Basics of Vortex flowmeter

Vortex flowmeter is a differential pressure measuring flowmeter which can measure both liquid and gas flow measurement. Vortexes are used for continuous flow measurement.

Principle:

When flow stream passes a stationary object or a bluff body and causes the formation of swirls, called vortices, downstream from the body. Each vortex will form, then detach from the object and continue to move with the flowing gas or liquid, one side at a time in alternating fashion. Differential pressure changes occur as the vortices are formed and shed. This pressure variation is used to actuate the sealed sensor at a frequency proportional to the vortex shedding

The principle of vortice measurement is based on the Theodore von Karman’s theory. The Karman’s frequency is f is proportional to the velocity V, Therefore it is possible to obtain the flow rate by measuring the Karman vortex frequency:

f=StV/d

f = vortex frequency

St = Strouhal’s number dimensionless

V = medium flow rate

d = the width of triangle prim

Construction and Working:

The bluff body is fixed at the centre of the flow stream, fluid gets passed aside the bluff body. Differential pressure changes occur as the vortices are formed and shed. This pressure variation is used to actuate the sealed sensor at a frequency proportional to the vortex shedding. The vortex flowmeter is available with a sensor attached to it.

The pressure sensors used in vortex flowmeters are not standard differential pressure transmitters since the vortex frequency is too high to be successfully detected by such bulky instruments. Instead, the sensors are typically piezoelectric crystals.

This is the metallic bar installed in the vortex instrument, otherwise the transducer part. The shedder bar is installed perpendicular to the flow. If the differential pressure sensor is installed immediately downstream of the stationary object in such an orientation that it detects the passing vortices as pressure variations, an alternating signal
will be detected:

The frequency of the alternate signal generated is proportional to the flow velocity of the fluid passing through the shedder bar. These pressure sensors need not be calibrated since the amplitude of the pressure waves detected is irrelevant. Only the frequency of the waves matter for measuring flow rate.

There are bluff bodies available in different shapes cylindrical, triangular, rectangular, swirl type etc.,

Applications:

• Transformation of natural gas metering
• Steam measurement
• General water metering
• In pharmaceutical & chemical industries

• Suitable for liquid, gas or steam.
• Used with non-conductive fluids.
•  No moving parts, they do not suffer the problems of wear and lubrication facing turbine meters.
• Sensors available to measure both gas and liquid.
• Not affected by viscosity, density, pressure or temperature.
• Low installation cost.
• Good accuracy.
• Linear response

• low flow cutoff, where the flowmeter simply stops working below a certain flow rate.
• Uni-directional measurement only.
• Clean fluids only.
• Not suitable for partial phase change.
• Not suitable for viscous liquids.
•  Large unrecoverable pressure drop.
•  Straight pipe runs required for installation