Coefficient of Discharge for Safety Relief Valve Sizing

[chemter]

Hello.
I have a question concerning Safety Relief Valve coefficients of discharge (Kd). Consider a case where an ASME stamped safety relief valve (for a Sec. VIII, Div. I application) is installed with liquid trim. Assume the conventional safety relief valve has a Kd value of 0.7 for this liquid trim. Also assume the same safety relief valve has a gas/vapor Kd value of 0.9 when installed with gas/vapor trim. My question is if one is to now consider a sizing case for all vapor relief, what Kd value would you use for this safety relief valve since it is installed with liquid trim? 0.9 or 0.7? A similar case would be just the opposite, where an existing SRV is installed with gas/vapor trim and is subject to all liquid relief. Is the Kd value a function of the installed trim? Thanks in advance for any help.

 

[techman81]

Hi,

For preliminary sizing you can use Kd=0,65

 

[Latexman]

For the vapor only case, use K_d = 0.9 and calculate the minimum required area, A_vapor.

Compare this to the documentation of the current worst case scenario. There should be a minimum required area, A_liquid, calculated for that.

If A_vapor > A_liquid, the worst case scenario changed and you must change the trim. If A_liquid > A_vapor, the liquid case is still the worst case scenario and don't change the trim.

If you had a 2-phase flow scenario, you'd use K_d = 0.9 for that and calculate A_2-phase. The largest required area of the three flow phases will determine the size and trim of the relief device.

Good luck,
Latexman