Q1. In the August 25, 2014 e-mail response, PHMSA’s Field Services Support Division stated that the "K" approval of DOT 4DS cylinder requalifiers prescribed in §§ 107.805 and 180.211 is required if a rupture disc in a pressure relief device (PRD) is replaced on a cylinder rebuild without heat treatment. You ask if the person rebuilding the cylinder is required to have a "K" approval before he or she can replace a rupture disc on the hermetically sealed DOT 4DS fire extinguishers you described?

A1. The answer is no. In the scenario you described, the PRD is removed from the DOT 4DS to perform a cylinder requalification. If the cylinder passes the requalification, a new PRD is placed on the cylinder by welding it to the "boss." Because the boss is fusion welded to the cylinder and the PRD is welded to the boss, the PRD does not come in direct contact with a pressure part of the cylinder and, therefore, its installation is not a "rebuild" or "repair" as these terms are defined under § 180.203 of the HMR. Persons who replace a rupture disc in a PRD on a DOT 4DS cylinder must be properly trained to perform this task in conformance with 49 CFR Part 172, Subpart H (Training).

Q2. If the procedure to replace the rupture disc on a DOT 4DS cylinder does fall under the rebuilding requirements specified in § 180.211 and a "K" approval is required, please clarify how the approval holder will not be in violation of the HMR when performing this action since it is impossible to comply simultaneously with the requirements prescribed in §§ 180.211(d)(2)(ii) (visual inspection after removal of a non-pressure component) and 180.211(d)(2)(iv) (welding and inspecting a rebuilt cylinder).

A2. As stated earlier, the procedure to replace a PRD that contains a rupture disc on a DOT 4DS cylinder is neither a "rebuild" or "repair" under the HMR criteria, and is not subject to the "K" approval requirements. As a result, the requalifier is required to perform only the visual inspection prescribed in § 180.211(d)(2)(ii).