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I am working on a flare gas recovery project. We understand purging of flare header is recommended to maintain positive pressure and air ingress. My query is, if I have liquid seal or any other device between knock out drum and flare stack to divert gases from K.O. Drum to flare gas recovey unit (FGRU), and maintain a positive pressure upstream of seal, do I still need to purge flare header, or only flare stack needs to be purged to provide air ingress?
If at all purging is required then what is correct reason for purging?
Although, as liquid seal/or any device (fast opening valve placed) will result in positive pressure upstream of seal and K.O. Drum and I feel that purging of flare header is not required and only flare stack needs to be purged.
 
Answers
08/11/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Mr Ragsdale's answer gets my vote as very good. I want to add--NO 'FAST ACTING VALVE' is allowed in an emergency pressure relief system. Water seal legs work , but do require freeze protection and automatic seal liquid replenishment facilities. A relieving event will usually blow all the seal liquid out and up the flare stack.
'Molecular Seal' flare tips dramatically lower purge gas requirements for the flare stack. You do not want to ever allow oxygen concentration in the flare stack or headers to even approach combustible limits.
08/11/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Theoretically, you are correct in thinking that only the stack will need purging. Some people, however, believe that, because the seal can be blown, and is usually not automatically re-established, it is wise to purge the entire header. If the flare gas compressor returns the material to a gas plant, you want the purge gas to be methane, not nitrogen.
08/11/2010 A: SImone Robinson, Tube Tech International Ltd, simone@tubetech.com
Any actions you take re the above may create an increased fouling issue which can now be solved by an 'online' cleaning method. Please review case studies on the Tube Tech website.