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 Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Danaca Jordan.
Danaca Jordan, CAP, is a Digital Manufacturing Center of Excellence engineer at Eastman Chemical Company.

Danaca Jordan’s Question

What is the effect of the pump on control valves at lower rates?

Greg McMillan’s Answer:

If the pump is only supplying the subject valves, at the lower rates the pump head developed increases causing the control valve to ride closer to its seat where the friction is greatest. Continual cycling at low rates is most likely caused by increased stick-slip most noticeable for valve positions less than 20 percent.

At low rates (low system frictional losses), you are also subject to a greater effect of changes in source and destination suction heads as seen in the intersection of system curve with the pump curve. Adaptive control can compensate for changes in process gain from installed characteristics or the different effect of split range valves. For a big-small valve, valve position control (VPC) is preferable to split range control if the VPC is fast enough by the use of feedforward.

See the posts How to Succeed – Part 5 and How to Succeed – Part 6 for how to get the control valve that will have the least stick-slip and the greatest linearity and rangeability.

ISA Mentor Program Posts & Webinars

Did you find this information of value? Want more? Click this link to view other ISA Mentor Program blog posts, technical discussions and educational webinars.

For Additional Reference:

Baumann, Hans D., Fluid Mechanics of Control Valves.
McMillan, Gregory K., and Vegas, Hunter, 101 Tips for a Successful Automation Career.

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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