The document summarizes a series of demonstrations showing the effects of valve stick-slip and deadband on process control loops. It discusses how stick-slip can create limit cycles that PID tuning alone cannot eliminate. Adding I-deadband to both primary and secondary controllers is necessary to remove the limit cycle in cascade control systems. Valve deadband also introduces deadtime and limit cycles, with amplitude and period inversely proportional to PID gain. The demos vary control mode, tuning, I-deadband settings, stick-slip, deadband, and process type to illustrate these challenges and solutions.

1. Interactive Opportunity Assessment Demo and Seminar (Deminar) Series for Web Labs – PID Control of Valve Sticktion and Backlash April 21, 2010 Sponsored by Emerson, Experitec, and Mynah Created by Greg McMillan and Jack Ahlers

3. The Latest on Valve Response and Smart Positioners Royalties are donated to the University of Texas Research Campus for Energy and Environmental Resources for Development of Wireless Instrumentation and Control

11. Valve Backlash (Deadband) and Sticktion (Stick-Slip) dead band Deadband Stick-Slip is worse near closed position Signal (%) 0 Stroke (%) Digital positioner will force valve shut at 0% signal Pneumatic positioner requires a negative % signal to close valve Valve deadband and stick-slip is greatest near the closed position Deadband is 5% - 50% without a positioner ! Deadband is the change in signal to reverse direction of stroke - principal sources are links and mechanisms for rotary valve actuation Stick-slip is the smallest possible change in valve stroke – principal sources are friction in packing and friction in closure element (trim) seal (can be seen as shaft windup and breakaway in rotary valves) Sensitivity limits associated with piston actuator and pneumatic positioners create a staircase response similar to stick-slip

13. Direct Connection of Piston Actuator to Rotary Valve Less backlash but wear of piston O-ring seal from piston pitch is concern See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

14. Link-Arm Connection of Piston Actuator to Rotary Valve Significant backlash from link pin points 1 and 2 See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

15. Rack and Pinion Connection of Piston Actuator to Rotary Valve Stick-slip from rack and gear teeth - particularly bad for worn teeth See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

16. Scotch-Yoke Connection of Piston Actuator to Rotary Valve Lots of backlash from slot See “ Best Practices for Valve Performance” slide for best solution to minimize backlash and stick-slip

18. Installed Characteristic (Linear) Valve pressure drop ratio (  P R ) for installed characteristic: Characteristic 1:  P R  0.5 Characteristic 2:  P R  0.25 Characteristic 3:  P R  0.125 Characteristic 4:  P R  0.0625

19. Installed Characteristic (Equal Percentage) Valve pressure drop ratio (  P R ) for installed characteristic: Characteristic 1:  P R  0.5 Characteristic 2:  P R  0.25 Characteristic 3:  P R  0.125 Characteristic 4:  P R  0.0625

21. Real Valve Rangeability Minimum fractional flow coefficient for an equal percentage trim and stick-slip: Minimum controllable fractional flow for installed characteristic and stick-slip: C xmin  minimum flow coefficient expressed as a fraction of maximum (dimensionless)  P r  valve pressure drop ratio (dimensionless) Q xmin  minimum flow expressed as a fraction of the maximum (dimensionless) R v  rangeability of control valve (dimensionless) R  range of the equal percentage characteristic (e.g. 50 - 200) X vmin  maximum valve stroke (%) S v  stick-slip near closed position (%)

23. Effect of Tuning on Limit Cycle from Stick-Slip in Self-Regulating Processes A o = S s  K o K o =  K mv  K pv  K cv T o = 4  T i  [1  ( K o  K c )  1] A o = oscillation amplitude (%) S s = valve stick-slip (%) K o = open loop gain (%/%) (more commonly known as process gain) K mv = manipulated variable gain (valve gain) (e.g. kg/sec per %) K pv = process variable gain (unit operation gain) (e.g. o C per kg/sec) K cv = controlled variable gain (measurement gain) (e.g. % per o C) T i = controller integral time (sec) T o = oscillation period (%) http://www.controldesign.com/articles/2003/164.html “ What’s Your Flow Control Valve Telling You?”, Control Design, May 2003

25. Effect of Step Size on Small Valve Response Results show sensitivity limitation of piston actuators and pneumatic positioners

31. Effect of Tuning on Limit Cycle from Valve Deadband in Integrating Processes A o = DB  K c T o = 5  T i  [ 1  2  (K c ) 0.5 ] A o = oscillation amplitude (%) DB = valve deadband (%) K c = controller gain T i = controller integral time (sec) T o = oscillation period (%) http://www.controldesign.com/articles/2003/164.html “ What’s Your Flow Control Valve Telling You?”, Control Design, May 2003

34. Summary of Demos Demo Sec PID Mode Sec PID Tuning Sec PID I-deadband Prim PID I-deadband Valve Stick- Slip Valve Dead-band Process Type 1 CAS FAST 0% 0% 0% 0% Self-Reg 2 AUTO FAST 0% 0% 5% 0% Self-Reg 3 AUTO SLOW 0% 0% 5% 0% Self-Reg 4 AUTO FAST 4% 0% 5% 0% Self-Reg 5 CAS FAST 4% 0% 5% 0% Self-Reg 6 CAS FAST 4% 4% 5% 0% Self-Reg 7 CAS FAST 0% 0% 0% 5% Self-Reg 8 AUTO FAST 0% 0% 0% 5% Self-Reg 9 AUTO HI GAIN 0% 0% 0% 5% Integ 10 AUTO LO GAIN 0% 0% 0% 5% Integ

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