Ultrasonic Flow Measurement

Ultrasonic meters have no moving parts, they suffer no pressure loss and they
provide maintenance-free operation - important advantages over conventional
mechanical meters such as positive displacement meters (PDs), turbines, orifice
plates and vortex meters, and also in many cases coriolis mass meters.

Moreover, ultrasonic flowmeters are invariably more accurate and reliable
than many competing systems. Price has been a primary stumbling block,
but the case for ultrasonics is now stronger than ever with the emergence
of 3-beam ultrasonic meters developed to replace the mass, vortex, positive
displacement (PD) and turbine flowmeters that are used to measure
non-conductive fluids.

Ultrasonic flow measurement uses the transit time principle, whereby opposite
sending and receiving transducers are used to transmit signals through the flow.
The signal travels faster when moving with the flow stream rather than against
the flow stream. The difference between the two transit times is used to calculate
the flow rate.




Diagram showing the difference between
two beam and three beam technology.
Three beam technology allows the
instrument to differentiate between laminar
and turbulent flow, and to compensate
for it with the help of intelligent software,
resulting in an accuracy of up to 0.3%.


The measured average flow velocity between acoustic sensors does not
always represent the required average flow velocity. This is because flow
velocity across the pipe is not uniform and can differ in character. There
are two main areas of flow velocity distribution (i.e. flow profile): laminar
and turbulent flow and these have posed potential problems in flow
measurement.

The Reynolds number reflects the behaviour of fluids flowing through a pipe.
The number indicates the ratio between the inertial forces and viscous forces
in a flowing stream. In theory, for Re >2300 there will be a turbulent flow profile
(flattened shape), whereas for Re < 2300 the laminar profile (parabolic shape)
will have been established. Also, a flow profile called the transition area
exists in a Reynolds range from 1500 to 4000.

Close examination of what is measured by the ultrasonic waves shows that
this is not the same for turbulent and laminar flow profiles.

The accuracy of single beam measurement - within 2-3% - has proved sufficient
for many applications, but a demand has grown for more accurate measurements.
This led to the creation of two beam ultrasonic meter technology, which gave
an accuracy of 0.5%, but the instruments still encountered problems in
differentiating between laminar and turbulent flow.

To improve accuracy, Krohne introduced the Altosonic V which uses five
measurement channels (ten sensors) and became the first ultrasonic flowmeter
to meet the stringent requirements for custody transfer of high value oil products.
The flowmeter measures the average flow velocity along five, in parallel,
measuring beams, thereby covering a large range of the flow profile across
the metering section. This approach results in a wealth of information on the
flow profile, both in laminar and in turbulent flow conditions.

Another advantage of the multiple measurement channels is redundancy, which
enables the meter to function accurately even if one of the sensors ceases
functioning.

Five beam meters are cost prohibitive in some applications, which is why
Krohne developed 3-beam meters, an affordable middle-ground solution, using
the knowledge gained in 5-beam meters to deliver cost effective accuracy with
the advantages of ultrasonic measurement.

The new 3-beam ultrasonic flowmeters are available in two versions - one for
the oil and gas industry and the other for chemicals and other processing
applications. The third beam allows the instrument to differentiate between
laminar and turbulent flow, and to compensate for it with the help of intelligent
software, resulting in accuracy up to 0.3%.

The 3-beam meters employ software using the Digital Signal Processing (DSP)
technique to convert analogue signals into digital signals (AID). These digital
signals can then be processed in numerous ways to generate information on
the measurement process.

The most obvious advantage of DSP in the 3-beam ultrasonic flowmeter is that
through intelligent signal processing the 3-beam can handle attenuations,
which cause measurement problems, much better than other ultrasonic
flowmeters.

Until now, ultrasonic flowmeters did not quite meet the accuracy required in
the petrochemical or chemical industry, or were too expensive. Three-beam
technology has overcome these limitations, and Krohne has developed the
Altosonic III to serve the petrochemical industry and the UFM 3030 for the
chemical industry.

The Altosonic ifi incorporates a special feature for calculating the actual
volume to standard volume. This procedure is based on the API standards
2540. Two inputs can be assigned to measure the process temperature and
pressure, and since there are no moving parts, nothing can drift.
The UFM 3030 has a batching function with the option of assigning one of
the outputs to operate valve when the programmed batch volume is reached.
It also has the ability to measure low flow efficiently, and an important
advantage over vortex flowmeters is the extended metering range.

In addition to the basic advantages of ultrasonics such as no moving parts,
wide rangeability, negligible pressure loss and bi-directional operation,
3-beam technology also offers better sampling spectrum, DSP and dedicated
software options.

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For more information, contact Krohne on Tel: +44(0) 1933 408500

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