Short answer
Purging is the controlled displacement of one gas by another inside pipework: air out and fuel gas in when putting an installation into service, or fuel gas out and air in when taking it out of service. It prevents a flammable mixture forming inside the pipe or at the appliance, and on commercial installations it must be planned and calculated rather than done by simply opening a burner.
Why purging matters
Natural gas is flammable between roughly 5% and 15% in air. When air and gas mix inside a pipe during commissioning, a section of the pipe passes through that range. Purging at a controlled velocity, venting to a safe place and monitoring the result keeps that mixture moving and out of the building.
Planning a purge
The engineer calculates installation volume, the purge volume required for each section, and the flow rate needed to achieve a minimum purge velocity so gas and air do not stratify. Purge points, hoses, flame traps or vent stacks and an exclusion zone are agreed, along with ignition-source controls.
Proving the purge
Purging into service is complete when a suitable gas detector at the purge point shows a consistently high gas concentration (commonly over 90% gas). Purging out of service is complete when the reading falls to a safe level well below the lower flammable limit. A flame test may be used only where the standard and risk assessment allow it.
Records
The purge procedure, volumes, readings and engineer are recorded on a test and purge certificate such as a CP16.
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.
Need this done on site?
Need a commercial gas engineer in London?
Gas Safe registered commercial engineers, 24 hours a day. Call now or book online and we'll confirm a time.