Diagnosing Final Control Elements

McMillan and Weiner Ask James Beall What Was the Biggest Step Forward that Helped Him Improve Valve Performance

By Greg McMillan, Stan Weiner

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Greg: It seems a lot of poor performance can be related to suppliers and users not realizing that response time, deadband, and resolution are important. The ISA Standards ISA-75-25 sheds light on how to measure these metrics but stops at revealing the implications and providing guidelines for various applications. I got burnt big time in 1975 as lead engineer for the world's largest acrylonitrile plant when the contract design and construction firm said we could save big bucks by omitting positioners on several hundred loops. The lead engineer pulled out a Nyquist plot study from a supplier that showed positioners would hurt performance and if speed was needed a booster should be used instead of a positioner. During startup, I ended up putting positioners on all the valves because many did not move unless the signal changed by 25%.  I got burnt again about ten years later when I tried to replace positioners with boosters on surge valves. The huge butterfly valves slammed shut when the compressor started. The instrument tech showed me how he could manually easily move the 24 inch butterfly valve by simply grasping the shaft. When the positioner was put on he could not budge the valve as you might expect. The high outlet port sensitivity of the booster led to positive feedback with the diaphragm actuator. A couple of years later at a factory acceptance test, the valve supplier made the same mistake. I went up to the 30 inch valves and showed how I could stroke them with my hand on the shaft.

James: Without a positioner, the deadband on a brand new valve in perfect condition can be anywhere from 5 to 25% and imperfect bench settings manifest themselves as offsets. I restated the old rule that "fast loops should not have positioners to "fast" loops have a chance to work satisfactorily without a positioner but will perform better with positioner, especially a modern DVC type positioner."

Greg: The main concern about the violation of the cascade rule where the valve positioner as a secondary loop needed to be faster than the flow loop as a primary loop was overstated because the flow loop was tuned with mostly reset action and not much gain action partly due to the unknown nonlinearity of the installed valve characteristic. The new positioners such as the DVC with excellent resolution can be tuned for exceptionally fast response. Also, external-reset feedback can be used if a fast position readback is available to prevent the burst of oscillations from the process loop output changing than the valve can respond allowing violation of the cascade rule.

Stan: What can go wrong with even a good valve, actuator, and positioner?

James: I once worked on a loop that had a "green" control valve with a DVC yet the data historian showed there was an 1/8% deadband. Normally I would have been happy with 1/8% dead band but the other "green" valves in the plant were doing better than this!  When I went out in the field (remember, you will get burnt every time you do not go to the field!) I saw the feedback arm spring clip on the bonnet of the actuator. The feedback arm was missing its spring clip in the slot to rotate the positioner resulting in the observed backlash. The spring clip got knocked off. Someone realized the part belonged to the valve but didn't notify anyone.

Greg: Much worse is when the control valve and actuator are inherently a poor design and a DVC connected to the actuator shaft says everything is OK. The November 2012 Control feature article "Is your Control Valve an Imposter" gives the details of perhaps the greatest deception in the automation business.

James: In this case, a good "on-off" valve had been modified in an attempt to make it also provide throttling action.  However, it wasn't performing well and was limiting production.  Just 0.008 inch slack in the key ways of a shaft connection combined with the slop in a scotch yoke actuator caused an 8% deadband. The DVC feedback measurement was on the actuator shaft position before the backlash took effect. The DVC said things were not that bad. The addition of integral action in the positioner helped the slow level control loop do better. As a temporary fix, we made some mechanical modifications to the valve and actuator and then planned to replace the valve with a "real" regulating control valve at the first opportunity!  When the new control valve was replaced, the process capacity could be increased by 10%!

Greg: The combination of the integrating response of the piston and in the positioner caused a fast limit cycle from actuator and valve dead band. I think the limit cycle average value from the filtering action of the process was closer to the desired position than the offset if there was no integral action in the positioner. Thankfully, the limit cycle wore out the valve.  There is a quadruple whammy with the use of on-off rotary valve designed for tight shutoff as a control valve. Due to overcapacity the valve is operating near the seat where the sealing friction is the greatest. Since the translation of liner to rotary motion and shaft to stem connections were designed for open-close rather than throttling service, precision was not a consideration resulting in rampant backlash. The valve drop is a small fraction of the system drop causing a gross distortion of the inherent characteristic. To top it off the piston actuator has a poor threshold and is prone to o-ring problems. Adding insult to injury, the positioner is lying to you.

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