Interpretation Response #25-0020
Below is the interpretation response detail and a list of regulations sections applicable to this response.
Interpretation Response Details
Response Publish Date:
Company Name: Boyd Pro Service, LLC
Individual Name: Floyd A. Boyd
Location State: OH Country: US
View the Interpretation Document
Response text:
September 23, 2025
Floyd A. Boyd
Boyd Pro Service LLC
993 Pruden Ave.
Dayton, OH 45403
Reference No. 25-0020
Dear Mr. Boyd:
This letter is in response to your February 13, 2025 letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to pressure relief devices (PRDs) on Motor Carrier (MC) 312 cargo tank motor vehicles (CTMVs). Specifically, you state that your company has a customer who owns several MC 312 CTMVs used exclusively for the transportation of "UN1791, Hypochlorite solutions, 8, II," each fitted with a single-use, non-reclosing, 35 psi PRD. You further state that these cargo tanks were built in the 1980's. You ask if single-use PRDs on MC 312 CTMVs can be replaced with 35 psi reclosing PRDs.
Section 173.33(d)(1) specifies that non-reclosing PRDs are not authorized in any cargo tank except when in series with a reclosing PRD; however, a cargo tank marked or certified before August 31, 1995, which is fitted with non-reclosing PRDs, may continue to be used in any hazardous material service for which it is authorized. Section 173.33(d)(3) specifies that for MC 312 CTMVs, PRDs or outlets may be modified to meet the applicable requirement for a Department of Transportation (DOT) 412 CTMV. Further, § 180.407(j)(1)(iii)(B) states that MC 312 cargo tanks modified with reclosing PRDs meeting DOT 412 specifications must meet the pressures set forth for a DOT 412 cargo tank in § 178.348-4. Paragraph (c) of § 178.348-4 states that the setting of PRDs must be in accordance with § 178.345-10(d), which specifies that 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, and that the valve must reclose at not less than 108 percent of the MAWP and remain closed at lower pressures.
I hope this information is helpful. Please contact us if we can be of further assistance.
Sincerely,
Alexander Wolcott
Acting Chief, Regulatory Review and Reinvention Branch
Standards and Rulemaking Division
173.33(d), 178.345-10(d), 178.348-4, 180.407(j)