# Control valves flow coefficients Cv Kv for valve sizing

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.

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

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

With ΔP in Psi, Qvin GPM, Cvin GPM