Why is a positioner required?


Thread Starter

Bronson, Robert

We have a 2" Fisher ET body valve with a 657 actuator (6 - 30 psi) with 2 and 5/16 inch trim on a compressor recycle (Fail Open). I have been told that the valve has been set up to ensure that under process conditions (8,000 kpa upstream - 1,200 kpa downstream) the valve should start to open as the controller output drops below 100%. (The bench set on the valve is 22psi.)

In practice, when the unit goes into low suction override control, the valve does not start to open until the controller output is at 65%.

We installed a Fieldvue positioner and the valve now opens at 95%.

It's great to solve a problem. It would be greater still to understand why a positioner is required. My guess is that 99% of compressor recycle valves in natural gas field production don't have them. I would like to keep it that way if possible.
Dear Robert,

My guess is that your air supply was insufficient and it took a long time to change the air pressure in the valve actuator and by adding a positioner you boosted it. If I am right, you don't need a positioner, omly a booster.

Best regards, Bela

Brian T. Smith


The best source for an answer to your question may be your local Fisher Controls representative. You may think that they just want to sell you a positioner so i'll respond as a user of Fisher valves and positioners.

The high pressure drop could result in different valve openings with the same input signal when the pressure drop changes. Without a positioner the flow differences that result would be detected by your flow controller and the valve would be adjusted to provide a flow that
meets setpoint. Your ET valve is a balanced trim so it does not need a positioner as much as an unbalanced trim would.

So having a positioner results in improved control by minimizing flow disturbances before they are detected by the flow controller. Flow
disturbances that are caused by the control valve performance. The choice is yours depending on the level of performance you need. Compressor recycle valves usually need to be fast acting, especially if you are operating near the surge line. Fortunately your valve is only a 2 Inch which should be fast acting.

If you are interested in improving control valve performance refer to the following web pages:


Brian Smith
Like said before, the positoiner is a booster and a controller in itself. The main thing to remember is that it delays the overall loop action. This is generally no problem as your valve is throttling device.