Control Valve Glossary

33 terms, in plain language, each traced to the conventions of IEC 60534 and ISA-75 and cross-linked so you can follow a concept from actuator to valve authority. This glossary grows continuously — suggest a term if one is missing.

Actuation & Instrumentation

• Actuator — The actuator is the muscle of a control valve: the device that converts a control signal — usually an air pres…
• Air-to-Open / Air-to-Close — These terms describe the fail-safe direction of a spring-opposed pneumatic actuator. Air-to-open (increasing a…
• Bench Set — The bench set is the air-pressure range applied to a diaphragm or piston spring actuator (measured on the benc…
• Booster (Volume Booster) — A volume booster (relay booster) is a pneumatic relay valve installed between the positioner output and the ac…
• Dead Band — Dead band is the range a control signal can change in either direction without producing any change in the val…
• Diaphragm Actuator — The diaphragm actuator is the classic pairing for throttling globe valves: a fabric-reinforced rubber diaphrag…
• Electric Actuator — An electric actuator uses a motor with a gear train to drive the valve stem or shaft, commanded by a control s…
• Fail-Safe Action — Fail-safe action is the position a valve moves to when its power source is lost: fail-closed (FC) , fail-open …
• Hunting (Valve Oscillation) — Hunting is the sustained cyclic motion of a valve — typically a few seconds to a minute in period — in which t…
• Hysteresis — Hysteresis is the difference in valve position for the same input signal depending on the direction of approac…
• Piston Actuator — A piston actuator replaces the diaphragm with a sealed piston sliding in a cylinder, fed by plant air (often 4…
• Positioner — A positioner is a closed-loop controller mounted on the valve: it compares the command signal (4–20 mA → press…
• Stiction — Stiction (static friction) is the extra force needed to start a valve moving from rest, over and above the for…

Trim & Internals

• Balanced Trim — Balanced trim equalizes the pressure acting on the two sides of the plug . Holes through the plug admit upstre…
• Cage — The cage is a perforated or slotted cylindrical sleeve surrounding the plug in a globe valve. The plug rides i…
• Leakage Class (FCI 70-2 / IEC 60534-4) — Seat leakage classes quantify how much a valve leaks when fully closed, under a standard test. The classificat…
• Packing — Packing is the deformable sealing system in the valve's stuffing box that prevents process leakage along the s…
• Plug — The plug is the movable closure member of a globe valve: the shaped element that approaches the seat ring to t…
• Seat (Seat Ring) — The seat ring is the fixed orifice ring in a globe valve against which the plug closes. Together, plug contour…
• Trim — Trim is the collective name for the internal parts of the valve that are in contact with the flowing fluid and…

Flow Phenomena

• Cavitation — Cavitation is the violent two-stage phenomenon that occurs when liquid flowing through a valve first drops bel…
• Cavitation Index (σ, Kc, xFz) — Several dimensionless numbers predict where a valve sits on the cavitation scale; the three most met in practi…
• Choked Flow — Choked (critical) flow is the condition in which increasing the pressure drop across a valve no longer increas…
• Flashing — Flashing occurs when the liquid passing a valve partially vaporizes because the pressure somewhere in the trim…
• Valve Noise — Control valve noise comes in three distinct mechanisms, and the treatment depends on which one you have: Aerod…

Flow Characteristics

• Equal Percentage Characteristic — The equal-percentage inherent characteristic produces a constant relative flow change for each increment of tr…
• Inherent Characteristic — The inherent flow characteristic is the flow-versus-travel curve of a valve measured at a constant pressure dr…
• Linear Characteristic — The linear inherent characteristic gives flow directly proportional to travel: at 50 % open the valve pas…
• Quick-Opening Characteristic — The quick-opening inherent characteristic delivers most of the valve's capacity within the first fraction of t…

Sizing & Selection

• Flow Coefficient (Cv, Kv) — The flow coefficient expresses a valve's capacity in one number. Kv is the flow of water, in m³/h, at a pressu…
• Rangeability — Rangeability is the ratio of the maximum to minimum controllable flow a valve can throttle within its specifie…
• Recovery Coefficient (FL) — F L (liquid pressure-recovery coefficient) states how much of the pressure drop a valve recovers after the ven…
• Valve Authority — Valve authority (pressure-drop ratio) is the share of the total circuit pressure drop that the valve takes, us…