Monday, September 11, 2017

Control Valve Positioner and Control Valve Actuator Basics

Control Valve Loop
Control Valve Loop*
Control valves control fluid in a pipe by varying the orifice size through which the fluid flows. Control valves contain three major components, the valve body, the positioner, and the valve actuator.

The valve body provides the fluid connections and movable restrictor comprised a valve stem and plug that is in contact with the fluid that varies the flow.

The valve actuator is the component that physically moves the restrictor to vary the fluid flow.

Their are two general categories of control valves - linear and rotary. Three actuator types are used in linear control valves including spring and diaphragm, solenoid, and motor operated. Three actuator types are used in rotary control valves including pneumatic, electric, and electro-hydraulic. Rotary actuators are sometimes referred to as "quarter-turn" or "partial-turn".
Pneumatic positioner on rotary vane actuator
Pneumatic positioner with
rotary vane actuator
on ball valve.

The valve disc (restrictor) controls flow through the valve body. A positioner receives information from a supervisory controller advising wether or not the flow condition is satisfactory. The positioner then provides a signal to the actuator that provides the force to open and close the valve.

Each type of positioner works in response to a process signal. Some positioners (linear) use a 3-15 PSI pneumatic process signal. The pressure is exerted on a large diaphragm creating downward force that is applied against a spring which moves the restrictor up and own. Other types of positioners use a 3-15 PSI pneumatic signal to regulate a higher supply pressure (such as 0-60 PSI) to move pistons or vanes back and forth (rotary). 

The variable 3-15 PSI control signal can be provided directly by a pneumatic controller connected directly to the process, or in other cases the 3-15 PSI is regulated by an electropneumatic device called an I/P or E/P (current to pressure or voltage to pressure) transmitter. These transmitters receive their signal from a supervisory control as a 0-10VDC or 4-20mA and then throttle the 3-15 PSI output to open/close the valve. 

* Image courtesy of Tony R. Kuphaldt from "Lessons In Industrial Instrumentation"