Home » Articles » McMillan & Weiner

Voices: McMillan & Weiner

RSS

Our Control Experts Deal with Process Dynamics

McMillan and Weiner Ask James Beall How He Approaches the Challenge of Intertwined Problems That Have Evaded Solution. See What He Had to Say

Stan: A process control specialist may be expected to walk into a control room and within a few hours develop wondrous solution to a problem whose real cause not apparent. I had to do this in my career repeatedly on startups when time was precious and patience was slim.

Greg: I also had this eye opening experience that got more technically complicated when I moved into Engineering Technology and was tackling problems that plagued production units for years. Process control improvement comes down to putting process knowledge in the control system. How you get this information from the plant documentation, and operations, process engineering, and research is a problem it itself. Documentation is more focused on details to build and operate the plant than how to control the plant. Research reports may stay in the research department. Chemists, operators, and chemical engineers don't generally understand dynamics or the basics of process control. What they can offer is very important in turns of what does and doesn't work and process relationships. I have found Tip #51 Seek Conversations with Knowledgeable People (7/13/2012) post on the ISA Interchange Site to be essential for success. To get another perspective, we asked James Beall, our guide in our recent December and January columns, how he approaches the challenge of intertwined problems that have evaded solution.

Read Also: Control Room Designs Revolutionize Work Environments

James: I have had to tackle a double effect evaporator system with downstream centrifuges and a lab analysis just once a day, over 20 pieces of equipment, multiple sources and destinations, and a recycle stream of unknown composition. I drew a process sketch that combined the automation system components from the Piping and Instrument Diagram (P&ID) and key streams from the Process Flow Diagram (PFD). Every week I asked more questions and used the answers to improve the sketch.

Greg: There is a non-self-regulating effect of recycle streams where impurities and inerts can accumulate. I know a plant whose capacity did not achieve the promises of debottlenecking projects for decades because a long-forgotten impurity was building up. A research report from over 50 years ago buried in the archives provided the revelation that could have saved millions of dollars and decades of frustration and speculation. There must be an analysis of all components in the recycle stream and an intelligent purge of impurities and inerts. Recycle streams can cause your thinking and the control system to go in circles. Feedback control adds self-regulation. A 1993 series of papers by Bjorn Tyrus and Bill Luyben "Dynamics and Control of Recycle Streams" concluded there must be a flow control of the recycle stream somewhere in the recycle system to prevent a "snowballing" effect. Also, the makeup of a reactant recovered from the recycle stream in a distillation column must fed back to the reactor by tight level control in the distillate receiver. A solution often needs to address the short term effects by feedback control and rely on optimization levels to find the best recycle flow. A paper Model Predictive Control for Process Improvement by ISA Mentor Program protégé Flavio Briguente in the OSU Automation Society Newsletter Spring 2013 showed how an innovative application of advanced process control greatly reduced the pH variability in a reactor with a recycle stream.

Stan: James, what do you think will be the key to a solution for your complex recycle system?

James: I will determine basic models of the process responses to better understand process relationships and operation. I did some initial step tests and found that steady state was reached in the composition response within 2 to 3 hours, instead of the 24 hours expected by operations. The temperatures were also lining out in the same time frame. Part of the misunderstanding may be caused by the once per day sample analysis. During the tests we took extra samples. Process engineers are not particularly in tune with dynamics. Operations may have a more realistic view.

Stan: Since operators spend 12 hours whereas process engineers an occasional few hours with the process, I would expect the operators would have a better sense of time. However, people in general have problems with anticipating the effect of dead time. I found this in retirement.

Greg: Operator graphics put too much focus on digital numbers and an obsession with values after the decimal place of no meaningful value. Operator displays showing the future trajectories of the controlled and manipulated variables is one of the advantages of model predictive control. I think all process variables should have an intelligent trend chart time span to show the trajectory of the past and into the future as noted in the Control Talk Blogs "Checklist for Loop Analysis by Trend Charts" (7/26/3012)  and "Future PV Values are the Future" (6/29/2012).

Stan: James, how do you intend to use the knowledge of dynamics gained?

James: I plan to use the step response models in a simulator to help develop and test my new MPC control scheme. This will allow process engineers and operators to view and confirm the desired response from the MPC control scheme.

Greg: I think there is a great opportunity for step response models gained from auto tuner and adaptive tuner software to provide models. You can save a lot of test time by using a near-integrator approximation for slow continuous processes. You get the dead time and an integrating process gain, which is the maximum ramp rate divided by the change in the manipulated variable. The integrating process gain can in turn be converted to a time constant. The steady state gain can be approximated as the difference in controlled variable divided by the difference in manipulated variable for two different operating points. The time constant is then this steady state gain divided by the integrating process gain. You may even want to use a near-integrator approximation for a true integrator to prevent a model from ramping to a limit before all of the controls are nailed down. All calculations are done in percent of scale of the controlled and manipulated variables.

Stan: I can't overemphasize the importance of including the total loop dead time besides the process time constant. Tieback models that focus on a steady state gain do not give any sense of dynamics. The controller gain depends on all three terms.

Greg: Without dead time, I would be out of job. The controller could immediately see and correct for any change whether a load or a setpoint within the limits of noise. The importance of dead time is emphasized in Tip #70 Minimize Dead Time (7/27/2012) post on the ISA Interchange Site. Of course just the act of putting the controller and a model in a virtual plant creates dead time from the controller, model, and interface execution times.

Stan: Often the models need to be sped up for analysis and operator training. In this case, both the dead time and time constant should be shortened by the same factor so the controller gain is the same. The reset time should be shortened by the same factor as well, since the reset time is proportional to the time constant for the Lambda self-regulating process and is proportional to the dead time for the Lambda integrating process tuning method.

Greg: If the process time constant is greater than 4 times the dead time, the process should be treated as near-integrating and lambda integrating process tuning rules used as noted in my Control Talk blog series "Processes with no Steady-State in PID Time Frame" (3/02/2013). A question remains. Before you can do the tests what is the best guess for step response model dynamics? For new plants, the steady state gain can be found from process flow diagrams, instrument scale ranges, and valve and pump sizing. Some rules of thumb can get you started for the dead time and time constant. The process dead time for the composition response from reactant or reagent addition is on the order of 1 minute for vessels and 10 minutes when evaporation or distillation is involved. For small reagent flows, the dead time estimate can be out the window due to injection delay from dip tubes. The dead time for temperature response is larger due to thermal lags. If there is an analyzer, the model dead time is increased by the sample transportation delay and 150% of the analyzer cycle time plus 100% of the multiplex time. A ballpark number for the composition process time constant for reagent and reactant addition is 20 times the process dead time. If evaporation or distillation is involved the process time constant is about 5 times the process dead time. Once the plant is running, the initial dynamics of the model can be improved. An adaptive controller can be readily setup to adjust the dynamics of the step response model online to better match the operating conditions of the plant. After a setpoint or manual output change, the adaptive controller can adjust the integrating process gain to make the ramp rate of the model PV match the ramp rate of the process PV. For self-regulating processes, the adaptive controller can adjust the model steady state gain to get the model PV to match the actual PV. For near-integrating processes, the process time constant can be estimated from the steady state and integrating process gains.

A rap song "P.I. Diddy" composed by Ken Lane while attending my short course "Effective Use of PID Controllers" for the ISA New Orleans Section:

P.I. Diddy
by Ken Lane

I've got the wrong kind of action
… The integral action
The insidious effect is gonna leave me in traction

Pulled from all angles,
Stick-slip, now I dangle

Oscillation, postulation
Overshoot … now I'm tangled

Tangled in a structure
Steps ahead to my destruction
Overshot the landing
Deviated from my function

Full throttle, bang-bang
There is dead time in my block
Loop it back around
And put your Shinskey on my clock

---

More Voices

Our Control Experts Deal with Process Dynamics
05/10/2013
McMillan and Weiner Ask James Beall How He Approaches the Challenge of Intertwined Problems That Have Evaded Solution. See What He Had to Say

Model Predictive Control -- Where Have We Been and Where Are We Going -- Part 3
04/11/2013
McMillan, Weiner and Darby Discuss Practical Considerations in MPC Setup, Maintenance and Improvement to Meet Economic Objectives

Model Predictive Control - Past, Present and Future - Part 2
03/14/2013
McMillan, Winer and Darby Discuss Model Development and Tuning

Model Predictive Control - Past, Present and Future, Part 1
02/11/2013
McMillan and Weiner Talk to Mark Darby About MPC Applications, Proper Use of the Regulatory Level, Inferential Measurements, Model Development, Economic Objectives, Support and Maintenance

Maximize pH Response, Accuracy and Reliability
02/05/2013
Best Practices to Keep Electrodes Running at Their Best

Tuning to Meet Process Objectives
01/08/2013
McMillan, Weiner and James Beall Talk About Tuning the Controller

Diagnosing Final Control Elements
12/07/2012
McMillan and Weiner Ask James Beall What Was the Biggest Step Forward that Helped Him Improve Valve Performance

Ruel Rules for Use of PID, MPC and FLC
11/12/2012
When Do You Need to Move Beyond PID Control?

Is Your Control Valve an Imposter?
11/06/2012
Beware the Disguised Positioner

Bringing Advanced Process Control Home
09/26/2012
McMilland and Weiner Talk With Arnold Martin About Model-Predictive Control. Read the Column and Learn About Martin's Accomplishments and Perspective

Get the Most Out of Your Batch
09/07/2012
The Same Techniques Used to Get the Most Ethanol Out of a Scarce Corn Crop Can Help You Optimize Other Batch Processes

Control Loop Improvement
08/30/2012
McMillan and Weiner Talk With Jacques Smuts About Improving Control Loops

Calibration Trends
08/13/2012
McMillan and Weiner Ask Glenn Gardner, Fluke Product Planner, to Help Them Complete Their Perspective on Calibration by Taking a Look at Future Trends

Smart Calibration
07/16/2012
How Can You Make Temperature Calibration Faster?

The Human Factor
06/18/2012
What Are the Biggest Problems That Control Engineers Face When Going into a Plant?

Control Valve Innovations
05/14/2012
McMillan and Weiner Asks Hans Baumann About His Most Notable Innovations in Control Valves.

New Paradigms for Lab Control Systems
03/29/2012
What a Lab Control Systems Can and Should Do for Commercialization of Biopharmaceuticals

Going with the Flow
03/08/2012
McMillan and Weiner Continue Their Interview With Ram Ramachandran. They Discuss What Flow Sensor Works Best in Various Applications

Radar Love
01/30/2012
Control Talk Editors, McMillan and Weiner, Ask Industrial Measurement Device Expert Ram Ramachandran About the Use of Radar on the Plant Floor

Gas Chromatographs Rule
01/10/2012
What Comes First--the Analyzer or the Analyzer Application?

Process Analyzers. Analyze This!
12/07/2011
Fewer Analyzer Specialists Are Left Working at the Plant Floor, Could It Be Because Newer Anaylyzer Systems Come Available as Complete Packages?

The Operations Key to Better Plant Performance
11/15/2011
What Have You Done to Help the Operator Get the Most Out of a Control System?

Don't Over Look PID in APC
11/09/2011
That Old Stalwart PID Can Do a Lot of Heavy Lifting in Your Process Control Efforts

The Maintenance Key to Improved Performance
10/17/2011
Getting the Inside Scoop from a Control Engineer on How to Integrate O&M Knowledge and Relationships to Provide Long-Term Highly Successful Solutions

The Key to Process Knowledge and Sustainable Manufacturing
09/12/2011
What Is the Future of Operator Training Systems, and What Are You Doing to Further Simulation Technologies?

PID World Tour--The Final Performance
08/15/2011
Read the Conclusion of McMillan, Weiner and Congiundi's Tour of PID World

More Fun with PID Controllers
07/11/2011
Exploring Just How Flexible and Powerful the PID Controller Can Be

More Extraordinary PID Innovations
06/13/2011
The Creative Use of PID Controllers for Process Improvement

35 Years of Extraordinary PID Innovations
05/12/2011
The Best Tuning of a PID Controller Is Knowledge of the Process Dynamics

Ultimate Limits to Performance
04/11/2011
Whether You Are Talking About Advanced PID Control or Model-Predictive Control, the Final Control Element Determine How Well You Can Achieve Your Control Objectives

The Future is Virtually Here
03/17/2011
What's leading to virtualization? The IT Industry Looked for Ways to Manage Costs, and Came Up With the Idea of "Server Farms"

Operators Unleashed
02/15/2011
If the People On the Front Line Have an Understanding of the Process Relationships, the Result Can Be Truly Remarkable

The Virtual Plant - Better than the Real Thing
01/14/2011
When the Virtual Plant Came, the Window to the Future Opened

The Final Word on Instrument Upgrade Projects
12/08/2010
A Wrap Up of Measurements and Actuators, Valuable Advice for Projects, and an Amusing, Insightful List of "Things You Don't Want to Hear on Start-up"

Retrofit Projects - Getting Flows Right
11/12/2010
What Are the Biggest Problems With Getting the Right Control Valves?

Successful Retrofit and Automation Projects
10/14/2010
Discussing Control System Design and Implementation for Retrofit and Automation Projects. What Are the Challenges in Meeting Plant and Corporate Requirements?

Process Performance Improvement-Part 3
09/02/2010
McMilland and Weiner Concluding Their Interview with Mike Brown. It Focused on the Bottom-Line Performance of Process Control Tools

Process Performance Improvement - Part 2
08/10/2010
Continuation of the Interview with Mike Brown on Performance of Process Control Tools

Process Performance Improvement - Part 1
07/12/2010
Gaining a Perspective on Tools and Metrics for Process Performance

What's Going On with Loop Performance?
06/07/2010
Do You Know What the Most Common Performance Problems Are?

Drowning in Data, Starving for Information, 4
05/05/2010
Concluding This Four-Part Series on Data Analytics with Special Guests Modeling and Control Specialist, Brian Hrankowsky and PAT expert, Randy Reiss. How Important Is a Better Use of Data?

Drowning in Data, Starving for Info-Part 3
04/13/2010
More Chat with Randy Reiss, Who Helped Develop a Data Analytics System for Batch Processes for the New Opportunities Created by the Federal Drug Administration's Process Analytical Technology (PAT) Initiative

Drowning in Data, Starving for Information-2
03/10/2010
McMillan and Weiner Tacked a Big Question, What Is Data Analytics?

Drowning in Data; Starving for Information - 1
02/16/2010
This Is the First of a Four-Part Series on Past, Present and Future Challenges and Opportunities Presented by the Deluge of Data Now Available to Automation Professionals

The Future is Now
01/12/2010
Process Control Is Open to Innovation!

Show Me the Money - Part 2
12/17/2009
Showing the Monetary Benefits of Projects as Much as the Technical Achievements

Show Me the Money – Part 1
11/18/2009
The Best Way to Keep Your Job, Save our Profession, and Do Wonderful Things Is to Quantify the Benefits of Process Control Improvements. Money Talks

Going, Going, Gone - Part 3
10/14/2009
If the Industry Wants Access to Its Consultants, It Should Insure Their Prospective Retirees Have No Outside Interests

Going, Going, Gone — Part 2
10/08/2009
What Can Be Done to Save and Promote Process Control Expertise?

Going, Going, Gone–Part 1
10/05/2009
When Greg and Stan Retired, There Was No Attempt from the Industry to Save Their Expertise. Over 100 of the Best Minds in Process Modeling and Control Retired, and the Indusrty Made No Attempt to Retain a Snippet of Their Knowledge

Downturn Turndown
07/29/2009
Are People Going to Be Put to Work Improving the Rangeability of the Plant or Let Go to Save Short-Term Costs?

Sensible Sensor Speed-Part 2
06/29/2009
Greg McMillan and Stan Weiner Continue Their Conversation on Sensor Speeds

Sensible Sensor Speed–Part 1
05/13/2009
Greg McMillan and Stan Weiner Talk Sensor Speeds

The Secret Life of pH Electrodes - Part 3
04/09/2009
Bringing Back Experts to Wind Up Our Discussion of One of the Grand Old Tools of Process Control- the pH Electrode

The Secret Life of pH Electrodes – Part 2
03/09/2009
Greg McMillan and Stan Weiner Continue Celebrating the 100th Anniversary of the Glass pH Electrode

The Secret Life of pH Electrodes–Part 1
02/11/2009
Celebrating the 100th Anniversary of the Glass pH Electrode

It’s 12 a.m. Do You Know What Your PID’s Doing?
01/12/2009
The Proportional-Integral-Derivative (PID)

APC and Wireless Rabbits
12/22/2008

Secure Answers for a Risky Business
11/03/2008
Why Is There Such an Increased Focus on Security?

Deltas Rule
09/15/2008
Deltas Rule in the Equation for the Digital Implementation of the PID Algorithm

Oneness
08/08/2008
Is There a Metaphysical Aspect to “Oneness?”

Disturbing Remarks
07/03/2008
The Memories of Loops Gone Bad and Processes Gone Wild

Feeding on Feedforward
05/05/2008
This Month’s Topic—Feedforward Control

Loops are Not Just for Continuous Processes
04/04/2008
Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

Up From the Ashes
03/13/2008
This Month’s Column Tells How One Intrepid Fellow Survived a hit From a Reorganization Meteorite and Went on to Find Happiness as Part of a Process Control Improvement Team.

Deal or No Deal
02/05/2008
Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

Straight Talk
01/04/2008
Greg McMillan and Stan Weiner Bring Their Wits and More Than 66 Years of Process Control Experience to Bear on Your Questions, Comments and Problems

Year End Puzzler Bonanza
12/10/2007
Greg McMillan and Stan Weiner, PE bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

One Man’s Story – Back to the Future
10/02/2007
Greg McMillan and Stan Weiner, PE bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

One Man’s Story – Part 1
08/01/2007
Greg McMillan and Stan Weiner, PE bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

Round Up Them Puzzler Answers
07/01/2007
Greg McMillan and Stan Weiner, PE bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

The best of the best, Part 6
06/07/2007
Control Talk columnists McMillan and Weiner continue their multipart series of talks with some of the Great Minds in Process Control. This month it’s Terry Blevins, 2004 Control Hall of Fame inductee.

The best of the best, Part 5
05/11/2007
Control Talk columnists McMillan and Weiner continue their multipart series of talks with some of the Great Minds in Process Control. This month it’s Sheldon Lloyd, past VP of technology for Fisher Controls.

Best of the best, Part 4
04/12/2007
Control Talk columnists McMillan and Weiner continue their interviews with the big names in process control. This time they talk with Bob Heider, adjunct professor at Washington University in St. Louis.

Best of the best, Part 3
03/08/2007
In the third installment of this series, Control Talk columnists Greg McMillan and Stan Weiner, PE, continue their interviews with the big names in process control. This time they talk with ISA Fellow Vernon Trevathan.

The best of the best — Part 2
02/07/2007
In the second installment of this two-part series, Control Talk columnists Greg McMillan and Stan Weiner, PE, bring their wits and more than 70 years of process control experience to bear on your questions.

The best of the best, Part 1
01/05/2007
Columnists Greg McMillan and Stan Weiner, PE, bring their wits and more than 70 years of process control experience to bear on your questions, comments and problems in this month’s installment of Control Talk.

Talking about talking
12/15/2006
Columnists Greg McMillan and Stan Weiner, PE, bring their wits and more than 70 years of process control experience to bear on your questions, comments and problems in this month’s installment of Control Talk.

Tuning rule bonanza
11/07/2006
Control Talk columnists McMillan and Weiner say in order to get good performance, you need to measure and track the process, the controls, and the equipment under actual real-time operating conditions.

Still life
10/13/2006
Control Talk columnists McMillan and Weiner invite Wendy Kramer, Mark Sowell and Control Hall of Fame inductee Terry Tolliver to comment on improving the control of batch distillation applications.

Flashbacks
09/15/2006
In conjunction with their retirement motto of better late than never, Control Talk columnists McMillan and Weiner offer answers to the May and June Puzzlers, and the Top 10 signs your project is behind schedule.

Intoxicating answers
08/10/2006
Control Talk columnists McMillan and Weiner describe the period of time between when you first take a drink and when you first recognize the effect and bypass the next round for coffee as dead time.

15 case-in-points of common control myths
04/18/2006
In a time-proven tradition of subjecting everything to scrutiny and ridicule, columnists McMillan and Weiner offer up the following 15 examples used to help illustrate and demystify control mythology.

The Bad Hall of Fame
03/20/2006
Control Talk columnists McMillan and Weiner induct some really bad instruments, final control elements, and systems into the Bad Hall of Fame, then proffer the Top 10 signs your life is like a Reality TV show.

Resolutions are made to be broken
02/13/2006
Control Talk columnists McMillan and Weiner provide their unique brand of commentary on process trends and dynamics, then offer up some humor with their Top 10 broken New Year’s resolutions.

Five rules for helping a middle-aged engineer
01/16/2006
Control Talk columnists McMillan and Weiner provide their unique brand of commentary on the handling of cascade loops, then offer up some humor with the Top 10 reasons you should migrate to a new DCS.

Are you grounded in reality?
12/23/2005
Control Talk columnists McMillan and Weiner get an insightful reply as to why a plant instrument engineer said the control schemes and instruments successfully used at other locations won't work in his plant.

Daytime talk is a hoot and a holler
11/06/2005
McMillan and Weiner imagine a transcript of a control engineer on a daytime talk show and offer up the Top 10 reasons why you won’t find a model-based control text book anywhere in today’s college classroom.

Top 10 signs your software is over the (leading) edge
10/19/2005
Control Talk columnists McMillan and Weiner discuss standard mixing design practices for neutralization control and offer up the Top Ten Signs you are over the edge with your leading-edge software.

Top ten signs you're an endangered species
09/11/2005
Why is the instrument engineer such a rare find? The answers may be in the standard dialog on the causes of endangerment. Here are the Top Ten Signs you are an Endangered Species.

Intrinsically wicked
08/22/2005
Control Talk columnists McMillan and Weiner rustle up answers to why an electrode changed when it was inserted, then provide a bit of humor with a Top Ten list of reasons not to retire.

Top 10 signs a startup has gone wrong
07/01/2005
Control Talk columnists Greg McMillan and Stan Weiner, PE, offer up a bit of humor regarding startups, and how Stan avoided being fired despite a recent Monday morning hangover.

Bonkers vortex meter spells double trouble
06/05/2005
Control Talk columnists Greg McMillan and Stan Weiner offer up a bit of humor along with the answer to April's Puzzler on why a vortex meter measuring toothpaste went bonkers.