Control Valve
A control valve is a power-operated valve that continuously modulates flow in response to a signal from a control system, holding a process variable such as pressure, flow, level or temperature at its set point. It comprises a valve body and trim, an actuator, and usually a positioner that compares the control signal (typically 4–20 mA or digital fieldbus) with the actual stem position. Common body types are globe, rotary ball, eccentric plug and butterfly. Sizing is based on the required flow coefficient (Cv or Kv) at maximum, normal and minimum flows, using methods in IEC 60534 and ISA 75.01. Other selection parameters include inherent flow characteristic (linear, equal percentage, quick opening), rangeability, allowable pressure drop, cavitation and flashing risk, noise, seat leakage class to ANSI/FCI 70-2 or IEC 60534-4, and fail-safe position.
Examples
- Pressure control valve on a steam header
- Level control valve on a separator liquid outlet
- Temperature control valve on a heat exchanger bypass