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.
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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.).