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 Muhammad Khalifah in Saudi Arabia.

Muhammad Khalifah’s Question

We are planning to install a flowmeter in 48-inch line to an oil storage tank. I thought of using an annubar flowmeter instead of an orifice for this application. What do you think is the best fit for our application in terms of the following?

  • Permanent pressure loss (minimum pressure loss is required)
  • Maintenance and construction cost noting size of the pipe is 48 inches
  • Rangeability (1:4)
  • Accuracy (not a big concern for our application we can tolerate some)

Greg McMillan’s Answer

As you probably expected, the permanent pressure loss is less for an annubar than an orifice. So far as performance, the ISA book Essentials of Modern Measurements and Final Elements in the Process Industry offers some guidance. On page 170, the accuracy of orifice flow meters is stated as 2 percent to 4 percent of full-scale flow, whereas the accuracy of annubars is 0.75 percent to 2.0 percent of actual flow.

Thus at low flows, the accuracy of the annubar is significantly better. The stated rangeability is 5:1 for orifices and 10:1 for annubars assuming the maximum flow meter flow matches exactly the maximum process flow and the upstream and downstream piping requirements are met. For greater accuracy, consider temperature and pressure compensation as described for the multivariable DP transmitter starting on page 177.

Ram Ramachandran’s Answer

You are correct not to select orifice flow for 48-inch line because of cost, accuracy and repeatability concerns. The pitot tube or averaging [Eagle] pitot tube is not the best choice though installed cost is minimal. Due to low d/p generated, accuracy, turn-down ratio and repeatability will suffer. Generally, mechanical integrity of flow sensor will be sacrificed due to size and velocity.

As you well know, error/accuracy of installed d/p measurement whether orifice flow or pitot tube is about 2 percent. You should consider other techniques readily available for large pipelines with minimum pressure loss while maximum velocity measurement is at the center of the pipe.

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Some possible alternate applications:

  • Target meter with insertion type sensor-cost similar to pitot tube, slightly higher.
  • Insertion type custom-made vortex shedding meter – accurate and repeatable but costlier to pitot tube but far cheaper to orifice installation. Check with vendors like Emerson and others.
  • Insertion type turbine meter with better accuracy but lesser dependability. Solids in crude and velocity sensitive may make it high maintenance.
  • Doppler flow meter – due to the presence of sand/water in crude oil – accurate and repeatable.
  • Fluidic Component heat-loss characterized probes – similar to pitot tube in cost, but is application-specific.
  • Time of flight sonic flow meter wrapped around the pipe line externally – has all the advantages of the above metering devices at minimum cost – noninvasive installation, no pressure loss, accurate, repeatable, high turn-down ratios.

These will suffice for storage tanks transfers if they are not custody transfers. Ship to shore or reverse. You can also have radar level gauges in storage tanks, which are very accurate for custody transfers. Fill and drain rates in barrels per day can be computed and put on a separate display for operator use, altogether avoiding any flow meter installation (see Control Talk column Radar Love for more details). This is accomplished by configuring the DCS as a secondary read out. You can also install single loop standalone digital display with scaling.

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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