*Posts on this page are from the Control Talk blog, which is one of the ControlGlobal.com blogs for process automation and instrumentation professionals and Greg McMillan’s contributions to the ISA Interchange blog.

Tips for New Process Automation Folks
    • 15 Apr 2019

    How to Implement Effective Safety Instrumented Systems for Process Automation Applications

    The post How to Implement Effective Safety Instrumented Systems for Process Automation Applications first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Hariharan Ramachandran.

    Hariharan starts an enlightening conversation introducing platform independent key concepts for an effective safety instrumented system with the Mentor Program resource Len Laskowski, a principal technical SIS consultant, and Hunter Vegas, co-founder of the Mentor Program.

    Hariharan Ramachandran, a recent resource added to the ISA Mentor Program, is a control and safety systems professional with various levels of experience in the field of Industrial control, safety and automation. He has worked for various companies and executed global projects for oil and gas and petrochemical industries gaining experience in the entire life cycle of industrial automation and safety projects.

    Len Laskowski is a principal technical SIS consultant for Emerson Automation Solutions, and is a voting member of ISA84, Instrumented Systems to Achieve Functional Safety in the Process Industries.

    Hunter Vegas, P.E., has worked as an instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he entered the systems integration industry and is currently working for Wunderlich-Malec as an engineering project manager in Kernersville, N.C. Hunter has executed thousands of instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms. Hunter earned a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University.

    Hariharan Ramachandran’s First Question

    How is the safety integrity level (SIL) of a critical safety system maintained throughout the lifecycle?

    Len Laskowski’s Answer

    The answer might sound a bit trite by the simple answer is by diligently following the lifecycle steps from beginning to end. Perform the design correctly and verify that it has been executed correctly. The SIS team should not blindly accept HAZOP and LOPA results at face value. The design that the LOPAs drive is no better than the team that determined the LOPA and the information they were provided. Often the LOPA results are based on incomplete or possibly misleading information. I believe a good SIS design team should question the LOPA and seek to validate its assumptions. I have seen LOPA’s declare that there is no hazard because XYZ equipment protects against it. But a walk in the field later discovered that equipment was taken out of service a year ago and had not yet been replaced. Obviously getting the LOPA/Hazop right is the first step.

    The second step is to make sure one does a robust design and specifies good quality instruments that are a good fit for the application. For example, a vortex meter may be a great meter for some applications but a poor choice for others. Similarly certain valve designs may have limited value as a safety shutdown valve. Inexperienced engineers may specify Class VI shutoff for on-off valves thinking they are making the system safer, but Class V metal seat valves would stand up to the service much better in the long run since the soft elastomer seats can easily be destroyed in less than month of operation. The third leg of this triangle is using the equipment by exercising it and routinely testing the loop. Partial stroke testing the valves is a very good idea to keep valves from sticking. Also for new units that do not have extensive experience with a process, the SIF components (valves and sensors) should be inspected at the first shutdown to assess their condition. This needs to be done until a history with the installation can be established. Diagnostics also fall into this category, deviation alarms, stroke time and any other diagnostics that can help determine the SIS health is important.

    Hariharan Ramachandran’s Feedback

    The safety instrumented function has to be monitored and managed throughout its lifecycle. Each layer in a safety protection system must have the ability to be audited. SIS verification and validation process provides a high level of assurance that the SIS will operate in accordance with its safety requirements specification (SRS). The proof testing must be carried out periodically at the intervals specified in the safety requirement specification. There should be a mechanism for recording of SIF life event data (proof test results, failures, and demands) for comparison of actual to expected performance. Continuous evaluation and improvement is the key concept here in maintaining the SIS efficiently.

    Hariharan Ramachandran’s Second Question

    What is the best approach to eliminate the common cause failures in a safety critical system?

    Hunter Vegas’ Answer

    There are many ways that common cause failures can creep into a safety system design. Some of the more common ways include:

    • Using a single orifice plate to feed redundant 2oo3 transmitters. Some make it even worse by using a single orifice tap to feed all three.  (Ideally it is best to get as much separation as possible – as a minimum have 3 different taps and individual impulse lines.  Better yet have completely different flow meters and if possible utilize different technologies to measure flow so that a single failure or abnormal process condition won’t affect them all.) 
    • If the impulse lines of redundant transmitters require heat trace, it is best to use different sources of heat. (If they are fed with a single steam line its failure might impact all three readings. This might apply to a boiler drum level or an orifice plate.)
    • Having the same technician calibrate all three meters simultaneously. (Sometimes he’ll get the calibration wrong and set up all three meters incorrectly.)  Some plants have the technician only calibrate one meter of the three each time. That way an incorrect calibration will stand out.
    • Putting redundant transmitters (or valves) on the same I/O card. If it freezes or fails all of the readings are lost.
    • Implementing SIS trips in the same DCS that controls the plant.
    • Just adding a SIS contact to the solenoid circuit of an existing on/off valve. If the solenoid or actuator fails such that the valve fails open neither the DCS or SIS can trip it.  At least add a second solenoid but it is far better to add a separate shutdown valve. (Some put a trip solenoid on a control valve.  However if the control valve fails open the trip solenoid might not be able close it either.)
    • Having a single device generate a 4-20mA signal for control and also generate a contact for a trip circuit. A single fault within the instrument might fail and take out both the 4-20mA and the trip signal.  (Using a SIS transmitter for control is really the same thing.)

    Hariharan Ramachandran’s Feedback

    Both, random and systematic events can induce common cause failure (CCF) in the form of single points of failure or the failure of redundant devices.

    Random hardware failures are addressed by Design architecture, diagnostics, estimation (analysis) of probabilistic failures, design techniques and measures (to IEC 61508‐7).

    Systematic failures are best addressed through the implementation of a protective management system, which overlays a quality management system with a project development process. A rigorous system is required to decrease systematic errors and enhance safe and reliable operation. Each verification, functional assessment, audit, and validation is aimed at reducing the probability of systematic error to a sufficiently low level.

    The management system should define work processes, which seek to identify and correct human error. Internal guidelines and procedures should be developed to support the day-to-day work processes for project engineering and on-going plant operation and maintenance. Procedures also serve as a training tool and ensure consistent execution of required activities. As errors or failures are detected, their occurrence should be investigated, so that lessons can be learned and communicated to potentially affected personnel.

    Hariharan Ramachandran’s Third Question

    An incident happened at a process plant, what are all the engineering aspects that needs to be verified during the Investigation?

    Len Laskowski’s Answer

    I would start at the beginning of the lifecycle look at Hazop and LOPA’s to see that they are done properly.  Look to see that documentation is correct; P&IDs, SRS, C&Es, MOC and test logs and procedures. Look to see where the break down occurred.  Were things specified correctly? Were the designs verified? Was the System correctly validated? Was proper training given? Look for test records once the system was commissioned.

    Hunter Vegas’ Answer

    Usually the first step is to determine exactly what happened separating conjecture from facts. Gather alarm logs, historian data, etc. while it is available. Individually interview any personnel involved as soon as possible to lock in the details. With that information in hand, begin to work backwards determining exactly what initiated the event and what subsequent failures occurred to allow it to happen. In most cases there will be a cascade of failures that actually enabled the event to happen. Then examine each failure to understand what happened and how it can be avoided in the future. Often there will be a number of changes implemented.  If the SIS system failed, then Len’s answer provides a good list of items to check.

    Hariharan Ramachandran’s Feedback

    Also verify if the device/equipment is appropriately used within the design intent.

    Hariharan Ramachandran’s Fourth Question

    What are all the critical factors involved in decommissioning a control systems?

    Len Laskowski’s Answer

    The most critical factor is good documentation. You need to know what is going to happen to your unit and other units in the plant once an instrument, valve, loop or interlock is decommissioned. A proper risk and impact assessment has to be carried out prior to the decommissioning. One must ask very early on in a project’s development if all units controlled by the system are planning to shut down at the same time. This is needed for maintenance and upgrades. Power distribution and other utilities are critical. One may not be able to demo a system because it would affect other units. In many cases, a system cannot be totally decommissioned until the next shutdown of the operating unit and it may require simultaneous shutdowns of neighboring units as well. Waste management strategy, regulatory framework and environmental safety control are the other factors to be considered.

    Hariharan Ramachandran’s Feedback

    A proper risk and impact assessment has to be carried out prior to the decommissioning. Waste management strategy, regulatory framework and environmental safety control are the other factors to be considered.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 8 Apr 2019

    Webinar Recording: The Amazing World of ISA Standards

    The post Webinar Recording: The Amazing World of ISA Standards first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    Historically, predictive maintenance required very expensive technology and resources, like data scientists and domain experts, to be effective. Thanks to artificial intelligence (AI) methods such as machine learning making its way into the mainstream, predictive maintenance is now more achievable than ever. Our webinar will explore how machine learning is changing the game and greatly reducing the need for data scientists and domain experts. These technologies self-learn and autonomously monitor for data pattern anomalies. Not only does this make predictive maintenance far more practical than what was historically possible, but now predictions 30 days in advance are the norm. Don’t let the old way of doing predictive maintenance cause you loss in productivity any longer.

    This webinar covers:

    • AI made real
    • Leveraging existing technology for a higher ROI
    • Learning from downtime event history
    • How to never be blindsided by breakdown again

    About the Featured Presenter
    Nicholas P. Sands, P.E., CAP, serves as senior manufacturing technology fellow at DuPont, where he applies his expertise in automation and process control for the DuPont Safety and Construction business (Kevlar, Nomex, and Tyvek). During his career at DuPont, Sands has worked on or led the development of several corporate standards and best practices in the areas of automation competency, safety instrumented systems, alarm management, and process safety. Nick is: an ISA Fellow; co-chair of the ISA18 committee on alarm management; a director of the ISA101 committee on human machine interface; a director of the ISA84 committee on safety instrumented systems; and secretary of the IEC (International Electrotechnical Commission) committee that published the alarm management standard IEC62682. He is a former ISA Vice President of Standards and Practices and former ISA Vice President of Professional Development, and was a significant contributor to the development of ISA’s Certified Automation Professional program. He has written more than 40 articles and papers on alarm management, safety instrumented systems, and professional development, and is co-author of the new edition of A Guide to the Automation Body of Knowledge. Nick is a licensed engineer in the state of Delaware. He earned a bachelor of science degree in chemical engineering at Virginia Tech.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 27 Mar 2019

    Missed Opportunities in Process Control - Part 3

    Here is the third part of a point blank decisive comprehensive list of what we really need to know in a detailed attempt to reduce the disparity between theory and practice. Please read, think and take to heart the opportunities to increase the performance and recognized value of our profession. The list is necessarily concise in detail. If you want more information on these opportunities, please join the ISA Mentor Program and ask the questions whose answers can be shared via Mentor Q&A Posts.

    You can also get a comprehensive resource focused on what you really need to know for a successful automaton project including nearly a thousand best practices in the 98% new 2019 Process/Industrial Instruments and Controls Handbook Sixth Edition capturing the expertise of 50 leaders in industry.

    The following list reveals common misconceptions that need to be understood to seek real solutions that actually address the opportunities.

    1. Dead time dominant loops need Model Predictive Control or a Smith Predictor.  There are many reasons for Model Predictive Control but dead time dominance is not really one of them. Dead time compensation can be simply done by inserting a dead time block in the external-reset feedback path making a conventional PID an enhanced PID and then tuning the PID much more aggressively. This enhanced PID is much easier to implement than a Smith Predictor because there is no need to identify and update an open loop gain or primary open loop time constant and there is no loss of the controlled variable seen on the PID faceplate.  An additional pervasive misconception is that dead time dominant processes benefit the most from dead time compensation. It turns out that reduction in integrated error for an unmeasured process input load disturbance is much greater for lag dominant processes, especially near-integrating processes. While the improvement is significant, the performance of a lag dominant process is often already impressive provided the PID is tuned aggressively (e.g., integrating process tuning rules with minimum arrest time). For more details see the ISA Mentor Program Q&A Post "How to Improve Loop Performance for Dead Time Dominant Systems"
    2. The model dead time should not be smaller than the actual loop dead time.  For Model Predictive Control, Smith Predictors and an enhanced PID, a model dead time larger than the actual dead time by just 40% can lead to fast oscillations. These controllers are less sensitive to a model dead time smaller than the actual dead time. For a conventional PID, tuning based on a model dead time larger than the actual dead time just causes a sluggish response so in general conventional PID tuning is based on largest possible loop dead time.
    3. Cascade control loops will oscillate if cascade rule is violated. For small or slow setpoint changes or unmeasured load disturbances, the loop may not break out into oscillations. While it is not a good idea to violate the rule that the secondary loop be at least five times faster than the primary loop, there are simple fixes. The simplest and easiest fix is to turn on external-reset feedback that will prevent the primary loop integral mode from changing faster than the secondary loop can respond. It is important that external-reset feedback signal be the actual process variable of secondary loop. There is no need to slow down the tuning of the primary loop, which is the most common quick fix if the secondary loop tuning cannot be made faster.
    4. Limit cycles are inevitable from resolution limits. While one or more integrators anywhere in the system can cause a limit cycle from a resolution limit, turning on external-reset feedback can stop the limit cycle. The feedback for the external reset feedback must be a fast readback of the actual manipulated valve position or speed. Often readback signals are slow and changes or lack of changes in the readback of actuator shaft position are not representative of the actual ball or disk movement for on-off valves posing as control valves. While external-reset feedback can stop the limit cycle, there is an offset from the desired valve position. For some exothermic reactors, it may be better to have a fast limit cycle in the manipulated coolant temperature than an offset because tight temperature control is imperative and the oscillation is attenuated (averaged out) by the well-mixed reactor volume.
    5. Fast opening and slow closing surge valves will cause oscillations unless PID is tuned for slower valve response. It is desirable that surge valve be fast in terms of increasing and slow in terms of decreasing vent or recycle flow for compressor surge control. Generally, this was done in the field by restricting the actuator fill rate or enhancing exhaust rate by a quick exhaust valve since the surge valves are fail open. The controller had to be tuned to deal with the crude unknown rate limiting. Using different setpoint up and down rate limits on the analog output block and turning on external-reset feedback via a fast readback of the actual valve position make the adjustment much more exact and visible. The controller does not need to be tuned for the slow closing rate because the integral mode will not outrun the response of the valve.
    6. Prevention of oscillations at split range point requires a deadband in the split range block. A dead band anywhere in the loop adds a dead time that is the dead band divided by the rate of change of signal. Dead band will cause a limit cycle if there are two or more integrators anywhere in the control loop including the positioner, process, and cascade control. The best solution is a precise properly sized control valves with minimal backlash and stiction and a linear installed flow characteristic. External-reset feedback with setpoint rate limits can be added in the direction of opening or closing a valve at the split range point to instill patience and eliminate unnecessary crossings of the split range point. For small and large valves, the better solution is a valve position controller that gradually and smoothly moves the big valve to ensure the small valve manipulated by the process controller is in a good throttle position.
    7. Valve position controller integral action must be ten times slower than process controller integral action to prevent oscillations. External-reset feedback in the valve position controller with fast readback of actual big valve position and up and down setpoint limits on analog output block for large valve can provide slow gradual optimization but a fast getaway for abnormal operation to prevent running out of the small valve. This is called directional move suppression and is generally beneficial when valve position controllers are used to maximize feed or minimize compressor pressure or maximize cooling tower or refrigeration unit temperature setpoints. One of the advantages of Model Predictive Control is move suppression to slow down changes in the manipulated variable that would be disruptive. Here we have the additional benefit of the move suppression being directional with no need to retune.
    8. High PID gains causing fast large changes in PID output upset operators and other loops. The peak and integrated errors for unmeasured load disturbances are inversely proportional to the PID gain. A high PID gain is necessary to minimize these errors and to get to setpoint faster for setpoint changes. Too low of a PID gain is unsafe for exothermic reactor temperature control and can cause slow large amplitude oscillations in near-integrating and true integrating processes.  Higher PID gains can be used to increase loop performance without upsetting operators or other loops by turning on external-reset feedback, putting setpoint rate limits on the analog output block or secondary loop and providing an accurate fast feedback of manipulated valve position or process variable.
    9. Large control valve actuators and VFD rate limiting to prevent motor overload requires slowing down the PID tuning to prevent oscillations. Turning on external-reset feedback and using a fast accurate readback of valve position or VFD speed enables faster tuning to be used that makes the response to small changes in PID output much faster. Of course, the better solution is a faster valve or larger motor. Since there is always a slewing rate or speed rate limit in VFD setup using external-reset feedback with fast readback is good idea in general.
    10. Large analyzer cycle times require PID detuning to prevent oscillations. While the additional dead time that is 1.5 times the cycle time is excessive in terms of ability of loop to deal with unmeasured load disturbances, when this additional dead time is greater than the 63% process response time, an intelligent computation of integral action using external-reset feedback can enable the resulting enhanced PID gain to be as large as the inverse of the open loop gain for self-regulating processes even if the cycle time increases. This means the enhanced PID could be used with an offline analyzer with a very large and variable time between results reported. While load disturbances are not corrected until an analytical result is available, the enhanced PID does not become unstable. The intelligent calculation of the proportional (P), integral (I) and derivative (D) mode is only done when there is a change in the measurement. The time interval between the current and last result is used in I and D mode computations. The input to I mode computation is the external-reset feedback signal. If there is no response of the manipulated variable, I mode contribution does not change. An analyzer failure will not cause a PID response since there is no change in P or D mode contribution unless there is a new result or setpoint. The same benefits apply to wireless loops (additional dead time is ½ update rate). For more details see the Control Talk Blog “Batch and continuous control with at-line and offline analyzer tips
    • 13 Mar 2019

    Solutions for Unstable Industrial Processes

    The post Solutions for Unstable Industrial Processes first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Caroline Cisneros.

    Negative resistance also known as positive feedback can cause processes to jump, accelerate and oscillate confusing the control system and the operator. These are characterized as open loop unstable processes. Not properly addressing these situations can result in equipment damage and plant shutdowns besides the loss of process efficiency. Here we first develop a fundamental understanding of the causes and then quickly move on to the solutions to keep the process safe and productive.

    Caroline Cisneros, a recent graduate of the University of Texas who became a protégé about a year ago, is gaining significant experience working with some of the best process control engineers in an advanced control applications group. Caroline asks a question about the dynamics that cause unstable processes. The deeper understanding gained as to the sources of instability can lead to process and control system solutions to minimize risk and to increase process performance.

    Caroline Cisneros’ Question

    What causes processes to be unstable when controllers are in manual?

    Greg McMillan’s Answer

    Fortunately, most processes are self-regulating by virtue of having negative feedback that provides a resistance to excursions (e.g., flow, liquid pressure, and continuous composition and temperature). These processes come to a steady state when the controller is in manual.  Somewhat less common are processes that have no feedback that will result in a ramp (e.g., batch composition and temperature, gas pressure and level). Fortunately, the ramp rate is quite slow except for gas pressure giving the operator time to intervene.

    There are a few processes where the deviation from setpoint can accelerate when in manual due to positive feedback. These processes should never be left in manual. We can appreciate how positive feedback causes problems in sound systems (e.g., microphones too close to speakers). We can also appreciate from circuit theory how negative resistance and positive feedback would cause an acceleration of a change in current flow. We can turn this insight into an understanding of how a similar situation develops for compressor, steam-jet ejector, exothermic reactor and parallel heat exchanger control.

    The compressor characteristic curves from the compressor manufacturer that are a plot of compressor pressure rise versus suction flow shows a curve of decreasing pressure rise for each speed or suction vane position whose  slope magnitude increases as the suction flow increases in the normal operating region. The pressure rise consequently decreases more as the flow increases opposing additional increases in compressor flow creating a positive resistance to flow. Not commonly seen is that compressor characteristic curve slope to the left of the surge point becomes zero as you decrease flow, which denotes a point on the surge curve, and then as the flow decreases further, the pressure rise decreases causing a further decrease in compressor flow creating a negative resistance to a decrease in flow.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

    When the flow becomes negative, the slope reverses sign creating a positive resistance with a shape similar to that seen in the normal operating region to the right of the surge point. The compressor flow then increases to a positive flow at which point the slope reverses sign creating negative resistance. The compressor flow jumps in about 0.03 seconds from the start of negative resistance to some point of positive resistance. The result is a jump in 0.03 seconds to negative flow across the negative resistance, a slower transition along positive resistance to zero flow, than a jump in 0.03 seconds across the negative resistance to a positive flow well to the right of the surge curve. If the surge valve is not open far enough, the operating point walks about 0.5 to 0.75 seconds along the positive resistance to the surge point. The whole cycle repeats itself with an oscillation period of 1 to 2 seconds. If this seems confusing, don’t feel alone. The PID controller is confused as well.

    Once a compressor gets into surge, the very rapid jumps and oscillations are too much for a conventional PID loop. Even a very fast measurement, PID execution rate and control valve response can’t deal with it alone. Consequently, the oscillation persists until an open loop backup activates and holds open the surge valves till the operating point is sustained well to the right of the surge curve for about 10 seconds at which point there is a bumpless transfer back to PID control. The solution is a very fast valve and PID working bumplessly with an open loop backup that detects a zero slope indicating an approach to surge or a rapid dip in flow indicating an actual surge. The operating point should always be kept well to the right of the surge point.

    For much more on compressor surge control see the article Compressor surge control: Deeper understanding, simulation can eliminate instabilities.

    The same shape but with much less of a dip in the compressor curve, sometimes occurs just to the right of the surge point. This local dip causes a jumping back and forth called buzzing. While the oscillation is much less severe than surge, the continual buzzing is disruptive to users.

    A similar sort of dip in a curve occurs in a plot of pumping rate versus absolute pressure for a steam-jet ejector. The result is a jumping across the path of negative resistance. The solution here is a different operating pressure or nozzle design, or multiple jets to reduce the operating range so that operation to one side or the other of the dip can be assured.

    Positive feedback occurs in exothermic reactors when the heat of reaction exceeds the cooling rate causing an accelerating rise in temperature that further increases the heat of reaction. The solution is to always insure the cooling rate is larger than the heat of reaction. However, in polymerization reactions the rate of reaction can accelerate so fast the cooling rate cannot be increased fast enough causing a shutdown or a severe oscillation. For safety and process performance, an aggressively tuned PID is essential where the time constants and dead time associated with heat transfer in cooling surface and thermowell and loop response are much less than the positive feedback time constant.

    Derivative action must be maximized and integral action must be minimized. In some cases a proportional plus derivative controller is used. The runway response of such reactors is characterized by a positive feedback time constant as shown in Figure 1 for an open loop response. The positive feedback time constant is calculated from the ordinary differential equations for the energy balance as shown in Appendix F of 101 Tips for a Successful Automation Career. The point of acceleration cannot be measured in practice because it is unsafe to have the controller in manual. A PID gain too low will allow a reactor to runaway since the PID controller is not adding enough negative feedback. There is a window of allowable PID gains that closes as the time constants from heat transfer surface and thermowell and the total loop dead time approach the positive feedback time constant.

    Figure 1: 1 Positive Feedback Process Open Loop Response

    Positive feedback can also occur when parallel heat exchanges have a common process fluid input each with outlet temperature controller(s) with a setpoint close to the boiling point or temperature resulting in vaporization of a component in the process fluid. Each temperature controller is manipulating a utility stream providing heat input. The control system is stable if the process flow is exactly the same to all exchangers. However, a sudden reduction in one  process flow causes overheating causing bubbles to form expanding back into the exchanger causing an increase in back pressure and hence a further decrease in process flow thru this hot exchanger.

    The increasing back pressure eventually forces all of the process flow into the colder heat exchanger making it colder. The high velocity in the hot exchanger from boiling and vaporization causes vibration and possibly damage to any discontinuity in its path from slugs of water. When nearly all of the water is pushed out of the hot exchanger, its temperature drops drawing feed that was going to the cold heat exchanger that causes the hot exchanger to overheat repeating the whole cycle. The solution is separate flow controllers and pumps for all streams so that changes in the flow to one exchanger do not affect another and a lower temperature setpoint.

    To summarize, to eliminate oscillations, the best solution is a process and equipment design that eliminates negative resistance and positive feedback. When this cannot provide the total solution, operating points may need to be restricted, loop dead time and thermowell time constant minimized and the controller gain increased with integral action decreased or suspended.

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 24 Feb 2019

    Missed Opportunities in Process Control - Part 2

    The post, Missed Opportunities in Process Control - Part 2, first appeared on the ControlGlobal.com Control Talk blog.

    Here is the second part of a point blank decisive comprehensive list of what we really need to know in a detailed attempt to reduce the disparity between theory and practice. Please read, think and take to heart the opportunities to increase the performance and recognized value of our profession. The list is necessarily concise in detail.

    If you want more information on these opportunities, please join the ISA Mentor Program and ask the questions whose answers can be shared via Mentor Q&A Posts.

    You can also get a comprehensive resource focused on what you really need to know for a successful automaton project including nearly a thousand best practices in the 98% new 2019 Process/Industrial Instruments and Controls Handbook Sixth Edition capturing the expertise of 50 leaders in industry.

    1. Ratio control instead of feedforward control.  Most of the literature focuses on feedforward control. This is like flying blind for the operator. In most  cases there is a flow measurement and the primary process loop dead time is large enough for there to be a cascade control using a secondary flow loop. A ratio controller is then setup whose input is the flow signal that would have been the feedforward signal. This could be a disturbance or wild flow or a feed flow in the applications involving most vessels (e.g., crystallizers, evaporators, neutralizers, reactors …) and columns. To provide a shortcut in categorization, I simply call it the “leader flow”. The secondary loop then is the “follower” flow whose setpoint is the “leader” flow multiplied by the ratio controller setpoint. A bias is applied to the Ratio controller output similar to what is done by a feedforward summer that is corrected by the primary loop.  The operator can change the ratio setpoint and see the actual ratio after correction. Improvements can be made to the ratio setpoint based on recognizable persistent differences between set and the actual ratio. Many vessels and most columns are started up on ratio control until normal operating conditions are reached.  When primary loops use an analyzer, ratio correction may be suspended when the analyzer misbehaves. If the flow measurement lacks sufficient rangeability, a flow can be computed from the installed flow characteristic and substituted for the flow measurement at low flows. A notable exception is avoidance of ratio control for steam header pressures since the dead time is too short for cascade control consequently necessitating feedforward control.
    2. Adaptation of feedforward gain and ratio control setpoint. A simple adaptive controller similar to a valve position controller (VPC) for optimization can be used. The adaptive controller setpoint is zero correction, its process variable is the current correction and its output is the feedforward gain or ratio setpoint. Like a VPC, the traditional approach would be a slow integral-only controller where the integral action is more than 10 times slower than in the primary loop controller. However, the opportunity for directional move suppression described next month can provide more flexibility and opportunity to deal with undesirable conditions.
    3. PID Form and Structure used in Industry. The literature often shows the “Independent” Form that computes the contribution of the P, I  and D modes in parallel with the proportional gain not affecting the I and  D modes. The name “Independent” is appropriate not only because the contribution of the modes are independent from each other but also because this Form is independent of what is normally used in today’s distributed control systems (DCSs). Often the tuning parameters for I and D are an integral and derivative gain, respectively rather than a time. The “Series” or “Real” Form necessarily used in pneumatic controllers was carried over to electronic controllers and is offered as an option in DCSs. The “Series” Form causes an interaction in the time domain that can be confusing but prevents the D mode contribution from exceeding the I mode contribution. Consequently, tuning where the D mode setting is larger the I mode setting do not cause oscillations. If these are settings are carried over to the “Ideal” Form more extensively today, the user is surprised by unsuspected fast oscillations. The different units for tuning settings also cause havoc. Some proportional modes still use proportional band in percent and integral settings could be repeats per minute repeats per second instead of minutes or seconds. Then you have the possibility of an integral gain and derivative gain in an Independent Form. Also, the names given by suppliers for Forms are not consistent. There are also 8 structures offering options to turn off the P and I mode or use setpoint  weight factors for the P and D modes. The D mode is simply turned off by a zero setting (zero rate time or derivative gain).  I am starting an ISA Standards Committee  for PID algorithms and performance to address these issues and many more.  For more on PID Forms see the ISA Mentor Q&A “How do you convert tuning settings of an independent PID?”     
    4. Sources of Deadband, Resolution, Sensitivity, and Velocity Limit. Deadband can originate from backlash in linkages or connections, deadband in split range configuration, and deadband in Variable Frequency Drive (VFD) setup. Resolution limitation can originate from stiction and analog to digital conversion or computation. Sensitivity limitations can originate from actuators, positioners, or sensors.  Velocity Limits can originate from valve slewing rate set by positioner or booster relay capacity and actuator volume and from speed rate limits in VFD setup.
    5. Oscillations from Deadband, Resolution, Sensitivity, and Velocity Limit. Deadband can cause a limit cycle if there are two or more integrators in the process or control system including the positioner.Thus, a positioner with integral action will create a limit cycle in any loop with integral action in the controller. Positioners should have high gain proportional action and possibly some form of derivative action. Resolution can cause a limit cycle if there are two or more integrators in the process or control system including the positioner. Positioners with poor sensitivity have been observed to create essentially a limit cycle. A slow velocity limit causes oscillations that can be quite underdamped.
    6. Noise, resonance and attenuation. The best thing to do is eliminate the source of oscillations often due to the control system as detailed in the Control Talk Blog “The Most Disturbing Disturbances are Self-Inflicted”. Oscillation periods faster than the loop dead time is essentially noise. There is nothing the loop can do so the best thing is to ignore it. If the oscillation period is between one and ten dead times cause resonance. The controller tuning needs to be less aggressive to reduce amplification. If the oscillation period is more than 10 times the dead time, the controller tuning needs to be more aggressive to provide attenuation.
    7. Control Loops Transfer Variability. We would like to think a control loop makes variability completely disappear. What it does is transfer the variability from the controlled variable to the manipulated variable. For many level control loops, we want minimization of this transfer and give it the more positive terminology “maximization of absorption” of variability. This is done by less aggressive tuning that still prevents activation of alarms. The user must be careful that for near-integrating and true-integrating processes, the controller gain must not be decreased without increasing the integral time so that the product of the controller gain and integral time is greater than twice the inverse of the integrating process gain to prevent large slow rolling nearly underdamped oscillations with a period forty or more times the dead time.
    8. Overshoot of Controller Output. Some articles have advocated that the PID controller should be tuned so its output never overshoots the final resting value (FRV). While this may be beneficial for balanced self-regulating process particularly seen in refineries, it is flat out wrong and potentially unsafe for near-integrating, true integrating and runaway processes. In order to get to a new setpoint or recover from a disturbance, the controller output must overshoot the FRV. This generally requires that the integral mode not dominate the proportional and derivative mode contributions. Integrating process tuning rules are used.
    9. Hidden Factor in Temperature, Composition and pH Loops. The process gain in these loops for continuous and fed-batch operations is generally plotted versus a ratio of manipulated flow to feed flow. To provide a process gain with the proper units, you need to divide by the feed flow. Most people don’t realize the process gain is inversely proportional to feed flow. This is particularly a problem at low production rates resulting in a very large hidden factor. For much more on this see the Control Talk Blog “Hidden factor in Our Most Important Loops”.     
    10. Variable Jacket Flow. If the flow to a vessel jacket is manipulated for temperature control, you have a double whammy. The low flow for a low cooling or heating demand causes an increase in the process gain per the hidden factor and an increase in the process dead time due to the larger transportation delay. The result is often a burst of oscillations from tuning that would be fine at normal operating conditions. A constant jacket flow should be maintained by recirculation and the manipulation of coolant or heating utility makeup flow (preferably steam flow to a steam  injector) for high heat demands. The utility return flow is made equal to  the makeup flow by a pressure controller on jacket output manipulating return flow.
    • 20 Feb 2019

    What Skill Sets Do You Need to Excel at IIoT Applications in an Automation Industry Career?

    The post What Skill Sets Do You Need to Excel at IIoT Applications in an Automation Industry Career? first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Angela Valdes.

    The Industrial Internet of Things (IIoT) is the hot topic as seen in the many feature articles published. The much greater availability of data is hoped to provide the knowledge needed to sustain and improve plant safety, reliability and performance. Here we look at what are some of the practical issues and resources in achieving the expected IIoT benefits.

    Angela Valdes is a recently added resource in the ISA Mentor Program. Angela is the automation manager of the Toronto office for SNC-Lavalin. She has over 12 years of experience in project leadership and execution, framed under PMI, lean, agile and stage-gate methodologies. Angela seeks to apply her knowledge in process control and automation in different industries such as pharmaceutical, food and beverage, consumer packaged products and chemicals.

    Angela’s question

    What skill sets and ISA standards shall I start building/referencing in order to grow in the IIoT space and work field?

    Nick Sands’ answer

    The ISA communication division is forming a technical interest group in IIoT. The division has had presentations on the topic for several years at conferences. The leader will be announced in InTech magazine. The ISA95 standard committee is working on updating the enterprise – control system communication to better support IIoT concepts.

    Jim Cahill’s answer

    One tremendous resource would be to read most of Jonas Berge’s LinkedIn blog posts. He writes about IIoT and digital communications and the impact they can have on reliability, safety, efficiency and production. I recommend you send him a connection request to see when he has new things to post. One other person to connect with includes Terrance O’Hanlon of ReliabilityWeb.com. Searching on the #IIoT hashtag in Twitter and LinkedIn is also a very good way to discover new articles and influencers in these areas.

    Greg McMillan’s answer

    One of the things we need to be careful about is to make sure there are people with the expertise to use the data and associated software, such as data analytics. There was a misrepresentation in a feature article that IIoT would make the automation engineer obsolete when in fact the opposite is true. We need more process control engineers besides process analytical technology and IIoT experts to make the most out of the data. The data by itself can be overwhelming as seen in the series of articles “Drowning in Data; Starving for Information”: Part 1, Part 2, Part 3, and Part 4.

    Process control engineers with a fundamental knowledge of the process and the automation system need to intelligently analyze and make the associated improvements in instrumentation, valves, setpoints, tuning, control strategies, and use of controller features whether PID or MPC. Often lacking is the recognition of the importance of dynamics in the process and particularly the automation system. The process inputs must be synchronized with the process outputs for continuous processes before true correlations can be identified.

    Knowledge of process first principles is also needed to determine whether correlations are really cause and effect. While the solution would seem to be employing expert rules to the IIoT results, a word of caution here is that the attempts to develop and use real time expert systems in the 1980s and 1990s were largely failures wasting an incredible amount of time and money. Deficiencies in conditions, interrelationships and knowledge in the rules of logic implemented plus lack of visibility of interplay between rules and ability to troubleshoot rules led to a lot of false alerts resulting in the systems being turned off and eventually abandoned.

    Hunter Vegas’ answer

    There have been multiple “data revolutions” over the years, and I consider IIoT to be just another wave where new information is made available that wasn’t available before. Unfortunately the problem that bedeviled the previous data revolutions still remains today. More data is not necessarily useful unless the right information is delivered at the right time to a person who can act on it.  In many cases the operators have too much information now – when something goes wrong they get 1000 alarms and have to wade through the noise to try to figure out what went wrong and how to fix it.  

    IIoT data can undoubtedly be useful, but it takes a huge amount of time and effort to create an interface than can effectively present that information and still more time and effort to keep it up. All too often management reads a few trendy articles and thinks IIoT is something you buy or install and savings should just appear. Unfortunately most fail to appreciate the effort required to implement such a system and keep it working and adding value. Usually money is spent, people celebrate the glorious new system, then it falls out of favor and use and gets eliminated a short time later. 

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

    As far as I know there aren’t any specific standards associated with IIoT.  I do think that there are several skill sets that can you help you implement it:

    • Knowledge the latest alarm standards will help you understand how to identify alarm information/data that IS useful and how to make sure the operators get the important information in a timely fashion and not get buried with useless alarm data that doesn’t matter.
    • Knowledge of some of the new HMI design standards are useful to learn how to present the information in a meaningful way that lets the operator quickly understand a situation and correctly react to it.
    • Knowledge of getting the information into the system. That particular topic will depend upon your particular control system and how data flows into it.  It might come in via OPC, wireless, Hart, Modbus, Ethernet, or any number of other paths.  Each communication type will have its own challenges and security issues that must be addressed.
    • Knowledge of what matters to your plant. In an aging acid plant corrosion can be a big issue.  If you can add a handful of small wireless pipe thickness gauges in a few key spots that might have significant value.  If you have environmental problems and sumps located all over your facility it might be possible to add wireless analyzers to detect solvent spills and quickly react to them rather than having a spill hit the river outfall before you detect it. The key to all of this is to understand the plant’s ‘pain points’ and then determine a way to address it.  IIoT may offer an answer or it may be as simple as retuning a controller or replacing a poorly specified control valve with a better one.  Regardless, if calling it an “IIoT Project” gets you funding and you solve a problem then you are a hero regardless.

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 11 Feb 2019

    Webinar Recording: Practical Limits to Control Loop Performance

    The post Webinar Recording: Practical Limits to Control Loop Performance first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    Part 2 provides a quick review of Part 1 and then discusses the contribution of each PID mode, why reset time is orders of magnitude too small for most composition and temperature loops, the ultimate and practical limits to control loop performance, the critical role of dead time, and when PID gain that is too high or too low causes more oscillation.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 11 Feb 2019

    Webinar Recording: Simple Loop Tuning Methods and PID Features to Prevent Oscillations

    The post Webinar Recording: Simple Loop Tuning Methods and PID Features to Prevent Oscillations first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    Part 3 (the final part) describes simple tuning methods and the PID features that can be used to prevent the oscillations that plague our most important loops and to achieve the desired degree of tightness or looseness in level control. A general procedure is offered and a block diagram of the most effective PID structure, not shown anywhere else, is given followed by questions and answers.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 18 Jan 2019

    Missed Opportunities in Process Control - Part 1

    The post, Missed Opportunities in Process Control - Part 1, first appeared on the ControlGlobal.com Control Talk blog.

    I had an awakening as to the much greater than realized disconnect between what is said in the literature and courses and what we need to know as practitioners as I was giving guest lectures and labs to chemical engineering students on PID control. We are increasingly messed up. The disparity between theory and practice is exponentially growing because of leaders in process control leaving the stage and users today not given the time to explore and innovate and the freedom to publish. Much of what is out there is a distraction at best.  I decided to make a decisive pitch not holding back for sake of diplomacy. Here is the start of a point blank decisive comprehensive list in a six part series.

    Please read, think and take to heart the opportunities to increase the performance and recognized value of our profession. The list is necessarily concise in detail. If you want more information on these opportunities, please join the ISA Mentor Program and ask the questions whose answers can be shared via Mentor Q&A Posts.

    1. Recognizing and addressing actual load disturbance location. Most of the literature unfortunately shows disturbances entering the process output when in reality disturbances enter mostly as process inputs (e.g., feed flow, composition and temperature changes) passing through the primary process time constant. Thinking of disturbances on the process output leads to many wrong conclusions and mistakes, such as large primary  time constants are bad, tuning can be done primarily for setpoint changes,  feedforward and ratio control is not important, and algorithms like Internal Model Control are good alternatives to PID control.
    2. Tuning and tests to first achieve good load disturbance rejection and then good setpoint response. While most of the literature focuses on setpoint response tuning and testing, the first objective should be good load disturbance rejection particularly in chemical processes. Such tuning generally requires more aggressive proportional action. Testing is simply done by momentarily putting the PID in manual, changing the PID output and putting the PID back in auto. Tuning should minimize peak and integrated error from load disturbances taking into account needs to minimize resonance. To prevent overshoot in the setpoint response, a setpoint lead-lag can be used with lag time equal to reset time or a PID structure of proportional and derivative action on PV and integral action on error (PD on PV and I on E) can be used. If a faster setpoint response is needed, setpoint lead can be increased to ¼ lag time or a 2 Degrees of Freedom (2DOF) PID structure used with setpoint weight factors for the  proportional and derivative modes equal to 0.5 and 0.25, respectively. Rapid changes in signals to valves or secondary loops upsetting other loops from higher PID gain setting can be smoothed by setpoint rate limits on analog output blocks and secondary PIDs and turning on external-reset feedback (ERF). We will note the many other advantages of ERF and its facilitation of directional move suppression to intelligently slow down changes of manipulated flows in a disruptive direction in subsequent months (hope you can wait). In Model Predictive Control move suppression plays a key role. Here we can enable it with additional intelligence of direction without retuning PID.
    3. Minimum possible peak error is proportional to dead time and actual peak error is inversely proportional to PID gain. Peak error is important to prevent relief, alarm and SIS activation and environmental violation. The ultimate limit to what you can achieve in minimizing peak error is proportional to the total loop dead time. The practical limit as to what you actually achieve is inversely proportional to the product of the PID gain and open loop process gain. The maximum PID gain is inversely proportional to the total loop dead time. These relationships hold best for near-integrating, true integrating and runaway processes.
    4. Minimum possible integrated error is proportional to dead time squared and actual peak error is proportional to reset time and inversely proportional to PID gain. The integrated absolute error is the most common criteria sited in literature. It does provide a measure of the amount of process material that is off-spec. The ultimate limit to what you can achieve in minimizing integrated error is proportional to the total loop dead time squared. The practical limit as to what you actually achieve is proportional to reset time and inversely proportional to the product of the PID gain and open loop process gain. The minimum reset time is proportional and the maximum PID gain is inversely proportional to the total loop dead time.  These      relationships hold best for near-integrating, true integrating and runaway processes.
    5. Detuning a PID can be evaluated as an increase in implied dead time. The relationships cited in items 3 and 4 above can be understood by realizing that a larger than actual total loop dead time is the effect  on loop performance of a smaller PID gain and larger reset time setting than needed to prevent oscillations. This implied dead time is basically ½ and ¼ the summation of Lambda plus the actual dead time, for self-regulating and integrating processes, respectively.
    6. The effect of analyzer cycle time and wireless update rate depends on implied dead time and consequently tuning. You can prove almost any point you want to make about whether the effect of a discontinuous update is important or not by how you tune the PID. The dead time from an analyzer cycle time is 1½ times the cycle time. The dead time from a wireless device update or PID execution rate or sample rate is ½ the time interval between updates assuming no latency. How important this  additional dead time is seen in how big it is relative to the implied dead time. The conventional rule of thumb is that the dead time from discontinuous updates should be less than 10% of the total loop dead time (wireless update rates and PID execution rates less than 20% of dead time). This is only really true if you are pursing aggressive control where the implied dead time is near the actual dead time. A better recommendation would be a wireless update rate or PID execution rate less      than 20% of “original” implied dead time. I use the work “original” to remind us not to spiral into slowing down update and execution rates by increasing implied dead time and then further slowing down update and execution rates.
    7. The product of the PID gain and reset time must be greater than the inverse of the integrating process gain. Violation of this rule cause very large and very slow oscillations that are slightly damped taking hours to days to die out for vessels and columns, respectively. This is a common problem because in control theory courses we learned that high controller gain causes oscillations and the actual PID gain permitted for near integrating, true integrating and runaway processes is quite large (e.g., > 100). Most don’t think such a high PID gain is possible and don’t like sudden large movements in valves. Furthermore, integral action  provides the gradual action that will always be in a direction consistent  with error sign and will seek to exactly match up PV and SP meeting common expectations. The result is a reset time frequently set that is orders of magnitude too small making the product of PID gain and reset time less than the inverse of the integrating process gain causing confusing slow oscillations.
    8. The effective rate time should be less than ¼ the effective reset time. While PID controllers with a Series Form effectively prevented this due to interaction factors in the time domain, this is not the case for the other PID Forms. Not enforcing this limit is a common problem in migration projects since older controllers had the Series Form and most modern controllers use the ISA Standard Form. The result is erratic fast oscillations.
    9. Automation system dynamics affect the performance of most loops. This should be good news for us since this is much more under the control of the automation engineer and easier and cheaper to fix than process or equipment dynamics. Flow, pressure, inline temperature and composition (e.g., static mixer), and fluidized bed reactors are affected by sensor response time and final control element (e.g., valve and VFD) response  time. Pressure and surge control loops are also affected by PID execution rate.
    10. Reserve feedforward multiplier and ratio controller ratio correction for sheet lines and plug flow systems.  The conventional rule that on a plot of manipulated variable versus feedforward variable, a change in slope demands a feedforward multiplier and a change in intercept demands a feedforward summer is not really relevant. A feedforward multiplier introduces a change in controller gain that is counteracts the change in process gain. However, this is only useful for sheet lines and plug flow (e.g., static mixers and extruders) because for vessels and columns, the effect of back mixing from agitation and reflux or recirculation creates a process time constant that is proportional to the residence time. For decreases in feed flow the increase in process time constant from an increase in residence time negates the increase in process gain. Also, the most important error is often a bias error in the measurements. Span errors are smitten by a large span showing up mostly as a change in process gain much less than the other sources of changes in process  gain.  Also, the scaling and filtering of a feedforward summer signal and its correction is much easier.     
    • 14 Jan 2019

    How to Get Started with Effective Use of OPC

    The post How to Get Started with Effective Use of OPC first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    Encouraged to ask general questions that would help share knowledge, Nikki Escamillas provided several questions on OPC. Initially, the OPC standard was restricted to the Windows operating system with the acronym originally designating OLE (object linking and embedding) for process control.  OPC is the acronym for open platform communications that is much more widely used playing a key role in automation systems. We are fortunate to have answers to Nikki’s questions from a knowledgeable expert in higher level automation system communications, Tom Freiberger, product manager for industrial Ethernet in R&D engineering for Emerson Automation Solutions.

    Nikki Escamillas is a recently added protégé in the ISA Mentor Program. Nikki is an Automation Process Engineer for Republic Cement and Building Materials – Batangas Plant. Nikki specializes in process optimization and automation control, committed in minimizing cost and product quality improvement through effective time management and efficient use of resources and data analytics. Nikki has an excellent knowledge and experience of advanced process control principles and its application to different plant processes more specifically on cement and building materials manufacturing.

    Nikki Escamillas’ First Question

    How does OPC work?

    Tom Freiberger’s Answer

    OPC is a client/server protocol. The server has a list of data points (normally in a tree structure) that it provides. A client can connect to a server and pick a set of data points it wishes to use. The client can then read or write to those data points.  OPC is meant to be a common language for integrating products from multiple vendors. The OPC Foundation has a good introduction of OPC DA and UA at their website.

    Nikki Escamillas’ Second Question

    Does configuration of OPC DA differs from OPC UA?

    Tom Freiberger’s Answer

    Yes and no. The core concept of client/server and working with a set of data points remains consistent between the two, but the details of how to configure differ. The security configuration is the primary difference. OPC DA is based off of Microsoft’s DCOM technology, which means the security settings in the operating system are used. OPC UA runs on many operating systems and therefore the security settings are embedded into the configuration of the OPC application. OPC UA applications should use common terminology in their configuration, to ease integration between multiple vendors

    Nikki Escamillas’ Third Question

    Do we have any guidelines to follow when installing and configuring one OPC based upon its type?

    Tom Freiberger’s Answer

    Installation and configuration guidelines are going to be specific to the products being used. Some products are going to be limited on the number of data points that can be exchanged by a license or other application limitation. Some products may have performance limits. All of these details should be supplied in the documentation of the product.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    Nikki Escamillas’ Fourth Question

    Could I directly make one computer to become OPC capable?

    Tom Freiberger’s Answer

    An OPC server or client by itself is just a means to transfer data. OPC is not very interesting without another application behind it to supply information. The computer you are attempting to add OPC to would need some other application to provide data. The vendor of that application would need to build OPC into their product. If the application with the data supports some other protocol to exchange data (like Modbus TCP, Ethernet/IP, or PROFINET) an OPC protocol converter could be used to interface with other OPC applications. If the application with the data has no means of extracting the information, there is nothing an OPC server or client can do.

    Nikki Escamillas’ Fifth Question

    Is it also possible to create a server to server communication between two OPC?

    Tom Freiberger’s Answer

    I believe there are options for this in the OPC protocol specification, but the details would be specific to the product being used. If it allows server to server connections, it should be listed in its documentation.

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 19 Dec 2018

    Webinar Recording: PID and Loop Tuning Options and Solutions for Industrial Applications

    The post Webinar Recording: PID and Loop Tuning Options and Solutions for Industrial Applications first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    This is Part 1 of a series on the benefits of knowing your process and PID capability. Part 1 focuses on process behavior, the many loop objectives and different worlds of industrial applications, and the loop component’s contribution to the dynamic response.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 19 Dec 2018

    Webinar Recording: PID and Loop Tuning Options and Solutions for Industrial Applications

    The post Webinar Recording: PID and Loop Tuning Options and Solutions for Industrial Applications first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    This is Part 1 of a series on the benefits of knowing your process and PID capability. Part 1 focuses on process behavior, the many loop objectives and different worlds of industrial applications, and the loop component’s contribution to the dynamic response.

    Join the ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about how you can join the ISA Mentor Program.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 17 Dec 2018

    How to Improve Loop Performance for Dead Time Dominant Systems

    The post How to Improve Loop Performance for Dead Time Dominant Systems first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    Dead time is the source of the ultimate limit to control loop performance. The peak error is proportional to the dead time and the integrated error is dead time squared for load disturbances. If there was no dead time and no noise or interaction, perfect control would be theoretically possible. When the total loop dead time is larger than the open loop time constant, the loop is said to be dead time dominant and solutions are sought to deal with the problem.

    Anuj Narang is an advanced process control engineer at Spartan Controls Limited. He has more than 11 years of experience in the academics and the industry with a PhD in process control. He has designed and implemented large scale industrial control and optimization solutions to achieve sustainable and profitable process and control performance improvements for the customers in the oil and gas, oil sands, power and mining industry. He is a registered Professional Engineer with the Association of Professional Engineers and Geoscientists of Alberta, Canada.

    Anuj’s Question

    Is there any other control algorithm available to improve loop performance for dead time dominant systems other than using Smith predictor or model predictive control (MPC), both of which requires identification of process model?

    Greg McMillan’s Answer

    The solution cited for deadtime dominant loops is often a Smith predictor deadtime compensator (DTC) or model predictive control. There are many counter-intuitive aspects in these solutions. Not realized is that the improvement by the DTC or MPC is less for deadtime dominant systems than for lag dominant systems. Much more problematic is that both DTC and MPC are extremely sensitive to a mismatch between the compensator and model deadtime versus the actual total loop deadtime for a decrease besides an increase in the deadtime. Surprisingly, the consequences for the DTC and MPC are much greater for a decrease in plant dead time. For a conventional PID, a decrease in the deadtime just results in more robustness and slower control. For a DTC and MPC, a decrease in plant deadtime by as little as 25 percent can cause a big increase in integrated error and an erratic response.

    Of course, the best solution is to decrease the many source of dead time in the process and automation system (e.g., reduce transportation and mixing delays and use online analyzers with probes in the process rather than at-line analyzers with a sample transportation delay and an analysis delay that is 1.5 times the cycle time). An algorithmic mitigation of consequences of dead time first advocated by Shinskey and now particularly by me is to simply insert a deadtime block in the PID external-reset feedback path (BKCAL) with the deadtime updated to be always be slightly less than the actual total loop deadtime. Turning on external-reset feedback (e.g., dynamic reset limit) on and off enables and disables the deadtime compensation. Note that for transportation delays, this means updating the deadtime as the total feed rate or volume changes. This PID+TD implementation does not require the identification of the open loop gain and open loop time constant for inclusion as is required for a DTC or MPC. Please note that the external-reset feedback should be the result of a positive feedback implementation of integral action as described in the ISA Mentor Program webinar PID Options and Solutions – Part 3.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    There will be no improvement from a deadtime compensator if the PID tuning settings are left the same as they were before the DTC or by a deadtime block in external-reset feedback (PID+TD).  In fact the performance can be slightly worse for even an accurate deadtime. You need to greatly decrease the PID integral time toward a limit of the execution time plus any error in deadtime. The PID gain should also be increased. The equation for predicting integrated error as a function of PID gain and reset time settings is no longer applicable because it predicts an error less than the ultimate limit that is not possible. The integrated error cannot be less than the peak error multiplied by the deadtime. The ultimate limit is still present because we are not making deadtime disappear.

    If the deadtime is due to analyzer cycle time or wireless update rate, we can use an enhanced PID (e.g., PIDPlus) to effectively prevent the PID from responding between updates. If the open loop response is deadtime dominant mostly due to the analyzer or wireless device, the effect of a new error upon update results in a correction proportional to the PID gain multiplied by the open loop error. If the PID gain is set equal to the inverse of the open loop gain for a self-regulating process, the correction is perfect and takes care of the step disturbance in a single execution after an update in the PID process variable.

    The integral time should be set smaller than expected (about equal to the total loop deadtime that ends up being the PID execution time interval) and the positive feedback implementation of integral action must be used with external reset feedback enabled. The enhanced PID greatly simplifies tuning besides putting the integrated error close to its ultimate limit. Note that you do not see the true error that could’/ have started at any time in between updates but only see the error measured after the update.

    For more on the sensitivity to both increases and decrease in the total loop deadtime and open loop time constant, see the ISA books Models Unleashed: A Virtual Plant and Predictive Control Applications (pages 56-70 for MPC) and Good Tuning: A Pocket Guide 4th Edition (pages 118-122 for DTC). For more on the enhanced PID, see the ISA blog post How to Overcome Challenges of PID Control and Analyzer Applications via Wireless Measurements and the Control Talk blog post, Batch and Continuous Control with At-Line and Offline Analyzers Tips.

    The following figures from Models Unleashed shows how a MPC with two controlled variables (CV1 and CV2)  and two manipulated variables for a matrix with condition number three (CN = 3) responds to a doubling and a halving of the plant dead time (delay) when the total loop dead time is greater than the open loop time constant.

    Figure 1: Dead Time Dominant MPC Test for Doubled Plant Delay

     

    Figure 2: Dead Time Dominant MPC Test for Halved Plant Delay

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 17 Dec 2018

    How to Improve Loop Performance for Dead Time Dominant Systems

    The post How to Improve Loop Performance for Dead Time Dominant Systems first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    Dead time is the source of the ultimate limit to control loop performance. The peak error is proportional to the dead time and the integrated error is dead time squared for load disturbances. If there was no dead time and no noise or interaction, perfect control would be theoretically possible. When the total loop dead time is larger than the open loop time constant, the loop is said to be dead time dominant and solutions are sought to deal with the problem.

    Anuj Narang is an advanced process control engineer at Spartan Controls Limited. He has more than 11 years of experience in the academics and the industry with a PhD in process control. He has designed and implemented large scale industrial control and optimization solutions to achieve sustainable and profitable process and control performance improvements for the customers in the oil and gas, oil sands, power and mining industry. He is a registered Professional Engineer with the Association of Professional Engineers and Geoscientists of Alberta, Canada.

    Anuj’s Question

    Is there any other control algorithm available to improve loop performance for dead time dominant systems other than using Smith predictor or model predictive control (MPC), both of which requires identification of process model?

    Greg McMillan’s Answer

    The solution cited for deadtime dominant loops is often a Smith predictor deadtime compensator (DTC) or model predictive control. There are many counter-intuitive aspects in these solutions. Not realized is that the improvement by the DTC or MPC is less for deadtime dominant systems than for lag dominant systems. Much more problematic is that both DTC and MPC are extremely sensitive to a mismatch between the compensator and model deadtime versus the actual total loop deadtime for a decrease besides an increase in the deadtime. Surprisingly, the consequences for the DTC and MPC are much greater for a decrease in plant dead time. For a conventional PID, a decrease in the deadtime just results in more robustness and slower control. For a DTC and MPC, a decrease in plant deadtime by as little as 25 percent can cause a big increase in integrated error and an erratic response.

    Of course, the best solution is to decrease the many source of dead time in the process and automation system (e.g., reduce transportation and mixing delays and use online analyzers with probes in the process rather than at-line analyzers with a sample transportation delay and an analysis delay that is 1.5 times the cycle time). An algorithmic mitigation of consequences of dead time first advocated by Shinskey and now particularly by me is to simply insert a deadtime block in the PID external-reset feedback path (BKCAL) with the deadtime updated to be always be slightly less than the actual total loop deadtime. Turning on external-reset feedback (e.g., dynamic reset limit) on and off enables and disables the deadtime compensation. Note that for transportation delays, this means updating the deadtime as the total feed rate or volume changes. This PID+TD implementation does not require the identification of the open loop gain and open loop time constant for inclusion as is required for a DTC or MPC. Please note that the external-reset feedback should be the result of a positive feedback implementation of integral action as described in the ISA Mentor Program webinar PID Options and Solutions – Part 3.

    Join the ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about how you can join the ISA Mentor Program.

    There will be no improvement from a deadtime compensator if the PID tuning settings are left the same as they were before the DTC or by a deadtime block in external-reset feedback (PID+TD).  In fact the performance can be slightly worse for even an accurate deadtime. You need to greatly decrease the PID integral time toward a limit of the execution time plus any error in deadtime. The PID gain should also be increased. The equation for predicting integrated error as a function of PID gain and reset time settings is no longer applicable because it predicts an error less than the ultimate limit that is not possible. The integrated error cannot be less than the peak error multiplied by the deadtime. The ultimate limit is still present because we are not making deadtime disappear.

    If the deadtime is due to analyzer cycle time or wireless update rate, we can use an enhanced PID (e.g., PIDPlus) to effectively prevent the PID from responding between updates. If the open loop response is deadtime dominant mostly due to the analyzer or wireless device, the effect of a new error upon update results in a correction proportional to the PID gain multiplied by the open loop error. If the PID gain is set equal to the inverse of the open loop gain for a self-regulating process, the correction is perfect and takes care of the step disturbance in a single execution after an update in the PID process variable.

    The integral time should be set smaller than expected (about equal to the total loop deadtime that ends up being the PID execution time interval) and the positive feedback implementation of integral action must be used with external reset feedback enabled. The enhanced PID greatly simplifies tuning besides putting the integrated error close to its ultimate limit. Note that you do not see the true error that could’/ have started at any time in between updates but only see the error measured after the update.

    For more on the sensitivity to both increases and decrease in the total loop deadtime and open loop time constant, see the ISA books Models Unleashed: A Virtual Plant and Predictive Control Applications (pages 56-70 for MPC) and Good Tuning: A Pocket Guide 4th Edition (pages 118-122 for DTC). For more on the enhanced PID, see the ISA blog post How to Overcome Challenges of PID Control and Analyzer Applications via Wireless Measurements and the Control Talk blog post, Batch and Continuous Control with At-Line and Offline Analyzers Tips.

    The following figures from Models Unleashed shows how a MPC with two controlled variables (CV1 and CV2)  and two manipulated variables for a matrix with condition number three (CN = 3) responds to a doubling and a halving of the plant dead time (delay) when the total loop dead time is greater than the open loop time constant.

    Figure 1: Dead Time Dominant MPC Test for Doubled Plant Delay

     

    Figure 2: Dead Time Dominant MPC Test for Halved Plant Delay

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 21 Nov 2018

    How to Setup and Identify Process Models for Model Predictive Control

    The post How to Setup and Identify Process Models for Model Predictive Control first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    Luis Navas is an ISA Certified Automation Professional and electronic engineer with more than 11 years of experience in process control systems, industrial instrumentation and safety instrumented systems. Luis’ questions on evaporator control are important to improve evaporator concentration control and minimize steam consumption

    Luis Navas’ Introduction

    The process depicted in Figure 1 shows a concentrator with its process inputs and outputs. I have the following questions regarding the process testing in order to generate process models for a MPC in the correct way. I know that MPC process inputs must be perturbed to allow an identification and modeling of each process input and output relationship.  

    Figure 1: Variables for model predictive control of a concentrator

     

    Luis Navas’ First Question

    Before I start perturbing the feed flow or steam flow, should the disturbance be avoided or at least minimized? Or simply let it be as usual in the process since this disturbance is always present?

    Mark Darby’s Answer

    If it is not difficult, you can try to suppress the disturbance. That can help the model identification for the feed and steam. To get a model to the disturbance, you will want movement of the disturbance outside the noise level (best is four to five times). If possible, this may require making changes upstream (for example, LIC.SP or FIC.SP).

    Luis Navas’ Second Question

    What about the steam flow? Should it be maintained a fix flow, (FIC in MAN with a fix % open FCV), while perturbing the feed flow and in the same way when perturbing the steam flow, should the feed flow be fixed? I know some MPC software packages excite its outputs in a PRBS (Pseudo Random Binary Sequence) practically at the same time while the process testing is being executed, and through mathematics catches the input and output relationships, finally generating the model.  

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    Mark Darby’s Answer

    Because the steam and feed setpoints are manipulated variables, it is best to keep them both in auto for the entire test. PRBS is an option, but it will take more setup effort to get the magnitudes and the average switching interval right. An option is to start with a manual test and switch to PRBS after you’ve got a feel for the process and the right step sizes. Note: a pretest should have already been conducted to identify instrument issues, control issues, tuning, etc. Much more detail is offered in my Section 9.3 on in the McGraw-Hill handbook Process/Industrial Instruments and Control Sixth Edition.

    Luis Navas’s Last Questions

    What are the pros & cons for process testing  if the manipulated variables are perturbed through FIC Setpoints, (closed loop), or through FIC Outputs, (open loop)? Or simply: should it be done according with the MPC design? What are the pros & cons if in the final design the FCVs are directly manipulated by the MPC block or through FICs, as MPC’s downstream blocks? I know in this case the FICs will be faster than MPC, so I expect a good approach is to retain them.

    Mark Darby’s Answers

    Correct – do according to the MPC design. Note sometimes the design will need to change during a step test as you learn more about the process. Flow controllers should normally be retained unless they often saturate. This is the same idea for justifying a cascade – to have the inner loop manage the higher frequency disturbances (so the slower executing MPC doesn’t have to). The faster executing inner loop also helps with linearization (for example, valve position to flow).  

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 21 Nov 2018

    How to Setup and Identify Process Models for Model Predictive Control

    The post How to Setup and Identify Process Models for Model Predictive Control first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    Luis Navas is an ISA Certified Automation Professional and electronic engineer with more than 11 years of experience in process control systems, industrial instrumentation and safety instrumented systems. Luis’ questions on evaporator control are important to improve evaporator concentration control and minimize steam consumption

    Luis Navas’ Introduction

    The process depicted in Figure 1 shows a concentrator with its process inputs and outputs. I have the following questions regarding the process testing in order to generate process models for a MPC in the correct way. I know that MPC process inputs must be perturbed to allow an identification and modeling of each process input and output relationship.  

    Figure 1: Variables for model predictive control of a concentrator

     

    Luis Navas’ First Question

    Before I start perturbing the feed flow or steam flow, should the disturbance be avoided or at least minimized? Or simply let it be as usual in the process since this disturbance is always present?

    Mark Darby’s Answer

    If it is not difficult, you can try to suppress the disturbance. That can help the model identification for the feed and steam. To get a model to the disturbance, you will want movement of the disturbance outside the noise level (best is four to five times). If possible, this may require making changes upstream (for example, LIC.SP or FIC.SP).

    Luis Navas’ Second Question

    What about the steam flow? Should it be maintained a fix flow, (FIC in MAN with a fix % open FCV), while perturbing the feed flow and in the same way when perturbing the steam flow, should the feed flow be fixed? I know some MPC software packages excite its outputs in a PRBS (Pseudo Random Binary Sequence) practically at the same time while the process testing is being executed, and through mathematics catches the input and output relationships, finally generating the model.  

    Join the ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about how you can join the ISA Mentor Program.

    Mark Darby’s Answer

    Because the steam and feed setpoints are manipulated variables, it is best to keep them both in auto for the entire test. PRBS is an option, but it will take more setup effort to get the magnitudes and the average switching interval right. An option is to start with a manual test and switch to PRBS after you’ve got a feel for the process and the right step sizes. Note: a pretest should have already been conducted to identify instrument issues, control issues, tuning, etc. Much more detail is offered in my Section 9.3 on in the McGraw-Hill handbook Process/Industrial Instruments and Control Sixth Edition.

    Luis Navas’s Last Questions

    What are the pros & cons for process testing  if the manipulated variables are perturbed through FIC Setpoints, (closed loop), or through FIC Outputs, (open loop)? Or simply: should it be done according with the MPC design? What are the pros & cons if in the final design the FCVs are directly manipulated by the MPC block or through FICs, as MPC’s downstream blocks? I know in this case the FICs will be faster than MPC, so I expect a good approach is to retain them.

    Mark Darby’s Answers

    Correct – do according to the MPC design. Note sometimes the design will need to change during a step test as you learn more about the process. Flow controllers should normally be retained unless they often saturate. This is the same idea for justifying a cascade – to have the inner loop manage the higher frequency disturbances (so the slower executing MPC doesn’t have to). The faster executing inner loop also helps with linearization (for example, valve position to flow).  

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant) and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 12 Nov 2018

    Webinar Recording: How to Use Modern Process Control to Maintain Batch-To-Batch Quality

    The post Webinar Recording: How to Use Modern Process Control to Maintain Batch-To-Batch Quality first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    Understanding the difficulties of batch processing and the new technologies and techniques offered can lead to solutions by better automation and control that offer much greater increases in efficiency and capacity than usually obtained for continuous process. Industry veteran and author Greg McMillan discusses analyzing batch data, elevating the role of the operator, tuning key control loops, and setting up simple control strategies to optimize batch operations. The presentation concludes with an extensive list of best practices.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 12 Nov 2018

    Webinar Recording: How to Use Modern Process Control to Maintain Batch-To-Batch Quality

    The post Webinar Recording: How to Use Modern Process Control to Maintain Batch-To-Batch Quality first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    Understanding the difficulties of batch processing and the new technologies and techniques offered can lead to solutions by better automation and control that offer much greater increases in efficiency and capacity than usually obtained for continuous process. Industry veteran and author Greg McMillan discusses analyzing batch data, elevating the role of the operator, tuning key control loops, and setting up simple control strategies to optimize batch operations. The presentation concludes with an extensive list of best practices.

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 29 Oct 2018

    What Types of Process Control Models are Best?

    The post What Types of Process Control Models are Best? first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the U.S. This question comes from Daniel Rodrigues.

    Daniel Rodrigues is one of our newest protégés in the ISA Mentor Program. Daniel has been working in research & development for Norsk Hydro Brazil since 2016 specializing in:

    • Development of a greener, safer, more accurate, and cheaper analytical method
    • Cost reduction, efficiency enhancement opportunities identification
    • Process modelling and advanced control logic development and assessment
    • Research methodology development, execution, and planning
    • Statistical analysis of process variables and test results

    Daniel Rodrigues’ Question

    What is your take on process control based on phenomenological models (using first-principle models to guide the predictive part of controllers)? I am aware of the exponential growth of complexity in these, but I’d also like to have an experienced opinion regarding the reward/effort of these.

    Greg McMillan’s Answer

    I prefer first principle models to gain a deeper understanding of cause and effects, process relationships, process gains, and the response to abnormal situations. Most of my control system improvements start with first principle models. The incorporation of the actual control system (digital twin) to form a virtual plant has made these models a more powerful tool.  However, most first principle models use perfectly mixed volumes neglecting mixing delays and are missing transportation delays and automation system dynamics. For pH systems, including all of the non-ideal dynamics from piping and vessel design, control valves or variable speed pumps, and electrodes is particularly essential, I have consequently partitioned the total vessel volume into a series of plug flow and perfectly back mixed volumes to model the mixing dead times that originate from agitation pattern and the relative location of input and output streams. I add a transportation delay for reagent piping and dip tubes due to gravity flow or blending. For extremely low reagent flows (e.g., gph), I also add an equilibration time in dip tube after closure of a reagent valve associated with migration of the reagent into the process followed by migration of process fluid back up into the dip tube. I add a transportation delay to electrodes in piping. I use a variable dead time block and time constant blocks in series to show the effect of velocity, coating, age, buffering and direction of pH change on electrode response. I use a backlash-stiction and a variable dead time block to show the resolution and response time of control valves. The important goal is to get the total loop dead time and secondary lag right.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    By having the much more complete model in a virtual plant, the true dynamic behavior of the system can be investigated and the best control system performance achieved by exploring, discovering, prototyping, testing, tuning, justifying, deploying, commissioning, maintaining and continuously improving, as described in the Control magazine feature article Virtual Plant Virtuosity.

     

    Figure 1: Virtual Plant that includes Automation System Dynamics and Digital Twin Controller

     

    Model predictive control is much better at ensuring you have the actual total dynamics including dead time, lags and lead times at a particular operating point. However, the models do not include the effect of backlash-stiction or actuator and positioner design on valve response time and consequentially on total loop dead time because by design the steps are made several times larger than the deadband and resolution or sensitivity limits of the control valve. Also, the models identified are for a particular operating point and normal operation. To cover different modes of operation and production rates, multiple models must be used requiring logic for a smooth transition or recently developed adaptive capabilities. I see an opportunity to use the results from the identification software used by MPC to provide a more accurate dead time, lag time and lead time by inserting these in blocks on the measurement of the process variable in first principle models. The identification software would be run for different operating points and operating conditions enabling the addition of supplemental dynamics in the first principle models. This addresses the fundamental deficiency of dead times, lag times and lead times being too small in first principle models.

    Statistical models are great at identifying unsuspected relationships, disturbances and variability in the process and measurements. However, these are correlations and not necessarily cause and effect. Also, continuous processes require dynamic compensation of each process input so that it matches the dynamic response timewise of each process output being studied. This is often not stated in the literature and is a formidable task. Some methods propose using a dead time on the input but for large time constants, the dynamic response of the predicted output is in error during a transient. These models are more designed for steady state operation but this is often an ideal situation not realized due to disturbances originating from the control system due to interactions, resonance, tuning, and limit cycles from stiction as discussed in the Control Talk Blog The most disturbing disturbances are self-inflicted. Batch processes do not require dynamic compensation of inputs making data analytics much more useful in predicting batch end points.

    I think there is a synergy to be gained by using MPC to find missing dynamics and statistical process control to help track down missing disturbances and relationships that are subsequently added to the first principle models. Recent advances in MPC capability (e.g., Aspen DMC3) to automatically identify changes in process gain, dead time and time constant including the ability to compute and update them online based on first principals has opened the door to increased benefits from the using MPC to improve first principle models and vice versa. Multivariable control and optimization where there are significant interactions and multiple controlled, manipulated and constraint variables are best handled by MPC. The exception is very fast systems where the PID controller is directly manipulating control valves or variable frequency drives for pressure control. Batch end point prediction might also be better implemented by data analytics. However, in all cases the first principle model should be accordingly improved and used to test the actual configuration and implementation of the MPC and analytics and to provide training of operators extended to all engineers and technicians supporting plant operation.

    I would think for research and development, the ability to gain a deeper and wider understanding of different process relationships for different operating conditions would be extremely important. This knowledge can lead to process improvements and to better equipment and control system design. For pH and biological control systems, this capability is essential.

    For a greater perspective on the capability of various modeling and control methodologies, see the ISA Mentor Program post with questions by protégé Danaca Jordan and answers by Hunter Vegas and I: What are the New Technologies and Approaches for Batch and Continuous Control?

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 29 Oct 2018

    What Types of Process Control Models are Best?

    The post What Types of Process Control Models are Best? first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the U.S. This question comes from Daniel Rodrigues.

    Daniel Rodrigues is one of our newest protégés in the ISA Mentor Program. Daniel has been working in research & development for Norsk Hydro Brazil since 2016 specializing in:

    • Development of a greener, safer, more accurate, and cheaper analytical method
    • Cost reduction, efficiency enhancement opportunities identification
    • Process modelling and advanced control logic development and assessment
    • Research methodology development, execution, and planning
    • Statistical analysis of process variables and test results

    Daniel Rodrigues’ Question

    What is your take on process control based on phenomenological models (using first-principle models to guide the predictive part of controllers)? I am aware of the exponential growth of complexity in these, but I’d also like to have an experienced opinion regarding the reward/effort of these.

    Greg McMillan’s Answer

    I prefer first principle models to gain a deeper understanding of cause and effects, process relationships, process gains, and the response to abnormal situations. Most of my control system improvements start with first principle models. The incorporation of the actual control system (digital twin) to form a virtual plant has made these models a more powerful tool.  However, most first principle models use perfectly mixed volumes neglecting mixing delays and are missing transportation delays and automation system dynamics. For pH systems, including all of the non-ideal dynamics from piping and vessel design, control valves or variable speed pumps, and electrodes is particularly essential, I have consequently partitioned the total vessel volume into a series of plug flow and perfectly back mixed volumes to model the mixing dead times that originate from agitation pattern and the relative location of input and output streams. I add a transportation delay for reagent piping and dip tubes due to gravity flow or blending. For extremely low reagent flows (e.g., gph), I also add an equilibration time in dip tube after closure of a reagent valve associated with migration of the reagent into the process followed by migration of process fluid back up into the dip tube. I add a transportation delay to electrodes in piping. I use a variable dead time block and time constant blocks in series to show the effect of velocity, coating, age, buffering and direction of pH change on electrode response. I use a backlash-stiction and a variable dead time block to show the resolution and response time of control valves. The important goal is to get the total loop dead time and secondary lag right.

    Join the ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about how you can join the ISA Mentor Program.

    By having the much more complete model in a virtual plant, the true dynamic behavior of the system can be investigated and the best control system performance achieved by exploring, discovering, prototyping, testing, tuning, justifying, deploying, commissioning, maintaining and continuously improving, as described in the Control magazine feature article Virtual Plant Virtuosity.

     

    Figure 1: Virtual Plant that includes Automation System Dynamics and Digital Twin Controller

     

    Model predictive control is much better at ensuring you have the actual total dynamics including dead time, lags and lead times at a particular operating point. However, the models do not include the effect of backlash-stiction or actuator and positioner design on valve response time and consequentially on total loop dead time because by design the steps are made several times larger than the deadband and resolution or sensitivity limits of the control valve. Also, the models identified are for a particular operating point and normal operation. To cover different modes of operation and production rates, multiple models must be used requiring logic for a smooth transition or recently developed adaptive capabilities. I see an opportunity to use the results from the identification software used by MPC to provide a more accurate dead time, lag time and lead time by inserting these in blocks on the measurement of the process variable in first principle models. The identification software would be run for different operating points and operating conditions enabling the addition of supplemental dynamics in the first principle models. This addresses the fundamental deficiency of dead times, lag times and lead times being too small in first principle models.

    Statistical models are great at identifying unsuspected relationships, disturbances and variability in the process and measurements. However, these are correlations and not necessarily cause and effect. Also, continuous processes require dynamic compensation of each process input so that it matches the dynamic response timewise of each process output being studied. This is often not stated in the literature and is a formidable task. Some methods propose using a dead time on the input but for large time constants, the dynamic response of the predicted output is in error during a transient. These models are more designed for steady state operation but this is often an ideal situation not realized due to disturbances originating from the control system due to interactions, resonance, tuning, and limit cycles from stiction as discussed in the Control Talk Blog The most disturbing disturbances are self-inflicted. Batch processes do not require dynamic compensation of inputs making data analytics much more useful in predicting batch end points.

    I think there is a synergy to be gained by using MPC to find missing dynamics and statistical process control to help track down missing disturbances and relationships that are subsequently added to the first principle models. Recent advances in MPC capability (e.g., Aspen DMC3) to automatically identify changes in process gain, dead time and time constant including the ability to compute and update them online based on first principals has opened the door to increased benefits from the using MPC to improve first principle models and vice versa. Multivariable control and optimization where there are significant interactions and multiple controlled, manipulated and constraint variables are best handled by MPC. The exception is very fast systems where the PID controller is directly manipulating control valves or variable frequency drives for pressure control. Batch end point prediction might also be better implemented by data analytics. However, in all cases the first principle model should be accordingly improved and used to test the actual configuration and implementation of the MPC and analytics and to provide training of operators extended to all engineers and technicians supporting plant operation.

    I would think for research and development, the ability to gain a deeper and wider understanding of different process relationships for different operating conditions would be extremely important. This knowledge can lead to process improvements and to better equipment and control system design. For pH and biological control systems, this capability is essential.

    For a greater perspective on the capability of various modeling and control methodologies, see the ISA Mentor Program post with questions by protégé Danaca Jordan and answers by Hunter Vegas and I: What are the New Technologies and Approaches for Batch and Continuous Control?

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant) and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 23 Oct 2018

    Many Objectives, Many Worlds of Process Control

    The post, Many Objectives, Many Worlds of Process Control first appeared on ControlGlobal.com's Control Talk blog.

    In many publications on process control, the common metric you see is integrated absolute error for a step disturbance on the process output. In many tests for tuning, setpoint changes are made and the most important criteria becomes overshoot of setpoint. Increasingly, oscillations of any type are looked at as inherently bad. What is really important varies because of the different loops and types of processes. Here we seek to open minds and develop a better understanding of what is important.

    Many Objectives

    • Minimum PV peak error in load response to prevent:

    –        Compressor surge, SIS activation, relief activation, undesirable reactions, poor cell health

    • Minimum PV integrated error in load or setpoint response to minimize:

    –        total amount of off-spec product to enable closer operation to optimum setpoint

    • Minimum PV overshoot of SP in setpoint response to prevent:

    –        Compressor surge, SIS activation, relief activation, undesirable reactions, poor cell health

    • Minimum Out overshoot of FRV* in setpoint response to prevent:

    –        Interaction with heat integration and recycle loops in hydrocarbon gas unit operations

    • Minimum PV time to reach SP in setpoint response to minimize:

    –        Batch cycle time, startup time, transition time to new products and operating rates

    • Minimum split range point crossings to prevent:

    –        Wasted energy-reactants-reagents, poor cell health (high osmotic pressure)

    • Maximum absorption of variability in level control (e.g. surge tank) to prevent:

    –        Passing of changes in input flows to output flows upsetting downstream unit ops

    • Optimum transfer of variability from controlled variable to manipulated variable to prevent:

    –        Resonance, interaction and propagation of disturbances to other loops

    * FRV is the Final Resting Value of PID output. Overshoot of FRV is necessary for setpoint and load response for integrating and runaway processes. However for self-regulating processes not involving highly mixed vessels (e.g., heat exchangers and plug flow reactors),  aggressive action in terms of PID output can upset other loops and unit operations that are affected by the flow manipulated by the PID. Not recognized in the literature is that external-reset feedback of the manipulated flow enables setpoint rate limits to smooth out changes in manipulated flows without affecting the PID tuning.

    Many Worlds

    • Hydrocarbon processes and other gas unit operations with plug flow, heat integration & recycle streams (e.g. crackers, furnaces, reformers)

    –        Fast self-regulating responses, interactions and complex secondary responses with sensitivity to SP and FRV overshoot, split range crossings and utility interactions.

    • Chemical batch and continuous processes with vessels and columns

    –        Important loops tend to have slow near or true integrating and runaway responses with minimizing peak and integrated errors and rise time as key objectives.

    • Utility systems (e.g., boilers, steam headers, chillers, compressors)

    –        Important loops tend to have fast near or true integrating responses with minimizing peak and integrated errors and interactions as key objectives.

    • Pulp, paper, food and polymer inline, extrusion and sheet processes

    –        Fast self-regulating responses and interactions with propagation of variability into product (little to no attenuation of oscillations by back mixed volumes) with extreme sensitive to variability and resonance. Loops (particularly for sheets) can be dead time dominant due to transportation delays unless there are heat transfer lags.

    • Biological vessels (e.g., fermenters and bioreactors)

    –        Most important loops tend have slow near or true integrating responses with extreme sensitivity to SP and FRV overshoot, split range crossings and utility interactions. Load disturbances originating from cells are incredibly slow and therefore not an issue.

    A critical insight is that most disturbances are on the process input not the process output and are not step changes. The fastest disturbances are generally flow or liquid pressure but even these have an 86% response time of at least several seconds because of the 86% response time of valves and the tuning of PID controllers. The fastest and most disruptive disturbances are often manual actions by an operator or setpoint changes by a batch sequence. Setpoint rate limits and a 2 Degrees of Freedom (2DOF) PID structure with Beta and Gamma approaching zero can eliminate much of the disruption from setpoint changes by slowing down changes in the PID output from proportional and derivative action. A disturbance to a loop can be considered to be fast if it has a 86% response time less than the loop deadtime.

    If you would like to hear more on this, checkout the ISA Mentor Program Webinar Recording: PID Options and Solutions Part1

    If you want to be able to explain this to young engineers, check out the dictionary for translation of slang terms in the Control Talk Column “Hands-on Labs build real skills.”

    • 15 Oct 2018

    How to Get Rid of Level Oscillations in Industrial Processes

    The post How to Get Rid of Level Oscillations in Industrial Processes first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the U.S. This question comes from Luis Navas.

    Luis Navas is an ISA Certified Automation Professional and electronic engineer with more than 11 years of experience in process control systems, industrial instrumentation and safety instrumented systems. Luis’ questions on evaporator control are important to improve evaporator concentration control and minimize steam consumption

    Luis Navas’ Questions

    For an MPC application I need to build a smoothed moving mean from a batch level to use as a controlled variable for my MPC, so the simple moving average is done as depicted below. However, I need to smooth the signal, due there is some signal ripple still. I tried with a low-pass filter achieving some improvement as seen in Figure 1. But perhaps you know a better way to do it, or I simply need to increase the filter time.

    Figure 1: Old Level Oscillations (blue: actual level and green: level with simple moving mean followed by simple moving mean + first order filter)

    Greg McMillan’s Initial Answer

    I use rate limiting when a ripple is significantly faster than a true change in the process variable. The velocity limit would be the maximum possible rate of change of the level. The velocity limit should be turned off when maintenance is being done and possibly during startup or shutdown. The standard velocity limit block should offer this option. A properl Save & Exit y set velocity limit introduces no measurement lag. A level system (any integrator) is very sensitive to a lag anywhere.

    If the oscillation stops when the controller is in manual, the oscillation could be from backlash or stiction. In your case, the controller appears to be in auto with a slow rolling oscillation possibly due to a PID reset time being too small.

    I did a Control Talk Blog that discusses good signal filtering tips from various experts besides my intelligent velocity limit.

    Mark Darby’s Initial Answer

    In many cases, I’ve seen signals overly filtered. Often, if the filtered signal looks good to your eye, it’s too much filtering. As Michel Ruel states: If period is known, moving average (sum of most recent N values divided by N) will nearly completely remove a uniform periodic cycle. So the issue is how much lag is introduced. Depending on the MPC, one may be able to specify variable CV weights as a function of the magnitude error, which will decrease the amount of MV movement when the CV weight is low; or the level signal could be brought in as a CV twice with different tuning or filtering applied to each.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    Greg McMillan’s Follow-Up Answer

    Since the oscillation is uniform in period and amplitude, the moving average as described my Michel Ruel is best as a starting point. Any subsequent noise from non-uniformity can be removed by an additional filter but nearly all of this filter time becomes equivalent dead time in near and true integrating processes. You need to be careful that the reset time is not too small as you decrease the controller gain either due to filtering or to absorb variability. The product of PID gain and reset time should be greater than twice the inverse of the integrating process gain (1/sec) to prevent the slow rolling oscillations that decay gradually. Slide 29 of the ISA webinar on PID options and solutions give the equations for the window of allowable PID gains. Slide 15 shows how to estimate the attenuation of an oscillation by a filter. The webinar presentation and discussion is in the ISA Mentor Program post How to optimize PID controller settings.

    If you need to minimize dead time introduced by filtering, you could develop a smarter statistical filter such as cumulative sum of measured values (CUSUM). For an excellent review of how to remove unwanted data signal components, see the InTech magazine article Data filtering in process automation systems.

    Mark Darby’s Follow-Up Answer

    My experience is that most times a cycle in a disturbance flow is already causing cycling in other variables (due to the multivariable nature of the process).  And advanced control, including MPC, will not significantly improve the situation and may make it worse.  So it is best to fix the cycle before proceeding with advanced control.  Making a measured cyclic disturbance a feedforward to MPC likely won’t help much.  MPC normally assumes the current value of the feedforward variables stays constant over the prediction horizon. What you’d want is to have the future prediction include the cycle.  Unfortunately this is not easily done with the MPC packages today.

    Often, levels are controlled by a PID loop, not in the MPC.  The exception can be if there are multiple MVs that must be used to control the level (e.g., multiple outlet flows), or the manipulated flow is useful for alleviating a constraint (see the handbook).  Another exception is if there is significant dead time between the flow and the level.

    Luis Navas’ Follow-up Response

    Thank you for the support. I think the ISA Mentor Program resources are a truly elite support team, by the way, I have already read the blogs about signal filtering.

    My comments and clarifications:

    1. The signal corresponds to a tank level in a batch process, due that it has an oscillating behavior (without noise).
    2. The downstream process is continuous, (evaporator) and the idea is control the Feed tank level with MPC (using the moving average), through evaporator flow input. The feed tank level is critical for the evaporator works fine.
    3. I have applied the Michel Ruel statement: If period is known, moving average (sum of most recent N values divided by N) will nearly completely remove a periodic cycle. Now the moving average is better as seen in Figure 2.

     

    Figure 2: New Level Oscillations (blue: actual level and green: level with Ruel moving average)

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 15 Oct 2018

    How to Get Rid of Level Oscillations in Industrial Processes

    The post How to Get Rid of Level Oscillations in Industrial Processes first appeared on the ISA Interchange blog site.

    The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

    Luis Navas is an ISA Certified Automation Professional and electronic engineer with more than 11 years of experience in process control systems, industrial instrumentation and safety instrumented systems. Luis’ questions on effectively reducing evaporator level oscillations from an upstream batch operation so that the level controller can see the true level trajectory represent a widespread concern in chemical plants where the front end for conversion has batch operations and back end for separation has continuous operations.

     Luis Navas’ Questions

    For the MPC application I need to build a smoothed moving mean from a batch level to use as a controlled variable for my MPC, so the simple moving average is done as depicted below. However, I need to smooth the signal, (due there is some signal ripple still), I tried with a low-pass filter achieving some improvement as seen in Figure 1. But perhaps you know a better way to do it, or I simply need to increase the filter time.

     

    Figure 1: Old Level Oscillations (blue: actual level and green: level with simple moving mean followed by simple moving mean + first order filter)

     

    Greg McMillan’s Initial Answer

    I use rate limiting when a ripple is significantly faster than a true change in the process variable. The velocity limit would be the maximum possible rate of change of the level. The velocity limit should be turned off when maintenance is being done and possibly during startup or shutdown. The standard velocity limit block should offer this option. A properly set velocity limit introduces no measurement lag. A level system (any integrator) is very sensitive to a lag anywhere.

    If the oscillation stops when the controller is in manual, the oscillation could be from backlash or stiction. In your case, the controller appears to be in auto with a slow rolling oscillation possibly due to a PID reset time being too small.

    I did a Control Talk Blog that discusses What are good signal filtering tips from various experts besides my intelligent velocity limit.

    Mark Darby’s Initial Answer

    In many cases, I’ve seen signals overly filtered.  Often, if the filtered signal looks good to your eye, it’s too much filtering. As Michel Ruel states: If period is known, moving average (sum of most recent N values divided by N) will nearly completely remove a uniform periodic cycle. So the issue is how much lag is introduced. Depending on the MPC, one may be able to specify variable CV weights as a function of the magnitude error, which will decrease the amount of MV movement when the CV weight is low; or the level signal could be brought in as a CV twice with different tuning or filtering applied to each.

    Join the ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about how you can join the ISA Mentor Program.

    Greg McMillan’s Follow-up Answer

    Since the oscillation is uniform in period and amplitude, the moving average as described my Michel Ruel is best as a starting point. Any subsequent noise from nonuniformity can be removed by an additional filter but nearly all of this filter time becomes equivalent dead time in near and true integrating processes. You need to be careful that the reset time is not too small as you decrease the controller gain either due to filtering or to absorb variability. The product of PID gain and reset time should be greater than twice the inverse of the integrating process gain (1/sec) to prevent the slow rolling oscillations that decay gradually. Slide 29 of the ISA WebEx on PID Options and Solutions give the equations for the window of allowable PID gains. Slide 15 shows how to estimate the attenuation of an oscillation by a filter. The WebEx presentation and discussion is in the ISA Mentor Program post How to optimize PID controller settings.

    If you need to minimize dead time introduced by filtering, you could develop a smarter statistical filter such as cumulative sum of measured values (CUSUM). For an excellent review of how to remove unwanted data signal components, see the InTech magazine article Data filtering in process automation systems.

    Mark Darby’s Follow-up Answer

    My experience is that most times a cycle in a disturbance flow is already causing cycling in other variables (due to the multivariable nature of the process).  And advanced control, including MPC, will not significantly improve the situation and may make it worse.  So it is best to fix the cycle before proceeding with advanced control.  Making a measured cyclic disturbance a feedforward to MPC likely won’t help much.  MPC normally assumes the current value of the feedforward variables stays constant over the prediction horizon. What you’d want is to have the future prediction include the cycle.  Unfortunately this is not easily done with the MPC packages today.

    Often, levels are controlled by a PID loop, not in the MPC.  The exception can be if there are multiple MVs that must be used to control the level (e.g., multiple outlet flows), or the manipulated flow is useful for alleviating a constraint (see the handbook).  Another exception is if there is significant dead time between the flow and the level.

    Luis Navas’ Follow-up Response

    Thank you for the support. I think the ISA Mentor Program resources are a truly Elite support team, by the way,I have already read the blogs about signal filtering.

    My comments and clarifications:

    1. The signal corresponds to a tank level in a batch process, due that it has an oscillating behavior, (without noise).
    2. The downstream process is continuous, (evaporator) and the idea is control the Feed tank level with MPC, (using the Moving average), through evaporator flow input. The feed tank level is critical for the evaporator works fine.
    3. I have applied the “Michel Ruel statement: If period is known, moving average (sum of most recent N values divided by N) will nearly completely remove a periodic cycle”, and now the moving average is better as seen in Figure 2.

     

    Figure 2: New Level Oscillations (blue: actual level and green: level with Ruel moving average)

    Additional Mentor Program Resources

    See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant) and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 1 Oct 2018

    Webinar Recording: Loop Tuning and Optimization

    The post Webinar Recording: Loop Tuning and Optimization first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    In this ISA Mentor Program presentation, Michel Ruel, a process control expert and consultant, provides insight and guidance as to the importance of optimization and how to achieve it through better PID control.

    ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about the ISA Mentor Program.

    About the Presenter
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg
    LinkedIn

    • 1 Oct 2018

    Webinar Recording: Loop Tuning and Optimization

    The post Webinar Recording: Loop Tuning and Optimization first appeared on the ISA Interchange blog site.

    This educational ISA webinar was presented by Greg McMillan in conjunction with the ISA Mentor Program. Greg is an industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical).

    In this ISA Mentor Program presentation, Michel Ruel, a process control expert and consultant, provides insight and guidance as to the importance of optimization and how to achieve it through better PID control.

    Join the ISA Mentor Program

    The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career.  Click this link to learn more about how you can join the ISA Mentor Program.

    About the Author
    Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

    Connect with Greg:

    LinkedIn