Control valves are devices that regulate pressure and the flow of fluids in a process system. They control pressure, temperature or liquid flow by partially or fully opening in response to signals received from system controllers.
Control valves consist of three major parts: the actuator, positioner and body. The actuator directly connects to the valve and provides the force necessary to move the control valve. Opening and closing of control valves occurs automatically through hydraulic, electrical and pneumatic actuators. The positioner provides enhanced control, stability and shut-off capability to the valves based on signals sent by the actuator. It receives external signals, compares them to the actual position of the valve plug and sends the correct control signal to the valve's actuator, thus positioning the plug for optimum flow modulation.
There are various categories of valves based on the type of stem movement that is either linear or rotary. For control valves to function properly, they must fulfil various key requirements, including the absence of external leakage, adequate capacity for system media, and resistance to corrosion and high temperatures. Factors to consider when choosing control valves for a system include the cost, energy consumption, maintenance and compatibility with the system. The relationship between the control valve opening and the flow of fluid through the valve determines the flow characteristic of that valve.