5 Must-Have Features in a Vortex Flow Meter

This measuring principle is based on the fact that vortices are formed downstream of an obstacle in a fluid flow, either in a closed pipe or in an open channel. This phenomenon can be observed by looking at the eddies (“vortex street”) formed downstream of a bridge pillar, for example (Fig. 1). The frequency of vortex shedding down each side of the pillar (bluff body) is proportional to the mean flow velocity and therefore to the volume flow. As early as , Leonardo da Vinci had sketched stationary vortices downstream of obstacles shedding flow.

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In , Strouhal was attempting to describe in scientific form the eddies that form behind bluff bodies. His studies revealed that a wire stretched tight in a jet of air will oscillate. He found that the frequency of this oscillation is proportional to the velocity of the air jet. This phenomenon can be observed in a car or house: the whistling tone of the wind is caused by vortex shedding and rises or falls as velocity changes. This is called the “aeolian tones”.

The physicist Theodore von Kármán laid down more of the theoretical groundwork for flow measurement with vortex meters in , when he described what has become known as the “vortex street”. His analysis of the double row of vortices behind a bluff body in a fluid flow revealed a fixed ratio between their transverse spacing (d) and longitudinal spacing (L). If the bluff body is cylindrical, this ratio is 0.281, for example. With a uniform pipe diameter, the volume of the individual vortices is therefore constant. Presuming that the vortices are of the same size despite differences in operating conditions, flow can therefore be derived directly from the number of vortices per unit of time.

The flow reaches its maximum velocity at the widest part of the bluff body and subsequently loses some of this speed. Figure 3 shows that the flow tries to break away (a) from the contour of the bluff body, instead of continuing to follow it. This causes localized low pressure, producing backflows and, ultimately, vortices (b). These vortices shed alternately down each side of the bluff body and are carried away by the fluid.

Bluff bodies vary in shape from manufacturer to manufacturer. They can be rectangular, triangular, round, delta-shaped or one of several proprietary and patented designs. The design must be such that the Strouhal number remains constant over the entire measuring range, in other words, the vortex frequency is independent of pressure, temperature and density. It is this constant range (Re > 10.000) that is utilized for measuring volume flow with vortex meters (see Fig. 4). Delta-shaped bluff bodies exhibit almost ideal linearity and have proved particularly reliable. NASA engineers have subjected these bluff-body designs to exhaustive studies. Measuring accuracy can be ±0.75% o.r., and reproducibility is around 0.1%.

It is usual to define the characteristics of vortex flowmeters in terms of the “K factor”. This factor represents the number of vortices in unit time (pulses per unit of volume). The manufacturer obtains this K factor by calibration and includes this information on the instrument name plate. It is dependent on bluff body shape and pipe size.

Vortex flowmeters are used in numerous branches of industry to measure the volume flow of steam, liquids and gas. Vortex meters are becoming more and more common in applications that were formerly the preserve of differential pressure flowmeters such as orifice plates. This trend is ongoing, for the simple reasons that vortex meters are easier to install and have a wider range of turndown. Figure 5 shows an example of such a case.

The focus of end customers has evolved from purely volumetric measurement to compensated mass measurement. This development makes it possible to draw up precise balance sheets. By taking pressure and temperature into account, accurate mass measurements can be achieved, which is essential for accurate balancing, process control and optimization.

Furthermore, special wet steam measurement (dryness fraction/steam quality) can help operators to understand the quality of their steam and detect potential accumulation of wetness online. This way, safety and efficiency can be improved reliably. Best accuracy results become possible in saturated/wet steam environments enabling customers to close potential gaps in their mass balances.

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Rosemount™ MultiVariable and Quad Vortex Flow Meters: A superior solution for compensated steam and redundant flow measurement

Steam applications are well known for their relentless demands and their inherent safety risks. With unparalleled features like the ability to perform maintenance any time without shutting down the process and the ability to compensate for changing process condi-tions, the Rosemount Vortex Flow Meter knows how to go to work for you and your operations.

The powerful combination of the Rosemount Multivariable Vortex Flow Meter with an integrated temperature sensor paired with a Rosemount S Pressure Transmitter delivers reliable pressure and temperature compensation for your mass flow measure-ments in superheated steam applications.

The Rosemount Quad Vortex is designed for SIS applications and provides the ut-most reliability to guard against spurious trips using two out of three voting and includes a fourth integrated transmitter for process control. The Quad Vortex removes installation complexity in an elegant solution you can quickly bolt in, wire up, and go.

Rosemount™ MultiVariable and Quad Vortex Flow Meters: A superior solution for compensated steam and redundant flow measurement

Steam applications are well known for their relentless demands and their inherent safety risks. With unparalleled features like the ability to perform maintenance any time without shutting down the process and the ability to compensate for changing process condi-tions, the Rosemount Vortex Flow Meter knows how to go to work for you and your operations.

The powerful combination of the Rosemount Multivariable Vortex Flow Meter with an integrated temperature sensor paired with a Rosemount S Pressure Transmitter delivers reliable pressure and temperature compensation for your mass flow measure-ments in superheated steam applications.

The Rosemount Quad Vortex is designed for SIS applications and provides the ut-most reliability to guard against spurious trips using two out of three voting and includes a fourth integrated transmitter for process control. The Quad Vortex removes installation complexity in an elegant solution you can quickly bolt in, wire up, and go.