Page 30 - Instrumentation Reference Book 3E

Fluid flow in closed pipes 15

1.3.1.6 Turget flowmeter where C is a new constant including the numer-
ical factors. Mass flow rate is
Although not strictly a differential-pressure
device, this is generally categorized under that C(D' - d')
general heading. The primary and secondary elem- w=Qr'= JFy'kg/s (1.43)
ents form an integral unit, and differential pres- d
sure tappings are not required. It is particularly The force F is balanced through the force bar and
suited for measuring the flow of high-viscosity
liquids: hot asphalt, tars, oils, and slurries at pres- measured by a balanced strain gauge bridge
whose output signal is proportional to the square
sures up to 100 bar and Reynolds numbers as low root of flow.
as 2000. Figure 1.15 shows the meter and working Flow ranges available vary from 0-52.7 to
principles. 0-123 literslminute for the 19mm size at tempera-
The liquid impinging on the target will be
brought to rest so that pressure increases by tures up to 400 "C to from 0-682 to 0-2273 liters/
minute for the 100 mm size at temperatures up to
V'12g in terms of head of liquid so that the force
F on the target will be 260 "F. Meters are also available for gas flow.
The overall accuracy of the meter is f0.5
percent with repeatability of &0.1 percent.
F = Ky' V: At12 N (1.41)
Target flow meters are in use in applications
as diverse as supersaturated two-phase steam and
where y is the mass per unit volume in kg/m3. municipal water distribution. Wet chlorine gas
The area of the target is At measured in m3, K is and liquefied chlorine gas are also applications
a constant, and VI is the velocity in m/s of the for this type of device. The shape of the target,
liquid through the annular ring between target and which produces the repeatability of the device, is
pipe. empirical, and highly proprietary among manu-
If the pipe diameter is D m, and the target facturers.
diameter dm, then area A of the annular space
equals ~(0' - d2)/4 m2.
Therefore volume flow rate is 1.3.2 Rotating mechanical meters for liquids
Rotating mechanical flowmeters derive a signal
Q=A.Vl= from a moving rotor that is rotated at a speed
4 proportional to the fluid flow velocity. Most of
these meters are velocity-measuring devices
except for positive-displacement meters, which
(1'42) are quantity or volumetric in operation. The prin-
cipal types are: positive-displacement, rotating
vane, angled propeller meter, bypass meter, helix
meter, and turbine meter.

1.3.2. I Positive-displacement
Positive-displacement meters are widely used on
applications where high accuracy and good
repeatability are required. Accuracy is not
affected by pulsating flow, and accurate measure-
ment is possible at higher liquid viscosities than
with many other flowmeters. Positive-displace-
ment meters are frequently used in oil and water
undertakings for accounting purposes.
The principle of the measurement is that as the
liquid flows through the meter, it moves a meas-
uring element which seals off the measuring
chamber into a series of measuring compartments
which are successively filled and emptied. Thus,
for each complete cycle of the measuring element
a fixed quantity of liquid is permitted to pass
from the inlet to the outlet of the meter. The seal
between the measuring element and the measur-
Figure 1.15 A Target flowmeter with an electronic
transmitter. Courtesy, the Venture Measurement Division ing chamber is provided by a Of the measured
of Alliant Inc. liquid. The number of cycles of the measuring