Glossary term
Valve Coefficient
A coefficient expressing the flow capacity of a valve under specified pressure drop conditions.
Definition
metricValve coefficient is a flow-capacity rating that relates valve opening, pressure drop, fluid properties, and flow rate under defined reference conditions.
The most common form, Cv, expresses the flow of water in US gallons per minute through a valve with a 1 psi pressure drop at specified reference conditions. Equivalent metric coefficients such as Kv are used in SI-oriented practice. Valve coefficient is central to control-valve sizing, pressure-drop allocation, cavitation assessment, noise estimation, and process controllability.
For an incompressible liquid under standard Cv convention, a simplified relation is:
where Q is flow rate in US gallons per minute, \Delta p is pressure drop in psi, and SG is specific gravity relative to water. This simplified equation is not a universal sizing method; gas, steam, flashing, cavitation, viscous flow, choked flow, and installed piping effects require correction factors and applicable standards.
Engineering use
Valve coefficient helps select a valve that can pass the required maximum flow while still giving stable control at normal and minimum flow. A valve that is too small starves the process. A valve that is too large operates nearly closed, loses resolution, becomes noisy, and can make the control loop unstable.
The inherent valve characteristic is measured under controlled conditions. The installed characteristic depends on the surrounding piping, pump curve, pressure losses, reducers, fittings, and downstream equipment. For control valves, rangeability, actuator resolution, dead band, hysteresis, leakage class, and fail position matter alongside Cv.
Common mistakes
A common mistake is sizing a valve from maximum flow alone and ignoring normal operating point, minimum controllable flow, pressure recovery, cavitation, flashing, noise, and actuator authority. Another is comparing Cv values between valve styles without checking opening position and test convention. A strong valve-coefficient review states fluid properties, temperature, vapor pressure, inlet and outlet pressures, required flow range, valve opening, piping geometry, cavitation check, noise limit, and installed control characteristic.