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# Resistance Coefficient K from Cv or Kv

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# Calculating Resistance Coefficient K from Cv or Kv

by Engineered Software, Inc.

For control valves, the valve capacity and flow characteristics are typically expressed in terms of a valve flow coefficient (Cv).  This article discusses how this flow coefficient can be used to calculate the equivalent resistance coefficient, or K value, used in the Darcy Weisbach equation.

The flow coefficient Cv is defined as the flow rate of 60°F water (in gpm) which can be passed by a valve with a pressure differential of 1 psid.

Similarly, the flow coefficient Kv represents the flow of water in m³/hr which can be passed by a valve with a pressure differential of 1 bar.

Kv is related to Cv by the following equation:

The resistance coefficient (K) that is calculated or entered by the user on PIPE-FLO®'s valve/fitting screen is

Where,
f = Darcy friction factor
$//$ = equivalent length of a resistance to flow, in pipe diameters.

K can be calculated from the valve coefficient Cv using the following equation:

Where,
d = inside diameter in inches

Or, using the coefficient Kv:

If you know the Cv flow coefficient for a valve, you can either calculate a K value using the equation above, or you can use the dP Calculator feature to quickly determine the corresponding K value.

To use the dP Calculator feature, simply specify the Cv value for the flow rate (in gpm) and a pressure drop of 1 psi in the dP Calculator.  The program calculates the corresponding K value.