Purging or cleaning operations is to drain the contents of a pipe or tank and replace it with another material (gas or liquid).
When a gas pipeline enters the service, it must be emptied and cleaned of the primary air that is present. If natural gas is used for piercing, air and gas are blended together, forming a flammable compound inside the pipe, which generally destroys safety.
Types of purging:
Cleansing with Neutral Materials: The action of replacing a neutral material with the original contents of a conveyor or tank completely or in such a way as to interfere with the two substances that are to be replaced to avoid mixing.
Dilution: Injection of a new substance that mixes slightly with the previous material.
Replacement: The complete replacement of the new material in place of the previous one, without any trace of the previous material or a mixture thereof.