CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This patent application is a continuation of U.S. patent application Ser. No. 10/718,365, filed Nov. 20, 2003, which is a continuation of U.S. patent application Ser. No. 10/377,382, filed Feb. 28, 2003, which claims the benefit of U.S. Provisional Patent Application No. 60/360,373, filed Mar. 1, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a lightweight storage tank for fluids. More specifically, the invention relates to a fabric reinforced, thermoplastic coated, flexible container utilized, for example, for storage of liquid fuel, potable water or liquid hazardous waste.  
       BACKGROUND OF THE INVENTION  
       [0003]     Flexible liquid storage tanks of relatively high capacity that exhibit a pillow- or sausage-like shape when filled are widely known as “pillow tanks.” They are typically composed of thermoplastic materials, such as polyether or polyester, and may include two or more layers of material. These tanks can be used to store, for example, gasoline, diesel fuel, jet fuel, potable water or hazardous liquid waste. Flexible storage tanks have the advantages of light weight and portability. Also, flexible storage tanks can be stored in a relatively small volume until needed.  
         [0004]     However, conventional flexible storage tanks typically include seams, which are often the source of leakage. In particular, conventional flexible storage tanks are constructed in shapes that subject the seams of their flexible walls to stresses oriented perpendicularly to the flexible walls. These perpendicular stresses, widely known as “normal stresses,” are more difficult to seal against than “shearing stresses” (also known as “tangential stresses”).  
         [0005]     For example, U.S. Pat. No. 3,453,164, issued to Gursky et al., describes a method of building fabric elastomeric containers in which a fabric is cut into strips and a tube is assembled by overlapping the edges of the strips in stitching to form individual seams. A liquid polyurethane reaction mixture and a material suitable for forming a fuel vapor barrier are applied to both sides of the assembled tube. Two end members are formed by folding pieces of the fabric into U-shapes. One of the U-shaped end members is cemented on each end of the tube to form a substantially rectangular container. Then each of the corners is trimmed to remove a triangle of fabric from each corner. Specially shaped and sized pieces of knit fabric are cemented over the trimmed areas to produce somewhat rounded corners, as indicated for knit fabric piece 41 in FIG. 4 of the Gursky et al. patent. However, as can be seen in FIG. 5 of the Gursky et al. patent, the finished container is still substantially rectangular in shape. Because neither this substantially rectangular shape nor the shape of fabric piece 41 matches the configuration of hydraulic forces within the container when it is filled with a liquid, at least some of the seams in the container of Gursky et al. patent are likely to be exposed to predominantly normal stresses.  
         [0006]     A collapsible storage tank is described in U.S. Pat. No. 4,573,508, issued to Knaus, as including a substantially rectangular tank body composed of vulcanized inner and outer envelope structures. As can be seen in FIG. 3 of the Knaus patent, the collapsible storage tank includes rectangular corners and a peripheral seam that do not conform to the configuration of hydraulic forces that arise when the collapsible storage tank is filled with a liquid.  
         [0007]     U.S. Pat. No. 3,919,030, issued to Jones, describes an elastic, fluid impervious storage tank having an intermediate section and a pair of end sections. Each of the end sections is reportedly formed from a single blank composed of a fiber-reinforced elastomer, which is cut and folded so that the corner portions are of rounded or arcuate configuration. The Jones et al. patent recites that this rounded corner portion greatly increases the strength of the storage tank, as compared to tanks having angular corner portions. According to the Jones patent, the cuts in the end section are closed by adhering a pre-formed arcuate inner attachment member 36 to the inside surface of the corner portion and, also adhering a pre-formed arcuate outer attachment 37 to the outer surface of each corner portion. The need to employ pre-formed members 36, 37 is a disadvantage in some situations. Also, it appears that any advantages associated with the storage tank of the Jones patent are limited to storage tanks that are small enough for the end sections to be constructed from one or two blanks of fabric material. Relatively larger tanks, which must necessarily be constructed of a number of fabric panels, are excluded.  
         [0008]     A need exists for an improved flexible storage tank constructed in a configuration that is less susceptible to leaking when constructed of commonly available materials. Preferably, the improved tank is formed in shapes of revolution having relatively greater radii, as compared to conventional flexible storage tanks. Ideally, the seams of the improved tank are substantially under shearing stress, rather than normal stress, when the tank is filled with a liquid.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The invention provides a soft shell, flexible storage tank, including corners of an improved configuration adapted to resist leaks. The corners are relatively more rounded and larger than those of conventional flexible tanks. Additionally, the walls of the tank are fabricated from thermoplastic panels joined in lapped seams by a heat treatment. The walls and the improved corners act together to resist leaks. The rounded corners tend to reduce the effective pressure in the tank walls by loading the seams predominantly in shearing stress when the tank is filled with a liquid.  
         [0010]     The tank is especially resistant to leaks which might otherwise occur in the seams and adjacent the ends and corners of the tank. The tank is configured in relatively large-radius shapes of revolution which tend to place the seams in shearing stress, as opposed to normal stress. The shapes of revolution are developed as panels, which are thermally bonded to produce secure and reliable liquid-tight seams. Each of the panels is bonded by lapped seams to others of the panels.  
         [0011]     In developing the panels, allowance may be made for panel stretching under the influence of gravity and hydraulic pressure, over a specific range of ambient temperature and storage fluid density. The total number of panels and length of seams are other factors to consider.  
         [0012]     The tank is unique in that it includes specially rounded corners, sometimes called “elegant corners.” In effect, the flexible storage tank functions as a pressure vessel, which tends to resist leakage. Each of the rounded corners includes four panels. One of the four panels is generally triangular, being bounded by three curved edges. Each of the curved edges is bonded by a lapped seam with one of the four panels. The rounded corner is generally ellipsoidal when the tank is filled with a liquid.  
         [0013]     The tanks are formed from panels composed of thermoplastic material. The panels are sealed, effectively welded together, by an application of heat. Flexible urethane material is used for the tank, preferably polyether or polyester, most preferably polyether. Additionally, the edges of the panels are filled up with a film of rubber gum to further reduce leakage. The resulting seams are superior to conventional seams that have previously been created by use of glue or solvents. The tanks are suitable for use with water and aromatic storage liquids.  
         [0014]     The corners of the tank are built-up by joining thermoplastic panels. While this may increase the time required to make the tank, the improved rounded corners reduce the effect of pressure from liquid that occurs in the corners of the tank. This reduces the possibility of leakage. The elegant corner does not concentrate stress at any point in the corner.  
         [0015]     In order to make a flexible storage tank of the present invention, one or more sheets composed essentially of a thermoplastic material and a fabric layer are cut to produce panels of various shapes. A number of top panels of approximately equal length and generally rectangular shape are produced in this manner. Bottoms panels of approximately the same length as the top panels and generally rectangular shape are also fashioned from one or more sheets. Side panels are cut to a length less than that of the top panels and the bottom panels. Generally triangular panels are shaped so as to be bounded by three curved edges.  
         [0016]     These panels are assembled by bonding the top panels to each other to produce a generally rectangular topside assembly. The bottom panels are bonded to each other to each other to produce a generally rectangular bottomside assembly. Each of the ends of the top panels is bonded, respectively, with one of the ends of the bottom panels. The triangular panels are attached by a lapped seam bonding each of the curved edges with one of the top panels, one of the bottom panels or one of the side panels to produce a flexible storage tank having rounded corners.  
         [0017]     Previously known similar products were called “pillow tanks”. They had somewhat rounded corners, but used cement to form the seams and were configured in a different geometry than the tank of the instant invention. They were more prone to leakage than the tanks of the present invention.  
         [0018]     The improved tank is made in various sizes. In order to reduce the cost of construction, the width of the various sizes of tank is fixed and the volume is adjusted by varying the length of the particular tank. In this way, the tank sizes are expandable. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a plan view of a 50,000-gallon flexible fuel storage tank  100 ;  
         [0020]      FIG. 2  is an elevation end view of the tank depicted in  FIG. 1 ;  
         [0021]      FIG. 3  is a development of panel  1  for the tank depicted in  FIG. 1 ;  
         [0022]      FIG. 4  is a development of panel  2  for the tank depicted in  FIG. 1 ;  
         [0023]      FIG. 5  is a development of panel  3  for the tank depicted in  FIG. 1 ;  
         [0024]      FIG. 6  is a development of panel  4  for the tank depicted in  FIG. 1 ;  
         [0025]      FIG. 7  is a development of panel  5  for the tank depicted in  FIG. 1 ;  
         [0026]      FIG. 8  is a development of panel  6  for the tank depicted in  FIG. 1 ;  
         [0027]      FIG. 9  is a development of panel  7  for the tank depicted in  FIG. 1 ;  
         [0028]      FIG. 10  is a development of panel  8  for the tank depicted in  FIG. 1 ;  
         [0029]      FIG. 11  is a development of panel  9  for the tank depicted in  FIG. 1 ;  
         [0030]      FIG. 12  is a development of panel  10  for the tank depicted in  FIG. 1 ;  
         [0031]      FIG. 13  is a development of panel  11  for the tank depicted in  FIG. 1 ;  
         [0032]      FIG. 14  is a development of accessories for the tank depicted in  FIG. 1 ;  
         [0033]      FIG. 15  is an assembly drawing for the tank depicted in  FIG. 1 ;  
         [0034]      FIG. 16  is a partial perspective view of tube  50  constituted by panels  1 - 11  according to the invention;  
         [0035]      FIG. 17  is a partial perspective view of tube  50  with top panels  1 ,  2 ,  3 ,  10  and  11  and bottom panels  5 - 8  joined in a closed end;  
         [0036]      FIG. 18  is a partial perspective view of tank  100  showing the placement of triangular panels  15 ,  16 ; and  
         [0037]      FIG. 19  is a partial side view of tank  100  drawn to scale, with a human figure (not part of the invention) included to convey the size of tank  100 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]     In a preferred embodiment, the invention provides a flexible, soft shell, fuel storage tank  100 , as depicted in  FIG. 1-15 . Tank  100  is useful for containing, for example, diesel fuel or jet fuel and has a capacity of 50,000 United States gallons. Tank  100  is adapted to resist leaks over a range of operating temperature up to about 130 degrees F.  
         [0039]     Referring now to  FIG. 1 , tank  100  is of lapped seam construction with seams  13  extending along the length of tank  100 . A “lapped seam,” also known as a lapped joint, means a seam made by lapping one piece or part over another and fastening them together. Corners  12  of tank  100  are noticeably rounded, and built-up employing several panels  1 - 11 ,  15 - 18  of definite shape. The improvement afforded by rounded corners  12  is analogous to that found in rounded pressure vessels.  
         [0040]     As depicted in  FIG. 2 , panels  1 - 11 ,  15 - 18  are composed of a layer of thermoplastic material, such as polyester or polyether, and a fabric layer. Other materials may be employed based, among other things, on the physical and chemical characteristics of the liquid intended for storage and the expected operating temperature conditions. Seams  13  of tank  100  are sealed by applying heat to the thermoplastic material. The resulting welded seam  13  is superior to seams formed by applying glues or solvents.  
         [0041]     While tank  100  is a 50,000 gallon tank, tanks of other capacities may be easily fashioned by employing the same end dimensions and adjusting the lengths of the respective panels.  
         [0042]     Tank  100  is 64½ feet long and 23 feet wide when filled. Tank  100  includes two manways  22  for inspection and cleaning. Each of the manways  22  is located six feet from an end of tank  100 . Tank  100  also includes two floor cutouts  24 , two floor drains  27  and a vent  28 , which is fitted with a flame arrestor (not shown). Filling is accomplished through one or more of the manways  22  via a flexible filler hose of 4 inches diameter.  
         [0043]     As shown in  FIG. 2 , tank  100  has a generally elliptical transverse cross-section, bounded by panels  1 - 11 ,  15 - 18 . Panels  1 - 11  are numbered sequentially beginning at the top center line of tank  100  and proceeding in a clockwise direction, as depicted in  FIG. 2 . Each of panels  1 - 11  is symmetrical with regard to a center line that is perpendicular to the length of the respective panel.  
         [0044]     In order to better communicate the invention, panels  1 ,  2 ,  3 ,  10  and  11  are referred to as top panels, together forming topside assembly  56  (best seen in  FIG. 16 ). Top panels  1 ,  2 ,  3 ,  10  and  11 , are all between 66 and 67 feet in length, and are about 4 and ½ feet in width, although not all are exactly rectangular. Panels  5 - 8  are referred to as bottom panels, together forming bottomside assembly  58  (best seen in  FIG. 16 ). Bottom panels  5 - 8  are all about 65 feet in length, and about 4 and ½ feet in width, although they are exactly rectangular. Panels  4  and  9  are referred to as side panels, each having a length of less than about 60 and a width of about 4 and ½ feet, although they are not exactly rectangular. Joining each of the side panels  4 ,  9 , respectively to topside assembly  56  and to bottomside assembly  58  produces flexible tube  50  (best seen in  FIG. 16 ).  
         [0045]     Each of panels  15 - 18  is generally triangular and bounded by three of curved edges  30 - 41 , as described below. Tank  100  is 5½ feet in height when filled and is provided with thirty-two handles, for use in folding, positioning or securing tank  100 .  
         [0046]      FIGS. 3-13  depict developments of panels  1 - 11 ,  15 - 18 , which illustrate details for each of panels  1 - 11 ,  15 - 18 . Referring to  FIG. 3 , panel  1  is in the shape of a rectangle 66 feet and 10¼ inches long and 4⅔ feet wide. A 3-inch wide seam area is designated along each end of panel  1 . A vent cutout  29  is located at the intersection of the center-lines of rectangular panel  1 .  
         [0047]     As shown in  FIG. 4 , panel  2  has a generally rectangular, six-sided shape cut from a rectangle 4⅔ feet wide and 66 feet and 10¼ inches long. Panel  2  has one straight edge 66 feet and 5⅝ inches in length. Panel  2  has a second straight edge of length 48 feet and 10½ inches in length, centered on and parallel to the first straight edge at a distance of 4 feet and 8 inches. The ends of panel  2  are slightly obtuse with respect to the first straight edge, each extending from the first straight edge to a terminus located 4 feet and 5¼ inches transversely from the first straight edge and 66 feet and 10¼ inches from the terminus of the opposite end. Two additional straight edges, each measuring 8 feet and  11⅞ inches connect the ends with the second straight edge. A  3-inch wide seam area is designated along each end of panel  2 . A manway cutout  26  is located at the intersection of the axial center of panel  2 , about six and a half feet from the nearest end.  
         [0048]      FIG. 5  depicts panel  3 , which may be cut from a rectangle 66 feet and 4 and 3/16 inches long and 4 feet and 5 and ¼ inches wide. Panel  3  has two centrally located, parallel edges of length 52 feet and 5 and ¾ inches and 48 feet and 3 and ¾ inches, respectively. A seam area 2 and ½ inches wide is designated along the shorter of these parallel edges. Each of the ends of panel  3  is an oblique straight edge having a length of 2 feet and 6 and 15/16 inches. A seam area having a width of 3 inches is designated along each of the straight edge ends of panel  3 . The straight edge ends are connected to the parallel edges by curves, as shown in  FIG. 5 .  
         [0049]     Panel  4  and separate triangular panels  15 ,  17  are depicted in  FIG. 6 . Panels  4 ,  15  and  17  are shown together in  FIG. 6  to emphasize that all three may be cut from a single rectangular sheet that is 66 feet and 5 inches long and 4 feet and 5 and ¼ inches wide. Panel  4 , which is one of the side panels  4 ,  9 , is generally rectangular with an overall length of 59 feet and 8 and ⅞ inches. A seam area having a width of 2 and ½ inches is designated along three of the edges of panel  4 .  
         [0050]     Continuing with  FIG. 6 , panels  15  and  17  are mirror images of each other. Each of the panels  15 ,  17  is generally triangular in shape and bounded by three curved edges. A triangle inscribed within and sharing the vertices of panel  15  would have one altitude of about 2 feet and 4 inches and another altitude of about 4 feet and 3 inches. A seam area 2 and ½ inches wide is designated along intermediate length curved edges  31 ,  37  of panels  15 ,  17 , respectively.  
         [0051]     As depicted in  FIG. 7 , panel  5  is generally rectangular with a length of 64 feet and 10 and 13/16 inches and a width of 4 feet and 8 inches. Drain cutout  24  is located 5 and ½ feet from the nearest end.  
         [0052]     Panel  6 , shown in  FIG. 8 , is generally rectangular with a length of about 64 feet and 10 and ½ inches and a width 4 feet and 8 inches.  
         [0053]     Panel  7 , which is depicted in  FIG. 9 , is the mirror image of panel  6 .  
         [0054]     Panel  8 , which is depicted in  FIG. 10 , is the mirror image of panel  5 .  
         [0055]     Panel  9 , which is depicted in  FIG. 11 , is the mirror image of panel  4 . Panels  16  and  17 , which are also depicted in  FIG. 11 , are the mirror images of panels  15  and  17 , respectively  
         [0056]     Panel  10 , which is depicted in  FIG. 12 , is the mirror image of panel  3 .  
         [0057]     Panel  11 , which is depicted in  FIG. 13 , is the mirror image of panel  2 .  
         [0058]      FIG. 14  illustrates accessory panels, which may be optionally be used in constructing tank  100 . For example, the legend “MW CHAF  37 ” in  FIG. 14  designates a chaffing pad to be located directly beneath one of the manways  22 .  
         [0059]      FIG. 15  is an assembly drawing, which depicts the manner in which panels  1 - 11  are joined with regard to each other. More specifically, a preferred method of making tank  100  includes joining each of panels  1 - 11  to two others of panels  1 - 11  by thermally bonded, lapped seams  13  in the relationship illustrated in  FIG. 15 . Seams  13  may be bonded in any order; preferably, panels  6  and panel  7  are the last of this group to be bonded.  
         [0060]     Bonding panels  1 - 11  along their edges produces tube  50 , as depicted in  FIG. 16 . Panels  1 ,  2 ,  3 ,  10 , and  11 , together, constitute topside assembly  56 . Panels  5 - 8 , together, constitute bottomside assembly  58 . Side panels  4 ,  9  join the two assemblies  56 ,  58  to each other. Tube  50  includes end  52 , which is shown in  FIG. 16 , and end  54  (not shown.).  
         [0061]     After tube  50  has been assembled, the ends of top panels  1 ,  2 ,  3 ,  10 , and  11  each may be bonded, respectively, with the ends of one of bottom panels  5 - 8 . This bonding substantially closes end  52  of tube  50 , as shown in  FIG. 17 , and end  54  (not shown). However, as can be seen in  FIG. 17 , openings still remain.  
         [0062]      FIG. 18  depicts triangular panels  15 ,  16  positioned and bonded to complete the closing of end  52  of tube  50 . Triangular panels  17 ,  18  (best seen in  FIG. 11 ) are similarly positioned and bonded to close end  54  (not shown). Curved edge  30 , which is the shortest edge of panel  15 , bonds to panel  3 . Curved edge  31 , which is of intermediate length in panel  15  bonds to panel  5 . Curved edge  32 , which is the longest edge of panel  15 , bonds to panel  4 . Each the curved edges  30 ,  31 ,  32 , respectively, is thermally bonded in a lapped seam  13  with one of panels  3 ,  5 ,  4  to complete one of the rounded corners  12 .  
         [0063]     When assembled, tank  100  comprises 3500 square feet of coated fabric and weighs 1,090 pounds. Because seams  13  of tank  100  are substantially under shearing stress, rather than normal stress, tank  100  tends to resist leakage when filled with a storage liquid.  
         [0064]     While only a few, preferred embodiments of the invention have been described above, those of ordinary skill in the art will recognize that these embodiments may be modified and altered without departing from the central spirit and scope of the invention. The preferred embodiments described above are to be considered in all respects as illustrative and not restrictive.
A soft shell, flexible storage tank is provided, including corners of an improved configuration adapted to resist leaks. The walls of the tank and the corners are relatively more rounded and larger in radius than those of conventional flexible tanks. The walls and the improved corners act together to resist leaks. The rounded corners tend to reduce the effective pressure in the tank walls by loading the seams predominantly in shearing stress when the tank is filled with a liquid. The rounded corners are developed as panels, which are thermally bonded to produce secure and reliable liquid-tight seams. Each of the panels is bonded by lapped seams to others of the panels. Each of the rounded corners includes a generally triangular corner that is bonded to at least three other panels. In effect, the flexible storage tank functions as a pressure vessel, which tends to resist leakage.