In transport, LNG
is housed in custom tanks on ships with reinforced hulls. Once it
arrives at a receiving terminal, LNG is stored in insulated tanks until
it is regasified and passed onto a pipeline for delivery to natural gas
On a smaller scale,
LNG may also be produced by liquefying gas taken from a pipeline,
storing it, and then regasifying it for pipeline distribution to
customers when demand is high, such as on cold winter days. These small
regasification plants are often called peakshaving plants, as they
provide additional supply during periods of peak use. Alternately, LNG
may be transported in special tanker trucks to small facilities where
it is stored and regasified as needed. Such facilities are called
satellite plants. According to the Energy Information Administration (www.eia.doe.gov
) of the Department of Energy (www.doe.gov
), there are more than 100 LNG satellite and peakshaving plants throughout the United States.
Why the Boom?
Natural gas is a
clean-burning fuel — far cleaner than coal and other fossil fuels — and
therefore a highly desirable energy source. Until recently, however,
natural gas has not been available wherever needed, at the right price.
This picture is changing rapidly though, as the economics improve for
transporting natural gas in liquid form.
LNG has been an
important part of the energy equation in some regions, notably Asia,
for decades. It received a flurry of attention in the United States
during the energy crisis of the late 1970s, but for the most part, LNG
has not played a significant role in the global energy picture.
The sudden spike in development around LNG is due in large part to
advances that are reducing the costs of liquefaction, shipping, and
regasification. Liquefaction costs have declined 35-50 percent in the
past 10 years, the cost of building an LNG tanker has fallen about 45
percent since the mid-1980s, and regasification costs have also
trade represents about 25 percent of international gas movements, and
it competes successfully with other sources of energy. Global demand is
increasing, especially in the United States and in Europe, and to meet
this demand many world-scale LNG projects are under development. New
production plants are being constructed in Egypt, Australia, Equatorial
Guinea, Indonesia, Norway, and Russia. Plants are being expanded in
Trinidad and Tobago, Oman, and Nigeria, and new projects are being
proposed in Nigeria, Angola, Qatar, and other locations.
As a result, global
LNG capacity has grown by 60 million tons per year over the past
several years, an increase that represents more than 50 percent of
existing global base-load capacity. At the same time, over 60 LNG ships
are on order or under construction — a 50 percent increase — and a
number of new LNG receiving terminals are under construction. If all of
these projects come to fruition, LNG production capacity could double
by the end of the decade.
As noted earlier, process efficiency is key to the realization of the LNG boom. According to Bechtel (www.bechtel.com
a leading designer of LNG facilities, one of its goals is to develop
technology for LNG plants with more than twice the production capacity
of today’s facilities (max. 4.5 million tons per year). Currently,
Bechtel says it has a design for an 8.25-million ton train that could
be incorporated in a Bechtel project currently under development.
cryogenic and natural gas measurement are among the enabling
technologies facility designers are counting on to help achieve
improved efficiency. Gas flowmeters are used to accurately measure and
allocate gas flows into liquefaction trains during LNG production. At
receiving terminals, after LNG is regasified, custody transfer gas
flowmeters measure the amount of natural gas sold to local pipeline
companies for further transportation and distribution. Gas flowmeters
are also used to measure the amount of natural gas flowing in vapor
return lines from storage tanks to ships while the tankers are
unloading. Liquid flowmeters capable of withstanding cryogenic
temperatures are used to measure the amount of LNG moved from
production facility liquefaction trains into storage and also to
monitor the movement of LNG pumped between tanks at storage terminals
to prevent damaging “rollover” that may occur if LNG isn’t continuously
mixed in storage tanks and becomes stratified. And designers of future
terminals will specify cryogenic liquid flowmeters to measure the flow
of LNG from ships into terminal storage tanks.
turbine, and ultrasonic designs have all proven capable of flow
measurement in LNG applications. Coriolis and ultrasonic systems, in
particular, have been generating significant uptake in recent years, as
these technologies can measure both cryogenic liquids and natural gas.
Other meter types can measure both gas and liquid, but Coriolis and
ultrasonic meters offer distinct advantages, including little pressure
drop and low maintenance. Coriolis and ultrasonic meters are also
a good fit for LNG applications because they are resistant to flashing
(i.e., vaporization resulting from pressure drop). Coriolis meters are
generally used on six-inch (outside diameter) and smaller pipes or in
parallel to support larger pipe sizes. Ultrasonic meters are typically
used on pipe sizes from six inches up to 42 inches or greater.
agencies have verified that Micro Motion Coriolis factory
calibration transfers directly to other fluids such as natural
gas, liquefied natural gas, and other cryogenic applications.|
In the 1980s,
Coriolis meters gained wide acceptance in liquid and gas applications,
achieving a worldwide installed base of over 400,000 units. For
cryogenic services, such as LNG, most Coriolis meters can be used
off-the-shelf with minor design modifications and operational changes.
Micro Motion (www.micromotion.com
), taking its support for LNG a step
farther, patented an algorithm for Young’s modulus of elasticity,
enabling its stainless steel Coriolis meters to compensate for the
elasticity of metal under cryogenic conditions. As a result, Micro
Motion’s Elite line of Coriolis meters can be plugged directly into a
cryogenic application with no changes at all.
The key advantage
Coriolis systems offer for LNG applications is that they measure mass
flow directly, rather than volume, and are unaffected by changing fluid
properties such as viscosity or density. As a result, the standard
water calibration or proving of Coriolis meters transfers to other
fluids, such as natural gas, or cryogenic applications, such as LNG.
Volumetric metering systems, on the other hand, need to be calibrated
or proved on fluids with the same properties that are to be measured to
achieve their stated accuracies. In addition, since Coriolis systems
measure mass directly, there is no need to compensate for changes
in pressure, Reynolds number, temperature, viscosity, etc. As a
secondary benefit, Coriolis meters measure online density and
temperature and provide diagnostics that can offer early warning of
upset conditions, such as gas breakout, warming of the LNG during
the liquefaction process, as well as product quality. The Coriolis
meter’s density measurement can also be used to potentially convert
volumetric measurement to mass units.
systems measure mass directly they do not require straight runs of
piping and/or flow conditioners. This reduces the overall cost
and size of the metering system.
In a clean fluid,
such as LNG, Coriolis performance is unlikely to deteriorate because
there are no moving parts and the structure of the tubes does not
change. Since the meter’s flow measurement is ultimately a function of
the tube twist, which in turn is a function of the structure of the
tubes, signals are resistant to drift over time. For LNG service, Micro
Motion Coriolis meters are manufactured of stainless steel, which is
very stable under cryogenic conditions, thus there are no changes in
tube properties, eliminating the need for recalibration over the life
of the product. Contractually, the meters will probably still need to
be proved if they are used in fiscal service. Coriolis meters may
also need to be zeroed over time to establish a zero flow point in the
transmitter. Over-pressuring the system (tube deformation), coating of
the tubes, loss of tube material, such as erosion or corrosion, will
generate a need for recalibration over time. However, these conditions
are not common to LNG applications since the fluid being handled is
agencies have verified that Micro Motion Coriolis factory
calibration transfers directly to other fluids such as natural
gas, liquefied natural gas, and other cryogenic applications. The
systems are capable of achieving 0.1 percent of mass rate accuracy over
a 20-to-one turndown. The flow performance of the company''s meters has
been certified by third-party measurement verification laboratories,
including the National Institute of Standards and Technologies
) and CERN (www.cern.ch
). In addition, Micro Motion
Coriolis meters have 10 years of successful LNG measurement in such
applications as custody transfer in Australia and cryogenic process
control in France and, more recently, high-capacity rundown flows in
Alaska. Micro Motion meters are also used for monitoring of local depot
deliveries to CNG-powered vehicles.
To combat icing in
cryogenic service, the flow tubes of Micro Motion Coriolis sensors are
surrounded in a hermetically sealed secondary metal enclosure,
preventing exposure of operating components to atmospheric moisture.
This enclosure seals the space around the flow tubes. Pickoff and drive
coils are filled with an inert gas, typically nitrogen or argon.
The line size and
flowrate capability of a Coriolis meter is dictated by the maximum
acceptable pressure drop. For example, a six-inch Coriolis meter can
achieve a flowrate with water of 681,000 kg/hr with a one bar pressure
drop. (LNG processes generally call for a pressure drop of one bar or
less.) Certain applications allow for multiple Coriolis meters in
parallel (typically no more than four to six meters). This would allow
a maximum flow capacity of a multiunit skid in the range of 2,275,000
kg/hr with pressure drops around one bar. In addition, Micro
Motion has developed skidded systems that include an inline master
meter that can continually monitor the other meters performance. Micro
Motion Coriolis meters are manufactured with tube diameters that can be
used in eight to 10-inch pipelines. Larger flowrates will typically
require multiple meters.
|The Panametrics Sentinel ultrasonic flowmeter can be used to measure
the flowrate of natural gas to liquefaction trains, and it provides the
accuracy necessary for custody-transfer measurements.|
flowmeters are considered a relatively new technology, they have been
used for natural gas measurement for more than 25 years. Prior to
moving into the realm of natural gas, transit-time ultrasonic
technology was primarily used for liquid flowmetering using both wetted
and clamp-on installation methods. At the time, ultrasonic systems had
not yet been commercialized for gases due to issues involved with
acoustic impedance between the ultrasonic transducers and the gas. This
acoustic impedance made the transfer of ultrasonic energy into a gas
difficult, much more difficult than transferring ultrasonic energy into
a liquid. Researchers at Panametrics, which was acquired by General
Electric in 2003 and is now part of GE Sensing (www.gesensing.com
developed a successful way of acoustically coupling ultrasonic
transducers to gases by adding an acoustic impedance-matching layer to
the transducer face. This invention is the cornerstone of ultrasonic
flow measurement for gases.
ultrasonic flowmeters offer several advantages to pipeline operators.
They create little pressure drop, have low maintenance requirements,
and can easily handle large pipe sizes. Because ultrasonic meters
create little pressure drop and do not wear, their operating costs and
cost of ownership are low.
ultrasonic meters can be used on pipes as large as 42 inches, with 12
and 24 inches being typical. (GE Sensing proves its larger meters using
a standard Reynolds Number equivalency technique.) In addition, because
of their wide turndown ratio, typically greater than 100-to-one, a
single ultrasonic meter can handle all the flow through a large pipe.
In the first phase
of LNG production, natural gas liquids, sulfur, and water vapor are
removed from the natural gas to improve gas quality and prevent
problems, including hydrate formation and pipeline blockage from
occurring. GE Sensing’s Panametrics Moisture Image Series (MIS)
hygrometers are designed for measuring the amount of water vapor
remaining after natural gas dehydration. The MIS solid-state aluminum
oxide moisture sensor probe continuously measures water vapor content
in the pipeline. The sensor probe is certified by BASEEFA (www.baseefa.com
) for hazardous areas and can be remotely mounted from the hygrometer electronics for ease of installation and use.
accomplished by a sampling system, containing the moisture sensor
probe, located at the measurement point after dehydration. The
hygrometer’s sensor operates at full line pressure, simplifying
installation and preventing difficulties and inaccuracies that may
result if the pressure is reduced. The MIS moisture analyzer has a wide
measurement range capable of handling normal-to-upset operation. It is
specific to water vapor and immune to other natural gas components.
Measurement units may be selected as pounds of water per million,
standard cubic feet of natural gas, or parts per million. While the MIS
system does not rely on ultrasonic technology, it is the first step of
GE Sensing’s ultrasonic approach to LNG flow measurement.
natural gas is liquefied in liquefaction trains. The GE Sensing
Panametrics Sentinel ultrasonic flowmeter can be used to measure the
flowrate of natural gas to the liquefaction trains for allocation and
Panametrics DigitalFlow ultrasonic flowmeters, using GE Sensing’s
patented Bundled Waveguide Technology (BWT) transducers, provide flow
measurement at cryogenic temperatures (-161 C/-258 F) in pipes from six
inches to 42 inches in diameter. The BWT system does not cause flashing
and is designed to withstand extreme cryogenic conditions.
The BWT system is a
collection of bundled cylindrical elements that provide a path for the
ultrasonic signal to travel from the transducer to its wetted tip. This
design provides two major advantages. First, keeping the piezoelectric
element of the transducer away from the process keeps the element near
ambient temperature, thus preventing any potential for temperature
damage. Second, bundling the elements allows transmission of a highly
collimated beam of ultrasound into the fluid producing a high
signal-to-noise ratio for robust, stable measurement.
The BWT system was
developed in 1995 to measure flow in petrochemical processes that
require accurate measurement under extreme conditions. The BWT system
has been used successfully since 2001 for cryogenic LNG flow
measurement in Malaysia, Trinidad and Tobago, Egypt, and Russia.
Panametrics DigitalFlow liquid ultrasonic flowmeters, with the BWT
system, can provide accurate and reliable flow measurement of LNG when
offloading from ships into storage tanks at receiving terminals. In
addition, DigitalFlow gas ultrasonic flowmeters equipped with low
temperature transducers are suited to measure natural gas vapor
recovery flow during the LNG unloading process.
When the LNG is
re-gasified and sold to local transmission or distribution companies,
the Sentinel ultrasonic flowmeter provides accurate measurement for
custody transfer of the natural gas to the local buyer. The Sentinel
meter has been calibrated at the Colorado Experiment Engineering
Station (CEESI, www.ceesi.com
) Southwest Research Institute (SwRI, www.swri.edu
) and Nederlands Meetinstituut (NMi, www.nmi.nl
), and it provides custody transfer-level accuracy in excess of the specifications of the American Gas Association’s (www.aga.org
) Report Number 9, Measurement of Gas by Multipath Ultrasonic Meters.
It is important to
note, there are several alternatives to Coriolis and ultrasonic for
flow measurement in LNG environments, namely orifice-based differential
pressure and turbine meters. These technologies offer their own set of
advantages in such applications. Likewise, there are a number of
Coriolis and ultrasonic systems of different manufacture than the
models represented here. These systems may also be viable options for
LNG flow measurement. The systems detailed here should in no way be
considered the only options for LNG flow measurement. They do, however,
provide solid examples of the advantages offered by two popular flow
measurement concepts for LNG applications.
Matt Migliore is the editor of Flow Control magazine. He can be reached at Matt@GrandViewMedia.com
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