Vortex shedding flow meter

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. 

It is also good practice to measure the liquid chlorine flow to the suction chiUer. Control can be directly from the hquid level, as assumed above, or by flow control in cascade from the level. Vortex-shedding flow meters have given good service here. 

Three independent methods of measuring released CO2 vapor were used. The first was the measurement of carbon dioxide flow rate by a vortex shedding flow meter, located just upstream of the 4-in. and 6-in. butterfly control valves. The second method was the measurement and recording of pressures and temperature at a 4.5-in. orifice installed downstream... 

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. 

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. 

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. 

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. 

Vortex shedding and magnetic flow meters preferred due to proven performance... 

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. 

Such a meter is shown in Fig. 8.18 and has been evaluated with liquid nitrogen. It consists of a bluff body located normal to the flow stream pipe, which basically measures the average velocity of the flow passing through the meter. The frequency / of the vortex shedding in hertz is given approximately by... 

When a bluff body is interspersed in a fluid stream, the flow is split into two parts. The boundary layer (see Chapter 11) which forms over the surface of the obstruction develops instabilities and vortices are formed and then shed successively from alternate sides of the body, giving rise to what is known as a von Karman vortex street. This process sets up regular pressure variations downstream from the obstruction whose frequency is proportional to the fluid velocity, as shown by Strouai. 9. Vortex flowmeters are very versatile and can be used with almost any fluid — gases, liquids and multi-phase fluids. The operation of the vortex meter, illustrated in Figure 6.27, is described in more detail in Volume 3, by Gjnesi(8) and in a publication by a commercial manufacturer, Endress and Hauser.10 ... 

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