Short answer
Nitrogen purging is required when a direct purge cannot be carried out safely: typically on large-diameter or long pipework where adequate purge velocity cannot be achieved, on higher pressure sections, where pipework runs through confined or occupied spaces, and before hot work on pipework that has contained gas. The decision should come from the risk assessment and IGEM/UP/1.
Typical triggers
Large mains and risers in multi-storey buildings; plant room headers serving megawatt-scale boiler plant; sections that cannot be vented safely to outside; welded modifications to existing gas-filled pipework; and situations where the client's own risk assessment prohibits a flammable mixture inside the pipe.
Hazards of nitrogen
Nitrogen is an asphyxiant. Venting must be to outside, oxygen monitoring is needed where personnel could be exposed, and cylinders or bulk supplies must be secured and regulated. The purge plan must cover both the fire risk it removes and the asphyxiation risk it introduces.
Proving
When inerting, monitor oxygen until it falls below the target level. When introducing gas behind nitrogen, monitor gas concentration at the vent until the purge is complete.
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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