The invention relates to flexible tanks for transporting liquids or semi-liquid materials. More particularly, the invention relates to flexible tanks having improved resistance to leakage and rupture.
Lengthy shipments of goods frequently involve multiple modes of transport, such as ships, railroad cars and trucks. Standardized intermodal shipping containers can be easily moved from place to place in ports and warehouses, and between ships and railroad cars. The standards dictate certain characteristics such as size, location of doors, and the use of specific corners or fittings so that a container can be securely gripped and moved by equipment. Some containers may comply with the standards while having additional unique characteristics, such as being insulated or designed to transport liquids. However, the ability to use any generic standards-compliant shipping container is an advantage because the logistics of making many shipments of different kinds of goods is simplified when a particular shipping container is not necessary.
Flexible tanks (flexitanks) are useful because they enable one to transport bulk liquids within a generic intermodal shipping container so that a shipping container specifically designed for the transport of liquids is not necessary. A primary concern associated with flexitanks is the possibility of rupture. In addition to the obvious loss of the liquid inside, the rupture or failure of a liquid during transport can damage the container in which it is located. If rupture occurs while in the cargo hold of a ship, it may be undiscovered for a long period of time during which the liquid is loose within the cargo hold possibly causing damage thereto or to other containers. A related concern associated with flexitanks is movement of the flexitank within the container during transport. Movement can cause a rupture of a flexitank (even if there is no defect or weakness in the flexitank) by, for example, causing the flexitank to be caught on a snag, abrasion, burr, bolthead, or other deformity on the floor or wall of the container.
Rupture is most frequently the result of stress produced by the liquid dynamics exerted on the flexitank as the container and flexitank is subjected to certain motions. There can be pressure exerted on side walls of the container for example by up and down movement of a ship in windy seas. In particular, sudden starts or stops on a railcar are to be expected, and the liquid is then subjected to dynamic forces and can develop its own wave action. The pressure of such a wave when it hits an end seam of a flexitank can be tremendous. The forces increase exponentially as the volume of liquid and the length of the flexitank increases. For large quantities of a liquid, such as more than 8,000 liters, the forces exerted are quite likely to be too much for the ends of a conventional flexible tank to withstand. For this reason, the flexitank is conventionally longer than the internal length of the container so that the ends of the flexitank are supported by the front inside wall of the container and a bulkhead panel placed across the door opening at the rear wall. Therefore, the flexitank for a 20 foot shipping container may be, for example, 23 feet long. There is a further concern that the flexitank does not deform any of the side or end walls of the container in which it is placed. Intermodal shipping containers are sometimes stacked or placed very close together in cargo holds or ports, with only a few inches of tolerance, and an outwardly deformed wall may interfere with or prevent placement of the container.
Some shipping containers may not be well suited to supporting the ends of a flexitank because, for example, a bulkhead cannot be easily installed or the front wall is corrugated or otherwise configured such that it might cause a rupture of the flexitank. These circumstances are frequently present in larger shipping containers, such as 40 foot or 53 foot containers, or in certain containers such as UMAX® containers recently introduced by North American railroads. Conventional flexitank materials and construction techniques cannot withstand the greater dynamic forces when there is no end support. The ends of the flexitank woven polypropylene layers are typically joined together in a cross-stitched seam as shown in
It is an objective of the preferred embodiments of the invention to provide an improved flexible tank with an improved capability of preventing leakage and rupture when making a long multi-modal shipment of large quantities of a liquid, including when the flexible tank is not supported by the end or side walls of a shipping container.
A preferred embodiment of a flexitank is shown in the accompanying figures.
A cover provides additional strength along the length of the flexitank that will absorb and control the internal liquid dynamics during transport. The cover for the flexitank is preferably constructed from layers of a 610 gram per square meter vinyl fabric on a base reinforcing scrim of either a 14×14 or 20×20 per centimeter polyester thread. Such a relatively high thread count of the scrim provides added strength for the carriage of liquids with a specific gravity higher than water. The diameter of the covering external layers is dependent on the required capacity of the flexitank.
Improved end closures shown in
A process of forming a bag according to a preferred embodiment of the invention is shown in
In the first step, long and narrow fabric layers are welded together longitudinally, preferably by radio frequency (RF) welding, to form the top and bottom external layers. The ends of the top and bottom layers are welded back onto itself as shown in
In the second step, the end flap is welded to the inside of the bottom layer about 30 to 36 inches from each end of the bottom layer. This end flap is preferably the same fabric as the top and bottom outer layers. The end flap has the same width as the top and bottom layers and a length of approximately 7 to 8 feet. At this point, the end flap extends past the end of the bottom layer as shown by dashed line A in
In the third step, the looped ends of the top and bottom layers are cut at the same points to form corresponding equal sized sections of the looped ends as shown in
In the fourth step, a top mounted load/discharge valve is attached to the inner liner through an opening on the top external layer centrally placed widthwise and near one end seam lengthwise, preferably about 30 to 36 inches from the end seam. The valve is preferably secured using a clamp. The inner liner, with its 2-4 layers already formed and welded together at the ends, is inserted through the open end of the bag nearer the valve and positioned between the top and bottom layers. Any “coupon” of the inner liner at the closed end of the bag is tucked so that it lays flat against the outer layers. Any “coupon” of the inner liner at the open end of the bag is tucked and then the additional layer of fabric is moved from the position of dashed line A in
In the final step, the nylon rope or similar securing element is threaded through the alternating interlaced loops of the open ends of the bag completely across the seams. The rope closes the seams and secures the flexitank into the cover. When the bag is filled with liquid as shown in
The flexitank is preferably kept relatively low in height. Two or three baffles, external to the flexitank, can optionally be installed in the shipping container to restrict waves during transit. The baffles offer low height channels (for example, from 2-4 inches) for the liquid to flow through and effectively divide a single liner into three or four sections. This controls the liquid dynamics of the liquid and thus reduces dynamic loading on the end-closures of the flexitank. The baffles may be constructed and secured to the container in any suitable manner. Although a shipping container may have the baffles welded or otherwise permanently installed, the presence of the baffles may be a detriment when the container is being used to transport goods without a flexitank. It is preferable that the external baffles may be easily installed in a standard shipping container when a flexitank is used and removed after use. A preferred example of a removable baffle is the compression bar shown in
A flexible tank having an end closure according to the invention may vary in multiple ways from the precise description provided herein. In particular, the flexitank with the end closure may be used without the optional baffles and may be used independently of a shipping container. The extra strength provided by the end closure may permit a flexible tank to be used in a variety of industries, purposes, circumstances, and environments not specifically identified herein.
This application claims priority to U.S. Provisional Patent Application No. 62/579,612 filed on Oct. 31, 2017, the contents of which are hereby incorporated by reference in their entirety.
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Number | Date | Country | |
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20190202631 A1 | Jul 2019 | US |
Number | Date | Country | |
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62579612 | Oct 2017 | US |