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|Figure 1. Currently, there are five LNG import terminals located in the
United States, Canada, the Bahamas, and Mexico, with one LNG
liquefaction train located in Alaska.|
These proposed facilities represent approximately 60 billion cubic feet per day of new natural gas supply. By 2012, as many as 12 new facilities are expected to go into service, and some analysts predict a new facility will be required every few years thereafter (Figure 3).
|Figure 2. There are 39 LNG terminals approved and/or proposed for construction in North America.|
flowmeters capable of supporting cryogenic liquids. Developers are aggressively pursuing meters that can accurately and efficiently measure natural gas as it is converted from gas to liquid and from liquid back to gas. As with any technology under consideration for LNG processes, cost effectiveness is key. In the past, cost has been the primary stumbling block limiting liquid natural gas development. Several metering concepts, however, are being touted as a means to an efficient end for LNG processing.
What Is LNG?
Natural gas in its natural state is, naturally, a gas. When it is cooled to an atmospheric temperature of - 260 F (-161 C), natural gas condenses to a liquefied state. As a liquid, natural gas consumes approximately 600 times less space than it does in a gaseous state, making it economically feasible to transport to locations far and wide where natural gas resources are scarce.
|Figure 3. Beyond the facilities that have been approved and/or
proposed, an additional 20 LNG terminals could potentially achieve such
status in the near future.|
Liquefaction plants and receiving terminals typically have their own ship docking facilities built as part of the project. The Everett, Mass. LNG receiving terminal, for example, passes ships through Boston Harbor escorted by the U.S. Coast Guard to its own private dock. However, security concerns are causing new facilities to be moved away from populated areas or offshore. The FPSO (Floating Processing Storage and Offloading) plant design, located offshore like a drilling rig, represents the next generation in plant design.
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 users.
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
technological 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 decreased.
Currently, LNG 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.
Flowmeters for 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.
Coriolis, orifice, 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.
|Independent 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 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.
Since Coriolis 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 clean.
Independent 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 (www.nist.org) 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.|
Today, transit-time 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.
Transit-time 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.
Installation is 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.
After processing, 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 processing.
After liquefaction, 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.
GE Sensing’s 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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