How to Avoid Using Multivariable Flow Transmitters

The post How to Avoid Using Multivariable Flow Transmitters first appeared on the ISA Interchange blog site.

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 Jeff Downen.

Jeff Downen is an I&C commissioning engineer with cross-training in DCS and high voltage electrical testing. His expertise is in start-up and commissioning of natural gas, combined cycle, power plants. 

Jeff Downen’s Question

Our multivariable flow transmitters on new construction sites fail a lot. If the transmitter loses the RTD, the whole 4-20 loop goes bad quality along with the HART variables. I like the three devices being separate and their signals joined in the DCS logic much more.  I understand that it is more expensive. I want to see if there was any other reasoning behind it on the engineering side and how I can help get a better up front design.

How can we avoid the increasing use of multivariable flow transmitters as an industry standard despite a significant loss in reliability, accuracy, and diagnostic and computational capability from not having individual separate pressure, temperature and flow sensors and transmitters?

Greg Brietzke’s Answer

I like Jeff’s question on multivariable flow transmitters, as it would be relevant to control engineers, maintenance/reliability engineers, as well as maintenance personnel. What is the application? What are the accuracy requirements? Can you bring the individual variables back to the DCS/PLC through additional variable assignment? Would the increased cost of infrastructure justify the increased expense of a true mass flowmeter? This could be addressed from so many different viewpoints it could be a great discussion topic.

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Greg McMillan’s Answer

I suggest you explain to plant and project personnel the advantages of separate measurements and true mass flowmeters. Separate flow, temperature and pressure measurements offer better diagnostics, reliability, sensors, and installation location that is particularly important for temperature (e.g., RTD in tapered thermowell with tip centered in pipe with good velocity profile). They can provide faster and perhaps more accurate and maintainable measurements that could be used for personalized performance monitoring calculations and safety instrumented systems.

Coriolis meters provide the only true mass flow measurements offering an incredibly accurate density measurement as well. Most people don’t realize that pressure and temperature compensation of volumetric flow meters to get a mass flow measurement only works if the concentration is constant and known. The Coriolis mass flow is not affected by component concentrations or physical properties in the same phase. Density can provide an inferential measurement of concentration for a two component process fluid. The Coriolis meter accuracy and rangeability is the best by far as noted in the Control Talk column Knowing the best is the best.

David De Sousa’s Answer

Using dedicated and separated measurements also allows for the use of hybrid virtual flowmeters in complex process applications where, for example, the technology for inline multiphase flow metering is not yet mature enough, or where physical units will greatly increase the cost of the associated facilities.

With the digital transformation initiatives associated with Industry 4.0, the use of distributed instrumentation, data-driven learning algorithms, and physical flow models, are being tested and explored more and more in the process industries, especially in upstream oil & gas wellsite applications.

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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