Positioner :
It is an instrumentation device used to regulate the air pressure supplied to a control valve actuator, thereby precisely controlling the flow of fluid through the valve until the desired setpoint is reached.
The flow control process via a control valve works as follows: a control command, based on the desired set point (or manually), is sent to the positioner mounted on the control valve. This command changes the position of the valve stem (opening or closing). Based on the new stem position after the command, the positioner sends a feedback signal to the control room, indicating the valve’s actual position. This allows the control room operator to ensure that the valve has opened or closed according to the command signal.
Types of Positioners
Pneumatic Positioner :
A 3–15 PSI signal from the control room is applied to this type of positioner. Based on the received pneumatic signal, the positioner regulates the air pressure applied to the valve stem, thereby controlling the valve opening and closing. Due to the obsolescence of these control systems, their usage is gradually declining. Typically, these devices feature three pressure gauges to display the supply pressure, the air output from the controller, and the air output from the positioner.
In this type of positioner, a 4–20 mA input signal is applied. The positioner converts this electrical signal into the corresponding air pressure, which is then applied to the control valve actuator. This ensures the valve is positioned correctly, allowing precise control of the flow.
In this type, the input signal from the control room is digital and applied to the positioner. The operation proceeds similarly to the analog I-to-P positioner. It is important to note that the positioner must support the control system’s communication protocol.
Advantages of Using a Positioner :
High Accuracy and Monitoring of Line Flow
Fast Actuator Response to Control Room Signal
Ability to Use Electrical Signals for Controlling Valve Opening and Closing
