Short answer
Direct purging replaces air with gas (or gas with air) in one step, so a flammable mixture briefly exists inside the pipe. Indirect purging inserts an inert slug — usually nitrogen — between the two, so a flammable mixture never forms. Direct purging is normal on most commercial pipework within the limits of IGEM/UP/1; indirect purging is used where those limits are exceeded or the risk is too high.
Direct purging
Used on the majority of low pressure commercial installations. It depends on achieving enough velocity in the pipe to prevent stratification, which becomes harder as pipe diameter and length increase. IGEM/UP/1 sets the conditions under which a direct purge is acceptable.
Indirect purging
The pipe is first filled with nitrogen until oxygen falls below a safe level, then gas is introduced to displace the nitrogen. The reverse applies when taking pipework out of service. It needs more equipment and planning — nitrogen supply, oxygen and gas monitoring, and an asphyxiation risk assessment for confined spaces — but removes the flammable-mixture stage.
How to decide
Large diameters, long runs where velocity cannot be achieved, high pressure sections, pipework in confined or occupied spaces, and situations where hot work will follow are the usual triggers for indirect purging. The decision belongs in the written purge procedure.
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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