PHMSA letter of interpretation

Safety Relief Devices

Pipeline and Hazardous Materials Safety Administration · Jun 19, 1996 · Reference No. 96-0147

PHMSA says to use caution with this letter. On August 8, 2002, RSPA finalized Docket HM-220D, which consolidated the requirements for qualification, use, and maintenance of cylinders in Part 180, Subpart C. As a result, the references provided in this guidance are no longer accurate. Please see Part 180, Subpart C for the current requirements for the qualification, use, and maintenance of cylinders. Although not all references to requirements previously codified in § 173.34 have parallel requirements in Part 180, Subpart C, you may wish to review § 173.301(f)(6) for additional information.

Reggie’s quick take

Someone asked whether a safety relief valve is allowed on a DOT specification cylinder holding a material shipped as "Toxic liquid, flammable, organic n.o.s., PG I, Hazard Zone B" that has an inhalation hazard. The agency said yes, that is correct. Under § 173.34(d)(3), the ban on safety relief devices applies only to toxic liquids in Division 6.1, Hazard Zone A, so a Hazard Zone B material is not covered by that ban.

Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.

This is in response to your letter dated May 9, 1996, regarding your understanding of the requirements in § 173.34 (d)(3) that a safety relief valve is not prohibited on a DOT specification cylinder which contains "Toxic liquid, flammable, organic n.o.s, PG I, Hazard Zone B" material, with an inhalation hazard.

Your understanding is correct. The requirements in § 173.34(d)(3) only prohibit the use of a safety relief device on a cylinder containing toxic liquid materials in "Division 6.1, Hazard Zone A".

A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.