control vavle flow rate calculation

[wormhhh]

I am calculating the flow rate through a control valve:

steam condensate,liquid, temperature difference between inlet and outlet is about 40 F.

Cv=127
Sg=0.924
Delta P=23.8

The equation I am using:Q=Cv*Sqrt(DeltaP/Sg),I got some number real big. I am wondering: am I using the right equation for this?


Thanks. Appreciate it.

 

[TD2K]

Units would help as well as the value of the flow you are getting.

The equation you are using is correct for non-flashing/cavitating liquid. Q is in gpm. dP is psi. SG is unitless.

What is the Cv you are using? Is it the installed Cv of the valve or the Cv of the valve at the % it is open?

Is this a single phase non-cavitating liquid?

 

[wormhhh]

TD2k:

yeah, Q is in gpm. Cv is assumed normal Cv. I think it's Is this a single phase non-cavitating liquid.

 

[25362]

Without flashing how do you explain the 40 deg F cooling effect ?

 

[sshep]

Your problem is clearly flashing. We think you are using a Cv based on the valve travel, but applying as if is only a liquid flow. The valve has to open farther than you think is neccessary because it is handling the flashing liquid.

best wishes,
sshep

 

[JimCasey]

Yep, you have the equation for noncritical liquid flow.
As identified earlier, you probably have critical conditions and the real flow deviates from the noncritical flow markedly. I ran it 3 different ways:

Temp(F) 352 265 60
P1 (PSIG) 125 24.8 100
P2 (PSIG) 101.2 1.0 76.2
Q (GPM) 457.2 218.8 618

These all meet your criteria of Cv of 127, and DP of 23.8
The first 2 conditions are slightly subcooled condensate flashing through a valve, and the third condition is noncritical cold water.

I ran this through the Flowserve program. The Fisher, Masoneilan, and Leslie programs should give similar results. You really need to state your temp, Inlet pressure, outlet pressure, and either Cv or Flowrate for the programs to calculate properly. The referenced sizing programs have steam tables built-in to account for critical conditions.

 

[wormhhh]

thanks for you guys help.I appreciate it.

JimCasey:

the inlet is at 53.2 pisa,280F and outlet is at 29.4psia 240F. inlet fluid is liquid(water)only. Could you help me to run it at flowserve? what are the equations for this case?

Thanks lot!

 

[JimCasey]

At those conditions I get 319.14 GPM, flashing.

Some of this will depend upon the specific parameters of your valve. The valve I used was the default unbalanced contoured globe valve. The flow is choked, and choking is dependent upon several geometric parameters such as Fl.

The flash is 3.3 percent. specific volume is 0.482 ft3/lbm downstream, or about a 30:1 expansion over liquid water.

Also, the T2 is 249.17, or 30.83 degrees F cooling.

I can't paste a document into this forum, or a graphic, and the equations for critical flow of liquids cover 2 pages.

 

[wormhhh]

Jim casey:

Thanks so much! I really appraciate it.