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Applied Manufactur ing Technologies AMT
APC9 069 – Experiences in building
and maintaining MPC
Advances in Process Control 9, York,September 2011
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Process Performance Specialists AMT
What do AMT do? “Maximize client profitability through expert consulting
and successful advanced control and real-timeoptimisation technology implementation”
We‟re a service company providing technology expertise
throughout the chemical and refining industries
Maintain a “technology-independent” position
Maintain a high standard of practice and skill
Ensure our client‟s long term success by addressing
both application “peak performance” and long term
“average application performance”
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Process Performance Specialists AMT
AMT – a Global Services Company
Houston
UK
Barcelona
Seoul Tokyo
Expertise is leveraged globally as needed for specific engagements
Currently the most experienced & largest independent APC service group
in the world – approximately 500 man-years of experience, nearly 30 engineers
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Process Performance Specialists AMT
Areas of Expertise and Offerings Well Established APC
Deployment Capability PID tuning & configuration
APC Project implementation
Optimization Capability Full scope/Reduced scope
optimization implementations
Automated Plant Testing Significant experience in rapid
controller development
Strategy & feasibility studies
Performance reviews (postaudits)
Assistance in basic design ofregulatory control configuration
Troubleshooting processissues
Regulatory configuration andtuning course, Advanced APC
user courses Operator Training
Process and/or Technology ledtraining
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Process Performance Specialists AMT
Project Implementation Strategy “Process first, technology second”
Primary focus is on process understanding
Quality Advanced Process Control solutions start at theregulatory control layer
Design application with appropriate complexity toachieve sustainable benefits at true plant/process
constraints Software vendor independent
AMT works with all technologies provided by APC andOptimization software vendors
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Process Performance Specialists AMT
What Tools are Needed for Building and
Maintaining Model Predictive Controllers?
Must have appropriate knowledge
Process and operation experience
Process control at regulatory and advanced layers Computer expertise in software and hardware
integration
Technical skills must be backed up by people
skills Working with operators, technicians, process
engineers, planners and managers
Success and failure are highly visible
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Process Performance Specialists AMT
War Stories
“Getting it wrong” Management telling operations that APC will reduce manning
levels (it doesn‟t)
Not including time to resolve the regulatory control problems(getting the underlying structure right is key to success with
multivariable control) Being a control engineer rather than a process engineer (it is no
good being a whiz at PID tuning or multivariable controller tuningif you don‟t design right in the first place)
Not investing time in model development and conditioning(getting a good prediction is only part of the story)
Poor set up of variable priorities (a multivariable controllercannot control everything all of the time) and optimizationobjectives
Not investing in on-going maintenance (all multivariablecontrollers require regular attention)
We can illustrate some of these stories with examples
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Process Performance Specialists AMT
Example 1: Regulatory control is a multivariable
problem also…
Objective is to provide rapid and smoothrejection of disturbances – this is not the role of
APC Flows should be easy
If they‟re not working, is it the valve & related
hardware?
Start with material balance – levels & pressuresNext do the energy balance – temperatures
Tuning software or rules of thumb?
With experience, tuning software is only needed indifficult circumstances
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Process Performance Specialists AMT
Process Issues & PID tuning/configurationHeat integration and process layout creates
complex interactions
Example is 3 loops interacting with each other: Column C38101 inlet temp controller TC210
Column C38101 bottom level controller LC101
Column C205 bottom level controller LC215
By hand it is easy to tune single loops and eventwo loops that interact, but three can becometedious…
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Process Performance Specialists AMT
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Process Performance Specialists AMTProcess Issues & PID tools
We need to bring this understanding of the processengineering issue to our software tools
After testing all loops in manual the model obtained isthe following (unit step responses)
Can simulate the interactions and tune manually (forexample using Cutler‟s [UPID] and training simulatorsoftware)
Using a commercial package called “Aptitune” we canderive tuning for all three loops simultaneously – hard to
do by hand!
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Process Performance Specialists AMT
BEFORE AFTER
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Process Performance Specialists AMT
Benefits of tuning By now, you‟re a hero with the operators – loops
that they have had trouble with for years becomestable in all situations reduced workload
Operators become generous…
Process is in situation where it can move towardconstraints
In one example a single temperature loop was tunedand made 0.25 million $/year
Having won the hearts of the operators and theunit managers you will have a good “advanced
control” project
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Process Performance Specialists AMT
Example 2: Plant Testing and ModellingObjective to move the process “independent
variables” to get responses in the “dependent
variables”
Can be done by hand, or in recent years withsome automated help
Beware the eager operator…
Beware the lazy operator…
Beware the helpful operator…
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Process Performance Specialists AMT
Model identification – it’s not just about prediction
A stabiliser column has two products
Bottom C5+, control the Reid Vapour Pressure
Top C4-, control the %C5 lost to the overhead Two handles available to do this
Reflux (top cooling) and reboil (bottom heating)
This is a naturally ill-conditioned problem
Can find models that predict fairly easily but inthis instance successful control requires morethan a good prediction
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Process Performance Specialists AMT
Two models give equivalent prediction of RVP
Red = measured
Blue = model 1 prediction
Pink = model 2 prediction
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Process Performance Specialists AMT
Control calculation to reduce RVP and %C5 Typical scenario is running both qualities at their
upper limits
Say the controller needs to make a 0.01 and 0.5reduction in both qualities to achieve targets
Calculate steady-state change in handles bymultiplying required change in CVs by inverse of
model gains
5.0
01.01
1
d c
baCV G MV
ss
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Process Performance Specialists AMT
Gains for model 1 and model 2 The values a, b, c , d in the equation are the
steady-state gains between the handles (reflux,reboil) and the qualities
Model 1
Model 2
RVP (bar) C5 (mol%)
Reflux (m3/h) 0.000741216 -0.058088677
Reboil (˚C) -0.007069334 0.871968881
RVP (bar) C5 (mol%)
Reflux (m3/h) 0.000588745 -0.084574698
Reboil (˚C) -0.00709939 0.772619199
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Process Performance Specialists AMT
Resulting control moves for model 1 & 2 For model 1
For model 2
Clearly model 1 would have the controllermoving in the wrong direction, it should increase
reflux and increase reboil to improve thequalities
dMV
Reflux (m3/h) 77.47
Reboil (˚C) 7.83
dMV
Reflux (m3/h) -52.00
Reboil (˚C) -4.04
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Process Performance Specialists AMT
How come model 2 gives better results? Both models gave adequate predictions
Success in multivariable control requires operationunderstanding to be mapped in to the mathematics
The current commercial packages are still “just maths” Special testing tricks help with this kind of problem
Normally we make independent moves on each handle
But for ill-conditioned problems we find making some of themoves in a correlated way actually helps the identification
software find the correct gain ratiosSo long as the software is a true MIMO identification, older MISOtechniques don‟t work so well
Often we have to “fix” models by hand, sacrificingprediction quality to obtain satisfactory control behaviour
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Process Performance Specialists AMT
Example 3: But it used to work…? Some MPC technology is robust to numerical problems,
others not
With the following gains a controller had worked for
many years on a vacuum unit furnace … but when the control software vendor was changed
and the model migrated the application was notaccepted by the operators
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Process Performance Specialists AMT
Robustification? To handle numerical issues with some control
technologies requires human intervention
Relatively small adjustment in gains to get stablecontrol behaviour
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Process Performance Specialists AMT
Example 4: Process Understanding is the Key Large gasoline feedstock producing unit
Unifiners (H2S removal), Platformer (reactor), Isom (reactor)
Large scope application with 37 MVs & 171 CVs
Has been an application in place for many years, full re-design and revamp with a change in multivariable controltechnology Integrated 14 separate small applications in to one overall
structure to enable plant wide optimization
Some interesting events when controllers taken off-line Operators had become dependent upon the applications and
found the unit hard to operate without the advanced controls –nearly had a shut-down when we turned off the old APC!
Operator training simulators become essential to maintain skills
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Process Performance Specialists AMT
Heart Cut Column
Pull valuablebenzeneproduct fromthe effluentfrom thePlatformer
reactor Process is very
non-linear
Light Plat
Heavy Plat
Combined
Platformate
Platformate
Steam
Benzene
Heart Cut
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Process Performance Specialists AMT
Benzene in Heart Cut
Toluene
Pentane+ lights
Concentrationincreasing withincreased heart
cut flow
Benzene
B e n z e n e
c o n c e n t r a
t i o n
Heart-cut flow
T o l u e n e
c o n c e n t r a t i o n
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Process Performance Specialists AMT
Toluene
Pentane+ lights
Concentration reducingwith too much heart cutflow pulling Toluene up
from base
Benzene
B e n z e n e
c o n c e n t r a t i o n
Heart-cut flow
T o l u e n e
c o n c e n t
r a t i o n
MPC objective is to sit on the optimum point – maximising side draw until breakthrough
of toluene then moving until there is no toluene. This leads to some cycling of theproduct flow, but this is not a problem because the run-down goes to storage.
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Process Performance Specialists AMT
Control design accommodates non-linear behaviour
The multivariable controller is designed so thatincreasing reboil will potentially always increase the wt%benzene in heart cut, this is to ensure enough duty to
drive benzene up the column Depending on the amount of toluene breakthrough in the
heart cut (side-draw) other MVs (Side-draw flow & TopTemp.) may move differently to maximise the benzenerecovery
Toluene breakthrough case: If wt% bz in heart cut is low, thenreduce Top Temp. and side-draw to compensate until side-drawmoves into toluene free mode
Toluene free case: If wt% of bz in heart cut is low, then increaseboth side-draw and top temp. to compensate, until toluene startsto break through
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Process Performance Specialists AMT
Benefit of Applying Process Understanding
After nearly 15 years of different attempts thereis now an application which works
During commissioning we gradually increasedthe benzene heart cut flow upper limit
Operators nervous at the amount of product we werepulling
Eventually needed to re-range the flow meterbecause of large increase in the average flow
Planning department were able to make significantsales increase
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Process Performance Specialists AMT
New Application Working – what about keeping it
working?Monitoring
Reporting
Operations need feedback and support
Tools
Keep talking
Track performance metrics
Strategy analysis
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Process Performance Specialists AMT
Who is APC Maintenance Engineer’s Key Customer?
Be available to operators to advise and respond toproblems
Meet with operators (at least weekly) to check they are
happy and have no problems with the MPC or relatedapplications, arranging and giving training as necessary Spending a lot of time in the control room is a significant factor in
building operator confidence, getting their feedback always leadsto improvements in design, strategy and acceptance
Finding one or two operators who are very interested in theapplication is a blessing, they can help with training and withkeeping documentation and „self learning‟ materials about the
MPC up to date
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Process Performance Specialists AMT
APC Maintenance Engineer’s Task List
Understand the process objectives
Understand the process in the context of the refineryplan
Ensure that the MPC design matches those needs and ifpossible at low cost make modifications as appropriate
Review application performance against the aboveobjectives on a regular basis (typically every month)
Review application performance in terms of KPIs andstability of operation (typically every week)
Update controller tuning and models on an as neededbasis to ensure acceptable performance
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Process Performance Specialists AMTExample 5: Adapting a strategy may show new
problems in an old controller… new tricks can help
old dogs
Crude Unit application commissioned andrunning for several years
CDU quality objectives changed, a part timequality constraint becomes a full time constraint
Activated different combination of MVs and CVs
Controller makes excessive movement on toptemperature
Model gains seem reasonable, no significantRGA issues, what could be wrong?
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Process Performance Specialists AMT
Modelling technology has improved
Re-check identification of the problem relationship fromoriginal test data New approach to modelling shows bigger dynamic overshoot in
the modelled response than had been identified previously New modelling software used true MIMO relationships, adding
MV PVs in to the identification gave improved results
Update model curve with better dynamics New dynamic model shows MPC it needs to move slowly to
prevent the oscillation from building as the tower top temperatureswings the qualities
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Process Performance Specialists AMT
Conclusions
Implementing process control solutions is atough business Front line customer facing, often 24 hours per day
Failure is visible very quickly Success can be hard won – APC may move the plant
more than the operators are used to and it may taketime to “bed in”
Success requires experience and teamworkOnce the application is accepted it must not be
left to die Local engineering and remote support are essential