Ultrasonic Flowmeter
Using ultrasonic flowmeter in flow measurements
The ultrasonic technique of measurement of
volume flow rate is, like the magnetic flowmeter, a non-invasive method.
It is not restricted to conductive fluids, however, and is particularly
useful for measuring the flow of corrosive fluids and slurries. A
further advantage over magnetic flowmeters is that the instrument is one
which clamps on externally to existing pipework rather than being
inserted as an integral part of the flow line, as in the case of the
magnetic flowmeter. As the procedure of breaking into a pipeline to
insert a flowmeter can be as expensive as the cost of the flowmeter
itself, the ultrasonic flowmeter has enormous cost advantages. Its
clamp-on mode of operation has significant safety advantages in avoiding
the possibility of personnel installing flowmeters coming into contact
with hazardous fluids such as poisonous, radioactive, flammable or
explosive ones. Also, any contamination of the fluid being measured (e.g
food substances and drugs) is avoided. The introduction of this type of
flowmeter is comparatively recent and its present market share is only
about 1% of flowmeters sold. In view of its distinct advantages,
however, this proportion is likely to increase in the future.
Two different types of ultrasonic flowmeter exist which employ distinct
technologies, one based on the Doppler shift and the other on transit
time. In the past, the existence of these alternative technologies has
not always been readily understood, and has resulted in ultrasonic
technology being rejected entirely when one of these two forms has been
found to be unsatisfactory in a particular application. This is
unfortunate, because the two technologies have distinct characteristics
and areas of application, and many situations exist where one form is
very suitable and the other unsuitable. To reject both, having only
tried out one, is therefore a serious mistake.
Particular care has to be taken to ensure a stable flow profile in
ultrasonic flowmeter applications. It is usual to increase the normal
specification of the minimum length of straight run of pipe prior to the
point of measurement, expressed as a number of pipe diameters, from a
figure of 10 up to 20 or in some cases even 50 diameters. Analysis of
the reasons for poor performance in many instances of ultrasonic
flowmeter application has shown failure to meet this stable flow profile
requirement to be a significant factor.
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