Why are medical gas copper pipes brazed with a nitrogen purge?

Medical gas copper pipes are brazed with oxygen-free nitrogen flowing through them so that no oxide forms on the inside of the pipes and fittings and the joint keeps a bright, clean bore; HTM 02-01 (2026) in England and the ACSS specification in Portugal both require this internal nitrogen shield.

Can the nitrogen purge be skipped?

Under HTM 02-01 (2026), only for certain joints, such as connections to existing pipelines, by agreement between the Authorised Person (MGPS), the Authorising Engineer (MGPS) and the pipeline contractor; purging is then still needed to clear debris.

Why is no flux used on site?

HTM 02-01 limits site joints to copper-to-copper brazing with an alloy that needs no flux; joints to brass, gunmetal or bronze use flux and are made by manufacturers, who chemically remove the residues and clean the assembly for oxygen service.

Why are medical gas pipes brazed with a nitrogen purge?

HTM 02-01 (2026), the NHS England guidance for medical gas pipelines, asks for oxygen-free nitrogen (OFN) to be used as an internal inert gas shield on all medical gas pipelines, to prevent oxides forming inside the pipes and fittings. With the shield, the joint leaves a bright, clean bore. Particles matter because they can adversely affect the quality of the delivered gas, which is why a new pipeline is particulate-tested at every terminal unit.

How the nitrogen purge is done

OFN is fed into the pre-assembled, unbrazed pipework through a pressure regulator and a flow controller or flow-regulating device.
The pipeline is first purged to remove air; a continuous, steady flow is then kept through the pipe during brazing so that air cannot get in.
Pipe ends may be capped to direct the nitrogen; T-joints need particular attention, and nearby pipes must not oxidise from the heat.
The connection between the pipework and the nitrogen supply must be leak-free.
After brazing, the line is still purged to remove the shield gas and any other particulate matter.
Pipework should not be dry-fitted and left unbrazed for more than 72 hours.

The brazing itself

On site, HTM 02-01 allows only copper-to-copper joints, apart from mechanical joints, made with a silver-copper-phosphorus brazing alloy (CP104 to BS EN ISO 17672) and no flux, and heated with LPG/oxygen torches; acetylene should not be used. Joints to brass, gunmetal or bronze are made by manufacturers with flux, then chemically cleaned and degreased for oxygen service. The Portuguese ACSS specification likewise requires joints brazed with an inert gas (nitrogen) passing inside, with a solder of at least 40% silver.

How joints are checked

Sample joints are cut lengthwise and inspected: the inside should be clean and free from oxides and particulate matter, and the brazing alloy should penetrate at least three times the tube wall thickness or 3 mm, whichever is greater. A particulate test with a membrane filter follows at every terminal unit of a new system.

Safety

In confined spaces the HTM asks for the space to be ventilated or the shield gas piped out, and for the oxygen content of the air to be monitored. Nitrogen cylinders should be kept separate from medical gas cylinders and removed from site at the end of the contract.