PurgingPurging is a process of gas replacement whereby an unacceptable atmosphere is replaced by an acceptable one. The main applications of the technique are the provision of an inert atmosphere in a vessel and back purging to protect welds from contamination and to reduce the level of a toxic, flammable, or explosive atmosphere in a vessel to a safe level prior to any work being undertaken. Purging of pipelines and containers may also be used to eliminate atmospheric contamination during welding operations. When welding certain materials it is necessary to prevent the reverse side of the weld area oxidising, particularly in pipework when post-weld cleaning of the root is impossible. This is usually achieved by passing a stream of inert gas, usually argon, through the pipe (or over the reverse side of a plate) to exclude oxygen from the area. Purging may be carried out with gas in several ways including, use of a pressure cycle, use of a vacuum cycle, or at atmospheric pressure. Foam inerting may be used to displace a flammable atmosphere with nitrogen when welding or cutting has to be carried out in a vessel. Careful control of the process and accurate monitoring are essential if desired results are to be achieved.
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