PHMSA letter of interpretation

Requalification of DOT 4BW Cylinders

Pipeline and Hazardous Materials Safety Administration · Feb 20, 1997 · Reference No. 97-0179

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 § 180.209(g) for additional information.

Reggie’s quick take

A company asked whether its DOT 4BW cylinders can be requalified by visual inspection instead of hydrostatic testing. The cylinders dispense either the "A" component (diisocyanate and R-22) or the "B" component (polyol and R-22) of a foam sealant. PHMSA's answer is no. The visual-inspection option under § 173.34(e)(13) covers only cylinders used exclusively for the materials listed in that table, and diisocyanate and polyol are not on it. So these cylinders must get a hydrostatic test and a visual inspection every five years, as set out in the § 173.34(e) table.

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 January 9, 1997, addressed to Mr. Ron Abis, Office of Hazardous Materials Enforcement, concerning the requalification of a DOT 4BW cylinder under 49 CFR 173.34(e)(13). You state that your cylinders are "in exclusive use or dedicated service to dispense either "A" component (Diisocyanate and R-22 Fluorocarbon) or "B" component (Polyol and R-22 Fluorocarbon) which are mixed at the application gun to create a rigid polyurethane insulating foam sealant." Your letter was referred to this office of response.

The provision allowing performance of a visual inspection instead of a hydrostatic test applies only to cylinders used exclusively for the materials specifically listed in the table in § 173.34(e)(13). No other material may be contained in the cylinder. The materials you identify as Diisocyanate and Polyol are not listed in the table. Therefore, a cylinder containing either of these materials and a gas must be requalified by performing a hydrostatic test and a visual inspection every five years. See § 173.34(e) 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.