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Welcome to the official blog of the International Society of Automation (ISA).

This blog covers numerous topics on industrial automation such as operations & management, continuous & batch processing, connectivity, manufacturing & machine control, and Industry 4.0.

The material and information contained on this website is for general information purposes only. ISA blog posts may be authored by ISA staff and guest authors from the automation community. Views and opinions expressed by a guest author are solely their own, and do not necessarily represent those of ISA. Posts made by guest authors have been subject to peer review.

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Annubar or Orifice Flowmeter: What Is the Best Application for an Oil Storage Tank?

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 Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. This question is from Muhammad Al-Khalifah in Saudi Arabia:

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)

Answer from Greg McMillan (CDI Process & Industrial):
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.

The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about how you can join the ISA Mentor Program.

Answer from Ram Ramachandran (Systems Research Int'l Inc.):
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. 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.

Greg McMillan
Greg McMillan
Greg McMillan has more than 50 years of experience in industrial process automation, with an emphasis on the synergy of dynamic modeling and process control. He retired as a Senior Fellow from Solutia and a senior principal software engineer from Emerson Process Systems and Solutions. He was also an adjunct professor in the Washington University Saint Louis Chemical Engineering department from 2001 to 2004. Greg is the author of numerous ISA books and columns on process control, and he has been the monthly Control Talk columnist for Control magazine since 2002. He is the leader of the monthly ISA “Ask the Automation Pros” Q&A posts that began as a series of Mentor Program Q&A posts in 2014. He started and guided the ISA Standards and Practices committee on ISA-TR5.9-2023, PID Algorithms and Performance Technical Report, and he wrote “Annex A - Valve Response and Control Loop Performance, Sources, Consequences, Fixes, and Specifications” in ISA-TR75.25.02-2000 (R2023), Control Valve Response Measurement from Step Inputs. Greg’s achievements include the ISA Kermit Fischer Environmental Award for pH control in 1991, appointment to ISA Fellow in 1991, the Control magazine Engineer of the Year Award for the Process Industry in 1994, induction into the Control magazine Process Automation Hall of Fame in 2001, selection as one of InTech magazine’s 50 Most Influential Innovators in 2003, several ISA Raymond D. Molloy awards for bestselling books of the year, the ISA Life Achievement Award in 2010, the ISA Mentoring Excellence award in 2020, and the ISA Standards Achievement Award in 2023. He has a BS in engineering physics from Kansas University and an MS in control theory from Missouri University of Science and Technology, both with emphasis on industrial processes.

Books:

Advances in Reactor Measurement and Control
Good Tuning: A Pocket Guide, Fourth Edition
New Directions in Bioprocess Modeling and Control: Maximizing Process Analytical Technology Benefits, Second Edition
Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance
101 Tips for a Successful Automation Career
Advanced pH Measurement and Control: Digital Twin Synergy and Advances in Technology, Fourth Edition
The Funnier Side of Retirement for Engineers and People of the Technical Persuasion
The Life and Times of an Automation Professional - An Illustrated Guide
Advanced Temperature Measurement and Control, Second Edition
Models Unleashed: Virtual Plant and Model Predictive Control Applications

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