# 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.

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**.

Q_{v}=Flow rate (m3/h)

C_{v}=valve flow coefficient (GPM)

K_{v}=valve flow coefficient (m3/h)

ΔP=Pressure drop accross the valve (bar)

d_{15}_{t}=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, Q_{v}in GPM, C_{v}in GPM