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]

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.

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.^[2] Control valve bodies may be categorized as below:^[3]

Globe valve with a manual hand-wheel

Globe control valve with pneumatic actuator and smart positioner

• 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

Valve Sizing

The choice of valve for a particular application depends on various factors such as the fluid flow rate, the process temperatures and pressures, and whether the fluid is corrosive or abrasive. An important design consideration for safety is the maximum pressure and temperature of the flowing fluid as this will dictate a safe thickness for the body material and may determine which materials can be used. The capacity of a valve is measured in Cv, Kv or Av and can be determined experimentally for a valve by testing it, for example by measuring the amount of water that flows through it at a certain pressure differential across the valve.

The valve needed for an application will depend on whether the fluid is a liquid, gas or multi-phase fluid and on the properties of the fluid. Normally a number of process conditions - each of which will specify the inlet and outlet pressure, temperature, flow rate and maybe other fluid details - are used to calculate a required capacity, and to estimate the noise and the cavitation. Valves often have interchangeable internal components, called the trim, which can be selected to modify the valve to accommodate particular process conditions. For example perforated trim can be used to reduce the noise level by splitting the flow into multiple independent streams.

Due to the complexity of the fluid flow, noise and cavitation calculations often required to size and select an appropriate control valve many manufacturers provide a computer program to assist their customers. For example Severn Glocon and Dresser provide downloadable sizing programs. A small number of software companies have specialist products for this market such as Conval and the Millstream Software Valve Sizing Program.

See also

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

References

1. ^ Bela G. Liptak (Editor) (2003). Instrument Engineers' Handbook (4th Edition ed.). CRC Press. ISBN 0-8493-1083-0. 
2. ^ Hagen, S. (2003) "Control valve technology" Plant Services
3. ^ 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.