Flow measurement
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Since its introduction in 2006, it has found particular acceptance in
tank farm operations for vehicle loading-arm applications. Road or
rail tankers are often weighed when entering the facility to establish
zero or ‘tare weight’, filled at the loading arm, and then weighed upon
leaving the facility to establish custody transfer amount.
The measurement objective here is safe, optimised filling and
maximised vehicle payload. Legal limits for tanker loading exist in
many counties eg 96% in Mexico. Without a reliable flow meter to
provide totalised flow rate, the operation becomes labour intensive
and potentially inaccurate via tank level or even manual sight glass
readings. Occasionally, a tanker will enter the facility and deliver
liquid products into the tank farm, thus demanding a bidirectional
flow meter.
The ultrasonic transit time flow meter satisfies these needs, and
provides a superior performance to the mechanical devices utilised
previously. A turbine meter was a common choice in the past but
mechanical wear, performance dependent upon liquid viscosity and
the inability tomeasure flow bi-directionally in some cases, demanded
improvement. Where even better measurement performance or
indeed a direct mass flow measurement is required, or where the
preferred installation location offers no straight pipe run, then the
two-wire loop-powered coriolis technology is ideal.
Another application area where these two modern flow meter
technologies bring great benefit is that of fuel supply monitoring to
steamboilers or process heaters. As stated earlier, the increasing need
to minimise operating costs may result in variations of fuel type pur-
chased for plant operation according to price and availability, which
in turn results in variations of fluid property, and sometimes fluid
quality in terms of cleanliness. A flow measurement device which is
both fluid property dependent, or rendered inoperable or damaged
by a dirty fluid, is not suitable in such circumstances.
All the older technologies described earlier will fall into this cat-
egory. An inline ultrasonic transit time ultrasonic meter has proven
to be a very acceptable technology for fuel oil monitoring, and it
has successfully replaced numerous mechanical meters during the
past few years. A large petrochemical manufacturer in Hungary has
replaced its process heater fuel oil meters in three phases since
2008, proving by the decision to continue with phase two, that the
technology is a success.
The two-wire loop-powered coriolis meter provides even better
flow control, being not only twice as accurate but also measuring
direct mass flow, thus eliminating the issue of volume correction in
respect of temperature and offering enhanced combustion control in
the face of fuel density or viscosity variation. Furthermore, the coriolis
meter will measure mass flow of gaseous fuels such as natural gas
where independence of fluid properties including pressure has added
benefit. A coriolis flowmeter can be installed without straight inlet or
outlet piping as velocity profile has no influence on its performance.
Technology to take us forward
It is worth reiterating that the modern flow technologies like ultra-
sonic, coriolis, electromagnetic and vortex functioning as two-wire,
loop powered devices all offer the capability for plant modernisation.
The cost effectiveness of process plant control is transformed with
improved data access, visualisation and interaction with remote
instrumentation. Through the use of 4-20 mA HART, it is possible
to remotely interrogate and configure the instrumentation across a
complete process plant from one central computer. The visualisation
and interaction increases further through advanced protocols like
Profibus and Foundation Fieldbus.
Ultrasonic and conolis technologies provide instrument engineers
with vital self-monitoring information. The format of such informa-
tion is easier to understand and act upon when presented in accord-
ance with harmonised international guidelines for online plant asset
management ie NAMUR. Through more precise specification of their
signals, modern flowmeter technologies providemore detailedmeter
diagnostic and process related information.
These detailsmake it possible for instrument engineers and opera-
tors to take action at the appropriate time - immediately or during the
next scheduled shutdown, saving time and money by knowing the
exact health of their measuring point. Wireless HART technology now
makes it possible to add a measuring device to an existing process
without any cabling whatsoever.
Conclusion
Where a process plant has a large installed base of older mechanical
technologies, the decision to embark upon a programme of replace-
ment toward plant modernisation cannot of course be taken easily.
There are the stocks of spare parts to consider along with the plant
Instrument technicians who are trained on, and at ease with, the old
devices.
Whilst the new ultrasonic or coriolis two-wire devices cannot be
installed without the pipe being emptied and perhaps a flange being
re-welded, the major cost of cable and power supply installation is
avoided, and the benefits of improved plant operation can be realised.
A process plant which retains the older technology going forward
will be at risk and may find it increasingly difficult or even impos-
sible to maintain a cost position that competes effectively in the
global market.
Steve Milford is a global business development manager for
Endress+Hauser AG of Reinach, Switzerland. He has been
involved in flow measurement since 1983, and specialises
in ultrasonic flow technology. He is actively involved with
standards development for this technology within ISO and
the British Standards Institute.
Enquiries: Frans van den Berg. Tel. 011 262 8000 or email
.
com.
Electricity+Control
July ‘13
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