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Control valves flow coefficients Cv Kv for valve sizing

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1. Cv and Kv definition - calculation of flow through a valve - SI units
2. Usual values
3. Calculation of flow through a valve - US units

1. What is the flow coefficient Cv and Kv - calculation of flow through a valve - SI units

The flow through a valve can be calculated thanks to the pressure drop through the valve and a coefficient Kv which represents the number of m3/h of water that goes through the valve at 15c and with a pressure drop of 1 bar.

Flowrate through control valve expressed as function of Kv

Manufacturers may specified the coefficient Cv instead of Kv for their valve. Cv is the number of US gallons that goes through the valve at 60F and with a pressure drop of 1 psi. It is actually possible to Kv and Cv and have the expression in the right unit for the calculation of interest. Cv=1.156Kv

Flowrate through control valve expressed as function of Cv

Note that these formula are only valid with Newtonian liquids.

Qv=Flow rate (m3/h)
Cv=valve flow coefficient (GPM)
Kv=valve flow coefficient (m3/h)
ΔP=Pressure drop accross the valve (bar)
d15t=density of the liquid referred to water at 15 degrees (-)

2. Usual values

In order to have a good control, the design of the control valve should be such that it represents around 1/3 of the total pressure drop of the line. The valve must also be designed to work at flows corresponding to 50 to 80 percent of the Cv of the valve.

3. Calculation of flow through a valve - US units

The flow through the valve can be directly expressed with Cv in US units

Flowrate through control valve expressed as function of Cv

With ΔP in Psi, Qvin GPM, Cvin GPM