Vortex Flow Meter: Basic Working Principle

Vortex flow meters may measure a variety of fluids, including liquids, gases, and steam. They should be considered a first option, subject to verification to ensure that they meet the criteria of a specific application. 
Because vortex meters detect the frequency of vortices formed by a “bluff body” or “shedder bar,” they are essentially frequency meters.

Vortices will only form once a specific velocity (Re-number) has been reached. Hence vortex flow meters will have an elevated zero, also known as the “cut-off” point. The flow meter output will be cut to zero before the velocity reaches zero.
Some vortex meters may give an output signal above a specific back-flow (above the cut-off point), leading to an erroneous interpretation.

Vortex meters, like orifice meters, measure actual volume flow. Because certain meters are intrusive, such as orifice meters, the pressure will drop as the flow increases, resulting in a permanent loss. As a result, liquids nearing their boiling point may cause cavitation as the pressure across the meter falls below the liquid’s vapor pressure.
The bubbles will implode as soon as the pressure rises over the vapor pressure. Cavitation malfunctions the meter and should be avoided at all costs.

The Vortex Flowmeter’s Basic Working Principle

This principle states that if an obstruction is placed in the course of a flow, it can cause a succession of vortices to form on both sides of the barrier.
The flow rate that is impeded is related to the frequency with which these vortices alternate.

The vortex flowmeter is a type that shreds vortices with a small rod called a “shredder bar” or “bluff bar,” and the number of vortices produced is proportional to the flow rate. A pressure flow sensor can be used to measure the vortices, and the proportional flow rate can be calculated. Various types of flow meters/sensors may be used in place of a pressure sensor in some applications.

Flowmeter With Vortex

f*d/V0 = St (without dimension)
Strouhal’s number is Strouhal’s number.
F is the wire’s frequency.
D is the wire’s diameter.
Velocity = V0

This is known as “vortex shedding,” and the train of vortices is referred to as “Karman’s Vortex street.”

The frequency of vortex flowmeter shedding is a direct linear function of fluid velocity, and the form and face breadth of the bluff body influence the frequency. The frequency is determined by the expression- since the breadth of the blockage and the inner diameter of the pipe will be more or less constant.

f=(St*V)/c*D
f= frequency of vortex, in Hz
St=number, Strouhal’s with a smaller dimension.
V=Velocity of fluid at the cheddar bar in m/s
D=Measurement of the pipe’s inner diameter D=Measurement of the pipe’s inner diameter D
c=constant (d/D ratio)
d= Sheddar bar face width, m

The pressure loss gradient across the vortex meter will resemble an orifice meter in shape. At the shedder bar, the pressure will be at its lowest position (comparable to vena contracta for orifice meter). The pressure downstream of this location will gradually recover, eventually resulting in permanent pressure loss. The pressure loss at the vena-contracta is important to avoid cavitation.

The Following Is The Minimal Back Pressure Required To Avoid Cavitation:

Pmin=3.2*Pdel + 1.25*Pv Pmin is the minimum needed pressure in the bar for five pipe diameters downstream of the flow meter.

Peel represents the calculated permanent pressure loss in the bar.
Pv = vapor pressure in bars at operational temperature

Remember that the d/D range for most vortex meters is 0.22–0.26, and the size of the meter determines the frequency of vortices; the larger the meter, the lower the frequency. As a consequence, the greatest diameter of the vortex meter is limited, as the meter’s resolution may become an issue. for the sake of maintaining control.

On-board digital multipliers are utilized to solve this problem, multiplying the vortex frequency without adding any additional inaccuracy.

Principle Of Frequency Sensing

A pair of piezo-electrical crystals are integrated inside the cheddar bar as piezo-electrical sensors. The piezo-crystals, like the cheddar bar, will be susceptible to alternating forces induced by shedding frequency.
A pair of variable capacitance sensors are incorporated within the shedder bar. The capacitors will change their capacitance as the shedder bar is subjected to alternate micro motions generated by forces due to the shedding frequency.

Changes in shedder bar geometry due to erosion affect the performance of Vortex flow meters.

Wax deposits cause a change in the shedder bar geometry.
Upstream piping corrosion
If the shedder bar is not adequately secured, it should be moved.
The sound of hydraulics.
In general, the electronics part of a vortex meter will be as follows:

Elements to be picked up Noise abatement features, AC-pre amplifiers, AC-amplifiers with filters, Schmitt Trigger, Schmitt Trigger, Schmitt Trigger, Schm Microprocessor

Conclusion

This is all you should know about the principle of the vortex flow Meter. When a column body is placed in a pipe filled with flowing fluids, a series of vortices will form alternatively on either side of the object, as seen below. These eddies are known as “Karman Vortices,” and the vortex shedding frequency is proportional to the flow rate.

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