Valve positioners are devices used to precisely control the position of a control valve actuator in response to a control signal. They ensure accurate positioning of the valve plug or disc to achieve the desired flow rate, pressure, temperature, or level in a process control system. There are several types of valve positioners available, including:

1. Pneumatic Positioners:

   • Pneumatic positioners are the simplest type of valve positioners and operate using pneumatic signals.
   • They typically receive an input signal from a pneumatic controller or process instrument, such as a pressure transmitter or controller.
   • The positioner then adjusts the air pressure supplied to the actuator to position the valve plug or disc accordingly.
   • Pneumatic positioners are commonly used in pneumatic control systems and are relatively simple and cost-effective.

2. Electro-Pneumatic Positioners:

   • Electro-pneumatic positioners combine pneumatic and electrical components to provide precise valve control.
   • They receive an electrical input signal from a control system, such as a distributed control system (DCS) or programmable logic controller (PLC).
   • The electrical signal is converted into a pneumatic signal by the positioner, which then adjusts the air pressure supplied to the actuator to position the valve.
   • Electro-pneumatic positioners offer greater accuracy and flexibility compared to pneumatic positioners and are suitable for more demanding control applications.

3. Double Acting Positioners:

   • Double acting positioners are designed to operate with double-acting pneumatic actuators, which require air pressure to move the actuator in both directions (open and close).
   • They receive two separate input signals, one for the open direction and one for the close direction.
   • The positioner adjusts the air pressure supplied to the actuator in response to each input signal, allowing precise control of the valve position in both directions.
   • Double acting positioners are commonly used in applications where bidirectional control of the valve is required, such as control valves used in process industries.

Each type of valve positioner has its own advantages and suitability for different applications, depending on factors such as accuracy requirements, control system compatibility, and environmental conditions. Proper selection and installation of the appropriate valve positioner are essential to ensure optimal performance and reliability of the control valve system.