The United States Pipeline and Hazardous Materials Safety Administration (“PHMSA”) addressed in a September 24th interpretive letter the application of the federal Hazardous Materials Regulations (“HMR”) to cargo tanks and the performance requirements to prevent accumulation of material impairing the operation of the pressure relief systems. See Reference No. 25-0137.
PHMSA was responding to an October 8, 2025 letter from the Public Utilities Commission of Ohio (“PUCO”) asking about the applicability of § 178.345-10(b)(6) as it relates to pressure relief device (“PRD”) components.
PUCO first asked what part of the pressure relief device, e.g., the “rain cap,” does § 178.345-10(b)(6) apply to?
PHMSA responds that the referenced Section refers to the entire pressure relief system, as opposed to an individual PRD or an individual PRD component. It further states that under § 178.345-10(b)(1), a primary pressure relief system consists of one or more reclosing pressure relief valves, and a “rain cap” or cover may be either a component of the PRD or a separate element of the broader system.
PUCO next asked whether missing an “upper housing hole plug” as part of a PRD as shown in the example schematic provided constitutes a violation of § 178-345-10(b)(6)?
PHMSA responds that the cited provision is intended to ensure the pressure relief system is “mounted, shielded and drainable” to minimize the accumulation of material that could impair its operation. It further states that noncompliance is determined on a case-by-case basis. This is stated to consider:
- Cargo tank design.
- Conditions and environment in which the cargo tank will be in use.
Further, noncompliance is stated to potentially result if the absence of a single component, such as the one PUCO described, affects the operation or discharge capability of the pressure relief system—by the accumulation of material that allows freezing, corrosion, or blockage.
Finally, PUCO asked if it a violation if the “rain cap” is in place but the rubber plug is missing.
PHMSA refers to the answer to the previous question.
A copy of the interpretive letter can be found here.
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