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Coefficient of Discharge for Safety Relief Valve Sizing

Coefficient of Discharge for Safety Relief Valve Sizing

Coefficient of Discharge for Safety Relief Valve Sizing

(OP)
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.

RE: Coefficient of Discharge for Safety Relief Valve Sizing

Hi,

For preliminary sizing you can use Kd=0,65

RE: Coefficient of Discharge for Safety Relief Valve Sizing

For the vapor only case, use Kd = 0.9 and calculate the minimum required area, Avapor.

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

If Avapor > Aliquid, the worst case scenario changed and you must change the trim.  If Aliquid > Avapor, 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 Kd = 0.9 for that and calculate A2-phase.  The largest required area of the three flow phases will determine the size and trim of the relief device.

Good luck,
Latexman

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