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Interpretation Response #16-0090 ([Texas Trailer Corporation] [Mr. Ray Williams])

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date:

Company Name: Texas Trailer Corporation

Individual Name: Mr. Ray Williams

Location State: TX Country: US

View the Interpretation Document

Response text:

January 12, 2017

Mr. Ray Williams
Texas Trailer Corporation
1310 Highway 82 West
Gainesville, TX  76240

Reference No. 16-0090

Dear Mr. Williams:

This letter is in response to your May 19, 2016, email and subsequent emails and phone calls requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to safety relief valves on cargo tanks.  Specifically, you ask why the language in § 173.315(i)(3) was changed to omit cargo tanks and what setting requirements apply for the start-to-discharge pressure on a safety relief valve for a cargo tank motor vehicle in compressed gas service.

Section 173.315(a) authorizes the use of DOT specification (MC 330 and MC 331) cargo tank motor vehicles for certain compressed gases.  Section 173.315(i) further describes requirements for pressure relief devices on “tanks,” applying to both cargo and portable tanks.  On June 21, 2001, the Pipeline and Hazardous Materials Safety Administration’s (PHMSA) predecessor—Research and Special Programs Administration (RSPA)—published a final rule titled, “Harmonization with the United Nations Recommendations, International Maritime Dangerous Goods Code, and International Civil Aviation Organization’s Technical Instructions,” in the Federal Register under Docket No. RSPA-2000-7702 (HM-215D).  This final rule amended § 173.315(i)(3) to add new requirements for United Nations (UN) portable tanks, which changed the term “tank” to specify a “portable tank, other than a UN portable tank,” and omitted cargo tanks from the requirement.

In accordance with § 173.33(d)(3), an MC 330 cargo tank may have pressure relief devices or outlets conforming to the applicable specification to which the tank was constructed, or the pressure relief devices or outlets may be modified to meet the applicable requirement for the specification MC 331.  Pressure relief devices for MC 331 cargo tanks are specified in § 178.337-9, which references § 173.315(i).  Section 180.407(j)(1)(iv) specifies each self-closing pressure relief valve for an MC 330 or MC 331 cargo tank must open at not less than the required set pressure and not more than 110% of the required set pressure, and must reseat to a leak-tight condition at no less than 90% of the required set pressure.

I hope this information is helpful.  Please contact us if we can be of further assistance.


T. Glenn Foster
Chief, Regulatory Review and Reinvention Branch
Standards and Rulemaking Division

173.315(i)(3), 173.315(a), 173.315(i), 173.33(d)(3), 180.407(j)(1)(iv), 178.337-9

Regulation Sections

Section Subject
173.315 Compressed gases in cargo tanks and portable tanks
173.315 Compressed gases in cargo tanks and portable tanks
173.315 Compressed gases in cargo tanks and portable tanks
173.33 Hazardous materials in cargo tank motor vehicles
178.337-9 Pressure relief devices, piping, valves, hoses, and fittings