Nitrogen Purging

Nitrogen (N₂) purging is the process of replacing oxygen, moisture or other unwanted gases inside equipment, pipelines, vessels or process systems with nitrogen. Because nitrogen is relatively inert and dry, it is widely used during commissioning, shutdown, maintenance and product changeover.

High-Pressure Nitrogen Purging (20 – 340 barg)

For a natural-gas well, the nitrogen injection pressure, rate, quantity, injection point and acceptance criteria cannot safely be selected from well depth alone — they depend on wellhead pressure, reservoir pressure, tubing / casing dimensions, completion configuration, wellbore fluids, MAWP, temperature and the intended operation. For high-pressure purging we raise pressure from 20 to 340 barg. Nitrogen may be generated from liquid nitrogen (LIN) and vaporized / pressurized as required, or supplied as high-pressure gaseous nitrogen.

A high-pressure nitrogen system may include:

• Liquid nitrogen storage tank / tanker
• Cryogenic nitrogen pump
• Ambient or steam / electric vaporizer
• High-pressure nitrogen manifold and pressure-control valve
• Check / non-return valves, isolation valves and relief protection
• Flexible high-pressure hoses or rigid piping
• Flowmeter, pressure and temperature instrumentation
• Gas analyzer / O₂ analyzer
• Emergency shutdown system

Low-Pressure Nitrogen Purging (0.5 – 12 barg)

Low-pressure nitrogen purging removes oxygen, combustible gas, moisture or air from a pipeline or LPG vessel before commissioning, maintenance, shutdown or return to service. Nitrogen provides an inert atmosphere and reduces the risk of fire and explosion. For low-pressure purging we raise pressure from 0.5 to 12 barg; the actual purge pressure is established from the equipment design and operating procedure.

Typical sequence for an LPG / natural-gas pipeline:

1. Isolate the pipeline from the LPG / gas source.
2. Drain or depressurize the line as required by the operating procedure.
3. Connect nitrogen through a regulated low-pressure connection.
4. Introduce nitrogen at a controlled rate.
5. Displace the existing atmosphere through a safe vent point.
6. Monitor the outlet for oxygen concentration, hydrocarbon concentration (where applicable) and nitrogen pressure.
7. Continue purging until the specified acceptance criterion is reached.
8. Isolate nitrogen and maintain the required inert condition.

For a line containing LPG, the vented LPG / nitrogen mixture is routed to a location where it cannot create an ignition or exposure hazard, and the purge pressure never exceeds the allowable pressure of the pipeline / tank or the rating of its weakest connected component.