· Vortex flow meter operates on the physical
principle of the Karman vortex street. When a fluid flows pass a bluff body,
vortices are alternately forced on the side of that body and then detached or
shed by the flow.
·
The frequency of vortex shedding is proportional
to the mean flow velocity and therefore the volumetric flow (with Reynolds >
4000) St = Fd/V
·
Vortex frequency = St.v/d
St = strouhal number (dimensionless)
v = velocity fluid (m/s)
d = width of bluff body (m)
·
Alternating pressure change caused by the vortex
are transmitted via lateral port (sensor) into the bluff body.
·
The sensor detects the pressure pulses and
converts these into electrical signals.
·
They are typically available in flange size from
½ inch to 12 inches.
·
In gas services frequencies are about 10 times
higher than in liquid applications.
·
The proportionally between object width (d) and
vortex street wavelength {(l) – (lambda)} is called the “strouhal number” (S),
approximately equal to 0.17
ls = d l» d/0.17
Type of vortex flow meter sensor
i.Thermal sensing
ii.Mechanical sensor
iii.Capacitive sensor
iv.Piezoelectric sensor
v.Strain gauge sensor
vi.Ultrasonic sensor
Thermal sensor
·
Thermostats are using (negative temperature
co-effective)
Mechanical sensor
·
Also called shuffle ball sensor
·
A magnetic ball or disc moves from side to side
due to vortices.
·
This movement is detected by a magnetic pick up.
·
The main problem of stream the movement of the
ball or disc can be slowed by condensation.
Capacitive sensor
·
A stain less steel diaphragms are welded onto
the side of bluff body and the assembled filled with oil and sealed.
·
Purring vortex shedding diaphragm deflects and transfers
through the internal port from one side to the other.
·
When diaphragm deflects there is a change in the
capacitance between the diaphragm and electrodes.
·
Capacitance is inversely proportional to the
distance between the electrodes and directly proportional to the plate area.
·
Modern capacitive sensors use with superheated
steam for temperature upto 427° C.
Piezoelectric sensor
·
Piezo element produces voltage output that is
proportional to applied pressure.
·
Whilst piezo ceramic material produces a high
output it’s hawing limited operating temperature range (about 250° C).
·
Lithium niobate (linbo3) piezoelectric material
offers only medium output but operating temperature range is above 300° C.
·
These type sensors are unsuitable for temp.
below -40° C
·
This sensor same like capacitive sensor.
Strain gauge sensor.
·
The vortex created by bluff body causes the body
itself to be displaced by small amount of the order of 10µm.
·
This elastic movement can be detected using
strain gauges attached directly or indirectly to the bluff body.
·
Movement of the body produces a change in
resistance of the strain gauge.
·
Main drawback is upper temperature limitation of
strain gauge (about 120° C).
Ultrasonic sensing
·
Ultrasonic transmitter and receiver placed
behind the bluff body.
·
The vortex modulates the ultrasonic beam and the
resultant output is the vortex signal.
·
This sensor system has a good turn down ratio.
·
The main problem associated with this technique
is that extraneous sound sources can affect measurement.
The majority of vortex meter use piezoelectric or
capacitance type sensor. to detect the pressure oscillation around the bluff body.
The strouhal number and bluff body width and the
cross sectional area of the flow meter are all constants (which is defined as
“K”) the equation becomes
Q
= F/K
“K” factor can be defined as pulses per unit volume
such as pulses per gallons, pulses per liter, pulses per cubic feet, therefore
one can determine flow rate by counting the pulses per unit time.
Vortex frequencies range from one to thousands of
pluses per second depending upon the flow viscosity the character of the
process fluid and the size of meter.
Hints of vortex
flow meter:
·
The pipe Reynolds number should be above 30,000
minimum. This means vortex meters can only be used on low viscosity liquids.
High viscous fluids (>3 pa.s (30cp)) and slurries are not recommended
applications. (higher viscous having head loss)
·
The vortex shedding meter provides a linear
digital output signal without the use of separate transmitter or converters.
·
There is no drift because this is a frequency
system
·
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.
·
Low pressure (low density) gases do not produce
a strong enough pressure pulse, especially if fluid velocity are low (if use
the meter will be poor and low flows will not be measurable.
·
Vortex meter accuracy is based on the known
value of the meter factor (K- factor).
