Valve flow capacity, also called flow coefficient or flow factor, is the valve's capacity for a liquid or gas to flow through it. Different equations for calculation of flow coefficient or factor in liquids and gases are provided. A flow coefficient chart or flow curve shows the relationship between the opening percentage and the amount or percentage of C v or flow rate. Three flow curves or characteristics exist: quick opening, linear, and equal percentage. Quick opening means that a large amount of flow passes through the valve as it begins to open. Linear flow means that the change in the flow is equal to the opening percentage. An equal percentage means that the valve releases a relatively low amount of fluid at the beginning of the opening stage. Rangeability is another flow rate definition for industrial valves used for valve sizing. Rangeability is defined as the ratio of maximum flow to a minimum controllable flow of a valve installed in a piping system. Valve authority and gain that are used for flow control valves are explained at the end of the chapter.
