be tested before installation to assurecompliance with the maximum hardness speciÞcation.

As an aid to leak detection, the external surfaces of allßanges in HF service, including valve bonnet ßanges, shouldbe coated with a paint that changes color when exposed toHF. The use of ßange covers, which can trap acid or acidicwater where it cannot be readily observed and removed,should be avoided.

3.3.5 Heat Exchangers

Only seamless tubes should be used in heat exchangers inHF service. Bends in U-tube exchangers should be stress-relieved after fabrication.

3.3.6 Gauge Glasses

The use of gauge glasses should be minimized in HF ser-vice. If gauge glasses are used, a chloroßuoroethylene poly-mer shield (e.g. Kel-F) should be used to protect the glassfrom exposure to HF. Only polymer coatings manufacturedby the air-cooled method should be used as they have a higherallowable temperature limit than do shields manufactured byother methods. Polymer shielded gauge glass manufacturedby the water-cooled method should be avoided due to theirsusceptibility to cracking. Gauge glass gaskets should bePTFE, Kel-F, or ßexible graphite.

3.4 INSPECTION OF COMMISSIONED HF UNIT EQUIPMENT

3.4.1 General

A written program should be developed to govern theinspection of equipment commissioned into HF service. Thisprogram should consist of the necessary inspection proce-dures required to assure the mechanical integrity of HF unitequipment. Procedure development should consider the con-ditions affecting the performance of materials in HF serviceas noted below. These procedures should deÞne the extent,method, frequency and, as applicable, the techniques requiredto perform the inspections speciÞed. Conditions that shouldbe considered in determining monitoring locations andinspection frequency include the following:

a. Carbon steel exposed to temperatures above 150¡F and HF.b. Services that involve water content greater than 2 percentin HF.c. Vapor spaces that contain HF.d. Dead legs or start-up lines exposed to HF.e. Alloy 400 exposed to HF contaminated with oxygen.f. Alloy 400 exposed to HF at temperatures above 300¡F.g. Bimetallic or dissimilar welds

The following paragraphs provide the minimum recom-mended guidelines for the inspection of commissioned HF unitequipment. These inspection practices are intended to supple-

ment rather than replace routine reÞnery inspection practices.Owner-operator data should be used to increase or decrease theinspection interval within the limits of the applicable stan-dards. Non-HF bearing process equipment should be inspectedin accordance with the applicable standards and requirements.

Inspection requirements should be re-assessed wheneveroperational changes or process upsets occur that could impactthe mechanical integrity of the equipment. Examples of suchchanges or upsets include excessive water content in acid,acid carryovers, and temperature excursions.

3.4.2 Pressure Vessels

Pressure vessels, including heat exchanger shells, should beinspected in accordance with the applicable jurisdiction require-ments. API 510; Pressure Vessel Inspection Code, and API RP572; Inspection of Pressure Vessels should be used for guidancein establishing pressure vessel inspection requirements.

3.4.2.1 External Inspection

Each pressure vessel shall receive a visual external inspec-tion at least every 5 years or at the same interval as therequired internal or onstream inspection, whichever is less.The inspection shall, at the least, determine the condition ofthe exterior insulation and its impact on under insulation sur-faces, the condition of the supports, the allowance for expan-sion, and the general alignment of the vessel on its supports.

3.4.2.2 Internal and Onstream Inspection

Owner-operator inspection programs should also includeprovisions for obtaining onstream ultrasonic thickness mea-surements. A representative number of thickness measure-ments should be conducted on each vessel to satisfy therequirements for the onstream inspection. A decision on thenumber and location of the thickness measurements shouldconsider the results from previous inspections and unit oper-ating history. These measurements are intended to establishgeneral and localized corrosion rates in different sections ofthe vessel. The interval for performing complete onstreamthickness measurement surveys should not exceed 5 years inHF service equipment. Provisions for the evaluation ofonstream thickness measurements are further deÞned in API510, Section 6.

Process surfaces of each pressure vessel in Main Acid ser-vice should receive an internal visual inspection not to exceedone half the estimated remaining life of the vessel based oncorrosion rate or 5 years, whichever is less. Each pressurevessel in Trace Acid service shall receive an internal visualinspection not to exceed one half the estimated remaining lifeof the vessel based on corrosion rate or 10 years. An increaseor decrease of the inspection intervals within the limits of API510 or other applicable standards shall be based on owner-operator data.

At each vessel entry, complete removal of the processscale for the purpose of internal inspection is not required.

10 API RECOMMENDED PRACTICE 751

Studies have shown this scale provides a measure of protec-tion in many applications. The owner-operator shall establishthe extent to which scale must be removed for inspection pur-poses. That decision should acknowledge that heavy scalebuildup can mask deterioration and create process problemsby accumulating in various gravity collecting points. Thedevelopment and execution of inspection procedures shouldconsider the impact of increasing HF concentrations in theupper elevations of fractionating towers.

Pressure vessel walls should be inspected for environmen-tal cracking and blistering. The extent of initial inspection byWMT/WFMT or shear wave UT shall be sufÞcient to providea representative sample of the areas of concern. The areas ofconcern include longitudinal, circumferential, and nozzlewelds, and internal attachment welds to the pressure bound-ary. Areas to be inspected should speciÞcally include repair orvessel alteration welds and portions of the vessel that exhibitvisible blistering or signiÞcant corrosion. If blistering or envi-ronmental cracks are found, the inspection coverage shouldbe increased as necessary to adequately deÞne the extent ofblistering or cracking.

Reinspection intervals should be based on prior inspectionresults, the disposition of indications, or the requirements ofAPI 510 or other applicable standards.

a. If no cracks or blisters were found, no weld repairs oralterations made, the vessel remains in the same service, rein-spection for cracks may be deferred within the requirementsof API 510 or other applicable standards.b. If environmental cracks or blisters were found, or weldrepairs or alterations were made, reinspection should be doneby WMT/WFMT at the next scheduled turnaround, or by on-stream monitoring using techniques such as UT.c. If blisters or cracks are not removed or repaired, the vesselshould be re-examined periodically to ascertain whethergrowth occurs during subsequent service.

3.4.3 Piping

Piping and welded pipe components should be inspectedat intervals established in accordance with API 570, PipingInspection Code. All piping circuits in HF acid service, bothMain and Trace acid, shall be categorized as Class 1 service.Auxiliary piping in Main Acid service shall be inspected tothe same requirements as the primary process piping. Inspec-tion procedures should consist of external visual and thick-ness measurement methods. Internal visual inspections arenot normally performed on piping. When possible and practi-cal, internal visual inspections may be scheduled for systemssuch as ßare lines and large diameter transfer piping.

3.4.3.1 External and Ultrasonic Thickness Inspection

A complete external inspection should be conducted on allpiping circuits in HF acid service at least every 5 years. In

addition to the criteria deÞned in API 570, Section 3, theseexternal inspections should focus on identifying process leak-age from nonwelded joints as well as abnormal vibration insmall bore piping components. Each piping circuit in HF acidservice, Main or Trace acid, shall receive an ultrasonic thick-ness measurement inspection not to exceed one half the esti-mated remaining life of the piping component based oncorrosion rate or 5 years, whichever is less. These inspectionsmay be performed while the equipment is in service.

3.4.3.2 Small Bore Piping

Small bore piping systems have a higher potential vulnera-bility to selected deterioration mechanisms. Threaded jointfatigue as well as corrosion product accumulation in inactivebranch connections require more frequent assessment.Inspection programs should also include NDT provisions toexamine and ensure the integrity of these small bore non-butt-welded piping circuits. A representative sampling of thesejoints should be radiographically examined every 5 years.Owner-operator data should be used to increase or decreasethe inspection interval within the limits of the applicable stan-dards. ProÞle radiographs can be used to determine the condi-tion of threaded joints including thread engagement and seal-weld coverage of exposed threads in seal-welded joints. Dis-assembly of nonseal-welded joints is an alternative to radiog-raphy. Male and female threads should be checked with a ringor plug gauge. PTFE tape or equivalent should be used onnonseal-welded threaded joints.

3.4.3.3 Valves

At each unit outage, a representative number of valves inHF service should be internally inspected for corrosion orother forms of damage. The selection of valves should bebased on process conditions as well as onstream inspectiondata from adjacent piping spools. Gate, globe, plug, check,and control valves should be included in the representativesampling and removed from their adjoining spool pieces toperform the inspection. ProÞle radiography has been provenan effective onstream tool for evaluating selected valve-related problems.

Safety relief valves in HF acid service should be tested atleast every 5 years and the results documented. Selected loca-tions such as the acid settler or the rerun column can beexpected to require shorter testing intervals.

3.4.3.4 Flanged Joints

The crevice created on the inside diameter of a ßangedjoint presents an inherent location for localized HF corrosionto occur. Carbon steel ßanges are particularly susceptible tosuch attack. Over time, the corrosion of the ßange surfacesmay extend into the gasket mating area potentially compro-mising the seal integrity of the ßanged joint. Several factorscan inßuence the rate at which the ßange surface may corrode.

SAFE OPERATION OF HYDROFLUORIC ACID ALKYLATION UNITS 11

The inspection frequency of ßanged joints should consider therespective corrosion rate in conjunction with the calculatedsealing surface requirements. In the absence of inspectiondata, all ßanges in Main Acid service should be inspectedevery ten years, and all ßanges in Trace Acid service every Þf-teen years. For ßanges in the same process circuit, the Þndingsfrom one ßange inspection may be applied to other ßanges ofequal size and age.

Tightly adherent scale can mask corrosion and should beremoved for ßange mating surface assessment. Spiral-woundgaskets should be replaced whenever the ßanged joint is bro-ken. Standard grade (B7) studbolts are susceptible to hydro-gen embrittlement cracking on exposure to HF and should bereplaced whenever the bolt exhibits evidence of corrosion,scaling or exposure to HF.

3.4.4 Pumps

Pumps in HF service should be inspected for corrosion orother deterioration to the case and rotating parts. For pumpswith 10 years of operating experience and documentedinspection Þndings, the frequency of this examination shouldbe based on the pumpÕs performance history. For new pumps,pumps that have been modiÞed, pumps with less service time,or pumps without written chronological documentation, aninternal inspection should be performed at least every Þveyears.

3.4.5 Fired Heaters

Fired heaters should be inspected in accordance with APIRP 573. In addition, for units burning unneutralized ASO, theßue gas surfaces of heaters and stacks should be inspected atleast every 5 years. Locations selected from heater tube sur-faces should be included in the thickness monitoring pro-gram. Inspection of furnace tubes should include checks forbulging and bowing.

3.4.6 Fireproofing

FireprooÞng of vessel supports, critical valves, instru-ments, and electrical runs should be inspected as outlined inAPI Publication 2218. Spot checking for corrosion underinsulation and ÞreprooÞng should be considered.

3.4.7 Inspector Qualification

Inspection of equipment covered by this document shouldbe performed either by qualiÞed inspection personnel orunder the direct supervision of such personnel. QualiÞcationshould be in accordance with API 510, API 570 and otherapplicable codes, as mandated by the state in which theequipment is located. Equivalent foreign credentials may alsobe accepted. Nondestructive testing certiÞcation through theAmerican Society for Nondestructive Testing is also beneÞ-cial. All inspection personnel should receive training in thepersonnel hazards associated with handling or entering equip-

ment exposed to HF. The training should include reÞnery andunit procedures that cover personal protective clothing, entryto conÞned spaces, and HF safety.

3.4.8 Operator Surveillance

Another key element of release prevention is surveillanceby the unit operators for evidence of minute HF leakage.Operators should be alert for such signs as HF-sensitive paintthat has changed color, buildup of corrosion productsbetween ßanges, and the distinctive odor of HF. Repairsshould be made in a timely manner. A checklist of remote orlimited-access valves and instrument connections that shouldbe regularly checked for leakage should be considered forinclusion in unit operating procedures. Dilute ammonia-watersprays and wet litmus paper are common means of detectingHF wisps.

3.4.9 Storehouse Materials

A quality assurance program should be used to identify,segregate, and ensure the quality and proper warehouse stor-age and delivery to the site of materials speciÞc to the HFalkylation unit. A segregated storage area should be consid-ered for these materials.

3.4.10 Inspection Records

For an effective inspection program, complete records ofinspection results should be kept. API 510, 570, RP 572, RP574 and RP 576 contain example record forms.

3.4.11 Equipment Work List

A list of valves, instruments, controls, and safety devicesthat require service should be maintained in the unit or atanother speciÞed location. This list should be reviewed atintervals which will ensure that the conditions are remedied atthe appropriate time.

3.4.12 Unit Documentation

A copy of the unit piping and instrument diagrams andappropriate electrical diagrams should be kept in the unit con-trol center to assist operators and maintenance and inspectionpersonnel in locating blinds, isolation valves, pressure-reliev-ing devices, tagouts, and other safety and control equipment.These documents should be kept up to date as part of an over-all management-of-change procedure.

3.5 EQUIPMENT MAINTENANCE

3.5.1 General

A preventive and predictive maintenance program shouldbe in effect for HF alkylation units. This program should beused to establish proper inspection and maintenance intervalsfor equipment in HF service. Particular attention should begiven to sealing devices on pumps in HF service and to

12 API RECOMMENDED PRACTICE 751

ßanged joints. The maintenance program should comply withall applicable lockout/tagout requirements (see 29 CFR Part1910.147).

3.5.2 Equipment in Service

Vessels and piping that contain HF should not be weldedor hot tapped while in service. These operations can introducelocalized corrosion sites and areas of uncontrolled hardnessthat may result in leaks.

3.5.3 Pressure Vessel, Piping and Heat Exchanger Repair

As a minimum, repairs or welded alterations to pressurevessels, piping and heat exchangers should meet the require-ments of the most recent edition of API 510, API 570, andother applicable codes, as mandated by the state in which theequipment is located. Steels exposed to HF may become satu-rated with hydrogen. Consideration should be given to adegassing heat treatment and preheating prior to any repairwelding to improve weld quality.

The hardness of repair welds and heat-affected zonesshould be controlled in accordance with NACE StandardRP04727. Whenever postweld heat treatment is used to meetthe hardness requirements, the procedure for PWHT shouldbe in accordance with the applicable ASME Code. ThePWHT temperature should be 1150¡F held for one hour perinch of thickness, with a one hour minimum. Use of a lowertemperature heat treatment held for a longer time should notbe permitted.

Repair welds performed on steels previously exposed toHF should be subjected to nondestructive testing. Radiogra-phy, wet magnetic particle or wet ßuorescent magnetic parti-cle testing, or ultrasonic inspection should be considered,based on the type and depth of repair. Postweld heat treatedrepair welds should be inspected after the Þnal heat treatmentto detect the presence of any delayed hydrogen cracking.Plugged heat exchanger tubes should be cut or drilled behindthe tubesheet.

3.5.4 Flanged Joints

Spiral-wound gaskets should be replaced whenever aßange joint is broken. The ßange gasket surfaces should becarefully inspected for corrosion or scale buildup and, ifrepairs are needed, machined to the proper surface roughness.Bolts should be replaced whenever the bolt shows evidenceof corrosion or has been exposed to HF.

Leakage at ßanged joints require increased surveillance.Owner-operator programs should have a written procedurefor the assessment of ßanged joints exhibiting leakage and anappropriate remediation response.

3.5.5 Valves

Maintenance procedures should include testing and quali-Þcation of shops used to repair valves in HF service. Repair

shop standard practices should include the inspection of stuff-ing box bores for enlargement due to corrosion and for thecondition of ßanged joints.

3.5.6 Safety Relief Valves

A written procedure (based on API RP 576) should bedeveloped for the maintenance of safety relief valves. Theprocedure should require management approval for anysafety relief valve to be taken out of service while the unit isoperating. Special procedures are needed to accomplish thissafely. To determine the actual relief pressure, prepopping ofsafety relief valves before cleaning or disassembly should beconsidered. Any failure to function as speciÞed should beexamined, its cause should be determined, the safety reliefvalve should be repaired, and if necessary, the testing fre-quency should be modiÞed.

If rupture disks are used below safety relief valves, a high-pressure alarm or another means of detecting disk leakagemust be included in each such installation. Rupture disksshould be replaced whenever the safety relief valve is removed.

To ensure that isolation valves are actually open, radiogra-phy or another veriÞcation method should be considered eachtime the isolation gate valves before and after a safety reliefvalve are reopened. No isolation block valves should be Þttedahead of a safety relief valve unless an alternative means ofrelief, such as a secondary safety relief valve or an adequatelysized bypass, is present.

3.5.7 Temporary Repairs

Temporary repairs on equipment in acid service, includingleak repair clamps, should be made only after approval by anappropriate management of change procedure. When tempo-rary piping or clamps are used, they should be documentedand monitored by operators or inspectors on a scheduledbasis and removed at the Þrst opportunity. Sealant materialcompatibility with HF acid should be conÞrmed and docu-mented prior to use. Operators and maintenance personnelwho may be called on to clamp HF leaks should be trained inthe use of leak-clamping and sealant-pumping equipment,including criteria for change-out of bolts exposed to HF, andin the use of personal protective clothing and breathing appa-ratus. Bolts exposed to HF should be replaced. The clampingdevice should be designed to maintain the integrity of the linein the event that the material under the clamp fails.

3.5.8 Isolation and Neutralization

Written procedures should be developed for preparingequipment for maintenance. These procedures should includeneutralization, purging, and isolation before the equipment isturned over to maintenance personnel.

3.5.9 Material Removed From the Unit

Valves and other equipment to be taken outside the batterylimits of the alkylation unit should be tagged, opened, the

SAFE OPERATION OF HYDROFLUORIC ACID ALKYLATION UNITS 13

packing removed, and the bonnet bolting loosened. Otherareas where pockets of HF may form should be disassembled.The equipment should be neutralized and identiÞed with acaution tag indicating that the equipment has been in HF ser-vice and has been neutralized. If neutralization inside the bat-tery limits (ISBL) is impractible, appropriate handlingprocedures should be developed. Requirements for protectiveclothing should be speciÞed for personnel who disassembleneutralized equipment in the unit or in the shop.

Forklifts, metal scaffolding, and other equipment used inareas of HF service should be hosed down or neutralized afteruse. The use of wooden scaffolding should be discouraged,because wood absorbs HF and cannot be fully neutralized.

Scrap materials potentially contaminated with HF shouldbe stored in a segregated HF scrap-weathering area. A systemshould be used to track scrap materials as they enter and leavethe HF scrap area. Persons who receive HF scrap materialsshould be advised in writing about the hazards of HF.

3.5.10 Lifts

Use of a crane to lift heavy materials over piping andequipment that contain HF should be avoided. If such a lift isconsidered to be of critical importance, a consequence analy-sis should be performed and appropriate managementapproval should be obtained. A written rigging plan should beprepared and reviewed and should cover details such as alter-native lifting schemes; placement, mechanical condition, andcapacity of the crane; and location of sewer lines. Syntheticslings should not be used.

4 Transportation and Inventory Control

4.1 REFINER-SHIPPER COOPERATION

4.1.1 General

During shipping and, to some extent, during unloading offresh HF, the shipperÕs equipment is not protected by thereÞnerÕs safety and leak-mitigation systems. The reÞner, theacid supplier, and the shipper should actively cooperate toensure that these operations are performed safely. Some ele-ments of that cooperation, primarily relating to truck trans-port, are suggested in 4.1.2 through 4.1.7.

4.1.2 Shipping Containers

HF suppliers and shippers must use equipment designed,inspected, and maintained in accordance with DOT regula-tions (49 CFR Parts 100 to 1856). Shipping containers shouldbe dedicated to anhydrous HF service.

4.1.3 Routes

HF shippers should plan a route to the reÞnery that willminimize risk and should adhere to the planned route. Con-tainers should be closely tracked enroute. Routes within the

reÞnery should be jointly planned and should include a safeholding area in the reÞnery for early deliveries.

4.1.4 Hoses and Valves

Trucks for HF transportation should be equipped withhoses suitable for acid unloading and for pressurizing thetransport container. The hoses should have a minimum work-ing pressure of at least 150 pounds per square inch gauge andshould be inspected and tested for leaks before each use. Thetest results should be recorded. Containers should have air-to-open remote-operated valves on all outlet connections and anair-to-open remote-operated valve or a check valve on thenitrogen-pressuring connection. Crossover provision betweenthe acid unloading and nitrogen pressurizing lines should beprovided to allow an inert gas sweep after the HF transfer,prior to disconnecting. All external root valves should beenclosed in a protective housing.

4.1.5 Personnel and Clothing

The reÞner-shipper agreement should include the numberof persons accompanying each shipment of acid. Shippersmust provide safety equipment and clothing for their drivers(see 2.5.1.2 for information on personal protective equipment).The agreement should include the number of backup personnelneeded during unloading and an outline of the responsibilitiesof each participant in case of upset or emergency.

4.1.6 Training

Driver training should include HF hazards, Þrst aid, per-sonal protective equipment, unloading procedures, and opera-tion of the safety features of the shipping container. Trainingand testing programs should be ongoing. CertiÞcation of suchtraining should be made available to the reÞner on request.

4.1.7 Emergency Procedures

The reÞner should notify and review with the drivers theappropriate reÞnery and unit safety and evacuation procedures.

4.2 HF UNLOADING

The safe unloading of acid into the unit is of utmost impor-tance to the success of the reÞnerÕs hazards management pro-gram. The unloading station should be carefully designed tominimize the risk of accidental release. Clearly written oper-ating procedures should be in place to ensure that all steps inthe unloading process are performed safely. Appendixes Cand E of this practice list some of the procedural and designconsiderations for truck transport unloading that may helpreÞners develop their own programs.

4.3 INVENTORY CONTROL

The plantÕs inventory of fresh HF should be kept to a mini-mum. Part of this effort should include close scheduling withthe acid supplier to ensure timely deliveries. A key part of

14 API RECOMMENDED PRACTICE 751

controlling acid inventory is careful monitoring of acid levelsin the unit. Radioactive level devices have been proven effec-tive in monitoring levels in acid-storage vessels.

5 Relief, Utility, And Mitigation Systems

5.1 RELIEF AND NEUTRALIZATION SYSTEMS

5.1.1 General

The volatile nature of HF means that it can be efÞcientlyrecovered from the fractionation columns in the unit. It alsomeans that the vented materials, products, and by-productsfrom the unit may contain acid or organic ßuorides and thuspresent the possibility of an HF release. Each operating unitshould have facilities to control, neutralize, or otherwise miti-gate any hazard from process by-products.

5.1.2 Neutralization of Acid-soluble Oil

Acid-soluble oil or polymer from the acid-regenerationsystem may contain a small amount of free HF, an HF-waterazeotrope, or both. This may make the acid-soluble oil corro-sive, a possibility that should be recognized when this mate-rial is handled. Neutralizing, washing, or otherwise treatingthe acid contained in acid-soluble oil should be considered tominimize potential corrosion problems. The method of treat-ment should be covered in the operating procedures.

5.1.3 Pressure Relief And Flare Systems

Overpressure vents from the acid-containing parts of thealkylation unit will contain some HF. These streams shouldbe routed through a scrubber to remove the acid before thehydrocarbons are released to the ßare. The capacity of thescrubber should be reviewed whenever signiÞcant processchanges are made in the unit. The ßare system should beinspected for corrosion if it is inadvertently exposed to HF.

Based upon lessons learned from a recent industrial HFalkylation incident, a continuous dry, inert gas purge shouldbe considered to minimize stagnant pockets and corrosiveatmospheres in the ßare header. If neutralizing chemicals areinjected into the ßare header (e.g. ammonia), it should benoted that solids could form from the neutralization reactionthat could plug the header.

5.1.4 Product Treatment

5.1.4.1 General

Products from an HF alkylation unit may contain smallamounts of organic ßuoride or free HF. These streams shouldbe treated to reduce the potential for downstream corrosion.Product speciÞcations may require testing to ensure adequateßuoride removal.

5.1.4.2 Alkylate

HF contamination of alkylate is unlikely, but the rundowntankage should be checked periodically for low pH in thewater heel. This check is particularly important if the alkylateis untreated or if a unit upset has occurred. Addition of asmall amount of alkali to the water heel in the tank should beconsidered as a means of reducing corrosion on the tank ßoor.

5.1.4.3 Propane and Butane

Propane and butane products are normally treated by Þrstbeing passed through alumina deßuorinators to destroyorganic ßuorides and then through alkali (caustic) to removeany remaining HF. Potential hazards that should be consid-ered when operating procedures are written for this part of theprocess include the following:

a. Misoperation in the unit may allow signiÞcant quantities ofHF into the alkali treater. The heat of reaction in the treatermay be high enough to cause product vaporization, which canincrease pressure and cause equipment damage. Operatingprocedures should provide instructions for minimizing thishazard. Alarms should be installed to warn of such a situation.b. Upon reaction with HF, solid alkali forms a thick brinethat can spatter violently if drained carelessly into the sewer.c. Arsenic introduced with the fresh acid may deposit on thedeßuorinator alumina and can present a hazard when spentalumina is handled.

5.1.5 Process Drains And Neutralization

Process drains may contain HF and should therefore bemonitored to determine the need to neutralize the drainagebefore it is released to the wastewater treatment plant. Unitsshould have a neutralization basin or pit for acid area drains.Alkali neutralization may result in insoluble ßuoride salts,which may cause plugging. If acidic drainage is allowed to mixwith sulÞdic drainage from other areas, a release of hydrogensulÞde can result. Operating procedures should address thesepossibilities, as well as the disposal of ßuoride salts.

5.2 UTILITY SYSTEMS

5.2.1 General

The utility systems in an HF alkylation unit are importantbecause unexpected contamination of the process by the utili-ties can have a signiÞcant inßuence on unit corrosivity andsafety. Conversely, contamination of the utility systems withHF may spread acid hazards beyond unit limits. Proceduresshould be in place that deÞne how utility connections to pro-cesses are to be made and monitored.

5.2.2 Cooling Water

Operation with the cooling-water pressure higher than theprocess pressure should be avoided where possible. Cooling

SAFE OPERATION OF HYDROFLUORIC ACID ALKYLATION UNITS 15

water leaking through exchangers into the process canquickly increase unit acid corrosivity and present the poten-tial for unit upsets. Acid sampling and inspection frequenciesshould reßect this possibility.

Operation with the cooling-water pressure below the pro-cess pressure is preferred but risks leakage of acid into thecooling-water system. Segregating the cooling-water systemfor the HF alkylation unit can isolate this problem. Installa-tion of pH, ßuoride ion, or other monitors in the cooling-water system should be considered to provide a warning ofthis occurrence.

5.2.3 Steam Systems

Where high-pressure steam exchangers are used, the pos-sibility of water leakage into the process should be consid-ered. As with cooling water, this possibility should beconsidered when schedules for acid sampling and equipmentinspection are set.

5.2.4 Condensate Systems

On-stream pH or other monitors should be installed in thecondensate system to provide an early indication of acid leak-age. Operating procedures should include manual testingwith pH paper.

5.2.5 Nitrogen Systems

Nitrogen used in the HF alkylation unit should be periodi-cally tested for oxygen, which accelerates unit corrosion.Special care should be taken to avoid leakage of HF into thenitrogen or purge gas system used to unload fresh acid.

5.2.6 Breathing Air

As a minimum, breathing air should meet the requirementsfor Grade D breathing air as deÞned in CGA G-7.15. Breath-ing air may be supplied from cylinders or from a special ded-icated compressor. If a compressor is used, provision shouldbe made for appropriate air quality monitors on the inlet air-stream and for an emergency backup supply in case of apower failure.

5.2.7 Electrical Systems

A secure power supply should be provided for criticalelectrically powered instruments and unit control systems.

An alternative means of remotely stopping pump driversshould be considered for location at least 50 feet from theaffected pumps to provide a way to stop critical pumps in anemergency.

5.2.8 Instrument Air Supply Systems

Backup air or another appropriate gas should be providedfor critical air-powered instruments and unit control systems.

5.3 MITIGATION SYSTEMS

5.3.1 General

Despite the highest level of training and the best programof design, maintenance, and inspection, there may still exist aremote possibility of an HF release. Early detection and rapidmitigation (water sprays, rapid deinventory, and emergencyisolation, etc.) of a release should be considered in the reÞnerÕsprocess hazards management program for the HF alkylationunit. Rapid intervention of such systems is of paramountimportance. To achieve this, a facility needs to conduct appro-priate operator training and drills/exercises of such systems.

5.3.2 Monitoring and Detection Systems

The reliable, early detection of an HF release is an impor-tant component of an effective system for protecting bothreÞnery employees and the surrounding community. Detec-tion is also critically important in implementing mitigationmeasures. Each HF alkylation unit should have an effectiveleak-detection system. Such a system may include closed-cir-cuit television, point sensors, open-path sensors, and othersystems deemed appropriate for the unit. The sensors and/orsystems should be located so that they will likely detect anypotential HF release under varying weather conditions,release rates, and potential leak sources. Operating proce-dures should include the steps necessary to mitigate a leakonce detected. The system should provide coverage for allprocess areas that contain signiÞcant quantities of HF, as wellas storage and loading and unloading areas. Proceduresshould also specify frequency for detection system calibrationand testing to ensure a reliable system that functions whenactivated. Appendix F outlines some factors that affect thedesign and selection of components for detection systems.

5.3.3 Release Mitigation Systems

Accidental HF releases can be mitigated by a number oftechniques including, but not limited to, water application,diking, foam or chemical application, acid refrigeration,remote isolation, and rapid deinventory systems. The selec-tion of one or more mitigation techniques to apply willdepend on a variety of site- and release-speciÞc factors. Forexample, a pressurized release of superheated liquid HF islikely to result in an aerosol vapor cloud, thus negating theuse of dikes or vaporization-reduction chemicals. However, alow-pressure release of subcooled liquid HF is likely to forma liquid pool, and as such, containment in conjunction withvapor-suppression techniques may provide adequate short-term mitigation.

5.3.4 Water Mitigation Systems

A tested, effective tool for mitigating the effects of an HFrelease is the application of large quantities of water to theleak. A reÞneryÕs hazards management program should con-

16 API RECOMMENDED PRACTICE 751

sider a remotely operated water-application system speciÞ-cally designed for HF mitigation. Water can be applied by asystem of remotely operated Þxed-spray curtains, water mon-itors, or some combination of the two, depending on localconsiderations. Appendix G discusses parameters that shouldbe considered in the design of a remotely operated water-application system for mitigating the impact of an HF release.

Emergency response time is critical in any HF release sce-nario, therefore procedures should be developed or facilitiesinstalled to rapidly start the water mitigation systems upondetection of HF with multiple HF detectors.

Any water mitigation system should be fully testable whilethe unit is onstream. Operating procedures should specify testprocedures (dry or wet). The test frequency should ensure areliable system that functions when activated.

Written instructions should be developed for training per-sonnel in the operation of the mitigation system. These proce-dures should address how the mitigation system should beoperated under different release scenarios and meteorologicalconditions.

5.3.5 Emergency Isolation Valves

The magnitude of any HF release from an HF alkylationunit can be reduced if valves exist that can quickly isolate themajor inventories of acid. A reÞneryÕs hazards managementprogram should consider installation of remotely operatedemergency block valves. These valves should be located sothat large inventories and credible potential leak sources canbe safely isolated. The goal is to be able to remove sources ofacid or pressure from the point of release. Provisions for over-pressure protection of equipment isolated by emergency blockvalves should be included in the design. Appendix H discussesparameters that may be helpful in placing emergency blockvalves and in designing the installation. As with any emer-gency system, testability of the emergency isolation elementswith the unit onstream is an important part of the design.

5.3.6 Acid Deinventory Systems

5.3.6.1 General

The duration and thus the magnitude of an HF release canbe reduced if the acid in the leaking section of the unit can bemoved quickly to a safe location. A reÞneryÕs hazards man-agement program should consider facilities to remotely per-mit rapid movement of acid and entrained hydrocarbon fromleaking equipment into safe equipment.

The fundamental purpose of any system designed toremove the acid content of a vessel is to reduce the time dur-ing which the vessel may be leaking acid. The number ofdesign variables in a deinventory system is too great topresent details in this recommended practice; however, anumber of general factors, listed below, should be consideredin the design of such a system. Licensors of HF alkylationtechnology may also be able to provide design assistance.

5.3.6.2 Deinventory Time

The time allowed for the acid movement should be reason-ably short. The decision on allowable time will also have animpact on the design of the water mitigation system.

5.3.6.3 Extent of Movement

The extent of acid movement should be established earlyin the design process. It may be sufÞcient to move most of theacid that could be involved in a release rather than trying tomove all of the trapped acid.

5.3.6.4 Motive Force

Acid movement requires a motive force that will be avail-able during a release. The options available include gravity,existing pumps manifolded into appropriate low spots, newpumps installed for this purpose, hydrocarbon pressure, vac-uum, and nitrogen pressure. Care should be taken to ensurethat the motive force does not exacerbate the leak resulting ina larger release.

5.3.6.5 Vessels Included

The selection of vessels that will be connected to the rapiddeinventory system depends on the location of crediblereleases and the installation of emergency block valves to iso-late release sources. The acid deinventory system and emer-gency block valve installations should be coordinated toensure that the overall mitigation system functions properly.

5.3.6.6 Receiving Vessels

A key consideration is where the acid and entrained hydro-carbons will go. The options, depending on the volume to bemoved and the sizes of vessels, include the acid-storagedrum, settler, isostripper, and depropanizer, or a dedicated on-or off-site vessel.

5.3.6.7 Venting

The vessel or vessels receiving the acid may require provi-sion for venting and neutralizing of vapors generated duringthe movement of the acid.

5.3.6.8 Pressure Relief

The capacity and location of safety relief valves should bereviewed during the design of a deinventory system. Chang-ing the service of a vessel during an emergency or installingadditional valves in a piping system may change the pressurerelief needs of the equipment involved.

5.3.7 Hazard Analysis

Mitigation systems represent changes in the facility andshould be subject to some form of process hazards analysisduring the management-of-change process, as outlined in1.1.3. The special considerations for mitigation systems

SAFE OPERATION OF HYDROFLUORIC ACID ALKYLATION UNITS 17

include the effects of high-pressure water sprays on instru-ments and other equipment, the effects of inadvertent closureof emergency block valves, and the potential risks of movingacid to on-site locations that do not normally contain largeacid inventories or to off-site locations.

A consequence analysis, using appropriate atmospheric-plume dispersion models, should be conducted to determinepotential off-site impacts associated with each release sce-nario of interest. The analysis should include the potentialdownwind dosage or concentration and associated averagingtime, as compared to the Emergency Response PlanningGuidelines (ERPG) acute effects endpoints in Appendix B.

6 References

6.1 STANDARDS, CODES, AND SPECIFICATIONS

The most recent editions of the following standards, codes,and speciÞcations are cited in this recommended practice:

API Std 510 Pressure Vessel Inspection Code: Mainte-

nance, Inspection, Rating, Repair, andAlteration

RP 570 Piping Inspection Code: Inspection,Repair, Alteration and Rerating of In-Ser-vice Piping Systems

RP 572 Inspection of Pressure VesselsRP 573 Inspection of Fired Boilers and HeatersRP 574 Inspection Practices for Piping System

ComponentsRP 576 Inspection of Pressure-Relieving DevicesStd 601 Metallic Gaskets for Raised-Face Pipe

Flanges and Flanged Connections (Dou-ble-Gasketed and Spiral-Wound) * (out ofprint)

RP 750 Management of Process HazardsPubl 2218 FireprooÞng Practices in Petroleum and

Petrochemical Processing Plants Std 2510 Design and Construction of LiqueÞed

Petroleum Gas (LPG) Installations

ACGIH1 Threshold Limit Values and BiologicalExposure Indices

AIHA2

Emergency Response Planning Guidelines

ANSI3

Z358.1 Emergency Eyewash and ShowerEquipment

B16.5 Pipe Flanges and Flanged Fittings

ASME4

B31.3 Chemical Plant and Petroleum ReÞneryPiping Boiler and Pressure Vessel Code,Section VIII, ÒPressure VesselsÓ

B16.20 Metallic Gaskets for Pipe Flanges, Ring-Joint Spiral Wound and Jacketed

CGA5

G-7.1 Commodity SpeciÞcation for Air

DOT6

49 CFR Parts 100 to 185 ShippersÑGeneralRequirements for Shipments andPackagings

NACE7

Publ 5A171 Materials for Receiving, Handling, andStoring Hydroßuoric Acid

Std RP0472 Methods and Controls to Prevent In-Ser-vice Environmental Cracking of CarbonSteel Weldments in Corrosive PetroleumReÞning Environments

NIOSH8

90-117 NIOSH Pocket Guide to Chemical HazardsOccupational Health Guideline for Haz-ardous Chemicals: Hydrogen Fluoride

OSHA9

29 CFR Occupational Safety and Health Standards1910.38 ÒEmployee Emergency Plans and Fire Pre-

vention PlansÓ 1910.119 ÒProcess Safety Management of Highly

Hazardous ChemicalsÓ 1910.120 ÒHazardous Waste Operations and Emer-

gency ResponseÓ 1910.132 ÒGeneral RequirementsÓ;

1American Conference of Governmental Industrial Hygienists,Kemper Meadow Center, 1330 Kemper Meadow Drive, Cincinnati,Ohio 45240. 2American Industrial Hygiene Association, 2700 Prosperity Avenue,Suite 250, Fairfax, Virginia 22301.

3American National Standards Institute, 11 West 42nd Street, NewYork, New York 10036.4American Society for Mechanical Engineers, 345 East 47th Street,New York, New York 10017. 5Compressed Gas Association, 1235 Jefferson Davis Highway,Arlington, Virginia 22202-3269.6U.S. Department of Transportation, available from the U.S. Gov-ernment Printing OfÞce, Washington, D.C. 20402. 7NACE International, 1440 South Creek Drive, P.O. Box 218340,Houston, Texas 77218-8340.8National Institute of Occupational Safety and Health, 4676 Colum-bia Parkway, Cincinnati, Ohio 45226. 9Occupational Safety and Health Administration, U.S. Departmentof Labor, Washington, D.C. 20402. *Out-of-print publications may be ordered from: Global Engineer-ing Documents, 15 Inverness Way, East P.O. Box 1154, Englewood,Colorado 80150-7754.

18 API RECOMMENDED PRACTICE 751

1910.133 ÒEye and Face ProtectionÓ 1910.134 ÒRespiratory ProtectionÓ 1910.135 ÒOccupational Head ProtectionÓ 1910.136 ÒOccupational Foot ProtectionÓ 1910.147 ÒControl of Hazardous Energy (Lockout/

Tagout)Ó 1910.1000 ÒAir ContaminantsÓ 1910.1200 ÒHazard CommunicationÓ 1910.1450 ÒOccupational Exposure to Hazardous

Chemicals in LaboratoriesÓ

6.2 BOOKS, ARTICLES, AND REPORTS

1. ÒThe Use of Hydroßuoric Acid in the Petroleum ReÞn-ing Alkylation ProcessÓ(Background Paper), AmericanPetroleum Institute, Washington D.C., April 1991. 2. Guidelines for Hazard Evaluation Procedures (2nded.), American Institute of Chemical Engineers, NewYork, 1992. 3. D.N. Blewitt, R.P. Koopman, T.C. Brown, and W.J.Hague, ÒEffectiveness of Water Sprays on Mitigating

Hydroßuoric Acid Releases,Ó Paper presented at the 1987Center for Chemical Process Safety Vapor Cloud Confer-ence, Cambridge, Massachusetts, November 1987. 4. K.W. Schatz and R.P. Koopman, Effectiveness of WaterSpray Mitigation Systems for Accidental Releases ofHydrogen FluorideÑSummary Report (Vol. I-VIII),Industry Cooperative HF Mitigation/Assessment Pro-gram, June 1989. 5. G. Heskestad et al., Dispersal of LNG Clouds withWater Spray Curtains, Annual ReportÑPhase 1 (NTISNo. TB83-126029), Factory Mutual Research Corporationfor GRI, August 1982. 6. P.A. Moore and W.D. Rees, ÒForced Dispersion ofGases by Water and Steam,Ó Paper presented at the Inter-national Chemical Engineering Symposium, Manchester,England, November 1981. 7. T. S. Glickman and A. M. Ujihara, ÒDeciding BetweenIn-Place Protection and Evacuation in Toxic Vapor CloudEmergencies,Ó Journal of Hazardous Materials, 23 1990.

19

APPENDIX A—ELEMENTS OF A COMPREHENSIVE AUDIT

A comprehensive audit is a periodic review of a representa-tive sample of the systems and procedures in place at an HFalkylation unit. To ensure that agreed-upon actions areresolved, management should establish a system to addressthe auditÕs Þndings and recommendations, to documentaction item resolution, and to communicate the Þndings andrecommendations to appropriate personnel. The followingelements should be considered when an audit process isdeveloped:

a. An initial walk-through of the unit to look at the followingitems:

1. Fire-Þghting access, escape routes, location of emer-gency discharge vents, distance to other areas of concern,housekeeping, isolation of surface drains, and access topotential leak sites.

2. Labeling of lines, key valves, instrument settings, alarmand trip systems, emergency shutoffs, and unit access limitsand warnings.

3. Safety equipment, including Þre extinguishers, hydrantsand hoses, respiratory gear, personal protective clothing,grounding provisions, emergency communications systems,safety showers, eyewash stations, mitigation equipment, andÞrst-aid equipment.b. A systems review that includes the following items:

1. Review of process control for speciÞcation of normalcontrol ranges, system software and backup, instrument andequipment redundancy, currency of piping and instrumenta-tion diagrams, and instrument reliability history.

2. Review of detection systems for mechanical condition,testing and repair history, calibration frequency, and fre-quency of nuisance trips.

3. Review of mitigation systems for mechanical condition,repair history, Þre protection, and testing.c. A review of operating procedures for all appropriate activ-ities that covers technical correctness, clarity, ease with whichthe procedures can be followed, and completeness. The dateof the last review should be checked, and the inclusion in the

review of all changes to the unit should be veriÞed. Log-books, checklists, work and entry permit procedures, trainingmanuals and materials, and other records should also bereviewed.d. A review of preventive and predictive maintenance proce-dures and maintenance records covering mechanicalequipment, piping, trip systems, vessels, interlocks, safetyrelief valves, detection devices, mitigation systems, equip-ment neutralization, and use of personal protective equipmentin the shop area.e. A review of inspection procedures, methods, and sched-ules. Records should be up to date and should reßect thecurrent condition of the unit. Reports of inspection Þndingsand the status of recommendations should also be reviewed.f. Interviews with key operating, engineering, maintenance,and inspection personnel concerning maintenance, qualitycontrol, safety, training, operating procedures, managementof change, emergency procedures and simulations, and othermatters not easily found in unit records. The interviewsshould be structured to elicit any concerns that people mayhave, to determine that they are familiar with applicable oper-ating and maintenance procedures and safety and emergencyplans, and to assess the effectiveness of training programs.g. A review of the emergency response plan and the docu-mentation of emergency drills to determine the planÕseffectiveness. Such matters as notiÞcation procedures,response, agency interaction, and community outreach shouldbe considered, as should visits to Þrst-aid and medical-response sites. First-aid kits should be checked to ensure thatthey contain up-to-date materials; it should also be veriÞedthat personnel have been trained in the use of the kits.h. A review of any injuries (notably any HF injuries) or inci-dent investigations (notably any HF releases) that occurred onthe unit since the last audit. This review should be conductedto ensure that any agreed-upon action items from injury and/or incident investigations are complete. i. Other items appropriate to the speciÞc site.

21

APPENDIX B—HF EXPOSURE LIMITS

The limits listed in Table B-1 were in effect when this rec-ommended practice was published; the most recent edition ofthe source for each limit should be consulted. The deÞnition

of each term and the application of each limit should be takendirectly from the appropriate regulation or reference.

Notes: ACGIH = American Conference of Governmental IndustrialHygienists AIHA = American Industrial Hygiene AssociationIDLH = Immediately Dangerous to Life and HealthNIOSH = National Institute for Occupational Safety and HealthOSHA = Occupational Safety and Health AdministrationPEL = Permissible Exposure LimitPPMV = Parts Per Million by VolumeSTEL = Short-Term Exposure LimitTLV = threshold limit valueTWA = time weighted average.

aSee Section 6 for reference information.bSee 29 CFR Part 1910.1000.cSee ACGIHÕs Threshold Limit Values and Biological

Exposure Indices1.dA ceiling is a concentration that should not be exceeded

during any part of the working exposure.eSee NIOSHÕs Pocket Guide to Chemical Hazards and

Occupational Health Guideline for Hazardous Chemicals:Hydrogen Fluoride8.

fSee AIHAÕs Emergency Response Planning Guidelines2. gERPG = Emergency Response Planning Guidelines2.

ERPG levels are as follows:

ERPG-1 = maximum airborne concentration below whichit is believed that nearly all individuals could be exposed forup to 1 hour without experiencing other than mild, transientadverse health effects or without perceiving a clearly deÞnedobjectionable odor.

ERPG-2 = maximum airborne concentration below whichit is believed that nearly all individuals could be exposed forup to 1 hour without experiencing or developing irreversibleor other serious health effects or symptoms that could impairan individualÕs ability to take protective action.

ERPG-3 = maximum airborne concentration below whichit is believed that near all individuals could be exposed for upto 1 hour without experiencing or developing life-threateninghealth effects.

Note: At the time of the preparation of this recommended practicethere is a proposal to expand exposure guideline limits to include arange of exposure times. The draft values recommended by theNational Advisory Committee on Acute Exposure Guideline Level(AEGLs) are as follows:

These recommended values have not yet been approved.ConÞrmation of these values, or any adjustment to them,should be made prior to their use. The deÞnition for AEGLvalues is similar to that for ERPGs.

Table B-1—Exposure Limits

Organization Reference Concentration

Numbera Level Duration (PPMV) Routine Workplace Exposure

OSHA b PEL 8 hours TWA 3 (as F)OSHA proposed 2 STEL 15 minutes TWA 6

ACGIH c TLV ceilingd 8 hours per day 3

Accidental Release ExposureNIOSH/OSHA e IDLH < 30 minutes 30

AIHA f ERPG-1g < 60 minutes 2f ERPG-2g < 60 minutes 20f ERPG-3g < 60 minutes 50

Average Exposure Concentration, PPMV10 Minutes 30 Minutes 1 Hour 4 Hours 8 Hours

AEGL-1 2 2 2 1 1AEGL-2 95 34 24 12 9 AEGL-3 170 94 67 38 24

23

APPENDIX C—PROCEDURES FOR UNLOADING ACID

Various hazard reviews of HF alkylation units have identi-Þed acid unloading as having a higher potential for HFrelease than that of most other procedures in the unit. Becauseof this, acid unloading should follow a carefully written pro-cedure. Nitrogen-pressured unloading is the most commonlyused procedure, but other procedures, such as pumpedunloading, are also used.

The following is one possible procedure for nitrogen-pres-sured HF unloading from tank trucks. The details may varyfrom unit to unit. This procedure is written to accommodatethe type of unloading station described in Appendix E, and isadaptable to other types of transport containers such as tankcars or cargo tanks (ISO containers).

1. Class C protective clothing should be available to reÞn-ery personnel and truck drivers. It should be worn by allpersonnel involved in critical phases of the acidunloading.2. At least two people should be available full time forHF unloading. At least one qualiÞed person from thereÞnery HF alkylation unit should participate.3. Before acid is delivered, unit operators should verifythat all acid-tank try cocks, process valves, and the nitro-gen pressure controller are properly conÞgured andoperable. The level in the acid-storage vessel should bechecked to ensure that there is room for the volume offresh HF to be received. The contents of the transport con-tainer should be veriÞed as being anhydrous HF.4. The safety shower and eyewash station in the transportunloading area should be tested before unloading.5. HF transport containers should be unloaded duringdaylight hours, except where lighting in the unloadingarea is at daylight levels and enough staff are available foremergency response.6. The unit operators should ensure that adequate neutral-ization capacity is available in the neutralization section.7. When the transport container is spotted at the unload-ing area, the brakes should be set, the wheels chocked, thebonding wire hooked up, warning signs posted, and thearea isolated.8. For truck deliveries, the keys should be removed fromthe tractor and kept in a safe location controlled by theunit operator.9. If pressure in the acid-storage vessel is high enough toimpede unloading, the operator may vent to the neutraliza-tion section to achieve a pressure low enough to speed upunloading.10. The operability of both the transport containerÕsremote-operated valves and the unitÕs remote isolationvalve should be veriÞed.

11. Safety relief valves on HF transport containers fromdifferent vendors may be set at different pressures. Toavoid venting HF-laden gas, the unitÕs nitrogen pressureregulator and safety relief valve should be set well belowthe set point of the transport vesselÕs safety relief valve.The nitrogen header should be blown down before hookupto conÞrm that it is free from water and other foreignmatter.12. Before a hookup is made, the unit operators and thetruck driver (where applicable) should verify that all sys-tems are depressured on their respective ends. Then theblind ßanges on the acid-unloading line and the nitrogenpressure inlet line should be carefully removed. Flangeconnections should be visually inspected. The acid-unloading and nitrogen lines should be connected to theirrespective openings using new, solid, 1/8-inch-thick PTFEor PTFE-encapsulated steel gaskets and new bolts. Beforethese lines are used, they should be purged with air, andboth the nitrogen and acid-unloading systems should betested by pressuring with nitrogen, using a crossoverbetween the nitrogen line and the acid-unloading line. Allconnections should be checked with a liquid leak detector,such as soapy water. After the leak test, the system shouldbe depressured to a safe location and secured.13. Unit operators should ensure proper lineup to theacid-storage vessel. The valve at the end of the unloadingline at the unitÕs unloading manifold should remain closedat this time.14. The dome on the transport container should be swungopen slowly in case the valve stems are leaking HF intothe dome.15. Before the liquid discharge valve is opened, the trans-port container should be pressured with nitrogen to apressure higher than that of the acid-storage vessel plusany liquid head. This will prevent a backßow of liquid orgas into the transport container from the acid-storagevessel. 16. The unitÕs manifold acid valve should be openedslowly, and the Þttings should be inspected for leaks. Allconnections should be checked with a leak detector, forexample, a 10-percent ammonia solution. If no leaks arepresent, the acid valve on the transport containerÕs domeshould be opened slowly, permitting acid to ßow into theacid-storage vessel.17. Using the pressure regulator, the nitrogen pressure tothe transport container should be increased as needed tomaintain ßow.18. Unit operators should check the nitrogen pressure ofthe transport container periodically during the unloadingoperation. They should also check the pressure and acidlevel in the acid-storage vessel to ensure that acid is being

24 API RECOMMENDED PRACTICE 751

received and that there is ample room for the amountbeing unloaded. When the transport container is empty,the storage vesselÕs pressure will equalize with the con-tainerÕs pressure. The transport containerÕs gauge pressurewill fall quickly, and the hose may move around when thecontainer is empty.19. When the acid transfer is completed, the acid-storagevessel should be blocked in by closing the unloading lineat the vessel. The acid level in the acid-storage vesselshould be checked to ensure that the amount unloaded isthe same as the amount speciÞed on the bill of lading.20. The nitrogen valve in the transport container domeshould be closed. The pressure in the transport containershould be vented to the neutralization section using theunloading line and the appropriate vent line. During theventing operation, the pressure should gradually fall to 20pounds per square inch gauge or less.

Note: Transportation regulations preclude shipment of emptied con-tainers as placarded at a pressure greater than 41 pounds per squareinch absolute.)

21. The transport containerÕs acid-unloading dome valveshould be closed. Using the crossover provision, theunloading hose and line should be purged with nitrogen

for a period of time sufÞcient to remove as much liquidand vapor as practicable. The transport containerÕs air-to-open remote-operated valve(s) should be closed. Then thevent line to the acid relief header and the valves at the endof the nitrogen line at the unit manifold should be closed.The unloading hose and lines should be depressurized byopening the appropriate drains. The acid unloading valveat the manifold and transport vessel hose block valveshould be closed.22. The hose Þttings should be cautiously disconnectedfrom the transport container and stowed away. The blindßanges at the transport container and the unloading mani-fold should be replaced, using new PTFE gaskets and newbolts. The transport containerÕs dome should be sealed forreturn shipment. Any spilled acid should be washed downinto the area acid drain.23. When the transport container is ready to depart, thewarning signs and the chocks on the wheels should beremoved, and the brakes should be released.24. For tank truck shipments, the keys should be returnedto the driver after all piping has been disconnected and thehoses have been secured.

25

APPENDIX D—EXAMPLES OF TASKS FOR EACH CLOTHING CLASS

Following are some examples of work appropriate for eachof the four classes of personal protective clothing deÞned inthis recommended practice.

a. Class A clothingÑExamples of routine work include butare not limited to reading meters and gauges in the Þeld; rou-tine visual inspection of the unit; unloading or dumpingalumina (not exposed to HF); repair of equipment that hasbeen opened, disassembled, and neutralized so that no acidcan be trapped within; work on nonacid-containing equip-ment in an acid area if there is no other acid-area work goingon nearby; reassembly of thoroughly cleaned acid equipment;welding on equipment that has been properly prepared forwelding; and painting.

b. Class B clothingÑExamples of routine work include butare not limited to greasing of valves; washing down; samplecollection of nonacid-containing materials; pump work afterblinding has been completed and the cover plate removed;reboiler work (after column manways have been opened);opening manways on vessels; dismantling safety relief valves(when there is no potential for trapped acid); disassembly ofacid equipment that has been opened and neutralized, includ-ing exchangers and condensers with HF-containing tubes thatare known to be unplugged.

c. Class C clothingÑExamples of routine work include col-lecting samples that contain a potentially harmful quantity ofHF; changing pressure gauges in acid areas; blinding andopening of lines where equipment has been depressured;work on any acid equipment that is not blinded at the Þrstßange; initial opening of equipment after blinding (includingexchanger heads, manways, and ßanges); work on small pip-ing manifolds before it is established that no HF can betrapped inside attached equipment (including work on metersand meter manifolds and valves, disassembly of pumps, anddisassembly of exchangers and reboilers that contain acidbefore tubes are unplugged); connecting and disconnectingHF loading hoses (see Appendix C for further details).d. Class D clothingÑExamples of work include repairingfailed pump seals and valve packing and working in closeproximity to leaks that require lines or equipment to beisolated.

The following stepwise procedure is recommended for theremoval of protective clothing:

a. Neutralize boots and gloves.b. Remove face shield and hard hat.c. Remove boots.d. Remove outer clothing.e. Remove gloves.

27

APPENDIX E—DESIGN FEATURES OF AN ACID-TRUCK UNLOADING STATION

Unloading of HF into an alkylation unit is one of the morehazardous procedures in the routine operation of the unit. Acarefully designed unloading station can greatly reduce therisk of an HF release during this operation. Nitrogen-pres-sured unloading is the most common method, although otherprocedures are also used.

The features listed below are desirable for a nitrogen-pres-sured HF unloading station. Details may vary from unit tounit. This discussion is not intended to preclude the use ofother unloading systems, such as pumped unloading.

a. The unloading vehicle should have clear access to theunloading facility at the unit.

b. The unloading facility or transportation vehicle shouldinclude means, such as wheel chocks, for positively Þxing thevehicle.

c. An emergency shower and eyewash station should belocated 10Ð50 feet from the unloading station. Alarms acti-vated by the shower and eyewash will alert operators in thecontrol room to a possible upset.

d. Utility water should be available via a hose station forwashdown and ßushing.

e. Nitrogen should be provided at a pressure suitable forunloading the transport container into the storage facility. Thenitrogen header should have a safety relief valve set at least20 percent below the transport containerÕs safety relief valvesetting. There should be provision, such as check valves, toexclude backup of acid or any material through the nitrogenor other systems to other reÞnery areas. The nitrogen supplyshould have a restrictive device that will allow unloading in a

reasonable time but prevent high nitrogen ßow through thetransfer hose when the transport vessel is empty.f. Where access to the transport is from a top platform,egress should be possible in at least two directions. Stairwaysare preferred over ladders.g. Water mitigation facilities should be located so that theycan be directed on the unloading facility and transport con-tainer in the event of a release.h. There should be a connection from the unloading line tothe acid scrubber for venting after unloading and for emer-gency depressuring.i. Provision should be made to monitor the pressure in theacid-storage vessel during unloading.j. Unloading piping should be arranged to minimize deadlegs where acid could accumulate.k. Valves should be provided to allow tightness testing withnitrogen of the connections from the unloading manifold tothe transport vessel. A pressure indicator, visible from theunloading station, should be included to facilitate testing andmonitoring during unloading.l. The connecting hoses or lines from the unitÕs unloadingmanifold to the transport container should allow for verticalmovement due to decreasing load on the containerÕs springsduring unloading.m. A remote-operated valve should be provided as the lastvalve before the acid-hose connection on the unitÕs unloadingmanifold.n. An atmospheric vent valve, facing downward and locatedat the low point of the unitÕs unloading manifold, should beinstalled to permit Þnal depressuring after unloading is com-pleted and venting to the acid neutralizer.

29

APPENDIX F—MONITORING AND DETECTION SYSTEMS

F.1 General

A system to provide for the early detection of an accidentalrelease is an essential part of a hazards management plan forany HF alkylation unit. The design of a detection systemshould reßect its particular purpose and site-speciÞc factors.

Some of the operating characteristics desired in a systemfor monitoring air quality may be quite different from thosewanted for the detection of an accidental release. For exam-ple, there will be less need for rapid-response and early-warn-ing capabilities for the air-monitoring system. Also,reasonably precise measurement of concentration is animportant factor in a monitoring system, but it is much lesscritical in the early detection of an accidental release, forwhich quick detection of potentially dangerous levels is themost important consideration.

In all cases, the design of a detection system and thechoice of speciÞc components should reßect the details of theprocess and the reÞnery setting. Possible interference fromother chemical compounds and site meteorological condi-tions should be taken into consideration. Factors such as therange of possible ambient temperatures and relative humiditylevels can signiÞcantly affect the performance of systemcomponents and should therefore be included in componentselection and design.

The deployment of the detection system should include areview of the unit areas having a higher leak probability andconsequence, such as areas with numerous piping connec-tions and also areas commonly frequented by personnel.

F.2 Performance Characteristics of Systems and Components

The desired performance characteristics should be consid-ered in the selection of the components and in the design ofthe detection system. Key design issues for a system includethe following:

a. Detector type.b. Detection range.c. Selectivity.d. Response time.e. Stability.f. Reliability, in terms of (1) maintainability, (2) compatibil-ity with the reÞnery environment, and (3) experience insimilar applications.

Similar issues should be addressed for both an air-qualitymonitoring and an accidental-release detection system.Because the focus of this recommended practice is risk reduc-tion, the remainder of this appendix addresses a system forearly identiÞcation and warning of an acute accidental release.

F.3 Detector Type

Many HF detection systems are based on the electrochemi-cal point sensor, but as of this writing, other detection tech-nologies are now commercially available or underdevelopment. Detection systems are generally based on oneor more of the following general detector types:

a. Point sensorÑThese devices respond to HF only at theirspeciÞc location. The response signal is usually proportionalto the HF concentration.

Some point sensor technologies have signiÞcant sensitivityto interfering gases, that may be present in the site back-ground. The interfering compounds should be identiÞed, andchecked against potential releases of the compounds fromadjacent units. For example, if a detector has a known chlo-rine interference and a water treatment facility using chlorineis adjacent to the HF alkylation unit, a detector with this typeof sensitivity would not be an appropriate choice.

The operating temperature range may be an issue for somelocations. Low temperature performance may be affectedwhen ambient temperatures are signiÞcantly below freezing.In addition, the maintenance frequency may be affected forsites where high humidity exists.

HF once released can react with ambient moisture to formhydrates and at different ambient temperatures, polymericforms of (HF) exists. The sensitivity of the detectors to the var-ious forms of HF polymers and hydrates should be reviewed.

b. Open pathÑThese detectors respond to the presence ofHF anywhere along a line-of-sight path between a transmit-ting device and a receiver. Such devices do not provide pointconcentrations but respond to the cumulative quantity of HFin the open path. Open-path detectors are useful for perimetermonitoring. These sensors provide the fastest response to thepresence of HF in the detection path.

Open path sensors in general are affected by fog and steamplumes, which cause scattering of the light used to detect thepresence of HF. As stand alone detection, these systems maynot be appropriate for sites that experience frequent fog con-ditions. The installation should be done by selecting line ofsight locations where steam clouds crossing the beam areminimized. Some commercial technologies can detect HFquickly enough to differentiate HF aerosol from steam. Thatis they will detect the leading edge of HF molecules beforethe beam is blocked by the aerosol. The rigidity of mountingposts used to install open path sensors increases in impor-tance with increasing distance covered along the line of site.Good mounting post design is essential for obtaining goodsystem performance.

SpeciÞcity is an important aspect of HF detection withopen path devices. Broad absorption band technology may

30 API RECOMMENDED PRACTICE 751

not be suitable for all the detection applications, withoutsome way to increase detection discrimination. c. ImagingÑThese detectors are based on the visual or infra-red image generated by a cold, dense HF release. Thesesystems are useful for identifying or conÞrming a largerelease, establishing the exact location and remotely directingthe initial mitigation activities.

Current passive infrared imaging systems depend on theability to detect a temperature difference between the releaseplume and the background. As such, it may not be suitable foruse in low ambient temperatures. These systems may have anassociated image digitizing hardware and software system.The ability of the system to quickly digitize the image, andthe development of algorithms to interpret the image and gen-erate alarms are key components for a sensor type applica-tion. Otherwise, the system can be used through a visualoperator interface, where some image interpretation isrequired. This mode of operation would be similar to currentvideo monitoring, but with the added feature of thermal dif-ferentiation between a cold HF aerosol cloud and other aero-sols such as a hot steam cloud.

F.4 Detection RangeWhen HF detectors are used for early leak detection, as

opposed to industrial hygiene monitoring, a set point of about10 parts per million is desirable to activate an HF alarm. Pri-mary detection devices typically operate over a range ofabout 0Ð20 parts per million. Although the local concentra-tion of an accidental HF release may substantially exceed 20parts per million, the objective is early detection of a release,not determination of exact concentrations.

F.5 SelectivityIf other compounds may be present in the area being moni-

tored, a detection system should be HF speciÞc to the maxi-mum extent possible so that the presence of other gases doesnot affect the detectorÕs response. Frequent spurious activa-tion of the detectorÕs alarm system is likely to result in theoperators losing conÞdence in the detection system.

F.6 Response TimeIt is important to note that device response time may be

signiÞcantly affected by ambient conditions. For example,humidity affects the response time of some detector types bymany seconds, particularly at the lower end of the detectionrange. A detector should be capable of responding effectivelyunder all conditions likely to be experienced at a given site.

A second consideration for response time involves systemresponse time, which is the length of time from a signiÞcantHF release until the mitigation procedure is activated. Thistime will be a function of system deployment, deviceresponse time, and activation strategy. System response time

should be as low as possible to minimize the downwindimpact of an HF release.

When site maintenance personnel service the sensors, themost common task performed is a calibration check. Whenthis is done, a response time to span gas should be recorded.

F.7 Stability Detectors should have good stability. If there is a signiÞ-

cant zero or span drift over time, the detector will require fre-quent calibration to minimize inaccurate readings andspurious alarms.

Whenever possible, sensor stability/calibration should bechecked with an HF standard at ambient conditions. Gas cyl-inders at low HF concentrations have a shelf life which maynot allow storage for long periods of time. The cylinder gas istypically dry and may not reßect operating ambient condi-tions. Portable permeation tube calibrators can also be used tovalidate system operation.

F.8 Reliability It is important to consider the reliability of a detector in

terms of maintainability, compatibility with the reÞnery envi-ronment, and proven performance. If a detector is not prop-erly calibrated and maintained, it may fail to respond in theevent of a release. Detector elements may deteriorate or beconsumed over time or may become poisoned by other gasesso that the response to a release is impaired. A schedule ofperiodic maintenance and recalibration should be established.The frequency of calibration/validation should be determinedlocally and should be frequent enough to identify system deg-radation prior to failure based on site historical statistical dataand/or manufacturer recommendations under normal opera-tion. Since the detector will most likely be installed outdoors,it should be able to withstand local weather and environmen-tal conditions and meet appropriate electrical classiÞcationsfor the location. The components that may come in contactwith HF should be constructed of suitable materials. Also,any device chosen should be suitable for the hazardous areaclassiÞcation of the location where it is installed.

For any system to function properly over time, the instru-ment mechanics who will maintain it should be well trainedin the purpose and design details of all components.

The reliability of the entire system, as well as the detectoritself, should be high. For example, an uninterruptible powersupply should be provided wherever possible.

F.9 DeploymentThe components of a detection system should be deployed

so that the performance criteria described above are met. Ingeneral, this requires consideration of reÞnery and unit geom-etry, prevailing weather conditions, potential leak sources andrelease rates, maintenance requirements and access needs,

SAFE OPERATION OF HYDROFLUORIC ACID ALKYLATION UNITS 31

mitigation system characteristics, and other features relevantto the site. In some cases, a single detector type will not pro-vide all the desired characteristics. An integrated system ofmultiple device types may be useful in such cases. Onedevice may provide a capability lacking in another, and viceversa. When multiple detector types are used, the relative fea-tures and capabilities of the different types should be reßectedin the system layout, and the features should complementeach other. In general, a detection system using more thanone technology type will be found to be more reliable, pro-vided no common failure modes exist for the detector typesemployed.

Prevailing wind conditions alone should not be used tojustify asymmetrical sensor deployment around a potentialsource. This may lead to reduced detection capabilities attimes of unusual meteorological conditions.

F.10 Visual DetectionVideo cameras can be useful tools for alerting operators to

vapor clouds and other emergency situations. When used inconjunction with the detection systems described above,video cameras can signiÞcantly improve the operatorÕs abilityto assess a potential emergency and determine its exact loca-tion and scope. Cameras are also valuable for directing andobserving the effects of mitigation measures. When videocameras are employed, they should have remote pan andzoom capability and be located to cover strategic portions ofthe unit. Video monitors should be located in the unit controlroom where operators can readily see them. Long term cam-era operation considerations should include maintenanceaccess and protection of components such as glass lensesfrom long term low level HF background exposure.

33

APPENDIX G—WATER MITIGATION SYSTEMS

G.1 GeneralWater-spray mitigation is one of several techniques avail-

able to reduce the consequences of the release of an HFcloud. In this context, water sprays are designed to remove orscrub HF from a vapor cloud For this purpose, water can beapplied by Þxed-spray curtain systems, water monitors, orboth, depending on site-speciÞc conditions. Both methodshave been tested and evaluated in a series of large-scale Þeldand chamber studies. ÒEffectiveness of Water Sprays on Miti-gating Hydroßuoric Acid ReleasesÓ [3] provides an overviewof tests conducted in 1986 at the U.S. Department of Energyfacility at Frenchman Flats, Nevada, and Effectiveness ofWater Spray Mitigation Systems for Accidental Releases ofHydrogen FluorideÑSummary Report [4] provides a detailedaccount of a series of more than 80 parametric tests con-ducted in 1988 at the same facility. Volume VIII of Effective-ness of Water Spray Mitigation Systems [4] contains data onover 200 wind tunnel tests on monitors. This series of testsevaluated the effects of system geometry, water-spray curtainand water monitor conÞguration, of water droplet size, andeffect of released water-to-acid volumetric ratios on the efÞ-ciency of acid removal.

G.2 Effectiveness of Water Mitigation Mechanisms

Studies evaluating water application systems have Identi-Þed the following two primary mitigation phenomena:

a. Direct removal of HF from the release plume by chemicalabsorption and subsequent deposition, resulting in a reductionin the amount of acid moving downwind. Thus reducing theresulting downwind concentrationb. Entrainment of air induced by the movement of the waterdroplets, resulting in dilution of the plume and reduction ofthe near Þeld downwind HF concentration.

For HF, direct removal appears to be the primary mecha-nism, owing to HFÕs hygroscopic nature.

Studies of released gases cited in Dispersal of LNG Cloudswith Water Spray Curtains, Annual ReportÑPhase 1 [5] andÒForced Dispersion of Gases by Water and SteamÓ [6] haveclearly indicated the beneÞts of air entrainment in reducingconcentrations in a plume. However, these dilution effects areonly important near the point of release and have little effectin mitigating the effects of an HF release in the far Þeld.

The mitigation tests cited in Effectiveness of Water SprayMitigation Systems [4] indicate that removal efÞciencies of90 percent or greater can be achieved using either Þxed watersprays or water monitors provided that the applied watereffectively contacts the HF cloud. For a given system design,the percentage removal achieved will primarily be a functionof the water-to-acid ratio supplied to the plume as well as

other important parameters. In this context, it is importantthat the water sprays are distributed over the entire cloud. Anywater that does not come in contact with the cloud will bewasted and will result in a lower than optimal rate of removal.In addition, if the cloud bypasses the water sprays due tomomentum differences, the sprays will not achieve optimalefÞciency. The design of a mitigation system needs to exam-ine the interaction of the water sprays with an HF cloud. Thiscan be done using ßuid modeling techniques or by otherapplicable methods. These type of analyses should be donefor different release and meteorological conditions.

G.3 Design ConsiderationsA series of factors should be analyzed in designing an

effective water mitigation consequence reduction system. Thedesign will be highly dependent on individual site details. Thedegree of mitigation desired will depend on the reÞneryÕs set-ting, local topography, and proximity to the public. The gen-eral analysis of consequence reduction should include thefollowing:

a. Compilation of process and mechanical designinformation.b. Conduct of a process hazards analysis.c. Development of credible release scenarios.d. Development of local weather design data.e. Compilation of site topography and population locations.f. Conduct of a consequence analysis.g. Design mitigation system to account for release condi-tions, meteorological effects and unique site-speciÞccharacteristics of the unith. Design of water-supply, application, and disposal systems.i. Considerations for initiation and control accessibility.j. Site-speciÞc considerations.

These factors need to be combined into an analysis to eval-uate the effectiveness of the mitigation system. These datacan be used as an input to a Hazards Assessment analysis todetermine the potential downwind beneÞts of the mitigationsystem.

G.4 Development of Credible Release Scenarios

A list of credible release scenarios can be developed basedon the Þndings of the process hazards analysis. For eachrelease scenario, information should be developed to deÞnethe potential release, including the rate and duration of therelease, the HF concentration in the release, the height of therelease, the jet velocity (if appropriate), the initial tempera-ture, and an estimate of the likelihood of the released acidforming an aerosol or a liquid pool. The duration of the

34 API RECOMMENDED PRACTICE 751

release may also be a function of the isolation and deinven-tory systems installed.

G.5 Layout of Water Application Systems

The layout of the water spray system (the location of thewater sprays or monitors in relation to places on the unitwhere a signiÞcant release could occur) is a critical part of thedesign of any water spray mitigation system. In designingsuch a system it is important to ensure that the cloud willinteract with the water sprays for all releases of concern andmeteorological conditions. If the cloud bypasses the sprays orthe cloud is not distributed over the sprays, the efÞciency ofthe system will not be optimal and is severely compromised.The design of water spray mitigation systems, using eitherÞxed sprays or monitors is very site speciÞc in nature. Properdesign to ensure good mixing of the water sprays and a cloudcontaining HF must address the following issues: momentumof the release; momentum of the water spray system; meteo-rological effects (especially wind speed and direction); loca-tions where signiÞcant releases of HF could occur; clouddensity of the release; release orientation (jet direction com-pared to the mean wind ßow); and the effect of buildings onair ßow through out the unit. These issues need to be consid-ered in an integrated manner as they pertain to the overall per-formance of the water sprays. Fluid dynamic techniques as

well as other possible approaches could be used to satisfythese goals.

G.6 Determination of Water EffectivenessOne method of determining the water-removal efÞciency

can be estimated using the information provided in Effective-ness of Water Spray Mitigation Systems [4]. However, theeffective contacting of water and HF must be considered inthe design of a water spray mitigation system. Other ßuiddynamic analysis tools can be used to more accurately quan-tify the effectiveness of such systems.

G.7 Special ConsiderationsOther factors to be considered in completing the design

include the following:

a. Compatibility of the desired water rates with the capacityof the facilityÕs Þre-Þghting system.b. Use of video monitors to help manually direct and controlwater-application systems.c. Impoundment and neutralization facilities for acidic runoffwater generated during mitigation of an HF release.d. Drainage requirements.e. Special designs for areas that require freeze protection.f. Provision for periodic testing of the full system.g. Any local regulations that apply to the installation.

35

APPENDIX H—EMERGENCY ISOLATION OF AN HF RELEASE

H.1 GeneralThe magnitude of an HF release can be reduced in both

volume and duration by providing block valves to isolate thevarious acid-containing parts of the unit. LPG releases canalso be reduced by isolating major inventory sources. An HFalkylation unitÕs process hazards management plan shouldconsider the installation of emergency block valves (EBVs)or remote operators on existing valves at appropriate loca-tions in accordance with the following guidelines.

H.2 Determining EBV LocationsThe following steps should be considered in establishing

the need for new or retroÞtted remote-operated EBVs in thealkylation unit:

a. Identify vessels or other containers in which HF is storedin quantities large enough to cause a signiÞcant downwindhazard if released.b. Identify other equipment for which some likelihood of arelease exists. Pumps in acid service are a good example.c. Identify all lines connected to the equipment and vesselsdescribed above that should be isolated and blocked toremove (1) sources of acid feeding the releases and (2)sources of pressure that would reduce the release rate if theywere blocked.d. Identify existing valves that could be retroÞtted withremote operators to provide EBVs.e. Identify any other locations where new EBVs aredesirable.

H.3 Designing EBV InstallationsThe following mechanical factors should be considered in

the design of EBV installations:

a. Quarter-turn valves may close more quickly and be lessliable to foul with ßuoride scale than gate valves would.However, quarter-turn valves with PTFE sleeves may leakthrough if they are exposed to an extended Þre; whereas gatevalves with metal-to-metal seats should not leak as a result ofÞre exposure. Consideration should be given to using a com-

bination of quarter-turn and gate valves in high risk areaswhere quick shutoff is desired.b. Valve operators should be provided with backup motivepower (second electrical source, battery, emergency genera-tor, air, nitrogen, etc.) so that they will be operable during anupset if the primary motive power is lost.c. EBVs should be fully testable while the unit is onstream,from the remote initiation switch to the valve movement. Theamount of movement necessary to verify operability shouldbe considered. Valves whose seating surfaces can trap scaleprobably need full stroking, whereas valves of other designsmay require only partial stroking. The test frequency of theEBVs should be established to ensure a reliable system thatfunctions activated. d. Addition of position switches to valves should be consid-ered so that the operator can be notiÞed when the desiredmovement has taken place, or that the desired movement hasfailed to occur and additional measures are necessary.e. Location of EBV controls and position lights, as well asother mitigation controls, on a single panel with a schematicßow diagram of the unit should be considered so that theoperator can quickly assess the status of emergency controlequipment during an incident.f. Combining EBVs into logical systems with a single initia-tion switch should be considered. For example, an acidpumpÕs suction and discharge valves and the driver powershould be capable of shutoff/shutdown from a single initia-tion switch to quickly isolate a potential leak.g. Both the EBV and its motive power source should be pro-tected from damage by Þre, explosion, or impact during anincident, such that they are operable for a reasonable timeduring an incident. This may be achieved by inherent safetymethods, such as use of fail-safe actuators or Þre safe cabling,or by the installation of additional protection.h. Adding valves in a unit may isolate a piece of equipmentfrom its safety relief valve. Safety relief and neutralizationsystems should be reviewed after EBV locations have beenestablished.i. Fitting quarter-turn valves with position-indicating ßagsthat allow the position of the valve to be more readily deter-mined from a distance or by video should be considered.

37

APPENDIX I—BIBLIOGRAPHY

Blewitt, D.N., Petersen, R.L., and Heskestad, G., ÒEvaluationof Water Spray Mitigation for an Industrial Facility,Ópaper presented at the 1991 International Conferenceand Workshop on Modeling and MitigationÑThe Con-sequences of Accidental Releases of Hazardous Materi-als, Center for Chemical Process Safety, New Orleans,1991.

Degnan, T.F., ÒMaterials of Construction for HydroßuoricAcid and Hydrogen Fluoride,Ó Process Industries Cor-rosion: The Theory and Practice, National Associationof Corrosion Engineers, Houston, Texas, 1986.

Diener, R., and Van Zele, R.L., Industry Cooperative Hydro-gen Fluoride Mitigation & Ambient Impact AssessmentProgramÑSummary Report (NTIS No. DE90011208),The Industry Cooperative HF Mitigation/AssessmentProgram Steering Committee, 1989.

Fthenakis, V.M., A Theoretical Study of Absorption of ToxicGases by Spraying, American Institute of ChemicalEngineers, New York, 1989.

Fthenakis, V.M., Schatz, K.W., and Zakkay, V., ÒModeling ofWater Spraying of Field Releases of Hydrogen Fluo-ride,Ó paper presented at the 1991 International Confer-ence and Workshop on Modeling and Mitigation,Center for Chemical Process Safety, New Orleans.

Hanna, S.R., and Drivus, P.J., Guidelines for Use of VaporCloud Dispersion Models, American Institute of Chem-ical Engineers, New York, 1987.

Hanna, S.R., and Strimatis, D.G., Workbook of Test Cases forVapor Cloud Dispersion Models, American Institute ofChemical Engineers, New York, 1989. ÒMaterials forReceiving, Handling, and Storing Hydroßuoric AcidÓ(Committee Report 5A171), National Association ofCorrosion Engineers, Houston, Texas, 1983.

McFarlane, K., ÒDevelopment of Plume and Jet Models,Ópaper presented at the 1991 International Conferenceand Workshop on Modeling and Mitigation, Center forChemical Process Safety, New Orleans.

Meroney, R.N., ÒNumerical Simulation of Mitigation of HFCloud Concentrations by Means of Vapor Barriers and

Water Spray Curtains,Ó paper presented at the 1991International Conference and Workshop on Modelingand Mitigation, Center for Chemical Process Safety,New Orleans.

Moser, J.H., Blewitt, D.N., Steinberg, K.W., and Petersen,R.L., ÒHF HEGADIS Simulation of Dense Gas Disper-sion From a Wind TunnelÑModeled Oil ReÞnery,Ópaper presented at the 1991 International Conferenceand Workshop on Modeling and Mitigation, Center forChemical Process Safety, New Orleans.

Puttock, J.S., McFarlane, K., Prothero, A., Roberts, P.T.,Rees, F.J., Witlox, H.W.M., and Blewitt, D.N.,ÒHGSYSTEM Modeling Package,Ó Paper presented atthe 1991 International Conference and Workshop onModeling and Mitigation - The Consequences of Acci-dental Releases of Hazardous Materials, Center forChemical Process Safety, New Orleans, 1991.

Roberts, P.T., Puttock, J.S., and Blewitt, D.N., GravitySpreading and Surface Roughness Effects in the Dis-persion of Dense-Gas Plumes, American Institute ofChemical Engineers, New York, 1990.

Schuyler, R. L., ÒHydrogen Blistering of Steel in AnhydrousHydroßuoric Acid,Ó Materials Performance, April1979.

Spice, T.O., and Havens, J., Modeling Aerosol Dispersion forAccidental Consequences Analyses, American Instituteof Chemical Engineers, New York, 1990.

Witlox, H.W.M., ÒRecent Development of Heavy-Gas Dis-persion Modeling,Ó paper presented at the 1991 Interna-tional Conference and Workshop on Modeling andMitigation, Center for Chemical Process Safety, NewOrleans.

Witlox, H.W.M., McFarlane, K., Rees, F.J., and Puttock, J.S.,Development and Validation of Atmospheric DispersionModels for Ideal Gases and Hydrogen Fluoride, PartsI, II, and III, Shell Research Ltd., Manchester, England,1990.

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