The relative flow capacity of a control valve is indicated by its flow coefficient. The flow coefficient Cv refers to the number of U.S. gallons water at 60 degrees F that will pass through the valve with a 1 psid pressure drop across the valve.
Valve manufacturers determine flow coefficients by test and by calculation according to ANSI and Fluid Controls Institute (FCI) standards. Manufacturers typically publish the Cv ratings in tabular formats as shown in Figure 38. Note that the table includes Cv ratings at different percentages of travel for different valve sizes and port sizes. For rotary-shaft control valves, capacities are published for different degrees of rotation.