Thursday, September 20, 2018

The Working Principle and Applications of Vortex Flowmeters

Vortex flowmeters are used to measure the flow of Gases, Vapours and liquids in completely filled pipes. The measuring principle behind vortex flowmeters is based on the Karman vortex street.
When a fluid moves with a high Reynolds number past a stationary object (a bluff body) there is tendency for the fluid to form vortices on either side of the object. 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. This phenomenon is known as vortex shedding, and the pattern of moving vortices carried downstream of the stationary object is known as a vortex street.
From the research that was first done by Vincenc Strouhal then later on, by Theodore Von Karman, It was established that the distance between the successive vortices downstream of the stationary object is relatively constant, and directly proportional to the width of the object, for a wide range of Reynold number values.
The working Principle of Vortex Flowmeters

If consider these vortices as crests of a continuous wave, the distance between vortices may be represented by the symbol of wavelength ‘’lambda’’ (λ)


d- Object width
λ- Vortex street wavelength
S- Strouhal number
(λS = d) where S is approximately equal to 0.17
The wavelength (λ) is equivalent to d/0.17
If a 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.
Working Principle of Vortex Flowmeters

The frequency of this alternating pressure signal is directly proportional to fluid velocity past the object, since the wavelength is constant.
Using the classic frequency velocity wavelength formula common to all travelling waves; λf = v, and since we know the wavelength from the above, we may substitute this into this formula.
Velocity (v) = wavelength (λ) x Frequency (f),
Velocity (v) = d/0.17 x f
Therefore, frequency (f) = 0.17v/d
Therefore the stationary object and pressure sensor installed in the middle of the pipe section constitute a flowmeter called a Vortex flowmeter; the output frequency of a vortex flowmeter is linearly proportional to volumetric flow rate.
The pressure sensors used in vortex flowmeters, are typically piezoelectric crystals.
The relationship between sensor frequency (f) and volumetric flow rate (Q) may be expressed as proportionality, with the letter k used to represent the constant of proportionality for any particular flowmeter:
Therefore, f = kQ
Where f = Frequency of output signal (Hz)
             Q =Volumetric flow rate (e.g. liters per second or gallons per second etc.)
             K = ‘’K’’ factor of the vortex shedding flow tube (e.g. pulses per gallon or pulses per a liter)
Each vortex flowmeter has a ‘’k’’ factor relating to the number of pulses generated per unit volume passed through the meter.
Counting the number of pulses over a certain time span yields total fluid volume passed through the meter over the same time span, making the vortex flowmeter readily adaptable for totalizing fluid volume.
The direct proportion between vortex frequency and volumetric flow rate means vortex flowmeters are linear-responding instruments.

Advantages of using Vortex Flowmeters
Vortex flowmeters have a wide turn down ratio or a wide range of flow measurement. They do not require signal characterization to function properly. Since they have no moving parts, they do not suffer the problems of wear and lubrication facing turbine or positive displacement meters and can measure erratic flows.

Disadvantages of using Vortex Flowmeters
The flowmeter may stop working below certain flow rate, known as low cut off. This is because, at low flow rates i.e. laminar flow (low Reynolds number values), fluid viscosity becomes sufficient to prevent vortices from forming, causing the vortex flowmeter to register zero flow even when there may be some flow in the pipe.

Industrial Applications of Vortex Flowmeters
Vortex flowmeters are used in measurement of saturated steam and super-heated steam. They are also used in measurement of consumption of industrial gases. You will find them commonly used in Steam boiler monitoring, measurement of consumption in compressed air systems, heat metering in steam & hot water and lastly in  SIP and CIP processes in the food, beverage and pharmaceutical industries among other areas not mentioned here. 

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