Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations...
Transcript of Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations...
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Page: 12011 ChemInnovations Conference
OptiControls Inc.Houston, TXPh: 713-459-6291
Analyzing Control Problems and
Improving Control Loop Performance
-by Jacques F. Smuts
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Presenter
Principal Consultant at OptiControls Inc.
– League City, TX
20 years experience in process control
– Loop optimization and troubleshooting
– Consulting and control strategy design
– Process control training
Senior member of ISA, P.E., Ph.D.
Author of the book:“Process Control for Practitioners”
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Poor Loop Performance
Loop is in manual
Poor performance in auto
– Loop oscillates or goes unstable
– Sluggish and/or large deviations from set point
Often not tuning related
Process VariableSet Point
Oscillations
Large DeviationsSluggishness
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
![Page 6: Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations Conference Page: 3 Poor Loop Performance Loop is in manual Poor performance in auto](https://reader034.fdocuments.in/reader034/viewer/2022042105/5e83249ced37a230773860af/html5/thumbnails/6.jpg)
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Controller in Manual
Some loops: Okay to be in manual
– Redundant, standby equipment
– Certain process modes
All other loops should be in auto
– Check reason for manual with operator
– Review historical performance
– Place in automatic mode to see response
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Cyclical or Random
Analyze process variable to determine– Visually
– Use frequency analysis software
Oscillations have a repeatable, constant period
Noise and disturbances are random
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Internal vs. External Oscillations
Diagnostic Test: Place loop in manual
PV continues to oscillate external source
Oscillation ceases internal problem
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Oscillating Set Point
E.g. steam flow loop oscillates and its set point oscillates
Place temperature loop in manual– Flow loop stops
oscillating: analyze temperature loop
– Flow loop still oscillates: analyze flow loop
SteamTT
TC
Heat exchanger
Temperature
controller
FT FC
Controller output
Set point
Steam-flow
controller
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Interaction through Process
Interactive process– Heat integration, recycle, etc.
Oscillating loop elsewhere
Affects other loops
Interaction analysis tools– PAS, ExperTune, Matrikon
P&ID and Process Historian
Look for leading oscillation
Look at shape of oscillation
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Stiction
Short for “Static Friction”
Final control element is sticky
Can cause a “stick-slip” cycle with loop in auto
Stiction test: small controller output changesStick-Slip Cycle
White water flow controller
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Positioner Problem
Defective positioner Incorrectly tuned
positioner Sticky valve Revealed through small
step tests HP Flare Scrubber Flow Control as found – oscillating severely
Step tests revealed positioner overshoot After tuning – still oscillating because of stiction, but much less
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Nonlinearity
Loop unstable under certain conditions
Nonlinear valve characteristic– Plot Flow vs. % Open
– Use characterizer
Nonlinear process– Tune under different
conditions
– Use gain scheduling
Steam Pressure
Controller Output
Pro
ce
ss V
aria
ble
Low
gain
High
gain
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
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Controller Tuning Methods
Tune controller according to the process characteristics
Tune controllers to meet control objective
Trial-and-error tuning
Tuning rules
Tuning software
Better
Resu
lts
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Tuning Example
Oil-Gas Separator Level Control
As Found StepTests
AfterTuning
With PV FilterQuickStabilization
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Page: 222011 ChemInnovations Conference
Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
![Page 23: Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations Conference Page: 3 Poor Loop Performance Loop is in manual Poor performance in auto](https://reader034.fdocuments.in/reader034/viewer/2022042105/5e83249ced37a230773860af/html5/thumbnails/23.jpg)
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Cyclical Interaction
Loops “fighting” with each other
Continuous, interactive oscillations– Strong interaction
– Similar dynamics
– Aggressive tuning
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Solutions for Interactive Loops
Dynamically separate loop response times
– Tune most important loop for fast response
– Tune interacting loop to respond 3 x slower
Apply dynamic decoupling
– Two cross-coupled feedforward controllers between loops
Implement MPC
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
![Page 26: Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations Conference Page: 3 Poor Loop Performance Loop is in manual Poor performance in auto](https://reader034.fdocuments.in/reader034/viewer/2022042105/5e83249ced37a230773860af/html5/thumbnails/26.jpg)
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Noise or Disturbance
Noise is fast changes
Disturbances are longer in duration
Can coexist
Noise can be filtered
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Page: 272011 ChemInnovations Conference
Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
![Page 28: Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations Conference Page: 3 Poor Loop Performance Loop is in manual Poor performance in auto](https://reader034.fdocuments.in/reader034/viewer/2022042105/5e83249ced37a230773860af/html5/thumbnails/28.jpg)
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Dead Band (Hysteresis)
Dead band between CO and PV Appears like sluggish control action Can result in very incorrect tuning calculations Dead-band test: 2 steps + 1 in opposite direction
20% Dead Band
Caustic Flow to Bleach Tower
Dead-Band Test
Dead Band
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Page: 292011 ChemInnovations Conference
Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
![Page 30: Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations Conference Page: 3 Poor Loop Performance Loop is in manual Poor performance in auto](https://reader034.fdocuments.in/reader034/viewer/2022042105/5e83249ced37a230773860af/html5/thumbnails/30.jpg)
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Tuning and Disturbance Rejection
Previous comments about tuning apply
Tuning (speed of response) has limits
– Disturbance rejection
– Settling time
Limits mostly affected by dead time
Other solutions are available
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Poor Performance in Auto
Oscillations
Externally Caused
Through Set Point
Through Process
Internally Caused
Valve Problem
Tuning
Cyclical Interaction
Random Deviations
Measurement Noise
Disturbance
Valve Problem
TuningFast
Disturbances
Controller In Manual
Problem Analysis
![Page 32: Analyzing Control Problems and Improving Control Loop … · 2011-09-16 · 2011 ChemInnovations Conference Page: 3 Poor Loop Performance Loop is in manual Poor performance in auto](https://reader034.fdocuments.in/reader034/viewer/2022042105/5e83249ced37a230773860af/html5/thumbnails/32.jpg)
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Control-Side Disturbances
Go unnoticed until PV has been affected
E.g. steam pressure to heat exchanger
Steam
Process Flow
Condensate
TT
TC
Heat Exchanger
Temperature
Controller
Controller OutputChange in Pressure =Disturbance
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Steam
Process Flow
Condensate
TT
TC
Heat Exchanger
Temperature
Controller
FT FCCascaded
Flow Controller
Controller Output
Set Point
Solution: Cascade Control
Cascade control isolates control-side disturbances, nonlinearities, valve problems
Good practice for any slow control loop manipulating a flow
Cascaded Inner Loop
Disturbance
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Process-Side Disturbances
Disturbance
Go unnoticed until PV has been affected
E.g. process flow through heat exchanger
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Steam
Process Flow
Condensate
TT
TC
Heat ExchangerFT
FF ´
Feedforward Controller Feedback Controller
Solution: Feedforward Control
Use disturbance to drive control action
Feedforward control action can cancel out effect of major disturbances
Disturbance
Feedforward Controller
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Summary
Some poorly performing loops are a challenge to tune
Realize that it might not be a tuning problem
Do process tests to detect valve problems
Valve problems require repairs, not tuning
Tune from step test data using rules or software
Know the limitations of tuning
The problem may originate from outside the loop
Eliminate variability at its source
Enhance the control strategy where needed
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Page: 372011 ChemInnovations Conference
OptiControls Inc.Houston, TXPh: 713-459-6291
Questions?