VOLUME 4, NO. 6
Transit Time
Measurement Method For Ultrasonic Metering
The transit time
method of measurement is the most commonly used in ultrasonic metering. It’s
based on the principle of sound energy traversing the fluid or gas in a pipe
both upstream and downstream, where the time difference is impacted
proportionally depending on the flow rate.
A pulse or pulses are transmitted to and from transducers through the liquid
to the opposing transducer positioned further downstream. Sound waves travel
faster with the direction of flow and slower against the direction of flow.
The resulting time difference between the upstream transmission and
downstream transmission is proportional to the flow velocity. Hence, a zero
time difference would cause the flow meter to report zero flow
Ultrasonic flow meters offer great advantages over traditional metering,
including:
· Non-intrusive measurement, allowing for virtually zero pressure drop
· No wear mechanism, reducing or eliminating maintenance costs
· Mounting sensors external to existing pipe greatly reduces installation costs
·
Very
large turn down ratio, typically 400:1
There are two
primary forms of transit time measurement available in today’s market;
externally mounted diametral and insertion type chordal.
Flow Profile Considerations
Ultrasonic meters
can be affected by distortions in the velocity flow profile that can, given
the amount of distortion, lead to erroneous measurement errors. Straight
upstream piping is an important factor when employing ultrasonic meters in
high accuracy applications, since valves and bends can produce vortices and
swirl. These disturbances can cause errors in the measurement of flow
profile and result in errors in the flow measurement.
Manufacturers of ultrasonic meters determine the flow profile and correct,
as best as possible, by means of Reynolds number. It’s well accepted that
laminar flow is generally found for Reynolds numbers less than 2.000 and
turbulent flow for Reynolds numbers greater than 10.000. The laminar and
turbulent regions are generally well known and proper compensation can be
made to produce highly accurate measurement.
The region between
these two is known as the transition region. This region is problematic in
terms of ultrasonic flow measurement since it is unpredictable and difficult
to measure.
Many users and
manufacturers have adopted the use of flow conditioners to help reduce or
completely eliminate flow profile distortions in applications where high
accuracy is required but straight pipe run is not available. However, use of
flow conditioners tends to negate some advantages of ultrasonic meters since
they are inserted into the flow stream and can increase the pressure drop.
WideBeam® Measurement Technique
The WideBeam
principle utilizes externally mounted transducers that inject an ultrasonic
beam into the pipe wall that matches the intrinsic sonic waveguide
properties of the metal pipe. This creates a collimated transverse wave in
the pipe wall that does not suffer from the internal pipe wall sonic
reflections that cause major distortion of sonic waves. Basically, this
technique rings the pipe at its resonant frequency.
To accomplish the
WideBeam mode of operation, the transducer must operate at the wave guide
frequency of the pipe wall, which is a function of the wall thickness and
the pipe material.
WideBeam
technology has numerous advantages that enhance the ability of the
Ultrasonic meter to maintain operation in difficult applications and improve
accuracy compared to normal shear mode ultrasonic meters.
Less Sensitive To Aerated Liquids
WideBeam
technology allows sonic energy to pass through a wide swath of the liquid
stream, lowering the potential for the sonic beam to be interrupted by air
bubbles or solids.
Auto zeroing
Zero drift, as a
result of temperature changes in the transducer crystals, has historically
been a concern as it can impact the error of ultrasonic measurement. This
drift is typically insignificant in applications with low accuracy
requirements. However, it has become a more critical concern as accuracy
requirements have increased with the advances in technology that allow
custody transfer performance to be achieved.
As a result of the wide beam technology, a method of effectively eliminating
zero drift has been developed that utilizes the sonic signature that travels
down the pipe wall to the receiving transducer as a marker to any drift in
the zero adjustment. This marker is fixed. Any difference between the
arrival time of the pipe signal relative to the liquid signal can be
adjusted on a continuous basis, effectively removing any drift not caused by
actual flow.
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