The following discussion is part of an occasional series, “Ask the Automation Pros,” authored by Greg McMillan, industry consultant, author of numerous process control books and 2010 ISA Life Achievement Award recipient. Program administrators will collect submitted questions and solicit responses from automation professionals. Past Q&A videos are available on the ISA YouTube channel; you can view the playlist here. You can read posts from this series here.

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Greg McMillan’s Question

What is the best inherent flow characteristic for controllability, considering the effect of valve resolution and lost motion, process gain, piping system design, system pressure drops, fouling, flashing and cavitation?

Michael Taube’s Thoughts

A control valve is but one part of a much larger system, which includes the pump/compressor (if there is one), piping, other “users” of the process stream, and other equipment (i.e., heat exchangers). Thus, choosing a valve based on its published characteristics alone is a very limited, even illusory way of achieving (or predicting) the desired behavior. Some process simulation vendors have recognized this and provide the means to model piping networks, which can be quite useful in assessing different control valves over the entire operating range (and conditions) to which it is subjected. This should really be a part of every process and control engineer’s “toolbox,” even if only implemented in a spreadsheet!

The important issue to recognize in such an analysis is that the process is rarely at the “design point”, thus one must recognize the changes in up/downstream pressure, frictional losses and control valve characteristics over the entire range — from zero flow to “maximum” flow. This is, strictly speaking, a steady-state analysis, which does not require knowledge or recognition of dynamic behavior. Hence, the emphasis of being a process and control engineer’s tool.

Michel Ruel’s Thoughts

When selecting a process control valve, I focus primarily on overall process gain and linearity. While I use software tools provided by valve manufacturers, the key is accurately estimating the pressure drops at different flow rates, specifically at minimum, normal, and maximum flows. The valve’s inherent characteristic should be chosen to ensure the installed valve operates as linearly as possible.

As a rule of thumb, I aim for a maximum overall process gain of 3, with the ratio of maximum to minimum overall process gain kept below 3.

Some engineers may size the valve wrongly to ensure operation within 30% to 80% of its capacity, without considering factors like linearity and process gain. In such cases, an equal percentage valve characteristic can be used to minimize the impact of these errors. Since process characteristics have not been taken into account, this control loop will not perform.

Finally, it’s important to account for resolution, deadband, backlash and other potential issues to ensure the valve meets the expected performance standards.

Mark Darby’s Thoughts

An equal percentage valve is often recommended for processes in which the process gain is inversely proportional to flow, which also applies to a load input (a throughput variable). This means the process gain decreases as flow increases. A typical example is temperature control in a plug flow volume where the temperature controller is manipulating the flow to the volume for a load upset, which is the case for jackets, coils and heat exchangers. If the controller outputs to an equal percentage valve, its valve characteristic (proportional to flow) will help offset the varying process gain, even if the pressure drop across the control valve is constant (in which, normally, one expects to use a linear valve). A modified equal percentage valve may help near the closed position.

But this result would not hold if the TIC outputs to an FIC in a cascade. Use of an equal percentage valve here could introduce a nonlinearity. For a flow loop cascade, an equal percentage valve would make sense if the pressure drop across the control valve dropped significantly over the required flow range.

Additional Thoughts from Russ Rhinehart and Greg McMillan

You can find Russ Rhinehart’s response and additional thoughts from Greg McMillan in the related PDF resource. Additional material and equations by Greg McMillan can be found in Annex A of ISA-TR75.25.02-2024. Rhinehart’s experience has been in the academic side of understanding phenomena, so he shares fundamentals that may help some folks understand the principles that underlie the best practices mentioned in other responses. McMillan alerts users to the problem stemming from conventional thinking about valve rangeability, provides additional advice from his personal experience and explains the usefulness of ISA-TR75.25.02-2024.

About the Authors

ISA Fellow Gregory K. McMillan (https://www.linkedin.com/in/greg-mcmillan-5b256514/)

retired as a Senior Fellow from Solutia Inc. in 2002 and retired as a senior principal software engineer in Emerson Process Systems and Solutions simulation R&D in 2023. McMillan is the author of more than 200 articles and papers, 100 Q&A posts, 80 blogs, 200 columns and 20 books. McMillan received the ISA Lifetime Achievement Award in 2010, the ISA Mentor Award in 2020 and the ISA Standards Achievement Award in 2023. He was also one of the first inductees into the Control Global Process Automation Hall of Fame in 2001.

Michael Taube (https://www.linkedin.com/in/michaeltaube/) is a principal consultant at S&D Consulting, Inc. Serving the greater process industries as an independent consultant since 2002, he pursues his passion to make things better than they were yesterday by identifying the problems no one else sees or is willing to admit to and willingly “gets his hands dirty” to solve the problems no one else can. Due to the continued occurrence of individual injuries and fatalities as well as large-scale industrial incidents, he collaborates with operational excellence and safety culture experts to promote a real and lasting cultural shift in the process industries to help make zero incidents a reality. He graduated from Texas A&M University in 1988 with a bachelor of science degree in chemical engineering.

ISA Fellow Michel Ruel is a recognized expert in process control and control performance monitoring. Now retired, he led a team that implemented innovative and highly effective control strategies across a wide range of industries, including mining and metals, aerospace, energy, pulp and paper and petrochemicals. An accomplished author of numerous books and publications, Ruel is also a software designer specializing in instrumentation and process control. He is the founding president of Top Control Inc. and has contributed to projects in multiple countries. In addition, Ruel is a sought-after lecturer for various professional associations.

Mark Darby (www.linkedin.com/in/mark-darby-5210921) is an independent consultant with CMiD Solutions. He provides process control-related services to the petrochemical, refining, and mid/upstream industries in the design and implementation of advanced regulatory and multivariable predictive controls. Mark is an ISA Senior Member. He served on the TR5.9 committee that produced the PID technical report and has presented at ISA technical conferences. Mark frequently publishes and presents on topics related to process control and real-time optimization. He is a contributing author to the McGraw-Hill Process/Industrial Instruments and Controls Handbook, Sixth Edition.

ISA Fellow R. Russell Rhinehart has experience in both the process industry (13 years) and academe (31 years). He is a fellow of ISA and AIChE, and a CONTROL Automation Hall of Fame inductee. He served as president of the American Automatic Control Council and editor-in-chief of ISA Transactions. Now "retired," Russ is working to disseminate engineering techniques with his website (www.r3eda.com), short courses, books and monthly articles. His 1968 B.S. in ChE and M.S. in NucE are both from the University of Maryland. His 1985 Ph.D. in ChE is from North Carolina State University.