Flare Capacity Calculation 1

[meister]

Any thoughts on how to calculate the capacity of a flare? I know how much natural gas I can feed and the dimensions of the flare and piping, as well as the waste gas composition. I would like to know the max capacity of the flare. Thanks!

 

[TD2K]

What type of flare is it? A conventional open pipe type flare is good to 50% mach at the outlet tip. Velocities down the flare header should not exceed 50% to 70% mach (70% mach more typically applies to laterals, not the main flare header). I'm not sure I know what you mean by 'I know how much NG I can feed and the waste gas composition'.

Then there is the question of radiation on surrounding equipment, property lines, operating areas that needs to be checked out.

Capcity of the flare drum should also be checked, that method is summarized in API 520/521.

If it's a sonic type type, I suggest getting the vendor involved.

 

[rbcoulter]

You should consult the manufacturer. Although a flare may be able to burn at rates for exceeding what one would normally expect from a process, the capacity may be limited by other issues like , heat effects on surroundings as TD2K said, or the limit that I have encountered - excessive smoke, which could violate your air permit.

 

[CHD01]

Good Stuff RD2K and rbcoulter: but would not 50-70% mach be a pretty high dP for a flare?

Also - should not we include the ability to disperse the gas to a safe ground level concentration if the flare is extinguished? The more you learn, the less you are certain of.

 

[TD2K]

The pressure profile up the flare header also needs to be checked to ensure that you do not affect any of the PSVs that need to relieve to the header.

I've seen several company specifications which limit flare head velocities to 50% of mach. When I've done any calculations, the 50% mach limit at the header outlet quickly results in back pressure building up upstream of this point which in turns reduces the velocity and even more quickly, reduces the pressure drop upstream of that point. The effect is that only the end of the flare header (or ends if you have increasing diameter flare header) will see the 'high' velocities.

I have not routinely seen vapor dispersion calculations done on a flare to prevent ground level gas concentrations from exceeding the LEL or some portion of the LEL. I've done radiation calculations for 'cold' vents which were designed only to disperse the gas into the atmosphere but not the other way around and the only gas dispersion calculation I was involved with was on a ground level open vent to atmosphere. Other experience out there?

 

[CHD01]

Actually I was referring to a safe TOXIC ground level concentration - assuming the gases were toxic; although the same calculation can be made to limit flammability. The more you learn, the less you are certain of.

 

[fdomin]

I have made SO2 dispersion calculations for a refinery H2S flare, but never for gases assuming an extinguished flare.
Nevertheless API RP521 4th raise that issue in paragraph 4.4.3.5.
As exposed there, if flared gases are toxic, pilot monitoring is critical to ensure flare is always ignited.

Hope this helps

 

[Flareman]

Getting in on this a little late but rbcoulter is right. Check with the vendor.
Rough rules of thumb are,
- if the gas is a rich flammable, (natural gas or better) then 20% sonic is probably just fine; 50% sonic needs good pilots and flame stabilizers, and anything over than (yes there a lot of sonic discharge flares) needs a special design,
- if the gas is leaner than natural gas, go to the rules of 40CFR60.18

http://www.gpo.gov/su_docs/aces/aces140.html

They're a reasonable guide (albeit very poorly crafted and badly misapplied .. in my opinion)
Look at API RP-521 for other guidelines and I'll send you a paper of my own if you drop me an email at flareman_xs@netzero.net

David