United States Office of Solid Waste EPA 550­B­99­009

Environmental Protection and March 2009

Agency Emergency Response www.epa.gov/emergencies

RISK MANAGEMENT

PROGRAM

GUIDANCE

FOR OFFSITE

CONSEQUENCE

ANALYSIS

Office of Emergency Management
This document provides guidance to the owner or operator of processes covered by the Chemical Accident 

Prevention Program rule in the analysis of offsite consequences of accidental releases of substances regulated 

under section 112(r) of the Clean Air Act. This document does not substitute for EPA's regulations, nor is it 

a regulation itself. Thus, it cannot impose legally binding requirements on EPA, States, or the regulated 

community, and may not apply to a particular situation based upon the circumstances. This guidance does 

not constitute final agency action, and EPA may change it in the future, as appropriate. 
TABLE OF CONTENTS
 

Chapter Page 

Table of Potentially Regulated Entities ............................................ viii
 

Roadmap to Offsite Consequence Analysis Guidance by Type of Chemical .................. x 
 

1 Introduction ................................................................... 1- 1 
 

1.1 Purpose of this Guidance .................................................. 1-1 
 

1.2 This Guidance Compared to Other Models .................................... 1-4 
 

1.3 Number of Scenarios to Analyze ............................................ 1-4 
 

1.4 Modeling Issues ......................................................... 1-5 
 

1.5 Steps for Performing the Analysis ........................................... 1-6 
 

1.5.1 Worst-Case Analysis for Toxic Gases ................................. 1-7 
 

1.5.2 Worst-Case Analysis for Toxic Liquids ................................ 1-7 
 

1.5.3 Worst-Case Analysis for Flammable Substances ........................ 1-8 
 

1.5.4 Alternative Scenario Analysis for Toxic Gases .......................... 1-8 
 

1.5.5 Alternative Scenario Analysis for Toxic Liquids ......................... 1-9 
 

1.5.6 Alternative Scenario Analysis for Flammable Substances .................. 1-9 
 

1.6 Additional Sources of Information .......................................... 1-10 
 

2 Determining Worst-Case Scenario ................................................. 2-1 
 

2.1 Definition of Worst-Case Scenario .......................................... 2-1 
 

2.2 Determination of Quantity for the Worst-Case Scenario .......................... 2-3 
 

2.3 Selecting Worst-Case Scenarios ............................................. 2-3 
 

3 Release Rates for Toxic Substances ................................................ 3-1 
 

3.1 Release Rates for Toxic Gases .............................................. 3-1 
 

3.1.1 Unmitigated Releases of Toxic Gas ................................... 3-2 
 

3.1.2 Releases of Toxic Gas in Enclosed Space .............................. 3-2 
 

3.1.3 Releases of Liquefied Refrigerated Toxic Gas in Diked Area ............... 3-3 
 

3.2 Release Rates for Toxic Liquids ............................................ 3-4 
 

3.2.1 Releases of Toxic Liquids from Pipes ................................. 3-5 
 

3.2.2 Unmitigated Releases of Toxic Liquids ................................ 3-5 
 

3.2.3 Releases of Toxic Liquids with Passive Mitigation ....................... 3-7 
 

3.2.4 Mixtures Containing Toxic Liquids .................................. 3-11 
 

3.2.5 Release Rate Correction for Toxic Liquids Released at Temperatures 

Be e o n o tw en 25 C a d 50 C .......................................... 3-12

3.3 Release Rates for Common Water Solutions of Toxic Substances and for Oleum ..... 3-14 
 

4 Estimation of Worst-Case Distance to Toxic Endpoint ................................. 4-1 
 

April 15, 1999 i 
TABLE OF CONTENTS 

(Continued) 

Chapter Page 

5 Estimation of Distance to Overpressure Endpoint for Flammable Substances ............... 5-1 
 

5.1 Flammable Substances Not in Mixtures ...................................... 5-1 
 

5.2 Flammable Mixtures ..................................................... 5-2 
 

Reference Tables of Distances for Worst-Case Scenarios ............................... 5-4

 Table
 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by
 

Endpoint, F Stability, Wind Speed 1.5 Meters per Second:
 

1 10-Minute Release, Rural Conditions ........................................ 5-4 
 

2 60-Minute Release, Rural Conditions ........................................ 5-5 
 

3 10-Minute Release, Urban Conditions ........................................ 5-6 
 

4 60-Minute Release, Urban Conditions ........................................ 5-7 
 

Dense Gas Distances to Toxic Endpoint, F Stability, Wind Speed 1.5 Meters per Second:
 

5 10-Minute Release, Rural Conditions ........................................ 5-8 
 

6 60-Minute Release, Rural Conditions ........................................ 5-9 
 

7 10-Minute Release, Urban Conditions ....................................... 5-10 
 

8 60-Minute Release, Urban Conditions ....................................... 5-11 
 

Chemical-Specific Distances to Toxic Endpoint, Rural and Urban Conditions, F Stability,
 

Wind Speed 1.5 Meters per Second:
 

9 Anhydrous Ammonia Liquefied Under Pressure ............................... 5-12 
 

10 Non-liquefied Ammonia, Ammonia Liquefied by Refrigeration, or Aqueous
 

Ammonia ............................................................. 5-13 
 

11 Chlorine .............................................................. 5-14 
 

12 Sulfur Dioxide (Anhydrous) ............................................... 5-15 
 

Vapor Cloud Explosion Distances for Flammable Substances:
 

13 Distance to Overpressure of 1.0 psi for Vapor Cloud Explosions
 

of 500 - 2,000,000 Pounds of Regulated Flammable Substances .................. 5-16 
 

6 Determining Alternative Release Scenarios ........................................... 6-1 
 

7 Estimation of Release Rates for Alternative Scenarios for Toxic Substances ................ 7-1 
 

7.1 Release Rates for Toxic Gases .............................................. 7-1 
 

7.1.1 Unmitigated Releases of Toxic Gases ................................. 7-1 
 

7.1.2 Mitigated Releases of Toxic Gases ................................... 7-4 
 

April 15, 1999 ii 
TABLE OF CONTENTS 

(Continued) 

Chapter	 Page 

7.2	 Release Rates for Toxic Liquids ............................................ 7-6 
 

7.2.1	 Liquid Release Rate and Quantity Released for Unmitigated Releases ........ 7-7 
 

7.2.2	 Liquid Release Rate and Quantity Released for Mitigated Releases ......... 7-10 
 

7.2.3	 Evaporation Rate from Liquid Pool .................................. 7-10 
 

7.2.4	 Common Water Solutions and Oleum ................................ 7-14 
 

8	 Estimation of Distance to the Endpoint for Alternative Scenarios for Toxic Substances ....... 8-1 
 

9	 Estimation of Release Rates for Alternative Scenarios for Flammable Substances ............ 9-1 
 

9.1 	 Flammable Gases ........................................................ 9-1 
 

9.2	 Flammable Liquids ....................................................... 9-2 
 

10	 Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable
 

Substances ................................................................... 10-1 
 

10.1	 Vapor Cloud Fires ...................................................... 10-1 
 

10.2	 Pool Fires ............................................................. 10-5 
 

10.3	 BLEVEs .............................................................. 10-6 
 

10.4	 Vapor Cloud Explosion .................................................. 10-6 
 

Reference Tables of Distances for Alternative Scenarios ............................... 10-9

 Table
 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by
 

Endpoint, D Stability, Wind Speed 3.0 Meters per Second:
 

14 10-Minute Release, Rural Conditions ....................................... 10-9 
 

15 60-Minute Release, Rural Conditions ...................................... 10-10 
 

16 10-Minute Release, Urban Conditions ...................................... 10-11 
 

17 60-Minute Release, Urban Conditions ...................................... 10-12 
 

Dense Gas Distances to Toxic Endpoint, D Stability, Wind Speed 3.0 Meters per Second:
 

18 10-Minute Release, Rural Conditions ...................................... 10-13 
 

19 60-Minute Release, Rural Conditions ...................................... 10-14 
 

20 10-Minute Release, Urban Conditions ...................................... 10-15 
 

21 60-Minute Release, Urban Conditions ...................................... 10-16 
 

April 15, 1999	 iii 
TABLE OF CONTENTS 

(Continued) 

Chapter Page 

 Table
 

Chemical-Specific Distances to Toxic Endpoint, D Stability, Wind Speed 3.0 Meters per
 

Second:
 

22 Anhydrous Ammonia Liquefied Under Pressure .............................. 10-17 
 

23 Non-liquefied Ammonia, Ammonia Liquefied by Refrigeration, or Aqueous
 

Ammonia ............................................................ 10-18 
 

24 Chlorine ............................................................. 10-19 
 

25 Sulfur Dioxide (Anhydrous) .............................................. 10-20 
 

Neutrally Buoyant Plume Distances to Lower Flammability Limit (LFL) for Release Rate
 

Divided by LFL:
 

26 Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second ............... 10-21 
 

27 Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second ............... 10-21 
 

Dense Gas Distances to Lower Flammability Limit:
 

28 Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second ............... 10-22 
 

29 Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second ............... 10-23 
 

BLEVE Distances for Flammable Substances:
 

30 Distance to Radiant Heat Dose at Potential Second Degree Burn Threshold Assuming
 

Exposure for Duration of Fireball from BLEVE .............................. 10-24 
 

11 Estimating Offsite Receptors ..................................................... 11-1 
 

12 Submitting Offsite Consequence Analysis Information for Risk Management Plan .......... 12-1 
 

12.1 RMP Data Required for Worst-Case Scenarios for Toxic Substances .............. 12-1 
 

12.2 RMP Data Required for Alternative Scenarios for Toxic Substances .............. 12-2 
 

12.3 RMP Data Required for Worst-Case Scenarios for Flammable Substances .......... 12-3 
 

12.4 RMP Data Required for Alternative Scenarios for Flammable Substances .......... 12-3 
 

12.5 Submitting RMPs ....................................................... 12-4 
 

12.6 Other Required Documentation ............................................ 12-4 
 

APPENDICES 

Appendix A: References for Consequence Analysis Methods .................................. A-1 
 

Appendix B: Toxic Substances .......................................................... B-1 
 

B.1 Data for Toxic Substances ................................................. B-1 
 

B.2 Mixtures Containing Toxic Liquids ......................................... B-10 
 

April 15, 1999 iv 
TABLE OF CONTENTS 

(Continued) 

APPENDICES Page 

Appendix C: Flammable Substances ...................................................... C-1 
 

C.1 Equation for Estimation of Distance to 1 psi Overpressure for Vapor Cloud 
 

Explosions ............................................................. C-1 
 

C.2 Mixtures of Flammable Substances .......................................... C-1 
 

C.3 Data for Flammable Substances ............................................. C-2 
 

Appendix D: Technical Background ....................................................... D-1 
 

D.1 Worst-Case Release Rate for Gases ......................................... D-1 
 

D.1.1 Unmitigated Release .............................................. D-1 
 

D.1.2 Gaseous Release Inside Building ..................................... D-1 
 

D.2 Worst-Case Release Rate for Liquids ........................................ D-1 
 

D.2.1 Evaporation Rate Equation .......................................... D-1 
 

D.2.2 Factors for Evaporation Rate Estimates ................................ D-2 
 

D.2.3 Common Water Solutions and Oleum ................................. D-4 
 

D.2.4 Releases Inside Buildings ........................................... D-5 
 

D.3 Toxic Endpoints ......................................................... D-7 
 

D.4 Reference Tables for Distances to Toxic and Flammable Endpoints ................ D-8 
 

D.4.1 Neutrally Buoyant Gases ........................................... D-8 
 

D.4.2 Dense Gases ..................................................... D-9 
 

D.4.3 Chemical-Specific Reference Tables ................................. D-10 
 

D.4.4 Choice of Reference Table for Dispersion Distances ..................... D-10 
 

D.4.5 Additional Modeling for Comparison ................................ D-12 
 

D.5 Worst-Case Consequence Analysis for Flammable Substances ................... D-12 
 

D.6 Alternative Scenario Analysis for Gases ..................................... D-13 
 

D.7 Alternative Scenario Analysis for Liquids .................................... D-15 
 

D.7.1 Releases from Holes in Tanks ...................................... D-15 
 

D.7.2 Releases from Pipes .............................................. D-17 
 

D.8 Vapor Cloud Fires ...................................................... D-18 
 

D.9 Pool Fires ............................................................. D-18 
 

D.10 BLEVEs .............................................................. D-21 
 

April 15, 1999 v 
TABLE OF CONTENTS 

(Continued) 

APPENDICES Page 

D.11 Alternative Scenario Analysis for Vapor Cloud Explosions ...................... D-23 
 

Appendix E: Worksheets for Offsite Consequence Analysis .................................... E-1 
 

Worksheet 1. Worst-case Analysis for Toxic Gas ..................................... E-1 
 

Worksheet 2. Worst-case Analysis for Toxic Liquid ................................... E-2 
 

Worksheet 3. Worst-case Analysis for Flammable Substance ............................ E-5 
 

Worksheet 4. Alternative Scenario Analysis for Toxic Gas .............................. E-6 
 

Worksheet 5. Alternative Scenario Analysis for Toxic Liquid ........................... E-9 
 

Worksheet 6. Alternative Scenario Analysis for Flammable Substance ................... E-13 
 

Appendix F: Chemical Accident Prevention Provisions ........................................ F-1 
 

April 15, 1999 vi 
LIST OF EXHIBITS
 

Exhibit Page
 

1 Required Parameters for Modeling ................................................. 1-3 
 

2 Generic Reference Tables of Distances for Worst-case Scenarios ......................... 4-3 
 

3 Chemical-Specific Reference Tables of Distances for Worst-case Scenarios ................ 4-3 
 

4 Generic Reference Tables of Distances for Alternative Scenarios ......................... 8-2 
 

5 Chemical-Specific Reference Tables of Distances for Alternative Scenarios ................. 8-2 
 

6 Reference Tables of Distances for Vapor Cloud Fires as Alternative Scenario for Flammable
 

Substances ................................................................... 10-2 
 

A-1 Selected References for Information on Consequence Analysis Methods ................... A-2 
 

B-1 Data for Toxic Gases ............................................................ B-2 
 

B-2 Data for Toxic Liquids ........................................................... B-4 
 

B-3 Data for Water Solutions of Toxic Substances and for Oleum ............................ B-7 
 

B-4 Temperature Correction Factors for Liquids Evaporating from Pools at Temperatures
 o o o o Between 25 C and 50 C (77 F and 122 F) ......................................... B-8 
 

C-1 Heats of Combustion for Flammable Substances ...................................... C-3 
 

C-2 Data for Flammable Gases ........................................................ C-6 
 

C-3 Data for Flammable Liquids ...................................................... C-9 
 

April 15, 1999 vii 
TABLE OF POTENTIALLY REGULATED ENTITIES
 

This table is not intended to be exhaustive, but rather provides a guide for 

readers regarding entities likely to be regulated under 40 CFR part 68. This 

table lists the types of entities that EPA is now aware could potentially be 

regulated by this rule (see Appendix B of the “General Guidance for Risk 

Management Programs” for a more detailed list of potentially affected NAICS 

codes). Other types of entities not listed in this table could also be affected. To 

determine whether your facility is covered by the risk management program rules 

in part 68, you should carefully examine the applicability criteria discussed in 

Chapter 1 of the General Guidance and in 40 CFR 68.10. If you have questions 

regarding the applicability of this rule to a particular entity, call the 

EPCRA/CAA Hotline at (800) 424-9346 (TDD: (800) 553-7672). 

Category NAICS 

Codes 

SIC 

Codes 

Examples of Potentially Regulated 

Entities 

Chemical 

manufacturers 

325 28 Petrochemicals 

Industrial gas 

Alkalies and chlorine 

Industrial inorganics 

Industrial organics 

Plastics and resins 

Agricultural chemicals 

Soap, cleaning compounds 

Explosives 

Miscellaneous chemical manufacturing 

Petroleum refineries 32411 2911 Petroleum refineries 

Pulp and paper 322 26 Paper mills 

Pulp mills 

Paper products 

Food processors 311 20 Dairy products 

Fruits and vegetables 

Meat products 

Seafood products 

Polyurethane foam 32615 3086 Plastic foam products 

Non-metallic mineral 

products 

327 32 Glass and glass products 

Other non-metallic mineral products 

Metal products 331 

332 

33 

34 

Primary metal manufacturing 

Fabricated metal products 

April 15, 1999 viii 
Category NAICS 

Codes 

SIC 

Codes 

Examples of Potentially Regulated 

Entities 

Machinery 

manufacturing 

333 35 Industrial machinery 

Farm machinery 

Other machinery 

Computer and 

electronic equipment 

334 36 Electronic equipment 

Semiconductors 

Electric equipment 335 36 Lighting 

Appliance manufacturing 

Battery manufacturing 

Transportation 

equipment 

336 37 Motor vehicles and parts 

Aircraft 

Food distributors 4224 

4228 

514 

518 

Frozen and refrigerated foods 

Beer and wines 

Chemical distributors 42269 5169 Chemical wholesalers 

Farm supplies 42291 5191 Agricultural retailers and wholesalers 

Propane dealers 454312 5171 

5984 

Propane retailers and wholesalers 

Warehouses 4931 422 Refrigerated warehouses 

Warehouse storing chemicals 

Water treatment 22131 4941 Drinking water treatment systems 

Wastewater treatment 22132 

56221 

4952 

4933 

Sewerage systems 

Wastewater treatment 

Waste treatment 

Electric utilities 22111 4911 Electric power generation 

Propane users Manufacturing facilities 

Large institutions 

Commercial facilities 

Federal facilities Military installations 

Department of Energy installations 

April 15, 1999 ix 
Roadmap to Offsite Consequence Analysis Guidance by Type of Chemical 

Type of Chemical and Release Scenario Applicable Sections and Ap

Toxic Gas 

Worst-Case Scenario 

1) Define Worst Case Section 2.1 

2) Select Scenario 3) Calculate Release Rates 

Sections 2.2 and 2.3 

Unmitigated Section 3.1.1 Passive Mitigation Section 3.1.2 Refrigerated Section 3.1.3 

4) Find Toxic Endpoint Appendix B (Exhibit B-1) 5) Determine Reference Table and Distance Section 3.1.3, 3.2.3 

Dense or Neutrally Buoyant Plume Chapter 4 and Appendix B (Exhibit B-1Chemical-Specific Tables (ammonia, chlorine, sulfur dioxide) Chapter 4 Urban or Rural Section 2.1 and Chapter 4 Release Duration Section 2.1 

Alternative Scenario 

1) Define Alternative Scenario Chapter 6 2) Select Scenario 3) Calculate Release Rates 

Chapter 6 

Unmitigated (from tanks and pipes) Section 7.1.1 Active or Passive Mitigation Section 7.1.2 

4) Find Toxic Endpoint 5) Determine Reference Table and Distance 

Appendix B (Exhibit B-1) 

Dense or Neutrally Buoyant Plume Chapter 8 and Appendix B (Exhibit B-1Chemical-Specific Tables (ammonia, chlorine, sulfur dioxide) Chapter 8 Urban or Rural Section 2.1 and Chapter 8 Release Duration Section 7.1 

April 15, 1999 x 
Roadmap to Offsite Consequence Analysis Guidance by Type of Chemical (continued

Type of Chemical and Release Scenario Applicable Sections and Ap

Toxic Liquid 

Worst-Case Scenario 

1) Define Worst Case Section 2.1 

2) Select Scenario 3) Calculate Release Rates 

Sections 2.2 and 2.3 

Releases from Pipes Section 3.2.1 Unmitigated Pool Evaporation Section 3.2.2 Passive Mitigation (dikes, buildings) Section 3.2.3 Release at Ambient Temperature Section 3.2.2, 3.2.3 Release at Elevated Temperature Section 3.2.2, 3.2.3 Releases of Mixtures Section 3.2.4 and Appendix B (Section Temperature Corrections for Liquids at 25-50 C o Section 3.2.5 and Appendix B (Exhibit 

Releases of Solutions 

4) Find Toxic Endpoint 

Section 3.3 and Appendix B (Exhibit B-

For Liquids/Mixtures Appendix B (Exhibit B-2) For Solutions 

5) Determine Reference Table and Distance 

Appendix B (Exhibit B-3) 

Dense or Neutrally Buoyant Plume (liquids) Chapter 4 and Appendix B (Exhibit B-2Dense or Neutrally Buoyant Plume (solutions) Chapter 4 and Appendix B (Exhibit B-3Chemical Specific Table (aqueous ammonia) Chapter 4 Urban or Rural Section 2.1 and Chapter 4 Release Duration (liquids) Section 3.2.2 Release Duration (solutions) Chapter 4 

April 15, 1999 xi 
Roadmap to Offsite Consequence Analysis Guidance by Type of Chemical (continued

Type of Chemical and Release Scenario Applicable Sections and Ap

Toxic Liquid 

Alternative Scenario 

1) Define Alternative Scenario Chapter 6 

2) Select Scenario Chapter 6 3) Calculate Release Rates Section 7.2 

Unmitigated (from tanks and pipes) Section 7.2.1 Active or Passive Mitigation Section 7.2.2 Release at Ambient Temperature Section 7.2.3 Release at Elevated Temperature Section 7.2.3 Release of Solution 

4) Find Toxic Endpoint 

Sections 7.2.4 and 3.3 and Appendix B 

For Liquids/Mixtures Appendix B (Exhibit B-2) For Solutions 

5) Determine Reference Table and Distance 

Appendix B (Exhibit B-3) 

Dense or Neutrally Buoyant Plume (liquids/mixtures) Chapter 8 and Appendix B (Exhibit B-2Dense or Neutrally Buoyant Plume (solutions) Chapter 8 and Appendix B (Exhibit B-3Chemical-Specific Table (aqueous ammonia) Chapter 8 Urban or Rural Section 2.1 and Chapter 8 Release Duration (liquids/mixtures) Section 7.2 Release Duration (solutions) Chapter 8 

April 15, 1999 xii 
Roadmap to Offsite Consequence Analysis Guidance by Type of Chemical (continued

Type of Chemical and Release Scenario Applicable Sections and Ap

Flammable Substance 

Worst-Case Scenario 

1) Define Worst Case Sections 5.1 and 2.1 

2) Select Scenario 3) Determine Distance to Overpressure Endpoint 

Sections 5.1, 2.2, and 2.3 

For Pure Flammable Substances Section 5.1 For Flammable Mixtures Section 5.2 

Alternative Scenario 

1) Define Alternative Scenario Chapter 6 

2) Select Scenario 3) For Vapor Cloud Fires 

Chapter 6 

Calculate Release Rates (gases) Section 9.1 and Appendix C (Exhibit C-Calculate Release Rates (liquids) Section 9.2 and Appendix C (Exhibit C-Find Lower Flammability Limit (gases) Appendix C (Exhibit C-2) Find Lower Flammability Limit (liquids) Appendix C (Exhibit C-3) Dense or Neutrally Buoyant (gases) Appendix C (Exhibit C-2) Dense or Neutrally Buoyant (liquids) Appendix C (Exhibit C-3) Urban or Rural Section 10.1 Release Duration Section 10.1 Determine Distance Section 10.1 

4) For Pool Fires Section 10.2 and Appendix C (Exhibit C5) For BLEVEs Section 10.3 6) For Vapor Cloud Explosions Section 10.4 

April 15, 1999 xiii 
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1 INTRODUCTION

1.1 Purpose of this Guidance

This document provides guidance on how to conduct the offsite consequence analyses for

Risk Management Programs required under the Clean Air Act (CAA). Section 112(r)(7) of the CAA

directed the U.S. Environmental Protection Agency (EPA) to issue regulations requiring facilities

with large quantities of very hazardous chemicals to prepare and implement programs to prevent the

accidental release of those chemicals and to mitigate the consequences of any releases that do occur.

EPA issued that rule, “Chemical Accident Prevention Provisions” on June 20, 1996. The rule is

codified at part 68 of Title 40 of the Code of Federal Regulations (CFR). If you handle, manufacture,

use, or store any of the toxic or flammable substances listed in 40 CFR 68.130 above the specified

threshold quantities in a process, you are required to develop and implement a risk management

program under part 68 of 40 CFR. The rule applies to a wide variety of facilities that handle,

manufacture, store, or use toxic substances, including chlorine and ammonia, and highly flammable

substances, such as propane. If you are not sure whether you are subject to the rule, you should

review the rule and Chapters 1 and 2 of EPA’s General Guidance for Risk Management Programs

(40 CFR part 68), available from EPA at www.epa.gov/emergencies/rmp.

If you are subject to the rule, you are required to conduct an offsite consequence analysis to

provide information to the state, local, and federal governments and the public about the potential

consequences of an accidental chemical release. The offsite consequence analysis consists of two

elements:

� A worst­case release scenario, and

� Alternative release scenarios.

To simplify the analysis and ensure comparability, EPA has defined the worst­case scenario

as the release of the largest quantity of a regulated substance from a single vessel or process line

failure that results in the greatest distance to an endpoint. In broad terms, the distance to the endpoint

is the distance a toxic vapor cloud, heat from a fire, or blast waves from an explosion will travel

before dissipating to the point that serious injuries from short­term exposures will no longer occur.

Endpoints for regulated substances are specified in 40 CFR 68.22(a) and Appendix A of part 68 and

are presented in Appendices B and C of this guidance.

Alternative release scenarios are scenarios that are more likely to occur than the worst­case

scenario and that will reach an endpoint offsite, unless no such scenario exists. Within these two

parameters, you have flexibility to choose alternative release scenarios that are appropriate for your

site. The rule, in 40 CFR 68.28 (b)(2), and the General Guidance for Risk Management Programs (40

CFR part 68), Chapter 4, provide examples of alternative release scenarios that you should consider

when conducting the offsite consequence analysis.

March 2009
 

 

 

 

Chapter 1 

Introduction 

RMP*CompTM 

To assist those using this guidance, the National Oceanic and Atmospheric Administration 

(NOAA) and EPA have developed a software program, RMP*CompTM, that performs the 

calculations described in this document. This software can be downloaded from the 

EPA/OEM website at www.epa.gov/emergencies/rmp. 

This guidance document provides a simple methodology for conducting offsite consequence 

analyses. You may use simple equations to estimate release rates and reference tables to determine 

distances to the endpoint of concern. This guidance provides generic reference tables of distances, 

applicable to most of the regulated toxic substances, and chemical-specific tables for ammonia, 

chlorine, and sulfur dioxide. This guidance also provides reference tables of distances for 

consequences of fires and explosions of flammable substances. In some cases, the rule allows users of 

this document to adopt generic assumptions rather than the site-specific data required if another model 

is employed (see Exhibit 1). 

The methodology and reference tables of distances presented here are optional. You are not 

required to use this guidance. You may use publicly available or proprietary air dispersion models to 

do your offsite consequence analysis, subject to certain conditions. If you choose to use models 

instead of this guidance, you should review the rule and Chapter 4 of the General Guidance for Risk 

Management Programs, which outline required conditions for use of models. In selected example 

analyses, this document presents the results of some models to provide a basis for comparison. It also 

indicates certain conditions of a release that may warrant more sophisticated modeling than is 

represented here. However, this guidance does not discuss the procedures to follow when using 

models; if you choose to use models, you should consult the appropriate references or instructions for 

those models. 

This guidance provides distances to endpoints for toxic substances that range from 0.1 miles 

to 25 miles. Other models may not project distances this far (and some may project even longer 

distances). One commonly used model, ALOHA, has an artificial distance cutoff of 6 miles (i.e., any 

scenario which would result in an endpoint distance beyond 6 miles is reported as “greater than 6 

miles”). Although you may use ALOHA if it is appropriate for the substance and scenario, you should 

consider choosing a different model if the scenario would normally result in an endpoint distance 

significantly greater than 6 miles. Otherwise, you should be prepared to explain the difference 

between your results and those in this guidance or other commonly used models. Also, you should be 

aware that the RMP*eSubmit system accepts only numerical entries (i.e., it will not accept a “greater 

than” distance). If you do enter a distance in RMP*eSubmit that is the result of a particular model’s 

maximum distance cutoff (including the maximum distance cutoff in this guidance), you can explain 

this in the executive summary of your RMP. 

March 2009 1 - 2 
Exhibit 1
 

Required Parameters for Modeling (40 CFR 68.22)
 

WORST CASE ALTERNATIVE SCENARIO 

Endpoints (§68.22(a)) 

Endpoints for toxic substances are specified in part 68 Appendix A. Endpoints for toxic substances are specified in part 68 Appendix A. 

For flammable substances, endpoint is overpressure of 1 pound per square 

inch (psi) for vapor cloud explosions. 

For flammable substances, endpoint is: 

�Overpressure of 1 psi for vapor cloud explosions, or 

�Radiant heat level of 5 kilowatts per square meter (kW/m ) for 40 2 

seconds for heat from fires (or equivalent dose), or 

�Lower flammability limit (LFL) as specified in NFPA documents or 

other generally recognized sources. 

Wind speed/stability (§68.22(b)) 

This guidance assumes 1.5 meters per second and F stability. For other 

models, use wind speed of 1.5 meters per second and F stability class 

unless you can demonstrate that local meteorological data applicable to 

the site show a higher minimum wind speed or less stable atmosphere at 

all times during the previous three years. If you can so demonstrate, these 

minimums may be used for site-specific modeling. 

This guidance assumes wind speed of 3 meters per second and D 

stability. For other models, you must use typical meteorological 

conditions for your site. 

Ambient temperature/humidity (§68.22(c)) 

This guidance assumes 25�C (77�F) and 50 percent humidity. For other 

models for toxic substances, you must use the highest daily maximum 

temperature and average humidity for the site during the past three years. 

This guidance assumes 25�C and 50 percent humidity. For other 

models, you may use average temperature/humidity data gathered at the 

site or at a local meteorological station. 

Height of release (§68.22(d)) 

For toxic substances, you must assume a ground level release. This guidance assumes a ground-level release. For other models, release 

height may be determined by the release scenario. 

Surface roughness (§68.22(e)) 

Use urban (obstructed terrain) or rural (flat terrain) topography, as 

appropriate. 

Use urban (obstructed terrain) or rural (flat terrain) topography, as 

appropriate. 

Dense or neutrally buoyant gases (§68.22(f)) 

Tables or models used for dispersion of regulated toxic substances must 

appropriately account for gas density. If you use this guidance, see Tables 

1-4 for neutrally buoyant gases and Tables 5-8 for dense gases, or Tables 

9-12 for specific chemicals. 

Tables or models used for dispersion must appropriately account for gas 

density. If you use this guidance, see Tables 14-1 7 for neutrally 

buoyant gases and Tables 18-21 for dense gases, or Tables 22-25 for 

specific chemicals. 

Temperature of released substance (§68.22(g)) 

You must consider liquids (other than gases liquefied by refrigeration) to 

be released at the highest daily maximum temperature, from data for the 

previous three years, or at process temperature, whichever is higher. 

Assume gases liquefied by refrigeration at atmospheric pressure to be 

released at their boiling points. This guidance provides factors for 

estimation of release rates at 25�C or the boiling point of the released 

substance, and also provides temperature correction factors. 

Substances may be considered to be released at a process or ambient 

temperature that is appropriate for the scenario. This guidance 

provides factors for estimation of release rates at 25 �C or the boiling 

point of the released substance, and also provides temperature 

correction factors. 

April 15, 1999 1 - 3 
Chapter 1 

Introduction 

1.2 This Guidance Compared to Other Models 

Results obtained using the methods in this document are expected to be conservative (i.e., they will 

generally, but not always, overestimate the distance to endpoints). The chemical-specific reference tables in 

this guidance provide less conservative results than the generic reference tables, because the chemical-specific 

tables were derived using more realistic assumptions and considering more factors. 

Complex models that can account for many site-specific factors may give less conservative estimates 

of offsite consequences than the simple methods in this guidance. This is particularly true for alternative 

scenarios, for which EPA has not specified many assumptions. However, complex models may be expensive 

and require considerable expertise to use; this guidance is designed to be simple and straightforward. You 

will need to consider these tradeoffs in deciding how to carry out your required consequence analyses. 

Appendix A provides information on references for some other methods of analysis; these references do not 

include all models that you may use for these analyses. You will find that modeling results will sometimes 

vary considerably from model to model. 

1.3 Number of Scenarios to Analyze 

The number and type of analyses you must perform depend on the “Program” level of each of your 

processes. The rule defines three Program levels. Processes are eligible for Program 1 if, among other 

criteria, there are no public receptors within the distance to the endpoint for the worst-case scenario. Because 

no public receptors would be affected by the worst-case release, no further modeling is required for these 

processes. For processes subject to Program 2 or Program 3, both worst-case release scenarios and 

alternative release scenarios are required. To determine the Program level of your processes, consult 40 CFR 

68.10(b), (c), and (d), or Chapter 2 of EPA’s General Guidance for Risk Management Programs (40 CFR 

part 68). 

Once you have determined the Program level of your processes, you are required to conduct the 

following offsite consequence analyses: 

� One worst-case release scenario for each Program 1 process; 

� One worst-case release scenario to represent all regulated toxic substances in Program 2 and 

Program 3 processes; 

� One worst-case release scenario to represent all regulated flammable substances in Program 

2 and Program 3 processes; 

� One alternative release scenario for each regulated toxic substance in Program 2 and 

Program 3 processes; and 

� One alternative release scenario to represent all regulated flammable substances in Program 

2 and Program 3 processes. 

NOTE: You may need to analyze additional worst-case scenarios if release scenarios for regulated 

flammable or toxic substances from other covered processes at your facility would affect different public 

April 15, 1999 1 - 4 
 

 

 

Chapter 1 

Introduction 

receptors. For example, worst-case release scenarios for storage tanks at opposite ends of your facility 

may potentially reach different areas where people could be affected. In that case, you will have to 

conduct analyses of and report on both releases. 

GUIDANCE FOR INDUSTRY-SPECIFIC RISK MANAGEMENT PROGRAMS 

EPA developed guidance for industry-specific risk management programs for the following industries: 

Ø Propane storage Ø Warehouses 

facilities Ø Ammonia refrigeration 

Ø Chemical distributors Ø Small propane retailers & 

Ø Waste water treatment users 

plants 

Industry-specific guidance is either appended to this guidance or is contained in stand-alone documents 

that you can obtain from EPA's website at www.epa.gov/emergencies/rmp. If an industry-specific 

appendix or guidance document exists for your process(es), you should consider using it because it will 

provide more information that is specific to your process(es), including dispersion modeling and 

prevention program elements. 

1.4 Modeling Issues 

The consequences of an accidental chemical release depend on the conditions of the release 

and the conditions at the site at the time of the release. This guidance provides reference tables of 

distances, based on results of modeling, for estimation of worst-case and alternative scenario 

consequence distances. Worst-case consequence distances obtained using these tables are not intended 

to be precise predictions of the exact distances that might be reached in the event of an actual 

accidental release. For this guidance, worst-case distances are based on modeling results assuming the 

combination of worst-case conditions required by the rule. This combination of conditions occurs 

rarely and is unlikely to persist for very long. To derive the alternative scenario distances, less 

conservative assumptions were used for modeling; these assumptions were chosen to represent more 

likely conditions than the worst-case assumptions. Nevertheless, in an actual accidental release, the 

conditions may be very different. Users of this guidance should remember that the results derived 

from the methods presented here are rough estimates of potential consequence distances. Other 

models may give different results; the same model also may give different results if different 

assumptions about release conditions and/or site conditions are used. 

The reference tables of distances in this guidance provide results to a maximum distance of 

25 miles. EPA recognizes that modeling results at such large distances are highly uncertain. Almost no 

experimental data or data from accidents are available at such large distances to compare to modeling 

results. Most data are reported for distances well under 10 miles. Modeling uncertainties are likely to 

increase as distances increase because conditions (e.g., atmospheric stability, wind speed, surface 

roughness) are not likely to remain constant over large distances. Thus, at large distances (e.g., greater 

than about 6 to 10 miles), the modeling results should be viewed as very coarse estimates of 

consequence distances. EPA believes, 

March 2009 1 - 5 
Chapter 1 

Introduction 

however, that the results, even at large distances, can provide useful information for comparison purposes. 

For example, Local Emergency Planning Committees (LEPCs) and other local agencies can use relative 

differences in distance to aid in establishing chemical accident prevention and preparedness priorities among 

facilities in a community. Since worst-case scenario distances are based on modeling conditions that are 

unlikely to occur, and since modeling of any scenario that results in large distances is very uncertain, EPA 

strongly urges communities and industry not to rely on the results of worst-case modeling or any modeling 

that results in very large toxic endpoint distances in emergency planning and response activities. Results of 

alternative scenario models are apt to provide a more reasonable basis for planning and response. 

1.5 Steps for Performing the Analysis 

This Chapter presents the steps you should follow in using this guidance to carry out an offsite 

consequence analysis. Before carrying out one or more worst-case and/or alternative release analyses, you 

will need to obtain several pieces of information about the regulated substances you have, the area 

surrounding your site, and typical meteorological conditions: 

� Determine whether each regulated substance is toxic or flammable, as indicated in the rule or 

Appendices B and C of this guidance. 

� For the worst-case analysis, determine the quantity of each substance held in the largest 

single vessel or pipe. 

� Collect information about any passive or active (alternative scenarios only) release 

mitigation measures that are in place for each substance. 

� For toxic substances, determine whether the substance is stored as a gas, as a liquid, as a gas 

liquefied by refrigeration, or as a gas liquefied under pressure. For alternative scenarios 

involving a vapor cloud fire, you may also need this information for flammable substances. 

� For toxic liquids, determine the highest daily maximum temperature of the liquid, based on 

data for the previous three years, or process temperature, whichever is higher. 

� For toxic substances, determine whether the substance behaves as a dense or neutrally 

buoyant gas or vapor (see Appendix B, Exhibits B-1 and B-2). For alternative scenarios 

involving a vapor cloud fire, you will also need this information for flammable substances 

(see Appendix C, Exhibits C-2 and C-3). 

� For toxic substances, determine whether the topography (surface roughness) of your site is 

either urban or rural as thse terms are defined by the rule (see 40 CFR 68.22(e)). For 

alternative scenarios involving a vapor cloud fire, you will also need this information for 

flammable substances. 

After you have gathered the above information, you will need to take three steps (except for 

flammable worst-case releases): 

(1) Select a scenario; 

April 15, 1999 1 - 6 
Chapter 1 

Introduction 

(2) Determine the release or volatilization rate; and 

(3) Determine the distance to the endpoint. 

For flammable worst-case scenarios, only steps one and three are needed. Sections 1.5.1 through 1.5.6 

outline the procedures to perform the analyses. In addition to basic procedures, these sections provide 

references to sections of this guidance where you will find detailed instructions on carrying out the applicable 

portion of the analysis. Sections 1.5.1 through 1.5.3 below provide basic steps to analyze worst-case 

scenarios for toxic gases, toxic liquids, and flammable substances. Sections 1.5.4 through 1.5.6 provide 

basic steps for alternative scenario analysis. Appendix E of this document provides worksheets that may help 

you to perform the analyses. 

1.5.1 Worst-Case Analysis for Toxic Gases 

To conduct worst-case analyses for toxic gases, including toxic gases liquefied by pressurization (see 

Appendix E, Worksheet 1, for a worksheet that can be used in carrying out this analysis): 

Step 1: Determine worst-case scenario. Identify the toxic gas, quantity, and worst-case release scenario, as 

defined by the rule (Chapter 2). 

Step 2: Determine release rate. Estimate the release rate for the toxic gas, using the parameters required by 

the rule. This guidance provides methods for estimating the release rate for: 

� Unmitigated releases (Section 3.1.1). 

� Releases with passive mitigation (Section 3.1.2). 

Step 3: Determine distance to endpoint. Estimate the worst-case consequence distance based on the release 

rate and toxic endpoint (defined by the rule) (Chapter 4). This guidance provides reference tables of 

distances (Reference Tables 1-12). Select the appropriate reference table based on the density of the 

released substance, the topography of your site, and the duration of the release (always 10 minutes 

for gas releases). Estimate distance to the endpoint from the appropriate table. 

1.5.2 Worst-Case Analysis for Toxic Liquids 

To conduct worst-case analyses for toxic substances that are liquids at ambient conditions or for 

toxic gases that are liquefied by refrigeration alone (see Appendix E, Worksheet 2, for a worksheet for this 

analysis): 

Step 1: Determine worst-case scenario. Identify the toxic liquid, quantity, and worst-case release scenario, as 

defined by the rule (Chapter 2). To estimate the quantity of liquid released from piping, see Section 

3.2.1. 

Step 2: Determine release rate. Estimate the volatilization rate for the toxic liquid and the duration of the 

release, using the parameters required by the rule. This guidance provides methods for estimating the 

pool evaporation rate for: 

April 15, 1999 1 - 7 
Chapter 1 

Introduction 

� Gases liquefied by refrigeration alone (Sections 3.1.3 and 3.2.3). 

� Unmitigated releases (Section 3.2.2). 

� Releases with passive mitigation (Section 3.2.3). 

� Releases at ambient or elevated temperature (Sections 3.2.2, 3.2.3, and 3.2.5). 

� Releases of mixtures of toxic liquids (Section 3.2.4). 

� Releases of common water solutions of regulated substances and of oleum (Section 3.3). 

Step 3: Determine distance to endpoint. Estimate the worst-case consequence distance based on the release 

rate and toxic endpoint (defined by the rule) (Chapter 4). This guidance provides reference tables of 

distances (Reference Tables 1-12). Select the appropriate reference table based on the density of the 

released substance, the topography of your site, and the duration of the release. Estimate distance to 

the endpoint from the appropriate table. 

1.5.3 Worst-Case Analysis for Flammable Substances 

To conduct worst-case analyses for all regulated flammable substances (i.e., gases and liquids) (see 

Appendix E, Worksheet 3, for a worksheet for this analysis): 

Step 1: Determine worst-case scenario. Identify the appropriate flammable substance, quantity, and worstcase scenario, as defined by the rule (Chapter 2). 

Step 2: Determine distance to endpoint. Estimate the distance to the required overpressure endpoint of 1 psi 

for a vapor cloud explosion of the flammable substance, using the assumptions required by the rule 

(Chapter 5). This guidance provides a reference table of distances (Reference Table 13) for worstcase vapor cloud explosions. Estimate the distance to the endpoint from the quantity released and the 

table. 

1.5.4 Alternative Scenario Analysis for Toxic Gases 

To conduct alternative release scenario analyses for toxic gases, including toxic gases liquefied by 

pressurization (see Appendix E, Worksheet 4, for a worksheet for this analysis): 

Step 1: Select alternative scenario. Choose an appropriate alternative release scenario for the toxic gas. This 

scenario should have the potential for offsite impacts unless no such scenario exists. (Chapter 6). 

Step 2: Determine release rate. Estimate the release rate and duration of the release of the toxic gas, based 

on your scenario and site-specific conditions. This guidance provides methods for: 

� Unmitigated releases (Section 7.1.1). 

� Releases with active or passive mitigation (Section 7.1.2). 

April 15, 1999 1 - 8 
Chapter 1 

Introduction 

Step 3: Determine distance to endpoint. Estimate the alternative scenario distance based on the release rate 

and toxic endpoint (Chapter 8). This guidance provides reference tables of distances (Reference 

Tables 14-25) for alternative scenarios for toxic substances. Select the appropriate reference table 

based on the density of the released substance, the topography of your site, and the duration of the 

release. Estimate distance to the endpoint from the appropriate table. 

1.5.5 Alternative Scenario Analysis for Toxic Liquids 

To conduct alternative release scenario analyses for toxic substances that are liquids at ambient 

conditions or for toxic gases that are liquefied by refrigeration alone (see Appendix E, Worksheet 5, for a 

worksheet for this analysis): 

Step 1: Select alternative scenario. Choose an appropriate alternative release scenario and release quantity 

for the toxic liquid. This scenario should have the potential for offsite impacts (Chapter 6), unless no 

such scenario exists. 

Step 2: Determine release rate. Estimate the release rate and duration of the release of the toxic liquid, based 

on your scenario and site-specific conditions. This guidance provides methods to estimate the liquid 

release rate and quantity of liquid released for: 

� Unmitigated liquid releases (Section 7.2.1). 

� Mitigated liquid releases (Section 7.2.2). 

The released liquid is assumed to form a pool. This guidance provides methods to estimate the pool 

evaporation rate and release duration for: 

� Unmitigated releases (Section 7.2.3). 

� Releases with passive or active mitigation (Section 7.2.3). 

� Releases at ambient or elevated temperature (Sections 7.2.3). 

� Releases of common water solutions of regulated substances and of oleum (Section 7.2.4). 

Step 3: Determine distance to endpoint. Estimate the alternative scenario distance based on the release rate 

and toxic endpoint (Chapter 8). This guidance provides reference tables of distances (Reference 

Tables 14-25) for alternative scenarios for toxic substances. Select the appropriate reference table 

based on the density of the released substance, the topography of your site, and the duration of the 

release. Estimate distance to the endpoint from the appropriate table. 

1.5.6 Alternative Scenario Analysis for Flammable Substances 

To conduct alternative release scenario analyses for all regulated flammable substances (i.e., gases 

and liquids) (see Appendix E, Worksheet 6, for a worksheet for this analysis): 

April 15, 1999 1 - 9 
 

 

 

 

Chapter 1 

Introduction 

Step 1: Select alternative scenario. Identify the flammable substance, and choose the quantity 

and type of event for the alternative scenario consequence analysis (Chapter 6). 

Step 2: Determine release rate. Estimate the release rate to air of the flammable gas or liquid, if 

the scenario involves a vapor cloud fire (Section 9.1 for flammable gases, Section 9.2 for 

flammable liquids). 

Step 3: Determine distance to endpoint. Estimate the distance to the appropriate endpoint 

(defined by the rule). This guidance provides methods for: 

•	 Vapor cloud fires (Section 10.1 and Reference Tables 26-29); select the appropriate 

reference table based on the density of the released substance and the topography of your 

site, and estimate distance to the endpoint from the appropriate table. 

•	 Pool fires (Section 10.2); estimate distance from the equation and chemical-specific 

factors provided. 

•	 BLEVEs (Section 10.3 and Reference Table 30); estimate distance from the quantity of 

flammable substance and the table. 

•	 Vapor cloud explosions (Section 10.4 and Reference Table 13); estimate quantity in the 

cloud from the equation and chemical-specific factors provided, and estimate distance 

from the quantity, the table, and a factor provided for alternative scenarios. 

1.6 Additional Sources of Information 

EPA’s risk management program requirements may be found at 40 CFR part 68. The 

relevant sections were published in the Federal Register on January 31, 1994 (59 FR 4478) and 

June 20, 1996 (61 FR 31667). Final rules amending the list of substances and thresholds were 

published on August 25, 1997 (62 FR 45130) and January 6, 1998 (63 FR 640). A consolidated 

copy of these regulations is available in Appendix F. 

EPA is working with industry and local, state, and federal government agencies to assist 

sources in complying with these requirements. For more information, refer to the General 

Guidance for Risk Management Programs Appendix E (Technical Assistance). Appendices C and 

D of the General Guidance also provide points of contact for EPA and Occupational Safety and 

Health Administration (OSHA) at the state and federal levels for your questions. Your LEPC also 

can be a valuable resource. 

Finally, if you have access to the Internet, EPA has made copies of the rules, fact sheets, 

and other related materials available from EPA’s website at www.epa.gov/emergencies/rmp. Please 

check the site regularly, as additional materials are posted when they become available. If you do 

not have access to the Internet, you can call EPA’s hotline at (800) 424-9346. 

March 2009	 1 - 10 
2 DETERMINING WORST-CASE SCENARIOS 

In Chapter 2 

� 2.1 EPA’s definition of a worst-case scenario. 

� 2.2 How to determine the quantity released. 

� 2.3 How to identify the appropriate worst-case scenario. 

2.1 Definition of Worst-Case Scenario 

A worst-case release is defined as: 

The release of the largest quantity of a regulated substance from a vessel or process line 

failure, and 

�

The release that results in the greatest distance to the endpoint for the regulated toxic or 

flammable substance. 

�

You may take administrative controls into account when determining the largest quantity. 

Administrative controls are written procedures that limit the quantity of a substance that can be stored or 

processed in a vessel or pipe at any one time or, alternatively, procedures that allow the vessel or pipe to 

occasionally store larger than usual quantities (e.g., during shutdown or turnaround). Endpoints for regulated 

substances are specified in the rule (40 CFR 68.22(a), and Appendix A to part 68 for toxic substances). For 

the worst-case analysis, you do not need to consider the possible causes of the worst-case release or the 

probability that such a release might occur; the release is simply assumed to take place. You must assume all 

releases take place at ground level for the worst-case analysis. 

This guidance assumes meteorological conditions for the worst-case scenario of atmospheric stability 

class F (stable atmosphere) and wind speed 1.5 meters per second (3.4 miles per hour). Ambient air o o temperature for this guidance is 25 C (77 F). If you use this guidance, you may assume this ambient 

temperature for the worst case, even if the maximum temperature at your site in the last three years is higher. 

The rule provides two choices for topography, urban and rural. EPA (40 CFR 68.22(e)) has defined 

urban as many obstacles in the immediate area, where obstacles include buildings or trees. Rural, by EPA’s 

definition, means there are no buildings in the immediate area, and the terrain is generally flat and 

unobstructed. Thus, if your site is located in an area with few buildings or other obstructions (e.g., hills, 

trees), you should assume open (rural) conditions. If your site is in an area with many obstructions, even if it 

is in a remote location that would not usually be considered urban, you should assume urban conditions. 

Toxic Gases 

o Toxic gases include all regulated toxic substances that are gases at ambient temperature (25 C, 77 o

F), with the exception of gases liquefied by refrigeration under atmospheric pressure and released into diked 

areas. For the worst-case consequence analysis, you must assume that a gaseous release of the total quantity 

occurs in 10 minutes. You may take passive mitigation measures (e.g., enclosure) into account in the analysis 

of the worst-case scenario. 

April 15, 1999 
Chapter 2 

Determining Worst-Case Scenarios 

Gases liquefied by refrigeration alone and released into diked areas may be modeled as liquids at 

their boiling points and assumed to be released from a pool by evaporation (40 CFR 68.25(c)(2)). Gases 

liquefied by refrigeration alone that would form a pool one centimeter or less in depth upon release must be 

modeled as gases. (Modeling indicates that pools one centimeter or less deep formed by gases liquefied by 

refrigeration would completely evaporate in 10 minutes or less, giving a release rate that is equal to or greater 

than the worst-case release rate for a gaseous release. In this case, therefore, it is appropriate to treat these 

substances as gases for the worst-case analysis.) 

Endpoints for consequence analysis for regulated toxic substances are specified in the rule (40 CFR 

part 68, Appendix A). Exhibit B-1 of Appendix B lists the endpoint for each toxic gas. These endpoints are 

used for air dispersion modeling to estimate the consequence distance. 

Toxic Liquids 

For toxic liquids, you must assume that the total quantity in a vessel is spilled. This guidance 

assumes the spill takes place onto a flat, non-absorbing surface. For toxic liquids carried in pipelines, the 

quantity that might be released from the pipeline is assumed to form a pool. You may take passive mitigation 

systems (e.g., dikes) into account in consequence analysis. The total quantity spilled is assumed to spread 

instantaneously to a depth of one centimeter (0.033 foot or 0.39 inch) in an undiked area or to cover a diked 

area instantaneously. The temperature of the released liquid must be the highest daily maximum temperature 

occurring in the past three years or the temperature of the substance in the vessel, whichever is higher (40 

CFR 68.25(d)(2)). The release rate to air is estimated as the rate of evaporation from the pool. If liquids at 

your site might be spilled onto a surface that could rapidly absorb the spilled liquid (e.g., porous soil), the 

methods presented in this guidance may greatly overestimate the consequences of a release. Consider using 

another method in such a case. 

Exhibit B-2 of Appendix B presents the endpoint for air dispersion modeling for each regulated toxic 

liquid (the endpoints are specified in 40 CFR part 68, Appendix A). 

Flammable Substances 

For all regulated flammable substances, you must assume that the worst-case release results in a 

vapor cloud containing the total quantity of the substance that could be released from a vessel or pipeline. 

For the worst-case consequence analysis, you must assume the vapor cloud detonates. If you use a TNTequivalent method for your analysis, you must assume a 10 percent yield factor. 

The rule specifies the endpoint for the consequence analysis of a vapor cloud explosion of a regulated 

flammable substance as an overpressure of 1 pound per square inch (psi). This endpoint was chosen as the 

threshold for potential serious injuries to people as a result of property damage caused by an explosion (e.g., 

injuries from flying glass from shattered windows or falling debris from damaged houses). (See Appendix D, 

Section D.5 for additional information on this endpoint.) 

April 15, 1999 2 - 2 
Chapter 2 

Determining Worst-Case Scenarios 

Effect of Required Assumptions 

The assumptions required for the worst-case analysis are intended to provide conservative worst-case 

consequence distances, rather than accurate predictions of the potential consequences of a release; that is, in 

most cases your results will overestimate the effects of a release. In certain cases, actual conditions could be 

even more severe than these worst-case assumptions (e.g., very high process temperature, high process 

pressure, or unusual weather conditions, such as temperature inversions); in such cases, your results might 

underestimate the effects. However, the required assumptions generally are expected to give conservative 

results. 

2.2 Determination of Quantity for the Worst-Case Scenario 

EPA has defined a worst-case release as the release of the largest quantity of a regulated substance 

from a vessel or process line failure that results in the greatest distance to a specified endpoint. For 

substances in vessels, you must assume release of the largest amount in a single vessel. For substances in 

pipes, you must assume release of the largest amount in a pipe. The largest quantity should be determined 

taking into account administrative controls rather than absolute capacity of the vessel or pipe. Administrative 

controls are written procedures that limit the quantity of a substance that can be stored or processed in a 

vessel or pipe at any one time, or, alternatively, occasionally allow a vessel or pipe to store larger than usual 

quantities (e.g., during turnaround). 

2.3 Selecting Worst-Case Scenarios 

Under part 68, a worst-case release scenario analysis must be completed for all covered processes, 

regardless of program level. The number of worst-case scenarios you must analyze depends on several 

factors. You need to consider only the hazard (toxicity or flammability) for which a substance is regulated 

(i.e., even if a regulated toxic substance is also flammable, you only need to consider toxicity in your analysis; 

even if a regulated flammable substance is also toxic, you only need to consider flammability). 

For every Program 1 process, you must report the worst-case scenario with the greatest distance to an 

endpoint. If a Program 1 process has more than one regulated substance held above its threshold, you must 

determine which substance produces the greatest distance to its endpoint and report on that substance. If a 

Program 1 process has both regulated toxics and flammables above their thresholds, you still report only the 

one scenario that produces the greatest distance to the endpoint. The process is eligible for Program 1 if there 

are no public receptors within the distance to an endpoint of the worst-case scenario for the process and the 

other Program 1 criteria are met. For Program 2 or Program 3 processes, you must analyze and report on one 

worst-case analysis representing all toxic regulated substances present above the threshold quantity and one 

worst-case analysis representing all flammable regulated substances present above the threshold quantity. 

You may need to submit an additional worst-case analysis if a worst-case release from elsewhere at the source 

would potentially affect public receptors different from those affected by the initial worst-case scenario(s). 

If you have more than one regulated substance in a class, the substance chosen for the consequence 

analysis for each hazard for Program 2 and 3 processes should be the substance that has the potential to cause 

the greatest offsite consequences. Choosing the toxic regulated substance that might lead to the greatest 

offsite consequences may require a screening analysis of the toxic regulated substances on site, because the 

potential consequences are dependent on a number of factors, including quantity, toxicity, and volatility. 

April 15, 1999 2 - 3 
Chapter 2 

Determining Worst-Case Scenarios 

Location (distance to the fenceline) and conditions of processing or storage (e.g., a high temperature process) 

also should be considered. In selecting the worst-case scenario, you may want to consider the following 

points: 

Toxic gases with low toxic endpoints are likely to give the greatest distances to the endpoint 

for a given release quantity; a toxic gas would be a likely choice for the worst-case analysis 

required for Program 2 and 3 processes (processes containing toxic gases are unlikely to be 

eligible for Program 1). 

�

Volatile, highly toxic liquids (i.e., liquids with high ambient vapor pressure and low toxic 

endpoints) also are likely to give large distances to the endpoint (processes containing this 

type of substance are unlikely to be eligible for Program 1). 

�

Toxic liquids with relatively low volatility (low vapor pressure) and low toxicity (large toxic 

endpoint) in ambient temperature processes may give fairly small distances to the endpoint; 

you probably would not choose such substances for the worst-case analysis for Program 2 or 

3 if you have other regulated toxics, but you may want to consider carrying out a worst-case 

analysis to demonstrate potential Program 1 eligibility. 

�

For flammable substances, you must consider the consequences of a vapor cloud explosion in the 

analysis. The severity of the consequences of a vapor cloud explosion depends on the quantity of the released 

substance in the vapor cloud, its heat of combustion, and other factors that are assumed to be the same for all 

flammable substances. In most cases, the analysis probably should be based on the regulated flammable 

substance present in the greatest quantity; however, a substance with a high heat of combustion may have a 

greater potential offsite impact than a larger quantity of a substance with a lower heat of combustion. In 

some cases, a regulated flammable substance that is close to the fenceline might have a greater potential 

offsite impact than a larger quantity farther from the fenceline. 

You are likely to estimate smaller worst-case distances for flammable substances than for similar 

quantities of most toxic substances. Because the distance to the endpoint may be relatively small, you may 

find it worthwhile to carry out a worst-case analysis for each process containing flammable substances to 

demonstrate potential eligibility for Program 1, unless there are public receptors close to the process. 

April 15, 1999 2 - 4 
3 RELEASE RATES FOR TOXIC SUBSTANCES 

In Chapter 3 

�	 3.1 Estimation of worst-case release rates for toxic gases. 

�	 3.2 Estimation of release rates for toxic liquids evaporating from pools. 

�	 3.3 Estimation of release rates for common water solutions of toxic substances 

and for oleum. 

This chapter describes simple methods for estimating release rates for regulated toxic substances for 

the worst-case scenario. Simple release rate equations are provided, and factors to be used in these equations 

are provided (in Appendix B) for each regulated substance. The estimated release rates may be used to 

estimate dispersion distances to the toxic endpoint for regulated toxic gases and liquids, as discussed in 

Chapter 4. 

3.1	 Release Rates for Toxic Gases 

In Section 3.1 

�	 3.1.1 Method to estimate worst-case release rates for unmitigated releases 

(releases directly to the air) of toxic gas. 

�	 3.1.2 Method to estimate worst-case release rates for toxic gas in enclosures 

(passive mitigation). 

�	 3.1.3 Method to estimate worst-case release rates for liquefied refrigerated 

toxic gases in diked areas (as toxic liquid - see Section 3.2.3), including 

consideration of the duration of the release. 

o o Regulated substances that are gases at ambient temperature (25 C, 77 F) should be considered 

gases for consequence analysis, with the exception of gases liquefied by refrigeration at atmospheric pressure. 

Gases liquefied under pressure should be treated as gases. Gases liquefied by refrigeration alone and released 

into diked areas may be treated as liquids at their boiling points if they would form a pool upon release that is 

more than one centimeter (0.033 foot) in depth. Gases liquefied by refrigeration alone that would form a pool 

one centimeter (0.033 foot) or less in depth should be treated as gases. Modeling shows that the evaporation 

rate from such a pool would be equal to or greater than the rate for a toxic gas, which is assumed to be 

released over 10 minutes; therefore, treating liquefied refrigerated gases as gases rather than liquids in such 

cases is reasonable. You may consider passive mitigation for gaseous releases and releases of gases liquefied 

by refrigeration. 

April 15, 1999 
Chapter 3 

Release Rates for Toxic Substances 

3.1.1 Unmitigated Releases of Toxic Gas 

If no passive mitigation system is in place, estimate the release rate for the release over a 10-minute 

period of the largest quantity resulting from a pipe or vessel failure, as required by the rule (40 CFR 

68.25(c)). For a release from a vessel, calculate the release rate as follows: 

QS QR	 � (3-1) 10 

where:	 QR = Release rate (pounds per minute) 

QS = Quantity released (pounds) 

Example 1. Gas Release (Diborane) 

You have a tank containing 2,500 pounds of diborane gas. Assuming the total quantity in the tank is released 

over a 10-minute period, the release rate (QR), from Equation 3-1, is: 

QR = 2,500 pounds/10 minutes = 250 pounds per minute 

3.1.2 Releases of Toxic Gas in Enclosed Space 

If a gas is released in an enclosure such as a building or shed, the release rate to the outside air may 

be lessened considerably. The dynamics of this type of release are complex; however, you may use the 

simplified method presented here to estimate an approximate release rate to the outside air from a release in 

an enclosed space. The mitigation factor (i.e., 55 percent) presented in this method assumes that the release 

occurs in a fully enclosed, non-airtight space that is directly adjacent to the outside air. If you are modeling a 

release in an interior room that is enclosed within a building, a smaller factor (i.e., more mitigation) may be 

appropriate. On the other hand, a larger factor (i.e., less mitigation) should be used for a space that has doors 

or windows that could be open during a release. If any of these special circumstances apply to your site, you 

may want to consider performing site-specific modeling to determine the appropriate amount of passive 

mitigation. In addition, you should not incorporate the passive mitigation effect of building enclosures into 

your modeling if you have reason to believe the enclosure would not withstand the force of the release or if 

the chemical is handled outside the building (e.g., moved from one building to another building). 

For the worst case, assume as before that the largest quantity resulting from a pipe or vessel failure is 

released over a 10-minute period. Determine the unmitigated worst-case scenario release rate of the gas as 

the quantity released divided by 10 (Equation 3-1). The release rate from the building will be approximately 

55 percent of the worst-case scenario release rate (see Appendix D, Section D.1.2 for the derivation of this 

factor). Estimate the mitigated release rate as follows: 

April 15, 1999	 3 - 2 
Chapter 3 

Release Rates for Toxic Substances 

QS QR × 0.55 � (3-2) 10 

where: QR = Release rate (pounds per minute) 

QS = Quantity released (pounds) 

0.55 = Mitigation factor (discussed in Appendix D, Section D.1.2) 

Example 2. Gas Release in Enclosure (Diborane) 

Suppose the diborane gas from Example 1 is released inside a building at the rate of 250 pounds per minute. 

The mitigated release to the outside air from the building would be: 

QR = 250 pounds/minute × 0.55 = 138 pounds per minute 

3.1.3 Releases of Liquefied Refrigerated Toxic Gas in Diked Area 

If you have a toxic gas that is liquefied by refrigeration alone, and it will be released into an area 

where it will be contained by dikes to form a pool more than one centimeter (0.033 foot) in depth, you may 

carry out the worst-case analysis assuming evaporation from a liquid pool at the boiling point of the liquid. If 

your gas liquefied by refrigeration would form a pool one centimeter (0.033 foot) or less in depth, use the 

methods described in Section 3.1.1 or 3.1.2 above for the analysis. For a release in a diked area, first 

compare the diked area to the maximum area of the pool that could be formed. You can use Equation 3-6 in 

Section 3.2.3 to estimate the maximum size of the pool. Density factors (DF), needed for Equation 3-6, for 

toxic gases at their boiling points are listed in Exhibit B-1 of Appendix B. If the pool formed by the released 

liquid would be smaller than the diked area, assume a 10-minute gaseous release, and estimate the release rate 

as described in Section 3.1.1. If the dikes prevent the liquid from spreading out to form a pool of maximum 

size (one centimeter in depth), you may use the method described in Section 3.2.3 for mitigated liquid 

releases to estimate a release rate from a pool at the boiling point of the released substance. Use Equation 3-

8 in Section 3.2.3 for the release rate. The Liquid Factor Boiling (LFB) for each toxic gas, needed to use 

Equation 3-8, is listed in Exhibit B-1 of Appendix B. See the example release rate estimation on the next 

page. 

After you have estimated the release rate, estimate the duration of the vapor release from the pool 

(the time it will take for the pool to evaporate completely) by dividing the total quantity spilled by the release 

rate. You need to know the duration of release to choose the appropriate reference table of distances to 

estimate the consequence distance, as discussed in Section 4. (You do not need to consider the duration of the 

release for chlorine or sulfur dioxide, liquefied by refrigeration alone. Only one reference table of distances is 

provided for worst-case releases of each of these substances, and these tables may be used regardless of the 

release duration. The principal reason for making no distinction between 10-minute and longer releases for 

the chemical-specific tables is that the differences between the two are small relative to the uncertainties that 

have been identified.) 

April 15, 1999 3 - 3 
Chapter 3 

Release Rates for Toxic Substances 

Example 3. Mitigated Release of Gases Liquefied by Refrigeration (Chlorine) 

You have a refrigerated tank containing 50,000 pounds of liquid chlorine at ambient pressure. A diked area 

around the chlorine tank of 275 square feet is sufficient to hold all of the spilled liquid chlorine. Once the o liquid spills into the dike, it is then assumed to evaporate at its boiling point (-29 F). The evaporation rate at 

the boiling point is determined from Equation 3-8. For the calculation, wind speed is assumed to be 1.5 meters 

per second and the wind speed factor is 1.4, LFB for chlorine (from Exhibit B-1) is 0.19, and A is 275 square 

feet. The release rate is: 

QR = 1.4 × 0.19 × 275 = 73 pounds per minute 

The duration of the release does not need to be considered for chlorine. 

3.2 Release Rates for Toxic Liquids 

In Section 3.2 

�	 3.2.1 Method to estimate the quantity of toxic liquid that could be released from 

a broken pipe. 

�	 3.2.2 Method to estimate the release rate of a toxic liquid evaporating from a 

pool with no mitigation (no dikes or enclosures), including: 

o -- Releases at ambient temperature (25 C),
 

-- Releases at elevated temperature, and
 

-- Estimation of the duration of the release.
 

�	 3.2.3 Method to estimate the release rate of a toxic liquid evaporating from a 

pool with passive mitigation, including: 

-- Releases in diked areas, 
 

-- Releases into other types of containment, and 
 

-- Releases into buildings.
 

�	 3.2.4 Estimation of release rates for mixtures containing toxic liquids. 

�	 3.2.5 Method to correct the estimated release rate for liquids released at o o temperatures between 25 C and 50 C.

 For the worst-case analysis, the release rate to air for toxic liquids is assumed to be the rate of 

evaporation from the pool formed by the released liquid. This section provides methods to estimate the 

evaporation rate. Assume the total quantity in a vessel or the maximum quantity from pipes is released into 

the pool. Passive mitigation measures (e.g., dikes) may be considered in determining the area of the pool and 

April 15, 1999	 3 - 4 
Chapter 3 

Release Rates for Toxic Substances 

the release rate. To estimate the consequence distance using this guidance, you must estimate how long it will 

take for the pool to evaporate (the duration of the release), as well as the release rate, as discussed below. 

The rule (40 CFR 68.22(g)) requires you to assume that liquids (other than gases liquefied by 

refrigeration) are released at the highest maximum daily temperature for the previous three years or at process o o temperature, whichever is higher. This chapter provides methods to estimate the release rate at 25 C (77 F) o or at the boiling point, and also provides a method to correct the release rate at 25 C for releases at o o temperatures between 25 C and 50 C. 

The calculation methods provided in this section apply to substances that are liquids under ambient 

conditions or gases liquefied by refrigeration alone that are released to form pools deeper than one centimeter 

(see Section 3.1.3 above). You must treat gases liquefied under other conditions (under pressure or a 

combination of pressure and refrigeration) or gases liquefied by refrigeration alone that would form pools one 

centimeter or less in depth upon release as gas rather than liquid releases (see Sections 3.1.1 and 3.1.2 

above). 

3.2.1 Releases of Toxic Liquids from Pipes 

To consider a liquid release from a broken pipe, estimate the maximum quantity that could be 

released assuming that the pipe is full of liquid. To estimate the quantity in the pipe, you need to know the 

length of the pipe (in feet) and cross-sectional area of the pipe (in square feet). Note also that liquid may be 

released from both directions at a pipe shear (both in the direction of operational flow and the reverse 

direction, depending on the location of the shear). Therefore, the length would be the full length of pipe 

carrying the liquid on the facility grounds. Then, the volume of the liquid in the pipe (in cubic feet) is the 

length of the pipe times the cross-sectional area. The quantity in the pipe (in pounds) is the volume divided 

by the Density Factor (DF) times 0.033. (DF values are listed in Appendix B, Exhibit B-2. Density in 

pounds per cubic foot is equal to 1/(DF times 0.033).) Assume the estimated quantity (in pounds) is released 

into a pool and use the method and equations described below in Section 3.2.2 (unmitigated releases) or 3.2.3 

(releases with passive mitigation) to determine the evaporation rate of the liquid from the pool. 

3.2.2 Unmitigated Releases of Toxic Liquids 

If no passive mitigation measures are in place, the liquid is assumed to form a pool one centimeter 

(0.39 inch or 0.033 foot) deep instantaneously. You may calculate the release rate to air from the pool (the 

evaporation rate) as discussed below for releases at ambient or elevated temperature. 

Ambient Temperature 

If the liquid is always at ambient temperature, find the Liquid Factor Ambient (LFA) and the Density o Factor (DF) in Exhibit B-2 of Appendix B. The LFA and DF apply to liquids at 25 C; if your ambient o o temperature is between 25 C and 50 C, you may use the method described here and then apply a 

Temperature Correction Factor (TCF), as discussed in Section 3.2.5 below, to correct the calculated release o rate. Calculate the release rate of the liquid at 25 C from the following equation: 

QR QS � × 1.4 × LFA × DF (3-3) 

April 15, 1999 3 - 5 
Chapter 3 

Release Rates for Toxic Substances 

where: QR = Release rate (pounds per minute) 

QS = Quantity released (pounds) 

1.4 = Wind speed factor = 1.50.78, where 1.5 meters per second (3.4 miles per 

hour) is the wind speed for the worst case 

LFA = Liquid Factor Ambient 

DF = Density Factor 

Example 4. Unmitigated Liquid Release at Ambient Temperature (Acrylonitrile) 

You have a tank containing 20,000 pounds of acrylonitrile at ambient temperature. The total quantity in the 

tank is spilled onto the ground in an undiked area, forming a pool. Assume the pool spreads out to a depth of 

one centimeter. The release rate from the pool (QR) is calculated from Equation 3-3. For the calculation, the 

wind speed is assumed to be 1.5 meters per second and the wind speed factor is 1.4. From Exhibit B-2, 

Appendix B, LFA for acrylonitrile is 0.018 and DF is 0.61. Then: 

QR = 20,000 × 1.4 × 0.018 × 0.61 = 307 pounds per minute 

The duration of the release (from Equation 3-5) would be: 

t = 20,000 pounds/307 pounds per minute = 65 minutes 

Elevated Temperature 

o If the liquid is at an elevated temperature (above 50 C or at or close to the boiling point), find the 

Liquid Factor Boiling (LFB) and the Density Factor (DF) in Exhibit B-2 of Appendix B (see Appendix D, o Section D.2.2, for the derivation of these factors). For temperatures up to 50 C, you may use the method 

above for ambient temperature and apply the Temperature Correction Factors, as discussed in Section 3.2.5. o If the temperature is above 50 C, or the liquid is at or close to its boiling point, or no Temperature Correction 

Factors are available for your liquid, calculate the release rate of the liquid from the following equation: 

QR QS � × 1.4 × LFB × DF	 (3-4) 

where:	 QR = Release rate (pounds per minute) 

QS = Quantity released (pounds) 

1.4 =	 Wind speed factor = 1.50.78, where 1.5 meters per second (3.4 miles per 

hour) is the wind speed for the worst case
 

LFB = Liquid Factor Boiling
 

DF = Density Factor
 

April 15, 1999	 3 - 6 
Chapter 3 

Release Rates for Toxic Substances 

Example 5. Unmitigated Release at Elevated Temperature (Acrylonitrile) 

You have a tank containing 20,000 pounds of acrylonitrile at an elevated temperature. The total quantity in the 

tank is spilled onto the ground in an undiked area, forming a pool. Assume the pool spreads out to a depth of 

one centimeter. The release rate from the pool is calculated from Equation 3-4. For the calculation, the wind 

speed factor for 1.5 meters per second is 1.4. From Exhibit B-2, Appendix B, LFB for acrylonitrile is 0.11 and 

DF is 0.61. Then: 

QR = 20,000 × 1.4 × 0.11 × 0.61 = 1,880 pounds per minute 

The duration of the release (from Equation 3-5) would be: 

t = 20,000 pounds/1880 pounds per minute = 11 minutes 

Duration of Release 

After you have estimated a release rate as described above, determine the duration of the vapor 

release from the pool (the time it will take for the liquid pool to evaporate completely). If you calculate a o corrected release rate for liquids above 25 C, use the corrected release rate, estimated as discussed in Section 

3.2.5 below, to estimate the release duration. To estimate the time in minutes, divide the total quantity 

released (in pounds) by the release rate (in pounds per minute) as follows: 

QS t � (3-5) QR 

where: t = Duration of the release (minutes) 

QR = Release rate (pounds per minute) (use release rate corrected for 

temperature, QR , if appropriate) C 

QS = Quantity released (pounds) 

You will use the duration of the vapor release from the pool to decide which table is appropriate for 

estimating distance, as discussed in Chapter 4 below. 

3.2.3 Releases of Toxic Liquids with Passive Mitigation 

Diked Areas 

If the toxic liquid will be released into an area where it will be contained by dikes, compare the diked 

area to the maximum area of the pool that could be formed; the smaller of the two areas should be used in 

determination of the evaporation rate. The maximum area of the pool (assuming a depth of one centimeter) 

is: 

April 15, 1999 3 - 7 
Chapter 3 

Release Rates for Toxic Substances 

A � QS × DF	 (3-6) 

where:	 A = Maximum area of pool (square feet) for depth of one centimeter 

QS = Quantity released (pounds) 

DF = Density Factor (listed in Exhibit B-2, Appendix B) 

Maximum Area Smaller than Diked Area. If the maximum area of the pool is smaller than the diked 

area, calculate the release rate as described for "no mitigation" above. 

Diked Area Smaller than Maximum Area. If the diked area is smaller than the maximum pool area, 

go to Exhibit B-2 in Appendix B to find the Liquid Factor Ambient (LFA), if the liquid is at ambient 

temperature, or the Liquid Factor Boiling (LFB), if the liquid is at an elevated temperature. For liquids at o o temperatures between 25 C and 50 C, you may use the method described here and then apply a 

Temperature Correction Factor (TCF), as discussed in Section 3.2.5 below, to correct the calculated release 

rate. For gases liquefied by refrigeration alone, use LFB from Exhibit B-1. Calculate the release rate from 

the diked area as follows for liquids at ambient temperature: 

QR � 1.4 × LFA × A	 (3-7) 

or, for liquids at elevated temperature or for gases liquefied by refrigeration alone: 

QR � 1.4 × LFB × A (3-8) 

where:	 QR = Release rate (pounds per minute) 

1.4 =	 Wind speed factor = 1.50.78, where 1.5 meters per second (3.4 miles per 

hour) is the wind speed for the worst case 

LFA = Liquid Factor Ambient (listed in Exhibit B-2, Appendix B) 

LFB = Liquid Factor Boiling (listed in Exhibit B-1 (for liquefied gases) or B-2 (for 

liquids), Appendix B)
 

A = Diked area (square feet)
 

Potential Overflow of Diked Area. In case of a large liquid spill, you also need to consider whether 

the liquid could overflow the diked area. Follow these steps: 

Determine the volume of the diked area in cubic feet from surface area times depth or length 

times width times depth (in feet). 

�

Determine the volume of liquid spilled in cubic feet from QS × DF × 0.033 (DF × 0.033 is 

equal to 1/density in pounds per cubic foot). 

�

Compare the volume of the diked area to the volume of liquid spilled. If the volume of 

liquid is greater than the volume of the diked area: 

�

-- Subtract the volume of the diked area from the total volume spilled to determine the 

volume that might overflow the diked area. 

April 15, 1999	 3 - 8 
Chapter 3 

Release Rates for Toxic Substances 

-- Estimate the maximum size of the pool formed by the overflowing liquid (in square 

feet) by dividing the overflow volume (in cubic feet) by 0.033 (the depth of the pool 

in feet). 

-- Add the surface area of the diked area and the area of the pool formed by the 

overflow to estimate the total pool area (A). 

-- Estimate the evaporation rate from Equation 3-7 or 3-8 above. 

After you have estimated the release rate, estimate the duration of the vapor release from the pool by 

dividing the total quantity spilled by the release rate (Equation 3-5 above). 

Example 6. Mitigated Liquid Release at Ambient Temperature (Bromine) 

o You have a tank containing 20,000 pounds of bromine at an ambient temperature of 25 C. Assume that the 

total quantity in the tank is spilled into a square diked area 10 feet by 10 feet (area 100 square feet). The dike 

walls are four feet high. The area (A) that would be covered to a depth of 0.033 feet (one centimeter) by the 

spilled liquid is given by Equation 3-6 as the quantity released (QS) times the Density Factor (DF). From 

Exhibit B-2, Appendix B, DF for bromine is 0.16. Then: 

A = 20,000 × 0.16, or 3,200 square feet 

The diked area is smaller than the maximum pool area. The volume of bromine spilled is 20,000 × 0.16 × 

0.033, or 106 cubic feet. The spilled liquid would fill the diked area to a depth of a little more than one foot, 

well below the top of the wall. You use the diked area to determine the evaporation rate from Equation 3-7. 

For the calculation, wind speed is 1.5 meters per second, the wind speed factor is 1.4, LFA for bromine (from 

Exhibit B-2) is 0.073, and A is 100 square feet. The release rate is: 

QR = 1.4 × 0.073 × 100 = 10 pounds per minute 

The maximum duration of the release would be: 

t = 20,000 pounds/10 pounds per minute = 2,000 minutes 

Other Containment 

If the toxic liquid will be contained by other means (e.g., enclosed catch basins or trenches), consider 

the total quantity that could be spilled and estimate the surface area of the released liquid that potentially 

would be exposed to the air. Look at the dimensions of trenches or other areas where spilled liquids would be 

exposed to the air to determine the surface area of pools that could be formed. Use the instructions above to 

estimate a release rate from the total surface area. 

April 15, 1999 3 - 9 
Chapter 3 

Release Rates for Toxic Substances 

Releases into Buildings 

If the toxic liquid is released inside a building, compare the area of the pool that would be formed 

(depending upon floor space or passive mitigation) to the maximum area of the pool that could be formed (if 

the liquid is not contained); the smaller of the two areas should be used in determining the evaporation rate. 

The maximum area of the pool is determined as described above for releases into diked areas, using Equation 

3-6. If the toxic liquid would spread to cover the building floor, you determine the area of the building floor 

as: 

A L � × W	 (3-9) 

where:	 A = Area (square feet) 

L =	 Length (feet) 

W =	 Width (feet) 

If there are obstacles such as dikes inside the building, determine the size of the pool that would be formed 

based on the area defined by the dikes or other obstacles. 

The evaporation rate is then determined for a worst-case scenario (i.e., wind speed is 1.5 meters per 

second (3.4 miles per hour)), using Equation 3-3 or 3-4, if the liquid spreads to its maximum area, or 

Equation 3-7 or 3-8, if the pool area is smaller than the maximum. The maximum rate of evaporated liquid 

exiting the building is taken to be 10 percent of the calculated worst-case scenario evaporation rate (see 

Appendix D, Section D.2.4 for the derivation of this factor), as follows: 

QR � B 0.1 ×	 QR (3-10) 

where:	 QRB = Release rate from building 

QR = Release rate from pool, estimated as discussed above 

0.1 = Mitigation factor, discussed in Appendix D, Section D.2.4 

Note that the mitigation factor (i.e., 0.1) presented in this method assumes that the release occurs in a 

fully enclosed, non-airtight space that is directly adjacent to the outside air. It may not apply to a release in 

an interior room that is enclosed within a building, or to a space that has doors or windows that could be open 

during a release. In such cases, you may want to consider performing site-specific modeling to determine the 

appropriate amount of passive mitigation. 

April 15, 1999	 3 - 10 
Chapter 3 

Release Rates for Toxic Substances 

Example 7. Liquid Release Inside Building (Bromine) 

Suppose that your tank of bromine from Example 6 is contained inside a storage shed 10 feet by 10 feet (area 

100 square feet). There are no dikes inside the shed. From Example 6, you see that the area covered by the 

bromine in an unenclosed space would be 3,200 square feet. The building area is smaller than the maximum 

pool area; therefore, the building floor area should be used to determine the evaporation rate from Equation 3-

7. For the calculation, first determine the worst-case scenario evaporation rate: 

QR = 1.4 × 0.073 × 100 = 10 pounds per minute 

The release rate to the outside air of the evaporated liquid leaving the building would then be: 

QRB = 0.1 × 10 pounds per minute = 1 pound per minute 

3.2.4 Mixtures Containing Toxic Liquids 

Mixtures containing regulated toxic substances do not have to be considered if the concentration of 

the regulated substance in the mixture is below one percent by weight or if you can demonstrate that the 

partial vapor pressure of the regulated substances in the mixture is below 10 millimeters of mercury (mm 

Hg). Regulated substances present as by-products or impurities would need to be considered if they are 

present in concentrations of one percent or greater in quantities above their thresholds, and their partial vapor 

pressures are 10 mm Hg or higher. In case of a spill of a liquid mixture containing a regulated toxic 

substance with partial vapor pressure of 10 mm Hg or higher (with the exception of common water solutions, 

discussed in the next section), you have several options for estimating a release rate: 

Carry out the analysis as described above in Sections 3.2.2 or 3.2.3 using the quantity of the 

regulated substance in the mixture and the liquid factor (LFA or LFB) and density factor for 

the regulated substance in pure form. This is a simple approach that likely will give 

conservative results. 

�

If you know the partial pressure of the regulated substance in the mixture, you may estimate 

a more realistic evaporation rate. An equation for the evaporation rate is given at the end of 

Section B.2 in Appendix B. 

�

-- In this case, estimate a pool size for the entire quantity of the mixture, for an 

unmitigated release. If you know the density of the mixture, you may use it in 

estimating the pool size; otherwise, you may assume the density is the same as the 

pure regulated substance (in most cases, this assumption is unlikely to have a large 

effect on the results). 

You may estimate the partial pressure of the regulated substance in the mixture by the 

method described in Section B.2 in Appendix B and use the equation presented there to 

estimate an evaporation rate. This equation is appropriate to mixtures and solutions in 

�

April 15, 1999	 3 - 11 
Chapter 3 

Release Rates for Toxic Substances 

which the components do not interact with each other. It is probably inappropriate for most 

water solutions. It is likely to overestimate the partial vapor pressure of regulated 

substances in water solutions in which hydrogen bonding may occur (e.g., solutions of acids 

or alcohols). As discussed above, use the pool size for the entire quantity of the mixture for 

an unmitigated release. 

Example 8. Mixture Containing Toxic Liquid (Acrylonitrile) 

You have a tank containing 50,000 pounds of a mixture of acrylonitrile (a regulated substance) and N,Ndimethylformamide (not regulated). The weight of each of the components of the mixture is known 

(acrylonitrile = 20,000 pounds; N,N-dimethylformamide = 30,000 pounds.) The molecular weight of 

acrylonitrile, from Exhibit B-2, is 53.06, and the molecular weight of N,N-dimethylformamide is 73.09. Using 

Equation B-3, Appendix B, calculate the mole fraction of acrylonitrile in the solution as follows: 

Xr

 = (20,000/53.06) 

 (20,000/53.06) + (30,000/73.09)
 

Xr

 = 377
 

 377 + 410 

Xr = 0.48 

Estimate the partial vapor pressure of acrylonitrile using Equation B-4 as follows (using the vapor pressure of 

acrylonitrile in pure form at 25�C, 108 mm Hg, from Exhibit B-2, Appendix B): 

VPm = 0.48 × 108 = 51.8 mm Hg 

Before calculating evaporation rate for acrylonitrile in the mixture, you must determine the surface area of the 

pool formed by the entire quantity of the mixture, using Equation 3-6. The quantity released is 50,000 pounds 

and the Density Factor for acrylonitrile is 0.61 in Exhibit B-2; therefore: 

A = 50,000 × 0.61 = 30,500 square feet 

Now calculate the evaporation rate for acrylonitrile in the mixture from Equation B-5 using the VPm and A 

calculated above: 

QR = 0.0035 × 1.0 × (53.06)� × 30,500 × 51.8 

 298
 

QR = 262 pounds per minute 

3.2.5 Release Rate Correction for Toxic Liquids Released at Temperatures o o Between 25 C and 50 C 

o o o o If your liquid is at a temperature between 25 C (77 F) and 50 C (122 F), you must use the higher 

temperature for the offsite consequence analysis. You may correct the release rate calculated for a pool at 25 

April 15, 1999 3 - 12 
Chapter 3 

Release Rates for Toxic Substances 

o

C to estimate from a pool at the higher temperature using Temperature Correction Factors (TCF) provided in 

Appendix B, Exhibit B-4. Calculate a corrected release rate as follows: 

o o Calculate the release rate (QR) of the liquid at 25 C (77 F) as described in Section 3.2.2 

(for unmitigated releases) or 3.2.3 (for releases with passive mitigation). 

�

� From Exhibit B-4 in Appendix B: 

-- Find your liquid in the left-hand column of the table. 

-- Find the temperature closest to your temperature at the top of the table. If your 

temperature is at the midpoint between two temperatures, go to the higher 

temperature; otherwise go to the closest temperature (higher or lower than your 

temperature). 

-- Find the TCF for your liquid in the column for the appropriate temperature. 

� Estimate a corrected release rate (QR ) by multiplying the estimated release rate by the TCF; C 

i.e., 

QR � C QR × TCF (3-11) 

where: QRC = Corrected release rate o QR = Release rate calculated for 25 C
 

TCF = Temperature Correction Factor (from Exhibit B-4, Appendix B)
 

The derivation of the Temperature Correction Factors is discussed in Appendix D, Section D.2.2. If 

you have vapor pressure-temperature data for a liquid not covered in Exhibit B-4, you may correct the 

evaporation rate using the method presented in Section D.2.2. 

April 15, 1999 3 - 13 
Chapter 3 

Release Rates for Toxic Substances 

o o Example 9. Liquid Release at Ambient Temperature Between 25 C and 50 C (Bromine) 

o Assume the tank containing 20,000 pounds of bromine, from Example 6, is at an ambient temperature of 35 C o (95 F). As in Example 6, the total quantity in the tank is spilled into a diked enclosure that completely contains o the spill. The surface area is 100 square feet. In Example 6, the release rate (QR) at 25 C was calculated from o Equation 3-7 to be 10 pounds per minute. To adjust the release rate for the temperature of 35 C, you find the o Temperature Correction Factor (TCF) for bromine at 35 C from Exhibit B-4 in Appendix B. The TCF at this o temperature is 1.5; the corrected release rate (QR ) at 35 C, from Equation 3-11, is C 

QRC = 10 × 1.5 = 15 pounds per minute 

The duration of the release (from Equation 3-5) would be: 

t = 20,000 pounds/15 pounds per minute = 1,300 minutes 

3.3 Release Rates for Common Water Solutions of Toxic Substances and for 

Oleum 

In Section 3.3 

�	 Methods to estimate the release rates for several common water solutions and 

for oleum, including: 

-- Evaporation from pools with no mitigation (see 3.2.2), 

-- Evaporation from pools with dikes (see 3.2.3), 

-- Releases at elevated temperatures of solutions of gases, and 

-- Releases at elevated temperatures of solutions of liquids. 

This section presents a simple method of estimating the release rate from spills of water solutions of 

several substances. Oleum (a solution of sulfur trioxide in sulfuric acid) also is discussed in this section. 

The vapor pressure and evaporation rate of a substance in solution depends on its concentration in 

the solution. If a concentrated water solution containing a volatile toxic substance is spilled, the toxic 

substance initially will evaporate more quickly than water from the spilled solution, and the vapor pressure 

and evaporation rate will decrease as the concentration of the toxic substance in the solution decreases. At 

much lower concentrations, water may evaporate more quickly than the toxic substance. There is one 

concentration at which the composition of the solution does not change as evaporation occurs. For most 

situations of interest, the concentration exceeds this concentration, and the toxic substance evaporates more 

quickly than water. 

For estimating release rates from solutions, this guidance lists liquid factors (ambient) for several 

common water solutions at several concentrations that take into account the decrease in evaporation rate with 

decreasing concentration. Exhibit B-3 in Appendix B provides LFA and DF values for several concentrations 

April 15, 1999	 3 - 14 
Chapter 3 

Release Rates for Toxic Substances 

of ammonia, formaldehyde, hydrochloric acid, hydrofluoric acid, and nitric acid in water solution. Factors for 

oleum are also included in the exhibit. Chlorine dioxide also may be found in water solutions; however, 

solutions of chlorine dioxide commonly are below one percent concentration. Solutions below one percent 

concentration do not have to be considered. Chlorine dioxide, therefore, is not included in Exhibit B-3. These 

factors may be used to estimate an average release rate for the listed substances from a pool formed by a spill 

of solution. Liquid factors are provided for two different wind speeds, because the wind speed affects the rate 

of evaporation. 

For the worst case, use the factor for a wind speed of 1.5 meters per second (3.4 miles per hour). 

You need to consider only the first 10 minutes of the release for solutions under ambient conditions in 

estimating the consequence distance, because the toxic component in a solution evaporates fastest during the 

first few minutes of a spill, when its concentration is highest. Modeling indicates that analysis considering 

the first 10 minutes of the release gives a good approximation of the overall consequences of the release. 

Although the toxic substance will continue to evaporate from the pool after 10 minutes, the rate of 

evaporation is so much lower that it can safely be ignored in estimating the consequence distance. (See 

Appendix D, Section D.2.3, for more information.) Estimate release rates as follows: 

Ambient Temperature 

Unmitigated. If no passive mitigation measures are in place, and the solution is at ambient 

temperature, find the LFA at 1.5 meters per second (3.4 miles per hour) and DF for the 

solution in Appendix B, Exhibit B-3. Follow the instructions for liquids presented in 

Section 3.2.2 above to estimate the release rate of the listed substance in solution. Use the 

total quantity of the solution as the quantity released (QS) in carrying out the calculation of 

release rate. 

�

Mitigated. If passive mitigation is in place, and the solution is at ambient temperature, find 

the LFA at 1.5 meters per second (3.4 miles per hour) in Appendix B, Exhibit B-3, and 

follow the instructions for liquids in Section 3.2.3 above. Use the total quantity of the 

solution to estimate the maximum pool area for comparison with the diked area. 

�

April 15, 1999 3 - 15 
Chapter 3 

Release Rates for Toxic Substances 

Example 10. Evaporation Rate for Water Solution at Ambient Temperature (Hydrochloric Acid) 

You have a tank containing 50,000 pounds of 37 percent hydrochloric acid solution, at ambient temperature. 

For the worst-case analysis, you assume the entire contents of the tank is released, forming a pool. The release 

occurs in a diked area of 9,000 square feet. 

From Exhibit B-3, Appendix B, the Density Factor (DF) for 37 percent hydrochloric acid is 0.42. From 

Equation 3-6, the maximum area of the pool would be 50,000 times 0.42, or 21,000 square feet. The diked 

area is smaller; therefore, the diked area should be used in the evaporation rate (release rate) calculation, using 

Equation 3-7. 

For the calculation using Equation 3-7, you need the pool area (9,000 square feet) and the Liquid Factor 

Ambient (LFA) for 37 percent hydrochloric acid; you assume a wind speed of 1.5 meters per second, so the 

wind speed factor is 1.4. From Exhibit B-3, Appendix B, the LFA is 0.0085. From Equation 3-7, the release 

rate (QR) of hydrogen chloride from the pool is: 

QR = 1.4 × 9,000 × 0.0085 = 107 pounds per minute 

You do not need to consider the duration of the release, because only the first ten minutes are considered. 

Elevated Temperature 

Known Vapor Pressure. If the solution is at an elevated temperature, the vapor pressure of 

the regulated substance and its release rate from the solution will be much higher. This 

guidance does not include temperature correction factors for evaporation rates of regulated 

substances from solutions. If you know the partial vapor pressure of the toxic substance in 

solution at the relevant temperature, you can carry out the calculation of the release rate 

using the equations in Appendix D, Sections D.2.1 and D.2.2. As for releases of solutions at 

ambient temperature, you only need to consider the first 10 minutes of the release, because 

the evaporation rate of the toxic substance from the solution will decrease rapidly as its 

concentration decreases. 

�

Unknown Vapor Pressure. If you do not know the vapor pressure of the substance in 

solution, as a conservative approach for the worst-case analysis, use the appropriate 

instructions, as follows: 

�

-- Solutions containing substances that are gases under ambient conditions. The 

list of regulated substances includes several substances that, in their pure form, are 

gases under ambient conditions, but that may commonly be found in water 

solutions. These substances include ammonia, formaldehyde, hydrogen chloride, 

and hydrogen fluoride. For a release of a solution of ammonia, formaldehyde, 

hydrochloric acid, or hydrofluoric acid above ambient temperature, if you do not 

have vapor pressure data for the temperature of interest or prefer a simpler method, 

assume the quantity of the pure substance in the solution is released as a gas over 10 

April 15, 1999 3 - 16 
Chapter 3 

Release Rates for Toxic Substances 

minutes, as discussed in Section 3.1 above. You may determine the amount of pure 

substance in the solution from the concentration (e.g., a solution of 37 percent 

hydrochloric acid by weight would contain a quantity of hydrogen chloride equal to 

0.37 times the total weight of the solution). 

Example 11. Evaporation Rate for Gas in Water Solution at Elevated Temperature (Hydrochloric 

Acid) 

You have 50,000 pounds of 37 percent hydrochloric acid solution in a high-temperature process. For the 

worst-case analysis, you assume the entire contents of the process vessel is released. In this case, because the 

solution is at an elevated temperature, you consider the release of gaseous hydrogen chloride from the hot 

solution. 

The solution would contain 50,000 × 0.37 pounds of hydrogen chloride, or 18,500 pounds. You assume the 

entire 18,500 pounds is released over 10 minutes. From Equation 3-1, the release rate is 18,500 divided by 10, 

or 1,850 pounds per minute. 

-- Liquids in solution. If you have vapor pressure data for the liquid in solution 

(including nitric acid in water solution and sulfur trioxide in oleum) at the 

temperature of interest, you may use that data to estimate the release rate, as 

discussed above. You only need to consider the first 10 minutes of the evaporation. 

For a release of nitric acid solution at a temperature above ambient, if you do not 

have vapor pressure data or prefer to use this simpler method, determine the 

quantity of pure nitric acid in the solution from the concentration. Assume the 

quantity of pure nitric acid is released at an elevated temperature and estimate a 

release rate as discussed in Section 3.2 above, using the LFB. For temperatures o o between 25 C and 50 C, you may use the LFA and the temperature correction 

factors for the pure substance, as described in Section 3.2.5. You do not need to 

estimate the duration of the release, because you only consider the first 10 minutes. 

Similarly, for a release of oleum at an elevated temperature, determine the quantity 

of free sulfur trioxide in the oleum from the concentration and assume the sulfur 

trioxide is released at an elevated temperature. Use the LFB or the LFA and 

temperature correction factors for sulfur trioxide to estimate a release rate as 

discussed in Section 3.2. You only need to consider the first 10 minutes of the 

release in your analysis. 

For a spill of liquid in solution into a diked area, you would need to consider the 

total quantity of solution in determining whether the liquid could overflow the diked 

area (see the steps in Section 3.2.3). If you find that the liquid could overflow the 

dikes, you would need to consider both the quantity of pure substance remaining 

inside the diked area and the quantity of pure substance spilled outside the diked 

area in carrying out the release rate analysis as discussed in Section 3.2.3. 

April 15, 1999 3 - 17 
Chapter 3 

Release Rates for Toxic Substances 

Example 12. Evaporation Rate for Liquid in Water Solution at Elevated Temperature (Nitric Acid) 

You have 18,000 pounds of 90 percent nitric acid solution in a high temperature process. The solution would 

contain 18,000 x 0.90 pounds of nitric acid, or 16,200 pounds. You assume 16,200 pounds of pure nitric acid 

is released at an elevated temperature. 

For the calculation using Equation 3-4, you need the quantity released (16,200); the Liquid Factor Boiling 

(LFB) for nitric acid (0.12 from Exhibit B-2); the Density Factor (DF) for nitric acid (0.32 from Exhibit B-2); 

and you assume a wind speed of 1.5 meter per second, so the wind speed factor is 1.4. From Equation 3-4, the 

release rate (QR) of hot nitric acid is: 

QR = 16,200 × 1.4 × 0.12 × 0.32 = 870 pounds per minute 

You do not need to estimate the duration of release, because you only consider the first 10 minutes. 

April 15, 1999 3 - 18 
4 ESTIMATION OF WORST-CASE DISTANCE TO TOXIC 

ENDPOINT 

In Chapter 4 

� Reference tables of distances for worst-case releases, including: 

-- Generic reference tables (Exhibit 2), and 

-- Chemical-specific reference tables (Exhibit 3). 

� Considerations include: 

-- Gas density (neutrally buoyant or dense), 

-- Duration of release (10 minutes or 60 minutes), 

-- Topography (rural or urban). 

This guidance provides reference tables giving worst-case distances for neutrally buoyant gases and 

vapors and for dense gases and vapors for both rural (open) and urban (obstructed) areas. This chapter 

describes these reference tables and gives instructions to help you choose the appropriate table to estimate 

consequence distances for the worst-case analysis. 

Neutrally buoyant gases and vapors have approximately the same density as air, and dense gases and 

vapors are heavier than air. Neutrally buoyant and dense gases are dispersed in different ways when they are 

released; therefore, modeling was carried out to develop separate reference tables. These generic reference 

tables can be used to estimate distances using the specified toxic endpoint for each substance and the 

estimated release rate to air. In addition to the generic tables, chemical-specific reference tables are provided 

for ammonia, chlorine, and sulfur dioxide. These chemical-specific tables were developed based on modeling 

carried out for industry-specific guidance documents. All the tables were developed assuming a wind speed 

of 1.5 meters per second (3.4 miles per hour) and F stability. To use the reference tables, you need the worstcase release rates estimated as described in the previous sections. For liquid pool evaporation, you also need 

the duration of the release. In addition, to use the generic tables, you will need to determine the appropriate 

toxic endpoint and whether the gas or vapor is neutrally buoyant or dense, using the exhibits in Appendix B. 

You may interpolate between entries in the reference tables. 

Generic reference tables are provided for both 10-minute releases and 60-minute releases. You 

should use the tables for 10-minute releases if the duration of your release is 10 minutes or less; use the tables 

for 60-minute releases if the duration of your release is more than 10 minutes. For the worst-case analysis, all 

releases of toxic gases are assumed to last for 10 minutes. You need to consider the estimated duration of the 

release (from Equation 3-5) for evaporation of pools of toxic liquids. For evaporation of water solutions of 

toxic liquids or of oleum, you should always use the tables for 10-minute releases. 

The generic reference tables of distances (Reference Tables 1-8), which should be used for 

substances other than ammonia, chlorine, and sulfur dioxide, are found at the end of Chapter 5. The generic 

tables and the conditions for which each table are applicable are described in Exhibit 2. Chemical-specific 

reference tables of distances (Reference Tables 9-12) follow the generic reference tables at the end of Chapter 

5. Each of these chemical-specific tables includes distances for both rural and urban topography. These 

tables are described in Exhibit 3. 

April 15, 1999 
Chapter 4 

Estimation of Worst-Case Distance to Toxic Endpoint 

Remember that these reference tables provide only rough estimates, not accurate predictions, of the 

distances that might be reached under worst-case conditions. In particular, although the distances in the 

tables are as great as 25 miles, you should bear in mind that the larger distances (more than six to ten miles) 

are very uncertain. 

To use the reference tables of distances, follow these steps: 

For Regulated Toxic Substances Other than Ammonia, Chlorine, and Sulfur Dioxide 

Find the toxic endpoint for the substance in Appendix B (Exhibit B-1 for toxic gases or 

Exhibit B-2 for toxic liquids). 

�

Determine whether the table for neutrally buoyant or dense gases and vapors is appropriate 

from Appendix B (Exhibit B-1 for toxic gases or Exhibit B-2 for toxic liquids). A toxic gas 

that is lighter than air may behave as a dense gas upon release if it is liquefied under 

pressure, because the released gas may be mixed with liquid droplets, or if it is cold. 

Consider the state of the released gas when you decide which table is appropriate. 

�

� Determine whether the table for rural or urban conditions is appropriate. 

-- Use the rural table if your site is in an open area with few obstructions. 

-- Use the urban table if your site is in an urban or obstructed area. The urban tables 

are appropriate if there are many obstructions in the area, even if it is in a remote 

location, not in a city. 

� Determine whether the 10-minute table or the 60-minute table is appropriate. 

-- Always use the 10-minute table for worst-case releases of toxic gases. 

-- Always use the 10-minute table for worst-case releases of common water solutions 

and oleum from evaporating pools, for both ambient and elevated temperatures. 

-- If you estimated the release duration for an evaporating toxic liquid pool to be 10 

minutes or less, use the 10-minute table. 

-- If you estimated the release duration for an evaporating toxic liquid pool to be more 

than 10 minutes, use the 60-minute table. 

April 15, 1999 4 - 2 
Chapter 4 

Estimation of Worst-Case Distance to Toxic Endpoint 

Exhibit 2
 

Generic Reference Tables of Distances for Worst-case Scenarios
 

Applicable Conditions Reference Table 

Number 

Gas or Vapor Density Topography Release Duration 

(minutes) 

Neutrally buoyant Rural 10 1 

60 2 

Urban 10 3 

60 4 

Dense Rural 10 5 

60 6 

Urban 10 7 

60 8 

Exhibit 3
 

Chemical-Specific Reference Tables of Distances for Worst-case Scenarios
 

Substance 

Applicable Conditions 

Table 

Reference 

Gas or Vapor Number 

Density 

Topography Release Duration 

(minutes) 

Anhydrous ammonia 

liquefied under pressure 

Dense Rural, Urban 10 9 

Non-liquefied ammonia, 

ammonia liquefied by 

refrigeration, or aqueous 

ammonia 

Neutrally buoyant Rural, Urban 10 10 

Chlorine Dense Rural, Urban 10 11 

Sulfur dioxide (anhydrous) Dense Rural, Urban 10 12 

April 15, 1999 4 - 3 
Chapter 4 

Estimation of Worst-Case Distance to Toxic Endpoint 

Neutrally Buoyant Gases or Vapors 

If Exhibit B-1 or B-2 indicates the gas or vapor should be considered neutrally buoyant, and 

other factors would not cause the gas or vapor to behave as a dense gas, divide the estimated 

release rate (pounds per minute) by the toxic endpoint (milligrams per liter). 

�

Find the range of release rate/toxic endpoint values that includes your calculated release 

rate/toxic endpoint in the first column of the appropriate table (Reference Table 1, 2, 3, or 

4), then find the corresponding distance to the right (see Example 13 below). 

�

Dense Gases or Vapors 

If Exhibit B-1 or B-2 or consideration of other relevant factors indicates the substance 

should be considered a dense gas or vapor (heavier than air), find the distance in the 

appropriate table (Reference Table 5, 6, 7, or 8) as follows; 

�

-- Find the toxic endpoint closest to that of the substance by reading across the top of 

the table. If the endpoint of the substance is halfway between two values on the 

table, choose the value on the table that is smaller (to the left). Otherwise, choose 

the closest value to the right or the left. 

-- Find the release rate closest to the release rate estimated for the substance at the left 

of the table. If the calculated release rate is halfway between two values on the 

table, choose the release rate that is larger (farther down on the table). Otherwise, 

choose the closest value (up or down on the table). 

-- Read across from the release rate and down from the endpoint to find the distance 

corresponding to the toxic endpoint and release rate for your substance. 

For Ammonia, Chlorine, or Sulfur Dioxide 

Find the appropriate chemical-specific table for your substance (see the descriptions of 

Reference Tables 9-12 in Exhibit 3). 

�

-- If you have ammonia liquefied by refrigeration alone, you may use Reference Table 

10, even if the duration of the release is greater than 10 minutes. 

-- If you have chlorine or sulfur dioxide liquefied by refrigeration alone, you may use 

the chemical-specific reference tables, even if the duration of the release is greater 

than 10 minutes. 

� Determine whether rural or urban topography is applicable to your site. 

-- Use the rural column in the reference table if your site is in an open area with few 

obstructions. 

April 15, 1999 4 - 4 
Chapter 4 

Estimation of Worst-Case Distance to Toxic Endpoint 

-- Use the urban column if your site is in an urban or obstructed area. The urban 

column is appropriate if there are many obstructions in the area, even if it is in a 

remote location, not in a city. 

� Estimate the consequence distance as follows: 

-- In the left-hand column of the table, find the release rate closest to your calculated 

release rate. 

-- Read the corresponding distance from the appropriate column (urban or rural) to the 

right. 

The development of Reference Tables 1-8 is discussed in Appendix D, Sections D.4.1 and D.4.2. 

The development of Reference Tables 9-12 is discussed in industry-specific risk management program 

guidance documents and a backup information document that are cited in Section D.4.3. If you think the 

results of the method presented here overstate the potential consequences of a worst-case release at your site, 

you may choose to use other methods or models that take additional site-specific factors into account. 

Examples 14 and 15 below include the results of modeling using two other models, the Areal 

Locations of Hazardous Atmospheres (ALOHA) and the World Bank Hazards Analysis (WHAZAN) 

systems. These additional results are provided for comparison with the results of the methods presented in 

this guidance. The same modeling parameters were used as in the modeling carried out for development of 

the reference tables of distances. Appendix D, Section D.4.5, provides information on the modeling carried 

out with ALOHA and WHAZAN. 

Example 13. Gas Release (Diborane) 

In Example 1, you estimated a release rate for diborane gas of 250 pounds per minute. From Exhibit B-1, the 

toxic endpoint for diborane is 0.0011 mg/L, and the appropriate reference table for diborane is a neutrally 

buoyant gas table. Your facility and the surrounding area have many buildings, pieces of equipment, and other 

obstructions; therefore, you assume urban conditions. The appropriate reference table is Reference Table 3, for 

a 10-minute release of a neutrally buoyant gas in an urban area. 

The release rate divided by toxic endpoint for this example is 250/0.0011 = 230,000. 

From Reference Table 3, release rate divided by toxic endpoint falls between 221,000 and 264,000, 

corresponding to about 8.1 miles. 

April 15, 1999 4 - 5 
Chapter 4 

Estimation of Worst-Case Distance to Toxic Endpoint 

Example 14. Gas Release (Ethylene Oxide) 

You have a tank containing 10,000 pounds of ethylene oxide, which is a gas under ambient conditions. 

Assuming the total quantity in the tank is released over a 10-minute period, the release rate (QR) from Equation 

3-1 is: 

QR = 10,000 pounds/10 minutes = 1,000 pounds per minute 

From Exhibit B-1, the toxic endpoint for ethylene oxide is 0.09 mg/L, and the appropriate reference table is the 

dense gas table. Your facility is in an open, rural area with few obstructions; therefore, you use the table for 

rural areas. 

Using Reference Table 5 for 10-minute releases of dense gases in rural areas, the toxic endpoint of 0.09 mg/L 

is closer to 0.1 than 0.075 mg/L. For a release rate of 1,000 pounds per minute, the distance to 0.1 mg/L is 3.6 

miles. 

Additional Modeling for Comparison 

The ALOHA model gave a distance of 2.2 miles to the endpoint, using the same assumptions. 

The WHAZAN model gave a distance of 2.7 miles to the endpoint, using the same assumptions and the dense 

cloud dispersion model. 

Example 15. Liquid Evaporation from Pool (Acrylonitrile) 

You estimated an evaporation rate of 307 pounds per minute for acrylonitrile from a pool formed by the release 

of 20,000 pounds into an undiked area (Example 4). You estimated the time for evaporation of the pool as 65 

minutes. From Exhibit B-2, the toxic endpoint for acrylonitrile is 0.076 mg/L, and the appropriate reference 

table for a worst-case release of acrylonitrile is the dense gas table. Your facility is in an urban area. You use 

Reference Table 8 for 60-minute releases of dense gases in urban areas. 

From Reference Table 8, the toxic endpoint closest to 0.076 mg/L is 0.075 mg/L, and the closest release rate to 

307 pounds per minute is 250 pounds per minute. Using these values, the table gives a worst-case 

consequence distance of 2.9 miles. 

Additional Modeling for Comparison 

The ALOHA model gave a distance of 1.3 miles to the endpoint for a release rate of 307 pounds per minute, 

using the same assumptions.

 The WHAZAN model gave a distance of 1.0 mile to the endpoint for a release rate of 307 pounds per minute, 

using the same assumptions and the dense cloud dispersion model. 

April 15, 1999 4 - 6 
5 ESTIMATION OF DISTANCE TO OVERPRESSURE ENDPOINT 

FOR FLAMMABLE SUBSTANCES 

In Chapter 5 

� Methods to estimate the worst-case consequence distances for releases of 

flammable substances. 

-- 5.1 Vapor cloud explosions of flammable substances that are not 

mixed with other substances, and 

-- 5.2 Vapor cloud explosions of flammable substances in mixtures. 

For the worst-case scenario involving a release of flammable gases and volatile flammable liquids, 

you must assume that the total quantity of the flammable substance forms a vapor cloud within the upper and 

lower flammability limits and the cloud detonates. As a conservative worst-case assumption, if you use the 

method presented here, you must assume that 10 percent of the flammable vapor in the cloud participates in 

the explosion. You need to estimate the consequence distance to an overpressure level of 1 pound per square 

inch (psi) from the explosion of the vapor cloud. An overpressure of 1 psi may cause partial demolition of 

houses, which can result in serious injuries to people, and shattering of glass windows, which may cause skin 

laceration from flying glass. 

This chapter presents a simple method for estimating the distance to the endpoint for a vapor cloud 

explosion of a regulated substance. The method presented here for analysis of vapor cloud explosions is 

based on a TNT-equivalent model. Other methods are available for analysis of vapor cloud explosions, 

including methods that consider site-specific conditions. You may use other methods for your worst-case 

analysis if you so choose, provided you assume the total quantity of flammable substance is in the cloud and 

use an endpoint of 1 psi. If you use a TNT-equivalent model, you must assume a yield factor of 10 percent. 

Appendix A includes references to documents and journal articles on vapor cloud explosions that may 

provide useful information on methods of analysis. 

5.1 Flammable Substances Not in Mixtures 

For the worst-case analysis of a regulated flammable substance that is not in a mixture with other 

substances, you may estimate the consequence distance for a given quantity of a regulated flammable 

substance using Reference Table 13. This table provides distances to 1 psi overpressure for vapor cloud 

explosions of quantities from 500 to 2,000,000 pounds. These distances were estimated by a TNTequivalent model, Equation C-1 in Appendix C, Section C.1, using the worst-case assumptions described 

above and data provided in Exhibit C-1, Appendix C. If you prefer, you may calculate your worst-case 

consequence distance for flammable substances from the heat of combustion of the flammable substance and 

Equation C-1 or C-2. 

April 15, 1999 
Chapter 5 

Estimation of Distance to Overpressure Endpoint for Flammable Substances 

Example 16. Vapor Cloud Explosion (Propane) 

You have a tank containing 50,000 pounds of propane. From Reference Table 13, the distance to 1 psi
 

overpressure is 0.3 miles for 50,000 pounds of propane.
 

Alternatively, you can calculate the distance to 1 psi using Equation C-2 from Appendix C:
 

D = 0.0081 × [ 0.1 × 50,000 × (46,333/4,680) ]1/3 

D = 0.3 miles 

5.2 Flammable Mixtures 

If you have more than 10,000 pounds of a mixture of flammable substances that meets the criteria o o o for listing under CAA section 112(r) (flash point below 22.8 C (73 F), boiling point below 37.8 C (100 o

F), National Fire Protection Association (NFPA) flammability hazard rating of 4), you may need to carry out 

a worst-case consequence analysis for the mixture. (If the mixture itself does not meet these criteria, it is not 

covered, and no analysis is required, even if the mixture contains one or more regulated substances.) You 

should carry out the analysis using the total quantity of all regulated flammable substance or substances in the 

mixture. Non-flammable components should not be included. However, if additional (non-regulated) 

flammable substances are present in the mixture, you should include them in the quantity used in the analysis. 

For simplicity, you may carry out the worst-case analysis based on the predominant regulated 

flammable component of the mixture or a major component of the mixture with the highest heat of 

combustion if the whole vapor cloud consists of flammable substances (see Exhibit C-1, Appendix C for data 

on heat of combustion). Estimate the consequence distance from Reference Table 13 for the major 

component with the highest heat of combustion, assuming that the quantity in the cloud is the total quantity 

of the mixture. If you have a mixture in which the heats of combustion of the components do not differ 

significantly (e.g., a mixture of hydrocarbons), this method is likely to give reasonable results. 

Alternatively, you may estimate the heat of combustion of the mixture from the heats of combustion 

of the components of the mixture using the method described in Appendix C, Section C.2, and then use 

Equation C-1 or C-2 in Appendix C to determine the vapor cloud explosion distance. This method may be 

appropriate if you have a mixture that includes components with significantly different heats of combustion 

(e.g., a mixture of hydrogen and hydrocarbons) that make up a significant portion of the mixture. 

Examples 17 and 18 illustrate the two methods of analysis. In Example 17, the heat of combustion 

of the mixture is estimated, and the distance to the endpoint is calculated from Equation C-2. In Example 18, 

the component of the mixture with the highest heat of combustion is assumed to represent the entire mixture, 

and the distance to the endpoint is read from Reference Table 13. For the mixture of two hydrocarbons used 

in the example, the methods give very similar results. 

April 15, 1999 5 - 2 
Chapter 5 

Estimation of Distance to Overpressure Endpoint for Flammable Substances 

Example 17. Estimating Heat of Combustion of Mixture for Vapor Cloud Explosion Analysis 

You have a mixture of 8,000 pounds of ethylene (the reactant) and 2,000 pounds of isobutane (a catalyst 

carrier). To carry out the worst-case analysis, estimate the heat of combustion of the mixture from the heats of 

combustion of the components of the mixture. (Ethylene heat of combustion = 47,145 kilojoules per kilogram; 

isobutane heat of combustion = 45,576). Using Equation C-3, Appendix C: 

HCm = [ (8,000/2.2) × 47,145 ] + [ (2,000/2.2) × 45,576 ] 

 (10,000/2.2) (10,000/2.2) 

HCm = (37,716) + (9,115) 

HCm = 46,831 kilojoules per kilogram 

Now use the calculated heat of combustion for the mixture in Equation C-2 to calculate the distance to 1 psi 

overpressure for vapor cloud explosion. 

D = 0.0081 × [ 0.1 × 10,000 × (46,831/4,680) ]�

D = 0.2 miles 

Example 18. Vapor Cloud Explosion of Flammable Mixture (Ethylene and Isobutane) 

You have 10,000 pounds of a mixture of ethylene (the reactant) and isobutane (a catalyst carrier). To carry out 

the worst-case analysis, assume the quantity in the cloud is the total quantity of the mixture. Use data for 

ethylene because it is the component with the highest heat of combustion. (Ethylene heat of combustion = 

47,145 kilojoules per kilogram; isobutane heat of combustion = 45,576, from Exhibit C-1, Appendix C). From 

Reference Table 13, the distance to 1 psi overpressure is 0.2 miles for 10,000 pounds of ethylene; this distance 

would also apply to the 10,000-pound mixture of ethylene and isobutane. 

April 15, 1999 5 - 3 
Reference Table 1 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

10-Minute Release, Rural Conditions, F Stability, Wind Speed 1.5 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 4.4 0.1 

4.4 - 37 0.2 

37 - 97 0.3 

97 - 180 0.4 

180 - 340 0.6 

340 - 530 0.8 

530 - 760 1.0 

760 - 1,000 1.2 

1,000 - 1,500 1.4 

1,500 - 1,900 1.6 

1,900 - 2,400 1.8 

2,400 - 2,900 2.0 

2,900 - 3,500 2.2 

3,500 - 4,400 2.4 

4,400 - 5,100 2.6 

5,100 - 5,900 2.8 

5,900 - 6,800 3.0 

6,800 - 7,700 3.2 

7,700 - 9,000 3.4 

9,000 - 10,000 3.6 

10,000 - 11,000 3.8 

11,000 - 12,000 4.0 

12,000 - 14,000 4.2 

14,000 - 15,000 4.4 

15,000 - 16,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

16,000 - 18,000 4.8 

18,000 - 19,000 5.0 

19,000 - 21,000 5.2 

21,000 - 23,000 5.4 

23,000 - 24,000 5.6 

24,000 - 26,000 5.8 

26,000 - 28,000 6.0 

28,000 - 29,600 6.2 

29,600 - 35,600 6.8 

35,600 - 42,000 7.5 

42,000 - 48,800 8.1 

48,800 - 56,000 8.7 

56,000 - 63,600 9.3 

63,600 - 71,500 9.9 

71,500 - 88,500 11 

88,500 - 107,000 12 

107,000 - 126,000 14 

126,000 - 147,000 15 

147,000 - 169,000 16 

169,000 - 191,000 17 

191,000 - 215,000 19 

215,000 - 279,000 22 

279,000 - 347,000 25 

>347,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 5 - 4 
Reference Table 2 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

60-Minute Release, Rural Conditions, F Stability, Wind Speed 1.5 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 5.5 0.1 

5.5 - 46 0.2 

46 - 120 0.3 

120 - 220 0.4 

220 - 420 0.6 

420 - 650 0.8 

650 - 910 1.0 

910 - 1,200 1.2 

1,200 - 1,600 1.4 

1,600 - 1,900 1.6 

1,900 - 2,300 1.8 

2,300 - 2,600 2.0 

2,600 - 2,900 2.2 

2,900 - 3,400 2.4 

3,400 - 3,700 2.6 

3,700 - 4,100 2.8 

4,100 - 4,400 3.0 

4,400 - 4,800 3.2 

4,800 - 5,200 3.4 

5,200 - 5,600 3.6 

5,600 - 5,900 3.8 

5,900 - 6,200 4.0 

6,200 - 6,700 4.2 

6,700 - 7,000 4.4 

7,000 - 7,400 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

7,400 - 7,700 4.8 

7,700 - 8,100 5.0 

8,100 - 8,500 5.2 

8,500 - 8,900 5.4 

8,900 - 9,200 5.6 

9,200 - 9,600 5.8 

9,600 - 10,000 6.0 

10,000 - 10,400 6.2 

10,400 - 11,700 6.8 

11,700 - 13,100 7.5 

13,100 - 14,500 8.1 

14,500 - 15,900 8.7 

15,900 - 17,500 9.3 

17,500 - 19,100 9.9 

19,100 - 22,600 11 

22,600 - 26,300 12 

26,300 - 30,300 14 

30,300 - 34,500 15 

34,500 - 38,900 16 

38,900 - 43,600 17 

43,600 - 48,400 19 

48,400 - 61,500 22 

61,500 - 75,600 25 

>75,600 >25* 

 *Report distance as 25 miles 

April 15, 1999 5 - 5 
Reference Table 3 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

10-minute Release, Urban Conditions, F Stability, Wind Speed 1.5 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 21 0.1 

21 - 170 0.2 

170 - 420 0.3 

420 - 760 0.4 

760 - 1,400 0.6 

1,400 - 2,100 0.8 

2,100 - 3,100 1.0 

3,100 - 4,200 1.2 

4,200 - 6,100 1.4 

6,100 - 7,800 1.6 

7,800 - 9,700 1.8 

9,700 - 12,000 2.0 

12,000 - 14,000 2.2 

14,000 - 18,000 2.4 

18,000 - 22,000 2.6 

22,000 - 25,000 2.8 

25,000 - 29,000 3.0 

29,000 - 33,000 3.2 

33,000 - 39,000 3.4 

39,000 - 44,000 3.6 

44,000 - 49,000 3.8 

49,000 - 55,000 4.0 

55,000 - 63,000 4.2 

63,000 - 69,000 4.4 

69,000 - 76,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

76,000 - 83,000 4.8 

83,000 - 90,000 5.0 

90,000 - 100,000 5.2 

100,000 - 110,000 5.4 

110,000 - 120,000 5.6 

120,000 - 130,000 5.8 

130,000 - 140,000 6.0 

140,000 - 148,000 6.2 

148,000 - 183,000 6.8 

183,000 - 221,000 7.5 

221,000 - 264,000 8.1 

264,000 - 310,000 8.7 

310,000 - 361,000 9.3 

361,000 - 415,000 9.9 

415,000 - 535,000 11 

535,000 - 671,000 12 

671,000 - 822,000 14 

822,000 - 990,000 15 

990,000 - 1,170,000 16 

1,170,000 - 1,370,000 17 

1,370,000 - 1,590,000 19 

1,590,000 - 2,190,000 22 

2,190,000 - 2,890,000 25 

>2,890,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 5 - 6 
Reference Table 4 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

60-Minute Release, Urban Conditions, F Stability, Wind Speed 1.5 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 26 0.1 

26 - 210 0.2 

210 - 530 0.3 

530 - 940 0.4 

940 - 1,700 0.6 

1,700 - 2,600 0.8 

2,600 - 3,700 1.0 

3,700 - 4,800 1.2 

4,800 - 6,400 1.4 

6,400 - 7,700 1.6 

7,700 - 9,100 1.8 

9,100 - 11,000 2.0 

11,000 - 12,000 2.2 

12,000 - 14,000 2.4 

14,000 - 16,000 2.6 

16,000 - 17,000 2.8 

17,000 - 19,000 3.0 

19,000 - 21,000 3.2 

21,000 - 23,000 3.4 

23,000 - 24,000 3.6 

24,000 - 26,000 3.8 

26,000 - 28,000 4.0 

28,000 - 30,000 4.2 

30,000 - 32,000 4.4 

32,000 - 34,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

34,000 - 36,000 4.8 

36,000 - 38,000 5.0 

38,000 - 41,000 5.2 

41,000 - 43,000 5.4 

43,000 - 45,000 5.6 

45,000 - 47,000 5.8 

47,000 - 50,000 6.0 

50,000 - 52,200 6.2 

52,200 - 60,200 6.8 

60,200 - 68,900 7.5 

68,900 - 78,300 8.1 

78,300 - 88,400 8.7 

88,400 - 99,300 9.3 

99,300 - 111,000 9.9 

111,000 - 137,000 11 

137,000 - 165,000 12 

165,000 - 197,000 14 

197,000 - 232,000 15 

232,000 - 271,000 16 

271,000 - 312,000 17 

312,000 - 357,000 19 

357,000 - 483,000 22 

483,000 - 629,000 25 

>629,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 5 - 7 
Reference Table 5

Dense Gas Distances to Toxic Endpoint

10-minute Release, Rural Conditions, F Stability, Wind Speed 1.5 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 2.2 1.7 1.5 1.1 0.8 0.7 0.5 0.5 0.3 0.2 0.2 0.2 0.1 

2 3.0 2.4 2.1 1.5 1.1 0.9 0.7 0.7 0.4 0.3 0.3 0.2 0.2 

5 4.8 3.7 3.0 2.2 1.7 1.5 1.2 1.0 0.7 0.5 0.4 0.3 0.3 

10 6.8 5.0 4.2 3.0 2.4 2.1 1.7 1.4 1.0 0.7 0.6 0.5 0.4 

30 11 8.7 6.8 5.2 3.9 3.4 2.8 2.4 1.7 1.3 1.1 0.9 0.7 50 14 11 9.3 6.8 5.0 4.2 3.5 3.0 2.2 1.7 1.4 1.1 0.9 

100 19 15 12 8.7 6.8 5.8 4.8 4.2 2.9 2.2 1.9 1.6 1.3 

150 24 18 15 11 8.1 6.8 5.7 5.0 3.6 2.7 2.3 1.9 1.6 250 >25 22 19 14 11 8.7 7.4 6.2 4.5 3.4 2.8 2.3 2.0 500 * >25 >25 19 14 12 9.9 8.7 6.2 4.7 3.8 3.1 2.7 

750 * * * 23 17 15 12 11 7.4 5.5 4.5 3.7 3.2 

1,000 * * * >25 20 17 14 12 8.1 6.2 5.2 4.2 3.6 

1,500 * * * * 24 20 16 14 9.9 7.4 6.2 5.0 4.3 

2,000 * * * * >25 23 19 16 11 8.7 6.8 5.6 4.8 

2,500 * * * * * >25 20 18 12 9.3 8.1 6.2 5.3 

3,000 * * * * * * 23 20 14 9.9 8.7 6.8 5.6 

4,000 * * * * * * >25 22 16 11 9.3 7.4 6.2 

5,000 * * * * * * * 25 17 13 11 8.7 6.8 

7,500 * * * * * * * >25 20 15 12 9.9 8.7 

10,000 * * * * * * * * 24 17 14 11 9.3 15,000 * * * * * * * * >25 20 17 13 11 20,000 * * * * * * * * * 23 19 15 12 

50,000 * * * * * * * * * >25 >25 21 18 

75,000 * * * * * * * * * * * >25 21 

100,000 * * * * * * * * * * * * 24 

150,000 * * * * * * * * * * * * >25 

200,000 * * * * * * * * * * * * * 

* > 25 miles (report distance as 25 miles) # <0.1 mile (

5 - 8 
Reference Table 6

Dense Gas Distances to Toxic Endpoint

60-minute Release, Rural Conditions, F Stability, Wind Speed 1.5 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 3.7 2.7 2.2 1.4 1.0 0.8 0.6 0.5 0.3 0.2 0.2 0.1 0.1 

2 5.3 4.0 3.2 2.2 1.6 1.2 1.0 0.8 0.5 0.4 0.3 0.2 0.2 

5 8.7 6.8 5.3 3.7 2.7 2.2 1.7 1.4 0.9 0.6 0.5 0.4 0.3 

10 12 9.3 8.1 5.3 4.0 3.3 2.7 2.2 1.4 1.0 0.8 0.6 0.5 30 22 16 14 9.9 7.4 6.1 4.9 4.1 2.9 2.1 1.6 1.2 1.0 50 >25 21 18 12 9.3 8.1 6.2 5.4 3.8 2.7 2.2 1.7 1.4 

100 * >25 >25 18 13 11 9.3 7.4 5.5 4.0 3.2 2.5 2.1 

150 * * * 22 17 14 11 9.9 6.8 4.9 4.0 3.1 2.7 250 * * * >25 22 18 14 12 8.7 6.2 5.2 4.1 3.5 500 * * * * >25 25 20 17 12 9.3 7.4 5.8 5.0 750 * * * * * >25 25 22 15 11 9.3 7.4 6.1 

1,000 * * * * * * >25 25 17 12 11 8.1 6.8 

1,500 * * * * * * * >25 20 16 12 9.9 8.7 

2,000 * * * * * * * * 24 17 14 11 9.9 

2,500 * * * * * * * * >25 20 16 13 11 

3,000 * * * * * * * * * 21 17 14 12 

4,000 * * * * * * * * * 24 20 16 14 

5,000 * * * * * * * * * >25 22 17 15 

7,500 * * * * * * * * * * >25 21 18 

10,000 * * * * * * * * * * * 24 20 15,000 * * * * * * * * * * * >25 24 20,000 * * * * * * * * * * * * >25 50,000 * * * * * * * * * * * * * 

75,000 * * * * * * * * * * * * * 

100,000 * * * * * * * * * * * * * 150,000 * * * * * * * * * * * * * 200,000 * * * * * * * * * * * * * 

* > 25 miles (report distance as 25 miles) # <0.1 mile (

5 - 9 
Reference Table 7

Dense Gas Distances to Toxic Endpoint

10-minute Release, Urban Conditions, F Stability, Wind Speed 1.5 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 1.6 1.2 1.1 0.7 0.6 0.4 0.4 0.3 0.2 0.2 0.1 0.1 0.1 

2 2.2 1.7 1.4 1.1 0.8 0.6 0.5 0.4 0.3 0.2 0.2 0.1 0.1 

5 3.5 2.7 2.2 1.6 1.2 1.0 0.8 0.7 0.5 0.4 0.3 0.2 0.2 

10 4.9 3.8 3.1 2.2 1.7 1.4 1.2 1.0 0.7 0.5 0.4 0.3 0.2 

30 8.1 6.2 5.3 3.7 2.9 2.4 2.0 1.7 1.2 0.9 0.7 0.6 0.4 

50 11 8.1 6.8 4.8 3.7 3.1 2.5 2.1 1.5 1.1 0.9 0.7 0.6 

100 15 11 9.3 6.8 5.2 4.2 3.5 3.0 2.1 1.6 1.3 1.0 0.9 

150 19 14 12 8.1 6.1 5.2 4.3 3.6 2.5 1.9 1.6 1.2 1.1 250 24 18 15 11 8.1 6.8 5.4 4.6 3.3 2.4 2.0 1.6 1.4 500 >25 >25 21 15 11 9.3 7.4 6.2 4.5 3.4 2.8 2.2 1.9 750 * * >25 18 14 11 9.3 8.1 5.5 4.1 3.3 2.6 2.2 

1,000 * * * 21 16 13 11 9.3 6.2 4.6 3.8 3.0 2.5 1,500 * * * >25 19 16 12 11 7.4 5.6 4.6 3.7 3.0 

2,000 * * * * 22 18 15 12 8.7 6.2 5.2 4.1 3.5 

2,500 * * * * 24 20 16 14 9.9 6.8 5.8 4.7 3.8 

3,000 * * * * >25 22 18 16 11 7.4 6.2 5.0 4.2 

4,000 * * * * * 25 20 17 12 8.7 6.8 5.6 4.8 

5,000 * * * * * >25 23 20 14 9.9 8.1 6.2 5.3 

7,500 * * * * * * >25 24 16 12 9.9 7.4 6.2 

10,000 * * * * * * * >25 19 14 11 8.7 7.4 15,000 * * * * * * * * 22 16 13 11 8.7 20,000 * * * * * * * * >25 19 15 12 9.9 50,000 * * * * * * * * * >25 23 17 15 

75,000 * * * * * * * * * * >25 21 17 

100,000 * * * * * * * * * * * 24 20 

150,000 * * * * * * * * * * * >25 23 

200,000 * * * * * * * * * * * * >25 

* > 25 miles (report distance as 25 miles) # <0.1 mile (

5 - 10 
Reference Table 8

Dense Gas Distances to Toxic Endpoint

60-minute Release, Urban Conditions, F Stability, Wind Speed 1.5 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 2.6 1.9 1.5 1.1 0.7 0.6 0.4 0.4 0.2 0.2 0.1 0.1 0.1 

2 3.8 2.9 2.3 1.5 1.1 0.9 0.7 0.6 0.4 0.2 0.2 0.1 0.1 

5 6.2 4.7 3.9 2.6 1.9 1.5 1.2 0.9 0.6 0.4 0.3 0.2 0.2 

10 9.3 6.8 5.6 3.9 2.9 2.3 1.8 1.5 0.9 0.7 0.5 0.4 0.3 

30 16 12 9.9 7.4 5.3 4.3 3.4 2.9 1.9 1.3 1.0 0.7 0.6 50 22 16 14 9.3 6.8 5.7 4.5 3.8 2.6 1.8 1.4 1.1 0.9 

100 >25 24 20 14 9.9 8.1 6.8 5.7 3.8 2.7 2.2 1.7 1.4 

150 * >25 24 17 12 11 8.1 6.8 4.8 3.5 2.8 2.2 1.8 250 * * >25 22 16 14 11 9.3 6.2 4.5 3.7 2.9 2.4 500 * * * >25 24 19 16 13 9.3 6.8 5.4 4.2 3.5 750 * * * * >25 24 19 16 11 8.1 6.8 5.2 4.3 

1,000 * * * * * >25 22 19 13 9.3 7.4 6.0 5.0 

1,500 * * * * * * >25 24 16 12 9.3 7.4 6.2 

2,000 * * * * * * * >25 19 13 11 8.7 7.4 

2,500 * * * * * * * * 20 15 12 9.3 8.1 

3,000 * * * * * * * * 22 16 13 11 8.7 

4,000 * * * * * * * * >25 19 16 12 9.9 

5,000 * * * * * * * * * 21 17 14 11 

7,500 * * * * * * * * * >25 20 16 14 

10,000 * * * * * * * * * * 24 19 16 15,000 * * * * * * * * * * >25 22 19 20,000 * * * * * * * * * * * >25 21 50,000 * * * * * * * * * * * * >25 

75,000 * * * * * * * * * * * * * 

100,000 * * * * * * * * * * * * * 150,000 * * * * * * * * * * * * * 200,000 * * * * * * * * * * * * * 

* > 25 miles (report distance as 25 miles) # <0.1 mile (

5 - 11 
Reference Table 9
 

Distances to Toxic Endpoint for Anhydrous Ammonia Liquefied Under Pressure
 

F Stability, Wind Speed 1.5 Meters per Second
 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

1 0.1 <0.1* 

2 0.1 0.1 

5 0.1 0.1 

10 0.2 0.1 

15 0.2 0.2 

20 0.3 0.2 

30 0.3 0.2 

40 0.4 0.3 

50 0.4 0.3 

60 0.5 0.3 

70 0.5 0.3 

80 0.5 0.4 

90 0.6 0.4 

100 0.6 0.4 

150 0.7 0.5 

200 0.8 0.6 

250 0.9 0.6 

300 1.0 0.7 

400 1.2 0.8 

500 1.3 0.9 

600 1.4 0.9 

700 1.5 1.0 

750 1.6 1.0 

800 1.6 1.1 

900 1.7 1.2 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

1,000 1.8 1.2 

1,500 2.2 1.5 

2,000 2.6 1.7 

2,500 2.9 1.9 

3,000 3.1 2.0 

4,000 3.6 2.3 

5,000 4.0 2.6 

6,000 4.4 2.8 

7,000 4.7 3.1 

7,500 4.9 3.2 

8,000 5.1 3.3 

9,000 5.4 3.4 

10,000 5.6 3.6 

15,000 6.9 4.4 

20,000 8.0 5.0 

25,000 8.9 5.6 

30,000 9.7 6.1 

40,000 11 7.0 

50,000 12 7.8 

75,000 15 9.5 

100,000 18 10 

150,000 22 13 

200,000 ** 15 

250,000 ** 17 

750,000 ** ** 

*Report distance as 0.1 mile ** More than 25 miles (report distance as 25 miles) 

April 15, 1999 5 - 12 
Reference Table 10

 Distances to Toxic Endpoint for Non-liquefied Ammonia, Ammonia Liquefied by Refrigeration, or 

Aqueous Ammonia
 

F Stability, Wind Speed 1.5 Meters per Second
 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

1 0.1 

<0.1* 

2 0.1 

5 0.1 

10 0.2 0.1 

15 0.2 0.1 

20 0.3 0.1 

30 0.3 0.1 

40 0.4 0.1 

50 0.4 0.1 

60 0.4 0.2 

70 0.5 0.2 

80 0.5 0.2 

90 0.5 0.2 

100 0.6 0.2 

150 0.7 0.2 

200 0.8 0.3 

250 0.8 0.3 

300 0.9 0.3 

400 1.1 0.4 

500 1.2 0.4 

600 1.3 0.4 

700 1.4 0.5 

750 1.4 0.5 

800 1.5 0.5 

900 1.5 0.6 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

1,000 1.6 0.6 

1,500 2.0 0.7 

2,000 2.2 0.8 

2,500 2.5 0.9 

3,000 2.7 1.0 

4,000 3.1 1.1 

5,000 3.4 1.2 

6,000 3.7 1.3 

7,000 4.0 1.4 

7,500 4.1 1.5 

8,000 4.2 1.5 

9,000 4.5 1.6 

10,000 4.7 1.7 

15,000 5.6 2.0 

20,000 6.5 2.4 

25,000 7.2 2.6 

30,000 7.8 2.8 

40,000 8.9 3.3 

50,000 9.8 3.6 

 75,000 12 4.4 

100,000 14 5.0 

150,000 16 6.1 

200,000 19 7.0 

250,000 21 7.8 

750,000 ** 13 

*Report distance as 0.1 mile ** More than 25 miles (report distance as 25 miles) 

April 15, 1999 5 - 13 
Reference Table 11
 

Distances to Toxic Endpoint for Chlorine
 

F Stability, Wind Speed 1.5 Meters per Second
 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

1 0.2 0.1 

2 0.3 0.1 

5 0.5 0.2 

10 0.7 0.3 

15 0.8 0.4 

20 1.0 0.4 

30 1.2 0.5 

40 1.4 0.6 

50 1.5 0.6 

60 1.7 0.7 

70 1.8 0.8 

80 1.9 0.8 

90 2.0 0.9 

100 2.2 0.9 

150 2.6 1.2 

200 3.0 1.3 

250 3.4 1.5 

300 3.7 1.6 

400 4.2 1.9 

500 4.7 2.1 

600 5.2 2.3 

700 5.6 2.5 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

750 5.8 2.6 

800 5.9 2.7 

900 6.3 2.9 

1,000 6.6 3.0 

1,500 8.1 3.8 

2,000 9.3 4.4 

2,500 10 4.9 

3,000 11 5.4 

4,000 13 6.2 

5,000 14 7.0 

6,000 16 7.6 

7,000 17 8.3 

7,500 18 8.6 

8,000 18 8.9 

9,000 19 9.4 

10,000 20 9.9 

15,000 25 12 

20,000 * 14 

25,000 * 16 

30,000 * 18 

40,000 * 20 

50,000 * * 

* More than 25 miles (report distance as 25 miles) 

April 15, 1999 5 - 14 
Reference Table 12

 Distances to Toxic Endpoint for Anhydrous Sulfur Dioxide
 

F Stability, Wind Speed 1.5 Meters per Second
 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

1 0.2 0.1 

2 0.2 0.1 

5 0.4 0.2 

10 0.6 0.2 

15 0.7 0.3 

20 0.9 0.4 

30 1.1 0.5 

40 1.3 0.5 

50 1.4 0.6 

60 1.6 0.7 

70 1.8 0.7 

80 1.9 0.8 

90 2.0 0.8 

100 2.1 0.9 

150 2.7 1.1 

200 3.1 1.3 

250 3.6 1.4 

300 3.9 1.6 

400 4.6 1.9 

500 5.2 2.1 

600 5.8 2.3 

700 6.3 2.5 

Release Rate Distance to Endpoint (miles) 

(lbs/min) 

Rural Urban 

750 6.6 2.6 

800 6.8 2.7 

900 7.2 2.9 

1,000 7.7 3.1 

1,500 9.6 3.8 

2,000 11 4.5 

2,500 13 5.0 

3,000 14 5.6 

 4,000 17 6.5 

5,000 19 7.3 

6,000 21 8.1 

7,000 23 8.8 

7,500 24 9.1 

8,000 25 9.5 

9,000 * 10 

10,000 * 11 

15,000 * 13 

20,000 * 16 

25,000 * 18 

30,000 * 19 

40,000 * 23 

50,000 * * 

* More than 25 miles (report distance as 25 miles) 

April 15, 1999 5 - 15 
Reference Table 13

Distance to Overpressure of 1.0 psi for Vapor Cloud Explosions of 500 - 2,000,000 Pounds of Regulated Flamm

Based on TNT Equivalent Method, 10 Percent Yield Factor

Quantity in Cloud (pounds) 500 2,000 5,000 10,000 20,000 50,000 100,000 200,000 500,000 

CAS No. Chemical Name Distance (Miles) to 1 psi Overpressure 

75-07-0 Acetaldehyde 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 74-86-2 Acetylene 0.07 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.7 

598-73-2 Bromotrifluoroethylene 0.02 0.04 0.05 0.06 0.08 0.1 0.1 0.2 0.2 

106-99-0 1,3-Butadiene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

106-97-8 Butane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

25167-67-3 Butene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

590-18-1 2-Butene-cis 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 624-64-6 2-Butene-trans 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

106-98-9 1-Butene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 107-01-7 2-Butene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 463-58-1 Carbon oxysulfide 0.04 0.06 0.08 0.1 0.1 0.2 0.2 0.3 0.4 

7791-21-1 Chlorine monoxide 0.02 0.03 0.04 0.05 0.06 0.08 0.1 0.1 0.2 

590-21-6 1-Chloropropylene 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 557-98-2 2-Chloropropylene 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 460-19-5 Cyanogen 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 

75-19-4 Cyclopropane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

4109-96-0 Dichlorosilane 0.04 0.06 0.08 0.1 0.1 0.2 0.2 0.3 0.4 

75-37-6 Difluoroethane 0.04 0.06 0.09 0.1 0.1 0.2 0.2 0.3 0.4 

124-40-3 Dimethylamine 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 

463-82-1 2,2-Dimethylpropane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

74-84-0 Ethane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

107-00-6 Ethyl acetylene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

75-04-7 Ethylamine 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 

April 15, 1999 5 - 16 
Reference Table 13 (continued) 

Quantity in Cloud (pounds) 500 2,000 5,000 10,000 20,000 50,000 100,000 200,000 500,000 

CAS No. Chemical Name Distance (Miles) to 1 psi Overpressure 

75-00-3 Ethyl chloride 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 74-85-1 Ethylene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.7 60-29-7 Ethyl ether 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 75-08-1 Ethyl mercaptan 0.05 0.09 0.1 0.2 0.2 0.2 0.3 0.4 0.5 

109-95-5 Ethyl nitrite 0.05 0.07 0.1 0.1 0.2 0.2 0.3 0.3 0.5 

1333-74-0 Hydrogen 0.09 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.9 

75-28-5 Isobutane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 78-78-4 Isopentane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 78-79-5 Isoprene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 75-31-0 Isopropylamine 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 75-29-6 Isopropyl chloride 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 

74-82-8 Methane 0.07 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.7 

74-89-5 Methylamine 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 

563-45-1 3-Methyl-1-butene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 563-46-2 2-Methyl-1-butene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 115-10-6 Methyl ether 0.05 0.09 0.1 0.1 0.2 0.3 0.3 0.4 0.5 107-31-3 Methyl formate 0.04 0.07 0.1 0.1 0.2 0.2 0.3 0.3 0.4 

115-11-7 2-Methylpropene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 504-60-9 1,3-Pentadiene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

109-66-0 Pentane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

109-67-1 1-Pentene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

646-04-8 2-Pentene, (E)- 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

627-20-3 2-Pentene, (Z)- 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

463-49-0 Propadiene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

April 15, 1999 5 - 17 
Reference Table 13 (continued) 

Quantity in Cloud (pounds) 500 2,000 5,000 10,000 20,000 50,000 100,000 200,000 500,000 

CAS No. Chemical Name Distance (Miles) to 1 psi Overpressure 

74-98-6 Propane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

115-07-1 Propylene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

74-99-7 Propyne 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

7803-62-5 Silane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

116-14-3 Tetrafluoroethylene 0.02 0.03 0.04 0.05 0.07 0.09 0.1 0.1 0.2 

75-76-3 Tetramethylsilane 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

10025-78-2 Trichlorosilane 0.03 0.04 0.06 0.08 0.1 0.1 0.2 0.2 0.3 

79-38-9 Trifluorochloroethylene 0.02 0.03 0.05 0.06 0.07 0.1 0.1 0.2 0.2 75-50-3 Trimethylamine 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.6 

689-97-4 Vinyl acetylene 0.06 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 

75-01-4 Vinyl chloride 0.05 0.08 0.1 0.1 0.2 0.2 0.3 0.4 0.5 

109-92-2 Vinyl ethyl ether 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 

75-02-5 Vinyl fluoride 0.02 0.04 0.05 0.06 0.08 0.1 0.1 0.2 0.2 

75-35-4 Vinylidene chloride 0.04 0.06 0.08 0.1 0.1 0.2 0.2 0.3 0.4 

75-38-7 Vinylidene fluoride 0.04 0.06 0.09 0.1 0.1 0.2 0.2 0.3 0.4 

107-25-5 Vinyl methyl ether 0.06 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.6 

April 15, 1999 5 - 18 
6 DETERMINING ALTERNATIVE RELEASE SCENARIOS 

In Chapter 6 

� Considerations for alternative release scenarios for regulated substances in 

Program 2 or Program 3 processes. 

� Potential alternative scenarios for releases of flammable substances. 

You are required to analyze at least one alternative release scenario for each listed toxic substance 

you have in a Program 2 or Program 3 process above its threshold quantity. You also are required to analyze 

one alternative release scenario for flammable substances in Program 2 or 3 processes as a class (i.e., you 

analyze one scenario involving a flammable substance as a representative scenario for all the regulated 

flammable substances you have on site in Program 2 or Program 3 processes). You do not need to analyze an 

alternative scenario for each flammable substance. For example, if you have five listed substances – chlorine, 

ammonia, hydrogen chloride, propane, and acetylene – above the threshold in Program 2 or 3 processes, you 

will need to analyze one alternative scenario each for chlorine, ammonia, and hydrogen chloride and a single 

alternative scenario to cover propane and acetylene (listed flammable substances). Even if you have a 

substance above the threshold in several processes or locations, you need only analyze one alternative 

scenario for it. 

According to the rule (40 CFR 68.28), alternative scenarios should be more likely to occur than the 

worst-case scenario and should reach an endpoint offsite, unless no such scenario exists. Release scenarios 

considered should include, but are not limited to, the following: 

� Transfer hose releases due to splits or sudden hose uncoupling; 

Process piping releases from failures at flanges, joints, welds, valves and valve seals, and 

drains or bleeds; 

�

� Process vessel or pump releases due to cracks, seal failure, or drain, bleed, or plug failure; 

Vessel overfilling and spill, or overpressurization and venting through relief valves or 

rupture disks; and 

�

� Shipping container mishandling or puncturing leading to a spill. 

Alternative release scenarios for toxic substances should be those that lead to concentrations above 

the toxic endpoint beyond your fenceline. Scenarios for flammable substances should have the potential to 

cause substantial damage, including on-site damage. Those releases that have the potential to reach the 

public are of the greatest concern. You should consider unusual situations, such as start-up and shut-down, in 

selecting an appropriate alternative scenario. 

For alternative release scenarios, you are allowed to consider active mitigation systems, such as 

interlocks, shutdown systems, pressure relieving devices, flares, emergency isolation systems, and fire water 

and deluge systems, as well as passive mitigation systems, as described in Sections 3.1.2 and 3.2.3. 

For alternative release scenarios for regulated substances used in ammonia refrigeration, chemical 

distribution, propane distribution, warehouses, or POTWs, consult EPA's risk management program guidance 

April 15, 1999 
Chapter 6 

Determining Alternative Release Scenarios 

documents for these industry sectors. 

You have a number of options for selecting release scenarios for toxic or flammable substances. 

You may use your worst-case release scenario and apply your active mitigation system to 

limit the quantity released and the duration of the release. 

�

You may use information from your process hazards analysis, if you have conducted one, to 

select a scenario. 

�

You may review your accident history and choose an actual event as the basis of your 

scenario. 

�

If you have not conducted a process hazards analysis, you may review your operations and 

identify possible events and failures. 

�

Whichever approach you select, the key information you need to define is the quantity to be released 

and the time over which it will be released; together, these allow you to estimate the release rate and use 

essentially the same methods you used for the worst-case analysis. 

For flammable substances , the choice of alternative release scenarios is somewhat more complicated 

than for toxic substances, because the consequences of a release and the endpoint of concern may vary. For 

the flammable worst case, the consequence of concern is a vapor cloud explosion, with an overpressure 

endpoint. For alternative scenarios (e.g., fires), other endpoints (e.g., heat radiation) may need to be 

considered. 

Possible scenarios involving flammable substances include: 

Vapor cloud fires (flash fires) may result from dispersion of a cloud of flammable vapor and 

ignition of the cloud following dispersion. Such a fire could flash back and could represent a 

severe heat radiation hazard to anyone in the area of the cloud. This guidance provides 

methods to estimate distances to a concentration equal to the lower flammability limit (LFL) 

for this type of fire. (See Sections 9.1, 9.2, and 10.1.) 

�

A pool fire, with potential radiant heat effects, may result from a spill of a flammable liquid. 

This guidance provides a simple method for estimating the distance from a pool fire to a 

radiant heat level that could cause second degree burns from a 40-second exposure. (See 

Section 10.2). 

�

A boiling liquid, expanding vapor explosion (BLEVE), leading to a fireball that may 

produce intense heat, may occur if a vessel containing flammable material ruptures 

explosively as a result of exposure to fire. Heat radiation from the fireball is the primary 

hazard; vessel fragments and overpressure from the explosion also can result. BLEVEs are 

generally considered unlikely events; however, if you think a BLEVE is possible at your site, 

this guidance provides a method to estimate the distance at which radiant heat effects might 

lead to second degree burns. (See Section 10.3.) You also may want to consider models or 

�

April 15, 1999 6 - 2 
Chapter 6 

Determining Alternative Release Scenarios 

calculation methods to estimate effects of vessel fragmentation. (See Appendix A for 

references that may provide useful information for estimating such effects.) 

For a vapor cloud explosion to occur, rapid release of a large quantity, turbulent conditions 

(caused by a turbulent release or congested conditions in the area of the release, or both), and 

other factors are generally necessary. Vapor cloud explosions generally are considered 

unlikely events; however, if conditions at your site are conducive to vapor cloud explosions, 

you may want to consider a vapor cloud explosion as an alternative scenario. This guidance 

provides methods you may use to estimate the distance to 1 psi overpressure for a vapor 

cloud detonation, based on less conservative assumptions than the worst-case analysis. (See 

Section 10.4.) A vapor cloud deflagration, involving lower flame speeds than a detonation 

and resulting in less damaging blast effects, is more likely than a detonation. This guidance 

does not provide methods for estimating the effects of a deflagration, but you may use other 

methods of analysis if you want to consider such events. (See Appendix A for references 

that may provide useful information.) 

�

A jet fire may result from the puncture or rupture of a tank or pipeline containing a 

compressed or liquefied gas under pressure. The gas discharging from the hole can form a 

jet that "blows" into the air in the direction of the hole; the jet then may ignite. Jet fires 

could contribute to BLEVEs and fireballs if they impinge on tanks of flammable substances. 

A large horizontal jet fire may have the potential to pose an offsite hazard. This guidance 

does not include a method for estimating consequence distances for jet fires. If you want to 

consider a jet fire as an alternative scenario, you should consider other models or methods 

for the consequence analysis. (See Appendix A for references that may provide useful 

information.) 

�

If you carry out an alternative scenario analysis for a flammable mixture (i.e., a mixture that meets 

the criteria for NFPA 4), you need to consider all flammable components of the mixture, not just the regulated 

flammable substance or substances in the mixture (see Section 5.2 on flammable mixtures). If the mixture 

contains both flammable and non-flammable components, the analysis should be carried out considering only 

the flammable components. 

Chapter 7 provides detailed information on calculating release rates for alternative release scenarios 

for toxic substances. If you can estimate release rates for the toxic gases and liquids you have on site based 

on readily available information, you may skip Chapter 7 and go to Chapter 8. Chapter 8 describes how to 

estimate distances to the toxic endpoint for alternative scenarios for toxic substances. Chapters 9 provides 

information on calculation release rates for flammable substances. Chapter 10 describes how to estimate 

distances to flammable endpoints. 

April 15, 1999 6 - 3 
Chapter 6 

Determining Alternative Release Scenarios 

This page intentionally left blank. 

April 15, 1999 6 - 4 
7 ESTIMATION OF RELEASE RATES FOR ALTERNATIVE 

SCENARIOS FOR TOXIC SUBSTANCES 

For the alternative scenario analysis, you may use typical meteorological conditions and typical 

ambient temperature and humidity for your site. This guidance assumes D atmospheric stability and wind 

speed of 3.0 meters per second (6.7 miles per hour) as conditions likely to be applicable to many sites. 

7.1	 Release Rates for Toxic Gases 

In Section 7.1 

�	 7.1.1 Methods for unmitigated releases of toxic gases, including: 

-- Release of toxic gas from a hole in a tank or pipe (for choked flow 

conditions, or maximum flow rate), 

-- Release of toxic gas from a pipe, based on the flow rate through the 

pipe, or based on a hole in the pipe (using the same method as for a 

hole in a tank), 

-- Puff releases (no method is provided; users are directed to use other 

methods), 

-- Gases liquefied under pressure, including gaseous releases from holes 

above the liquid level in the tank and releases from holes in the liquid 

space, and 

--	 Consideration of duration of releases of toxic gas. 

�	 7.1.2 Methods for adjusting the estimated release rate to account for active or 

passive mitigation, including: 

-- Active mitigation to reduce the release duration (e.g., automatic 

shutoff valves), 

--	 Active mitigation to reduce the release rate to air, and 

--	 Passive mitigation (using the same method as for worst-case 

scenarios). 

7.1.1	 Unmitigated Releases of Toxic Gases 

Gaseous Releases 

Gaseous Release from Tank. Instead of assuming release of the entire contents of a vessel containing 

a toxic gas, you may decide to consider a more likely scenario as developed by the process hazards analysis, 

such as release from a hole in a vessel or pipe. To estimate a hole size you might assume, for example, the 

hole size that would result from shearing off a valve or pipe from a vessel containing a regulated substance. 

If you have a gas leak from a tank, you may use the following simplified equation to estimate a release rate 

based on hole size, tank pressure, and the properties of the gas. This equation applies to choked flow, or 

maximum gas flow rate. Choked flow generally would be expected for gases under pressure. (See Appendix 

D, Section D.6 for the derivation of this equation.) 

April 15, 1999 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

QR � HA × Pt × 

1 

Tt 

× GF (7-1) 

where: QR = Release rate (pounds per minute) 

HA = Hole or puncture area (square inches) (from hazard evaluation or best 

estimate) 

Pt = Tank pressure (pounds per square inch absolute (psia)) (from process 

information; for liquefied gases, equilibrium vapor pressure at 25 C is o 

included in Exhibit B-1, Appendix B) 

Tt = Tank temperature (K), where K is absolute temperature in kelvins; 25 C o 

(77 F) is 298 K o 

GF = Gas Factor, incorporating discharge coefficient, ratio of specific heats, 

molecular weight, and conversion factors (listed for each regulated toxic gas 

in Exhibit B-1, Appendix B) 

You can estimate the hole area from the size and shape of the hole. For a circular hole, you would 2 use the formula for the area of a circle (area = �r , where � is 3.14 and r is the radius of the circle; the radius 

is half the diameter). 

This equation will give an estimate of the initial release rate. It will overestimate the overall release 

rate, because it does not take into account the decrease in the release rate as the pressure in the tank decreases. 

You may use a computer model or another calculation method if you want a more realistic estimate of the 

release rate. As discussed below, you may use this equation for releases of gases liquefied under pressure if 

the release would be primarily gas (e.g., if the hole is in the head space of the tank, well above the liquid 

level). 

Example 19. Release of Toxic Gas from Tank (Diborane) 

You have a tank that contains diborane gas at a pressure of 30 psia. The temperature of the tank and its 

contents is 298 K (25�C). A valve on the side of the tank shears off, leaving a circular hole 2 1⁄2 inches in 2 diameter in the tank wall. You estimate the area from the formula for area of a circle (�r , where r is the 2 radius). The radius of the hole is 1 1/4 inches, so the area is � × (1 1/4) , or 5 square inches. From Exhibit 

B-1, the Gas Factor for diborane is 17. Therefore, the release rate, from Equation 7-1, is: 

QR = 5 × 30 × 1/(298)1⁄2 × 17 = 148 pounds per minute 

Gaseous Release from Pipe. If shearing of a pipe may be an alternative scenario for a toxic gas at 

your site, you could use the usual flow rate through the pipe as the release rate and carry out the estimation of 

distance as discussed in Chapter 8. 

April 15, 1999 7 - 2 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

If you want to consider a release of toxic gas through a hole in a pipe as an alternative scenario, you 

may use the method described above for a gas release from a hole in a tank. This method neglects the effects 

of friction along the pipe and, therefore, provides a conservative estimate of the release rate. 

Puff Releases. If a gaseous release from a hole in a tank or pipe is likely to be stopped very quickly 

(e.g., by a block valve), resulting in a puff of toxic gas that forms a vapor cloud rather than a plume, you may 

want to consider other methods for determining a consequence distance. A cloud of toxic gas resulting from a 

puff release will not exhibit the same behavior as a plume resulting from a longer release (e.g., a release over 

10 minutes). 

Liquefied Gases 

Gases Liquefied Under Pressure. Gases liquefied under pressure may be released as gases, liquids, 

or a combination (two-phase), depending on a number of factors, including liquid level and the location of the 

hole relative to the liquid level. The resulting impact distances can vary greatly. 

For releases from holes above the liquid level in a tank of gas liquefied under pressure, the release 

could be primarily gas, or the release may involve rapid vaporization of a fraction of the liquefied gas and 

possibly aerosolization (two-phase release). It is complex to determine which type of release (i.e., gas, twophase) will occur and the likely mix of gases and liquids in a two-phase release. The methods presented in 

this guidance do not definitively address this situation. As a rule of thumb, if the head space is large and the 

distance between the hole and the liquid level is relatively large given the height of the tank or vessel, you 

could assume the release is gaseous and, therefore, use Equation 7-1 above. (Exhibit B-1, Appendix B, o includes the equilibrium vapor pressure in psia for listed toxic gases liquefied under pressure at 25 C; this 

pressure can be used in Equation 7-1.) However, use of this equation will not be conservative if the head 

space is small and the release from the hole is two-phased. In situations where you are unsure of whether the 

release would be gaseous or two-phase, you may want to consider other models or methods to carry out a 

consequence analysis. 

For a hole in the liquid space of a tank, you may use Equation 7-2 below to estimate the release rate. o Exhibit B-1 in Appendix B gives the equilibrium vapor pressure in psia for listed toxic gases at 25 C; this 

is the pressure required to liquefy the gas at this temperature. You can estimate the gauge pressure in the 

tank from the equilibrium vapor pressure by subtracting the pressure of the ambient atmosphere (14.7 psi). 

Exhibit B-1 also gives the Density Factor (DF) for each toxic gas at its boiling point. This factor can be used o to estimate the density of the liquefied gas (the density at 25 C would not be significantly different from the 

density at the boiling point for most of the listed gases). The equation to estimate the release rate is (see 

Appendix D, Section D.7.1, for more information): 

11.7 669 QR � HA × 6.82 × LH � × P

g (7-2) 

DF 2 DF 

April 15, 1999 7 - 3 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

where: QR = Release rate (pounds per minute) 

HA = Hole or puncture area (square inches) (from hazard evaluation or best 

estimate) 

DF = Density Factor (listed for each regulated toxic gas in Exhibit B-1, Appendix 

B; 1/(DF x 0.033) is density in pounds per cubic foot) 

LH = Height of liquid column above hole (inches) (from hazard evaluation or best 

estimate) 

Pg = Gauge pressure of the tank pressure (pounds per square inch gauge (psig), 

from vapor pressure of gas (listed in Exhibit B-1, Appendix B) minus 

atmospheric pressure (14.7 psi) 

This equation gives the rate of release of liquid through the hole. For a gas liquefied under pressure, assume 

that the released liquid will immediately flash into vapor (or a vapor/aerosol mixture) and the release rate to 

air is the same as the liquid release rate. This equation gives an estimate of the initial release rate. It will 

overestimate the overall release rate, because it does not take into account the decrease in the release rate as 

the pressure in the tank and the height of the liquid in the tank decrease. You may use a computer model or 

another calculation method if you want a more realistic estimate of the release rate. 

For a release from a broken pipe of a gas liquefied under pressure, see equations 7-4 to 7-6 below for 

liquid releases from pipes. Assume the released liquid flashes into vapor upon release and use the calculated 

release rate as the release rate to air. 

Gases Liquefied by Refrigeration. Gases liquefied by refrigeration alone may be treated as liquids. 

You may use the methods described in Section 7.2 for estimation of release rates. 

Duration of Release 

The duration of the release is used in choosing the appropriate generic reference table of distances, as 

discussed in Chapter 8. (You do not need to consider the duration of the release to use the chemical-specific 

reference tables.) You may calculate the maximum duration by dividing the quantity in the tank or the 

quantity that may be released from pipes by your calculated release rate. You may use 60 minutes as a 

default value for maximum release duration. If you know, and can substantiate, how long it is likely to take 

to stop the release, you may use that time as the release duration. 

7.1.2 Mitigated Releases of Toxic Gases 

For gases, passive mitigation may include enclosed spaces, as discussed in Section 3.1.2. Active 

mitigation for gases, which may be considered in analyzing alternative release scenarios, may include an 

assortment of techniques including automatic shutoff valves, rapid transfer systems (emergency deinventory), 

and water/chemical sprays. These mitigation techniques have the effect of reducing either the release rate or 

the duration of the release, or both. 

April 15, 1999 7 - 4 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

Active Mitigation 

Active Mitigation to Reduce Release Duration. An example of a mitigation technique to reduce the 

release duration is automatic shutoff valves. If you have an estimate of the rate at which the gas will be 

released and the time it will take to shut off the release, you may estimate the quantity potentially released 

(release rate times time). You must be able to substantiate the time it will take to shut off the release. If the 

release will take place over a period of 10 minutes or more, you may use the release rate to estimate the 

distance to the toxic endpoint, as discussed in Chapter 8. For releases stopped in less than 10 minutes, 

multiply the initial release rate by the duration of release to estimate the quantity released, then divide the new 

quantity by 10 minutes to estimate a mitigated release rate that you may apply to the reference tables 

described in Chapter 8 to estimate the consequence distance. If the release would be stopped very quickly, 

you might want to consider other methods that will estimate consequence distances for a puff release. 

Active Mitigation to Directly Reduce Release Rate to Air. Examples of mitigation techniques to 

directly reduce the release rate include scrubbers and flares. Use test data, manufacturer design 

specifications, or past experience to determine the fractional reduction of the release rate by the mitigation 

technique. Apply this fraction to the release rate that would have occurred without the mitigation technique. 

The initial release rate, without mitigation, may be the release rate for the alternative scenario (e.g., a release 

rate estimated from the equations presented earlier in this section) or the worst-case release rate. The 

mitigated release rate is: 

QR � � R (1 FR)× QR	 (7-3) 

where: QRR = Reduced release rate (pounds per minute) 

FR	 = Fractional reduction resulting from mitigation 

QR	 = Release rate without mitigation (pounds per minute) 

Example 20. Water Spray Mitigation (Hydrogen Fluoride) 

A bleeder valve on a hydrogen fluoride (HF) tank opens, releasing 660 pounds per minute of HF. Water sprays 

are applied almost immediately. Experimental field and laboratory test data indicate that HF vapors could be 

reduced by 90 percent. The reduced release rate is: 

QRR	 = (1 - 0.9) × (660 pounds per minute)
 

= 66 pounds per minute
 

In estimating the consequence distance for this release scenario, you would need to consider the release both 

before and after application of the water spray and determine which gives the greatest distance to the endpoint. 

You need to be able to substantiate the time needed to begin the water spray mitigation. 

April 15, 1999	 7 - 5 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

Passive Mitigation 

The same simplified method used for worst-case releases may be used for alternative release 

scenarios to estimate the release rate to the outside air from a release in an enclosed space. For alternative 

scenarios, you may use a modified release quantity, if appropriate. You may also adjust the mitigation factor 

to account for the effects of ventilation, if appropriate for the alternative scenario you have chosen. Use the 

equations presented in Section 3.1.2 to estimate the release rate to the outside air. 

Duration of Release 

You should estimate the duration of the release either from your knowledge of the length of time it 

may take to stop the release (be prepared to substantiate your time estimate) or by dividing the quantity that 

may be released by your estimated release rate. (You do not need to consider the release duration to use the 

chemical-specific reference tables of distances.) 

7.2	 Release Rates for Toxic Liquids 

In Section 7.2 

�	 7.2.1 Methods for estimating the liquid release rate and quantity released for 

toxic liquids released without mitigation, including: 

-- Release of toxic liquid from a hole in a tank under atmospheric 

pressure (including toxic gases liquefied by refrigeration alone), 

-- Release of toxic liquid from a hole in the liquid space of a pressurized 

tank (the user is referred to equations provided in the section on toxic 

gases or in the technical appendix), and 

--	 Release of toxic liquid from a broken pipe. 

�	 7.2.2 Methods for estimating the liquid release rate and quantity released for 

toxic liquids released with mitigation measures that reduce the duration of the 

liquid release or the quantity of liquid released (e.g., automatic shutoff valves), 

�	 7.2.3 Methods for estimating the evaporation rate of toxic liquids from pools, 

accounting for: 

-- Ambient temperature,
 

-- Elevated temperature, 
 

-- Diked areas,
 

-- Releases into buildings,
 

-- Active mitigation to reduce the evaporation rate of the liquid, 
 o o -- Temperatures between 25 C and 50 C, and
 

-- Duration of the release.
 

�	 7.2.4 Methods for estimating the evaporation rate for common water solutions 

of regulated toxic substances and for oleum. 

April 15, 1999	 7 - 6 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

This section describes methods for estimating liquid release rates from tanks and pipes. The released 

liquid is assumed to form a pool, and the evaporation rate from the pool is estimated as for the worst-case 

scenario. For the alternative scenario, you may assume the average wind speed in your area in the calculation 

of evaporation rate, instead of the worst-case wind speed of 1.5 meters per second (3.4 miles per hour). For 

the reference tables in this guidance, the wind speed for alternative scenarios is assumed to be 3.0 meters per 

second (6.7 miles per hour). 

If you have sufficient information to estimate the quantity of liquid that might be released to an 

undiked area under an alternative scenario, you may go directly to Section 7.2.3 to estimate the evaporation 

rate from the pool and the release duration. After you have estimated the evaporation rate and release 

duration, go to Chapter 8 for instructions on estimating distance to the toxic endpoint. 

7.2.1 Liquid Release Rate and Quantity Released for Unmitigated Releases 

Release from Tank 

Tank under Atmospheric Pressure. If you have a liquid stored in a tank at atmospheric pressure 

(including gases liquefied by refrigeration alone), you may use the following simple equation to estimate the 

liquid release rate from a hole in the tank below the liquid level. (See Appendix D, Section D.7.1, for the 

derivation of this equation.) 

QRL � HA × LH × LLF (7-4) 

where: QRL = Liquid release rate (pounds per minute) 

HA = Hole or puncture area (square inches) (from hazard evaluation or best 

estimate) 

LH = Height of liquid column above hole (inches) (from hazard evaluation or best 

estimate) 

LLF = Liquid Leak Factor incorporating discharge coefficient and liquid density 

(listed for each toxic liquid in Exhibit B-2, Appendix B). 

Remember that this equation only applies to liquids in tanks under atmospheric pressure. This 

equation will give an overestimate of the release rate, because it does not take into account the decrease in the 

release rate as the height of the liquid above the hole decreases. You may use a computer model or another 

calculation method if you want a more realistic estimate of the liquid release rate. 

You may estimate the quantity that might be released by multiplying the liquid release rate from the 

above equation by the time (in minutes) that likely would be needed to stop the release. You should be able 

to substantiate the time needed to stop the release. Alternatively, you may assume the release would stop 

when the level of liquid in the tank drops to the level of the hole. You may estimate the quantity of liquid 

above that level in the tank from the dimensions of the tank, the liquid level at the start of the leak, and the 

level of the hole. Assume the estimated quantity is released into a pool and use the method and equations in 

Section 7.2.3 below to determine the evaporation rate of the liquid from the pool and the duration of the 

release. As discussed in Section 7.2.3, if you find that your estimated evaporation rate is greater than 

estimated liquid release rate, you should use the liquid release rate as the release rate to air. 

April 15, 1999 7 - 7 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

Example 21. Liquid Release from Atmospheric Tank (Allyl Alcohol) 

You have a tank that contains 20,000 pounds of allyl alcohol at ambient temperature and pressure. A valve on 

the side of the tank shears, leaving a hole in the tank wall 5 square inches in area. The liquid column is 23 

inches above the hole in the tank. From Exhibit B-2, the Liquid Leak Factor for allyl alcohol is 41. Therefore, 

from Equation 7-4, the liquid release rate is: 

QRL = 5 × (23)1⁄2 × 41 = 983 pounds per minute 

It takes 10 minutes to stop the release, so 10 minutes × 983 pounds per minute = 9,830 pounds of allyl alcohol 

released. 

Pressurized Tank. If you have a liquid stored in a tank under pressure, you may estimate a release 

rate for liquid from a hole in the liquid space of the tank using the equation presented above for gases 

liquefied under pressure (Equation 7-2 in Section 7.1.1) or the equations in Appendix D, Section D.7.1. 

Release from Pipe 

If you have a liquid flowing through a pipe at approximately atmospheric pressure, and the pipeline 

remains at about the same height between the pipe inlet and the pipe break, you can estimate the quantity of 

liquid released from the flow rate in the pipe and the time it would take to stop the release by multiplying the 

flow rate by the time. For liquids at atmospheric pressure, assume the liquid is spilled into a pool and use the 

methods in Section 7.2.3 below to estimate the release rate to air. 

For the release of a liquid under pressure from a long pipeline, you may use the equations below (see 

Appendix D, Section D.7.2 for more information on these equations). These equations apply both to 

substances that are liquid at ambient conditions and to gases liquefied under pressure. This method does not 

consider the effects of friction in the pipe. First estimate the initial operational flow velocity of the substance 

through the pipe using the initial operational flow rate as follows: 

FR × DF × 0.033 V � a	 (7-5) A 

p 

where:	 Va = Initial operational flow velocity (feet per minute) 

FR = Initial operational flow rate (pounds per minute) 

DF = Density Factor (from Exhibit B-2, Appendix B) 

Ap = Cross-sectional area of pipe (square feet) 

You can estimate the cross-sectional area of the pipe from the diameter or radius (half the diameter 2 of the pipe) using the formula for the area of a circle (area = �r , where r is the radius). 

The release velocity is then calculated based on the initial operational flow, any gravitational 

April 15, 1999	 7 - 8 
 

Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

acceleration or deceleration effects resulting from changes in the height of the pipeline, and the pressure 

difference between the pressure in the pipe and atmospheric pressure, using a form of the Bernoulli equation: 

�197 × [28.4 × (PT 14.7) × DF] � [5.97 × (Z )] � [2.58×10�5 V � � × V b a Zb a 

2

] (7-6) 

where: Vb = Release velocity (feet per minute) 

PT = Total pressure on liquid in pipe (psia) 

DF = Density factor, see Exhibit B-1 or Exhibit B-2 

Za = Height of pipeline at inlet (feet) 

Zb = Height of pipeline at break (feet) 

Va = Operational velocity (feet per minute), calculated from Equation 7-5 

Please note that if the height of the pipe at the release point is higher than the initial pipe height, then Z -Z is a b 

negative, and the height term will cause the estimated release velocity to decrease. 

The release velocity can then be used to calculate a release rate as follows: 

Vb × A 

p QR � L (7-7) DF × 0.033 

where: QRL = Release rate (pounds per minute) 

Vb = Release velocity (feet per minute) 

DF = Density Factor 

Ap = Cross-sectional area of pipe (square feet) 

You may estimate the quantity released into a pool from the broken pipe by multiplying the liquid 

release rate (QR ) from the equation above by the time (in minutes) that likely would be needed to stop the L 

release (or to empty the pipeline). Assume the estimated quantity is released into a pool and use the method 

and equations described in Section 7.2.3 below to determine the evaporation rate of the liquid from the pool. 

You must be able to substantiate the time needed to stop the release. 

As noted above in Section 7.1.1, for a release from a pipe of gas liquefied under pressure, assume 

that the released liquid is immediately vaporized, and use the calculated liquid release rate as the release rate 

to air. If the release duration would be very short (e.g., because of active mitigation measures), determine the 

total quantity of the release as the release rate times the duration, then estimate a new release rate as the 

quantity divided by 10. This will give you a release rate that you can use with the 10-minute reference tables 

of distances in this guidance to estimate a distance to the endpoint. 

In the case of very long pipes, release rates from a shear or hole will be lower than the estimates from 

this method because of pipe roughness and frictional head loss. If friction effects are deemed considerable, an 

established method for calculating frictional head loss such as the Darcy formula may be used. 

April 15, 1999 7 - 9 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

7.2.2 Liquid Release Rate and Quantity Released for Mitigated Releases 

For alternative release scenarios, you are permitted to take credit for both passive and active 

mitigation systems, or a combination if both are in place. Active mitigation techniques that reduce the rate of 

liquid release or the quantity released into the pool are discussed in this section. Active and passive 

mitigation to reduce the evaporation rate of liquid from a pool are discussed in the next section. 

Active Mitigation to Reduce Quantity Released 

Examples of mitigation techniques to reduce the quantity released into the pool include automatic 

shutoff valves and emergency deinventory. You may use the equations in Section 7.2.1 above for calculating 

liquid release rate, if applicable. Estimate the approximate time needed to stop the release by the mitigation 

technique (you must be able to justify your estimate). Multiply the release rate times the duration of release 

to estimate quantity released. Assume the estimated quantity is released into a pool and use the method and 

equations described in Section 7.2.3 below to determine the evaporation rate of the liquid from the pool. You 

should also consider mitigation (active or passive) of evaporation from the pool, if applicable, as discussed in 

Section 7.2.3 below. 

Example 22. Mitigated Liquid Release 

A bromine injection system suffers a hose failure; the greatly lowered system pressure triggers an automatic 

shutoff valve within 30 seconds of the release. The flow rate out of the ruptured hose is approximately 330 

pounds per minute. Because the release occurred for only 30 seconds (0.5 minutes), the total quantity spilled 

was 330 x 0.5, or 165 pounds. 

7.2.3 Evaporation Rate from Liquid Pool 

After you have estimated the quantity of liquid released, assume that the liquid forms a pool and 

calculate the evaporation rate from the pool as described below. You may account for both passive and active 

mitigation in estimating the release rate. Passive mitigation may include techniques already discussed in 

Section 3.2.3 such as dikes and trenches. Active mitigation to reduce the release rate of liquid in pools to the 

air may include an assortment of techniques including foam or tarp coverings and water or chemical sprays. 

Some methods of accounting for passive and active mitigation are discussed below. 

If the calculated evaporation rate from the pool is greater than the liquid release rate you have 

estimated from the container, no pool would be formed, and calculating the release rate as the evaporation 

rate from a pool would not be appropriate. If the pool evaporation rate is greater than the liquid release rate, 

use the liquid release rate as the release rate to air. Consider this possibility particularly for relatively volatile 

liquids, gases liquefied by refrigeration, or liquids at elevated temperature that form pools with no mitigation. 

April 15, 1999 7 - 10 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

Unmitigated 

Ambient temperature. For pools with no mitigation, if the liquid is always at ambient temperature, 

find the Liquid Factor Ambient (LFA) and the Density Factor (DF) in Exhibit B-2 of Appendix B (see o Appendix D, Section D.2.2 for the derivation of these factors). If your ambient temperature is between 25 C o and 50 C, you may use this method to calculate the release rate, and then use the appropriate Temperature 

Correction Factor from Exhibit B-4, Appendix B, to adjust the release rate, as described below. For gases 

liquefied by refrigeration, use the Liquid Factor Boiling (LFB) and DF from Exhibit B-1. Calculate the 

release rate from the following equation for liquids at ambient temperature with no mitigation: 

QR QS � × 2.4 × LFA × DF	 (7-8) 

where:	 QR = Release rate (pounds per minute) 

QS = Quantity released (pounds) 

2.4	 = Wind speed factor = 3.00.78, where 3.0 meters per second (6.7 miles per 

hour) is the wind speed for the alternative scenario for purposes of this 

guidance 

LFA = Liquid Factor Ambient
 

DF = Density Factor
 

This method assumes that the total quantity of liquid released spreads out to form a pool one 

centimeter in depth; it does not take into account evaporation as the liquid is released. 

Example 23.	 Evaporation from Pool Formed by Liquid Released from Hole in Tank (Allyl Alcohol) 

In Example 21, 9,830 pounds of allyl alcohol were estimated to be released from a hole in a tank. From Exhibit 

B-2, the Density Factor for allyl alcohol is 0.58, and the Liquid Factor Ambient is 0.0046. Assuming that the 

liquid is not released into a diked area or inside a building, the evaporation rate from the pool of allyl alcohol, 

from Equation 7-8, is: 

QR = 9,830 × 2.4 × 0.0046 × 0.58 = 63 pounds per minute 

Elevated temperature. For pools with no mitigation, if the liquid is at an elevated temperature (above o 50 C or at or close to its boiling point), find the Liquid Factor Boiling (LFB) and the Density Factor (DF) in 

Exhibit B-2 of Appendix B (see Appendix D, Section D.2.2, for the derivation of these factors). For liquids o o at temperatures between 25 C and 50 C, you may use the method above for ambient temperature and apply 

the appropriate Temperature Correction Factor from Appendix B, Exhibit B-4, to the result, as discussed o below. For liquids above 50 C, or close to their boiling points, or with no Temperature Correction Factors 

available, calculate the release rate of the liquid from the following equation: 

QR QS � × 2.4 × LFB × DF	 (7-9) 

April 15, 1999	 7 - 11 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

where:	 QR = Release rate (pounds per minute) 

QS = Quantity released (pounds) 

2.4	 = Wind speed factor = 3.00.78, where 3.0 meters per second (6.7 miles per 

hour) is the wind speed for the alternative scenario for purposes of this 

guidance 

LFB = Liquid Factor Boiling
 

DF = Density Factor
 

Mitigated 

Diked Areas. If the toxic liquid will be released into an area where it will be contained by dikes, 

compare the diked area to the maximum area of the pool that could be formed, as described in Section 3.2.3 

(see Equation 3-6). Also verify that the quantity spilled will be totally contained by the dikes. The smaller of 

the two areas should be used in determination of the evaporation rate. If the maximum area of the pool is 

smaller than the diked area, calculate the release rate as described for pools with no mitigation (above). If the 

diked area is smaller, and the spill will be totally contained, go to Exhibit B-2 in Appendix B to find the 

Liquid Factor Ambient (LFA), if the liquid is at ambient temperature, or the Liquid Factor Boiling (LFB), if o o the liquid is at an elevated temperature. For temperatures between 25 C and 50 C, you may use the 

appropriate Temperature Correction Factor from Exhibit B-4, Appendix B, to adjust the release rate, as 

described below. For gases liquefied by refrigeration, use the LFB. Calculate the release rate from the diked 

area as follows for liquids at ambient temperature: 

QR � 2.4 × LFA × A	 (7-10) 

or, for liquids at elevated temperatures or gases liquefied by refrigeration alone: 

QR � 2.4 × LFB × A (7-11) 

where:	 QR = Release rate (pounds per minute) 

2.4	 = Wind speed factor = 3.00.78, where 3.0 meters per second (6.7 miles per 

hour) is the wind speed for the alternative scenario for purposes of this 

guidance 

LFA = Liquid Factor Ambient (listed in Exhibit B-2, Appendix B)
 

LFB = Liquid Factor Boiling (listed in Exhibit B-1 or B-2, Appendix B)
 

A = Diked area (square feet)
 

Releases Into Buildings. If a toxic liquid is released inside a building, compare the area of the 

building floor or any diked area that would contain the spill to the maximum area of the pool that could be 

formed; the smaller of the two areas should be used in determining the evaporation rate, as for the worst-case 

scenario. The maximum area of the pool is determined from Equation 3-6 in Section 3.2.3 for releases into 

diked areas. The area of the building floor is the length times width of the floor (in feet) (Equation 3-9). 

If the floor area or diked area is smaller than the maximum pool size, estimate the outdoor 

evaporation rate from a pool the size of the floor area or diked area from Equation 7-10. If the maximum 

pool area is smaller, estimate the outdoor evaporation rate from a pool of maximum size from Equation 7-8. 

April 15, 1999	 7 - 12 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

Estimate the rate of release of the toxic vapor from the building as five percent of the calculated outdoor 

evaporation rate (multiply your evaporation rate by 0.05). See Appendix D, Section D.2.4 for more 

information on releases into buildings. 

Active Mitigation to Reduce Evaporation Rate. Examples of active mitigation techniques to reduce 

the evaporation rate from the pool include water sprays and foam or tarp covering. Use test data, 

manufacturer design specifications, or past experience to determine the fractional reduction of the release rate 

by the mitigation technique. Apply this fraction to the release rate (evaporation rate from the pool) that 

would have occurred without the mitigation technique, as follows: 

QR � RV (1�FR)× QR (7-12) 

where: QRRV = Reduced evaporation rate (release rate to air) from pool (pounds per 

minute) 

FR = Fractional reduction resulting from mitigation 

QR = Evaporation rate from pool without mitigation (pounds per minute) 

o Temperature Corrections for Liquids at Temperatures between 25 and 50 C 

o o o o If your liquid is at a temperature between 25 C (77 F) and 50 C (122 F), you may use the 

appropriate Temperature Correction Factor (TCF) from Exhibit B-4, Appendix B, to calculate a corrected o o release rate. Calculate the release rate (QR) of the liquid at 25 C (77 F) as described above for unmitigated 

releases or releases in diked areas and multiply the release rate by the appropriate TCF as described in 

Section 3.2.5. 

Evaporation Rate Compared to Liquid Release Rate 

If you estimated the quantity of liquid in the pool based on an estimated liquid release rate from a 

hole in a container or pipe, as discussed in Sections 7.2.1 and 7.2.2, compare the evaporation rate with the 

liquid release rate. If the evaporation rate from the pool is greater than the liquid release rate, use the liquid 

release rate as the release rate to air. 

Duration of Release 

After you have estimated a release rate as described above, determine the duration of the vapor 

release from the pool (the time it will take for the liquid pool to evaporate completely). To estimate the time 

in minutes, divide the total quantity released (in pounds) by the release rate (in pounds per minute) (see 

Equation 3-5 in Section 3.2.2). If you are using the liquid release rate as the release rate to air, as discussed 

in the preceding paragraph, estimate a liquid release duration as discussed in Section 7.2.1 or 7.2.2. The 

duration could be the time it would take to stop the release or the time it would take to empty the tank or to 

release all the liquid above the level of the leak. If you have corrected the release rate for a temperatures o above 25 C, use the corrected release rate to estimate the duration. 

April 15, 1999 7 - 13 
Chapter 7 

Estimation of Release Rates for Alternative Scenarios for Toxic Substances 

7.2.4 Common Water Solutions and Oleum 

You may use the methods described above in Sections 7.2.1, 7.2.2, and 7.2.3 for pure liquids to 

estimate the quantity of a solution of a toxic substance or oleum that may be spilled into a pool. LFA, DF, 

and LLF values for several concentrations of ammonia, formaldehyde, hydrochloric acid, hydrofluoric acid, 

and nitric acid in water solution and for oleum are listed in Appendix B, Exhibit B-3. The LFA for a wind 

speed of 3.0 meters per second (6.7 miles per hour) should be used in the release rate calculations for 

alternative scenarios for pools of solutions at ambient temperature. 

For unmitigated releases or releases with passive mitigation, follow the instructions in Section 7.2.3. 

If active mitigation measures are in place, you may estimate a reduced release rate from the instructions on 

active mitigation in Section 7.2.2. Use the total quantity of the solution as the quantity released from the 

vessel or pipeline (QS) in carrying out the calculation of the release rate to the atmosphere. 

If the solution is at an elevated temperature, see Section 3.3. As discussed in Section 3.3, you may 

treat the release of the substance in solution as a release of the pure substance. Alternatively, if you have 

vapor pressure data for the solution at the release temperature, you may estimate the release rate from the 

equations in Appendix D, Sections D.2.1 and D.2.2. 

If you estimated the quantity of solution in the pool based on an estimated liquid release rate from a 

hole in a container or pipe, as discussed in Sections 7.2.1 and 7.2.2, compare the evaporation rate with the 

liquid release rate. If the evaporation rate from the pool is greater than the liquid release rate, use the liquid 

release rate as the release rate to air. 

April 15, 1999 7 - 14 
8 ESTIMATION OF DISTANCE TO THE ENDPOINT FOR
 

ALTERNATIVE SCENARIOS FOR TOXIC SUBSTANCES
 

In Chapter 8 

� Reference tables of distances for alternative releases, including: 

-- Generic reference tables (Exhibit 4), and 

-- Chemical-specific reference tables (Exhibit 5). 

� Considerations include: 

-- Gas density (neutrally buoyant or dense), 

-- Duration of release (10 minutes or 60 minutes), 

-- Topography (rural or urban). 

For estimating consequence distances for alternative scenarios for toxic substances, this guidance 

provides four generic reference tables for neutrally buoyant gases and vapors and four for dense gases. The 

generic reference tables of distances (Reference Tables 14-21) are found at the end of Chapter 10. The 

generic tables and the conditions for which each table is applicable are described in Exhibit 4. Four chemicalspecific tables also are provided for ammonia, chlorine, and sulfur dioxide. The chemical-specific reference 

tables follow the generic reference tables at the end of Chapter 10. These tables, and the applicable 

conditions, are described in Exhibit 5. 

All the reference tables of distances for alternative scenarios were developed assuming D stability 

and a wind speed of 3.0 meters per second (6.7 miles per hour) as representative of likely conditions for many 

sites. Many wind speed and atmospheric stability combinations may be possible at different times in 

different parts of the country. If D stability and 3.0 meters per second are not reasonable conditions for your 

site, you may want to use other methods to estimate distances. 

For simplicity, this guidance assumes ground level releases. This guidance, therefore, may 

overestimate the consequence distance if your alternative scenario involves a release above ground level, 

particularly for neutrally buoyant gases and vapors. If you want to assume an elevated release, you may want 

to consider other methods to determine the consequence distance. 

The generic reference tables should be used for all toxic substances other than ammonia, chlorine, 

and sulfur dioxide. To use the generic reference tables, you need to consider the release rates estimated for 

gases and evaporation from liquid pools and the duration of the release. For the alternative scenarios, the 

duration of toxic gas releases may be longer than the 10 minutes assumed for the worst-case analysis for 

gases. You need to determine the appropriate toxic endpoint and whether the gas or vapor is neutrally 

buoyant or dense, using the tables in Appendix B and considering the conditions of the release. You may 

interpolate between entries in the reference tables. 

April 15, 1999 
Chapter 8 

Estimation of Distance to the Endpoint for Alternative Scenarios for Toxic Substances 

Exhibit 4
 

Generic Reference Tables of Distances for Alternative Scenarios
 

Applicable Conditions Reference Table 

Number 

Gas or Vapor Density Topography Release Duration 

(minutes) 

Neutrally buoyant Rural 10 14 

60 15 

Urban 10 16 

60 17 

Dense Rural 10 18 

60 19 

Urban 10 20 

60 21 

Exhibit 5
 

Chemical-Specific Reference Tables of Distances for Alternative Scenarios
 

Substance 

Conditions of Release 

Table 

Reference 

Gas or Vapor Number 

Density 

Release Duration 

(minutes) 

Topography 

Anhydrous ammonia 

liquefied under pressure 

Dense 10-60 Rural, urban 22 

Non-liquefied ammonia, 

ammonia liquefied by 

refrigeration, or aqueous 

ammonia 

Neutrally buoyant 10-60 Rural, urban 23 

Chlorine Dense 10-60 Rural, urban 24 

Sulfur dioxide (anhydrous) Dense 10-60 Rural, urban 25 

April 15, 1999 8 - 2 
Chapter 8 

Estimation of Distance to the Endpoint for Alternative Scenarios for Toxic Substances 

Note the following concerning the use of the chemical-specific reference tables for ammonia, 

chlorine, and sulfur dioxide: 

The table for anhydrous ammonia (Reference Table 22) applies only to flashing releases of 

ammonia liquefied under pressure. Use Table 23 for release of ammonia as a gas (e.g., 

evaporation from a pool or release from the vapor space of a tank). 

�

� You may use these tables for releases of any duration. 

To use the reference tables of distances, follow these steps: 

For Regulated Toxic Substances Other than Ammonia, Chlorine, and Sulfur Dioxide 

Find the toxic endpoint for the substance in Appendix B (Exhibit B-1 for toxic gases or 

Exhibit B-2 for toxic liquids). 

�

Determine whether the table for neutrally buoyant or dense gases and vapors is appropriate 

from Appendix B (Exhibit B-1 for toxic gases or Exhibit B-2, column for alternative case, 

for toxic liquids). A toxic gas that is lighter than air may behave as a dense gas upon release 

if it is liquefied under pressure, because the released gas may be mixed with liquid droplets, 

or if it is cold. Consider the state of the released gas when you decide which table is 

appropriate. 

�

� Determine whether the table for rural or urban conditions is appropriate. 

-- Use the rural table if your site is in an open area with few obstructions. 

-- Use the urban table if your site is in an urban or obstructed area. 

� Determine whether the 10-minute table or the 60-minute table is appropriate. 

-- Use the 10-minute table for releases from evaporating pools of common water 

solutions and of oleum. 

-- If you estimated the release duration for gas release or pool evaporation to be 10 

minutes or less, use the 10-minute table. 

-- If you estimated the release duration for gas release or pool evaporation to be more 

than 10 minutes, use the 60-minute table. 

Neutrally Buoyant Gases or Vapors 

If Exhibit B-1 or B-2 indicates the gas or vapor should be considered neutrally buoyant, and 

other factors would not cause the gas or vapor to behave as a dense gas, divide the estimated 

release rate (pounds per minute) by the toxic endpoint (milligrams per liter). 

�

April 15, 1999 8 - 3 
Chapter 8 

Estimation of Distance to the Endpoint for Alternative Scenarios for Toxic Substances 

Find the range of release rate/toxic endpoint values that includes your calculated release 

rate/toxic endpoint in the first column of the appropriate table (Reference Table 14, 15, 16, 

or 17), then find the corresponding distance to the right. 

�

Dense Gases or Vapors 

If Exhibit B-1 or B-2 or consideration of other relevant factors indicates the substance 

should be considered a dense gas or vapor (heavier than air), find the distance in the 

appropriate table (Reference Table 18, 19, 20, or 21) as follows; 

�

-- Find the toxic endpoint closest to that of the substance by reading across the top of 

the table. If the endpoint of the substance is halfway between two values on the 

table, choose the value on the table that is smaller (to the left). Otherwise, choose 

the closest value to the right or the left. 

-- Find the release rate closest to the release rate estimated for the substance at the left 

of the table. If the calculated release rate is halfway between two values on the 

table, choose the release rate that is larger (farther down on the table). Otherwise, 

choose the closest value (up or down on the table). 

-- Read across from the release rate and down from the endpoint to find the distance 

corresponding to the toxic endpoint and release rate for your substance. 

For Ammonia, Chlorine, or Sulfur Dioxide 

Find the appropriate chemical-specific table for your substance (see the descriptions of 

Reference Tables 22-25 in Exhibit 5). 

�

-- If you have ammonia liquefied by refrigeration alone, you may use Reference Table 

23, even if the duration of the release is greater than 10 minutes. 

-- If you have chlorine or sulfur dioxide liquefied by refrigeration alone, you may use 

the chemical-specific reference tables, even if the duration of the release is greater 

than 10 minutes. 

� Determine whether rural or urban topography is applicable to your site. 

-- Use the rural column in the reference table if your site is in an open area with few 

obstructions. 

-- Use the urban column if your site is in an urban or obstructed area. 

� Estimate the consequence distance as follows: 

-- In the left-hand column of the table, find the release rate closest to your calculated 

release rate. 

April 15, 1999 8 - 4 
Chapter 8 

Estimation of Distance to the Endpoint for Alternative Scenarios for Toxic Substances 

-- Read the corresponding distance from the appropriate column (urban or rural) to the 

right. 

The development of the generic reference tables is discussed in Appendix D, Sections D.4.1 and 

D.4.2. The development of the chemical-specific reference tables is discussed in industry-specific risk 

management program guidance documents and a backup information document that are cited in Section 

D.4.3. If you think the results of the method presented here overstate the potential consequences of a your 

alternative release scenario, you may choose to use other methods or models that take additional site-specific 

factors into account. 

Examples 24 and 25 below include the results of modeling using two other models, ALOHA and 

WHAZAN, for comparison with the results of the methods presented in this guidance. Appendix D, Section 

D.4.5 provides additional information on this modeling. 

Example 24. Gas Release of Chlorine 

Assume that you calculated a release rate of 500 pounds per minute of chlorine from a tank. A chemicalspecific table is provided for chlorine, so you do not need to consult Appendix B for information on chlorine. 

The topography of your site is urban. For a release of chlorine under average meteorology (D stability and 3 

meters per second wind speed), go to Reference Table 24. The estimated release rate of 500 pounds per 

minute, with urban topography, corresponds to a consequence distance of 0.4 miles. 

Additional Modeling for Comparison 

The ALOHA model gave a distance of 3.0 miles to the endpoint, using the same assumptions. 

The WHAZAN model gave a distance of 3.2 miles to the endpoint, using the same assumptions and the dense 

cloud dispersion model. 

April 15, 1999 8 - 5 
Chapter 8 

Estimation of Distance to the Endpoint for Alternative Scenarios for Toxic Substances 

Example 25. Allyl Alcohol Evaporating from Pool 

In Example 23, the evaporation rate of allyl alcohol from a pool was calculated as 63 pounds per minute. The total 

quantity in the pool was estimated as 9,830 pounds; therefore, the pool would evaporate in 9,830/63 or 156 

minutes. You would use a 60-minute reference table to estimate the distance to the endpoint. From Exhibit B-2 in 

Appendix B, the toxic endpoint for allyl alcohol is 0.036 mg/L, and the appropriate reference table for the 

alternative scenario analysis is a neutrally buoyant plume table. To find the distance from the neutrally buoyant 

plume tables, you need the release rate divided by the endpoint. In this case, it is 63/0.036, or 1,750. Assuming 

the release takes place in a rural location, you use Reference Table 15, applicable to neutrally buoyant plumes, 60-

minute releases, and rural conditions. From this table, you estimate the distance as 0.4 mile. 

Additional Modeling for Comparison 

The ALOHA model gave a distance of 0.7 mile to the endpoint for a release rate of 63 pounds per minute, using 

the same assumptions and the dense gas model. 

The WHAZAN model gave a distance of 0.5 mile to the endpoint for a release rate of 63 pounds per minute, using 

the same assumptions and the buoyant plume dispersion model. 

April 15, 1999 8 - 6 
9 ESTIMATION OF RELEASE RATES FOR ALTERNATIVE 

SCENARIOS FOR FLAMMABLE SUBSTANCES 

In Chapter 9 

�	 Methods to estimate a release rate to air for a flammable gas (9.1) or liquid 

(9.2). 

9.1	 Flammable Gases 

Gaseous Release from Tank or Pipe 

An alternative scenario for a release of a flammable gas may involve a leak from a vessel or piping. 

To estimate a release rate for flammable gases from hole size and storage conditions, you may use the method 

described above in Section 7.1.1 for toxic gases. This release rate may be used to determine the dispersion 

distance to the lower flammability limit (LFL), as described in Section 10.1. Exhibit C-2 in Appendix C 

includes Gas Factors (GF) that may be used in carrying out the calculations for each of the regulated 

flammable gases. 

Example 26. Release Rate of Flammable Gas from Hole in Tank (Ethylene) 

A pipe tears off a tank containing ethylene. The pipe is in the vapor space of the tank. The release rate from 

the hole can be estimated from Equation 7-1 in Section 7.1. You estimate that the pipe would leave a hole with 

an area (HA) of 5 square inches. The temperature inside the tank (T , absolute temperature, Kelvin) is 282 K, t 

9�C, and the square root of the temperature is 16.8. The pressure in the tank (P ) is approximately 728 pounds t 

per square inch absolute (psia). From Exhibit C-2, Appendix C, the gas factor (GF) for ethylene is 18. From 

Equation 7-1, the release rate (QR) is: 

QR = 5 × 728 × (1/16.8) × 18 = 3,900 pounds per minute 

Gases Liquefied Under Pressure 

A vapor cloud fire is a possible result of a release of a gas liquefied under pressure. You may use the 

methods described in Section 7.1.1 for toxic gases liquefied under pressure to estimate the release rate from a 

hole in a tank for a flammable gas liquefied under pressure. The estimated release rate may be used to 

estimate the dispersion distance to the LFL for a vapor cloud fire. 

Flammable gases that are liquefied under pressure may be released very rapidly, with partial 

vaporization of the liquefied gas and possible aerosol formation. Section 10.4 presents a method for 

estimating the consequences of a vapor cloud explosion from such a release of a gas liquefied under pressure. 

April 15, 1999 
Chapter 9 

Estimation of Release Rates for Alternative Scenarios for Flammable Substances 

Gases Liquefied by Refrigeration 

Flammable gases liquefied by refrigeration alone can be treated as liquids for the alternative scenario 

analysis, as discussed in Section 9.2 and Section 10.2, below. 

9.2 Flammable Liquids 

You may estimate a release rate for flammable liquids by estimating the evaporation rate from a 

pool. Release rates also can be estimated for flammable gases liquefied by refrigeration alone by this method, 

if the liquefied gas is likely to form a pool upon release. You first need to estimate the quantity in the pool. 

You may use the method discussed in Section 7.2.1 to estimate a rate of liquid release for flammable 

liquids into a pool from a hole in a tank or from a pipe shear. Exhibit C-3 in Appendix C includes liquid leak 

factors (LLF) for calculating release rate from a hole. Note that the LLF is appropriate only for atmospheric 

tanks. LLF values are not provided for liquefied flammable gases; you will need to estimate the quantity in 

the pool from other information for liquefied flammable gases. 

Once you have an estimate of the quantity of flammable liquid in a pool, you may use the methods 

presented in Section 7.2.3 to estimate the evaporation rate from the pool. Liquid factors at ambient and 

boiling temperature (LFA and LFB) for liquids for the calculation are listed in Exhibit C-3 in Appendix C, 

and LFBs for liquefied gases are listed in Exhibit C-2. Both passive and active mitigation measures 

(discussed in Sections 7.2.2 and 7.2.3) may be taken into account. You do not need to estimate the duration 

of the release, because this information is not used to estimate distance to the LFL, as discussed in the next 

chapter. 

As for toxic liquids, if the rate of evaporation of the liquid from the pool is greater than the rate of 

release of the liquid from the container, you should use the liquid release rate, not the pool evaporation rate, 

as the rate of release to the air. You should expect rapid evaporation rates for liquefied flammable gases 

from a pool. All of the regulated flammable liquids are volatile, so the evaporation rate from a pool may be 

expected to be relatively high, particularly without mitigation. 

April 15, 1999 9 - 2 
10 ESTIMATION OF DISTANCE TO THE ENDPOINT FOR 

ALTERNATIVE SCENARIOS FOR FLAMMABLE SUBSTANCES 

In Chapter 10 

� 10.1 Method to estimate the dispersion distance to the LFL for vapor cloud 

fires. 

� 10.2 Method to estimate the distance to the heat radiation endpoint for a pool 

fire involving a flammable liquid, based on the pool area and factors provided 

in the appendix. 

� 10.3 Method to estimate the distance to the heat radiation endpoint for a 

fireball from a BLEVE, using a reference table of distances. 

� 10.4 Alternative scenario analysis for vapor cloud explosions, using less 

conservative assumptions than for worst-case vapor cloud explosions. 

10.1 Vapor Cloud Fires 

The distance to the LFL represents the maximum distance at which the radiant heat effects of a vapor 

cloud fire might have serious consequences. Exhibit C-2, Appendix C, provides LCL data (in volume percent 

and milligrams per liter) for listed flammable gases; Exhibit C-3 provides these data for flammable liquids. 

This guidance provides reference tables for the alternative scenario conditions assumed in this guidance (D 

stability and wind speed 3.0 meters per second, ground level releases) for estimating the distance to the LCL. 

Release rate is the primary factor for determining distance to the flammable endpoint. Because the methods 

used in this guidance assumes that the vapor cloud release is in a steady state and that vapor cloud fires are 

nearly instantaneous events, release duration is not a critical factor for estimating vapor cloud fire distances. 

Thus, the reference tables for flammable substances are not broken out separately by release duration (e.g., 

10 minutes, 60 minutes). The development of these tables is discussed further in Appendix D, Section D.4. 

The reference tables for flammable substances (Reference Tables 26-29 at the end of Chapter 10) are listed in 

Exhibit 6. 

To use the reference tables of distances to find the distance to the LFL from the release rate, follow 

these steps: 

Find the LFL endpoint for the substance in Appendix C (Exhibit C-2 for flammable gases or 

Exhibit C-3 for flammable liquids). 

�

Determine from Appendix C whether the table for neutrally buoyant or dense gases and 

vapors is appropriate (Exhibit C-2 for flammable gases or Exhibit C-3 for flammable 

liquids). A gas that is lighter than air may behave as a dense gas upon release if it is 

liquefied under pressure, because the released gas may be mixed with liquid droplets, or if it 

is cold. Consider the state of the released gas when you decide which table is appropriate. 

�

April 15, 1999 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

Determine whether the table for rural or urban conditions is appropriate. 

-- Use the rural table if your site is in an open area with few obstructions. 

-- Use the urban table if your site is in an urban or obstructed area. 

�

Exhibit 6 

Reference Tables of Distances for Vapor Cloud Fires of Flammable Substances 

Applicable Conditions Reference Table 

Number 

Gas or Vapor Density Topography Release Duration 

(minutes) 

Neutrally buoyant Rural 10 - 60 26 

Urban 10 - 60 27 

Dense Rural 10 - 60 28 

Urban 10 - 60 29 

Neutrally Buoyant Gases or Vapors 

If Exhibit C-2 or C-3 indicates the gas or vapor should be considered neutrally buoyant, and 

other factors would not cause the gas or vapor to behave as a dense gas, divide the estimated 

release rate (pounds per minute) by the LFL endpoint (milligrams per liter). 

�

Find the range of release rate/LFL values that includes your calculated release rate/LFL in 

the first column of the appropriate table (Reference Table 26 or 27), then find the 

corresponding distance to the right. 

�

Dense Gases or Vapors 

If Exhibit C-2 or C-3 or consideration of other relevant factors indicates the substance 

should be considered a dense gas or vapor (heavier than air), find the distance in the 

appropriate table (Reference Table 28 or 29) as follows: 

�

-- Find the LFL closest to that of the substance by reading across the top of the table. 

If the LFL of the substance is halfway between two values on the table, choose the 

value on the table that is smaller (to the left). Otherwise, choose the closest value to 

the right or the left. 

-- Find the release rate closest to the release rate estimated for the substance at the left 

of the table. If the calculated release rate is halfway between two values on the 

April 15, 1999 10 - 2 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

table, choose the release rate that is larger (farther down on the table). Otherwise, 

choose the closest value (up or down on the table). 

-- Read across from the release rate and down from the LFL to find the distance 

corresponding to the LFL and release rate for your substance. 

Example 27. Flammable Gas Release (Ethylene) 

In Example 26, you estimated a release rate for ethylene from a hole in a tank of 3,900 pounds per minute. You 

want to estimate the distance to the LFL for a vapor cloud fire resulting from this release. 

From Exhibit C-2, Appendix C, the LFL for ethylene is 31 mg/L, and the appropriate table for distance 

estimation is a neutrally buoyant gas table for flammable substances. Your site is in a rural area, so you would 

use Reference Table 26. 

To use the neutrally buoyant gas tables, you need to calculate release rate/endpoint. In this case, release 

rate/LFL = 3,900/31 or 126. On Reference Table 26, 126 falls in the range of release rate/LFL values 

corresponding to 0.2 miles. 

April 15, 1999 10 - 3 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

Example 28. Vapor Cloud Fire from Evaporating Pool of Flammable Liquid 

You have a tank containing 20,000 pounds of ethyl ether. A likely scenario for a release might be shearing of a 

pipe from the tank, with the released liquid forming a pool. You want to estimate the consequences of a vapor 

cloud fire that might result from evaporation of the pool and ignition of the vapor. 

You first need to estimate the rate of release of the liquid from the tank. You can do this using Equation 7-4, 

Section 7.2.1. For this calculation, you need the area of the hole that would result from shearing the pipe (HA), 

the height of the liquid in the tank above the hole (LH), and the liquid leak factor (LLF) for ethyl ether, from 

Exhibit C-3 in Appendix C. The pipe diameter is 2 inches, so the cross sectional area of the hole would be 3.1 

square inches. You estimate that the pipe is 2 feet, or 24 inches, below the level of the liquid when the tank is 

full. The square root of LH (24 inches) is 4.9. LLF for ethyl ether is 34. From Equation 7-4, the rate of release 

of the liquid from the hole is calculated as: 

QRL	 = 3.1 × 4.9 × 34 

= 520 pounds per minute 

You estimate that the release of the liquid could be stopped in about 10 minutes. In 10 minutes, 10 × 520, or 

5,200 pounds, would be released. 

The liquid would be released into an area without dikes. To estimate the evaporation rate from the pool formed 

by the released liquid, you use Equation 7-8 from Section 7.2.3. To carry out the calculation, you need the 

Liquid Factor Ambient (LFA) and the Density Factor (DF) for ethyl ether. From Exhibit C-3, Appendix C, 

LFA for ethyl ether is 0.11 and DF is 0.69. The release rate to air is: 

QR	 = 5,200 × 2.4 × 0.11 × 0.69 

= 950 pounds per minute 

The evaporation rate from the pool is greater than the estimated liquid release rate; therefore, you use the liquid 

release rate of 520 pounds per minute as the release rate to air. To estimate the maximum distance at which 

people in the area of the vapor cloud could suffer serious injury, estimate the distance to the lower flammability 

limit (LFL) (in milligrams per liter) for ethyl ether, from the appropriate reference table. From Exhibit C-3, 

Appendix C, LFL for ethyl ether is 57 mg/L, and the appropriate reference table is a dense gas table. Your site 

is in a rural area with few obstructions, so you use Reference Table 28. 

From Reference Table 28, the closest LFL is 60 mg/L. The lowest release rate on the table is 1,500 pounds per 

minute, which is higher than the evaporation rate estimated for the pool of ethyl ether. For a release rate less 

than 1,500 pounds per minute, the distance to the LFL is less than 0.1 miles. 

April 15, 1999	 10 - 4 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

10.2 Pool Fires 

Pool fires may be considered as potential alternative scenarios for flammable liquids, including gases 

liquefied by refrigeration alone. You may find, however, that other scenarios will give a greater distance to 

the endpoint and, therefore, may be more appropriate as alternative scenarios. A "Pool Fire Factor" (PFF) 

has been derived for each of the regulated flammable liquids and most of the flammable gases to aid in the 

consequence analysis. The derivation of these factors is discussed in Appendix D, Section D.9. The PFF, 

listed in Appendix C, Exhibit C-2 for flammable gases and C-3 for flammable liquids, may be used to 

estimate a distance from the center of a pool fire where people could potentially receive second degree burns 

from a 40-second exposure. The heat radiation endpoint for this analysis is 5 kilowatts per square meter 2 o o (kW/m ). Ambient temperature is assumed to be 25 C (77 F) for calculation of the PFF for flammable 

liquids. 

To estimate a distance using the PFF, you first need to estimate the size of the pool, in square feet, 

that might be formed by the release of a flammable substance. You may use the methods described above for 

toxic liquids to estimate pool size. Density factors (DF) for the estimation of pool size in undiked areas may 

be found for flammable gases and flammable liquids in Exhibits C-2 and C-3 of Appendix C. For flammable 

gases, the DF is based on the density at the boiling point. You may want to consider whether the released 

substance may evaporate too quickly to form a pool of the maximum size, particularly for liquefied gases. 

Distances may be estimated from the PFF and the pool area as follows: 

d � PFF × A (10-1) 

where: d = Distance (feet) 

PFF = Pool Fire Factor (listed for each flammable substance in Appendix C, 

Exhibits C-2 and C-3) 

A = Pool area (square feet) 

April 15, 1999 10 - 5 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

Example 29. Pool Fire of Flammable Liquid 

For a tank containing 20,000 pounds of ethyl ether, you want to estimate the consequences of a pool fire. You 

estimate that 15,000 pounds would be released into an area without dikes, forming a pool. Assuming the liquid 

spreads to a depth of 1 centimeter (0.39 inches), you estimate the area of the pool formed from Equation 3-6, 

Section 3.2.3. For this calculation, you need the density factor (DF) for ethyl ether; from Exhibit C-3, 

Appendix C, DF for ethyl ether is 0.69. From Equation 3-6, the area of the pool is: 

A = 15,000 × 0.69 = 10,400 square feet 

You can use Equation 10-1 to estimate the distance from the center of the burning pool where the heat radiation 

level would reach 5 kW/m . 2 For the calculation, you need the square root of the pool area (A) and the pool fire 

factor (PFF) for ethyl ether. The square root of A, 10,400 square feet, is 102 feet. From Exhibit C-3, 2 Appendix C, PFF for ethyl ether is 4.3. From Equation 10-1, the distance (d) to 5 kW/m is: 

d = 4.3 × 102 = 440 feet (about 0.08 miles) 

If you have a gas that is liquefied under pressure or under a combination of pressure and 

refrigeration, a pool fire is probably not an appropriate alternative scenario. A fire or explosion involving the 

flammable gas that is released to the air by a sudden release of pressure is likely to have the potential for 

serious effects at a greater distance than a pool fire (e.g., see the methods for analysis of BLEVEs and vapor 

cloud explosions in Sections 10.3 and 10.4 below, or see Appendix A for references that provide more 

information on consequence analysis for fires and explosions). 

10.3 BLEVEs 

If a fireball from a BLEVE is a potential release scenario at your site, you may use Reference Table 

30 to estimate the distance to a potentially harmful radiant heat level. The table shows distances for a range 

of quantities to the radiant heat level that potentially could cause second degree burns to a person exposed for 

the duration of the fire. The quantity you use should be the total quantity in a tank that might be involved in a 

BLEVE. The equations used to derive this table of distances are presented in Appendix D, Section D.10. If 

you prefer, you may use the equations to estimate a distance for BLEVEs, or you may use a different 

calculation method or model. 

10.4 Vapor Cloud Explosion 

If you have the potential at your site for the rapid release of a large quantity of a flammable vapor, 

particularly into a congested area, a vapor cloud explosion may be an appropriate alternative release scenario. 

For the consequence analysis, you may use the same methods as for the worst case to estimate consequence 

distances to an overpressure endpoint of 1 psi (see Section 5.1 and the equation in Appendix C). Instead of 

assuming the total quantity of flammable substance released is in the vapor cloud, you may estimate a smaller 

April 15, 1999 10 - 6 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

quantity in the cloud. You could base your estimate of the quantity in the cloud on the release rate estimated 

as described above for gases and liquids multiplied by the time required to stop the release. 

To estimate the quantity in the cloud for a gas liquefied under pressure (not refrigerated), you may 

use the equation below. This equation incorporates a "flash fraction factor" (FFF), listed in Appendix C, 

Exhibit C-2 for regulated flammable gases, to estimate the quantity that could be immediately flashed into 

vapor upon release. A factor of two is included to estimate the quantity that might be carried along as spray 

or aerosol. See Appendix D, Section D.11 for the derivation of this equation. The equation is: 

QF � FFF × QS ×2 (10-2) 

where: QF = Quantity flashed into vapor plus aerosol (pounds) (cannot be larger than 

QS) 

FFF = Flash fraction factor (unitless) (listed in Appendix C, Exhibit C-2) (must be 

less than 1) 

QS = Quantity spilled (pounds) 

2 = Factor to account for spray and aerosol 

o o For derivation of the FFF, the temperature of the stored gas was assumed to be 25 C (77 F) (except 

as noted in Exhibit C-2). You may estimate the flash fraction under other conditions using the equation 

presented in Appendix D, Section D.11. 

You may estimate the distance to 1 psi for a vapor cloud explosion from the quantity in the cloud 

using Reference Table 13 (at the end of the worst-case analysis discussion) or from Equation C-1 in 

Appendix C. For the alternative scenario analysis, you may use a yield factor of 3 percent, instead of the 

yield factor of 10 percent used in the worst-case analysis. As discussed in Appendix D, Section D.11, the 

yield factor of 3 percent is representative of more likely events, based on data from past vapor cloud 

explosions. If you use the equation in Appendix C, use 0.03 instead of 0.1 in the calculation. If you use 

Reference Table 13, you can incorporate the lower yield factor by multiplying the distance you read from 

Reference Table 13 by 0.67. 

April 15, 1999 10 - 7 
Chapter 10 

Estimation of Distance to the Endpoint for Alternative Scenarios for Flammable Substances 

Example 30. Vapor Cloud Explosion (Propane) 

You have a tank containing 50,000 pounds of propane liquefied under pressure at ambient temperature. You 

want to estimate the consequence distance for a vapor cloud explosion resulting from rupture of the tank. 

You use Equation 10-2 to estimate the quantity that might be released to form a cloud. You base the 

calculation on the entire contents of the tank (QS = 50,000 pounds). From Exhibit C-2 of Appendix C, the 

Flash Fraction Factor (FFF) for propane is 0.38. From Equation 10-2, the quantity flashed into vapor, plus the 

quantity that might be carried along as aerosol, (QF) is: 

QF = 0.38 × 50,000 × 2 = 38,000 pounds 

You assume 38,000 pounds of propane is in the flammable part of the vapor cloud. This quantity falls between 

20,000 pounds and 50,000 pounds in Reference Table 13; 50,000 pounds is the quantity closest to your 

quantity. From the table, the distance to 1 psi overpressure is 0.3 mile for 50,000 pounds of propane for a 10 

percent yield factor. To change the yield factor to 3 percent, you multiply this distance by 0.67; then the 

distance becomes 0.2 mile. 

April 15, 1999 10 - 8 
Reference Table 14 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

10-Minute Release, Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 64 0.1 

64 - 510 0.2 

510 - 1,300 0.3 

1,300 - 2,300 0.4 

2,300 - 4,100 0.6 

4,100 - 6,300 0.8 

6,300 - 8,800 1.0 

8,800 - 12,000 1.2 

12,000 - 16,000 1.4 

16,000 - 19,000 1.6 

19,000 - 22,000 1.8 

22,000 - 26,000 2.0 

26,000 - 30,000 2.2 

30,000 - 36,000 2.4 

36,000 - 42,000 2.6 

42,000 - 47,000 2.8 

47,000 - 54,000 3.0 

54,000 - 60,000 3.2 

60,000 - 70,000 3.4 

70,000 - 78,000 3.6 

78,000 - 87,000 3.8 

87,000 - 97,000 4.0 

97,000 - 110,000 4.2 

110,000 - 120,000 4.4 

120,000 - 130,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

130,000 - 140,000 4.8 

140,000 - 160,000 5.0 

160,000 - 180,000 5.2 

180,000 - 190,000 5.4 

190,000 - 210,000 5.6 

210,000 - 220,000 5.8 

220,000 - 240,000 6.0 

240,000 - 261,000 6.2 

261,000 - 325,000 6.8 

325,000 - 397,000 7.5 

397,000 - 477,000 8.1 

477,000 - 566,000 8.7 

566,000 - 663,000 9.3 

663,000 - 769,000 9.9 

769,000 - 1,010,000 11 

1,010,000 - 1,280,000 12 

1,280,000 - 1,600,000 14 

1,600,000 - 1,950,000 15 

1,950,000 - 2,340,000 16 

2,340,000 - 2,770,000 17 

2,770,000 - 3,240,000 19 

3,240,000 - 4,590,000 22 

4,590,000 - 6,190,000 25 

>6,190,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 10 - 9 
Reference Table 15 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

60-Minute Release, Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 79 0.1 

79 - 630 0.2 

630 - 1,600 0.3 

1,600 - 2,800 0.4 

2,800 - 5,200 0.6 

5,200 - 7,900 0.8 

7,900 - 11,000 1.0 

11,000 - 14,000 1.2 

14,000 - 19,000 1.4 

19,000 - 23,000 1.6 

23,000 - 27,000 1.8 

27,000 - 32,000 2.0 

32,000 - 36,000 2.2 

36,000 - 42,000 2.4 

42,000 - 47,000 2.6 

47,000 - 52,000 2.8 

52,000 - 57,000 3.0 

57,000 - 61,000 3.2 

61,000 - 68,000 3.4 

68,000 - 73,000 3.6 

73,000 - 79,000 3.8 

79,000 - 84,000 4.0 

84,000 - 91,000 4.2 

91,000 - 97,000 4.4 

97,000 - 100,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

100,000 - 108,000 4.8 

108,000 - 113,000 5.0 

113,000 - 120,000 5.2 

120,000 - 126,000 5.4 

126,000 - 132,000 5.6 

132,000 - 140,000 5.8 

140,000 - 150,000 6.0 

150,000 - 151,000 6.2 

151,000 - 171,000 6.8 

171,000 - 191,000 7.5 

191,000 - 212,000 8.1 

212,000 - 233,000 8.7 

233,000 - 256,000 9.3 

256,000 - 280,000 9.9 

280,000 - 332,000 11 

332,000 - 390,000 12 

390,000 - 456,000 14 

456,000 - 529,000 15 

529,000 - 610,000 16 

610,000 - 699,000 17 

699,000 - 796,000 19 

796,000 - 1,080,000 22 

1,080,000 - 1,410,000 25 

>1,410,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 10 - 10 
Reference Table 16 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

10-Minute Release, Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to Endpoint 

(miles) 

0 - 160 0.1 

160 - 1,400 0.2 

1,400 - 3,600 0.3 

3,600 - 6,900 0.4 

6,900 - 13,000 0.6 

13,000 - 22,000 0.8 

22,000 - 31,000 1.0 

31,000 - 42,000 1.2 

42,000 - 59,000 1.4 

59,000 - 73,000 1.6 

73,000 - 88,000 1.8 

88,000 - 100,000 2.0 

100,000 - 120,000 2.2 

120,000 - 150,000 2.4 

150,000 - 170,000 2.6 

170,000 - 200,000 2.8 

200,000 - 230,000 3.0 

230,000 - 260,000 3.2 

260,000 - 310,000 3.4 

310,000 - 340,000 3.6 

340,000 - 390,000 3.8 

390,000 - 430,000 4.0 

430,000 - 490,000 4.2 

490,000 - 540,000 4.4 

540,000 - 600,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

600,000 - 660,000 4.8 

660,000 - 720,000 5.0 

720,000 - 810,000 5.2 

810,000 - 880,000 5.4 

880,000 - 950,000 5.6 

950,000 - 1,000,000 5.8 

1,000,000 - 1,100,000 6.0 

1,100,000 - 1,220,000 6.2 

1,220,000 - 1,530,000 6.8 

1,530,000 - 1,880,000 7.5 

1,880,000 - 2,280,000 8.1 

2,280,000 - 2,710,000 8.7 

2,710,000 - 3,200,000 9.3 

3,200,000 - 3,730,000 9.9 

3,730,000 - 4,920,000 11 

4,920,000 - 6,310,000 12 

6,310,000 - 7,890,000 14 

7,890,000 - 9,660,000 15 

9,660,000 - 11,600,000 16 

11,600,000 - 13,800,000 17 

13,800,000 - 16,200,000 19 

16,200,000 - 23,100,000 22 

23,100,000 - 31,300,000 25 

>31,300,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 10 - 11 
Reference Table 17 

Neutrally Buoyant Plume Distances to Toxic Endpoint for Release Rate Divided by Endpoint 

60-Minute Release, Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 200 0.1 

200 - 1,700 0.2 

1,700 - 4,500 0.3 

4,500 - 8,600 0.4 

8,600 - 17,000 0.6 

17,000 - 27,000 0.8 

27,000 - 39,000 1.0 

39,000 - 53,000 1.2 

53,000 - 73,000 1.4 

73,000 - 90,000 1.6 

90,000 - 110,000 1.8 

110,000 - 130,000 2.0 

130,000 - 150,000 2.2 

150,000 - 170,000 2.4 

170,000 - 200,000 2.6 

200,000 - 220,000 2.8 

220,000 - 240,000 3.0 

240,000 - 270,000 3.2 

270,000 - 300,000 3.4 

300,000 - 320,000 3.6 

320,000 - 350,000 3.8 

350,000 - 370,000 4.0 

370,000 - 410,000 4.2 

410,000 - 430,000 4.4 

430,000 - 460,000 4.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

460,000 - 490,000 4.8 

490,000 - 520,000 5.0 

520,000 - 550,000 5.2 

550,000 - 580,000 5.4 

580,000 - 610,000 5.6 

610,000 - 640,000 5.8 

640,000 - 680,000 6.0 

680,000 - 705,000 6.2 

705,000 - 804,000 6.8 

804,000 - 905,000 7.5 

905,000 - 1,010,000 8.1 

1,010,000 - 1,120,000 8.7 

1,120,000 - 1,230,000 9.3 

1,230,000 - 1,350,000 9.9 

1,350,000 - 1,620,000 11 

1,620,000 - 1,920,000 12 

1,920,000 - 2,250,000 14 

2,250,000 - 2,620,000 15 

2,620,000 - 3,030,000 16 

3,030,000 - 3,490,000 17 

3,490,000 - 3,980,000 19 

3,980,000 - 5,410,000 22 

5,410,000 - 7,120,000 25 

>7,120,000 >25* 

 *Report distance as 25 miles 

April 15, 1999 10 - 12 
Reference Table 18

Dense Gas Distances to Toxic Endpoint

10-minute Release, Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 0.6 0.4 0.4 0.2 0.2 0.1 0.1 0.1 <0.1 <0.1 # # # 

2 0.9 0.6 0.5 0.4 0.3 0.2 0.2 0.1 0.1 0.1 <0.1 <0.1 # 

5 1.4 1.1 0.9 0.6 0.4 0.4 0.3 0.2 0.2 0.1 0.1 0.1 <0.1 

10 2.0 1.5 1.2 0.9 0.6 0.5 0.4 0.4 0.2 0.2 0.1 0.1 0.1 

30 3.7 2.7 2.2 1.5 1.1 0.9 0.7 0.7 0.5 0.3 0.3 0.2 0.2 

50 5.0 3.7 3.0 2.1 1.9 1.2 1.0 0.9 0.6 0.4 0.4 0.3 0.2 

100 7.4 5.3 4.3 3.0 2.3 1.7 1.4 1.2 0.9 0.6 0.6 0.4 0.4 

150 8.7 6.8 5.5 3.8 2.8 2.3 1.9 1.6 1.1 0.8 0.7 0.6 0.5 250 12 8.7 7.4 5.0 3.7 3.0 2.4 2.1 1.4 1.1 0.9 0.7 0.5 500 17 13 11 7.4 5.3 4.5 3.6 3.0 2.1 1.6 1.3 1.1 0.9 750 22 16 13 9.3 6.8 5.6 4.5 3.8 2.7 1.9 1.6 1.3 1.1 

1,000 >25 19 16 11 8.1 6.8 5.2 4.5 3.1 2.3 2.2 1.5 1.3 

1,500 * 23 19 13 9.9 8.1 6.8 5.6 3.9 2.9 2.4 1.9 1.6 

2,000 * >25 22 15 12 9.3 7.4 6.8 4.5 3.4 2.7 2.2 1.9 

2,500 * * 25 17 13 11 8.7 7.4 5.2 3.8 3.2 2.5 2.1 

3,000 * * >25 19 14 12 9.3 8.1 5.7 4.2 3.5 2.8 2.4 

4,000 * * * 22 17 14 11 9.3 6.8 4.9 4.1 3.3 2.8 

5,000 * * * >25 19 16 12 11 7.4 5.6 4.7 3.7 3.1 

7,500 * * * * 24 19 16 13 9.3 6.8 5.8 4.7 4.0 

10,000 * * * * >25 22 18 16 11 8.1 6.8 5.3 4.6 15,000 * * * * * >25 22 19 13 9.9 8.1 6.8 5.7 

20,000 * * * * * * >25 22 16 11 9.3 7.4 6.8 50,000 * * * * * * * >25 24 18 15 12 10 

75,000 * * * * * * * * >25 22 18 15 13 

100,000 * * * * * * * * * >25 21 17 14 

150,000 * * * * * * * * * * >25 20 17 

200,000 * * * * * * * * * * * 23 19 

* > 25 miles (report distance as 25 miles) # <0.1 mile (re

10 - 13 
Reference Table 19

Dense Gas Distances to Toxic Endpoint

60-minute Release, Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 0.5 0.4 0.3 0.2 0.2 0.1 0.1 0.1 <0.1 # # # # 

2 0.8 0.6 0.5 0.3 0.2 0.2 0.2 0.1 0.1 <0.1 <0.1 <0.1 # 

5 1.6 1.0 0.8 0.5 0.4 0.3 0.2 0.2 0.2 0.1 0.1 0.1 <0.1 

10 2.0 1.4 1.2 0.8 0.6 0.5 0.4 0.3 0.2 0.2 0.1 0.1 0.1 

30 4.0 2.8 2.2 1.5 1.1 0.9 0.7 0.6 0.4 0.3 0.2 0.2 0.2 

50 5.5 3.9 3.1 2.1 1.5 1.2 1.0 0.8 0.6 0.4 0.3 0.3 0.2 

100 8.7 6.1 4.8 3.2 2.2 1.8 1.4 1.2 0.8 0.6 0.5 0.4 0.3 

150 12 8.1 6.2 4.1 2.9 2.3 1.8 1.6 1.1 0.7 0.6 0.5 0.4 250 17 11 8.7 5.6 4.0 3.2 2.5 2.1 1.4 1.1 0.9 0.7 0.6 

500 >25 19 14 9.3 6.2 5.0 3.9 3.3 2.2 1.6 1.3 1.0 0.9 750 * 25 19 12 8.7 6.8 5.1 4.2 2.8 2.0 1.6 1.3 1.1 

1,000 * >25 24 15 11 8.1 6.1 5.2 3.4 2.4 1.9 1.5 1.3 

1,500 * * >25 20 14 11 8.1 6.8 4.3 3.0 2.5 1.9 1.7 

2,000 * * * 24 17 13 9.9 8.1 5.2 3.7 2.9 2.3 1.9 2,500 * * * >25 19 15 12 9.3 6.0 4.3 3.4 2.7 2.2 

3,000 * * * * 22 17 13 11 6.8 4.8 3.8 3.0 2.5 

4,000 * * * * >25 21 16 14 8.7 5.8 4.7 3.6 3.0 

5,000 * * * * * 25 19 16 9.9 6.8 5.3 4.1 3.5 

7,500 * * * * * >25 25 20 13 9.3 6.8 5.4 4.5 

10,000 * * * * * * >25 25 16 11 8.7 6.8 5.4 15,000 * * * * * * * >25 21 14 11 8.7 7.4 20,000 * * * * * * * * 25 17 14 11 8.7 50,000 * * * * * * * * >25 >25 25 19 16 

75,000 * * * * * * * * * * >25 25 20 

100,000 * * * * * * * * * * * >25 24 

150,000 * * * * * * * * * * * * >25 

200,000 * * * * * * * * * * * * * 

* > 25 miles (report distance as 25 miles) # <0.1 mile (re

10 - 14 
Reference Table 20

Dense Gas Distances to Toxic Endpoint

10-minute Release, Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 0.5 0.3 0.2 0.2 0.1 0.1 0.1 0.1 <0.1 # # # # 

2 0.7 0.5 0.4 0.3 0.2 0.2 0.1 0.1 0.1 <0.1 <0.1 # # 

5 1.1 0.8 0.6 0.5 0.3 0.3 0.2 0.2 0.1 0.1 0.1 <0.1 <0.1 

10 2.1 1.2 1.0 0.7 0.5 0.4 0.3 0.3 0.2 0.1 0.1 0.1 0.1 

30 3.0 2.2 1.9 1.2 0.9 0.8 0.6 0.6 0.4 0.3 0.2 0.2 0.1 

50 4.1 3.0 2.5 1.6 1.2 1.0 0.8 0.7 0.5 0.3 0.3 0.2 0.2 

100 5.8 4.3 3.5 2.7 1.8 1.4 1.2 1.0 0.7 0.6 0.4 0.4 0.3 

150 7.4 5.5 4.5 3.1 2.2 1.9 1.4 1.2 0.9 0.7 0.6 0.4 0.4 250 9.9 7.4 5.8 4.1 3.0 2.5 2.0 1.7 1.1 0.9 0.7 0.6 0.5 500 14 11 8.7 5.9 4.3 3.6 2.9 2.5 1.7 1.2 1.0 0.8 0.7 

750 17 13 11 7.4 5.5 4.5 3.6 3.1 2.1 1.6 1.2 1.0 0.9 

1,000 20 15 12 8.7 6.2 5.3 4.3 3.5 2.5 1.8 1.5 1.2 1.0 

1,500 >25 19 16 11 8.1 6.2 5.2 4.5 3.0 2.2 1.8 1.5 1.2 

2,000 * 22 18 12 9.3 7.4 6.2 5.2 3.7 2.7 2.2 1.7 1.4 

2,500 * 24 20 14 11 8.7 6.8 6.0 3.8 3.0 2.2 1.9 1.7 

3,000 * >25 22 16 11 9.3 7.4 6.8 4.5 3.3 2.7 2.1 1.9 

4,000 * * >25 18 14 11 8.7 7.4 5.3 4.0 3.2 2.6 2.1 

5,000 * * * 20 15 12 9.9 8.7 5.8 4.4 3.6 2.9 2.4 

7,500 * * * >25 19 16 12 11 7.4 5.5 4.5 3.6 3.0 

10,000 * * * * 22 18 14 12 8.7 6.2 5.2 4.2 3.6 

15,000 * * * * >25 22 18 16 11 8.1 6.8 5.2 4.4 20,000 * * * * * >25 20 18 12 9.3 7.4 6.0 5.2 

50,000 * * * * * * >25 >25 20 15 12 9.7 8.3 

75,000 * * * * * * * * 25 18 15 12 10 

100,000 * * * * * * * * >25 21 17 14 12 

150,000 * * * * * * * * * >25 21 17 14 

200,000 * * * * * * * * * * 24 19 16 

* > 25 miles (report distance as 25 miles) # <0.1 mile (re

10 - 15 
Reference Table 21

Dense Gas Distances to Toxic Endpoint

60-minute Release, Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second

Release 

Rate 

Toxic Endpoint (mg/L) 

0.0004 0.0007 0.001 0.002 0.0035 0.005 0.0075 0.01 0.02 0.035 0.05 0.075 0.1 

(lbs/min) Distance (Miles) 

1 0.4 0.3 0.2 0.2 0.1 0.1 0.1 <0.1 # # # # # 

2 0.7 0.5 0.4 0.2 0.2 0.2 0.1 0.1 <0.1 <0.1 # # # 

5 1.1 0.8 0.7 0.4 0.3 0.2 0.2 0.2 0.1 0.1 <0.1 <0.1 <0.1 

10 1.7 1.2 1.0 0.7 0.5 0.4 0.3 0.3 0.2 0.1 0.1 0.1 0.1 

30 3.3 2.4 1.9 1.3 0.9 0.7 0.6 0.5 0.3 0.2 0.2 0.2 0.1 

50 4.7 3.3 2.6 1.7 1.2 1.0 0.8 0.7 0.4 0.3 0.3 0.2 0.2 

100 7.4 5.2 4.1 2.7 1.9 1.5 1.2 1.0 0.7 0.5 0.4 0.3 0.3 

150 9.9 6.8 5.3 3.4 2.4 1.9 1.5 1.3 0.9 0.6 0.5 0.4 0.3 250 14 9.3 7.4 4.7 3.4 2.7 2.1 1.7 1.1 0.8 0.7 0.5 0.4 500 22 16 12 7.4 5.2 4.2 3.2 2.7 1.7 1.2 1.0 0.8 0.7 750 >25 20 16 9.9 6.8 5.4 4.2 3.5 2.2 1.6 1.3 1.0 0.9 

1,000 * 24 19 12 8.1 6.8 5.0 4.2 2.7 1.8 1.6 1.2 1.0 

1,500 * >25 >25 16 11 8.7 6.8 5.5 3.5 1.9 2.0 1.6 1.3 

2,000 * * * 19 14 11 8.1 6.8 4.2 3.0 2.2 1.9 1.6 2,500 * * * 23 16 12 9.3 7.4 4.9 3.4 2.7 2.1 1.7 

3,000 * * * >25 18 14 11 8.7 5.5 3.8 3.0 2.4 2.0 

4,000 * * * * 22 17 13 11 6.8 4.7 3.1 2.8 2.4 

5,000 * * * * >25 20 16 12 8.1 5.3 4.3 3.3 2.7 

7,500 * * * * * 25 20 17 11 6.8 5.6 4.3 3.5 

10,000 * * * * * >25 24 20 13 8.7 6.8 5.2 4.3 

15,000 * * * * * * >25 >25 17 11 8.7 6.8 5.6 20,000 * * * * * * * * 20 14 11 8.1 6.8 50,000 * * * * * * * * >25 >25 20 15 13 

75,000 * * * * * * * * * * >25 20 16 

100,000 * * * * * * * * * * * 24 20 

150,000 * * * * * * * * * * * >25 >25 

200,000 * * * * * * * * * * * * * 

* > 25 miles (report distance as 25 miles) # <0.1 mile (re

10 - 16 
Reference Table 22 

Distances to Toxic Endpoint for Anhydrous Ammonia Liquefied Under Pressure
 

D Stability, Wind Speed 3.0 Meters per Second
 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

<10 <0.1* 

<0.1* 

10 0.1 

15 0.1 

20 0.1 

30 0.1 

40 0.1 

50 0.1 

60 0.2 0.1 

70 0.2 0.1 

80 0.2 0.1 

90 0.2 0.1 

100 0.2 0.1 

150 0.2 0.1 

200 0.3 0.1 

250 0.3 0.1 

300 0.3 0.1 

400 0.4 0.2 

500 0.4 0.2 

600 0.5 0.2 

700 0.5 0.2 

750 0.5 0.2 

800 0.5 0.2 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

900 0.6 0.2 

1,000 0.6 0.2 

1,500 0.7 0.3 

2,000 0.8 0.3 

2,500 0.9 0.3 

3,000 1.0 0.4 

4,000 1.2 0.4 

5,000 1.3 0.5 

7,500 1.6 0.5 

10,000 1.8 0.6 

15,000 2.2 0.7 

20,000 2.5 0.8 

25,000 2.8 0.9 

30,000 3.1 1.0 

40,000 3.5 1.1 

50,000 3.9 1.2 

75,000 4.8 1.4 

100,000 5.4 1.6 

150,000 6.6 1.9 

200,000 7.6 2.1 

250,000 8.4 2.3 

* Report distance as 0.1 mile 

April 15, 1999 10 - 17 
Reference Table 23 

Distances to Toxic Endpoint for Non-liquefied Ammonia, Ammonia Liquefied by Refrigeration, or 

Aqueous Ammonia
 

D Stability, Wind Speed 3.0 Meters per Second
 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

<8 <0.1* 

<0.1* 

8 0.1 

10 0.1 

15 0.1 

20 0.1 

30 0.1 

40 0.1 

50 0.2 0.1 

60 0.2 0.1 

70 0.2 0.1 

80 0.2 0.1 

90 0.2 0.1 

100 0.2 0.1 

150 0.3 0.1 

200 0.3 0.1 

250 0.4 0.2 

300 0.4 0.2 

400 0.4 0.2 

500 0.5 0.2 

600 0.6 0.2 

700 0.6 0.2 

750 0.6 0.2 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

800 0.7 0.2 

900 0.7 0.3 

1,000 0.8 0.3 

1,500 1.0 0.4 

2,000 1.2 0.4 

2,500 1.2 0.4 

3,000 1.5 0.5 

4,000 1.8 0.6 

5,000 2.0 0.7 

7,500 2.2 0.7 

10,000 2.5 0.8 

15,000 3.1 1.0 

20,000 3.6 1.2 

25,000 4.1 1.3 

30,000 4.4 1.4 

40,000 5.1 1.6 

50,000 5.8 1.8 

75,000 7.1 2.2 

100,000 8.2 2.5 

150,000 10 3.1 

200,000 12 3.5 

* Report distance as 0.1 mile 

April 15, 1999 10 - 18 
Reference Table 24
 

Distances to Toxic Endpoint for Chlorine
 

D Stability, Wind Speed 3.0 Meters per Second
 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

1 <0.1* 

<0.1* 

2 0.1 

5 0.1 

10 0.2 0.1 

15 0.2 0.1 

20 0.2 0.1 

30 0.3 0.1 

40 0.3 0.1 

50 0.3 0.1 

60 0.4 0.2 

70 0.4 0.2 

80 0.4 0.2 

90 0.4 0.2 

100 0.5 0.2 

150 0.6 0.2 

200 0.6 0.3 

250 0.7 0.3 

300 0.8 0.3 

400 0.8 0.4 

500 1.0 0.4 

600 1.0 0.4 

700 1.1 0.4 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

750 1.2 0.4 

800 1.2 0.5 

900 1.2 0.5 

1,000 1.3 0.5 

1,500 1.6 0.6 

2,000 1.8 0.6 

2,500 2.0 0.7 

3,000 2.2 0.8 

4,000 2.5 0.8 

5,000 2.8 0.9 

7,500 3.4 1.2 

10,000 3.9 1.3 

15,000 4.6 1.6 

20,000 5.3 1.8 

25,000 5.9 2.0 

30,000 6.4 2.1 

40,000 7.3 2.4 

50,000 8.1 2.7 

75,000 9.8 3.2 

100,000 11 3.6 

150,000 13 4.2 

200,000 15 4.8 

* Report distance as 0.1 mile 

April 15, 1999 10 - 19 
Reference Table 25 

Distances to Toxic Endpoint for Sulfur Dioxide
 

D Stability, Wind Speed 3.0 Meters per Second
 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

1 <0.1* 

<0.1* 

2 0.1 

5 0.1 

10 0.2 0.1 

15 0.2 0.1 

20 0.2 0.1 

30 0.2 0.1 

40 0.3 0.1 

50 0.3 0.1 

60 0.4 0.2 

70 0.4 0.2 

80 0.4 0.2 

90 0.4 0.2 

100 0.5 0.2 

150 0.6 0.2 

200 0.6 0.2 

250 0.7 0.3 

300 0.8 0.3 

400 0.9 0.4 

500 1.0 0.4 

600 1.1 0.4 

700 1.2 0.4 

Release Rate 

(lbs/min) 

Distance to Endpoint (miles) 

Rural Urban 

750 1.3 0.5 

800 1.3 0.5 

900 1.4 0.5 

1,000 1.5 0.5 

1,500 1.9 0.6 

2,000 2.2 0.7 

2,500 2.3 0.8 

3,000 2.7 0.8 

4,000 3.1 1.0 

5,000 3.3 1.1 

7,500 4.0 1.3 

10,000 4.6 1.4 

15,000 5.6 1.7 

20,000 6.5 1.9 

25,000 7.3 2.1 

30,000 8.0 2.3 

40,000 9.2 2.6 

50,000 10 2.9 

75,000 13 3.5 

100,000 14 4.0 

150,000 18 4.7 

200,000 20 5.4 

* Report distance as 0.1 mile 

April 15, 1999 10 - 20 
Reference Table 26 

Neutrally Buoyant Plume Distances to Lower Flammability Limit (LFL)
 

For Release Rate Divided by LFL
 

Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second
 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 28 0.1 

28 - 40 0.1 

40 - 60 0.1 

60 - 220 0.2 

220 - 530 0.3 

530 - 860 0.4 

860 - 1,300 0.5 

1,300 - 1,700 0.6 

1,700 - 2,200 0.7 

2,200 - 2,700 0.8 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

2,700 - 3,300 0.9 

3,300 - 3,900 1.0 

3,900 - 4,500 1.1 

4,500 - 5,200 1.2 

5,200 - 5,800 1.3 

5,800 - 6,800 1.4 

6,800 - 8,200 1.6 

8,200 - 9,700 1.8 

9,700 - 11,000 2.0 

11,000 - 13,000 2.2 

Reference Table 27
 

Neutrally Buoyant Plume Distances to Lower Flammability Limit (LFL)
 

For Release Rate Divided by LFL
 

Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second
 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

0 - 68 0.1 

68 - 100 0.1 

100 - 150 0.1 

150 - 710 0.2 

710 - 1,500 0.3 

1,500 - 2,600 0.4 

2,600 - 4,000 0.5 

4,000 - 5,500 0.6 

Release Rate/Endpoint 

[(lbs/min)/(mg/L)] 

Distance to 

Endpoint 

(miles) 

5,500 - 7,300 0.7 

7,300 - 9,200 0.8 

9,200 - 11,000 0.9 

11,000 - 14,000 1.0 

14,000 - 18,000 1.2 

18,000 - 26,000 1.4 

26,000 - 31,000 1.6 

31,000 - 38,000 1.8 

April 15, 1999 10 - 21 
Reference Table 28

Dense Gas Distances to Lower Flammability Limit

Rural Conditions, D Stability, Wind Speed 3.0 Meters per Second

Release 

Rate 

(lbs/min) 

Lower Flammability Limit (mg/L) 

27 30 35 40 45 50 60 70 100 

Distance (Miles) 

<1,500 # # # # # # # # # 

1,500 <0.1 <0.1 # # # # # # # 

2,000 0.1 0.1 <0.1 # # # # # # 

2,500 0.1 0.1 0.1 <0.1 # # # # # 

3,000 0.1 0.1 0.1 0.1 <0.1 <0.1 # # # 4,000 0.1 0.1 0.1 0.1 0.1 0.1 <0.1 # # 5,000 0.1 0.1 0.1 0.1 0.1 0.1 0.1 <0.1 # 7,500 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 <0.

10,000 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

# < 0.1 mile (report distance as 0.1 mile) 

April 15, 1999 10 - 22 
Reference Table 29
 

Dense Gas Distances to Lower Flammability Limit
 

Urban Conditions, D Stability, Wind Speed 3.0 Meters per Second
 

Release 

Rate 

(lbs/min) 

Lower Flammability Limit (mg/L) 

27 30 35 40 >40 

Distance (Miles) 

<5,000 # # # # # 

5,000 <0.1 <0.1 # # # 

7,500 0.1 0.1 <0.1 # # 

10,000 0.1 0.1 0.1 <0.1 # 

# < 0.1 mile (report distance as 0.1 mile) 

April 15, 1999 10 - 23 
Reference Table 30 

Distance to Radiant Heat Dose at Potential Second Degree Burn Threshold Assuming Exposure for Duration of Fir

(Dose = [5 kW/m ]2 4/3 x Exposure Time) 

Quantity in Fireball (pounds) 1,000 5,000 10,000 20,000 30,000 50,000 75,000 100,000 200,000 

Duration of Fireball (seconds) 3.5 5.9 7.5 9.4 10.8 12.7 14.8 15.5 17.4 

CAS No. Chemical Name Distance (miles) at which Exposure for Duration of Fireball May Cause Second Degree Bur

75-07-0 Acetaldehyde 0.04 0.08 0.1 0.1 0.2 0.2 0.3 0.3 0.4 74-86-2 Acetylene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

598-73-2 Bromotrifluoroethylene 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.1 

106-99-0 1,3-Butadiene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

106-97-8 Butane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

106-98-9 1-Butene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

107-01-7 2-Butene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

25167-67-3 Butene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

590-18-1 2-Butene-cis 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

624-64-6 2-Butene-trans 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

463-58-1 Carbon oxysulfide 0.02 0.05 0.06 0.09 0.1 0.1 0.2 0.2 0.2 

7791-21-1 Chlorine monoxide 0.01 0.02 0.02 0.03 0.03 0.04 0.05 0.06 0.08 

557-98-2 2-Chloropropylene 0.03 0.07 0.1 0.1 0.2 0.2 0.2 0.3 0.4 

590-21-6 1-Chloropropylene 0.03 0.07 0.1 0.1 0.2 0.2 0.2 0.3 0.4 460-19-5 Cyanogen 0.03 0.07 0.1 0.1 0.2 0.2 0.2 0.3 0.4 

75-19-4 Cyclopropane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

4109-96-0 Dichlorosilane 0.02 0.04 0.06 0.08 0.1 0.1 0.2 0.2 0.2 

75-37-6 Difluoroethane 0.02 0.05 0.07 0.1 0.1 0.1 0.2 0.2 0.3 

124-40-3 Dimethylamine 0.04 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.5 

463-82-1 2,2-Dimethylpropane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

74-84-0 Ethane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

107-00-6 Ethyl acetylene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

April 15, 1999 10 - 24 
Reference Table 30 (continued) 

Quantity in Fireball (pounds) 1,000 5,000 10,000 20,000 30,000 50,000 75,000 100,000 200,000 

Duration of Fireball (seconds) 3.5 5.9 7.5 9.4 10.8 12.7 14.8 15.5 17.4 

CAS No. Chemical Name Distance (miles) at which Exposure for Duration of Fireball May Cause Second Degree Bur

75-04-7 Ethylamine 0.04 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.5 

75-00-3 Ethyl chloride 0.03 0.07 0.09 0.1 0.2 0.2 0.2 0.3 0.3 74-85-1 Ethylene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

60-29-7 Ethyl ether 0.04 0.09 0.1 0.2 0.2 0.2 0.3 0.3 0.5 

75-08-1 Ethyl mercaptan 0.04 0.08 0.1 0.2 0.2 0.2 0.3 0.3 0.4 

109-95-5 Ethyl nitrite 0.03 0.06 0.09 0.1 0.1 0.2 0.2 0.3 0.3 

1333-74-0 Hydrogen 0.08 0.2 0.2 0.3 0.4 0.5 0.6 0.6 0.9 

75-28-5 Isobutane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 78-78-4 Isopentane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

78-79-5 Isoprene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

75-31-0 Isopropylamine 0.04 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.5 

75-29-6 Isopropyl chloride 0.04 0.07 0.1 0.1 0.2 0.2 0.3 0.3 0.4 

74-82-8 Methane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.6 

74-89-5 Methylamine 0.04 0.08 0.1 0.2 0.2 0.2 0.3 0.3 0.4 

563-45-1 3-Methyl-1-butene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 563-46-2 2-Methyl-1-butene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 115-10-6 Methyl ether 0.04 0.08 0.1 0.2 0.2 0.2 0.3 0.3 0.4 107-31-3 Methyl formate 0.03 0.06 0.08 0.1 0.1 0.2 0.2 0.2 0.3 115-11-7 2-Methylpropene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 504-60-9 1,3-Pentadiene 0.05 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 109-66-0 Pentane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

109-67-1 1-Pentene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 646-04-8 2-Pentene, (E)- 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

April 15, 1999 10 - 25 
Reference Table 30 (continued) 

Quantity in Fireball (pounds) 1,000 5,000 10,000 20,000 30,000 50,000 75,000 100,000 200,000 

Duration of Fireball (seconds) 3.5 5.9 7.5 9.4 10.8 12.7 14.8 15.5 17.4 

CAS No. Chemical Name Distance (miles) at which Exposure for Duration of Fireball May Cause Second Degree Bur

627-20-3 2-Pentene, (Z)- 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

463-49-0 Propadiene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

74-98-6 Propane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

115-07-1 Propylene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

74-99-7 Propyne 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

7803-62-5 Silane 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

116-14-3 Tetrafluoroethylene 0.01 0.02 0.02 0.03 0.04 0.05 0.06 0.07 0.09 

75-76-3 Tetramethylsilane 0.05 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 

10025-78-2 Trichlorosilane 0.01 0.03 0.04 0.06 0.07 0.08 0.1 0.1 0.2 

79-38-9 Trifluorochloroethylene 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.1 75-50-3 Trimethylamine 0.04 0.09 0.1 0.2 0.2 0.3 0.3 0.4 0.5 

689-97-4 Vinyl acetylene 0.05 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.5 

75-01-4 Vinyl chloride 0.03 0.07 0.09 0.1 0.2 0.2 0.2 0.3 0.3 

109-92-2 Vinyl ethyl ether 0.04 0.09 0.1 0.2 0.2 0.2 0.3 0.3 0.4 

75-02-5 Vinyl fluoride 0.01 0.02 0.03 0.04 0.05 0.06 0.08 0.09 0.1 

75-35-4 Vinylidene chloride 0.02 0.05 0.07 0.09 0.1 0.1 0.2 0.2 0.3 75-38-7 Vinylidene fluoride 0.02 0.05 0.07 0.09 0.1 0.1 0.2 0.2 0.3 

107-25-5 Vinyl methyl ether 0.04 0.08 0.1 0.2 0.2 0.2 0.3 0.3 0.4 

April 15, 1999 10 - 26 
11 ESTIMATING OFFSITE RECEPTORS
 

In Chapter 11 

� How to estimate the number of offsite receptors potentially affected by your 

worst-case and alternative scenarios. 

� Where to find the data you need. 

The rule requires that you estimate residential populations within the circle defined by the endpoint 

for your worst-case and alternative release scenarios. In addition, you must report in the RMP whether 

certain types of public receptors and environmental receptors are within the circles. 

To estimate residential populations, you may use the most recent Census data or any other source of 

data that you believe is more accurate. Local authorities may be able to provide information on offsite 

receptors. You are not required to update Census data or conduct any surveys to develop your estimates. 

Census data are available in public libraries and in the LandView system, which is available on CD-ROM 

(see box below). The rule requires that you estimate populations to two significant digits. For example, if 

there are 1,260 people within the circle, you may report 1,300 people. If the number of people is between 10 

and 100, estimate to the nearest 10. If the number of people is less than 10, provide the actual number. 

How to obtain Census data and LandView 

Census data can be found in publications of the Bureau of the Census, available in public libraries, including 

County and City Data Book. 

LandView ®III is a desktop mapping system that includes database extracts from EPA, the Bureau of the Census, 

the U.S. Geological Survey, the Nuclear Regulatory Commission, the Department of Transportation, and the 

Federal Emergency Management Agency. These databases are presented in a geographic context on maps that 

show jurisdictional boundaries, detailed networks of roads, rivers, and railroads, census block group and tract 

polygons, schools, hospitals, churches, cemeteries, airports, dams, and other landmark features. 

CD-ROM for IBM-compatible PCS 

CD-TGR95-LV3-KIT $99 per disc (by region) or $549 for 11 disc set 

U.S. Department of Commerce
 

Bureau of the Census
 

P.O. Box 277943
 

Atlanta, GA 30384-7943 
 

Phone: 301-457-4100 (Customer Services -- orders)
 

Fax: (888) 249-7295 (toll-free)
 

Fax: (301) 457-3842 (local)
 

Phone: (301) 457-1128 (Geography Staff -- content)
 

http://www.census.gov/ftp/pub/geo/www/tiger/
 

Further information on LandView and other sources of Census data is available at the Bureau of the Census web 

site at www.census.gov. 

April 15, 1999 
Chapter 11 

Estimating Offsite Receptors 

Census data are presented by Census tract. If your circle covers only a portion of the tract, you 

should develop an estimate for that portion. The easiest way to do this is to determine the population density 

per square mile (total population of the Census tract divided by the number of square miles in the tract) and 

apply that density figure to the number of square miles within your circle. Because there is likely to be 

considerable variation in actual densities within a Census tract, this number will be approximate. The rule, 

however, does not require you to correct the number. 

Other public receptors must be noted in the RMP. If there are any schools, residences, hospitals, 

prisons, public recreational areas, or commercial, office, or industrial areas within the circle, you must report 

that. Any of these locations inhabited or occupied by the public at any time without restriction by the source 

is a public receptor. You are not required to develop a list of all institutions and areas; you must simply 

check off which types of receptors are within the circle. Most of these institutions or areas can be identified 

from local street maps. Recreational areas include public swimming pools, public parks, and other areas that 

are used for recreational activities (e.g., baseball fields). Commercial and industrial areas include shopping 

malls, strip malls, downtown business areas, industrial parks, etc. See EPA’s General Guidance for Risk 

Management Programs (40 CFR part 68) for further information on identifying public receptors. 

Environmental receptors are defined as national or state parks, forests, or monuments; officially 

designated wildlife sanctuaries, preserves, or refuges; and Federal wilderness areas. All of these can be 

identified on local U.S. Geological Survey (USGS) maps (see box below). You are not required to locate 

each of these specifically. You are only required to check off in the RMP that these specific types of areas are 

within the circle. If any part of one of these receptors is within your circles, you must note that in the RMP. 

Important: The rule does not require you to assess the likelihood, type, or severity of potential 

impacts on either public or environmental receptors. Identifying them as within the circle simply indicates 

that they could be adversely affected by the release. 

April 15, 1999 11 - 2 
Chapter 11 

Estimating Offsite Receptors 

How to obtain USGS maps 

The production of digital cartographic data and graphic maps comprises the largest component of the USGS 

National Mapping Program. The USGS's most familiar product is the 1:24,000-scale Topographic Quadrangle 

Map. This is the primary scale of data produced, and depicts greater detail for a smaller area than 

intermediate-scale (1:50,000 and 1:100,000) and small-scale (1:250,000, 1:2,000,000 or smaller) products, which 

show selectively less detail for larger areas. 

U.S. Geological Survey 

508 National Center 

12201 Sunrise Valley Drive 

Reston, VA 20192 

Phone: (703) 648-4000 

http://mapping.usgs.gov 

To order USGS maps by fax, select, print, and complete one of the online forms and fax to 303-202-4693. 

A list of the nearest commercial dealers is available at: http://mapping.usgs.gov/esic/usimage/dealers.html 

For more information or ordering assistance, call 1-800-HELP-MAP, or write: 

USGS Information Services 

Box 25286 

Denver, CO 80225 

For additional information, contact any USGS Earth Science Information Center or call 1-800-USA-MAPS. 

April 15, 1999 11 - 3 
Chapter 11 

Estimating Offsite Receptors 

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April 15, 1999 11 - 4 
 

 

 

 

 

 

12 SUBMITTING OFFSITE CONSEQUENCE ANALYSIS 

INFORMATION FOR RISK MANAGEMENT PLAN 

In Chapter 12 

	 12.1 Information you are required to submit for worst-case scenarios for 

toxic substances. 

	 12.2 Information you are required to submit for alternative scenarios for 

toxic substances. 

	 12.3 Information you are required to submit for worst-case scenarios for 

flammable substances. 

	 12.4 Information you are required to submit for alternative scenarios for 

flammable substances. 

For the offsite consequence analysis (OCA) component of the RMP you must provide 

information on your worst-case and alternative release scenario(s) for toxic and flammable regulated 

chemicals held above the threshold quantity. The requirements for what information you must submit 

differ if your source has Program 1, Program 2, or Program 3 processes. 

If your source has Program 1 processes, you must submit information on a worst-case 

release scenario for each Program 1 process. If your source has Program 2 or Program 3 processes, 

you must provide information on one worst-case release for all toxic regulated substances present 

above the threshold quantity and one worst-case release scenario for all flammable regulated 

substances present above the threshold quantity. You may need to submit an additional worst-case 

scenario if a worst-case release from another part of the source would potentially affect public 

receptors different from those potentially affected by the initial worst-case scenario(s) for 

flammable and toxic regulated substances. 

In addition to a worst-case release scenario, sources with Program 2 and Program 3 

processes must also provide information on alternative release scenarios. Alternative releases are 

releases that could occur, other than the worst-case, that may result in concentrations, overpressures, 

or radiant heat that reach endpoints offsite. You must present information on one alternative release 

scenario for each regulated toxic substance, including the substance used for the worst-case release, 

held above the threshold quantity and one alternative release scenario to represent all flammable 

substances held above the threshold quantity. The types of documentation to submit are presented 

below for worst-case scenarios involving toxic substances, alternative scenarios involving toxic 

substances, worst-case scenarios involving flammable substances, and alternative scenarios involving 

flammable substances. 

12.1 RMP Data Required for Worst-Case Scenarios for Toxic Substances 

For worst-case scenarios involving toxic substances, you will have to submit the 

following information. See the RMP*eSubmit User Manual for complete instructions. 

•	 Chemical name; 

•	 Percentage weight of the regulated liquid toxic substance (if present in a mixture); 

March 2009 
Chapter 12 

Submitting Offsite Consequence Analysis Information for Risk Management Plan 

Physical state of the chemical released (gas, liquid, refrigerated gas, gas liquefied by 

pressure); 

�

Model used (OCA or industry-specific guidance reference tables or modeling; name of other 

model used); 

�

� Scenario (gas release or liquid spill and vaporization); 

� Quantity released (pounds); 

� Release rate (pounds per minute); 

Duration of release (minutes) (10 minutes for gases; if you used OCA guidance for liquids, 

indicate either 10 or 60 minutes); 

�

Wind speed (meters per second) and stability class (1.5 meters per second and F stability 

unless you can show higher minimum wind speed or less stable atmosphere at all times 

during the last three years); 

�

� Topography (rural or urban); 

� Distance to endpoint (miles, rounded to two significant digits); 

Population within distance to endpoint (residential population rounded to two significant 

digits); 

�

Public receptors within the distance to endpoint (schools, residences, hospitals, prisons, 

recreation areas, commercial, office or industrial areas); 

�

Environmental receptors within the distance to endpoint (national or state parks, forests, or 

monuments; officially designated wildlife sanctuaries, preserves, or refuges; Federal 

wilderness areas); and 

�

� Passive mitigation measures considered (dikes, enclosures, berms, drains, sumps, other). 

12.2 RMP Data Required for Alternative Scenarios for Toxic Substances 

For alternative scenarios involving toxic substances held above the threshold quantity in a Program 2 

or Program 3 process, you will have to submit the following information. See the Risk Management Plan 

Data Elements Guide for complete instructions. 

� Chemical name; 

� Percentage weight of the regulated liquid toxic substance (if present in a mixture); 

Physical state of the chemical released (gas, liquid, refrigerated gas, gas liquefied by 

pressure); 

�

Model used (OCA or industry-specific guidance reference tables or modeling; name of other 

model used); 

�

Scenario (transfer hose failure, pipe leak, vessel leak, overfilling, rupture disk/relief valve, 

excess flow valve, other); 

�

� Quantity released (pounds); 

� Release rate (pounds per minute); 

� Duration of release (minutes) (if you used OCA guidance, indicate either 10 or 60 minutes); 

Wind speed (meters per second) and stability class (3.0 meters per second and D stability if 

you use OCA guidance, otherwise use typical meteorological conditions at your site); 

�

� Topography (rural or urban); 

� Distance to endpoint (miles, rounded to two significant digits); 

Population within distance to endpoint (residential population rounded to two significant 

digits); 

�

April 12, 1999 12 - 2 
Chapter 12 

Submitting Offsite Consequence Analysis Information for Risk Management Plan 

Public receptors within the distance to endpoint (schools, residences, hospitals, prisons, 

recreation areas, commercial, office, or industrial areas); 

�

Environmental receptors within the distance to endpoint (national or state parks, forests, or 

monuments; officially designated wildlife sanctuaries, preserves, or refuges; Federal 

wilderness areas); 

�

� Passive mitigation measures considered (dikes, enclosures, berms, drains, sumps, other); and 

Active mitigation measures considered (sprinkler system, deluge system, water curtain, 

neutralization, excess flow valve, flares, scrubbers, emergency shutdown system, other). 

�

12.3 RMP Data Required for Worst-Case Scenarios for Flammable Substances 

For worst-case scenarios involving flammable substances, you will have to submit the following 

information. See the Risk Management Plan Data Elements Guide for complete instructions. 

� Chemical name; 

Model used (OCA or industry-specific guidance reference tables or modeling; name of other 

model used); 

�

� Scenario (vapor cloud explosion); 

� Quantity released (pounds); 

� Endpoint used (for vapor cloud explosions use 1 psi); 

� Distance to endpoint (miles, rounded to two significant digits); 

Population within distance to endpoint (residential population rounded to two significant 

digits); 

�

Public receptors within the distance to endpoint (schools, residences, hospitals, prisons, 

recreation areas, commercial, office, or industrial areas); 

�

Environmental receptors within the distance to endpoint (national or state parks, forests, or 

monuments, officially designated wildlife sanctuaries, preserves, or refuges, Federal 

wilderness areas); and 

�

� Passive mitigation measures considered (blast walls, other). 

12.4 RMP Data Required for Alternative Scenarios for Flammable Substances 

For alternative scenarios involving flammable substances held above the threshold quantity in a 

Program 2 or Program 3 process, you will have to submit the following information. See the Risk 

Management Plan Data Elements Guide for complete instructions. 

� Chemical name; 

Model used (OCA or industry-specific guidance reference tables or modeling; name of other 

model used); 

�

� Scenario (vapor cloud explosion, fireball, BLEVE, pool fire, jet fire, vapor cloud fire, other); 

� Quantity released (pounds); 

Endpoint used (for vapor cloud explosions, the endpoint is 1 psi overpressure; for a fireball 

the endpoint is 5 kw/m2 for 40 seconds. A lower flammability limit (expressed as a 

percentage) may be listed as specified in NFPA documents or other generally recognized 

sources; these are listed in the OCA Guidance); 

�

� Distance to endpoint (miles, rounded to two significant digits); 

April 12, 1999 12 - 3 
 

 

 

 

 

 

Chapter 12 

Submitting Offsite Consequence Analysis Information for Risk Management Plan 

•	 Population within distance to endpoint (residential population rounded to two significant 

digits); 

•	 Public receptors within the distance to endpoint (schools, residences, hospitals, prisons, 

recreation areas, commercial, office, or industrial areas); 

•	 Environmental receptors within the distance to endpoint (national or state parks, forests, or 

monuments, officially designated wildlife sanctuaries, preserves, or refuges, Federal 

wilderness areas); 

•	 Passive mitigation measures considered (e.g., dikes, fire walls, blast walls, enclosures, other); 

and 

•	 Active mitigation measures considered (e.g., sprinkler system, deluge system, water curtain, 

excess flow valve, other). 

12.5 Submitting RMPs 

EPA has made RMP*eSubmit available to complete and file your RMP. RMP*eSubmit does 

the following: 

•	 Provides a user-friendly, web-based RMP Submission System via EPA’s secure Central 

Data Exchange (CDX). 

•	 Performs data quality checks, accepts limited graphics, and provides on-line help 

including defining data elements and providing instructions. 

•	 Online reporting simplifies the process. It saves you time, and improves data quality and 

security. 

•	 EPA uses industry-standard technology, including encryption used by most commercial 

banks, as well as stringent user ID and password protocols to protect your information. 

•	 You will be able to access your RMP online at any time. 

•	 For a facility to submit their RMP, the certifier will first be required to set up a CDX 

account (or use their current one if the facility already submits data via CDX). 

Instructions for obtaining a CDX account and using RMP*eSubmit can be found at: 

www.epa.gov/emergencies/rmp. 

•	 Upon approval of the document in submission, a confirmation e-mail will be sent to the 

certifier. 

12.6 Other Required Documentation 

Besides the information you are required to submit in your RMP, you must maintain other 

records of your offsite consequence analysis on site. Under 40 CFR 68.39, you must maintain the 

following records: 

•	 For worst-case scenarios, a description of the vessel or pipeline and substance selected as the 

worst case, the assumptions and parameters used, and the rationale for selection. Assumptions 

include any administrative controls and any passive mitigation systems that were used to limit 

the quantity that could be released. You must document that anticipated effects of these 

controls and systems on the release quantity and rate. 

March 2009	 12 - 4 
•	 For alternative release scenarios, a description of the scenarios identified the assumptions and 

parameters used, and the rationale for selection of the specific scenarios. Assumptions include 

any administrative controls and any passive mitigation systems that were used to limit the 

quantity that could be released. You must document that anticipated effects of these controls 

and systems on the release quantity and rate. 

•	 Documentation of estimated quantity released, release rate, and duration of the release. 

•	 Methodology used to determine distance to an endpoint. 

•	 Data used to estimate populations and environmental receptors potentially affected. 

You are required to maintain these records for five years. 

March 2009 12 - 5 
Chapter 12 

Submitting Offsite Consequence Analysis Information for Risk Management Plan 

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April 12, 1999 12 - 6