Control Valves

A control valve is a hydraulic device that regulates or manipulates the flow of fluids such as gas, oil, water, and steam. A control valve regulates fluid flow by adjusting the size of the flow path in response to a controller signal.
 This allows for direct control of flow rate and, as a result, control of process quantities such as pressure, temperature, and liquid level. A control valve is referred to as a "final control element" in automatic control language.