Liquid PRV formula 2

[rww88]

I am finding variations of the liquid relief valve sizing formula constant, or coefficient, with a range from 27 to 28.5. Is this constant valve manufacturer dependent? What is the latest formula?

 

[CHD01]

What you are probably looking at are simplified formulas incorporating different correction factors into one factor. Yes, this would change with manufacturer, also with the type of liquid trim (standard trim or ASME certified for 10% overpressrue) and whether you are basing area calculation on ASME or API methods (Kd differs by a factor of about 0.9 between the two methods). The more you learn, the less you are certain of.

 

[rww88]

Thank you very much CHD01. The 1972 formula I am presently using is: A=(Q (G)**0.5)/(27.2K Kp Kw Kv(delta P)**0.5). Is this a viable formula in 2002?

 

[CHD01]

Its a valid formula, but:

1. For a certified liquid trim valve, Kp always = 1.0, only for valves with older trim (older Farris models) is Kp required.
2. The part of your formula "27K" is actually derived from an ASME/API formula based on the manufacture's calculated Kd. In the API equation, this is replaced with the term "38*Kd". So if Kd is equal to 0.74 for a Crosby valve (for example), your K factor is equal to 38*0.74 = 28.14; which is what CROSBY shows in their engineering handbook:

A = GPM * SQRT(G) / 28.14 * Kw * Kv * SQRT(dP).


I have copied a segment from an excel program I wrote to size a relief valve. Technically, you should be able to take any vendor's equation and with manipulation obtain the formulas I am forwarding to you. Please be aware that it could contain typo's since I had to re-structure it after pasting.

"Begin Paste:

The following equations are used:
The equations may be used for either ASME or API area depending on the Kd used. For ASME area, Kd = 0.9*Kd determined by vendor.
For Gases and Vapors:
(1) Area Orifice = (W*SqRt(T*z)) / (Kd*C*P1*Kb*SqRt(M))
For Steam:
(2) Area Orifice = (W) / (51.5*Kd*P1*Kb*Ksh*Kn)


For Liquids
(where 23.56 = (38*K) if Kd is assumed to be = 0.62):
A. Standard Trim for ASME Non-Certified Liquid Capacity:
(3)Area Orifice = (Q*SqRt(G)) / (23.56*Kp*Kw*Kv*SqRt(P1-Pb))
Equation Requires P1=(1.25 * Set Pressure Ps in PSIG)+ 14.7
Kp factor adjusts for overpressure other than 25%

B. Liquid Trim for Certified Capacity at 10% OverpressureRequired for Valve Capacity per ASME Code in Liquid Service)
(4) Area Orifice = (Q*SqRt(G)) / (23.56*Kw*Kv*SqRt(P1-Pb))
Note: Kp = 1.00 & P1 includes allowable overpressure

See API 520 for reference on liquid sizing formulas. Note that equation (3) for P1 at 25% overpressure is equivalent to:

(5)Area Orifice=(Q*SqRt(G))/(23.56*Kp*Kw*Kv*SqRt(1.25P1-Pb))
End of Paste"
I am also attaching info on the Kd factor that I have found to be true:

Begin Paste:

Relief Valve Coefficient: I
If not known use API Kd of:

0.62 for non-certified liquids (Farris = 0.64)
0.65 for certified liquids
(varies from 0.724 to 0.744 depending on vendor)
0.975 for vapor, gas or steam
(varies from 0.950 to 0.975 depending on vendor)

Note: ASME Kd = 0.9*API Kd if ASME Area is desired

Gets confusing sometimes, Good Luck. Charlie D.


The more you learn, the less you are certain of.

 

[rww88]

Thanks very much Charlie D. Your information has been very helpful.