AIR-TO-OPEN: An increase in air pressure to the actuator is required to cause the valve to open.

BOOSTER: A pneumatic relay that is used to reduce the time lag in pneumatic circuits by reproducing pneumatic signals with high-volume and or high-pressure output.

CLOSURE MEMBER: The movable part of the valve, which is positioned in the flow path to modify the rate of flow through the valve.

COEFFICIENT FLOW: A constant (Cv) that is used to predict the flowrate through a valve. It is related to the geometry of the valve at a given valve opening.

DYNAMIC UNBALANCE: The total force produced on the valve plug in any stated open position by the fluid pressure acting upon it.

FEEDBACK SIGNAL: The return signal that results from a measurement of the directly controlled variable.

FLOW CHARACTERISTIC: The relationship between valve capacity and valve travel, usually expressed graphically in the form of a curve.

HYSTERESIS: The difference between up-scale and down-scale results in instrument response when subjected to the same input approached from the opposite direction.

INSTRUMENT PRESSURE: The output pressure from an automatic controller that is used to operate a control valve. It is the input signal to the valve.

LEAK-OFF: A threaded connection located on the bonnet of a valve that allows for the detection of leakage of the process fluid past the packing area.

POSITION SWITCH: A switch that is linked to the valve stem to detect a single, preset valve stem position.

RANGEABILITY: The range over which a control valve can control. It is the ratio of the maximum to minimum controllable flow coefficients.

SEAT LOAD: The contact force between the seat and the valve plug.

SEAT RING: A part of the flow passageway that is used in conjunction with the closure member to modify the rate of flow through the valve.

STATIC UNBALANCE: The net force produced on the valve stem by the fluid pressure acting on the closure member and stem within the pressure-retaining boundary.

TRANSDUCER: An element or device, which receives information in the form of one quantity and coverts it to information in the form of the same or another quantity.

VENA CONTRACTA: The location where the cross-sectional area of the flow stream is at its minimum size, where fluid velocity is at its highest level, and where fluid pressure is at its lowest level.

Glossary courtesy of Valtorc International,, a U.S. supplier of industrial valves and actuators.