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# Convert cv to cg

## Convert cv to cg

(OP)
Can anyone help....

How do convert cv to cg ?

I know that cv = constant x kg

Cv [US Gallons/min/psi] = 0,03445 KG [m³/h/bar] (related to natural gas, density = 0,83 kg/m³) ( by manufacture)

Note: But the service that valve is for H2. Is valid the same relation or the constant is different ??.

Thanks.

### RE: Convert cv to cg

Cv is GPM/(psi^0.5)
Divide by the specific gravity of your gas to adjust for gases other than air.

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### RE: Convert cv to cg

(OP)
The gass is H2 (200 bara e 35ºC)
(densidade = 14,55 kg/m3 ;  Z=1,084; cp/cv =1,45)

cv ou kv ?? please is urgent

considering :
Cv [US Gallons/min/psi] = 0,03445 KG [m³/h/bar] (related to natural gas, density = 0,83 kg/m³) ( by manufacture)

### RE: Convert cv to cg

Cg=Cv*C1

C1=39.76*(Xt)^0.5

(OP)
Xt ???

### RE: Convert cv to cg

"Xt ???"

I should have clairified.  This formula is from Fisher Control Valve. Xt is another coefficient they assumed we needed.

### RE: Convert cv to cg

(OP)
"Xt is another coefficient they assumed we needed..."

How??

### RE: Convert cv to cg

psafety, do you have the reference for C1 = 39.76*(Xt)^0.5, and do you know if it is an exact relationship or approximate ?

### RE: Convert cv to cg

PA83, are you talking about converting between Cv and Cg where Cg is the Fisher valve coefficient for gases?

As said, Cg = C1 * Cv.

I'll typically look up the C1 as it varies depending on the type of control valve, conventional style globe valves are usually around 30 but it can be quite a bit lower

If you can clarify what you want it would help, the hydrogen conditions by itself doesn't have anything to do with the Cv or Cg.

### RE: Convert cv to cg

CJKruger,

I clipped this from the bottom of one of Fisher's coefficient's pages.  It is in each of their control valve pdf files.

=========================================================
Conversion of Sizing Coefficients
Following are conversions for use with other common sizing equations.
Kv = (0.865)Cv
C1= 39.76(XT)^0.5
Cg = CvC1
Km = FL^2
Cs = 1/20(Cg). Cs is only applicable for inlet pressures up to 70 bar(a)

==========================================================

psafety

### RE: Convert cv to cg

Fisher published tables with the obsolete gas sizing coefficient long before ISA and IEC established the current control valve sizing formuli.  Consider avoiding obsolete vendor constants such as Cg since these International organizations publish concensus standards using Cv or Kv.

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