Air Assisted Flare Question

[Mike6158]

Can anyone give me an idea of how much excess air is typically used for an air assisted flare? I am trying to reverse engineer a smokeless flare design. I calculated the stoichiometric air requirments for our design and by using the design capacity of the blower I came up with over 300% excess air. That seems high to me. I suppose that it is possible that the designer used a little brute force and supplied us with a blower that far exceeds our design needs. I wish that he would have done as much for the tip ) Is there a good reference manual for flare design available for purchase?

 

[juw]

There are two good references available, John Zinks Combustion Handbook and API RP 521, both cover the basics. The flare will entrain the majority of the required air at the tip as it exits and only needs a small proportion of forced air to go smokeless. Typically you should expect 10 to 30 % of stoichiometric air ratio (SAR) for saturated hydrocarbons, and 30 to 40% SAR for unsaturated hydrocarbons.

 

[Mike6158]

Thanks for the reply. I've read API RP521 but I did not find anything specific concerning the SAR. I am having a problem backing into the design criteria for one of our air assisted flares. I calculated the amount of air that I need for stoichiometric combustion, assumed 50% excess air, and came up with a number in the 9,000 #/hr range. Let me back up a little. The flare stack was supposed to be designed for smokeless operation at 13,000 mscfd of propane. It has never met design. Not even close. I'm a rather hard headed individual. To test what the capacity actually was I connected a meter tube and valve to the vapor space of our propane tanks and to the flare system. Flow tests indicated that 250 mscfd was enough to make the flare smoke. At first I thought that we did not have enough air. After doing the calcs it appears to be a problem with the tip. The contractor is going to make the flare right for us by changing the tip and installing a new blower that is designed to flow 180,000+ #/hr. Now I am really confused. That is a considerable amount of air. I find it a little hard to believe that they are going to supply that much air for this case.

To get stoichiometric I used this formula in a spreadsheet to get #/hr rqd at 50% excess air. Did I miss something?

28.964*(C1mols*2)+(C2mols*3.5)+(C3mols*5)+(iC4mols*6.5)+(nC4mols*6.5)+(iC5mols*8)+(nC5mols*8)+(C6mols*9.5)*(1.5)

The component multipliers were derived from:
2CnH(2n+2) + (3n+1)O2 ---> 2nCO2 + (2n+2)H20

Thanks

Mike