Friday, July 6, 2012
Flow Measurement - 3
EPC-School 2012
I for Instrumentation
I for Instrumentation
Vortex 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.
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.
Vortices Formation due to bluff body
Another illustration of Vortices
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".
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 Principal -
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.
Labels: bluff body, cavirtation, cavitation, karman's effect, orifice, piezo-electric, Rahul Kapoor, sheddar bar, strouhal's number, Vortex meter, vortices
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