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.[1] 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
valve available 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:
·
Valve's actuator
·
Valve's positioner
·
Valve's body
The most common and versatile types of control valves are
sliding-stem globe and angle valves. Their popularity derives from rugged
construction and the many options available that make them suitable for a
variety of process applications, including severe service.[3] Control valve
bodies may be categorized as below
Angle Valves
1.
Cage-style valve bodies
2.
DiskStack style valve
bodies
3.
Angle seat piston valves
Globe valves
1.
Single-port valve bodies
2.
Balanced-plug cage-style
valve bodies
3.
High capacity, cage-guided
valve bodies
4.
Port-guided single-port
valve bodies
5.
Double-ported valve bodies
6.
Three-way valve bodies
Diaphragm Valves
Rotary valves
1.
Butterfly valve bodies
2.
V-notch ball control valve
bodies
3.
Eccentric-disk control
valve bodies
4.
Eccentric-plug control
valve bodies
Sliding cylinder valves
1.
Directional control valve
2.
Spool valve
3.
Piston valve
Air-operated valves
1.
Air-operated valve
2.
Relay valve
3.
Air-operated pinch valve
