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. Control Valve is also termed as the Final Control Element. 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, HART, Fieldbus Foundation, and Profibus being the more common protocols. Some of the control valves are Reverse Double-Ported Globe-Style Valve Body, Three-Way Valve with Balanced Valve Plug, Flanged Angle-Style Control Valve Body, Valve Body with Cage-Style Trim, Balanced Valve Plug, and Soft Seat.
A control valve consists of three main parts in which each part exist in several types and designs:
Automatic Flow Control Valve
A flow control valve regulates the flow or pressure of a fluid. Flow Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges. Control valves are normally fitted with actuators and positioners. Pneumatically-actuated globe valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball and butterfly valves are also used.
Control valves can also work with hydraulic actuators (also known as hydraulic pilots). These types of valves are also known as Automatic Control Valves. The hydraulic actuators will respond to changes of pressure or flow and will open/close the valve.
Automatic Control Valves do not require an external power source, meaning that the fluid pressure is enough for opening and closing the valve :
Globe Control Valves
Trunnion-Mounted Control Ball Valves
Eccentric Rotary Plug Control Valves
Segmented Ball Control Valves
Integrated Flow Controllers
Butterfly Control Valves
V-Ball Control Valves