Transcript Control Loop Troubleshooting

CHE 185 – PROCESS
CONTROL AND DYNAMICS
Control Loop Troubleshooting
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.
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
LOOP TROUBLESHOOTING
• THE INITIAL SETTINGS FOR A NEW
CONTROL LOOP ARE ALSO CRITICAL
http://www.expertune.com/tutor.html#FineTune
CONTROL DIAGRAM FOR A
TYPICAL CONTROL LOOP
COMPONENTS AND SIGNALS OF A
TYPICAL CONTROL LOOP
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?
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
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
CHECKING THE FINAL CONTROL
ELEMENT
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
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
COMMON PROBLEMS WITH THE
ACTUATOR SYSTEM
• EXCESSIVE VALVE DEADBAND
• IMPROPERLY SIZED CONTROL VALVE
• VALVE PACKING IS TIGHTENED TOO
MUCH
• IMPROPERLY TUNED VALVE
POSITIONER
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
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/Stict
ionMR.htm
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
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
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
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
CHECKING THE SENSOR
• TEMPERATURE SENSORS ALSO ARE
SUBJECT TO LOSS OF CONTACT WITH THE
SYSTEM
• HTTP://THERMOWELLS.COM/NEWPD8.HTML
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.
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://COOPS.NOS.NOAA.GOV/LEVELHOW.HTML
– MEASURING ATMOSPHERIC PRESSURE WITH
REMOTE SENSORS
HTTP://WWW.COAPS.FSU.EDU/COARE/LEVEL2
CONTROLLER MALFUNCTIONS
• THIS IS NORMALLY THE LAST
COMPONENT CHECKED
• MANY SYSTEMS ARE SPECIFIED
SO THAT ALL DCS COMPONENTS
ARE REDUNDANT
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.
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
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_M
ODELING_AND_CONTROL
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
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.
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.c
om/artCharact.html
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.c
om/artCharact.html
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.c
om/artCharact.html
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
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
CHECKING THE COMPLETE
LOOP
• CLOSED-LOOP BLOCK SINE WAVE TEST
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.
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
• IF ALL 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