ISA Saint Louis Short Course Process Control Opportunities
1. ISA Saint Louis Short Course Dec 6-8, 2010 Exceptional Process Control Opportunities - An Interactive Exploration of Process Control Improvements - Day 2
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4. Block Diagram of “Series”, “Real”, or “Interacting” PID Form Nearly all analog controllers used the “Series” or “Real” Form SP proportional integral derivative Gain Reset (1 T i ) Rate CO filter filter CV filter Filter Time Rate Time Improving Controllers
5. Block Diagram of “Standard”, “Ideal”, or “Non-interacting” PID Form Nearly all digital controllers have the “standard” or “Ideal” form as the default Improving Controllers SP proportional integral derivative Gain Reset (1 T i ) Rate CO filter filter CV filter Filter Time Rate Time
6. Positive Feedback Implementation of Integral Mode Improving Controllers SP proportional derivative Gain Rate CO filter filter CV filter Filter Time Rate Time filter Filter Time = Reset Time ER is external reset (e.g. secondary PV) Dynamic Reset Limit ER Positive Feedback
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8. PID Controller Forms PB = 100% K c CO n = P n I n + D n + CO i CO i = controller output at transition to AUTO, CAS, or RCAS modes (%) CO n = controller output at execution n (%) CV n = controlled variable at execution n (%) D n = contribution from derivative mode for execution n (%) I n = contribution from integral mode for execution n (%) K c = controller gain (dimensionless) P n = contribution from proportional mode for execution n (%) R i = reset setting (repeats/minute) PB = controller proportional band (%) SP n = set point at execution n (%) T d = derivative (rate) time setting (seconds) T i = integral (reset) time setting (seconds/repeat) = rate time factor to set derivative filter time constant (1/8 to 1/10) = set point weight for proportional mode (0 to 1) = set point weight for derivative mode (0 to 1) R i = (60 T i ) Conversion of settings: Improving Controllers (K c T d ) SP n – SP n-1 ) – (CV n CV n-1 ) ] T d D n-1 D n = ------------------------------------------------------------------------------- T d t P n = K c SP n – CV n ) I n = (K c T i ) SP n – CV n ) t I n-1 Standard P n = K c SP n – D n ) I n = (K c T i ) SP n – D n ) t I n-1 Series
19. Control Valve Watch-outs 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 The dead band and stick-slip is greatest near the closed position Deadband is 5% - 50% without a positioner ! Plugging and laminar flow can occur for low Cv requirements and throttling near the seat Consider going to reagent dilution. If this is not possible checkout out a laminar flow valve for an extremely low Cv and pulse width modulation for low lifts Improving Valves
20. Direct Connection Piston Actuator Less backlash but wear of piston O-ring seal from piston pitch is concern Improving Valves
21. Significant backlash from link pin points 1 and 2 Link-Arm Connection Piston Actuator Improving Valves
22. Stick-slip from rack and gear teeth - particularly bad for worn teeth Rack & Pinion Connection Piston Actuator Improving Valves
23. Lots of backlash from slot Scotch Yoke Connection Piston Actuator Improving Valves
24. Diaphragm Actuator with Solenoid Valves Improving Valves Port A Port B Supply ZZZZZZZ Control Signal Digital Valve Controller Must be functionally tested before commissioning! SV Terminal Box
25. Piston Actuator with Solenoid Valves Improving Valves Port A Port B Supply Digital Valve Controller SV SV Volume Tank Must be functionally tested before commissioning! Piston W Check Valve Air Supply Terminal Box
26. Size of Step Determines What you See Improving Valves Maintenance test of 25% or 50% steps will not detect dead band - all valves look good for 10% or larger steps
27. Effect of Step Size Due to Sensitivity Limit Improving Valves
28. Response to Small Steps (No Sensitivity Limit) Improving Valves Stroke (%) Time (sec)
29. Response to Large Steps (Small Actuator Volume) Improving Valves Time (sec) Stroke (%)
30. Installed Characteristic (Linear Trim) Improving Valves 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
31. Installed Characteristic (Equal Percentage Trim) Improving Valves 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
32. Improving Valves Installed Characteristic (Modified Parabolic Trim) 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
33. Limit Cycle in Flow Loop from Valve Stick-Slip Improving Valves Controller Output (%) Saw Tooth Oscillation Process Variable (kpph) Square Wave Oscillation
35. Real Rangeability Improving Valves Minimum fractional flow coefficient for a linear trim and stick-slip: 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) X vmin maximum valve stroke (%) S v stick-slip near closed position (%)
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37. Volume Booster with Integral Bypass (Furnace Pressure and Surge Control) Improving Valves Signal from Positioner Air Supply from Filter-Regulator Air Loading to Actuator Adjustable Bypass Needle Valve
38. Booster and Positioner Setup (Furnace Pressure and Surge Control) Improving Valves Port A Port B Supply ZZZZZZZ Control Signal Digital Valve Controller Must be functionally tested before commissioning! 1:1 Bypass Volume Booster Open bypass just enough to ensure a non-oscillatory fast response Air Supply High Capacity Filter Regulator Increase air line size Increase connection size Terminal Box
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44. Dynamic Response to Step Improving Measurements Time (seconds) Theoretical Transmitter Response Actual Transmitter Response True Process Variable m m deadtime measurement time constant Process Variable and Measurement
45. Dynamic Response to Ramp Improving Measurements Time (seconds) Actual Transmitter Response True Process Variable m m Process Variable and Measurement
46. Attenuation of Oscillation Amplitude by Transmitter Damping or Signal Filters: When a measurement or signal filter time ( f ) becomes the largest time constant in the loop, the above equation can be solved for (A o ) to get the Amplitude of the original process variability from the filtered amplitude (A f ) Improving Measurements Effect of Transmitter Damping and Signal Filters
47. Effect of Transmitter Damping or Filter for Surge Improving Measurements m m m m
56. Elimination of Ground Noise by Wireless pH Improving Measurements Wired pH ground noise spike Temperature compensated wireless pH controlling at 6.9 pH set point Incredibly tight pH control via 0.001 pH wireless resolution setting still reduced the number of communications by 60%
65. Effect of PZ Solvent on pH Improving Measurements
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67. Cascade Loop Block Diagram p1 p2 p2 K p2 p1 m2 m2 K m2 c2 f2 Primary Process K v v v K L2 L2 L2 Primary Load Upset CV p CO p MV PV p2 Delay Lag Delay Delay Delay Delay Delay Lag Lag Lag Lag Lag Gain Gain Gain Gain Local Set Point DV p2 % % % Delay <=> Dead Time Lag <=>Time Constant K L1 L1 L1 Delay Lag Gain DV p1 Secondary Load Upset CO s Secondary PID Cascade Set Point % % K p1 Gain CV s m2 m2 K m2 Delay Lag Gain c2 f2 Delay Lag Secondary Process Primary PID Primary: o2 v p1 p2 m2 c2 f2 v p1 Secondary: o1 v p1 m1 c1 f1 v Improving Loops - Part 1 K c2 T i2 T d2 K c1 T i1 T d1
68. Cascade Control Benefit (self-regulating process) Improving Loops - Part 1 i o i o i o i inner loop process time constant o outer loop process time constant i inner loop process deadtime o outer loop process deadtime
69. Cascade Control Benefit (integrating process) Improving Loops - Part 1 i inner loop process time constant o outer loop process time constant i inner loop process deadtime o outer loop process deadtime i o i o i o
70. Cascade Control Benefit (runaway process) Improving Loops - Part 1 i o i o i o i inner loop process time constant o outer loop process time constant i inner loop process deadtime o outer loop process deadtime
71. Secondary loop slowed down by a factor of 5 Secondary SP Secondary CO Primary PV Secondary SP Primary PV Secondary CO Effect of Slow Secondary Tuning (cascade control) Improving Loops - Part 1
72. Triple Cascade Loop Block Diagram Improving Loops - Part 1 Control Valve AO PID PID AI AI Flow Meter Process Process Sensor Secondary (Inner) Loop Feedback Primary (Outer) Loop Feedback Process SP Flow SP Out PV PV Relay PID * Position Loop Feedback DCS Valve Positioner Position (Valve Travel) I/P Drive Signal * most positioners use proportional only
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76. Bias Correction of Ratio Control For information on Ratio Control, see April 7, 2009 Post on website http://www.modelingandcontrol.com/2009/04/what_have_i_learned_-_ratio_co_1.html Improving Loops - Part 1
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Notas do Editor
The self-organizing network and automatic reporting of alerts makes it easy to maintain and revise a control system for flexibility and adaptability to changing experimental conditions and requirements in the dynamic environment of a process development lab.
Battery Life is extended by using exception reporting (signal is transmitted only when it exceeds the resolution setting) unless the elapsed time since the last transmission exceeds the refresh time, which can be set to large value.