Ask Greg McMillan Series

Greg's Response:

The biggest problem facing nearly all control loops stems from control valve specifications not requiring the control valve internal closure member to actually move and the flow to reasonably change for a change in valve signal. The lack of specification entries for precision, response time, and installed flow characteristic is baffling. The entries for capacity and leakage and emphasis on reducing pressure drop and cost, has led to valves with excessive stroking time, stiction, lost motion, shaft windup, and nonlinearity. The problem is accentuated by the term “high performance” valves that have higher capacity, tighter shutoff, lower pressure drops, and lower cost but are actually “on-off” valves posing as “throttling” valves. Even the best positioner cannot overcome the inherent detrimental results in terms of poor control. Often the positioner is being lied to because the feedback mechanism is moving in response to changes in actuator signal but the internal closure member is not moving due to stiction, lost motion, and shaft windup.

I have made it my goal for the last 40 years to turn this around. I have developed models that simulate the stiction and lost motion, pre-stroke deadtime, stroking time, time constants, and installed valve characteristic for control valves. I have published many articles most notably “How to specify valves and positioners that do not compromise control”. I have also written the ISA-TR75.25.02 Annex A - Valve Response and Control Loop Performance - Sources, Consequences, Fixes, and Specifications and ISA-TR5.9-2023 Annex C Valve positioners.  All process control applications with control valves should use these models and the Annexes I have written for ISA Technical Reports.

The best control valves for throttling are single plug, “flow-to-open” globe valves with high performance Teflon packing, valve to system pressure drop ratio greater than 0.25, oversized diaphragm actuators, and digital positioners with high gain setting and no integral action, and if necessary to reduce stroking time a volume booster on positioner output with bypass valve slightly open. If a rotary valve must be used, it should use direct splined shaft connections and not have any seal friction. Not realized is that integral action in the positioner reduces the allowable gain used in the positioner and increases the amplitude and period of oscillations from stiction and lost motion. To further drive home the critical importance of using dynamic models of valve response to ascertain and justify the best control valve here are the Top 10 Mistakes in Control Valve Specification:

1. Excessive flow capacity decreasing rangeability and increasing the oscillations from stiction and lost motion.
2. Low valve to system pressure drop ratio increasing nonlinearity, decreasing rangeability, and increasing the oscillations from stiction and lost motion.
3. Low leakage decreasing rangeability and increasing the oscillations from stiction and lost motion.
4. Keylock connection of actuator to rotary valve shaft increasing lost motion. 
5. Link-arm connection of actuator to rotary valve shaft increasing lost motion.
6. Rack and Pinion connection of actuator to rotary valve shaft increasing lost motion.
7. Scotch-Yoke connection of actuator to rotary valve shaft increasing lost motion.
8. Graphite packing increasing stiction.
9. Booster instead of positioner causing unstable diaphragm actuator.
10. Integral action and low gain setting in positioner.