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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Interpretation Response #23-0065

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

Interpretation Response Details

Response Publish Date:

Company Name: W. R. Grace & Co.

Individual Name: Mr. Paul Errichetti

Location State: MD Country: US

View the Interpretation Document

Response text:

April 16, 2024

Mr. Paul Errichetti
Sr. Mgr., Fleet and Dangerous Goods
W. R. Grace & Co.
7500 Grace Drive
Columbia, MD  21044

Reference No. 23-0065

Dear Mr. Errichetti:

This letter is in response to your July 7, 2023, letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to pressure relief devices on specification Department of Transportation (DOT) 51 portable tanks. In your letter, you state that you have a fleet of DOT 51 portable tanks—comprised of both 250 gallon and 430 gallon capacities—with a test pressure of 300 psig and a maximum allowable working pressure (MAWP) of 200 psig. Specifically, you ask whether the required pressure relief devices may start to discharge at less than 110% of two-thirds of the test pressure.

The answer is no. As provided by § 178.275(g)(6)(ii)—and described in a previous Letter of Interpretation (LOI) Ref. No. 21-0035—the required pressure relief devices must be set to start to discharge at a nominal pressure of five-sixths of the test pressure for shells having a test pressure of not more than 4.5 bar (450 kPa) and 110% of two-thirds of the test pressure for shells having a test pressure of more than 4.5 bar (450 kPa). Since the applicable portable tanks have a test pressure of more than 4.5 bar (450 kPa), the start to discharge pressure for the pressure relief devices is required to be set at 110% of two-thirds of the test pressure (220 psig).

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


Steven Andrews
Acting Chief, Regulatory Review and Reinvention Branch
Standards and Rulemaking Division


Regulation Sections

Section Subject
178.275 Specification for UN Portable Tanks intended for the transportation of liquid and solid hazardous materials