I have an applications and need som

[Jack_D]

I have an applications and need some assistance in validating a proper ball valve Cv is sufficient for the application:

Application:

Natural Gas
62,000 lbs/hr Min
68,000 Lbs/hr Max
MaxP = 560 PSIG
MaxT = 770 Deg F
Diff P = 560 PSI
Density = 0.044
MW = Min (16) Nor (17.5) Max = (19)

The Pipe size is 8" schedule 40

I believe an 8" full port ball valve is sufficient that has a manufactures published Cv of 9300

I have ran a few calculations but the math does not appear to be correct and I am hoping someone can help explain to me how to calculate Gas flow in LBS per hour to determine if the Cv of the valve is sufficient. I came up with a required Cv of 3328, which would allow me to use a smaller valve. However I just want to learn how to properly size the valve and validate my calculations.

I was trying to convert to SCFM first but again I am confused how to determine if the 8" valve is properly sized.

Thank you for your time and input.

Have a great day !

Jack

 

[LittleInch]

Calculation or determination of a valve CV needs to include the maximum differential pressure you want or can tolerate at max flow.

You don't say what that is.

Try this sitem (scroll down for gas non critical flow)

https://www.engineeringtoolbox.com/flow-coefficients-d_277.html

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack_D]

The P1 = 560 PSIG and P2 pressure = 0 . This valve is to serve as an emergency trip valve so the differential pressure across the valve is zero. The valve will either be full open or full closed. The Cv of 9300 is a full port ball valve.

 

[LittleInch]

If your pipe is 8" sch 80 all the way and your valve is a full bore ball valve, then you can't really get any less differential pressure as the valve is essentially the same as a similar length of 8" pipe.

Like I said, valve CV needs to have some sort of differential pressure otherwise you're dividing by zero somewhere.

How did you arrive at a required Cv of 3328?

I've got lost as to where to go next in this thread.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack_D]

Thanks Appreciate Your assistance.

The schedule pipe is 40. I was attempting to use 1 lb-mole/MW) X 359.05 to determine SCFM and then 0.0177 to determine the actual Cv requirement.

 

[georgeverghese]

An emergency trip valve size should be based on the max permissible dp when in max normal flow, and surely this cannot be 560psi. It is not sized on the dp when closed. If you really must size this valve, would suggest you allow a max dp of 1% of the min operating line pressure.
Your density seems unusually low for natural gas at 560psig, 770degF - I get approx 0.85lb/ft3, assuming z=1.
Use the closing dp for the torque to be developed at the ESV actuator, not for the Cv of the valve.
BTW, how does this gas get to 770degF?