Ask Greg McMillan

Greg's Response:

Stick and slip generally occur together and have a common cause of friction in the actuator design, stem packing, and seating surfaces. Rotary valves with high temperature packing and tight shutoff (the so-called high-performance valve) exhibit the most stick-slip. Rotary valves tend to also have shaft windup where actuator shaft moves but the ball, disc, or plug does not move. It is much worse at positions less than 20% where the ball, disc, or plug is starting to rotate into the sealing surfaces. For sliding stem (globe) valves, the stick-slip increases below 10% travel as the plug starts to move into the seating ring. These problems are more deceptive and problematic in rotary valves because the smart positioner is measuring actuator shaft position and not the ball, disc, or plug stem. If there is stick-slip, the controller will never get to the set point and there will always be a limit cycle. The biggest culprits are low leakage classes and the big squeeze from graphite and environmental packing particularly when they are tightened without a torque wrench. A bigger actuator may help but does not eliminate the problem.

For the best throttling valves (globe valves with diaphragm actuators), the stick-slip is normally only about 0.1% and its effect is typically observable only in pH system trend recordings.  The control valve resolution will clearly show up as a large sustained oscillation for a set point on the steep portion of the titration curve because of the high process gain. The extreme sensitivity of the pH process requires a valve resolution that goes well beyond the norm. The number of stages of equipment needed for neutralization may be dependent on the capability of the control valve. It is difficult to effectively use more than one control valve per stage. An extremely small and precise control valve is necessary to keep limit cycle within the control band. To achieve the large range of reagent addition and extreme precision required, several stages are used with the largest control valve on the first stage and the smallest control valve on the last stage.

Dynamic simulations with control valve resolution and lost motion included are needed to determine the best control valve and number of neutralization stages.

For much more knowledge, see the ISA book Advanced pH Measurement and Control Fourth Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).

The achievement of a fast and precise final control element is critical for pH control. The focus on control valve resolution and rangeability is an essential starting point but there are many other considerations. Also, variable frequency drives (VFDs) are often touted as offering tighter control but there are many design and installation choices made that can make this expectation unrealistic. To help us all to be aware of potential problems and recognized solutions, the following list of best practices is offered based on the content in Chapter 7 of the ISA book Essentials of Modern Measurements and Final Elements in the Process Industry.

1. Use sizing software with physical properties for worst case operating conditions.
2. Include effect of piping reducer factor on effective flow coefficient.
3. Select valve location and type to eliminate or reduce damage from flashing.
4. Preferably use a sliding stem valve (size permitting) to minimize backlash and stiction unless crevices and trim causes concerns about erosion, plugging, sanitation, or accumulation of solids particularly monomers that could polymerize and for single port valves install “flow to open” to eliminate bathtub stopper swirling effect.
5. If a rotary valve is used, select diaphragm actuator with splined actuator shaft to stem connection, integral cast ball or disk stem, and minimal seal and packing friction to minimize lost motion deadband and resolution limitation.
6. Use conventional Teflon packing, and for higher temperature ranges use Ultra Low Friction (ULF) Teflon packing, avoid overtightening of packing, and consider possible use of compatible stem lubricant. 
7. Compute the installed valve flow characteristic for worst case operating conditions.
8. Size actuator to deliver more than 150% of the maximum torque or thrust required.
9. Select actuator and positioner with threshold sensitivities of 0.1% or better.
10. Ensure total valve assembly deadband is less than 0.4% over the entire throttle range.
11. Ensure total valve assembly resolution is better than 0.2% over the entire throttle range.
12. Choose inherent flow characteristic and valve to system pressure drop ratio that does not change the valve gain more than 4:1 over entire process operating point range and flow range. 
13. Tune the positioner aggressively (high proportional action gain) for application without integral action with readback that indicates actual plug, disk or ball travel instead of just actuator shaft movement.
14. Never replace positioners with volume boosters. Instead put volume boosters on the positioner output to reduce valve 86% response time for large signal changes with booster bypass valve opened just enough to assure stability.
15. Use small (0.2%) as well as large step changes (20%) to test valve 86% response time to see if changes need to be made to meet desired 86% response time.
16. See ISA TR75.25.02 Annex A for more details on valve response and relaxing expectations on travel gain and 86% response time for small and large signal changes, respectively. 
17. Counterintuitively increase the PID gain to reduce oscillation period and/or amplitude from lost motion, stick-slip, and from poor actuator or positioner sensitivity. 
18. Use external-reset feedback detailed in Chapter 8 with accurate and fast valve position readback to stop oscillations from poor precision and slow response time.
19. Use input and output chokes and isolation transformers to prevent EMI from the VFD inverter.
20. Use PWM to reduce torque pulsation (cogging) at VFD low speeds.
21. Use a VFD inverter duty motor with class F insulation and 1.15 service factor, and totally enclosed fan cooled (TEFC) motor with a constant speed fan or booster fan or totally enclosed water cooled (TEWC) motor in high temperature applications to prevent overheating.
22. Use a NEMA Design B motor instead of Design A motor to prevent a steep VFD torque curve.
23. Use bearing insulation or path to ground to reduce bearing damage from electronic discharge machining (EDM). Damage from EDM is worse for the 6-step voltage older VFD technology.
24. Size the pump to prevent it from operating on the flat part of the VFD pump curve.
25. Use a recycle valve to keep the VFD pump discharge pressure well above static head at low flow and a low-speed limit to prevent reverse flow for highest destination pressure. 
26. Use at least 12-bit signal input cards to improve the VFD resolution limit to 0.05% or better
27. Use drive and motor with a generous amount of torque for the application so that speed rate-of-change limits in the VFD setup do not prevent changes in speed from being fast enough to compensate for the fastest possible disturbance.
28. Minimize VFD deadband introduced into the drive configuration (often set in misguided attempt to reduce response to noise) causing delay and limit cycling. 
29. For VFD tachometer control, use magnetic or optical pickup with enough pulses per shaft revolution to meet the speed resolution requirement.
30. For tachometer control, keep the speed control in the VFD to prevent cascade rule violation where the secondary speed loop is not 5 times faster than the primary process loop. 
31. To increase rangeability to 80:1, use fast cascade control of speed to torque in the VFD to provide closed loop slip control.
32. Use external- reset feedback with accurate and fast speed readback to stop oscillations from poor VFD resolution and excessive deadband and rate limiting in VFD configuration.
33. Use foil braided shield and armored cable for VFD output spaced at least one foot from signal wires never any crossing of signal wires, ideally via separate cable trays.