In fire and explosion prevention engineering, purging refers to the introduction of an inert (i.e. non-combustible) purge gas into a closed system (e.g. a container or a process vessel) to prevent the formation of an ignitable atmosphere.
Assume a closed system containing a flammable gas, which shall be prepared for safe ingress of air, for instance as part of a shut-down procedure.
It is useful with two terms for purging because purge-out-of-service requires much larger quantities of inert agent than purge-into-service.
Purging with an inert gas provides a higher degree of safety however, because the practice ensures that an ignitable mixture never forms.
The most common purge gases commercially available in large quantities are nitrogen and carbon dioxide.