Prior to the introduction or removal of flammable gases or liquid to/from a pipeline, vessel or pipe work system, it is a standard practice to render the atmosphere within the equipment incapable of supporting combustion. There are three basic ways of purging a system with Nitrogen: Displacement Purging (at atmospheric pressure), Pressure Cycle Purging and Vacuum Purging.
Nitrogen is passed continuously into the pipeline, vessel or system at one point, while the air or gas being purged leaves from another point. The system is normally purged to atmosphere, so the system remains at atmospheric pressure. In practice, Nitrogen performs its function by displacement or dilution of the contents, typically, both these processes occur simultaneously. For pipeline purging the mixing interface is usually quite small, therefore purging takes place mainly by displacement. For vessel purging the mixing of gases is usually complete and purging takes place mainly by dilution.
PRESSURE CYCLE PURGING
With this method of purging, a system is pressurised with Nitrogen then allowed to mix for a period before venting. This method is usually adopted when the configuration of a system is complex and the simple.
This method is basically similar to pressure cycle purging except that a system is evacuated before Nitrogen is introduced to break the vacuum and mix with residual gas.