Visual Inspection of Frangible Disc of Tank Car
Pipeline and Hazardous Materials Safety Administration · Aug 25, 2000 · Reference No. 99-0249
Interprets: 49 CFR § 173.31
Reggie’s quick take
Someone asked about the rule in 49 CFR 173.31(d)(1)(vi) that requires a careful check of a tank car's frangible (rupture) disc before each hazardous material shipment. The agency explained that the check looks for corrosion or damage that could change how the device works. It also said that a full inspection means removing the disc from the vent device, since a disc can look normal on top but be badly corroded underneath. The agency said it plans a future rulemaking on the concerns about tank cars that are forwarded loaded or returned with residue.
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.
I apologize for the delay in responding to your letter concerning the requirement in 49 CFR 173.31(d)(1)(vi) to carefully inspect a frangible (rupture) disc in a pressure relief device prior to each hazardous material shipment. This requirement has its origins in regulations of the Interstate Cornmerce Commission issued in 1921. The wording of this requirement was most recently revised in a final rule published on September 21, 1995, under RSPA's Docket Nos. HM-175A and 201 (60 FR 49098).
As the language of § 173.31(d)(1)(vi) states, the purpose of this type of inspection. is to check "for corrosion or damage that may alter the intended operation of the device." For that reason, in response to a comment submitted in a separate rule-makings proceeding under Docket no. MI-216 (61 FR 28666,28671; June 5, 1996), we stated in the preamble that RSPA and FRA believe in order to fully inspect a rupture disc (both top and bottom), the disc must be removed from the safety vent device. It has been FRA's experience that a rupture disc may appear normal on the top side, but be severely damaged or corroded on the bottom side.
You and others have raised concerns about the language of the present rule and its application to persons that forward a loaded tank car received from another location or return a tank car with residue. We anticipate initiating a rule-makings in the near future to address these concerns.
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.