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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.
Stan: Given that you need to use a rotary valve what do you look for?
James: A diaphragm actuator, splined shaft connection, an offset of stem to closure member, low sealing friction, low packing friction, a contoured disk butterfly or segmented V-notch ball with an equal percentage characteristic, and rotary feedback of stem position to a high performance digital positioner. Surprisingly, there is a lot technology in the packing. A "Mom and Pop" service shop unbeknownst to us in plant put GraphoilTM packing, not made by the valve manufacturer, in a valve creating horrendous resolution (stiction). However, we ran diagnostics on the valve and the signature plot looked dandy - the full scale traverse of position did not reveal much of a resolution issue. Only when we slowed the ramp rate of the input signal during the signature test did we see the stair step response of stick and slip caused by poor resolution. Apparently the generic packing had a large difference between the static and dynamic friction coefficients causing the "stick and slip" issue and resulting poor control. The problem was solved by putting in the low friction high temperature packing from the control valve manufacturer that was carefully designed to have a small difference between the static and sliding friction coefficient. By the way, most pneumatic positioners can cause a limit cycle similar to the packing induced "stick-slip" but is actually caused by a slow response of the positioner when the valve position is "close" to the target. This problem is easily solved with a high performance digital poistioner that has a consistent (fast) speed of response for small changes.