Working Principle of Vortex Flow Meter

Vortex Meters can be used for a wide range of fluids, i.e. liquids, gases and steam. They are to be seen as first choice, subject to verification to cover the requirements of a particular application. 

Vortex meters are essentially frequency meters, since they measure the frequency of vortices generated by a “bluff body” or”shedder bar”. Vortices will only occur from a certain velocity (Re-number) on-wards, consequently vortex meters will have an elevated zero referred to as the “cut-off” point. Before the velocity becomes nil, the meter output will be cut to zero.

At a certain back-flow (above cut off point) some vortex meters could produce an output signal, which could lead to a false interpretation.

Vortex meters are actual volume flow meters, like orifice meters. These being intrusive meters like orifice meters, will cause the pressure drop as flow is increased, resulting in a permanent loss. consequently, liquids near their boiling point, could introduce cavitation as the pressure across the meter drops below the vapour pressure of the liquid. As soon as the pressure recovers above the vapour pressure the bubbles will impode. cavitation causes the meter to malfunction and should be avoided at all times.

Principal – A fluid flowing with a certain velocity and passing a fixed obstruction generates vortices. The generation of vortices is known as Karman’s Vortices and culmination point of vortices will be approx. 1.2D downstream of bluff body. Strouhal discovered that as soon as a stretched wire starts vibrating in an air flow, frequency will be directly proportional to air velocity,

St= f*d/V0 (without dimention)

St= Strouhal’s number

f=frequency of wire

d=diameter of wire

V0= Velocity

This phenomena is called “vortex shedding” and the train of vortices is known as “Karman’s Vortex street”.

The frequency of vortex shedding is a direct linear function of fluid velocity and frequency depends upon the shape and face width of bluff body. Since the width of obstruction and inner diameter of the pipe will be more or less constant, the frequency is given by the expression-

f=(St*V)/c*D

f= vortex frequency, Hz

St=strouhal’s number, dimention less

V=Fluid velocity at the sheddar bar, m/s

D=Inner diameter of the pipe, m

c=constant (ratio d/D)

d= Face width of sheddar bar, m

The pressure loss gradient across the vortex meter will have a similar shape to that of an orifice meter. the lowest point in pressure will be at the sheddar bar (comparable to vena contracta for orifice meter). downstream of this point of pressure will recover gradually, finally resulting in permanent pressure loss. To avoid cavitation, the pressure loss at vena-contracta is of interest.

The minimum back pressure required to ensure cavitation doesn’t occur is:

Pmin=3.2*Pdel + 1.25*Pv

Pmin= minimum required pressure at five pipe diameters downstream of the flow meter in bar

Pdel= calculated permanent pressure loss in bar

Pv= vapour pressure at operating temperature in bar

Remember- for most vortex meters d/D will have range, 0.22 – 0.26, & frequency od vortices will depend on sizre of meter, larger the meter, lower the frequency. So the maximum diameter of vortex meter is restricted, because resolution of meter could become a problem.for control purposes. To overcome this problem, on-board digital multipliers are used which will multiply the vortex frequency without additional error.

Frequency Sensing Principle – 

Piezo-electrical Sensors- a pair of piezo-electrical crystals is built into the sheddar bar. as the sheddar bar will be subject to alternating forces caused by shedding frequency, so will the piezo-crystals.

Variable capacitance Sensors- a pair of variable capacitance sensors is built into the sheddar bar. As the sheddar bar will be subject to alternating micro movements caused by forces as a result of the shedding frequency, the capacitors will change their capacitance accordingly.

Performance of Vortex meters is influenced by-

change in sheddar bar geometry owning to erosion

change in sheddar bar geometry owning to deposits, i.e. Wax

corrosion of upstream piping

change in position of sheddar bar if not properly secured

Hydraulic noise.

In-general votex meter will consist of following electonics part-

pick-up elements, AC-pre amplifiers, AC-amplifier with filters, Noise abatement features, Schmitt Trigger, Microprocessor

Features-

The vortex shedding meter provides a linear digital (or analog) output signal without the use of separate transmitters or converters, simplifying equipment installation. Meter accuracy is good over a potentially wide flow range, although this range is dependent upon operating conditions. The shedding frequency is a function of the dimensions of the bluff body and, being a natural phenomenon, ensures good long term stability of calibration and repeatability of better than ±0.15% of rate. There is no drift because this is a frequency system.

The meter does have any moving or wearing components, providing improved reliability and reduced maintenance. Maintenance is further reduced by the fact that there are no valves or manifolds to cause leakage problems. The absence of valves or manifolds results in a particularly safe installation, an important consideration when the process fluid is hazardous or toxic.

If the sensor utilized is sufficiently sensitive, the same vortex shedding meter can be used on both gas and liquid. In addition, the calibration of the meter is virtually independent of the operating conditions (viscosity, density, pressure, temperature, and so on) whether the meter is being used on gas or liquid.

The vortex shedding meter also offers a low installed cost, particularly in pipe sizes below 6 in. (152 mm) diameter, which compares competitively with the installed cost of an orifice plate and differential pressure transmitter.

The limitation include meter size range. Meters below 0.5 in. (12 mm) diameter are not practical, and meters above 12 in. (300 mm) have limited application due their high cost compared to an orifice system and their limited output pulse resolution. The number of pulses generated per unit volume decreases on a cube law with increasing pipe diameter. Consequently, a 24 in. (610 mm) diameter vortex shedding meter with a typical blockage ratio of 0.3 would only have a full scale frequency output of approximately 5 Hz at 10 ft/s (3 m/s) fluid velocity.

Selection and Sizing :

 As the first step in the selection process, the operating conditions (process fluid temperature, ambient temperature, line pressure, and so on) should be compared with the meter specification. The meter wetted materials (including bonding agents) and sensors should then be checked for compatibility with the process fluid both with regard to chemical attack and safety. On oxygen, for example, non ferrous material should be used avoided or approached with extreme caution. The meter minimum and maximum flow rates for the given application should then be established.

The meter minimum flow rate is established by a Reynolds number of 10,000 to 10,500, the fluid density, and a minimum acceptable shedding frequency for the electronics. The maximum flow rate is governed by the meter pressure loss (typically two velocity heads), the onset of cavitation with liquids, and sonic velocity flow (choking) with gases. Consequently, the flow range for any application depends totally upon the operating fluid viscosity, density, and the vapour pressure, and the applications maximum flow rate and line pressure. On low viscosity products such as water, gasoline, and liquid ammonia, and with application maximum velocity of 15 ft/s (4.6 m/s), vortex shedding meters can have a rangeability of about 20:1 with a pressure loss of approximately 4 PSIG (27.4 kPa).

The meter’s good (“of rate”) accuracy and digital linear output signal make its application over wide flow ranges a practical proposition. The rangeability declines proportionally with increase in viscosity, decrease in density, or reductions in the maximum flow velocity of the process. Vortex shedding meters are therefore unsuitable for use on high viscosity liquids.

Vortex Meter Advantages

? Vortex meters can be used for liquids, gases and steam

? Low wear (relative to turbine flow meters)

? Relatively low cost of installation and maintenance

? Low sensitivity to variations in process conditions

? Stable long term accuracy and repeatability

? Applicable to a wide range of process temperatures

? Available for a wide variety of pipe sizes

Vortex Flow Meter Limitations

? Not suitable for very low flow rates

? Minimum length of straight pipe is required upstream and downstream of the vortex meter

Vortex flow meters are suitable for a variety of applications and industries but work best with clean, low-viscosity, medium to high speed fluids.

Some of the main uses include:

? Custody transfer of natural gas metering

? Steam measurement

? Flow of liquid suspensions

? General water applications

? Liquid chemicals & pharmaceuticals