Control Valve Positioners, and its types

Primarily, there are three types of valve positioners.

Pneumatic positioners

It receive a pneumatic signal (typically 3-15) from either a pneumatic controller or a I/P converter (in case of electronic controller). The output from the positioner is also a pneumatic signal, send it to the valve actuator.

Benefits of Pneumatic positioner

  • Easily repairable.
  • Lower cost of ownership than other type of positioners.

Electro-pneumatic positioners

It receive an electric signal (normally a 4-20mA) from a controller. The signal from the controller is connected to the coil inside the positioner.

The coil gets magnetized and moves the flapper attached to it. As with the changes in the current signal, the flapper movement takes place that results in nozzle back pressure. The output from the positioner is a pneumatic signal, which is sent to the valve actuator. 

Analog signal based positioners may also be referred to as “analog positioners.” This is because the electrical input to the positioner is a 4-20 mA analog signal.

Benefits of Electro-pneumatic positioner

  • 4-20 mA can be used in split range application as 4-12mA and 12-20 mA .
  • Lower air consumption.

Digital positioners

It receive an electric signal of normally a 4-20mA, HART, PROFIBUS, or Foundation Fieldbus from a controller.  A Digital signal allows more advanced capabilities for the user. Digital positioners are popularly known as “smart positioners”. Control valve diagnostics information can be obtained.  

Benefits of Digital Positioner

  • Reduced commissioning time and associated costs.
  • Advanced diagnostics for preventive maintenance.
  • More accurate calibration results in highly accurate control.
  • Improves control valve reliability and performance.

Author: Surya Vamshi

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