Flowmeter Turndown

Did you ever wonder what it would be like to be a billionaire? If nothing else, it sounds like an exciting lifestyle. But how would you like to be a billionaire stranded on a desert island? In this situation, all of the money in the world will not help you survive. You may be wondering what this has to do with flow measurement. Simply put, it has a lot to do with turndown.

Turndown is the ratio of the maximum to the minimum flowrate that a flowmeter can measure within a stated performance. Without getting into the subtleties of performance statements, this ratio can often be large. How many times have you heard vendor claims of "up to 40-to-1" or "up to 100-to-1"? The key operative words here are "up to." Interpretation of these statements means that you may achieve as high as 40-to-1 or 100-to-1, but the flowmeter could provide 4-to-1 or 10-to-1 (or less) and still be within the limits of the claim.

For example, magnetic flowmeters can typically measure the flow of a liquid traveling at a velocity of 30 feet per second. In slurry service, velocities above about 15 feet per second are sometimes recommended to prevent solids from accumulating. However, at these velocities, energy costs can increase and the pipes generally wear more rapidly. Therefore, in typical process applications, liquid velocities of six to eight feet per second are more common.

In another example, it is common for ultrasonic flowmeters to accurately measure velocities from approximately one to 40 feet per second. This means that such a flowmeter can have a turndown of "up to 40-to-1." This may sound great, but in a typical application, only about 6-to-1 or 8-to-1 turndown would be achieved because the full-scale flow is typically six to eight feet per second. In short, the stated maximum turndown is based on a range of flowrates (eight to 40 feet per second) that will not be encountered.

Conversely, piping systems for abrasive liquids are often designed to operate at much lower velocities to reduce abrasion. Maximum flowrates in these applications can be as low as two feet per second (or lower). If the aforementioned ultrasonic flowmeter applied to this application, the flowmeter would operate accurately over a turndown of only 2-to-1. This turndown is far from the 40-to-1 turndown implied by its "up to 40-to-1" specification.

When selecting a flowmeter, be sure that you define the turndown that you need, determine the turndown that the flowmeter will provide based on the actual flowrates or velocities encountered, and then make your decision. One more thing — be sure to stay away from desert islands where you might get stranded.

About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME MFC and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.

For More Information: www.spitzerandboyes.com