The main purpose of having a positioner is to guarantee that the valve does in fact move to the position where the controller wants it to be.
Pneumatic valve positioners are one of the most commonly used valve accessories.
The addition of a positioner can correct for many variations.
# Changes in packing friction because of dirt in the valve body.
# Corrosion effects of the control valve cause sluggish movement, that can be overcome with the addition of a positioner.
# Lack of lubrication.
# Variation in dynamic forces of the process.
# Non-linearities in the valve actuator.
It is the job of the positioner to protect the controlled variable from being upset by any of the above variations.
Depending upon the design and type of positioner, the linearization of non-linear flow characteristics can be accomplished by switching a mechanical cam or by digitally or electronically reprogramming a new performance curve into the positioner. It can be a more cost-effective solution than installing a new valve or changing out the valve trim.
The positioner is a high gain plain proportional controller which measures the valve stem position ( to within 0.1 mm) compares that measurement to its set point ( controller output signal) and if there is a drift, correct the error.
When the valve is in open-loop operation, it will always benefit from the addition of a positioner. Because a positioner increases the valve response by bringing down the valve’s hysteresis and dead band.
When the valve is under closed-loop control, the positioner is very helpful in most cases for slow loops like temp, liquid level, blending, slow flow, and a large volume gas flow.
However, it will degrade loop response, contribute to proportional offsets, limit cycling in fast loops ( fast flow, liquid pressure, small volume gas pressure)
An actuator without springs always requires positioners.