Positioners are used to control the air load pressure on the valve actuators. The positioner forces the actuator to move the valve until the desired position is reached and balanced according to the output signal from the controller. The control signal to the positioner could be electrical or pneumatic. They can be 3-15psi, 4-20 mA, 0-10V, digital or different protocols like HART or Profibus.
Why use positioners
The main purpose of a positioner is to enhance the control resolution of the valve. They ensure the valve position gets as close as possible to commanded position by the control system. They can mitigate the effect of dynamic variations that cause adverse controllability, such as changes in packing friction arising from corrosion, dirt, lubrication, sloppy linkage (causing dead band) and non-linearities in the valve actuators. They can also improve the response speed based on the received signal enabling faster air loading and venting.
How positioners work in the control loop
Depending on the type of valve, positioners are normally mounted either on the side-yoke for linear valves or at the top of the rotary valves body. However, there is always a mechanical linkage to the valve stem. When the process variable (e.g. flowrate, temperature, pressure) differs from the set point, a control signal from the controller inputs the positioner. The positioner sends a corresponding air pressure load to the actuator. Once the valve position is altered and sensed by the mechanical linkage, the feedback signal is sent to the controller and compared to the set point to make sure that the valve has reached the desired position. No position feedback could be achieved without a positioner in the control loop, resulting in less control precision.
Depending on the actuation configurations, positioners can fit both double acting and single acting (spring return) actuators. Positioners for single acting actuators have only one air output to the actuator while double acting types have two air outputs to be used with actuators without spring.
Ramén offers a number of analog and digital valve positioners from Flowserve, Siemens and Schneider Electric. Please contact us for more information on the positioners in our control flow packages.