Boyd Pro Service, LLC — Hazardous Materials Safety Interpretation

Page 11200 New Jersey Avenue, SE Washington, DC 20590 U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration 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#

Page 2Feb.13.2025 2:26PM No.1607 P. 1 Febuary 13,2025 25-00020 Larson Boyd Pro Service LLC. 993 Pruden Ave. Dayton, Oh. 45403 (937) 751-5964 Boydproservice@gmail.com U.S. DOT Research and Special Programs Attn: Approvals PHH-31 1200 New Jersey Ave, SE Washington, D.C. 20590-0001 To Whom it may concern: Ref: HMR 178.345�10 We have a customer that has a few MC-312 cargo tanks that were built in the B0's that transport Sodium Hypochlorite ( UN 1791) only. They currently have single use/changeable 35 P .S.I. pressure relief (graphite) Disk on them. I am writing to you to ask if that single use disk may be removed & replaced with a Teflon Lined 35 P.S.I. rec/osing Pressure relief valve. Will you please clarify. Sincerely, �,d/?,./ Floyd A. Boyd 1#