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Flare Stack Rating Consideration

Flare Stack Rating Consideration

(OP)
dear all,

i need to evaluate the flare stack performance, the existing flare is designed to dispose around 50 MMSCFD natural gas , due to demand increasement from buyer, the flare shall be capable to handle 65 MMSCFD disposal gas (block discharge scenario governs the flow rate)
i have some questions regarding this flare rating :

a. Flare Mach Number :
as per API RP 521, mach number in flare tip shall be 0,5 at emergency scenario. based on the condition at 65 MMSCFD, the mach number is 0,7 for the existing diameter of flare. what will happen if the mach number is 0,7 ? what is the negative effect ?

b. Flare Velocity :
according to API RP 521, there is phenomenon called blowoff (high flare velocity) or flashback (low flare tip velocity). since there is increasement in flowrate ( up to 65 MMSCFD), blowoff may happen in the flare stack. the problem is... i can't find the velicity limit to avoid blowoff (or flashback) in API RP 521
could you please point me out in what paragraph that this limit available in API RP 521 ? (or other references)

c. Noise Limit
due to higher velocity, excessive noise may become a problem, again, i can't find the noise limit for flare in API RP 521. could you please point me out in what paragraph that this limit available in API RP 521 ? (or other references)


d. additional flare
is it ok to just add another flare in a flare system network ? i mean, since the existing flare is 50 MMSCFD, i am considering to buy another flare with 15 MMSCFD (50+15 = 65 MMSCFD). what should i consider for this option ? or is it better if i just buy one flare in 65 MMSCFD capcacity ? i don't think back pressure in PRV will be a problem since only one flare header available


My reference : API RP 521, 1997 edition.



thanks in advance, i really need your help.

RE: Flare Stack Rating Consideration

Have you talked to your vendor? There might not be a problem at the higher Mach number or you might be able to get by with a tip replacement. Depending where you live, there might also be governmental regulations on your flare similar to the EPA in the US.

API RP 521, 1997 edition is a little dated now but I doubt API 521 is going to help you with blow-off past 0.5 Mach or noise levels, I'd be talking to the vendor.

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