"U" stamp on imported portable tanks
Pipeline and Hazardous Materials Safety Administration · Nov 20, 2008 · Reference No. 08-0197
Interprets: 49 CFR § 171.23
Reggie’s quick take
Someone asked whether a tank holding dimethyl sulfate, a toxic liquid, could be imported into the U.S. if it was built outside the country to the ASME Code but lacked the "U" stamp, and was being moved under the IMDG Code. PHMSA said no. For this material, a UN portable tank imported into the U.S. must be certified and stamped to the ASME Code under §§ 171.23(b)(10)(ii), 173.244, 178.273(b)(6) and 178.274(b)(1). The "U" stamp can only be applied at manufacture, and it is the only recognized proof that the ASME quality system was properly carried out. The tank must also meet any special provisions listed in § 172.101, such as TP 38 and TP 45.
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 electronic transmission requesting clarification of the Hazardous Materials Regulations (HMR: 49 CFR Parts 171-180) regarding the import of Dimethyl sulphate or Dimethyl sulfate, UN1595, 6.1, 8, PG I, Hazard Zone B in a UN portable tank that was manufactured outside the United States. You state that the portable tank is manufactured to the ASME Code, Section VIII, but is not "U" stamped. You ask whether the shipment, which is being transported in accordance with the International Maritime Dangerous Goods (IMDG) Code, may be imported into the United States without the "U" stamp.
The answer is no. A UN portable tank used to import Dimethyl sulfate, UN1595, 6.1, 8, PG I, Hazard Zone B into the United States is required to be certified and stamped to the ASME Code as specified in §§ 171.23(b)(10)(ii), 173.244, 178.273(b)(6) and 178.274(b)(1). The U.S. Department of Transportation requires the shipment of toxic-inhalation-hazard liquids to be transported in ASME Code "U" stamped portable tanks regardless of what other regulatory standards may allow. The "U" stamp can only be applied at the time of manufacture and is the only recognized form of certification that verifies the ASME quality system has been properly implemented. Additionally, the UN portable tank must conform to any applicable special provisions, such as Codes TP 38 and TP 45, listed in Column 7 of the § 172.101 Hazardous Materials Table.
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.