Monday, September 8, 2014

Control Valves in Industrial Control Systems

Cashco rotary control valve
Cashco rotary
control valve

One of the most ubiquitous final control elements in industrial control systems is the control valve. A control valve attempts to regulate flow through a pipe, reacting to a signal from some type of controlling device or balancing circuit, such as a simple on/off sensor, PID loop controller, or a programmable logic device (PLC). Control valves designs are available for simple (on/off) control of flow, or for more sophisticated applications that are intended to regulate flow between a fully opened valve and a fully closed valve.
Valtek linear control valve
Valtek linear
 control valve

Control valves are made up of two major components: 
  • A valve body, containing the seats, disks, body trim and other mechanical components that actually regulate and manage the process media
  • The valve actuator which provides the power for movement to open or close the valve, or vary its position for desired flow conditions. 

Rotork electric actuator
Rotork electric actuator
The kinds of actuators used for on/off control and throttling control are generally different. An on/off actuator provides 90 degree rotational movement to rotate a ball, butterfly or plug a full 90 degrees to fully open or close the valve. A throttling actuator (generally) provides linear movement to lift a disk from a seat in varying distances to allow more or less flow. Control valve actuators are available with ether pneumatic or electric power source.

Valve "trim" refers to the parts of the control valve that come in contact with the process media and actually perform the regulation of the flow. Valve trim comes in many variations, depending on the process application. The process media, pressure, temperature, erosive nature and abrasive nature dictate the materials used for the control valve body, seats, and disks. The style of control valve - whether gate, ball, butterfly or globe - is dictated by the material nature of the media (slurries, clean liquids, viscous), the type of control required, the flow rates, and budget.