Vortex-shedding meters

Occasionally, a nonrotational measurement device can generate pulse outputs. One example is the vortex shedding meter, where a pulse can oe generated when each vortex passes over the detec tor. 

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VORTEX-SHEDDING METERS. In a vortex-shedding meter the target is a bluff body, often trapezoidal in cross section (Fig. 8.24). This body is designed to create, when flow is turbulent, a vortex street in its wake. (See Chap. 7, p. 148.) Sensors... 

The steam flow should be measured with a vortex-shedding meter. Hre linear characteristic of such meters provides greater rangeability. A fail-closed steel globe valve with an equal percentage characteristic and a positioner should be used. Tire controller would be reverse-acting with proportional and integral control functions. 

The vortex shedding meter is a rate-velocity meter, like the turbine meters, but in a distinct class by itself. The phenomenon upon which it is based is the Karman vortex trail. Its application to the measurement of flow of liquids and gases is fairly recent. 

Vortex-shedding flow meters typically provide 1% of flow rate accuracy over wide ranges on Hquid, gas, and steam service. Sizes are available from 25 to 200 mm. The advantages of no moving parts and linear digital output have resulted in wide usage in the measurement of steam, water, and other low viscosity Hquids. 

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. 

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

While fishing in Transylvania, Theodore von Karman noticed that downstream of the rocks the distance between the shed vortices was constant, regardless of flow velocity (Figure 3.104). From that observation evolved the three types of vortex meters the vortex shedding, the vortex precession, and the fluidic oscillation (Coanda) versions. All three types detect fluid oscillation. They have no moving components and can measure the flow of gas, steam, or liquid. 

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. 

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. 

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