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.
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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.