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How to size the flowrate for the nitrogen purge line

How to size the flowrate for the nitrogen purge line

How to size the flowrate for the nitrogen purge line

(OP)
Dear Sir

A distillation column with operating pressure is 24kg/cm2g on the overhead, and discharge to the reactor.
And, there is a nitrogen purge line(4") between the overhead pipe line (8") to the reactor.

Known:
The flow rate for the overhead line about 50000kg/hr.

How can we size the nitrogen flow rate for the 4" nitrogen purge line?

There are such line size criteria as follow,
1)the maximum allowable velocity in pressured overhead line,
compressor lines and transfer line is 100 ft/sec.
2)or relief and blow down lines 0.1~0.7 mach.

Could you please kindly guide me which criteria is proper?
And how to get the flow rate for this nitrogen line.

Thank you very much.

RE: How to size the flowrate for the nitrogen purge line

This is a relatively low pressure purge scenario, hence those velocities are going to be way too high.

Many times the purge flowrates and velocity will depend on how much time you have between loadings, a recharge, some valve opening or closing, or time between some other events to fill up a vessel and lines with N2 in this case.  If you have 100 seconds to purge a 100 cu meter tank, you would at least need a flowrate of 1 cms.  Check that volumetric rate against whatever velocity that would give you through your inlet pipe and be sure it will be within a range of say 5 to 10 m/s.   

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

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