Kv value

[Destroyer]

Hello,

can anyone tell me the Kv equation for air. All the equations are for water and I would like to calculate it for air. And in the equation the term DeltaP (pressure drop) what does it stand for. Is it a property of the valve itself and how can I find the term???

Thanks a lot

 

[TD2K]

By Kv, I assume you mean the valve capacity coefficient used in Europe defined as the cubic metres of water per hour per bar DP that will pass through a control valve?
Cv = 1.166 * Kv

For the capacity in air, one option is to consult the manufacturer of the valve that you are trying to calculate it for as different manufacturers will have different formulas, at least in North America although you can use different manufacturer's formulas but the numbers will vary somewhat. Alternatively, you can use the ISA valve formula although you might have to estimate some of the valve's properties such as choked pressure drop ratio.

The term dP is simply the pressure drop across the valve, inlet pressure minus the outlet pressure and comes from the system the valve is located in.

 

[TD2K]

The ISA equation for a control valve is:

q = 7320*Fp*Cv*P1*Y*(x/(M*T1*Z))^0.5

q - scfh
Fp - piping geometry factor
Cv - valve sizing coefficient
P1 - inlet pressure, psia
Y - expansion coefficient
x - pressure drop ratio, dP/P1
M - molecular weight
T1 - inlet temperature, deg R
Z - gas compressibility factor