Short answer
Engineers use combustible gas detectors to find leaks and confirm purge completion, purge hoses and fittings to vent gas safely outdoors, and on larger installations a flare stack to burn off the purged gas where venting is not safe.
Detectors
A detector reading in %LEL or % gas confirms the purge end point and helps trace leaks. It must be bump-tested before use.
Purge kit
Purge hoses must be sized for the required flow rate and discharge to a safe place. Flame traps and flare stacks are used where venting near openings is unavoidable.
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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