i

Table of Contents

___________________________________________

Pages

Acknowledgments 1

Introduction 3

Purpose 3

Background 3

Authority 3

Exclusions 4

Applications 5

Frequently Asked Questions 5

Chapter 1 7

Overview: Organization of the Guidebook

Chapter 2 10

Regulatory Framework

Chapter 3 17

Compliance Process

Chapter 4 20

Evaluations and Findings

Chapter 5 40

Mitigation Options

Chapter 6 51

Extraordinary Circumstances

Appendices 53

A – Flowcharts

B – Worksheets

C – Data Resources

D – Bibliography and Photographic Credits

E – Index

F – Glossary
1

Acknowledgements

While numerous individuals contributed to the development of this

guidebook we wish to acknowledge the following:

Region 10 – NW/Alaska

Deborah Peavler-Stewart – Regional Environmental Officer

Sara Jensen – Field Environmental Officer

Region 8 – Rocky Mountains

Region 7 – Great Plains

Paul F. Mohr – Regional Environmental Officer

Region 4 – Southeast/Caribbean

Lenwood E. Smith – Field Environmental Officer

Region 3 – Mid Atlantic

Paul J. Lehmann – Regional Environmental Officer

Susanne Sortichos – Field Environmental Officer
2

1. The South Hill play area and homes are located near above ground stationary

storage containers in the South Hill neighborhood of Chesapeake, Virginia.
3

Introduction

This Guidebook focuses on the hazardous operations that can potentially harm HUDassisted projects and the people who live or work there. It reflects the parameters

established by 24 CFR Part 51 Subpart C, which regulates the technical requirements of

determining Acceptable Separation Distances (ASDs) from HUD-assisted projects in

proximity to hazardous operations.

Purpose

This Guidebook provides HUD staff, planners, developers, engineers and grantees with

specific steps to determine the Acceptable Separation Distance (ASD) between a HUDassisted project and a hazardous operation. It provides environmental review

professionals with information about the implementation of regulation 21 CFR Part 51

Subpart C. The Guidebook also provides both the analytical foundation and

recommended strategy for determining the distance between a proposed HUD-assisted

project and a hazardous operations site.

Background

The technical material referenced in previous editions of this Guidebook was obtained

from a December 1975 study1by Arthur D. Little, Inc., and a December 19, 1981 study2

by Rolf Jensen and Associates (RJ&A), Deerfield, Illinois, for HUD. The previous

iterations of these guidelines are:

 Urban Development Siting with Respect to Hazardous Commercial/ Industrial

Facilities – HUD –777-CPD)

 Regulation 24 CFR Part 51C;HUD Handbook 1390.2, “Environmental

Assessment Guide for Housing Projects”

 HUD Handbook 1390.4, ‘ A Guide to HUD Environmental Criteria and Standards

contained in 24 CFR Part 51” and,

 Siting of HUD-Assisted Projects Near Hazardous Facilities.

Authority

HUD’s authority to implement the information in this Guidebook is found in 24 CFR Part

51, Subpart C, “Siting of HUD – Assisted Projects Near Hazardous Operations Handling

Conventional Fuels or Chemicals of an Explosive or Flammable Nature.”

HUD-assisted projects are required to meet criteria, standards and guidelines for site

acceptability and a suitable living environment as provided under the following statues:

a) The National Housing Act, which was enacted “to encourage

improvements in housing standards and conditions, to provide a

system of mutual mortgage insurance, and for other purposes, “thus

1

Safety Considerations in Siting Housing Projects

2 Urban Development in Siting with Respect to Hazardous Commercial/Industrial Facilities
4

providing the basis for HUD’s Minimum Property Standards (MPS)

which have evolved as required by legislation over the past 44 years;

b) The Housing and Community Development Act of 1974, as amended,

which sets forth the national goal of “a decent home and a suitable

living environment for every American family”; and

c) The National Environmental Policy Act of 1969, as amended, which

directs Federal agencies to develop procedures to carry out the

purposes of the Act.

The information in this Guidebook can be applied to the following activities:

 HUD-assisted projects as defined in 24 CFR 51.201.

o Generally speaking, these are projects that are intended for residential,

institutional (this category includes public and government buildings),

recreational, commercial or industrial use that are planned for

development, construction, rehabilitation, modernization or conversion

(change of one state of phase to another) with HUD subsidy, grant

assistance, loan, loan guarantee, or mortgage insurance.

o The interpretation of the terms “rehabilitation” and “modernization” refer

only to such repairs and renovation of the proposed HUD-assisted project

that will result in an increased number of people being exposed to

hazardous operations by increasing residential densities, converting the

use of a building to human habitation, or making a vacant building

habitable.

Exclusions

The information contained in this Guidebook and in the ASD assessment tool do not

apply in the following situations, which are excluded from Part 51 Subpart C:

a. Underground Storage Containers: If the container is buried (see glossary),

there is no need for an ASD.

b. Stationary above-ground containers of 100 gallons or less capacity that

hold common liquid industrial fuels: Results from the December 19, 1981

study3by Rolf Jensen and Associates (RJ&A), Deerfield, Illinois, demonstrated

that stationary containers of 100 gallons or less capacity that store common

liquid industrial fuels (such as gasoline, fuel oil, kerosene and crude oil) do not

emit thermal radiation heat flux effects at levels that would pose a danger to

HUD-assisted projects. Note: This exception does not apply to above-ground

stationary containers that store hazardous gases, such as those listed in

Appendix I of the Regulation or in Appendix C of this guidebook.

c. Natural gas holders with floating tops: These are stationary above-ground

storage containers used to store natural gas. These containers are less

susceptible to corrosion and tank perforations that can cause Bleves (rupture

explosions).

3 Urban Development in Siting with Respect to Hazardous Commercial/Industrial Facilities
5

d. Mobile conveyances (tank trucks, barges, railroad tank cars): These are

containers that are mobile and have the capacity to store common liquid

industrial fuels or hazardous gases as listed in Appendix I of the regulation 24

CFR Part 51 Subpart C and in Appendix C of this Guidebook.

e. Pipelines, such as high pressure natural gas transmission pipelines or

liquid petroleum pipelines: Pipelines that transmit hazardous substances are

not considered a hazard under 24 CFR Part 51 Subpart C if they are located

underground or if they comply with applicable Federal, State or local safety

standards.

Applications

Regulation 24 CFR Part 51, Subpart C, the ASD assessment tool, and the information in

this Guidebook apply only to above-ground stationary containers of more than 100

gallon capacity that hold common liquid industrial fuels (for more information, please see

Appendix I of the Regulation and Appendix C of this Guidebook); and above-ground

stationary containers of any capacity that hold hazardous liquids or gases (see glossary)

that are not common liquid industrial fuels (see also the list of hazardous substances in

Appendix C of this Guidebook).

Frequently Asked Questions

To help you understand and apply the information in this Guidebook, what follow are

questions HUD-field environmental officers ask most frequently.

1. 1a. Is there a list of steps to assess whether a proposed HUD-assisted project

is too close to a hazardous facility?

1b. Is there a procedure for calculating the ASD?

1a. Yes. That information is located in Chapter 4. There you will find the steps

necessary for assessing the proposed HUD-assisted project and examples using the

Nomographs (Worksheets #1 through #4).

1b. Yes. Chapter 4 has manual calculation procedures. Chapter 4 and Appendix A

provide detailed flowcharts on how to calculate the ASD for blast-overpressure, thermal

radiation or both. You can also calculate the ASD by using the ASD assessment tool

available on HUD’s environmental webpage at

http://www.hud.gov/offices/cpd/environment/asdcalculator.cfm .

2. 2a. Which guidelines apply when the proposed HUD-assisted project site does

not meet the standard and mitigation is required?

2b. Is there detailed information available on mitigation analysis?

2a. Chapter 5 (Mitigation Options) provides the basic information and steps required for

a mitigation analysis. The results of this analysis provide key information that will help

you determine if mitigation is required. Chapter 5 also provides flowcharts on the

mitigation analysis for a site that has natural and man-made barriers if the ASD cannot

be achieved between the site and the hazard being assessed.
6

2b. Yes. Key information on mitigation analysis can be found in Chapter 5.

3. Are there guidelines for chemicals that are held in above-ground stationary

containers?

Yes. These guidelines can be found in Appendix C (Data Resources).

4. Aside from the manual calculation procedure (Nomographs) or the ASD

assessment tool, is there another way to access already calculated ASD

results based on the volume of the container?

Yes. There are 2 charts with ASD results (one for blast-overpressure, the other for

thermal radiation) based on the volume of the container. These charts are located in

Appendix C.

5. Is there a form to request a 51 C analysis regarding a potential hazardous site

near a proposed HUD-assisted project?

No, however, there is a form in Appendix C to Request for Headquarters Analysis of

Potential Hazardous Site, that details the information required to begin a 51 C analysis.

6. Is there a list of commonly asked questions on Regulation 24 CFR Part 51

Subpart C?

Yes. Those questions and answers can be found in Chapter 4, Evaluations and

Guidelines. This section will also help you understand important changes that have

occurred since the publication of the September 1996 “Siting of HUD-Assisted Projects

Near Hazardous Facilities.”

This edition of the Guidebook updates information and unresolved policy statements and

principles. There are no 24 CFR Part 51 Subpart C policy changes in this guidebook.

7. How are Self-Contained Above Ground Containers (SCAC’s) evaluated for

calculation of the ASD?

For calculation of the ASD, SCAC’s have two external walls. The first wall serves to

contain the product; the second serves to prevent spills outside of the container if the

first wall ruptures. The interstitial space (the space between the first and the second

wall) serves to control spillage if a rupture in the container’s internal wall occurs. Note:

SCAC’s are considered containers without a dike, and calculations must be done for

containers without a dike area.
7

Chapter 1 – Overview: Organization of the Guidebook

_____________________________________________

Petrochemical gases and liquids are used worldwide. Their demand varies depending

on the consumer. Domestic household consumers use petrochemicals to cook, heat

their homes and water tanks and run appliances. Commercial consumers such as

restaurants, office buildings, hotels and gas stations use petrochemicals to run their

businesses. Industrial consumers such as power stations, material manufacturers,

pharmaceutical and medical researchers, automotive and tool manufacturers use

petrochemicals to support the demands of purpose of their industries.

Regulation 24 CFR Part 51 Subpart C applies specifically to the petrochemical liquids

and gases that could ignite and explode and that are stored in above-ground, stationary

containers such as those ones listed in Appendix I of the Regulation and in Appendix C

of this Guidebook.

This Guidebook, as well as 24 CFR Part 51 Subpart C and the ASD assessment, are

tools designed to determine whether a proposed HUD-assisted project site is too close

to a facility with potentially hazardous stationary above-ground containers. They are

tools that also help environmental review professionals understand and implement the

regulation 24 CFR Part 51 Subpart C.

This latest edition of the Guidebook is a revision of the September 1996 “Siting of HUDAssisted Projects Near Hazardous Facilities.” This latest edition corrects inconsistencies

and removes either questionable or unresolved policy statements and principles. There

are no changes in this Guidebook to 24 CFR Part 51 Subpart C.

Additional features in this revision include:

 Flowcharts that illustrate

1. Site analysis steps for ASD calculation (procedures and findings for a HUDassisted project);

2. Procedural calculation of the ASD for thermal radiation (fire) or blastoverpressure (explosion);

3. Procedural calculation of the ASD for blast-overpressure (explosion) and

thermal radiation provided the substance being assessed has flammable or

combustible properties and is pressurized; and

4. Mitigation analysis for a proposed HUD-assisted project.

 Examples of multiple mitigation options involving natural and man-made barriers.

 A section on “Frequently Asked Questions” about 24 CFR Part 51 Subpart C.

This Guidebook contains an Introduction, 6 Chapters and 6 Appendices.

Introduction: Outlines purpose, background, authority, exclusions, applications and

limitations and frequently asked questions.
8

Chapter 1: Overview reviews the purpose and background of the Guidebook, providing

important basic information, as well as frequently asked questions.

Chapter 2: Regulatory Framework discusses the foundation and structure of 24 CFR

Part 51 Subpart C and the need to develop the standards of blast-overpressure and

thermal radiation from which the ASD is calculated for the proposed HUD-assisted

project.

Chapter 3: Compliance Process outlines the process to follow in order to comply with

the Regulation.

Chapter 4: Evaluations and Findings explains and outlines the procedures required to

evaluate proposed HUD-assisted projects that are in close proximity to a potentially

hazardous site.

Chapter 5: Mitigation Options provides information about mitigation analysis and how to

determine if mitigation is required on a proposed HUD-assisted project.

Chapter 6: Extraordinary Circumstances identifies extraordinary circumstances under

which proposed HUD-assisted projects are subject to consultation by HUD-Regional and

Field Environmental Officers. Under these circumstances, additional detailed

information and procedures that are not provided in 24 CFR Part 51 Subpart C would be

required.
9

2. A house sits near storage tanks in the South Hill neighborhood of Chesapeake,

Virginia
10

Chapter 2 - Regulatory Framework

_______________________________________________________________________

History:

Before the development of 24 CFR Part 51 Subpart C, HUD received inquiries from

developers, planners, engineers, HUD field staff and grantees about how to determine

Acceptable Separation Distances (ASDs) for HUD-assisted projects near stationary

facilities that store, handle or process explosive or fire prone substances. Answering

those inquiries involved determining the potential blast-overpressure and thermal

radiation effects of the hazard. The “area impacted from the blast-overpressure or

thermal radiation” was called a “hazard to buildings or people zone.” Most of the

proposed HUD-assisted projects were either greatly reduced in scope by the need to

eliminate the “hazards to buildings or people zone” of the project or not approved.

The analysis involved in determining how the hazard might delay HUD-assisted project

approval because there were no standards established from which an ASD could be

determined for explosives or fire-prone substances. As a result, HUD decided that there

was a need for the development of two standards:

1. Thermal Radiation – Applicable to people and buildings

2. Blast Overpressure – Applicable to buildings, building occupants and outdoor

unprotected facilities

HUD researched the most relevant sources of information at the time regarding how

stored flammable substances affect people and structures. The research, which

included a review of federal agencies, professional organizations, regulatory agencies,

and councils and boards, revealed that the existing standards specific to each entity’s

function and purpose were limited in both scope and application.

The following are some of the entities that were involved in the research study:

1. Federal Fire Safety Standards: The Federal Fire Safety standards for Liquified

Natural Gas (LNG) Facilities establish thermal radiation standards that are

applicable only to LNG facilities.

2. National Fire Protection Association (NFPA): The NFPA fire standards are limited

to fire hazards and are for the specific use of industry in the determination the

internal configuration of petrochemical facilities.

The first research project was meant to develop appropriate standards for blastoverpressure and thermal radiation and a method for calculating Acceptable Separation

Distances (ASD’s) for HUD-assisted projects. The study,4prepared by Arthur D. Little,

Inc. and titled “A Notice of Proposed Rulemaking (NPR),” was completed in December of

1975. After extensive review and commentary, the standards for blast-overpressure and

thermal radiation were established and published in the Federal Register on August 19,

1980.

4

Safety Considerations in Siting Housing Projects
11

Due to additional comments gathered from the NPR, as well as comments from other

industry experts, HUD decided to do a second research study,5 with the purpose of

addressing two concerns:

1. Simplifying the method of calculating of the Acceptable Separation Distance

(ASD) between HUD-assisted projects and stationary hazards and,

2. Modifying standards already developed by the first study to include additional

substances that were mostly used as fuels in the industry.

A second research project, done by Rolf Jensen and Associates, Inc., of Deerfield,

Illinois, under contract to HUD, provided the required technical support and was used to

address HUD concerns and to complete the current 24 CFR Part 51 Subpart C and the

previous editions of this Guidebook. Regulation 24 CFR Part 51 Subpart C became

effective on February 10, 1984, and amended on March 26, 1996. The research from

the Rolf Jensen and Associates study were encapsulated in a report titled Urban

Development Siting with Respect to Hazardous Commercial/ Industrial Facilities.

Average fire data at stationary bulk liquid fuel storage facilities from 1994 to 1998. Data6

provided from the National Fire Protection Association (NFPA).

Total : Damage off premises :

Annual average fire incidents : 387 Annual average fire incidents : 288

Casualties : 1 Casualties: 0

Injuries : 8 Injuries: 3

Direct Property Damage : $5,540,000 Direct Property Damage: $948,000

Average fire data at stationary LP-gas fuel storage facilities from 1994 to 1998. Data7

provided from the NFPA.

Total : Damage off premises :

Annual average fire incidents : 49 Annual average fire incidents : 26

Casualties : 1 Casualties: 0

Injuries : 5 Injuries: 1

Direct Property Damage : $722.00 Direct Property Damage: $117,000

The data presented supports the reason behind the development of the ASD under the

regulation 24 CFR Part 51 Subpart C, which is to avoid injuring people or destroying

property by properly assessing the distance between project sites and facilities that

store, handle or process flammable or combustible chemicals.

This edition of the Guidebook incorporates the results of the 1981 Rolf Jensen and

Associates study. Later experiments at the National Institutes of Health (see footnote

10) confirm the Regulation standards of thermal radiation for people (450 BTU/ft2-hr )

and the effect of heat on exposed human skin.

5 Urban Development Siting with Respect to Hazardous Commercial/Industrial Facilities

6

Fires at Flammable or Combustible Liquid Tank Storage Facilities: Statistical Analysis

7

Fires at LP-Gas Bulk Storage Plants: Statistical Analysis
12

A review of the Thermal Radiation Standard - Buildings

The standard for the determination of an Acceptable Separation Distance (ASD) for

thermal radiation is based on heat flux. A report8from ABS consulting indicates that

exposures of as little as 4,000 BTU/ft2-hr can cause ignition of wooden structures.

Referenced studies9show that the maximum thermal radiation exposure that

combustible materials can tolerate for a long period of time, without being susceptible to

pilot ignition, is 4,000 BTU/ft2-hr. From 5,000 BTU/ft2-hr to 9,000 BTU/ft2-hr, the

tolerance on combustible materials on the maximum thermal radiation exposure reduces

gradually as the thermal heat flux increases.

According to the Department of Homeland Security, United States Fire

Administration/National Fire Data Center, response times for structure fires are generally

less than 5 minutes 50 percent of the time, regardless of region, season, or time of day.

The nationwide 90th percent response time to structural fires is generally less than 11

minutes.

The maximum thermal radiation heat flux exposure to buildings is 10,000 BTU/ft2-hr for a

maximum duration of 15 minutes.

This is based upon the assumption that there will be fire department response to protect

exposed combustible buildings within 15 minutes and that the exposed combustible

materials will not spontaneously ignite before the fire department responds.

Modern multi-occupant buildings and homes made from wood are more fire resistant

than before. (Research and development of fire resistant wood started in 1985 and was

fully implemented in the construction of buildings and residences by 1997). With updated

developments on advanced additives incorporated into wood and the substitution of

plastic wood in homes and multi-occupant buildings, it takes more than 15 minutes for

exposed combustible materials to ignite spontaneously, which is well within fire

department response times. For multi-occupant buildings constructed before 1985, the

fire department response time is well within the time limits at which exposed combustible

materials spontaneously ignite.Therefore, a parameter standard of 10,000 BTU/ft2-hr

is considered the acceptable level of thermal radiation for buildings.

A Review of the Thermal Radiation Standard - People

Human skin reacts to heat by perspiring and increasing blood flow to the affected area.

Pain is felt when skin temperature rises to just above 44° C or 111° F over a depth of

0.1mm (Normal is 37° C or 98.4° F). 44° C is an approximate value due to various skin

depths, body fat, etc.

The National Library of Medicine and the National Institutes of Health and in

collaboration with the Food and Drug Administration, performed an oximeter study10 to

8

ABS consulting, Risk consulting Division, table 2.9, page 34, various Thermal Radiation Limits

for Structure from Lees (1996).

9

Loss Prevention in the Process Industries

10 “Temperature Threshold for burn Injury, an Oximeter Safety Study”, National Library of

Medicine and the National Institutes of Heath report, 2004., Greenhalgh, G.H., Lawless, B.B.

Chew, W.A. Crone, M.E. Fein, T.L. Palmieri.
13

determine the temperature threshold for burn injury. Pulse oximeters, essential devices

for evaluating and monitoring patient oxygenation, emit a small amount of heat into the

skin then detect a signal.

The standard for thermal radiation flux level for open spaces such as parks and

playgrounds where people of all ages congregate is based on the time it would take to

react and find protection from a fire hazard.

At 41° C or 106° F, the thermal heat flux is 175.3 BTU/ft2-hr. At this heat flux level,

people in open spaces will experience some degree of “redness” on their skin depending

on their distance from the hazard, exposure time and atmospheric conditions such as

wind temperature and speed, etc. This set parameter of thermal heat flux (175.3

BTU/ft2-hr) is the normal standard for protection of people in outside environments.

At 55° C or 228° F, the thermal heat flux is 450 BTU/ft2-hr. At 55° C or 228° F, people in

open spaces will experience a much greater degree of “redness” on the skin, depending

on the distance from the hazard, exposure time and atmospheric conditions such as

wind temperature and speed, etc. This set parameter of thermal heat flux (450 BTU/ft2-

hr) is the maximum limit standard for protection of people in open spaces.

Therefore, the maximum set level parameter of 450 BTU/ft2-hr is considered the

acceptable level of thermal radiation for people in open spaces where people

congregate, such as parks and playgrounds, etc.

A Review of the Blast-Overpressure Standard - Buildings

The criteria for determining Acceptable Separation Distances (ASD’s) for blastoverpressure is based on the pressure wave exerted by detonation of a set mass

quantity of TNT.

The method of comparison, also known as the “TNT Equivalent,” is used because the

blast-overpressure produced by 1 pound of TNT is known. This is the TNT equivalent

weight of the actual explosive.

The TNT equivalent weight is defined by W (in pounds) = mass of the explosive in

question times its relative strength.

For reference purposes, virtually all explosives have TNT equivalent weights within a

factor of 1.4 of their physical weight.

Therefore, the mathematical formula to calculate the scaled distance (Z) is Z = R/W 1/3,

where

W = pounds of TNT equivalent weight at a distance R (the stand off distance) from the

vertical exterior wall of a building (example).

Below is a visual representation of a hazard (TNT equivalent weight – W) and the

standoff distance (R) between the explosive hazard and a structure.
14

The following table illustrates several test results developed by a Department of Defense

and Sandia National Laboratories study11 of explosives detonation and the blastoverpressure effects on different materials. The amount and type of explosive used and

the distance between the explosives and the structural element were not released as

part of the results of the subject study.

Structural element Overpressure effects on

structural element

Approximate peak

positive incident

pressure recorded on

structural element

(psi)

Glass windows, large

and small

Shattering usually,

occasional frame failure

0.5- 1

Corrugated asbestos

siding

Shattering 1.0- 2.0

Corrugated steel or

aluminum paneling

Connection failure following

by buckling

1.0-2.0

Brick wall panel, 8in, or

12 in thick (not

reinforced)

Shearing and flexure

failures

3.0-10.00

Wood siding panels,

standard house

construction

Usually failure occurs at the

main connections allowing a

whole panel to be blown in

1.0- 2.0

Concrete or cinderblock wall panels, 8 in,

or 12 in thick (not

reinforced)

Shattering of the wall 1.5- 5.5

11 “Effects of Nuclear Weapons, Conditions of failure of overpressure-sensitive elements”, Table

5.145, page 221, 1997., Glasstone, S., and P.J. Dolan
15

The most current standard developed from testing on window systems, as used on GSA

buildings and Department of State embassies (Applied Research Associates, Inc.),

reflects a standard for glass overpressure of 0.5 psi.

Therefore, the parameter standard of 0.5 psi is the maximum allowable pressure

that can be measured at a distance from an explosive hazard when selecting a site

for HUD- assisted housing and occupants.
16

3. The aftermath of the January 30, 2007 propane blast at the Little General Store in

Ghent, West Virginia. Enhancements note the approximate location of the store, the

propane tanks and the ambulance overturned in the explosion.
17

Chapter 3 - Compliance Process

_____________________________________________________________________

HUD-assisted projects (as defined in 24 CFR 51.201) shall be in compliance with 24

CFR Part 51 Subpart C. The implementation of the Regulation is the responsibility of

the HUD approving official (certifying officer) or responsible entity.

The Regulation addresses protection of buildings and people when siting HUD-assisted

projects near hazardous operations that handle conventional fuels or chemicals of an

explosive, combustible or flammable nature.

For a HUD-assisted project to be in compliance with the regulation 24 CFR Part 51

Subpart C, the project location must be within the Acceptable Separation Distance

(ASD) established by HUD from facilities that store, handle or process explosive or fire

prone substances.

The key steps to complying with the regulation 24 CFR Part 51 Subpart C are as follows:

Step 1: Identify hazardous operations/facilities with stationary, aboveground storage

tanks in proximity of the proposed HUD-assisted project site.

Step 2: Calculate the ASD between the proposed HUD-assisted project site and the

hazardous facility using thermal radiation and blast overpressure standards.

Step 3: Determine whether the project site meets the standard.

Step 4: If the project meets the standard, no further action is required. However, if the

project does not meet the standard, mitigation (covered in detail in Chapter 5) may be

required.

The ASD is prescribed from the standards set out in the regulation 24 CFR Part 51

Subpart C, as follows:

 Thermal Radiation, 10,000 BTU/ft2- hr

o Applicable to buildings

 Thermal Radiation, 450 BTU/ft2– hr

o Applicable to people

 Blast-Overpressure, 0.5 psi

o Applicable to buildings, building occupants and outdoor unprotected

facilities

To protect buildings and housing units from thermal radiation, HUD established the

thermal radiation standard of 10,000 BTU/ft2– hr.
18

To protect people in outside areas, such as patios or common areas, or in places where

communities congregate, like parks or recreation areas, HUD established the thermal

radiation standard of 450 BTU/ft2-hr. If no mitigation exists or is implemented, it is

required to build HUD-assisted projects to the ASD at which the thermal radiation flux

will not exceed 450 BTU/ft2-hr.

Blast overpressure can harm people or destroy buildings if this pressure is higher than

0.5 psi. For proposed HUD-assisted project sites where there are stored hazards that

can cause blast overpressures and no mitigation, it is required to build to the ASD at

which this pressure is no higher than 0.5 psi.

HUD will not approve applications for assistance unless 1) the proposed project meets

the required ASD from the facility assessed as hazardous; or and 2) appropriate

mitigation measures or plans for appropriate mitigation are implemented.

Appropriate mitigation measures are classified as either natural or man-made structures

that shield or serve as an abatement to the HUD-assisted project site from the blast

overpressure and/or thermal radiation effects from the hazard. Whether natural or manmade, mitigation measures shall be stationary, of adequate size and strength, and

located between the hazard and the proposed HUD-assisted project site, and shall

shield all points of the project from the line of sight exposure of the hazard. (Chapter 5 of

this Guidebook covers mitigation measures in detail.)

If the HUD-assisted project is a facility that will process, manufacture or store hazardous

substances, HUD will not approve applications for assistance unless the proposed

project is located at the required ASD from housing developments and from facilities or

areas where people congregate or engage in outdoor activities.

In the case for all applications for HUD-assisted projects, the Department shall evaluate

projected development plans for the installation or existence of hazardous operations

near the location of the proposed HUD-assisted project. If such hazardous facility exists,

or there are plans for the development of such facility, the project must comply with the

standards of the Regulation, or adequate mitigation measures must exist, or the

Department must obtain satisfactory assurances that adequate mitigation measures will

be taken where the hazardous operation is installed before the Department approves the

applications for development of such projects.
19

4. Fire and smoke caused by the explosion of a propane tank at a maintenance facility

for Como Oil, located at Duluth, Minnesota.
20

Chapter 4 – Evaluations and Findings

_____________________________________________________________________

To determine if the proposed HUD-assisted project meets the established standards for

blast-overpressure or thermal radiation, HUD or the responsible entity is required to

calculate the Acceptable Separation Distance (ASD) between the proposed project and

the hazardous facility.

The following steps are intended to help the user of this Guidebook understand the

required process necessary for evaluating a HUD-assisted project that is located in close

proximity to a potentially hazardous site.

Step 1: Obtain a map, site plan or photos of the proposed HUD-assisted project

site and surrounding areas.

 Maps

o Scaled maps with site contours (topographic maps) are highly

recommended for the analysis of proposed HUD-assisted project sites.

These maps make it easier to determine the existence of natural or

man- made barriers and to perform an analysis of these barriers with

reference to the location of the facility storing hazardous or explosive

hazards.

o Topographic maps or site maps can be obtained from the United States

Geological Survey (USGS), City or County engineer’s or planning office

related to the proposed HUD-assisted project.

o Digitized topographic maps (free of cost) can be obtained from the

World Wide Web (WWW) for general public use from the following

sources. (HUD does not endorse any of the referenced electronic

sources of information.)

 Yahoo Maps, http://developer.yahoo.com/maps/

 Google Earth, http://earth.google.com/

o Site maps provide geographic site information for recognizance and

analysis of the various facilities near proposed HUD-assisted project

sites.

 Site plans

o Site plans provide detailed existing land-use information for proper

identification and analysis of the areas comprising the proposed HUDassisted project site. Site plans may be obtained from the engineer’s,

planning or developer’s office working on the HUD-assisted project.

 Photos

o Aerial photographs may be available from the developer of the HUDassisted project, local commercial survey firms or planning departments.

o Site photographs provide actual images of the facilities near proposed

HUD-assisted project sites and nearby land uses when current. Site

photographs provide valuable information for site evaluation. This

information can be gathered (camera required by the person gathering

the information) by scheduling a visit to the hazardous facility (making
21

sure that photographs are allowed) to be assessed and contacting the

facility site manager.

Step 2: Use the maps, photos and plans of the proposed HUD-assisted project

site, identify the following:

 HUD-assisted project site boundaries – to determine the area where facilities

that store, manufacture or process hazardous substances are located, plot a

mile-radius perimeter from the center of the proposed HUD-assisted project site.

o Consider all the stationary, above-ground containers containing

hazardous substances that can pose a hazard to the buildings or open

areas where people congregate, play, etc., associated with the HUDassisted project.

o Collect the following information for each container to be considered:

 Container’s capacity (in gallons).

 Container’s product chemical name (not the trade name).

 Container’s product phase of state (gases or liquids).

 If the container is unpressurized (containing liquids), find out if the

container is diked or undiked. If it is diked, determine the dike

area (Length times the Width - in square feet units).

o Measure the distance between the proposed HUD-assisted project site

and the container(s) being assessed.

Actual distances from the hazards to the buildings or open areas

where people congregate should be plotted in “feet” units. This

distance must be measured from the center of the above-ground

storage container(s) to the perimeter of the proposed HUD-assisted

project site.

o Determine which container(s) to use in making ASD calculations.

Guidelines on container evaluation (applies to pressurized and nonpressurized containers) for ASD calculations

 When there is a facility with stationary above-ground storage

containers and diked areas of the same size, the ASD needs to

be calculated for the container or diked area closest to the

proposed HUD-assisted project site.

 When there is a facility with stationary above-ground containers

and diked areas of different sizes, the ASD needs to be

calculated for the container or diked area of largest capacity

closest to the proposed HUD-assisted project site.

 When there is a facility with the same type of stationary aboveground containers of different capacity and the smaller capacity

container is the one closest to the proposed HUD-assisted project

site, the ASD must be calculated for the container that is closest

When plotting

the actual

distances from

the hazards in

stationary,

above ground

storage

containers,

always

measure the

distance from

the center of

the containers

to the perimeter

of the proposed

HUD-assisted

project site.

How do you

evaluate your

containers for each

facility?
22

to the proposed HUD-assisted project site, as well as the largest

capacity container.

 Calculate the ASD for the stationary container of largest capacity

(whether pressurized or unpressurized) closest to the proposed

HUD-assisted project. Always calculate the ASD for the

pressurized container, regardless of its location. Consider only

aboveground storage tanks.

Information on containers:

Within the regulation 24 CFR Part 51 Subpart C, there are 5 types of

above-ground stationary containers, as follow:

1. Pressurized containers (gases) designed to store a liquidgas mixture substance under pressure. If the substance

contained is a hazardous substance, the ASD for this

container must be calculated for blast-overpressure

(explosion) and thermal radiation (fire). If the substance

contained is not of a flammable or combustible nature, the

container does not need to be considered for the ASD

analysis.

2. Pressurized containers (gases) holding cryogenic

hazardous substances kept in a liquid state by

temperature control, consider ASD calculations for fire.

For hazardous cryogenic gases kept in gas phase,

consider ASD calculations for fire and explosion.

Hazardous cryogenic gases are kept in their liquid state at

very low temperatures by refrigeration, insulation, etc.

Hazardous cryogenic gases (e.g., Liquefied Natural Gas

and Hydrogen) are listed under the “Hazardous Gases” list

located in Appendix I of Subpart C and at Appendix C of

this Guidebook.

3. Unpressurized containers designed to store a substance

in a liquid phase of state (for liquids), calculate the ASD

for thermal radiation (fire) only if the substance is of a

hazardous nature. Since the substance contained is not

under pressure, there is no need to calculate the ASD for

blast-overpressure (explosion). If the liquid substance

contained is not of a hazardous nature, the container does

not need to be considered for the ASD analysis.

How do you

evaluate your

containers for each

facility?

This

Guidebook,

Regulation 24

CFR Part 51

Subpart C, and

the ASD

assessment

tool all apply to

these types of

containers.
23

4. Underground storage containers (liquids or gases) that are

designed to store liquids or gases that are placed under

the ground do not apply under this Guidebook, the

regulation 24 CFR Part 51 Subpart C, or the ASD

assessment tool.

5. Retrofitted mobile containers (tank trucks, barges, railroad

tank cars containing liquids or gases) that have the

capacity to store common liquid industrial fuels or

hazardous gases as listed in Appendix I do not apply

under this Guidebook, the regulation 24 CFR Part 51

Subpart C, and the ASD assessment tool.

Recommended guideline on calculating the ASD for pressurized and nonpressurized containers:

1. The ASD calculation will not be affected by the diked

area on pressurized containers (hazardous or nonhazardous substances) because gases occupy the

volume of a container. If a leak occurs, the gas

escapes to the atmosphere while the liquid boils and

turns into vapor. Therefore, the diked area is not

considered in the analysis.

ASD calculation facts on unpressurized containers:

1. If the container is diked: the ASD will be of a smaller

numerical value for diked containers vs. undiked

containers storing hazardous liquids because the

diked area defines the fire width, limiting the flames,

heat dispersion and extension.

2. If the container is undiked: the ASD will be of a larger

numerical value for undiked containers storing

hazardous liquids. Without the dike area, the fire has

no boundary or controlled perimeter.

Step 3: Calculate the ASD for the proposed HUD-assisted project:

The ASD can be calculated between a proposed HUD-assisted project and a

facility that stores, handles or processes hazardous substances for:

 Thermal radiation (fire)

 Blast-Overpressure (explosion)

 Or both

Steps 1 and 2 of this Chapter, provide the user of this Guidebook with the steps required

to gather the data necessary for calculating the ASD between a proposed HUD-assisted

project and a facility that stores, handles or processes hazardous substances.

This

Guidebook,

Regulation 24

CFR Part 51

Subpart C and

the ASD

assessment

tool do not

apply to these

types of

containers.

This is an ASD calculation

guideline for pressurized

containers. The diked area is

never used for calculation of

the ASD in pressurized

containers.

These are ASD

calculation facts for

non-pressurized

containers. Undiked ,

unpressurized

containers have a larger

ASD numerical value

than diked because the

dike defines the fire

width as a controlled fire.
24

Flowcharts illustrating procedural calculation of the ASD for Thermal radiation, blastoverpressure or both can be found in Appendix A.

The ASD can be calculated using one of the following methods:

Manual Calculation (Nomographs)

Using the Nomographs located in Appendix B, calculate the ASD by plotting the required

information (represented by the x- axis) to the respective graph (graphs are represented

by a logarithmic line, as in worksheets #1 through #4, Appendix B) and then plotting the

intersection point (between the x-axis and the graph) to the y- axis. The y-axis

represents (depending on the worksheet) an interim calculation result required in the

procedure to obtain the ASD or the ASD.

Electronic based calculation (The ASD Assessment Tool)

The web-based tool for calculation of the ASD offers an electronic application of the

manual approach to calculation requirements contained in regulation 24 CFR Part 51

Subpart C and this Guidebook. Mathematical equations were derived from the

Nomographs located in the Regulation and applied to the component parts of the

assessment to determine the ASD between a proposed HUD-assisted project site and

facilities that store, handle or process hazardous substances. This tool is accessible

through the following URL:http://www.hud.gov/offices/cpd/environment/asdcalculator.cfm
25

Acceptable Separation Distance Principles

Most of these principles have been cited through the previous chapters of this

Guidebook. However, because of their significance in the Regulation, they are all

reiterated here:

1. There are two standards from which ASD calculations are produced:

o Thermal Radiation (fire)

 Buildings – 10,000 BTU/ft2– hr

 People – 450 BTU/ft2– hr

o Blast-Overpressure (explosion)

 Buildings, building occupants and outdoor unprotected facilities–

0.5 psi

2. When measuring the ASD between a proposed HUD-assisted project and a facility

that stores, handles or processes hazardous substances, always measure from the

center of the stationary container containing the hazardous substance to the

perimeter of the proposed project site. The perimeter is defined as the path or

boundary that surrounds an area. In the case of proposed HUD-assisted projects,

this area is the site where the project will be developed.

3. ASD calculations must be made between a proposed HUD-assisted project and a

facility that stores, handles or processes hazardous substances. When calculating

the ASD for thermal radiation for people, outdoor areas where people congregate

must be assessed, such as:

o Playgrounds

o Outdoor recreation areas (parks, yards, planned open space)

o Balconies

o Residential parking lots (only parking lots associated to residential

projects are considered applicable to the Regulation, since they are open

spaces for congregation)

4. For stationary, above-ground pressurized containers that hold hazardous

substances, the ASD calculations are generally required for blast-over pressure and

thermal radiation.

5. For stationary, above-ground unpressurized containers that hold hazardous

substances, the ASD calculations are generally required only for thermal radiation.

6. The diked area (in square feet) has an effect only on ASD calculations involving

stationary, above-ground unpressurized containers that store hazardous

substances. The diked area does not have an effect on ASD calculations involving

stationary, above-ground pressurized containers that hold hazardous substances.

7. The information contained in this Guidebook and the ASD assessment tool do not

apply in the following situations, which are excluded from Part 51 Subpart C (see

page 4 for details):

o Underground storage containers
26

o Stationary containers of 100 gallons or less capacity containing common

liquid industrial fuels

o Natural gas holders with floating tops

o Mobile conveyances (tank trucks, barges, railroad tank cars). Mobile

conveyances, while performing fuel operations (loading or unloading fuel,

etc.) into a gas station, fuel transfer or storing facility, are not to be

considered as part of the ASD analysis for the proposed HUD-assisted

project if the proposed project is the fuel transfer or storing facility, since

in the fuel operation process (mobile conveyance being stationary), there

is the release of fumes from the fueling operation procedure, therefore,

moving conveyances do not apply to the Regulation.

o Pipelines, such as high pressure natural gas transmission pipelines or

liquid petroleum pipelines.

o Release of toxic gases or liquids, distribution piping associated with a

container or process vessel

8. If a proposed HUD-assisted project has more than one ASD, the ASD which assures

the greatest separation distance will be applied to the proposed project.

9. If the actual separation distance between the hazardous facility-tank and the

proposed HUD-assisted project is greater than the ASD, then the actual separation

distance is acceptable for the proposal.

10. If the actual separation distance between the hazardous facility-tank and the

proposed HUD-assisted project is less than the ASD, then the site is unacceptable

unless natural or man-made mitigation measures already exist or are implemented

between the above-ground stationary hazard and the proposed project site.

11. Consider the ASD for thermal radiation (fire) only for gases kept in a liquid phase of

state by a low temperature setting (known as hazardous cryogenic gases).

Hazardous cryogenic gases (e.g., Liquefied Natural Gas and Hydrogen) are listed

under the “Hazardous Gases” list located in the Appendix I of Subpart C and in

Appendix C. For hazardous cryogenic gases kept in gas phase, consider ASD

calculations for fire and explosion.

12. Regulation 24 CFR Part 51 Subpart C, this Guidebook, and the ASD assessment

tool apply to HUD-assisted projects located near hazardous operations which store,

handle or process hazardous substances. No provision was made for waiver of the

Regulation or for waivers of the ASD standard.

13. Barriers for blast-overpressure should be constructed as close to the hazard source

as possible. The barrier design, location and construction must be site specific.

14. Gasoline service stations usually do not fall within the purview of 24 CFR Part 51,

Subpart C. In most gasoline service stations, the containers where the fuel is stored

are underground, and underground storage containers do not apply to the

Regulation. However, if a gasoline service station does have above-ground

stationary containers of capacities of greater than 100 gallons that store common

liquid industrial fuels (such as gasoline, fuel oil, kerosene and crude oil), then those

containers would be considered under the Regulation.
27

15. If the ASD cannot be achieved between a hazard involving thermal radiation effects

and a proposed HUD-assisted project, please refer to Chapter 5 – Mitigation Options

of this Guidebook. If difficulties are found calculating the ASD or performing a

mitigation analysis for the proposed HUD-assisted project site, contact the respective

HUD Field or Regional Environmental Officer. The thermal heat flux effects on the

proposal need to be calculated by a licensed professional engineer on a case by

case basis, and depending on the results of the analysis, if a fire mitigation wall is

required, it may not be feasible for the developer due to the size requirements to

mitigate the fire ball produced by the hazard.

16. If the ASD cannot be achieved on a hazard involving blast-overpressure effects on a

proposed HUD-assisted project, please refer to Chapter 5 – Mitigation Options of this

Guidebook. If difficulties are found calculating the ASD or performing a mitigation

analysis for the proposed HUD-assisted project site, contact the HUD Field or

Regional Environmental Officer. Blast barriers should be designed and implemented

based on the calculation of the peak positive incident pressure (blast-overpressure)

produced by the hazard. The blast barrier should be implemented as close as

possible to the hazard producing the blast-overpressure. Only licensed professional

engineers (civil or structural) should design and implement blast barriers. Design and

implementation of blast barriers is site specific.

17. The Regulation does not apply to safety requirements of employees at facilities that

store, handle or process flammable or explosive substances. Such standards and

guidance or safety requirements would fall within the purview of the National Fire

Protection Association (NFPA), local fire codes, and permitting requirements, and the

Occupational Safety and Health Administration.

Important Questions and Answers Regarding 24 CFR Part 51 Subpart C:

The following questions are often asked of HUD-Headquarters personnel regarding the

regulation 24 CFR Part 51 Subpart C:

1. Are above-ground, stationary tanks that contain flammable or explosive

petrochemical fuels and are ancillary to the operation of a building (e.g., comfort

heating, cooking, water heating) that is not a 1-4 family - FHA insured property

subject to the Regulation?

Answer : Yes, unless the tanks have a capacity of 100 gallons or less and

contain common liquid industrial fuels, the ASD must be calculated for aboveground, stationary tanks containing flammable or explosive petrochemical fuels

ancillary to the operation of a building if is not a 1-4 family - FHA insured

property.

2. Does the Regulation consider tank designs (e.g., double wall, fire resistant,

protected) on the ASD calculation procedures for a proposed HUD-assisted

project site?

Answer: No, the Regulation does not consider tank designs on the ASD

calculation procedures for a proposed HUD-assisted project.
28

3. Does the Regulation consider fire suppression systems (e.g., water, high

expansion foam, Halon gas, Aqueous Film Forming Foam) as a substitute for the

ASD or mitigation?

Answer: No, the Regulation does not consider fire suppression systems as a

substitute for mitigation or the ASD.

4. When should above-ground, stationary tanks containing hazardous substances

be moved from a proposed HUD-assisted project in order for the project to be in

compliance with the Regulation?

Answer: The tanks must be moved before the HUD-assisted building or site is

occupied or used.

5. Does the Regulation provide standards for the location of above-ground

stationary storage tanks for the protection of construction workers?

Answer: No, the Regulation does not regulate or provide standards for the

location of storage tanks for the safety requirements of construction workers.

Such standards and guidance or safety requirements would fall within the

purview of the National Fire Protection Association (NFPA), local fire codes and

permitting requirements, and the Occupational Safety and Health Administration,

respectively.

6. Does the Regulation apply to protecting HUD-assisted hazardous facilities from

the facility’s own tanks?

Answer: No, the Regulation does not apply to the protection of the proposed

HUD-assisted industrial facility from the facility’s own tanks. For a proposed

HUD-assisted project involving the installation of a hazardous facility, the

Department shall ensure that such hazardous facility is located at an acceptable

distance from residences and from any other facility or area where people may

congregate.

7. Does the Regulation apply to public facilities, such as sewer and water

treatment plants that may have water tanks, underground sewer lines, and

pumping facilities?

Answer: Public facilities like the ones described above are not subject to

consideration under the Regulation as it applies to above-ground stationary

storage containers that hold hazardous substances. The Regulation does not

generally apply to public facilities unless there are above-ground stationary

containers on site that would meet the criteria contained in the Regulation.
29

What are the steps to follow after the ASD of a proposed HUD-assisted project has been

calculated?

 If the ASD meets the standard requirement, no further action is required.

 If the ASD does not meet the standard requirement, mitigation (explained in

Chapter 5) may be required.

 If the ASD does not meet the standard requirement and mitigation is not

possible, consult Chapter 5 (mitigation analysis #9).
30

Example 1: On the determination of the ASD for blast-overpressure (explosion) and thermal

radiation (fire):

In this hypothetical case, a proposed HUD-assisted project is to be sited 850 feet from a

stationary above-ground, 30,000 gallon liquid propane gas (LPG) container. The objective is to

determine the Acceptable Separation Distance (ASD) from the proposed HUD-assisted project

to the LPG container. Since LPG has flammable properties and it is under pressure (gases), the

ASD must be evaluated for blast-overpressure and thermal radiation.

Follow the flowchart below for a flammable, non-cryogenic substance contained in a stationary

above-ground container under pressure in order to determine the ASD for Thermal Radiation

(fire) and Blast-Overpressure while knowing the substance properties and the volume of the

container.

Proceed to calculate the fire width, using Worksheet 1. Use Worksheet 1 because the dike area

on a pressurized container does not have any effect on the ASD calculations. Following with the

procedure, the volume of the container (30,000 gallons) is plotted in the x-axis toward the line

graph, making an intersection with the line graph. This intersection point is then plotted toward

the y-axis, providing the fire width (see figure 1). The fire width was determined to be 350 feet.

Proceeding with ASD calculation procedures for people and buildings, use Worksheet 3. The

fire width (x-axis) is plotted toward the first line graph (bottom graph), making an intersection

with the graph. This intersection point is plotted toward the y-axis, providing the ASD for

buildings (see figure 2). The ASD for buildings was determined to be 240 feet. Further on, the

first intersection point (bottom graph), is (continued in page 33)

Calculating

ASD’s for

Blast -

Overpressure

(Start)

Data

requirements:

Substance

properties and

container’s

volume

Stationary

above-ground

pressurized

container ?

ASD calculation procedure

complete (Stop)

Substance

properties:

Cryogenic

liquified gas ?

Calculate ASD’s for

Bldgs, go to

worksheet #4

Calculate

Fire width,

go to

worksheet

#2

Calculate ASD

for People and

Bldgs, go to

worksheet #3

No

Yes

No

Yes

Diked

container?

Yes

Calculate:

Fire width,

go to

worksheet

#1

No

Calculate:

Fire width, go to

worksheet #1

Calculate ASD for

People and Bldgs, go to

worksheet #3
31

Figure 1: Fire Width calculation (Undiked Container)

350 feet

30,000 gallons
32

Figure 2: Acceptable Separation Distance Calculation for Buildings and People (Thermal

Radiation)

1,150 feet

240 feet

350 feet
33

plotted toward the second line graph (top graph), making an intersection with the graph. This

intersection is plotted toward the y-axis, providing the ASD for people (see figure 2). The ASD

for people was determined to be 1,150 feet.

In this case, the actual distance from the center of the LPG container to the perimeter of the

proposed HUD-assisted project is 850 feet. The proposed HUD-assisted project complies with

the thermal radiation standard for buildings, but not for people. Since the project cannot achieve

the ASD for thermal radiation for people (the ASD is 1,150, which is not within the actual

distance of the proposal, measuring from the center of the LPG container), a mitigation analysis

(presented and explained in detail in Chapter 5) would be required.

Since the product is contained under pressure (gases) and does have flammable properties, the

ASD for blast-overpressure must be calculated. Referring to the flowchart used in this example,

and knowing the volume of the LPG container (in gallons), use Worksheet 4. Plot the volume of

the container (x axis) toward the line graph and make an intersection with the graph. Further on,

plot this intersection toward the y axis to determine the ASD for blast-overpressure. In this case

this value is 660 feet (see figure 3). This proposed HUD-assisted project complies with the

blast-overpressure standard for buildings (the ASD is 660 feet, which is within the actual

distance of the proposal, measuring from the center of the LPG container).
34

Figure 3: Acceptable Separation Distance calculation for Buildings (Blast-Overpressure) ,

Building Occupants and outdoor unprotected facilities

660 feet

30,000 gallons
35

Example 2: On the determination of the ASD for thermal radiation (fire)

In this hypothetical case, a proposed HUD-assisted project is to be sited 850 feet from a diked,

stationary, above-ground 30,000 gallon gasoline container. The objective is to determine the

ASD from the proposed HUD-assisted project to the gasoline container. Since gasoline has

flammable properties and is not under pressure (liquid), the ASD must be evaluated only for

thermal radiation.

Follow the flowchart below for a hazardous substance contained in an above-ground stationary

container not under pressure for the determination of the ASD for Thermal Radiation (fire),

knowing the substance properties, diked area, and the volume of the container.

Knowing the diked area (length times the width in square feet) of 60,000 square feet (200ft

times 300ft), calculate the fire width using Worksheet 2. The diked area of the container (60,000

square feet) is plotted in the x-axis toward the line graph, making an intersection with the line

graph. This intersection point is then plotted toward the y-axis, providing the fire width (see

figure 4). The fire width was determined to be 235 feet.

Proceeding with calculation procedures for the ASD for people and buildings, use Worksheet 3.

The fire width (x-axis) is plotted toward the first line graph (bottom graph), making an

intersection with the graph. This intersection point is plotted toward the y-axis, providing the

ASD for buildings (see figure 5). The ASD for buildings was determined to be 160 feet. Further

on, the first intersection (bottom graph) is plotted toward the second line graph (top graph),

making an intersection with the graph.

Calculating

ASD’s for

Thermal

Radiation

(Start)

Data

requirements:

Substance

properties and

container’s

volume

Stationary

above-ground

unpressurized

container ?

Substance

properties:

Hazardous

substance ?

Diked

container?

Calculate:

Fire width,

go to

worksheet

#2

Calculate:

Fire width,

go to

worksheet

#1

Calculate:

ASD for

People and

Bldgs, go to

worksheet

#3

ASD calculation procedure

for Thermal Radiation

complete (Stop)

No

Yes

No

Yes Yes

No
36

Figure 4: Fire Width calculation (Diked Container)

235 feet

60,000 square feet
37

Figure 5: Acceptable Separation Distance Calculation for Buildings and People (Thermal

Radiation)

790 feet

160 feet

235 feet
38

This intersection is plotted toward the y-axis, providing the ASD for people (see figure 5). The

ASD for people was determined to be 790 feet.

In this case, the actual distance from the center of the gasoline container to the perimeter of the

proposed HUD-assisted project is 850 feet. The proposed HUD-assisted project complies with

the thermal radiation standard for buildings and for people.
39

5. View of the aftermath of an explosion (caused by a chemical contained under

pressure) at the Georgia Pacific Plant located in Ohio.
40

Chapter 5 – Mitigation Options

_________________________________________________________________

This chapter provides information about mitigation analysis and how to determine if mitigation is

required on a proposed HUD-assisted project. This chapter identifies the data points and

explains the analytic guidelines for a proposed HUD-assisted project, focusing on HUD

standards for blast-overpressure and thermal radiation produced by stationary hazards covered

under 24 CFR Part 51 Subpart C.

Mitigation is required to protect buildings and people when the ASD cannot be met between the

specific stationary hazardous operation and the proposed project site. Mitigation applies only

for HUD-assisted projects in proximity to stationary hazardous operations that have aboveground stationary storage containers of 1) more than 100 gallon capacity that contain common

liquid industrial fuels (see Appendix I of the Regulation and Appendix C of this Guidebook); and

2) of any capacity that contain hazardous liquids or gases that are not common liquid industrial

fuels (see also the list of hazardous substances in Appendix C of this Guidebook).

Mitigation analysis:

The best mitigation is based on site analysis using the following 11 questions:

1. Has the Acceptable Separation Distance (ASD) been calculated?

If the ASD has not been calculated, then calculate the ASD. The ASD is the first step of

the mitigation analysis for HUD-assisted projects near hazardous operations that store,

handle or process flammable or explosive substances.

If the ASD has been calculated, two results are possible: 1) The ASD is

achievable using the proposed site and no further action required, or 2) The ASD

is not achievable using the proposed site and so mitigation may be required.

2. Where is the technical information for the ASD determination available?

Technical assistance is available from the following sources:

o This guidebook,

o Regulation 24 CFR Part 51 Subpart C, and

o HUD field environmental staff (Regional Environmental Officers and Field

Environmental Officers)

3. What properties make the substances stored at the site hazardous?

Substances in general are classified as liquids, solids or gases. The physical state of

matter of existent substances are in liquid, gas or solid form. Gases are stored under

pressure, liquids cannot be compressed and take the form of the storage container in

question. Solids are incompressible and have a solid structure.

If the substances being assessed are hazardous and/or have blast-overpressure

properties and are in stationary above-ground containers, a hazardous analysis is

required for the proposed HUD-assisted project site. For ASD determination and
41

mitigation hazard analysis of a proposed HUD-assisted project site, only hazardous

liquids or gases and/or blast-overpressure properties are considered. Pressurized

containers with hazardous substances, if ruptured, can cause steam gas explosions or

gas explosions. For products (substances) stored under pressure, two results are

possible:

o Blast-overpressure or pressure wave

o A fireball accompanied by a pressure wave (blast-overpressure)

4. What differences are there between a diked and an undiked container?

If containers are diked:

o The ASD calculation will not be changed by the diked area on pressurized

containers (hazardous substances); and

o The ASD will be of a smaller numerical value for diked containers vs. undiked

containers storing hazardous liquids.

If containers are undiked:

o The ASD will be of a larger numerical value for undiked containers vs. diked

containers storing hazardous liquids.

(Chapter 4 includes calculation guidelines and facts for pressurized and non-pressurized

containers.)

5. What role does topography have in influencing ASD calculations? Do natural or

man-made barriers between the proposed HUD-assisted project and the hazard

make a difference?

Natural barriers are hills, mountains, earthen elevations, etc. Man made barriers are

buildings, housing developments and other structures. Natural and man-made barriers

may serve as abatement from thermal radiation or blast-overpressure effects that can

have an impact on HUD-assisted projects and the people who live and work there.

If there are natural or man-made barriers between the proposed HUD-assisted project

site and the hazard, the available barrier must serve to abate the effects of thermal

radiation, blast-overpressure or both from the hazard.

The following points provide valuable information to evaluate the available barrier

between the proposed HUD-assisted project and the hazard:

 Man-made or natural barriers may serve to abate the effects of thermal radiation or

blast-overpressure on HUD-assisted projects and the people who live and work

there.

o If the ASD is not achievable between the proposed HUD-assisted project site

and the hazardous operation/facility, but there is no clear line of sight

between the proposed HUD-assisted project and the hazard, mitigation may

not be required. Under the regulation 24 CFR Part 51 Subpart C, if there is a

natural or man-made abatement between the proposed HUD-assisted project
42

and the hazard that impedes a clear view, the abatement might serve as

mitigation for the proposed HUD-assisted project.

 If it has been determined that mitigation may not be required using the

above mentioned analysis, the natural or man-made abatement must

be further analyzed to ensure it will provide an acceptable level of

mitigation for the proposed HUD-assisted project site. Only a licensed

professional engineer should analyze and confirm the acceptability of

preexisting barriers based on the hazard being analyzed.

o If the ASD is not achievable between the site to be developed and the hazard

and there is a clear line of sight between the proposed HUD-assisted project

and the hazard, mitigation is required.

If there are no natural or man-made barriers between the proposed HUD-assisted

project site and the hazard and the ASD is not achievable, there are mitigation

options described in this Guidebook to achieve abatement and compliance with HUD

standards.

6. Where can I get maps or other geographic information?

 Sources of information:

i. Maps and related site (topographic) information are available from the

United States Geological Survey (USGS), City or County engineer’s or

planning office related to the proposed HUD-assisted project.

ii. Although HUD does not endorse these sources, free digitized topographic

maps and other relevant information can be found at

a. Yahoo Maps, http://developer.yahoo.com/maps/

b. Google Earth, http://earth.google.com/

 Types of information:

i. Scaled maps with site contours (topographic maps) are highly

recommended for the analysis of proposed HUD-assisted project sites.

ii. Site maps similar in content to the scaled maps, but without site contours,

are recommended as an initial type of geographic based information for

analysis of proposed HUD-assisted project sites.

iii. Site photos provide actual images of the facilities near proposed HUDassisted project sites and are recommended for use with scaled and site

maps for the analysis of proposed HUD-assisted project sites.
43

7. What sources can I use if the facility storing these hazards will not release subject

information?

 Fire Department: the local fire department can provide information about facilities

and operations

 Local Emergency Planning Committee Database: the governor of each state has

designed a State Emergency Response Commission Contact (SERC) whose

responsibility is to implement the Emergency Planning Community Right to Know

Act (EPCRA) provisions within that State. The SERC supervises and coordinates

the activities of the Local Emergency Planning Committee (LEPC) for each

district, establishes procedures for receiving and processing public requests for

information collected under EPCRA, and reviews local emergency response

plans. The local LEPC database for public use can be found at

http://YOSEMITE.EPA.GOV/OSWER/LEPCDB.NSF/SearchfORM?OpenForm

8. Fire suppression systems, used along with sirens and fire alarms for protection of

buildings and people, are designed to extinguish fires by automatically discharging

fire suppressing media (e.g., water, high expansion foam, Halon gas) at areas that

require fire protection.

 Can fire suppression systems modify or change the analysis to obtain Acceptable

Separation Distances (ASDs)?

o No (not at this time).

 Can fire suppression systems modify or change the analysis as an alternative to

ASDs?

o Neither the Regulation, the Guidebook nor the ASD assessment tool

consider the use of fire suppression systems as an alternative or

modification to ASDs or as part of the ASD analysis.

9. What are the options if you cannot achieve the ASD?

In addition to a barrier, other options include:

 Burying the hazard, an alternative that is often less expensive than building a

mitigation barrier.

 Modifying the building design to compensate for the ASD. The building design

can be modified by using heat retardant and high tensile strength materials in the

direction where the hazardous facility is located in order to compensate for the

ASD. Buildings can also be re-arranged and their exterior shapes modified. A

combination of these approaches may be used to provide an acceptable level of

mitigation (e.g., a horseshoe-shaped building can be oriented with the convex

curve facing the hazard and the structure augmented with heat retardant and

tensile strength materials).

 Choosing a different site.
44

 Resorting to a barrier.

10. How does a barrier work?

 A barrier works as an abatement for thermal radiation and blast-overpressure

and provides mitigation to HUD-assisted projects when the Acceptable

Separation Distance (ASD) is not achievable.

11. Who should design a barrier?

 Only a licensed professional engineer (civil or structural) should design and

oversee the construction of mitigation barriers
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Flowchart illustrating a mitigation analysis for a proposed HUD-assisted project site

Mitigation

Analysis

24 CFR

51C

(Start)

ASD achievable

for proposed

HUD-assisted

project site from

the hazard site ?

Existent natural

or man-made

barriers between

the proposed

HUD-assisted

project site and

the hazard site ?

Can the hazard

site be seen

standing from

the proposed

HUD-assisted

project site ?

No further action

required (Stop) Suggested

Mitigation

Options:

1. Burying the

hazard

2. Modifying

the building

design to

compensate

for the ASD

3. Choosing

another site

4. Resorting

into a barrier

Natural or man

made barrier of

adequate size and

strength to shield

the proposed

HUD-assisted

project ?

(Only a licensed

professional

engineer (civil or

structural) can

make this

determination)

Is a Barrier

required for the

proposal ?

Provide the 24 CFR Part 51 Subpart C

Barrier Design Guidance to the licensed

professional, for calculation of the

required barrier design criteria, based on

the Regulation standards of blast -

overpressure and thermal radiation

Apply one of

the

suggested

mitigation

options other

than barriers

Yes

No Yes

No

No

Yes

No

Yes

Yes

No
46

The following scenarios illustrate mitigation options involving natural and man-made

barriers if the ASD cannot be achieved between the proposed HUD-assisted project and

the hazard being assessed.

Scenario 1: A man-made barrier (warehouse) is located between a proposed housing

project and a stationary above-ground storage tank as a mitigation measure, following

the regulation 24 CFR Part 51 Subpart C.

Stationary Aboveground Storage Container

Barrier for hazards of fire or explosive prone nature

Housing project

Line of Sight

_______________________________________________________________________

Available distance between the housing project and the container

Acceptable Separation Distance (ASD)
47

Scenario 2: A natural barrier (mountain or hill) is located between a proposed housing

project and a stationary above-ground storage tank, providing mitigation to the housing

project if the tank explodes or catches on fire.

Stationary Aboveground Storage Container

Housing Project

Mountain or Hill

Line of Sight

Available distance between the housing project and the container

Acceptable Separation Distance (ASD)
48

Scenario 3: Reconfigure/relocate proposed HUD-assisted project to increase the

available distance between the project and the hazard and achieve the required

Acceptable Separation Distance (ASD) in accordance with the regulation 24 CFR Part 51

Subpart C.

Proposed HUD-assisted project

Project was relocated to achieve

the required ASD Stationary Aboveground Storage Tank (hazard)

Required ASD
49

Scenario 4: Modification of the building design to compensate for the ASD.

Stationary Above Ground Storage Container

The wall facing the storage container has been redesigned

to mitigate the effects of explosion or fire if the container

explodes or catches fire.

Housing project

Line of Sight

_______________________________________________________________________

Available distance between the housing project and the container

Acceptable Separation Distance (ASD)
50

6. The aftermath of the January 10, 2008 propane blast near a unit townhouse complex in

Brampton, California. More than a dozen propane containers had been stored near the

townhouse complex.
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Chapter 6 – Extraordinary Circumstances

_________________________________________________________________

This chapter identifies extraordinary circumstances which, due to a proposal’s complexity,

require consultation with HUD-Regional and Field Environmental Officers and additional

information and procedures that are not provided in the regulation 24 CFR Part 51 Subpart C.

Applicants for HUD funding assistance must comply with the standards and the Acceptable

Separation Distance (ASD) stipulated under the Regulation between specific stationary

hazardous facilities that store, handle, or process hazardous substances.

Standards under 24 CFR Part 51.203:

 Thermal Radiation, 10,000 BTU/ft2- hr

o Applicable to buildings

 Thermal Radiation, 450 BTU/ft2– hr

o Applicable to people

 Blast Overpressure, 0.5 psi

o Applicable to buildings, building occupants and outdoor unprotected facilities

Extraordinary circumstances include:

 Analysis of containers that hold hazardous substances (liquids or gases) not listed in

Appendix I of the Regulation or in Appendix C of this Guidebook in close proximity to

proposed HUD-assisted projects.

 Analysis of existing natural or man-made barriers that may serve as mitigation for HUDassisted projects from flammable or explosive hazards.

 Calculation of design specifications for barriers used to mitigate flammable and

explosive hazards that are located near proposed HUD-assisted projects.

Every proposed HUD-assisted project is different. Therefore, each one should be analyzed

according to site conditions such as container size and location, and proximity to residential

structures, etc. Mitigation and/or safety measures may be required at some sites, but only

suggested at others.
52

7. Firefighters extinguishing fires from jet fuels tanks that were burning out of control at Apra

Harbor, Guam.
53

Appendices