Vacuum insulated pipe seeks increased use with LNG.
VIP systems actually are a pipe within a pipe. Depending on service requirements, a wide range of pipe materials is available. However, the inner LNG pipe normally is ASTM A312 304/304L stainless steel pipe while the outer pipe may be made of the same stainless, carbon steel or one of many other alloys now available.
The annular space between the inner and outer pipe is evacuated in the factory to a vacuum level in the 10-3 Torr range and is then sealed to create a static vacuum that will last the life of the plant. This vacuum nearly eliminates the convective heat transfer into the LNG pipe and makes this system thermally efficient. It is also an important step in eliminating or substantially reducing all other means of heat transfer in the LNG.
This is done by installing multilayer insulation around the inner pipe to reduce the radiation heat transfer into the LNG. The VIP design further reduces conductive heat transfer into the LNG by using special purpose bayonet end connections. These are some of the standard VIP design innovations that are incorporated for all cryogenic fluids to achieve a highly efficient insulation method that cannot be achieved by any other insulation method available today.
VIP can be used in many applications that cannot use MIP. These include sub-sea, underground, and as vertical risers up the side of storage tanks. VIP can offer processors significantly lower capital costs at the initial stages of implementation and will always afford tremendous opportunity cost savings through greatly increased efficiency and reliability over traditional MIP.
Historically, LNG terminals field fabricate all necessary cryogenic pipe and then insulate it with polyurethane foam, foam glass or other mechanically insulated materials. The thickness of this insulation varies from 6-12 inches, depending on a customer's requirements and the insulation type.
Insulation subsequently is installed in many different environments--most being near water. Thus, it is difficult to control the quality of installation and oftentimes, contamination results. Additionally, the support system for pipe that utilizes mechanical insulation uses expensive cold shoes and increases the heat leak into the pipe. It is difficult to seal the vapor barrier at these supports.
Over time, the insulation degrades, and the vapor barrier allows moisture into the insulation space that eventually destroys the insulation itself. Such degradation is critical in LNG export terminals where all vapor is re-condensed. The initial sizing of these re-condensers is based on certain insulation values for the pipe system. It is also critical in receiving terminals for safety and reduces maintenance and operational costs.
VIP has been manufactured for more than 45 years as a low-heat leak transfer system for many cryogenic fluids. The primary fluid that initiated development of VIP was liquid helium. This is the coldest cryogenic fluid (-269[degrees]C/-452[degrees]F) and has a very low latent heat of vaporization of about 9 Btu/lb. As such, it is impossible to transfer liquid helium from one point to another with a standard MIP system since it would boil almost immediately upon entering it.
VIP was developed in the early 1950s by Ohio State University Professor Herrick Johnson for transferring fluids as part of a hydrogen liquefier system. Since then, it has been used extensively for most other cryogenic fluids (including methane) as well as by all industrial gas suppliers, national laboratories, aerospace companies, NASA, food-processing operations and the entertainment industry.
Static vacuum insulated, laminar radiation-shielded pipe is offered in rigid and flexible styles with inner-line diameters ranging from 3/8 inch through 48 inches. Its stainless steel construction, TIG welding, mass spectrometer leak checking and extensive vacuum retention testing help to ensure reliable, maintenance-free service.
Another key benefit of VIP is the ability to bury it, thus keeping it out of sight and preventing potential tampering.
Overall, VIP affords owners and operators a lower total cost than MIP. Consequently, system owners and operators can avoid high installation labor costs, ongoing maintenance, liquid losses and lost revenue due to longer deployment time that are inherent in MIP installations. Traditionally, VIP has slightly higher initial equipment costs, but tremendous savings are realized through lower installation and life-cycle costs. Figure 1 illustrates a side-by-side comparison of characteristics associated with VIP and MIP.
The recent introduction of VIP for LNG transfer service is a significant development for the industry. VIP was first used to transfer LNG in 1998 through a 2,493-foot, 4-inch diameter cool-down line in Trinidad. The line is still operational with no degradation in performance or maintenance requirements. The time needed to install the entire line and to be ready for operation was less than one week. Since then, other installations have occurred in the U.S., Egypt and Australia. Sizes now being built range from one-to-36 inches. VIP uses in LNG applications include:
* LNG Tank Riser. The long distance between the ground and the top of a tank requires strong support that VIP provides. The "pipe within a pipe" requires fewer supports and the built-in bellows makes installation easier. Due to its complete shop fabrication nature, the tank riser can be provided as a single module that is easy to install.
* Terminal Ship Jetty Marker. High reliability and low maintenance make VIP ideal for underwater applications such as this.
* Storage Tank To Vaporizer Pipe. VIP minimizes bi-phase flow. This increases useful product flow and vaporizer pump life.
Service And Support
It is wise to consider VIP manufacturers who offer comprehensive on-site installation, repair and replacement. Repair to damaged pipe should also be accompanied by full damage assessment and testing procedures. All work should be carried out to the original manufacturing quality criteria and be in full compliance with ASME B31.3 and NFPA 59A, as well as any additional customer specifications. The material should also be ASTM A312, 304/304L stainless steel.
Many economic models indicate that any cryogenic transfer line more than 200 meters long is a viable candidate for VIP. Furthermore, the models indicate that the greater the length, the quicker the payback.
Comparing only the cost of a single-wall pipe to VIP's double wall construction does not accurately evaluate the respective designs for cryogenic transfer. However, a proper evaluation should include the total costs of the installed pipe systems. Any evaluation must include all insulation and lagging costs, the minimized support requirements, reduced life cycle maintenance (upkeep and replacement) costs, and due consideration for the higher plant efficiencies that will result from reduced liquid losses through heat leak and pressure drop as well as re-liquefaction costs. An impartial economic analysis that includes these considerations will reveal which technology--mechanically insulated pipe or VIP--is the better solution for your application. Circle #201 or click www.thru.to/pgj
Figure 1: VIP MIP Life Designed to last 20 to 30 Typically lasts 6 to 8 Expectancy years. Requires little years. Requires maintenance. consistent, regular maintenance. Bellows Shop installed, internal Requires extensive field or external application. welding, expansion loops No field installation and complex mechanical work required. insulation and pipe support systems. Assembly When equipped with Must be field welded, bayonet assemblies, set x-rayed, leak tested, at elevation, mate the cleaned, finish painted bayonet assemblies and and insulated at seal weld one joint. customer's plant site. Installation Requires only 15% of Takes more than six times Time conventional pipeline longer than VIP. installation labor and timeframe. Extent of Shop Fully factory fabricated, Mechanical insulation as Fabrication cleaned, finish painted, well as the piping is and tested prior to completely done in the shipment. field including examination and testing. Bowing Internal bellows design Frequently occurs. eliminates all squirm. System does not bow during cool down. Anchors Rigid carbon steel. Requires special Easily welded to the insulated anchors. carbon steel outer jacket. Support System Simple carbon steel Must be manufactured from (costs less than special materials. mechanically insulated pipe supports). Supports Requires fewer supports. Requires two-times more Carbon/stainless steel supports. jacket allows for twice the distance between supports. Performance None Begins a short time after Degradation first cooldown of the line. Continues to increase as the system ages. Heat Leak Is 10% to 12% that of Eight to ten times that mechanically insulated of VIP systems. Increases pipe based on spool as the system ages. lengths of 24.5 meters or longer. Vacuum Pumping Vacuum systems are sealed Mechanical insulation requiring no maintenance. requires periodic System vacuum will remain servicing that constant and stable progressively increases throughout life of pipe over time and eventually system unless it is will require replacement. physically damaged. Continuous pumping is eliminated. Capital Costs Initial system Total Total Installed Costs Installed Costs (TIC) are (TIC) of the pipe and about the same as insulation are about the mechanically insulted same as VIP. pipe. However, significant life-cycle costs savings will be realized. Commercial Industry leaders assume Multiple system suppliers Responsibility single source (pipe. bellows, responsibility, and full insulation and guarantee and warranty installers). responsibility. Avoids conflicts.
Editor's Note: Pipeline & Gas Journal does not endorse products or services.
Author: John Bonn is a sales engineer for Chart Industries, Energy and Chemicals Group. He has more than 40 years of experience in cryogenic equipment design and development and holds several patents.
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|Title Annotation:||Tech Notes|
|Publication:||Pipeline & Gas Journal|
|Date:||Nov 1, 2004|
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