Short answer
Purging into service displaces the air in newly tested pipework with fuel gas so appliances receive gas, not an air-gas mixture. It is done after a successful strength and tightness test, section by section, venting to a safe outside location through a purge hose, with a gas detector confirming when the purge point reads high gas concentration.
Before you start
Confirm the tightness test has passed, all appliance isolation valves are closed, open ends are capped, the purge points and vent route are set up, no smoking or ignition sources are present, and the people in the building know what is happening.
Sequence
Purge from the supply end outwards, starting with the main run and then each branch. Open the purge valve and control the flow to achieve the required velocity, monitoring with a detector. When the reading is consistently high, close the purge valve and move to the next section.
Appliances
Each appliance is then purged at its inlet test point or by the manufacturer's procedure, before lighting and commissioning. Large burners with gas trains follow the burner manufacturer's purge and valve-proving sequence.
Finish
Re-check for leaks at the purge points, record the purge, then commission appliances and issue the CP16 and commissioning records.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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