Gas purging: why it is done, where it goes and what follows

Purging changes what is inside a defined section of pipework so that only the intended contents remain: air displaced by fuel gas when commissioning, or gas displaced by air when decommissioning. It is not about raising pressure or cleaning burners.

Why it is hazardous

As the contents change, the mixture can pass through its flammable range inside the pipe, and gas leaving the purge point mixes with air outside it. A flammable mixture needs an ignition source to burn. Prevent gas from accumulating: concentrated methane can leave too little oxygen to breathe as well as create a fire or explosion risk. A gas can be hazardous to inhale without acting like carbon monoxide.

Before you start

1. Prove tightness first. When purging into service, test for tightness immediately before the purge. Permanent decommissioning is the exception (below).
2. Calculate the volume. Under IGEM/UP/1B Edition 4, calculate the installation volume (IV) for every purge and set the purge volume (PV) at 1.5 × IV. An IV of 12 litres gives a PV of 18 litres; an IV of 0.014 m³ gives 0.021 m³, which is 21 litres. Keep units consistent. The volume is a quantity, not a time.
3. Choose the discharge point. When purging to air, discharge to a safe point in the open air where gas can disperse. Never into a confined or below-ground space, near drains or into an unventilated area, where it can build up to a flammable level. An openable window does not make an indoor purge point suitable, and natural-gas practice does not carry over to LPG or LPG/air.
4. Control ignition sources at and around the discharge point, including switches, pilot lights and phones.

An exception for the tightness test does not carry over to the purge. An existing-appliance loss below the smallest limit for its fuel may not need an IV calculation for that comparison, but if you then purge, you must still calculate IV.

Purging to air or to ignition

A purge is either released unburnt, which is purging to air, or burnt off as it arrives, which is purging to ignition. To purge to ignition, the gas is taken to an open burner, fitted temporarily at the purge point if needed, and lit as soon as possible, with ignition attempted throughout the purge.

• LPG and LPG/air: always purged to ignition. LPG is heavier than air, so gas released unburnt sinks and can collect in drains, basements and other low spots.
• Natural gas under UP/1B Edition 4: a calculated purge volume no greater than 0.02 m³ (20 litres) may be purged to air using the permitted arrangement. Above that quantity, purge to ignition. Compare this threshold with PV, not IV: the 18-litre example is below it, whereas the 21-litre example is above it. Meeting the volume limit does not make an unsuitable discharge point acceptable.

Finishing the purge

• Deliver the calculated volume and complete the required checks using the applicable method.
• If you purged through a loosened connection, retighten it and test it for tightness, and test every seal fitted after purging (GSIUR regulation 22(2)).
• A smell of gas at the outlet before the calculated volume has passed does not mean the purge is complete.
• Never check for gas with a naked flame at an open pipe end (a purge to ignition burns the gas at a burner set up for it). The calculated volume and the required checks finish a purge, not a remembered time or a one-size gas reading.

After purging into service

Relight each appliance to the manufacturer's instructions and commission it: confirm the operating pressure (or gas rate/heat input) and the combustion are correct. A burner that lights is not a commissioned appliance, and a tightness test alone is not enough.

Make these checks before the appliance is left in use, not at its next service: gas that does not burn properly produces carbon monoxide, which you cannot see, taste or smell.

Decommissioning

Before a redundant section is cut, capped or removed, purge the gas out with air. The work may use a tool that sparks, and the pipe may later be handled by someone else, such as a scrap yard. Purge whether or not the section will ever be reconnected.