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# ISA Control valve sizing equations

## ISA Control valve sizing equations

(OP)
Hi All.

I am busy using the ISA equations for sizing a control valve.
I know my mass flow rate and I have my density, compressibility factor and molecular mass values.
However, when I use Eq. 9 and 10, I get very different results. Cv 0.39 and Cv 12.38 for each equation respectively.

I have used coolprops to get my values with inlet conditions pressure 40 bar @ 60K. Dp 34 bar. Helium gas.

Should the equations using mass flow rate be similar? Or can I expect large differences?
Its just throwing me off since I dont know which value to use to size a valve correctly.

### RE: ISA Control valve sizing equations

That looks like one value is not compensating for the pressure. If you can post the equations itself and the numbers you're using we might be able to spot what's going on.

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

### RE: ISA Control valve sizing equations

(OP)
Ive double and triple checked everything. My values are as follows:

Fluid: Helium
Inlet pressure: 40 Bara
Outlet pressure: 6 Bara
Mass flow rate: 234 Kg/h
Inlet temp: 60K
Fluid density @ inlet: 29.614 Kg/m3
Ratio of specific heat: 1.6669
Molecular mass: 0.004002 Kg/mole
Compressibility factor: 1.08374

I get the following other factors from valve size etc.
Y: 0.667 The calculations shows I have choked flow.
Fy: 1.214
Xtp: 0.391

N numbers used:
N6: 27.3
N8: 94.8

I have attached an image of the Cv equations. I use Eq. 15 & 16 since I have mass flow and is choked.

### RE: ISA Control valve sizing equations

Daniel
Rio de Janeiro - Brazil

### RE: ISA Control valve sizing equations

(OP)
Thanks Danschwind.

I colleague here also corrected me. ISA asks for Kg/mol in Symbols table so I convert to 0.004.

### RE: ISA Control valve sizing equations

Are you sure that ISA is indicating M as kg/mol? Can you post a screenshot of it?

Daniel
Rio de Janeiro - Brazil

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