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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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.

Stan: Looks like another example of the consequences of poor valve dynamic performance not being commonly understood. If your service shop or control valve supplier cannot talk intelligently about response time, deadband (backlash), and resolution (stick-slip) you should look for one that can.

Greg: The positioner like any other controller performs only as good as the tuning. The pneumatic positioner was a proportional only controller with a gain of about 150. Today's positioner offers incredible flexibility and something akin to PID control. There is even a lead-lag to help make the rise time faster and overshoot smaller similar to the lead-lag advocated by Shinskey for the setpoint response in process PID controllers. How do you tune the DVC?

James: The DVC has recommended tuning sets for different actuator and valve combinations but many applications do not fit a set of rules just as in tuning process controllers. I first knock the heck out it maximizing the speed of response with proportional action. Often a positioner has a hidden gain multiplier, like 15, which means that a gain factor of 10 is effectively a proportional mode gain of 150. Once the valve is responding well, I reduce the overshoot with derivative action from the shaft position. The minor loop feedback helps by providing very fast motion control of the pneumatic relay kind of like a very fast inner loop. We still have a lot to learn about tuning positioners.  In most applications I do not recommend integral action in the positioner.  

Stan: What can users do to better understand stick-slip?

James: Static friction sets stick and dynamic friction determines slip. The static friction of tight shutoff rotary valves can be so large that there is shaft windup where the actuator shaft moves but the internal closure member (e.g., ball or disc) does not move. When the change in actuator torque becomes large enough, the shaft unwinds.  We have seen as much as 1-2% shaft windup!

Greg: The plots I have seen show a stair step response indicating a slip. If the movement did not try to make up for lost ground but simply tracked a continuing change in the same direction, the response would be more like backlash. Both backlash and sticktion create a deadtime that is the deadband or resolution limit divided by the rate of change of the controller output.

Stan: What have you learned about the valve characteristics?

James: Gas pressure loops can benefit from a linear trim characteristic when the valve has a pressure drop that is much larger than other pressure drops or when the valve is operating at a critical pressure drop. Linear trim is desirable in these cases because the effective pressure drop available for the valve does not change with flow resulting in an installed flow characteristic that is close to the inherent flow characteristic. This is because the valve drop is so large compared to other pressure drops resulting in an installed characteristic close to the inherent characteristic. In general we need to pay attention to the change in the flow characteristic when the valve drop is small compared to frictional losses in the piping system.

Greg: I also recommend linear trim for reagent valves for pH control since the valve drop is the system drop. For most other loops, a linear inherent characteristic is undesirable because it distorts to a quick opening characteristic as the valve drop as a fraction of the system drop decreases. Today's emphasis on minimizing energy has taken a wrong term in allocating insufficient pressure drop to the control valve causing severe distortion and loss of rangeability.

Stan: Where do you like to do signal characterization to deal with the installed characteristic?

James: I prefer to do the characterization in the DCS so the linearization is more visible, flexible, and maintainable. Cascaded signal characterizers can be used to provide incredible resolution. The input and output of the signal characterizers can be clearly shown on the operator graphics. For split ranged outputs, the characterizers can be addressed for each valve. In some DCS's a "swap output parameter" is available in the characterizer block that provides another input to reverse the output to the input to provide the back calculate signal needed for bumpless PID action. Also, with characterization in the DCS, if the valve is changed out in the middle of the night, you don't have to worry about whether the positioner has the right characterization.  

"Top Ten Signs Your Control Valve is an Imposter"

(10) The piping supplier says he has an incredible great deal to meet your control valve needs
(9) The valve supplier asks "What is deadband and resolution?"
(8) The valve looks suspiciously like the valve the operator uses for isolation
(7) The actuator looks suspiciously like the actuator on an on-off valve
(6) The packing is the same as used in the piping valves
(5) The leakage rate is phenomenally low
(4) You can standardize on parts in valves supplied with packaged equipment
(3) Integral action needs to be used in the positioner
(2) Loops oscillate regardless of tuning settings
(1) The positioner creed is "What Me Worry?" 

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