Why gas pipework is purged after meter or pipework work, how to size the purge (PV = 1.5 × IV) and the order of checks around it.
Opening or replacing a meter can let air into the pipework. Purging clears it, so no dangerous air-gas mixture is left and the installation goes back into use proven sound and free of air.
Before you purge
- Tightness test the section first. For commissioning, the test comes immediately before purging; never leave soundness until after the purge.
- Establish the section being purged. Include every component inside the procedure's boundary; a normal test from the ECV includes the meter.
- Calculate the installation volume (IV) of that section.
- Work out the purge volume: under UP/1B Edition 4, PV = 1.5 × IV whenever purging is done. This still applies when an earlier appliance-loss comparison did not need IV because the loss was below the smallest limit for the fuel.
Worked example
An installation volume of 0.018 m³ gives PV = 1.5 × 0.018 = 0.027 m³, which is 27 litres.
Common mistakes
- Using the meter's flow rating as a volume: a U6 rating of about 6 m³/h is a maximum flow, not an internal volume.
- Using a fixed purge time because the meter model is familiar.
- Treating a pressure reading as the installation volume or the purge quantity.
Purge
- Choose a discharge method permitted for the fuel, volume and setting.
- Before starting, decide where the gas will go, how that space is ventilated and how ignition sources are controlled. Gas or mixture must not collect in an unsafe space, and opening a window alone does not make every indoor purge safe.
- Deliver the full purge volume. Smelling some gas early does not prove the purge is complete.
- Never use a naked flame to detect gas.
Close out
- Close and secure the purge and test connections, and check them with leak detection fluid.
- Relight and check the appliances under the manufacturer's instructions.
- Record the test and purge, and explain any restrictions or unresolved defects to the responsible person.