Acceptable separation distance from explosive and flammable hazards

Subpart C of 24 CFR part 51 keeps projects assisted by the Department of Housing and Urban Development (HUD) apart from stationary containers of explosive or fire prone substances. HUD will not approve a project nearer to such a hazard than the acceptable separation distance (ASD) unless mitigating measures are implemented or already in place. The distance rests on a heat standard and a blast standard and is determined with a HUD guidebook.

Who the rule binds

Part 51's standards "shall apply to all HUD actions except where special provisions and exemptions are contained in each subpart" (24 CFR 51.4). Subpart C speaks of the HUD-assisted project: "the development, construction, rehabilitation, modernization or conversion with HUD subsidy, grant assistance, loan, loan guarantee, or mortgage insurance, of any project which is intended for residential, institutional, recreational, commercial or industrial use" (24 CFR 51.201). Here rehabilitation and modernization count only where the repairs and renovation will expose more people to hazardous operations, by raising residential density, changing a building's use to habitation or making a vacant one habitable (24 CFR 51.201).

The subpart "shall be implemented for each proposed HUD-assisted project by the HUD approving official or responsible entity responsible for review of the project", within the environmental review under 24 CFR parts 50 and 58 (24 CFR 51.206). Where the assisted project itself installs hazardous facilities, the Department shall ensure they stand at an ASD from residences and other places where people may be present (24 CFR 51.204).

What it asks

A hazard. It is "any stationary container which stores, handles, or processes hazardous substances of an explosive or fire prone nature" (24 CFR 51.201).
What the definition leaves out. Pipelines carrying hazardous substances, if they are underground or meet the Federal, State and local safety standards that apply; containers "with a capacity of 100 gallons or less when they contain common liquid industrial fuels, such as gasoline, fuel oil, kerosene, and crude oil"; facilities that the topography shields from the proposed project; "All underground containers"; and liquefied petroleum gas containers "with a volumetric capacity not to exceed 1,000 gallons water capacity, if they comply with National Fire Protection Association (NFPA) 58", 2017 edition (24 CFR 51.201).
Hazardous substances. They are petroleum products and chemicals able to produce blast overpressure or thermal radiation above the standards of 51.203. Appendix I to the subpart lists them, and the Secretary or a designee may, case by case and when circumstances warrant, require the subpart's application to an unlisted one that would exceed them (24 CFR 51.201, 51.207).
The distance. The ASD is the distance beyond which a hazard's explosion or combustion is not likely to expose structures or people to blast overpressure or thermal radiation above the safety standards (24 CFR 51.201).
The approval rule. It is quoted below (24 CFR 51.202(a)). The Department shall also evaluate projected development plans near every proposed project; if they show a plan to install a hazardous operation close by, it shall not approve assistance "unless the Department obtains satisfactory assurances that adequate mitigating measures will be taken when the hazardous operation is installed" (24 CFR 51.202(b)).
The safety standards. The thermal radiation flux allowed at the building "shall not exceed 10,000 BTU/sq. ft. per hr." (24 CFR 51.203(a)(1)). The level "for outdoor, unprotected facilities or areas of congregation shall not exceed 450 BTU/sq. ft. per hour" (24 CFR 51.203(a)(2)). Blast overpressure at buildings and at outdoor, unprotected facilities or areas "shall not exceed 0.5 psi" (24 CFR 51.203(b)). A substance that is both a thermal radiation and a blast hazard gets an ASD for each, and the larger is used (24 CFR 51.203(c)).
How the distance is determined. Sections 51.200 to 51.208 print no distance. The ASD "is determined by applying the safety standards established by this subpart C to the guidance set forth in HUD Guidebook", which the definition names as Siting of HUD-Assisted Projects Near Hazardous Facilities (24 CFR 51.201). Appendix II to the subpart holds background and charts (24 CFR 51.203(d)) and works one example: for an undiked 30,000 gallon liquid propane tank, 660 feet for explosion and, for fire, 1,150 feet for people in exposed areas and 240 feet for buildings.
Mitigating measures. The standards assume "level topography with no intervening object(s) between the hazard and the project". The section lists four cases in which their application "can be eliminated or modified": the topography shields the project; an existing permanent structure, fire resistant and of adequate size and strength, shields it; a barrier is built around the hazard, at the project site or between the two; the structure and the outdoor areas people use are designed to withstand the anticipated blast overpressure and thermal radiation (24 CFR 51.205).

What it means for the Phase I and the consultant

The rule turns on these facts: whether a stationary, aboveground container of a covered substance stands near the project; what it holds and how much; where it stands relative to the project's buildings and outdoor gathering areas; what lies between; and whether a hazardous operation is planned nearby.

A Phase I Environmental Site Assessment under ASTM E1527-21 has another aim: to identify hazardous substances or petroleum products released, likely released or posing a material threat of release at the subject property (E1527-21, 1.1.1). Its site visit looks for storage tanks at the subject property, and their description should give contents and capacity (E1527-21, 9.4.12). Adjoining properties and the surrounding area are observed from the subject property's edge and from public thoroughfares (E1527-21, 9.2.5). The required search of registered tank lists covers the subject property and adjoining properties (registered storage tanks; E1527-21, 8.2.2, Table 2).

So a Phase I may describe the subject property's own tanks and registered tanks on adjoining properties. A container farther off, or one on no registry, may not appear in it. The two also sort tanks differently: an underground tank is outside the definition of a hazard, and that definition does not turn on any release (24 CFR 51.201).

The ASD itself, whether a container is a hazard, and whether a mitigating measure is appropriate are for the HUD approving official or the responsible entity (24 CFR 51.206).

What the rule says

The Department will not approve an application for assistance for a proposed project located at less than the acceptable separation distance from a hazard, as defined in § 51.201, unless appropriate mitigating measures, as defined in § 51.205, are implemented, or unless mitigating measures are already in place.

24 CFR 51.202(a), edition revised as of April 1, 2025

This page is general reference for environmental professionals. It does not review any project. The regulation is the authority, and the determinations of HUD or of the entity responsible for the review govern.