Flow Control: Technology information & new products for fluid handling engineers

September 2006

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Differentiating Vortex Flowmeters
Despite Similarities, Not All Shedder Designs Are Created Equal

David W. Spitzer, P.E.

There are over 50 suppliers of vortex-shedding flowmeters worldwide. Some of these flowmeters are designed for specific applications, such as small liquid flows and pure fluids. In fact, one design uses an extremely small-diameter shedder that is similar to a piano wire. Most vortex shedders used in the process industries, however, have a metal or plastic body and a shedder bar whose width is approximately 20 percent of the inside diameter of the flowmeter. This is primarily because the physics involved with this shedder design is essentially the same regardless of minor differences in detail.

The vortex shedders commonly used in the process industry can usually be applied to measure the flow of liquids, gases, and vapors. The caveat is that the flowmeter operates when the fluid flows at a pipe Reynolds number that exceeds its minimum Reynolds number constraint. Operating below this constraint will cause the flowmeter to become erratic and/or turn off. To operate accurately, the fluid must operate at a pipe Reynolds number that exceeds its minimum linear Reynolds number constraint, which is often higher than its minimum Reynolds number constraint.

Given that different vortex-shedding flowmeters have shedder bars with similar widths, it should not be surprising to note that the pressure drops associated with vortex-shedding flowmeters of different manufacture are similar. In particular, the pressure drop associated with water flowing at 15 feet per second is approximately five PSID.

Despite the similarities, there are a number of factors that differentiate vortex-shedding flowmeters of different manufacture. For example, shedder bar shapes and sensing system techniques vary with manufacture. They are usually proprietary in order to preclude others from copying the technology. Often, the shedder bar and sensing system designs are totally welded to eliminate leak paths or incorporate techniques that reduce or eliminate the effects of vibration.

Some designs are superior to others in their ability to generate and measure vortices at low flowrates. In water applications, the minimum velocity at which vortices can be measured is approximately one foot per second (0.3 meters per second). Most vortex-shedding flowmeters have a min. velocity constraint within approximately 20 percent of this nominal velocity. Similarly, Reynolds number constraints will also vary somewhat with shedder design.

The difference in velocity constraints between flowmeters can be important in liquid applications, but they are often critical in gas/vapor applications where the operating density of the gas/vapor is relatively low as compared to liquid. The minimum velocity constraint for vortex shedders tends to increase as fluid density decreases because less momentum is available to operate the sensing system. As a result, the turndown of a vortex shedder in gas/vapor applications is typically lower than its turndown in liquid applications.

Not all vortex shedders are created equal. There are some similarities and many differences to consider. Differences that may seem subtle can mean the difference between a vortex-shedding meter that needs to be replaced and one that works properly.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, 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. He can be reached at 845 623-1830.

www.spitzerandboyes.com

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