API 2000 discharge coefficients for preliminary psv sizing

I usually just use the capacity charts from Enardo or Varec or others. I might curve fit them to automate the calculation process. Then the discharge coefficients are unimportant to me.

In the old days, we used an orifice discharge coefficient of 0.5 for preliminary sizing. We also used a "rule of thumb" that the sizing pressure had to be 2X the set pressure to guarantee the pallet was wide open.

Nowadays, in the 6th Ed. (Nov. 2009) of API 2000 I glanced at, they show a graph of discharge coefficient (Y) versus absolute pressure ratio (X) of inlet P / outlet P. For X from 0.5 to 1.0, Y ranges from 0.825 to 0.65, respectively. This is from the curved labeled valve discharge coefficient for published capacity.


Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.