Does backpressure affect blowdown valve's peak flow in a closed flare

[CalCa]

Hi,

I was confused on this topic described as in the subject when I was doing the depressuing calculation via Flarenet for blow down valves in a closed flare system.

Normally I picked up the peak flow of depressuring calculation from either Hysys or certain spreadsheet to input the blowdown scenario in Flarenet. However that peak flow is obtained from initial pressure depressuring to atmosperic pressure, which there is no back pressure issue. What I really concerned is in the closed flare system, if there is much higher downstream pressure for blowdown, sometimes even higher than the depressuring pressure, can I still use that peak flow as a correct one or not?

What I had done for those cases is to assume a backpressure, then calculate the peak flow in Hysys and input it into flarenet, then get a new backpressure from flarenet to calculate the new peak flow in Hysys. Using Trial and Error method (iteration) until both of backpressure and peak flow are match in Hysys and Flarenet.

I don't know if I am in the correct path regarding to this matter. Hope to get your feedback or advices.

Thanks for your time in advance.

Regards,
CalCa

 

[JimCasey]

Usually it is assumed that a SRV exceeds critical pressure drop. It would be difficult for that not to happen since critical flow happens at about a 2:1 pressure ratio and the minimum setting for a code valve is 15 psig. (15 psig is 29.7 psia, and atmospheric pressure is 14.7 psia. i.e. >2:1 pressure ratio.

If there is enough backpressure to cause the flow to be less than critical, then the flowrate is affected.

The SRV >>must<< (not an option) be bellows-balanced if the downstream pressure will fluctuate otherwise the unbalanced valve will read the decreased DP as blowdown. Chattering or inadvertent overpressure can occur, both high on the list of bad things. With a balanced valve, the valve acts by responding to vessel pressure differntial with atmosphere, regardless of developed backpressure.
A pilot actuated valve would need an assured atmospheric reference.

Crosby, Farris, Consolidated, etc. literature all give the procedure for calculating flow corrected for backpressure.