Requirements Applicable to Pressure Relief Devices on Cargo Tank Motor Vehicles
Pipeline and Hazardous Materials Safety Administration · Oct 23, 2013 · Reference No. 13-0147
Interprets: 49 CFR § 178.345-10(d), 49 CFR § 180.407
Reggie’s quick take
A company asked what federal rules cover testing and inspection of pressure relief valves on DOT 400 series cargo tanks. The agency said every reclosing pressure relief valve must be inspected externally for corrosion or damage that could prevent safe operation (§ 180.407(d)(3)). If the tank carries lading that corrodes the valve, the valve must be removed for inspection and testing. A removed valve must open between 100 and 110 percent of its required set pressure and reseat tightly at no less than 90 percent of its start-to-discharge pressure. Set pressure rules are in § 178.345-10(d). The primary valve's set pressure must be 120 to 132 percent of the MAWP, and a secondary valve's must be at least 120 percent.
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
This responds to your July 16, 2013 letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to pressure relief devices on specification DOT 400 series cargo tank motor vehicles. Specifically, you ask what the requirements are for testing and inspection of specification DOT 400 series cargo tank motor vehicles pertaining to pressure relief valves and pressure relief systems.
As provided in § 180.407(d)(3), all reclosing pressure relief valves must be externally inspected for any corrosion or damage which might prevent safe operation. Additionally, all reclosing pressure relief valves on cargo tanks carrying lading corrosive to the valve must be removed from the cargo tank for inspection and testing. Finally, each reclosing pressure relief valve required to be removed and tested must open at no less than the required set pressure and no more than 110 percent of the required set pressure, and must reseat to a leak-tight condition at no less than 90 percent of the start-to-discharge pressure or the pressure prescribed for the applicable cargo tank specification.
Under the general design and construction requirements applicable to specification DOT 400 series cargo tank motor vehicles (see § 178.345), the pressure relief system requirements provided in § 178.345-10(d) state that the set pressure of the pressure relief system is the pressure at which it starts to open, allowing discharge. The set pressure of each primary relief valve must be no less than 120 percent of the maximum allowable working pressure (MAWP), and no more than 132 percent of the MAWP. Additionally, the valve must reclose at not less than 108 percent of the MAWP and remain closed at lower pressures. Finally, the set pressure of each pressure relief valve used as a secondary relief device must be not less than 120 percent of the MAWP. This standard equally applies to original construction and requalification under parts 178 and 180 of the HMR, respectively.
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A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.