HOW TO PURGE WITH NITROGEN

Often times we are faced with the question

Calculate the volume of Nitrogen required to purge a system. Be it in a design setting where we need to estimatethe nitorgen system capacity or in a process plant, to calaculte the quantity of nitrogen cylinder required during a shutdown.        

A quick answer is provided:

Nitrogen purging is easily adapted to any process installation. Different?methods are used depending on the type and shape of the equipment to?be purged and on the location of the purging inlets and outlets.??

1. Displacement Purging??

This method is used for equipment with simple cross sections such as?pipelines. The volume of nitrogen required corresponds to the physical?volume of the pipe. In many pipelines, a rubber scraping piston, or "pig,"?is introduced and propelled through the pipe by the nitrogen pressure to?clean the line.??

The nitrogen volume required to purge equipment with a simple cross?section is determined using the following formula:??

V = VoP/14.7??

Where: V = Total nitrogen volume required (scf)?

?Vo = Water volume of pipeline (cf)?

?P = Absolute pressure of nitrogen in the pipeline during?purging (psia)?

?2. Pressurization Purging??

This method is used when conditions do not permit a sweeping action of?nitrogen through the vessel. The vessel is repeatedly pressurized and?mixed with nitrogen gas and then the mixture is exhausted. The total?volume of nitrogen depends on the number of pressurizing purges?required to reduce the contaminant to an acceptable level and can be?determined by using this formula:??

V = 1.2nVoP/Pa??

Where: V = Total nitrogen volume required (scf)?

?Vo = Water volume of vessel or tank (cf)?

?P = Absolute pressure after pressurization with nitrogenn(psia)??

?Pa = Absolute pressure after exhaust (psia)?

?n = Number of purges = C log Co/(log Pa log P)?

?Co = Initial content of gas to be removed?

?C = Final content of gas to be removed?

3. Dilution Purging??

This method is used for equipment cross sections such as distillation?columns, kilns, reactors, etc. Nitrogen partially mixes with the gas to be?purged out, and then the mixture exits through an outlet located as far as?possible from the inlet. The nitrogen required to reduce a contaminate to?a desired level can be found using the graph below and the total volume?of nitrogen needed can be computed from the following formula:??

V = nVo??

Where: V = Total nitrogen volume required (scf)?

?Vo = Water volume of equipment (cf)?

?n = Number of nitrogen volumes required?

A relevant portion from NFPA 69 on nitrogen purging is attached: