Short answer
Purging out of service removes fuel gas from pipework, replacing it with air (direct) or nitrogen (indirect), so the pipe can be cut, altered or removed safely. The supply is isolated and locked off, the section vented to a safe location, and the purge continued until a detector shows the gas concentration is safely below the flammable range.
Isolation
Close and lock the upstream valve, prove it is not letting by, and fit a physical disconnection or spade where the work requires positive isolation. Label the valve so nobody re-opens it during the work.
Purge medium
Air is acceptable for most small low pressure sections. Nitrogen is used where the section is large, where a flammable mixture inside the pipe would be unacceptable, or where hot work such as welding will follow immediately.
Proving and leaving safe
Monitor at the vent until readings are consistently safe. Cap all open ends with appropriate fittings, leave redundant pipework clearly labelled or removed, and record the decommissioning so future engineers know the section is dead.
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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