flow and level measurement - Omega Engineering

length in the direction of the flow
must be a certain multiple of the
bluff body width.
Today, the majority of vortex
meters use piezoelectric or capacitance-type
sensors to detect the
pressure oscillation around the
bluff body. These detectors
respond to the pressure oscillation
with a low voltage output signal
which has the same frequency as
the oscillation. Such sensors are modular,
inexpensive, easily replaced, and
can operate over a wide range of
temperature ranges—from cryogenic
liquids to superheated steam.
Sensors can be located inside the
meter body or outside. Wetted sensors
are stressed directly by the vortex
pressure fluctuations and are
enclosed in hardened cases to withstand
corrosion and erosion effects.
External sensors, typically piezoelectric
strain gages, sense the vortex
shedding indirectly through the force
exerted on the shedder bar. External
sensors are preferred on highly erosive/corrosive
applications to reduce
maintenance costs, while internal sensors
provide better rangeability (better
low flow sensitivity). They are also
less sensitive to pipe vibrations. The
electronics housing usually is rated
explosion- and weatherproof, and
contains the electronic transmitter
module, termination connections, and
optionally a flow-rate indicator
and/or totalizer.
• Sizing & Rangeability
Vortex shedding frequency is directly
proportional to the velocity of the
fluid in the pipe, and therefore to volumetric
flow rate. The shedding frequency
is independent of fluid properties
such as density, viscosity, conductivity,
etc., except that the flow
must be turbulent for vortex shedding
to occur. The relationship between
vortex frequency and fluid velocity is:
St = f(d/V)
Where St is the Strouhal number, f is
the vortex shedding frequency, d is
Concentric
Reducer
A) Length = Size of Meter
Flow
Flow
Figure 4-6: Installation Recommendations
Upstream
Straight
Pipe Run
the width of the bluff body, and V is
the average fluid velocity. The value
of the Strouhal number is determined
experimentally, and is generally
found to be constant over a wide
range of Reynolds numbers. The
Strouhal number represents the ratio
of the interval between vortex shedding
(l) and bluff body width (d),
which is about six (Figure 4-4). The
Strouhal number is a dimensionless
calibration factor used to characterize
various bluff bodies. If their
Strouhal number is the same, then
two different bluff bodies will perform
and behave similarly.
Because the volumetric flowrate
Q is the product of the average fluid
velocity and of the cross-sectional
area available for flow (A):
B) Upward C) Downward D) Horizontal
Q = AV = (A f d B)/St
where B is the blockage factor,
defined as the open area left by the
bluff body divided by the full bore
area of the pipe. This equation, in
turn, can be rewritten as:
Q = f K
4 Electronic Flowmeters
where K is the meter coefficient,
equal to the product (A f d B). As with
turbine and other frequency-producing
flowmeters, the K factor can be
defined as pulses per unit volume
(pulses per gallon, pulses per cubic
foot, etc.). Therefore, one can determine
flowrate by counting the pulses
per unit time. Vortex frequencies
range from one to thousands of pulses
per second, depending upon the
flow velocity, the character of the
process fluid, and the size of the
meter. In gas service, frequencies are
TRANSACTIONS Volume 4 51
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Vortex
Meter
Downstream
Straight
Pipe Run
Concentric
Expander
Flow
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