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Apr
09

Valves that can Turn on a Dime, or at Least a Quarter – Part I

I have seen some of the best and the worst of control valves. I have also found out that not all positioners are created equal and responsible responsiveness depends upon the whole package.

Here is a checklist for making sure your valve is nimble enough to respond to every demand of your loop if you don’t have time to read my positioner horror stories.

Checklist for Responsive Responsible “Right On” Control Valves

(1) Digital positioner with high gain and rate action that is properly tuned

(2) Low friction packing that is properly tightened

(3) Low seating and sealing friction at closure

(4) Diaphragm or floating piston actuator that is properly sized

(5) Spline shaft connections for rotary valves

(6) Ball, disk, or plug stem (not actuator shaft) position feedback for rotary valves

If you are an optimist and don’t really want to know whether a control valve is really moving as the controller output changes, then make sure there is no positioner and check the valve stroke manually in the field for 0%, 50%, and 100% signals. This is a time honored tradition that started in the 1980s when the old timer instrument engineers who knew the importance of a positioner and how to adjust them had retired. I have seen a large plant responsible for the most profitable herbicide in modern times demand that projects not buy control valves with positioners because they were too troublesome to calibrate and maintain. If there is no position feedback in the control room and the smallest change made by the instrument technician is 50%, it may seem like the right conclusion. The project manager is not going to argue ($500 or more per valve savings adds up). It is amazing to me no one ever reasoned that hopefully your loop is not making 50% changes but more like 0.5% changes from execution to execution even during the most upsetting times (except for surge control).

Today we have position feedback from Hart or Fieldbus positioners and we are much more enlightened or are we? On a 1999 project, the plant insisted on using a valve that was great as an isolation valve that was also relatively inexpensive, which for a 18 inch valve was certainly appreciated by the project. Other loops with these valves oscillated continually but this was attributed to some strange behavior in the process. As a concession to the fact this on-off valve really needed to be a throttling valve, a digital positioner with all types of internal analysis and plotting capability and a customer witnessed acceptance test at the manufacturer was specified.

When I arrived for the test, days of fancy plots and data had been gathered to show the positioner feedback for changes as small as 0.5%. Wow, it looked great until I noticed the disk wasn’t moving. A travel gauge installed on the disk itself revealed that the disk would not budge until the change in signal was 8% or more. The positioner feedback was actuator shaft position. Even though the shaft had two pin connections to the disk stem position there was enough play that the high sealing (rubbing) friction for the tight shutoff valve meant the when the shaft moved the disk did its own thing, which was usually nothing. Also, there was a tendency for the stem to eventually jump form shaft windup. The application here was air so there were plenty of choices for real control valves.

In another application, a similar thing occurred but it was even worse in that the ball seal was so tight that the ball stem connection didn’t even follow the actual ball position. The digital positioners in the control room said everything was OK but the loop PV said otherwise. Unfortunately, since this was liquid phosphorous, it was difficult to find a throttling control valve that would not seize up. However, one plant did get a special v-notch ball valve to work after I configured a periodic pulse to flush it out. In other applications I have avoided solids accumulation, fouling, and plugging from low flow throttling by a transition to pulse duration control for throttle positions less than 25%. In other words, there often ways to make a control valve do its job without having to go to valves that piping has specified as isolation or block valves. Next week we will offer the top signs your control valve is really an on-off valve in disguise.