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 Hiten Dalal.

Hiten Dalal, PE, PMP, is senior automation engineer for products pipeline at Kinder Morgan, Inc. Hiten has extensive experience in pipeline pressure and flow control.

Hiten Dalal’s First Question

I have been trying to get a handle on small ripples in one of the pipelines by using a rule of thumb to successively reduce proportional action by 20 percent and integral action by 50 percent.  Using the same rule, I could stabilize the ripples on Friday. On Sunday, the product changed in the pipeline and with that back came those 4 percent ripples. There is one control valve that impacts line pressure. I could stretch ripples a bit but could not eliminate them. Output going to zero is natural scheduled shutdown of pipeline. I know it is a lot of information that I am providing but perhaps you can glance through and pinpoint something that stands out. I am learning since I started tuning the control valve that it is product sensitive as well.   

Greg McMillan’s Answer

I meant no integral in the valve positioner that for Fisher is called a digital valve controller (DVC). You should use integral action in most process controllers (e.g., flow and pressure). Integral action in the process controllers is essential for the PID control of many processes. So far as tuning the process controller for pipeline control, the integral time also known as reset time (seconds per repeat) should generally be greater than four times the deadtime for an ISA Standard Form.

You must be careful about what PID form, structure and tuning setting units are being used. If the integral setting is an integral gain, such as what is used in the "parallel" PID form depicted in textbooks and used in some PLCs, the integral setting may not just be a simple factor of the deadtime (e.g., four times deadtime) but will also depend upon other dynamics. Also, some integral settings are in repeats per minute instead of seconds.

Please make sure you extensively test any tuning settings by making small changes in the setpoint with the controller in automatic or in the controller output by momentarily putting the controller in manual. There should be little to no oscillation. The tests should be done at different valve positions particularly if the valve installed flow characteristic is nonlinear. Oscillations may be most prone near the shutoff positioner where stiction is greatest from seat/seal friction.

If there is interaction between loops, the least important loop must be made slower or decoupling used by means of a feedforward signal. If you are going to do some optimization via a controller that seeks to minimize or maximize a valve position, the proportional gain divided by the reset time for this controller doing optimization must be an order of magnitude smaller than process controller to prevent interaction. These PID controllers used for optimizing a valve position are called "valve position controllers" (VPC).

I hesitated to mention this to avoid confusion because these are not valve positioners and are only used for optimization. Also, nonlinear or notch gains and directional move suppression via external reset feedback are used to keep the VPC from responding too much or too little so the process controller does not oscillate or run out of valve. Many newer smart positioners have added integral action to positioners in the last two decades. In some cases, integral action is enabled as the default. This prompted me to write the Control Talk blog post Getting the Most Out of Positioners. This blog does not address setting integral action in process controllers (e.g., flow and pressure controllers).

Hiten Dalal’s Second Question

When you say no integral action, do you mean in valve positioner or in controller? I don’t think our positioner has any PID setup. Only PID action is in controller. Since it is liquid pressure and flow, we use P&I. Are you suggesting we use only P action in my controller?

Greg McMillan’s Answer

I meant no integral in the valve positioner that for Fisher is called a digital valve controller (DVC). You should use integral action in most process controllers (e.g., flow and pressure). Integral action in the process controllers is essential for the PID control of many processes. So far as tuning the process controller for pipeline control, the integral time also known as reset time (seconds per repeat) should generally be greater than four times the deadtime for an ISA Standard Form.

You must be careful about what PID form, structure and tuning setting units are being used. If the integral setting is an integral gain, such as what is used in the “parallel” PID form depicted in textbooks and used in some PLCs, the integral setting may not just be a simple factor of the deadtime (e.g., four times deadtime) but will also depend upon other dynamics. Also, some integral settings are in repeats per minute instead of seconds.

Please make sure you extensively test any tuning settings by making small changes in the setpoint with the controller in automatic or in the controller output by momentarily putting the controller in manual. There should be little to no oscillation. The tests should be done at different valve positions particularly if the valve installed flow characteristic is nonlinear. Oscillations may be most prone near the shutoff positioner where stiction is greatest from seat/seal friction.

If there is interaction between loops, the least important loop must be made slower or decoupling used by means of a feedforward signal. If you are going to do some optimization via a controller that seeks to minimize or maximize a valve position, the proportional gain divided by the reset time for this controller doing optimization must be an order of magnitude smaller than process controller to prevent interaction. These PID controllers used for optimizing a valve position are called “valve position controllers” (VPC).

I hesitated to mention this to avoid confusion because these are not valve positioners and are only used for optimization. Also, nonlinear or notch gains and directional move suppression via external reset feedback are used to keep the VPC from responding too much or too little so the process controller does not oscillate or run out of valve. Many newer smart positioners have added integral action to positioners in the last two decades. In some cases, integral action is enabled as the default. This prompted me to write the Control Talkblog post Getting the Most Out of Positioners. This blog does not address setting integral action in process controllers (e.g., flow and pressure controllers).

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Hiten Dalal’s Third Question

Do you teach a control valve tuning class? Is there a specific method you recommend for a pipeline control valve?

Greg McMillan’s Answer

I do not offer a class on tuning positioners. Supplier courses on tuning positioners are good but you will need to insist on turning off integral action. You can have them talk to me if they disagree. In general you should make sure you do not use integral action and that you use the highest valve positioner gain that does not cause oscillation since for pipeline flow and pressure control, oscillations are not filtered. If you have an Emerson Digital Valve Controller (DVC),

I recommend "travel control" with no integral action and with the highest gain that still gives an overdamped response. The valve must be a true throttling valve and not an on-off valve posing as a throttling valve as discussed in the Control Talk blog Getting the Most out of Valve Positioners. Note that in this blog we are going for a more aggressive response than what you need. Because of the lack of a significant process time constant in a pipeline, you need a smooth valve response. In the blog, the valve positioner gain is described to be set high enough to cause a slight overshoot and oscillation that quickly settles out. 

Oscillations in the valve response are useful to get a faster response for vessels and columns since there is a a large process time constant to filter out oscillations. You want to still use a high gain and no integral action in the positioner but seek an overdamped (non-oscillatory) response of valve position.

Hiten Dalal’s Follow-up Reply

I have bought Tuning and Control Loop Performance Fourth Edition. I reference tables from there for suggested PID values. I have removed derivative from several pressure and flow loops and observed them to be equally efficient. In the process of tuning I have learned that operations installations have impact on loop tuning. I have made the following types of corrections,

1. As installed, the logic had the PID getting initiated as soon as block valve No. 1 was fully opened but block valve #2 was getting commanded to open after #1 causing PID output to ramp off to high output limit since the control valve was not seeing full flow. We solved this by setting temporary upper clamp in PID output at safe limit to avoid overshoot until block valve #2 was fully opened.
2. Transmitter range was high and margin of error was not acceptable by operations. Re-ranged transmitter to suitable range and brought error within acceptable margin.
3. EIM Controls Electric and REXA electrohydraulic actuators have a limit on number of actuations. I added an acceptable dead band to reduce number of actuations.

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.

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