Pipeline purge

HTM 02-01 (2026) says oxygen-free nitrogen (OFN) should be used as an internal inert gas shield for all medical gas pipelines. The pipeline to be brazed should first be purged to remove air, and purging is still required to remove the internal shield gas and other particulate matter not associated with the brazing operation. When two or more terminal units are removed in a partial decommissioning, the system should be purged using OFN; replacing one terminal unit needs no purge during brazing.

The engineer's particulate test (form B20) should identify inadequately purged pipelines or debris. Where purging will not clear particulate matter, a decision should be made by the Authorised Person (MGPS), with the Quality Controller (MGPS), on whether to accept the contamination, agree a procedure to remove it or condemn the system. All hoses should be purged before testing: at terminal units supplied via flexible hoses, water-vapour levels often exceed the limits on initial testing, through absorption of water vapour in the absence of flow.

In the dental chapter of the 2021 edition of the US code NFPA 99, as reproduced in an August 2021 NFPA committee report, each oxygen and nitrous oxide outlet was to be purged with an intermittent high-volume flow of test gas until the purge produced no discoloration in a clean white cloth, and the verifier then carried out a heavy, intermittent purge of the piping. NFPA lists 2027 as the current edition of NFPA 99.