Vortex shedding flowmeters

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

Vortex-Shedding Flowmeters These flowmeters take advantage of vortex shedding, which occurs when a fluid flows past a non-streamlined objec t (a Blunt body). The flow cannot follow the shape of the object and separates from it, forming turbulent vortices or eddies at the object s side surfaces. As the vortices move downstream, they grow in size and are eventually shed or detached from the objec t. 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

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Flo. 6.2. Vortex-shedding flowmeter (a) basic principle of meter (b) general view... 

The basic design categories of vortex flowmeters include (1) vortex shedding,... 

The swirl meter operates in most of the same applications as the vortex-shedding flowmeter but has the advantage that, because flow conditioning is done at the inlet and outlet of the meter body, virtually no upstream or downstream straight run is required for optimal installation. 

Vortex Shedding Flowmeter. At Reynolds numbers greater than 1000, vortices are shed behind bluff bodies giving an oscillating... 

Full-bore meters include variable-head meters such as venturi and orifice meters and variable-area meters such as rotameters. These will be described in some detail. Briefer descriptions are given of other full-bore measuring devices V-element, magnetic, vortex shedding, turbine and positive-displacement meters, ultrasonic meters, and mass flow devices such as Coriolis and thermal flowmeters. 

The experimental set-up (Fig. 1) was arranged in order to measure vortex shedding frequencies from very thin circular cylinders, perpendicular to the flow. A 500 liter constant-head tank was used to supply a steady flow of water or of. polymer solution. The flow was controlled by a valve at the extreme downstream end of the conduit and measured by an accurate magnetic flowmeter. 

Vortex shedding flowmeters (Fig. 18.12) introduce a shedding body into the flow to cause production (shedding) of vortices. The sound accompanying the production and collapse of the vortices is monitored and analyzed. The dominant frequency of the sound is indicative of the rate of vortex production and collapse, and hence an indication of flow rate. Vortex shedding flowmeters are useful in low-velocity, nonturbulent flows. 

A better way of measuring flows than the ordinary orifice plate method is by inducing vortex shedding across a tube in the flowing liquid and then measuring the velocity of the vortices. This is a nice method, as there are no orifice taps to plug. Then there are Doppler meters, which measure the velocity of a fluid based on how the speed of sound is affected by the flow in a pipe. More commonly, we have rotometers, which measure how far a ball or float is lifted in a vertical tube by the velocity of the liquid. But regardless of their relative merits, perhaps simply for historical reasons, the vast majority of flows in most process plants are measured with the orifice plate flowmeter, shown in Fig. 10.7. 

The basic types of flowmeters which find application in cryogenic service involve positive displacement types, pressure types (including orifice and venturi), turbine types, momentum types, vortex shedding, and various miscellaneous concepts. Each will be discussed below. 

J. A. Brennan, R. W. Stokes, C. H. Kneebone, and D. B. Mann, An Evaluation of Selected Angular Momentum, Vortex Shedding and Orifice Cryogenic Flowmeters, NBS Technical Note. No. 650, 1974. 

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