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 and the corresponding flow rate. Coefficent of discharge is ratio between the actual flow rate to Theoretical discharge. In mathematical terms, the flow coefficient Cv is the volume (in US gallons) of water at 60° F that will flow per minute through a valve with a pressure drop of 1 psi across the valve. Numerically this definition can be expressed as:
<math>C_v = F \sqrt{\dfrac{SG}{\Delta P}}</math>
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 as follows:
Definitions
Cv = Flow coefficient or flow capacity rating of valve. F = Rate of flow (US gallons per minute). SG = Specific gravity of fluid (Water = 1). ΔP = Pressure drop across valve (psi).
