Flow coefficient


The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the pressure drop across an orifice valve or other assembly and the corresponding flow rate.
Mathematically the flow coefficient Cv can be expressed as :
where:
In more practical terms, the flow coefficient Cv is the volume of water at 60 °F that will flow per minute through a valve with a pressure drop of 1 psi across the valve.
The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry. The general definition of the flow coefficient can be expanded into equations modeling the flow of liquids, gases and steam using the discharge coefficient.
For gas flow in a pneumatic system the Cv for the same assembly can be used with a more complex equation. Absolute pressures must be used for gas rather than simply differential pressure.
For air flow at room temperature, when the outlet pressure is less than 1/2 the absolute inlet pressure, the flow becomes quite simple. With Cv = 1.0 and 200 psia inlet pressure the flow is 100 standard cubic feet per minute. The flow is proportional to the absolute inlet pressure, so the flow in scfm would equal the Cv flow coefficient if the inlet pressure were reduced to 2 psia and the outlet were connected to a vacuum with less than 1 psi absolute pressure.

Flow factor

The metric equivalent flow factor is calculated using metric units :
where
Kv can be calculated from Cv using the equation: