Purge gas rate for flare sub header 1

[gks]

hey all,

As per literature (Oil Field Processing ; Vol.II : Crude Oil by F.S. Manning et all page –368 ) the recommended purge gas velocity for flare stack with molecular seal is 0.01 ft/sec t0 0.03 ft/sec. Can the same velocity criteria can be used for Unit flare header end purge of individual units in a large gas processing complex? We could not find it in API RP-521 . Will any one help us in finding the recommended purge gas velocities for Flare SUB Headers?

 

[TD2K]

What I've done in the past is set the minimum purge velocity at the tip and then work back up the headers distributing out the purge gas flow to each header and then seeing what that purge velocity is in each lateral. You'll then need to use your judgement whether you want to keep a certain minimum velocity in a lateral even if it means increasing the total purge rate out the tip. Whether that will be the case will depend on the size and number of laterials you have.

Personally, I'd be more concerned about keeping the minimum purge velocity at the tip as that is based on preventing air infiltration by diffusion back into the system. It doesn't really apply to the minimum purge velocities in each lateral. There is also the arguement that no flare system is 100% tight and any leakage effectively increases your purge.

Note, I've seen some company standards override significantly the purge requirements for molecular seals (your numbers are in line with what I have seen from the vendors).

Note, one other thing you may want to look at is if you flow hot gas down through the flare system. Once that flow stops, the hot gas in the system cools and shrinks which can pull air back in through the tip if the shrinkage rate is greater than your purge rate. We had to install a purge system for this case at one plant.