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 Mohd Zhafran A. Hamid.

Mohd Zhafran A. Hamid is a senior instrument engineer from Malaysia working in an EPC company, Toyo Engineering Corporation. He has worked in the field of control and instrumentation for about 10 years mostly in both engineering design and involvement at site/field.

Mohd Zhafran’s First Question

If you have selected a control valve whose installed flow characteristics significantly deviates from linear (either by mistake or forced to select due to certain circumstances), what is a practical way in the field after installation to linearize the installed flow characteristic?

Greg McMillan’s Answer

You need a sensitive flow measurement to identify the installed flow characteristic online. If you have a flow measurement and make changes in the manual controller output five times larger than dead band or resolution limit spaced out by a time interval greater than the response time, the slope of the installed flow characteristic is the change in per cent flow divided by the change in per cent signal. You need at least 20 points identified on the installed flow characteristic.

A signal characterizer is then inserted on the controller output to convert the flow in percent of scale to percent signal to get a piecewise linear fit that would linearize the characteristic so far as the controller is concerned. The controller output and linearized signal to the valve should be displayed. This linearization can be done in a positioner, but I prefer it being done in the DCS or PLC for better visibility and maintainability. For much more on signal characterizers see my Control Talk blog Unexpected benefits of signal characterizers.

Mohd Zhafran’s Second Question

I recently read the addendum “Valve Response Truth or Consequences” in Greg’s article How to specify valves and positioners that do not compromise control. I am curious for fast loop whereby the control valve is used with volume booster but without positioner, how come you can move the stem/shaft by hand only even though the valve size is big. Would you mind sharing the overall schematic? Also, would you also share the schematic of using positioner with booster and booster bypass?

Greg McMillan’s Answer

Positive feedback from a very sensitive booster outlet port is greatly assisting attempts to move the shaft either manually or due to fluid forces on a butterfly disk as described in item five of my Control Talk blog Missed opportunities in process control - Part 6. There is a schematic of the proper installation in slide 18 of the ISA Mentor Program webinar How to Get the Most out of Control Valves. I don’t have a schematic of the wrong thing to do where the volume booster input is connected to current to pneumatic transducer (I/P) output.

For new high pressure diaphragm actuators or boosters with lower outlet port sensitivity, this may not happen since diaphragm flexure and consequential change in pressure from change in actuator volume may be less than booster outlet port sensitivity but it is not worth the risk in my book. The rule positioners should not be used on fast loops is mostly bogus as explained in my point 4 in the same Control Talk blog.  If you need a response time faster than 0.5 seconds, you should use a variable frequency drive with a pulse width modulated inverter.

Mohd Zhafran’s Third Question

Greg highlighted the importance to specify valve gain requirement. Is there any publicly available modeling software that we design engineer can utilize to perform valve gain analysis? So far, I have encountered only one valve manufacturer that provides control valve sizing software (publicly available) with feature of valve gain graph. This manufacturer calculates process model based on the principle that the pressure losses in a piping system are approximately equal to flow squared.

Greg McMillan’s Answer

The Control Talk column Why and how to establish installed flow characteristic describes how one practitioner uses Excel to compute the installed flow characteristic. The analysis of all the friction losses in a piping system can be quite complicated because of the effect of process fluid properties and fouling determined by process conditions and operating history and the piping system including fittings, elbows, inline equipment (e.g., heat exchangers and filters), and valves.

A dynamic model in a Digital Twin that includes system pressure drops and the effect of fouling and the ability to enter the inherent flow characteristic perhaps by a piecewise linear fit can show how the valve gain changes for more complex and realistic scenarios. Ideally, there would be flow and pressure measurements to show key pressure drops particularly where fouling is a concern so that resistance coefficients can be back calculated. 

The fouling of heat transfer surfaces can be detected by an increase in the difference needed between the process and utility temperature to compensate for the decrease in heat transfer coefficient. A slow ramp of the valve signal followed by a slow ramp in a flow measurement could reveal the installed flow characteristic by a plot of flow ramp versus the signal ramp assuming there are no pressure disturbances and flow measurement has sufficient signal to noise ratio and rangeability.

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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