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Assist Gas for Flares

Assist Gas for Flares

Assist Gas for Flares

(OP)
How can I estimate the flowrate of assist gas (a gas with a higher heating value such as propane or natural gas required for complete combustion of a waste gas stream going to a flare?  I have seen guidelines on assist steam for smokeless operation, but I have not run into guidelines for assist gas.  
Any information would be greatly appreciated.  

RE: Assist Gas for Flares

I worked on one flare that used assist gas (horizontal ground flare) that was essentially heavy NG, maybe 19 MW.  The assist gas max design flow was 10 MMscfd, I believe the smokeless capacity of the tip was about 40 MMscfd from the vendor which would be about 0.25 lbs assist gas/lb flare gas.

This was off an oil processing plant.  Flareman likely has the best information for you.  What type of gas are you flaring?

RE: Assist Gas for Flares

(OP)
Diesel vapors

RE: Assist Gas for Flares

It really depends on the reason for adding gas. In flares its normally supporting combustion of a low HV gas or providing assistance to achieve smokeless flaring. A good guide line is to use 300 Btu/Scf for combustion support for low HV gases to ensure stable combustion. For complete combustion try and aim for 600 Btu/scf but at the same time try and flare at a higher velocity, the closer to 0.5 Mach the better as the air ingress in to the flame at this exit velocity helps combustion also.
I seem to remember that API 521 does give some guidance on the issue although not alot.

RE: Assist Gas for Flares

Flaring diesel, you must be boiling off diesel at high temperature.  Won't the diesel heating value be in the thousand of btu per scf.  I am not a flare specialist but I believe you will need air to assist burning rather then natural gas and definitely not propane.  

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