Use of a Positioner for a Control Valve

PS2 by Semens Positioner on a Control Valve

A Positioner for a Control Valve

A positioner for a control valve is used to adjust a valve’s position based on a desired setpoint for a process variable, whether it be pressure, temperature, or flow. Types of valve positioners include pneumatic, electro-pneumatic, and digital.

In this post, we’ll look at the basics of valve positioners and an overview of these three types.

Producers typically install valve positioners on the yolk or top casing of a pneumatic actuator for linear control valves. On rotary control valves, the valve positioner is installed in line with the valve and actuator stems on top of the actuator or on the side of the actuator. Installation depends on what type of actuator one uses. Affixing the positioner to the actuators allows the positioner to measure the stem travel (linear valves) or degree of rotation (rotary valves). This also changes the position of the valve as required based on the input signal from the instrument controller.

When the process variable differs from the desired setpoint, the instrument controller sends an electrical or pneumatic signal to the positioner. This varies its pneumatic output to the actuator to move the valve open or closed accordingly. This occurs until the process variable reaches the desired setpoint.

Types of Positioner for a Control Valve

There are three primary types of positioners:

  • Pneumatic positioners receive a pneumatic signal (typically 3-15 or 6-30 psi) from a controller. They then send a pneumatic signal to the valve actuator.
  • Electro-pneumatic positioners receive an electric signal (typically 4-20mA or 0-10 VDC) from a controller. They then send a pneumatic signal to the valve actuator.  Analog valve positioners may also be referred to as “analog positioners.” This is because the electrical input to the positioner is an analog signal.
  • Digital positioners receive an electric signal (typically 4-20mA, HART, PROFIBUS, or Foundation Fieldbus) from a controller. They then send a pneumatic signal to the valve actuator. The electrical input to a digital positioner can be a digital signal for these positioners. This digital signal allows more advanced capabilities for the user. Some refer to digital valve positioners as “smart positioners.”

The Siemens PS2 is one example of a digital valve positioner. The PS2 can remotely monitor and control any pneumatic valve. We offer it with many different communication protocols to meet the needs of all end users.

For further information about how a control valve positioner can improve your operation, contact your local Kimray store or authorized distributor.

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