High-pressure flare system sizing

[zhwil31]

I work on a flare system to collect the blowdown and other relief gases from compressor stations. The flow rates are quite high about 500 Mscfm and the reliefing pressure is about 800 psi. The initial sizing for main flare header shows that 44” diameter pipe is needed to limit the outlet flow speed below 70 percent of sonic velocity. For the purpose of reducing cost, I was thinking to use a combination of flare piping, which is composed of 44” main header from knockout drum to flare stack, 20” sub-header connected to design pressure of 50 psi knockout drum and associated upstream discharging pipes. Since the relief pressures are high, higher pressure drops in the flare system are allowable. This is the reason I choose a knockout drum operating at design pressure of 50 psig, which is still higher than the critical pressure for 20” upstream sub-header. Can you give some suggestions to minimize the length of main header from a knockout drum to flare stack, which still can handle the pressure drop from 50 psi to atmosphere pressure.

Is this plan practical? Are there any other options that can handle large and high-pressure flash loads in a cost-efficient way? Thank you for your comments.