PURGE WITH NITROGEN Page 1 of 3
HOW TO PURGE WITH NITROGEN
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 nitrogen (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
PURGE WITH NITROGEN Page 2 of 3
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
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