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GTM (Gas Transportation Modules)

GTMs are high pressure storage and transportation systems for compressed gases. Initially used for compressed natural gas (CNG) in locations and countries where there was no infrastructure in place to move the gas from a central pipeline, refinery or well, the usage of GTMs has increased to include a broader variety of gases, including helium, hydrogen, nitrogen and breathing air. In general, the pressure rating for these systems is 3600 psi (248 bar), but can be upwards of 6250 psi (431 bar)

EARLY GTMs

The first GTMs were typically called "tube trailers", as they were constructed of bundles of steel cylinders (or tubes) mounted on a trailer or chassis, or in an ISO shipping container. These systems, manufactured by such companies in the US as Weldship Corp.., Fiba and Western Sales and Testing, continue to be in wide spread use today. Due to the weight of the steel and the corresponding highway weight limits imposed by the US Department of Transportation, these systems can only go so far with respect to capacity. Basically, because more steel is required to contain increased pressure, the systems quickly max out because they bump up against the over-the-road weight restrictions.

To further understand the underlying technology of GTMs, it is important to understand that there are four types of high pressure gas cylinders, all of which were initially designed for use as stand alone compressed gas storage products for either the compressed gas industry or for the rapidly growing field converting cars and trucks to alternative fuels, namely CNG.

UNDERLYING TECHNOLOGY

GTMs rely on bundles of high pressure cylinders manifolded together to a common fill/off load port(s). Since over 70% of the cost of these systems is in the cylinders, it is importnat to understand the underlying technology behind them. Driven to increase capacity, yet be light in weight, the technology grew from products that were initially used for stand alone compressed gas storage; to SCBA (self contained breathing apparatus) devices for fire fighters and for the storage of propellant for paint ball guns; then finally for CNG fueled fleet vehicles.

There are basically four types of cylinders in use today:
 * 1) Type I: These are cylinders that are simply made of steel or aluminum.
 * 2) Type II: These are the same steel or aluminum cylinders (also sometimes referred to as "liners") that are reinforced with either a fiber glass or carbon fiber (or combination thereof) wrap. Type IIs are simply "hoop wrapped", as they do not have any wrapping around the end domes or end caps of the cylinders. These cylinders are lighter in weight than a Type I because the cylinder or liner wall does not need to be as thick, thereby providing the same capacity, but at a lesser weight. The largest manufacturer of these cylinders for use in GTMs is GTM Manufacturing.
 * 3) Type III: Like Type IIs, these cylinders (or liners) are wrapped with carbon fiber and/or fiber glass, though now the ends are also enclosed. This further enables a reduced wall thickness for the cylinders and thus, a reduction in overall weight enabling greater storage capacity. While a Type III cylinder could have a steel liner, in practice, all Type III cylinders in use today have an aluminum liner, thereby maximizing the weight savings. As stand alone cylinders for alternative fuel powered vehicles, these have become common in CNG fueled fleet vehicles (garbage trucks, street sweepers, utility trucks, etc.) and buses, where the considerable weight savings enables the fuel tanks to be mounted on the top of the coaches (rather than underneath on the frame) and consequently provide a lower loading platform for passengers. Leaders in this technology are Luxfer Cylinders, Worthington Cylinders (formerly Structural Composites Industries or SCI) and Dynetek. Type III GTMs are made with cylinders by GTM Technologies and by Dynetek.
 * 4) Type IV: Moving the technology forward, Type IV cylinders replace the metal liner with one made of plastic, theoretically reducing limits to costs (especially of aluminum) and size. With respect to size, the largest Type III cylinder has a diameter of 16" (due to the maximum diameter of commonly available aluminum tube stock) and length of 10'. Conversely, the largest Type IV cylinder, manufactured by the leader in this area, Lincoln Composites, is the TitanTM, which is 42" in diameter and 38 feet long. Lincoln has a sister company which also makes GTMs with Type IV cylinders, Raufoss Fuel Systems.

The "buy decision" of a Type I, II, III or IV GTM system is influenced by such factors as
 * Cost
 * Capacity
 * Cost of Operation (when considering transportation factors such as fuel usage, traffic, etc.)
 * Ease of Filling
 * Safety

In general, Type I systems will be less expensive, however their capacity is limited and their weight is the highest of all Types. Further, due to the weight, their cost to operate is greatest. Type IIs will cost more and weigh marginally less than a Type I system. Type IIIs and Type IV GTMs are the most expensive systems, but provide the greatest capacity options and the lowest cost of operations. Type IV systems represent the most progressive technology, however there have been problems of late resulting from overheating during "fast fill" operations that prevent a capacity fill, and the plastic liner does leak ever so slightly. This leaching of product is permissible within the ISO guidelines, but not an issue with Type IIIs. Finally, when considering the gas to be stored and transported, those with high moisture content can cause rusting and corrosion in all steel systems, and if a vacuum is required to maintain the purity of the gas, the plastic liner of a Type IV can collapse during this process, and thereby ruin the structural integrity of the cylinder.

CERTIFICATION

There is no overall certification, approval or design process for GTMs. However, all cylinders are designed and tested with at least one, if not multiple, standards in mind. These include ISO 11439, NGV2 and TPED. Design criteria is very detailed, and testing procedurs include bonfire, cycle, rupture, drop and gunfire tests. Based on these standards (and the corresponding, independently witnessed testing), the US Department of Transportation (USDOT) will approve cylinders for use in vehicles, but will only provide approval for use in GTMs under a Special Permit. While GTMs have been in use for sometime internationally (especially in Southeast Asia), they are only just now becoming available in the US, as the USDOT has begun to issue Special Permits for the cylinders available from the major manufacturers.

Besides the cylinder design and manufacturing tests, there are periodic retests required by the various agencies. Type I cylinders have to be hydrostatically retested every 5 years, and as a result, can have a very lengthy useful life. The other three cylinder Types must be inspected visually every 3 years, and be taken out of service after 15 years.

It is also important to note that the cylinders and corresponding GTMs are not certified or approved for use by the American Society of Mechanical Engineers (ASME) for stationary storage of compressed gases, but simply for applications where temporary storage is required and/or transportation of the underlying gas.

USES FOR GTMs

From the outset, Type I GTMs have been in use as a bulk transport mechanism for compressed gases, usually from an air separation plant or gas fabrication facility to a remote site for either direct use or to fill smaller storage cylinders. The growing demand for CNG in emerging growth countries, such as those in Southeast Asia (Indonesia, Malaysia, Vietnam and Thailand) where there was no existing natural gas distribution infrastructure, has led to increased usage of GTMs. At the outset, GTMs were used (and still are) to further the "Mother-Daughter Station" fueling concept, whereby GTMs are taken from a well-head or filling station to secondary locations to fuel buses and cars. Increasingly, industries (especially those that require large amounts of fuel, such as glass, brick, paper and tire manufacturers as well as food processors) have been migrating to CNG from the more expensive and dirtier liquid fuels. This is a transition that is likely to take place in the US Northeast and eastern Canadian Provinces, which have traditionally been regions dependent upon heating oil and other liquid fuels.

The United States military has been involved in an active GTM procurement program for helium usage, which has been needed in the Middle East war zones for observation balloons. These GTMs are designed for all forms of transportation, including by air. Nitrogen GTMs have been used to supply oil field servicing projects, and small GTMs are being specified to supply breathing air to workers in confined and potentially contaminated spaces during maintenance operations in refineries and chemical plants. As the hydrogen economy begins to grow, GTMs will certainly be used to move this gas around to more convenient filling locations or to refill the growing number of fuel cells that are powered by hydrogen.