CHE 185 – PROCESS CONTROL AND DYNAMICS

1. CHE 185 – PROCESS CONTROL AND DYNAMICS Control Loop Troubleshooting

2. WhY Control Loop Troubleshooting? • Control loop is suspected of not functioning properly. • Poor overall control performance • Erratic behavior • Control loop was removed from service. • Identify the source of the problem. • Correct the problem. • Retune the controller and monitor.

3. LOOP TROUBLESHOOTING • APPROACHES TO TROUBLESHOOTING • BASIC APPROACH IS TO HAVE A HYPOTHESIS ABOUT WHAT COULD GO WRONG AND THEN CREATE A TEST TO CHECK THE HYPOTHESIS • THIS TYPICALLY IS EASIEST BY ISOLATING THE COMPONENTS OF THE LOOP AND TESTING THEM INDEPENDENTLY IN SEQUENCE • AFTER EACH TEST IS COMPLETED, THE SYSTEM IS RETUNED TO THE REVISED OR ORIGINAL CONDITIONS • THE FINAL STEP AT THE END REQUIRES TESTING THE ENTIRE LOOP

4. LOOP TROUBLESHOOTING • THE INITIAL SETTINGS FOR A NEW CONTROL LOOP ARE ALSO CRITICAL http://www.expertune.com/tutor.html#FineTune

5. Control Diagram for a Typical Control Loop

6. Components and Signals of a Typical Control Loop

7. Overall Approach to Troubleshooting Control Loops • Check subsystems separately. • Actuator system • Controller • Sensor • Process • Then check performance of the entire control loop • What’s been changed lately?

8. CHECKING THE FINAL CONTROL ELEMENT • THE FINAL ELEMENT IS TYPICALLY A VALVE OR A VALVE WITH A POSITIONER • WHEN VALVE MOVEMENT IS RESTRICTED, THIS IS NORMALLY TERMED STICTION, WHICH COMBINES STICKING AND FRICTION

9. CHECKING THE FINAL CONTROL ELEMENT • A BLOCK SINE WAVE TEST WILL BE ABLE TO SHOW IF THERE IS A MALFUNCTION IN THE FINAL ELEMENT • THE TEST REQUIRES CHANGES THAT ARE BEYOND THE DEADBAND OF THE CONTROL VALVE AND POSITIONER - 0.5% WITH POSITIONER, 25% WITHOUT • A TYPICAL OUTPUT FOR THIS TEST IS SHOWN ON THE NEXT PAGE

10. CHECKING THE FINAL CONTROL ELEMENT

11. CHECKING THE FINAL CONTROL ELEMENT • THE FINAL ELEMENT IS TYPICALLY A VALVE OR A VALVE WITH A POSITIONER • WHEN VALVE MOVEMENT IS RESTRICTED, THIS IS NORMALLY TERMED STICTION, WHICH COMBINES STICKING AND FRICTION • A BLOCK SINE WAVE TEST WILL BE ABLE TO SHOW IF THERE IS A MALFUNCTION IN THE FINAL ELEMENT • THE TEST REQUIRES CHANGES THAT ARE BEYOND THE DEADBAND OF THE CONTROL VALVE AND POSITIONER - 0.5% WITH POSITIONER, 25% WITHOUT

12. Checking the Actuator System • Apply block sine wave input changes to the setpoint for the flow controller. • Determine the deadband of the flow control loop from a block sine wave test. Also, estimate the time constant for the flow control loop from the block sine wave test. • If the time constant is less than 2 seconds and the deadband is less than 0.5%, there is no need to evaluate the actuator system further

13. Common Problems with the Actuator System • Excessive valve deadband • Improperly sized control valve • Valve packing is tightened too much • Improperly tuned valve positioner

14. CHECKING THE FINAL CONTROL ELEMENT • THE EVIDENCE IS IN THE PATTERN OF THE RESPONSE • THE CONTROLLER OUTPUT INCREASES UNTIL THE VALVE “BREAKS FREE” AND THEN THERE IS A JUMP TO THE NEW CONTROL POSITION • THERE IS TYPICALLY SOME OVERSHOOT AND THE VALVE TENDS TO STICK IN THE OPPOSITE DIRECTION AS WELL • THIS TEST IS CONDUCTED ON-LINE/CLOSED LOOP

15. CHECKING THE FINAL CONTROL ELEMENT • AN ALTERNATE TEST CAN BE CONDUCTED ON-LINE OPEN-LOOP (MANUAL CONTROL SETTING) • THE SETPOINT IS INCREASE IN SMALL INCREMENTS UNTIL THE VALVE BREAKS LOOSE • A TYPICAL PATTERN FOR THIS TEST IS http://www.expertune.com/articles/isa2001/StictionMR.htm

16. CHECKING THE FINAL CONTROL ELEMENT • IF THE SYSTEM INCLUDES A POSITIONER, THAT MAY NEED TO BE TESTED SEPARATELY IF THE DEADBAND IS ABOVE THE NORMAL 0.5% UPPER LIMIT • See table 10.1 for other final control problems

17. CHECKING THE SENSOR • SENSORS ARE CHECKED BY EXAMINING THEIR REPEATABILITY, TIME CONSTANT, OR ACCURACY. • THE ACCURACY IS CHECKED THROUGH A CALIBRATION • THIS CAN BE DONE ON-LINE OR OFF-LINE

18. Common Sensor Failures • Transmitter • Improperly calibrated • Excessive signal filtering • Temperature sensor • Off calibration • Improperly located thermowell • Buildup of material on the thermowell • Pressure • Plugged line to pressure sensor

19. Common Sensor Failures • Sampling system for GC • Plugged line in sampling system • Flow indicator • Plugged line to differential pressure sensor • Level indicator • Plugged line to differential pressure sensor

20. CHECKING THE SENSOR • TEMPERATURE SENSORS ALSO ARE SUBJECT TO LOSS OF CONTACT WITH THE SYSTEM • http://thermowells.com/newpd8.html

21. CHECKING THE SENSOR • TEMPERATURE SENSORS • THE GAP BETWEEN THE THERMCOUPLE OR RTD CAN BE OPEN OR FILLED AND THAT AFFECTS THE DEVICE RESPONSE TIME • POSITION IN THE FLOW STREAM IS ANOTHER FACTOR, BUT THIS SHOULD NOT CHANGE UNDER NORMAL OPERATION • CALIBRATION OF TEMPERATURE SYSTEMS TYPICALLY USES WATER ICE POINT AND/OR WATER BOILING POINT FOR THE UNIT.

22. CHECKING THE SENSOR • OTHER SENSOR PROBLEMS ARE SUMMARIZED IN TABLE 10.2 • TWO ARTICLES OF INTEREST ARE • MEASURING THE WATER LEVEL IN OCEANS AND LAKES http://co-ops.nos.noaa.gov/levelhow.html • MEASURING ATMOSPHERIC PRESSURE WITH REMOTE SENSORS http://www.coaps.fsu.edu/COARE/level2

23. CONTROLLER MALFUNCTIONS • THIS IS NORMALLY THE LAST COMPONENT CHECKED • MANY SYSTEMS ARE SPECIFIED SO THAT ALL DCS COMPONENTS ARE REDUNDANT

24. CONTROLLER MALFUNCTIONS • Check the filtering on the measured value of the controlled variable. • Check the cycle time for the controller. • Check the tuning on the controller.

25. CONTROLLER MALFUNCTIONS • THE EASIEST WAY TO CHECK OUT THE CONTROLLER IS TO INSERT A SECOND UNIT WITH THE CONTROL PARAMETERS SET TO MIRROR THE ORIGINAL. • IF THE PERFORMANCE FAILS TO IMPROVE, THEN EITHER • THE PROCESS HAS CHANGED SO THE CONTROL PARAMETERS ARE NO LONGER STABLE AND A RETUNING IS REQUIRED • OR THE CONTROLLER CARD HAS ACTUALLY FAILED

26. PROCESS EFFECTS • MALFUNCTIONS TRACED BACK TO THE PROCESS ARE TYPICALLY THE RESULT OF NON-LINEARITIES • .A GROUP AT THE UNIVERSITY OF DELAWARE HAS SET UP A RESEARCH PROJECT TO STUDY NON-LINEARITIES FOR PARTICULAR SYSTEMS • http://www.che.udel.edu/systems/index.html • .general control system designs area also available at the cache virtual control site • http://www.cse.sc.edu/~gatzke/cache/#Discrete_Modeling_and_Control

27. PROCESS EFFECTS • ONE COMMON EXAMPLE OF NON-LINEARITIES IS RELATED TO pH CONTROL • .THE TYPICAL TITRATION CURVE FOR pH LOOKs LIKE: http://www.expertune.com/artCharact.html

28. PROCESS EFFECTS • THE PROCESS GAIN FOR THIS VARIABLE CHANGES DRAMATICALLY BELOW A VALUE OF ABOUT 4.5 AND ABOVE A VALUE OF ABOUT 9. • SINCE MANY OF THESE CONTROLLERS ARE USED WITH NEUTRALIZATION PROCESSES, THEY WOULD TYPICALLY BE TUNED TO OPERATE AROUND A pH OF 7, WHERE A SMALL ADDITION RESULTS IN A SIGNIFICANT CHANGE IN VALUE.

29. PROCESS EFFECTS • the simulated response of a pH control loop based on the previous titration curve (without nonlinear compensation), following a step decrease in acid load to the neutralization vessel. http://www.expertune.com/artCharact.html

30. PROCESS EFFECTS • The loop can be linearized by placing a complementary nonlinear function in the path of the pH measurement and set point—that function is shown in Fig. 3. In essence, this characterizer converts pH values into equivalent concentration of caustic in solution, linear with the delivery of caustic by the controller. . • . http://www.expertune.com/artCharact.html

31. PROCESS EFFECTS • Figure 4 repeats the step change in acid load with the characterizer applied to the controller. The recovery is much quicker and damping uniform—observe that the trajectory of the controller output is representative of a linear loop. • . http://www.expertune.com/artCharact.html

32. CHECKING THE COMPLETE LOOP • The type and magnitude of disturbances • Primarily affects variability in CV • Can affect nonlinear behavior • The lag associated with the components of the feedback control loop (actuator, process, and sensor) • Results in slower disturbance rejection which affects variability • Precision of the feedback components • Directly affects variability

33. CHECKING THE COMPLETE LOOP • VARIABLES USED TO CHARACTERIZE THE LOOP ARE • CLOSED-LOOP DEADBAND • SETTLING TIME • LOOP REPEATABILITY • .CLOSED-LOOP BLOCK SINE WAVE TEST CAN PROVIDE DATA FOR CALCULATING DEADBAND AND SETTLING TIME • .ATV (AUTOTUNE VARIATION) MAY BE MORE USEFUL BECAUSE IT RESULTS IN LESS PROCESS UPSET

34. CHECKING THE COMPLETE LOOP • Closed-Loop Block Sine Wave Test

35. Closed-Loop Block Sine Wave Test • Closed-loop deadband • Indication of the effect of actuator deadband, sensor noise, and resolution of A/D and D/A converters • Closed-loop settling time • Indication of the combined lags of the control loop components • A means of determining if all the major problems with in a control loop have been corrected.

36. CHECKING THE COMPLETE LOOP • IN EACH OF THESE analyses, THERE IS A HYPOTHESIS ABOUT WHAT COULD BE CAUSING A PARTICULAR TYPE OF UPSET • TEST METHODS CHECK FOR THAT SOURCE INDEPENDENT OF THE OTHER FACTORS • iFALL TESTS OF THE CONTROL LOOP PROVE NEGATIVE, THEN IT MUST BE ASSUMED THAT THERE IS ANOTHER SOURCE • PROCESS-BASED • EXTERNAL TO THE SYSTEM, SUCH AS RF INTERFERENCE