Page 33 - Instrumentation Reference Book 3E

18 Measurement of flow

Figure 1.21 Oval-gear meter.

possible down to 20 percent of the rated capacity
of the meter.

Oval-gear type This type of meter consists of
two intermeshing oval gearwheels which are
rotated by the fluid passing through it. This
means that for each revolution of the pair of
wheels a specific quantity of liquid is carried
through the meter. This is shown diagrammat-
ically in Figure 1.21. The number of revolutions
is a precise measurement of the quantity of liquid
passed. A spindle extended from one of the gears
can be used to determine the number of revolu-
tions and convert them to engineering units by
suitable gearing.
'FlOW,"g
liquid Oval-gear meters are available in a wide range
Figure 1.20 Sliding-vane type meter. Courtesy, of materials, in sizes from 10 to 400mm and
WayneTank & Pump Co. suitable for pressures up to 60 bar and flows up
to 1200m3/h. Accuracy of *0.25 percent of rate
of flow can be achieved.
vanes are in contact with a fixed cam. The four
cam-followers follow the contour of the cam, 1.3.2.2 Rotating vane
causing the vanes to move radially. This ensures
that during transition through the measuring This type of meter operates on the principle that
chamber the vanes are in contact with the cham- the incoming liquid is directed to impinge tangen-
ber wall. tially on the periphery of a free-spinning rotor.
The liquid impact on the blades causes the The rotation is monitored by magnetic, or photo-
rotor to revolve, allowing a quantity of liquid to electric pick-up, the frequency of the output being
be discharged. The number of revolutions of the proportional to flow rate, or alternatively by a
rotor is a measure of the volume of liquid passed mechanical register connected through gearing to
through the meter. the rotor assembly as shown in Figure 1.22.
Accuracy is dependent on calibration, and turn-
down ratios up to 20:1 can be achieved. This
Rotating-vane type This meter is similar in device is particularly suited to low flow rates.
principle to the sliding-vane meter, but the meas-
uring chambers are formed by four half-moon- 1.3.2.3 Angled-propeller meter
shaped vanes spaced equidistant on the rotor
circumference. As the rotor is revolved, the vanes The propeller flowmeter comprises a Y-type body
turn to form sealed chambers between the rotor with all components apart from the propeller
and the meter body. Accuracy of 10.1 percent is being out of the liquid stream. The construction