What is a positioner in the control system?

759 Views
Chirag Shah
8th July 2021
Marine Services

A valve positioner is an important device primarily used to enhance or decrease the air load pressure driving the actuator until the valve’s stem reaches a position balanced to the output signal from the process variable instrument controller.

The prime use of valve positioner is in controlling valves where accurate and rapid control is required without error or hysteresis.

Positioners are mounted on the side-yoke or top casing of the pneumatic actuator for linear sliding stem control valves. They are also mounted at or near the end-of-shaft for rotary control valves. For basic design types, mechanical feedback linkage is connected directly to the valve’s steam. This provides feedback to the controller. The process controller instructs the positioner to change position; the feedback linkage reports back to the positioner confirming that a change has happened and gives a sense of the magnitude of the change in position.

The valve positioner is a force-balanced instrument, with a pneumatic module installed on a double-acting actuator for air to open action. Positioning is based on a balance of two forces; one proportional to the instrument signal and the other proportional to the stem position.

Positioner Advantages

· Accurate positioning

· can accustom to large variations in forces acting on the plug

· Rapid positioning

· Removes stiction and friction effects of the gland

· Eliminates effects of large distances between valve and positioner

· Eliminates hysteresis

When should a positioner be fitted?

You should consider a positioner in the following circumstances:

· When accurate valve positioning is compulsory

· To boost up the speed of the valve response. The positioner uses higher pressure and greater airflow to adjust the valve position

· To enhance the pressure that a particular actuator and valve can close against

· When the valve pressure drops at the maximum operating flowrate

· To linearize a non-linear actuator

· Where varying differential pressures within the fluid would cause the plug position to vary

· When controlling with a wide strangling range

· When valves are handling solids in suspension.Ship Automation holds an astonishing market reputation in the field of the marine industry. The company supplies and manufactures various ship spare parts like ship automation system, marine automation system, valve positioners, and marine control system. The company has an array of positioners including Disparco PPL-B154HL, Siemens C73451-A430-B30, Nakakita Seisakusho, etc. Just visit the official website of the company and know more about it.