What is nitrogen purging?

Nitrogen purging replaces whatever is inside a pipeline, vessel, or process system with nitrogen, a gas that will not burn, will not support combustion, and does not react with most of what runs through industrial equipment. It is done before a system is opened, before it is put into service, and before it is filled with product, so that there is never a moment when fuel and oxygen are in the same pipe.

Where it is used

• Before start-up. A new or repaired line is purged of air so that the first product in does not meet oxygen.
• Before maintenance. A line that carried gas or liquid hydrocarbons is purged to remove them before anyone cuts into it.
• Drying and preservation. Dry nitrogen carries moisture out and leaves a line inert while it waits, which is why long lay-ups are often held under a nitrogen blanket.
• Pressure testing. Where water cannot be used, nitrogen is the safest gas for a pneumatic test, and it is also used to push the water out after a hydrotest.

How it is supplied

On a job site, nitrogen arrives either as a liquid in a cryogenic tank that is vaporized on site, or as a compressed gas in tube trailers and cylinder banks. Liquid gives the largest volume per truck. High-pressure tube trailers give pressure without a pump and are quicker to set up for smaller volumes. Nitrogen generators, which separate nitrogen from air on site, are used where the job runs long and the purity requirement allows it.

The numbers that matter

Purity, usually stated as a percentage of nitrogen, decides whether the purge meets the specification. Volume, in standard cubic feet, decides how much of the job one delivery covers. Pressure decides what the equipment can do without additional compression. A crew plans a purge by the volume of the system, the number of volume changes needed to reach the target oxygen level, and the flow the equipment can deliver.

Related terms: hydrostatic testing, pigging.