Control valves are valves used to control conditions
such as flow, pressure, temperature, and liquid level by fully or
partially opening or closing in response to signals received from
controllers that compare a “setpoint” to a “process variable” whose
value is provided by sensors that monitor changes in such conditions.
The opening or closing of control valves is
usually done automatically by electrical, hydraulic or pneumatic
actuators. Positioners are used to control the opening or closing of the
actuator based on electric, or pneumatic signals. These control
signals, traditionally based on 3-15psi (0.2-1.0bar), more common now
are 4-20mA signals for industry, 0-10V for HVAC systems, and the
introduction of “Smart” systems.
A control valve consists of three main parts in which each part exist in several types and designs:
