Control valves

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 valves 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.^[2]

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

Types of control valve bodies

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:^[4]

Globe valve with a manual hand-wheel

Globe control valve with pneumatic actuator and smart positioner

Types of control valves

• Angle Valves
  • Cage-style valve bodies
  • DiskStack style valve bodies
• Angle seat piston valves
• Globe valves
  • Single-port valve bodies
  • Balanced-plug cage-style valve bodies
  • High capacity, cage-guided valve bodies
  • Port-guided single-port valve bodies
  • Double-ported valve bodies
  • Three-way valve bodies
• Diaphragm Valves
• Rotary valves
  • Butterfly valve bodies
  • V-notch ball control valve bodies
  • Eccentric-disk control valve bodies
  • Eccentric-plug control valve bodies
• Sliding cylinder valves
  • Directional control valve
  • Spool valve
  • Piston valve
• Air-operated valves
  • Air-operated valve
  • Relay valve
  • Air-operated pinch valve

See also

• Control engineering
• Control system
• Fieldbus Foundation
• Flow control valve
• HART Protocol
• Instrumentation
• Instrumentation engineering
• Pneumatic flow control
• Process control
• PROFIBUS

References

1. Bela G. Liptak (Editor) (2003). Instrument Engineers' Handbook (4th ed.). CRC Press. ISBN 0-8493-1083-0.
2. Emerson Process Management "Control valve handbook, fourth edition, Fisher Controls International LLC, 2005.
3. Hagen, S. (2003) "Control valve technology" Plant Services
4. Fisher Controls International Emerson Process Management website.

External links

• Control valve tutorials Tutorials covering the sizing, capacity and characteristics of control valves. Actuators, positioners, controllers and sensors are also discussed - Spirax Sarco
• Control Valve Handbook (4th Edition) A 297-page online book.
• Process Instrumentation (Lecture 8): Control valves Article from a University of South Australia website.
• Pressure Independent Control Valve Animation Flash animations demonstrating conventional and pressure independent control valve operation.
• Samson AG Demo Valve Sizing Software. Basic Trial valve sizing programme.
• Control Valve Sizing Calculator Control Valve Sizing Calculator to determine Cv for a valve.