Patent Publication Number: US-RE37144-E

Title: Above-ground fire-resistant storage tank system and fabrication method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an above-ground fire-resistant tank system for flammable and combustible material. More specifically, the invention relates to a tank, fabricated from a specific metal of a minimum thickness which is welded in a specific manner, which can withstand 2000° F. environment for a minimum of two hours. 
     2. Description of the Related Technology 
     Numerous applications exist for above-ground tanks for flammable and combustible materials, i.e. gas stations, airports, and construction sites. In all these applications, and more, tanks storing flammable and combustible materials have a potential for fire and explosion, which should obviously be minimized. It has been determined that the risk is sufficiently minimized when a tank can withstand a 2000° F. environment for two hours, as per Underwriters Laboratories test procedure 2085. 
     A number of different designs of above-ground fire-resistant tanks have been developed which meet the Underwriters Laboratories test procedures. One type of tank is encased in a thickness of cement, which increases the cost of a tank. Besides the economic penalty of adding another component to the tank, the cement is a minimum six inches thick which decreases its maneuverability. Another design requires constructing a storage tank, surrounding the storage tank with insulation, and constructing a secondary containment tank around the insulation. The insulation adds material cost to the tanks and increases the construction labor costs. 
     It is clear that there has existed a long and unfilled need in the prior art for an above-ground fire-resistant tank for flammable and combustible liquids which does not require additional components in order to withstand 2000° F. for two hours. 
     SUMMARY OF THE INVENTION 
     Accordingly, is it an object of the invention to provide a fire-resistant above-ground tank system for storing flammable and combustible material, such as gasoline, by fabricating a tank from steel plates of at least 10 gauge, the steel having a maximum of approximately 0.15% carbon and a maximum of approximately 0.8% manganese. The plates being welded together either with joggle joints or butt weld joints. Further, at least one of the welds contains weld metal having a maximum of approximately 0.15% carbon and a maximum of approximately 1.4% manganese. The tank system contains tank fittings to transfer material into and out of the tank. 
     It is a further object to provide a double-wall fire-resistant above-ground tank system for storing flammable and combustible material, such as gasoline, by fabricating a product storage tank from steel plates of at least 7 gauge and nesting the storage tank within a secondary containment tank fabricated from steel plates of at least 10 gauge. Both tanks being made out of steel plates having a maximum of approximately 0.15% carbon and a maximum of approximately 0.8% manganese. The plates being welded together either with joggle joints or butt weld joints. Further, at least one of the welds contains weld metal having a maximum of approximately 0.15% carbon and approximately 1.4% manganese. 
     It is another object to provide a method to fabricate a fire-resistant above-ground tank system for storing flammable and combustible material, such as gasoline, by fabricating a product storage tank of at 10 gauge steel plates. The steel having a maximum of approximately 0.15% carbon and a maximum of approximately 0.8% manganese. The plates being welded together either with joggle joints or butt weld joints. The welds are made either using a submerged arc welder at 225 to 280 amps with L 61 wire and 761 flux or a hot gas metal arc welder with the gas shield being carbon dioxide, the wire feed being an AWS E71T-1 class, titania type flux cored wire designed for use with 100% carbon dioxide gas shielding, the wire having a maximum of approximately 0.15% carbon and a maximum of approximately 1.4% manganese, and the amperage used during welding is 180 to 220 amps. The tank system contains tank fittings to transfer material into and out of the tank. 
     It is another object to provide a method to fabricate a fire-resistant above-ground tank system for storing flammable and combustible material, such as gasoline, by fabricating a double wall tank with an inner product storage tank made from steel plates of at least 7 gauge and an outer secondary containment tank made from steel plates of at least 10 gauge. The steel for both tanks having a maximum of approximately 0.15% carbon and a maximum of approximately 0.8% manganese. The plates being welded together either with joggle joints or butt weld joints. The welds are made either using a submerged arc welder at 225 to 280 amps with L 61 wire and 761 flux or a hot gas metal arc welder with the gas shield being carbon dioxide, the wire feed being an AWS E71T-1 class, titania type flux cored wire designed for use with 100% carbon dioxide gas shielding, the wire having a maximum of approximately 0.15% carbon and a maximum of approximately 1.4% manganese, and the amperage used during welding is 180 to 220 amps. The tank system contains tank fittings to transfer material into and out of the tank. 
     In order to achieve the above and other objects of the invention, an above-ground fire-resistant storage tank system for storing combustible material, such as gasoline, is fabricated from steel plates of a minimum thickness, the sheets being made from a special metal alloy, and the plates welded in a specific way, in order to withstand a 2000° F. environment for a minimum of two hours. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective view of a single wall tank lacking end panels; 
     FIG. 2 illustrates an elevation of a single wall tank; 
     FIG. 3 illustrates an end view of a single wall tanks; 
     FIG. 4 illustrates an end view of a skid; 
     FIG. 5 illustrates an elevation of a double wall tank; and 
     FIG. 6 illustrates a perspective view of a skid for a double wall tank. 
     FIGS. 7 and 8 illustrate a butt weld joint and a joggle joint, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Referring to FIG. 1, cylindrical wall  10  of single wall storage tank  12  is comprised of rolled metal sheet  14  and rolled metal sheet  16 . Both rolled metal sheets  14  and  16  are formed by cutting a rectangular piece of metal and rolling the metal until two opposite butted ends  18  come together to form a ring. Butted ends  18  are welded together by weld joint  20 . Rolled metal sheet  14  has a joggle joint rolled end  22  and an outside end  28 . The formation of a joggle joint rolled end  22  is discussed below. Rolled metal sheet  16  has an inside end  24  and an outside end  28 . When rolled metal sheets  14  and  16  are assembled, joggle jointed rolled end  22  is welded to inside end  24 , thus forming joggle joint  26 . Attention is drawn to the non-alignment of butt-welded joints  20  of rolled metal sheets  14  and  16 . The purpose of non-alignment of butt-welded joint  20  is to increase the strength of single wall storage tank  12 . 
     Rolled metal sheets  14  and  16  are comprised of a special ferrous alloy which provides fire-resistant support to single wall storage tank  12 . The ferrous alloy has a maximum of approximately 0.15% carbon and a maximum of approximately 0.8% manganese. The maximum limits on carbon and manganese is to limit the brittleness of the alloy. A brittle alloy will not as effectively withstand the stresses placed on the tank when exposed to an elevated temperature. In the preferred embodiment, the ferrous alloy also has a maximum of approximately 0.04% phosphorous, and a maximum of approximately 0.05% sulfur. Attention is drawn to the fact that this ferrous alloy composition is not a standard composition. The closest structural grade steel available to this composition is ASTM A 36, which has a carbon percent maximum of approximately 0.25%. 
     Referring now to FIGS. 2 and 3, an assembled single wall storage tank  12  is comprised of cylindrical wall  10  and storage tank end panels  30 . Storage tank end panels  30  are cut from the same metal as used for rolled metal sheets  14  and  16 . Storage tank end panels are cut and flanged and attached to outside ends  28  forming joint  32 . In the preferred embodiment, joints  32  are joggle joints. 
     The thickness of rolled metal sheets  14  and  16  and storage tank end panels  30  are based on the size of the tank. Table I, Plate Thickness Chart for Single Wall Fire-Resistant Tanks, lists the plate thickness for various size tanks. The thickness of the steel is critical to the invention. When the tank is heated to over 1000° F., the outside of the steel becomes porous forming a protective “skin.” As a result, the steel sheet must be thick enough and be of a consistent quality to allow the “skin” to form and have enough mass to support the skin and to provide maintain the integrity of single wall storage tank  12 . Further, the thickness affects the performance of the assembly when exposed to high temperatures by allowing for increased expansion without fatal stresses that result in tank rupture. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Single Wall Fire-Resistant Tanks 
               
               
                 Plate Thickness Chart 
               
            
           
           
               
               
               
            
               
                 Gallons 
                 Size 
                 Plate Thickness 
               
               
                   
               
            
           
           
               
               
               
            
               
                 300 
                 38″ .times. 5′ 
                 10 ga. 
               
               
                 550 
                 48″ .times. 6′ 
                 10 ga. 
               
               
                 1,000 
                 48″ .times. 12′ 
                 10 ga. 
               
               
                 1,000 
                 64″ .times. 6′ 
                 10 ga. 
               
               
                 2,000 
                 64″ .times. 12′ 
                 7 ga. 
               
               
                 3,000 
                 64″ .times. 18′ 
                 7 ga. 
               
               
                 4,000 
                 64″ .times. 24′ 
                 7 ga. 
               
               
                 5,000 
                 8′ .times. 14′ 
                 ¼″ 
               
               
                 6,000 
                 8′ .times. 16′ 
                 ¼″ 
               
               
                 8,000 
                 8′ .times. 21′ 
                 ¼″ 
               
               
                 10,000 
                 8′ .times. 27′ 
                 ¼″ 
               
               
                 10,000 
                 9′ .times. 21′ 
                 ¼″ 
               
               
                 12,000 
                 9′ .times. 25′ 
                 ¼″ 
               
               
                 15,000 
                 9′ .times. 32′ 
                 ¼″ 
               
               
                 20,000 
                 10′ .times. 34′ 
                 ¼″ 
               
               
                 30,000 
                 10′ .times. 51′ 
                 ¼″ 
               
               
                   
               
            
           
         
       
     
     Single wall storage tank  12  also comprises welded couplings  34  and tank skids  36 . FIGS. 2 and 3 depict couplings  34  on the top of single wall storage tank  12 . However, couplings can be placed where desired, depending on the application for which single wall storage tank  12  is used. 
     Tank  12  is supported by skids  36 . In the preferred embodiment, there are two skids  36  which run longitudinally on the bottom of single wall storage tank  12 . Skids  36  on single wall storage tank  12  perform multiple functions. Skids  36  stabilize the tank  12  during its normal use. Skids  36  provide structural support to single wall storage tank  12  as the temperature of the tank increases and the “skin” develops. The structural strength of single wall storage tank  12  diminishes as the temperature of the tank increases beyond 1000° F. Skids  36  help give structural support to the steel in this temperature range. In the preferred embodiment, skids  36  have a similar coefficient of expansion as tank  12 , thereby expanding at a similar rate as tank  12  when exposed to elevated temperatures, further reducing the chance of tank rupture. 
     Now referring to FIG. 4, skid  36  has a generally unshaped cross section comprising a base  38 , short vertical member  40 , and tall vertical member  42 . Generally, skid  36  is designed such that single wall storage tank  12  rests on both short vertical member  40  and tall vertical member  42 , as is shown in FIG.  3 . Short vertical member  40  is at an angle  44  from base  38 . In the preferred embodiment, angle  44  is approximately 100.degree. Similarly, tall vertical member  42 , which is taller than short vertical member  40 , is at an angle  46  from base  38 . In the preferred embodiment, angle  46  is approximately 100°. Extending from the top of tall vertical member  42  is tank resting element  48 . Tank resting element  48  is a band of metal which extends the entire length of tall vertical member  42 . Tank resting element  48  makes angle  50  with the outside surface of tall vertical member  42 . In the preferred embodiment, angle  50  is approximately 142.degree. Tank resting element  48  has an upper surface  52 , which, along with edge  54  of short vertical member  40 , comprises the two points upon which single wall tank  12  rests. This configuration of skids  36  allows two parallel skids to be placed equally distant from the center of single wall tank  12  and supports single wall tank  12 . 
     Referring to FIG. 5, double wall storage tank  60  is comprised of a secondary containment tank  62  and a product storage tank  64 . Double wall storage tank  60  is fabricated in the same manner as single wall storage tank  12 , except that product storage tank  64  is nested inside secondary containment tank  62 . In the preferred embodiment, the diameter of secondary containment tank  62  is a half inch larger than the diameter of product storage tank  64 . Additionally, secondary containment tank  62  has a length which is four inches longer than the length of product storage tank  64 . These differences in diameter and length allow for expansion and contraction without rupturing either secondary containment tank  62  or product storage tank  64 . Additionally, the chance of tank rupture caused by thermal expansion is reduced by fabricating secondary containment tank  62  and product storage tank  64  from metal sheets having similar coefficients of expansion, so that both tanks expand at similar rates when exposed to elevated temperatures. 
     In order to maintain structural integrity during elevated temperatures, the walls of both secondary containment tank  62  and product storage tank  64 , which are listed on Table II, Plate Thickness Chart of Double Wall Fire Resistant Tanks. 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 Double Wall Fire-Resistant Tanks 
               
               
                 Plate Thickness Chart 
               
            
           
           
               
               
               
            
               
                   
                 Plate Thickness 
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Product 
                 Secondary 
               
               
                   
                   
                 Storage Tank 
                 Containment 
               
               
                 Gallons 
                 Size 
                 Tank 
                 Tank 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 300 
                 38″ .times. 5′ 
                 7 ga. 
                 10 ga. 
               
               
                 550 
                 48″ .times. 6′ 
                 7 ga. 
                 10 ga. 
               
               
                 1,000 
                 48″ .times. 12′ 
                 7 ga. 
                 10 ga. 
               
               
                 2,000 
                 64″ .times. 6′ 
                 7 ga. 
                 10 ga. 
               
               
                 3,000 
                 64″ .times. 18′ 
                 7 ga. 
                 10 ga. 
               
               
                 4,000 
                 64″ .times. 24′ 
                 7 ga. 
                 10 ga. 
               
               
                 5,000 
                 8′ .times. 14′ 
                 ¼″ 
                 7 ga. 
               
               
                 6,000 
                 8′ .times. 16′ 
                 ¼″ 
                 7 ga. 
               
               
                 8,000 
                 8′ .times. 21′ 
                 ¼″ 
                 7 ga. 
               
               
                 10,000 
                 8′ .times. 27′ 
                 ¼″ 
                 7 ga. 
               
               
                 10,000 
                 9′ .times. 21′ 
                 ¼″ 
                 7 ga. 
               
               
                 12,000 
                 9′ .times. 25′ 
                 ¼″ 
                 7 ga. 
               
               
                 15,000 
                 9′ .times. 32′ 
                 ¼″ 
                 7 ga. 
               
               
                 20,000 
                 10′ .times. 34′ 
                 ¼″ 
                 7 ga. 
               
               
                 30,000 
                 10′ .times. 51′ 
                 ¼″ 
                 7 ga. 
               
               
                   
               
            
           
         
       
     
     Double wall storage tank  60  has couplings  66  mounted through secondary containment tank  62  to product storage tank  64 . As in single wall storage tank  12 , couplings  66  are found on the top of double wall storage tank  60 , but can be located anywhere depending on the use of the tank. Double wall storage tank  60  also rests up and is supported by skid  70 . 
     Other embodiments of the invention include a double wall multiple product tank, with double bulk heads (not shown) welded inside the primary storage tank or multiple primary storage tanks nested inside a secondary containment tank (not shown). An additional embodiment of the invention includes a single wall multiple product tank with double bulk heads (not shown) welded inside the tank. 
     Now referring to FIG. 6, skid  70  comprises belly bands  72 , vertical support members  74 , 45.degree. support members  76 , horizontal members  78 , and angle member  80 . Belly bands  72  are constructed of a curved band of steel that conforms to the underside of double wall storage tank  60 , thus stabilizing it. Belly bands  72  have belly bands ends  82  which are supported by vertical support members  74 . Vertical support members have an upper end  84  and a lower end  86 . Upper end  84  is adjacent to the convex surface of belly band  72  near belly band end  82 , respectively. Lower ends  86  are directly below upper ends  84  and are on the ends of horizontal member  78  at a 90.degree. angle to horizontal member  78 . Horizontal member  78  is a flat band of metal that extends between lower ends  86  of each belly band  72  and upon which the center of belly band  72  rests. Belly bands  72  are also supported by 45.degree. support members  76 . 45.degree. support members  76  are flat bands of steel which extend at a 45.degree. angle to horizontal member  78  and extend from lower end  86  to the convex surface of belly bands  72 , respectively. To further support belly bands  72 , the corner formed from vertical member  74  meeting horizontal member  78  rests in the inside angle  90  of angle support  80 . An angle support  80  runs along each side of secondary containment tank  62 , providing additional support to structures which support belly bands  72 . Angle support  80  is illustrated with a gap to represent that angle  80  extends between belly bands  72  regardless of how far apart they are. Further, skids of other embodiments of the invention may have more than two belly bands  72  to sufficiently support longer or heavier tanks. Additionally, skid  70  performs the similar functions as skid  36 . 
     The single wall storage tank  12  and double wall storage tank  60  are constructed per UL 142—Standard for Steel Aboveground Tanks for Flammable and Combustible Liquids. All joints are either butt weld joints  94  or joggle joints  94 . Butt weld joints  96  are so named because the weld  96  fuses edges of plates that “butt” up against each other. In the present invention, a weld is made by welding both sides with a hot gas metal arc welder to ensure complete and thorough fusion. When using the hot gas metal arc welder, the gas shield is carbon dioxide. Additionally, the wire feed for the hot gas metal arc welder is an AWS E71T-1 class, titania type flux cored wire designed for use with 100% carbon dioxide gas shielding, the wire in the preferred embodiment having a typical composition of approximately 0.05% carbon and approximately 1.28% manganese. However, the wire can have a maximum of approximately 0.15% carbon and a maximum of approximately 1.4% manganese, of which excess manganese will be burnt off due to excess heat used during welding. Further, in the preferred embodiment, the wire has approximately 0.05% carbon, 1.28% manganese, 0.50% silicon, 0.013% phosphorus, and 0.009% sulfur. Further, the amperage used during welding is 180 to 220 amps. Also of importance is the relative tensile strength, yield strength, coefficient of expansion, and composition of the steel alloy in relation to the weld metal. 
     All the joints in tanks  12  and  60  which are not butt weld joints are joggle joints. Unlike a butt weld joint, in joggle joint, an edge overlap portion one overlaps an edge portion of another plate  112 . Further, weld  120 , which fuses joggle joint  94  together, is between edge  122  of plate  112  and plate  114  such that surface  124 , which is on the opposite side of plates  112  and  114  from edge overlap portion  116 , is substantially flat. A submerged arc welder is used to weld all joggle joints  94 . Additionally, and L 61 wire is used with a 761 flux and the amperage used during welding is 225 to 280 amps. 
     AS per UL 142, the tank is pressure tested by soaping all the welds, observing any leaks while the tank is under pressure, and rewelding when necessary. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.