Moving to Digital Control Valve Positioners?

[TiCl4]

Deleted the thread over at the valve engineering forum because it didn't get any responses...

We currently use Siemens 760 electropneumatic positioners for various control valves on the site. While they are generally reliable, I've seen a few issues with the valves. First is that the zero tends to drift over time, often eventually leading to the valve leaking by when commanded to 0% closed. Adjustment of the zero is finicky, and when rebuilding valves and setting up positioners, the setting of span and zero on the bench is also quite a headache; monkeying with the cams and spring tension is a troublesome endeavor that also seems to not translate well from bench to field install.

Last year I installed some low flow (<1 gpm) control valves from Badger with digital positioners and had very good results; the valve zero has not yet drifted and the deadband on commanded position is much, much smaller. I only have a year's worth of operation, so I don't have long-term data, but they have performed well enough that I thought I'd ask on this site about other's experiences with digital positioners. What have you liked - or not liked - about digital positioners that you've seen or installed?

 

[georgeverghese]

May get more feedback from the measurement and control engg forum or the control systems engg forum.

https://www.eng-tips.com/threadminder.cfm?pid=830

https://www.eng-tips.com/threadminder.cfm?pid=698

See which one of these 2 has more activity

New to me - grew up with electro pneumatic positioners with cam for position feedback.

Nice - so no mechanical feedback cams. Presumably it has some self diagnostics that will call up an alarm at DCS in case of minor faults, and drives the valve to safe position for serious faults.

 

[RVAmeche]

In my industrial work, many clients prefer Fisher control valves which commonly use the DVC6200 digital controller. Can't speak to pros/cons but I can tell you most control valves I see these days use digital controllers.

Old school cam type positioners like Masonealian and whatnot seem to be much cheaper and less reliable, but I'm not an end user.

 

[btrueblood]

Ti, does the zero drift occur due to seal wear/compression, or is the positioner itself actually drifting off for some reason?

 

[TiCl4]

Btrueblood,

The positioner itself actually drifts - it will be commanded to an output of 0% but actually be a few % open. I don't know the reason. The setup can be finicky - simply adjusting the zero screw can drive output back down to 0% again, but since this does not alter span it reduces the max % open of the valve. Likewise, if the mechanic "overdrives" the zero-set screw, it can be 0% at a commanded position of 10, 15%, etc, which limits the upper range of the valve to 100% - x, where x is the percentage of drive past 0% closed at 0% command.

These digital positioners are much more expensive, so I was hoping someone had some experience to share before I tried one out in a different setting as a trial.