Q
&
A
Flow
in some meter designs. Viscosity limitations in some meter designs
can produce excessive pressure loss leading to higher energy
costs. Lowering viscosity can have a dramatic effect on slippage or
product loss if not planned for in other meter designs.
As modern flow technologies have been introduced and
gained acceptance into gathering systems, branch tie-ins and
direct pipeline measurements, the requirements to accommodate
fluid viscosity changes inherently in the meter design and
calibration process was a necessity. Recent product innovations
highlight how viscosity influences can be accounted for – even
under low Reynolds number or laminar flow conditions.
Flexim, USA
Flowmeter technology, selecting a
flow measurement tool
There are many different flowmeter technologies and companies
that manufacture flow measurement tools. In oil and gas
measurement the most accurate flowmeters are ultrasonic,
turbines, coriolis and positive displacement. Ultrasonic flowmeters
(UFMs) are widely used for custody measurement of gas. For liquid
hydrocarbons, turbine and coriolis meters are more frequently
used. Clamp-on meters are utilised more often for pipeline leak
detection for liquids, and lost unaccounted for or segmentation
for gas pipelines. Clamp-on ultrasonic flowmeters can be installed
with little installation costs, making them attractive as a meter
replacement or where an additional metering need arises. In gas
measurement, clamp-on meters often replace orifice meters for
non-custody transfer gas measurement applications.
Diagnosing meter performance
Ultrasonic flowmeters are the only types of meters that have rich
diagnostic capabilities and software to complement this feature. A
turbine only spins and provides very little in the way of diagnostic
information. UFM meters utilise sound diagnostics to retrieve
information about the medium and flow profile characteristics
inside the pipe.
UFM meters can tell the difference between fuel types
in a multi-product pipeline. Knowing the fuel type allows the
UFM to utilise API and ASTM standards that allow the UFM to
compensate gross volume to net standard volume, which is
volume corrected to a standard temperature typically 60˚F. The
volume correction factors are computed in accordance with
industry standard algorithms, such as TP-25, ASTM1250 and D4311.
UFM and clamp-on meters are now utilised for product interface
detection and specific product identity. They can even detect
where a pig is in the pipeline. They are capable of providing an API
gravity output, which enables pipeline operators to know when to
divert the product into the proper storage tanks.
UFM gas flowmeters utilise diagnostic software that has
self-diagnosis and will warn of a possible flow or meter problem.
This is most utilised in the gas transportation pipeline sector. One
aspect of the rich diagnostics is the ability to get a complete
diagnostic overview (diagnostic log) of the health of the meter
and application conditions. AGA10 speed of sound calculations
are performed often in real-time by the diagnostic software.
Compared to the speed of sound measured by the meter,
the speed of sound calculated using AGA10 algorithms provides
exceptionally good confirmation that the UFM is measuring
reliable. One aspect of the advanced level of diagnostic
information and self-diagnostic software is that UFM meters can
be kept in service longer before the need for expensive removal
for calibration.
Leak detection
Most pipelines were built without leak detection capability. As
a result of concern over the environmental impact of spills from
leaks, regulations have been implemented to ensure that leaks
are detected quickly. Clamp-on meters are increasingly being
utilised for leak detection applications. Retrofitting a pipeline for
leak detection is very expensive. The choice between installing
technology like coriolis or clamp-on ultrasonic will be a cost/
benefit decision the pipeline company will have to make.
Coriolis meters are capable of 0.25% accuracy, but can be very
expensive for larger diameter pipes. Also, the cost to install an
inline meter will be multiple the cost of the meters. Clamp-on
meters can deliver 0.5% accuracy with the cost of the meters
and installation only a fraction the cost of installing an inline
meter. The difference between 0.25% is important in pipeline
leak detection, but the cost to achieve that accuracy can be
substantially higher.
Reverse flow
All ultrasonic flowmeters deliver the capability of positive and
reverse. They contain outputs for both directions of flow.
NEL, UK
Pipeline flow modelling
There is currently a debate within the flow measurement
community with regard to the merits of using real fluids for the
development, testing and calibration of multiphase flowmeters.
However, a consideration of thermophysical properties of fluids
and basic metrological principles indicates that the use of stable,
well-characterised substitute fluids offers a better solution.
Figure 3.
Coriolis flowmeters.
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