Plastic Tank Panels and Joint Assembly

1. Field of the Invention

This invention relates to the structural loading of materials having large differences in their moduli of elasticity. It particularly relates to panels comprising a composite system of steel bands laminated into fiberglass-polyester sheets. It further relates to butt joints for conjoining and interconnecting such panels. It additionally relates to modular construction of storage tanks by conjoining fiberglass panels with creep-resistant joints therebetween.

2. Review of the Prior Art

Storage tanks are very common types of equipment in the chemical and allied industries for accumulating reserve stocks of raw materials against a disturbance in regular deliveries or because shipment is periodical and in large quantity. In addition, storage tanks are used for intermediate storage in a process or for storage of a product. If a tank serves as one of the steps in a manufacturing process, it is designated a process tank, such as dissolving tanks, blending tanks, and feeding tanks, and may include other functions such as thickening, settling, clarifying, and fermenting. Tanks built at grade are of flat-bottom type and may rest on a foundation of sand or on both sand and a curbing of concrete slightly larger than the tank diameter.

In addition to such liquids as water, brine, acid solutions, caustic solutions, wood pulps, mining pulps, petroleum, naphtha, gasoline, and drilling muds that are commonly stored in storage and process tanks, flowable solids can also be stored therein and can similarly generate hydrostatic heads. Such flowable solids include grains, flour, powdered coal, fertilizer, powdered sulfur, sugar, salt, and various other powdered and particulate products.

Most tanks are made of steel, but corrosion-resistant tanks have frequently been made from thin-walled composite materials such as a composition containing a phenolformaldehyde resin and an acid-digested asbestos fiber or graphite. Such tanks resist the action of any acid and of many salt solutions. They are tough, strong, and impact resistant and resist sustained temperatures of 265.degree. F. (130.degree. C.).

During recent years, composite panels constructed of fiberglass and polyester resin have become widely used and highly preferred. Fiberglass-polyester resin has been used in the manufacture and installation of storage tanks because of its excellent resistance to a wide range of chemicals, good impact strength, and light weight.

Unfortunately, random-chopped fiberglass-resin composite panels have a tendency to creep under heavy loads over long periods of time. This slow movement creates stress concentrations, particularly near the bottom sides and at the joints or butted edges of a tank under load. Because of the concentrated loads at such places, there is a distinct possibility of eventual failure of the plastic structure. Furthermore, it is known that when reinforced polyester and epoxy resins are placed in contact with liquids over a long period of time, there is a gradual reduction in tensile strength. Designers are forced, by these characteristics of the reinforced panels that are currently in the market, to design and fabricate under considerable uncertainty and with exaggerated safety features.

Non-pressurized storage and process tanks create unique design problems because of uneven loading of the walls and the wall intercepts in accordance with the pressure created by the head of the liquid therein. When a tank is filled with liquid, there is no pressure at the top and maximum pressure at the bottom along the sides of the tank. Consequently, stretching along the walls is greatest near the bottom and makes the bottom circumference greater than the circumference of the base or of the top of the tank. Because of such concentrated stretching and bending, stress concentrations are also greatest near the bottom of the tank, and higher stress means accelerated corrosion, as indicated in FIGS. 1 and 2 of the drawings.

Another design factor arises because hydrostatic pressure tends to force the sides of a tank to assume a prefectly round shape. Therefore, any configuration, such as a field-assembled joint, that is not aligned along the circumference of a circle creates concentrated bending and stretching.

All joints under loads will generate stress concentrations and thus accelerate corrosion, especially in field-erected tanks constructed from thin-walled, creep-susceptible panels. During modular erection of a tank on site, the curved panels are generally assembled with a lap joint, a bolted butt joint, or a welded butt joint. As the walls of the tank increase in diameter because of stretching by hydrostatic pressure, the bends straddling a bolted butt joint particularly tend to become straightened out by the pressure exerted along the walls and become loci of accelerated corrosion, as indicated in FIG. 3. Furthermore, added fixtures in a tank wall, such as a valve or hatch, create stress concentrations at the opening in the wall because the opening can bear no load and must transmit its share of the load to the adjoining wall above and beneath its sides, as indicated in FIG. 4.

It is recognized that any material stretches at a rate defined by its modulus of elasticity. A fiberglass-resin composite of randomly oriented short fiber construction stretches approximately 30 times per unit area more than steel under any given load. Steel and fiberglass bonded in a plastic composite will stretch at a rate that is defined by their composite modulus of elasticity, i.e., whether the force is considered to be exerted on the steel or on the composite or on both, the movement will be the same.

The object of this invention is to provide a steel-fiberglass resin storage tank that is not susceptible to creep.

It is another object to provide a steel-fiberglass resin storage tank that will not deteriorate because of stress concentrations.

It is an additional object to provide a butt joint, for conjoining steel-fiberglass-resin panels, which will be subjected to substantially no tension from hydrostatic head.

It is a further object to provide a means for accurately balancing and tensioning the load-accepting means for this butt joint.

It is also an object to provide a steel-fiberglass resin storage tank having a reinforcing means which will permit design and fabrication without uncertainty and/or the exaggerated safety features necessary in fiberglass resin tanks currently in the market.

It is another important object to provide a method of fabricating any size storage tank to a given safety factor.

It is still another important object to provide a bottom or base for a storage tank which forms an expansion joint with the lower most ring module and relieves bending stresses.

In accordance with these objects and the spirit of this invention, a modular-construction tank is provided which is fabricated from thin-walled, creep-susceptible materials such as reinforced plastics, particularly fliberglass-polyester plastics. The tank comprises a base, one or more ring modules which are vertically stacked on top of the base, and, selectively, a top.

The ring modules are constructed from a plurality of curved wall panels which are interconnected along each vertical seam with a butt joint free of tension comprising straps of a relatively rigid and non-stretchable material, such as steel. The straps are attached to and embedded within each panel. The ends of the straps are interconnected by means of a rigid member extending across the vertical seam, such as a pair of steel bolts. The panels on each side of the butt joint diverge from the straps, in pairs, as inwardly recurved portions which terminate in adjacent radially disposed and outwardly extending flanges, thus creating a paired shape in section that is somewhat like the number "7". The straps are tightened across the butt joint by taking up the bolts to put the joint into circumferentially directed compression or at least free of tension.

While the tank is empty, the ends of the flanges are spaced inwardly from the bolts, but as the tank is filled and the curved wall panels receive an outwardly directed pressure load, the ends of the flanges move outwardly until they contact and receive support from the bolts. The pressure within the tank tends to bend the pair of recurved portions outwardly and straighten them so that a cam action occurs that squeezes the flanges even more tightly together.

The base comprises a raised central section and a low rim section which are joined by a recurved perimeter section extending from the upper surface of the central section to the rim section and comprising two expansion bends. The curved wall panels of each ring module have horizontally disposed flanges along their top and bottom sides with the bottom flanges having downward edges.

The lowermost ring module is conjoined to the base with a movement-permitting base joint. Its bottom curved wall panel rests upon the rim of the base, forming in combination therewith a bottom knuckle expansion joint. This joint eliminates bending moments that would otherwise be generated at the juncture of the bottom of the curved wall panel and the base if connection therebetween were totally immobile.

The ring modules are conjoined by a circumferential or horizontal joint that is relieved from stress concentrations by the designs of the curved panels of each module. The bottom flange of the curved wall panels are slightly wider than the top flange of the curved wall panels upon which they rest so that when one wall panel is in position on top of a second wall panel, the downward edge extends over the circumferential or horizontal joint.

The top, as well as the base, can be in two or several pie-shaped panels, depending upon the size thereof and the on-site handling facilities. Because the base rests on the ground and cannot move downwardly under pressure and because the top is not under pressure, a movement-permitting joint is not needed between the pie-shaped panels of either. The pie-shaped panels of the top do, however, have an outwardly disposed arcuate face along their outermost edge. This arcuate face rests upon the top flanges of the curved wall panels of the topmost ring module.

The tanks of this invention are suitable for use as storage tanks and as process tanks and can be constructed in any size indicated by design calculations for the component materials. These tanks are principally intended to be field erected from factory-constructed curved wall panels, using a permanent sealant for all joints. However, they can be satisfactorily constructed with gaskets in all joints for temporary use, such as for large drums that are suitable for one-way shipment and disassembled return.

FIG. 1 shows in sectional elevation a filled tank of the prior art whose walls have been stretched by hydrostatic pressure.

FIG. 2 is a plan view of the filled tank of FIG. 1.

FIG. 3 is an enlarged plan view of one of the butt joints of the tank of FIG. 2.

FIG. 4 is an enlarged perspective view of the tank wall and valve shown in FIGS. 1 and 2.

FIG. 5 is an exploded perspective view of a modular-construction tank of this invention.

FIG. 6 is a detailed perspective view of a vertical butt joint interconnecting two adjoining curved wall panels of a ring module.

FIG. 7 is an exploded plan view of the compressed butt joint of this invention.

FIG. 8 shows a portion of one vertical side of a curved wall panel for forming a compressed butt joint of this invention by means of separate laminations, as one embodiment of joint and panel construction.

FIG. 9 is a cross-sectional representation of a curved wall panel, constructed as the preferred embodiment from polyester resin, optional polyester foam, chopped fiberglass-polyester gel, and steel straps.

FIG. 10 is a plan view of a compressed butt joint in use in an empty tank.

FIG. 11 is a plan view of the compressed butt joint of FIG. 10 after the tank has been filled with liquid to put the joint under hydrostatic load.

FIG. 12 is a perspective view in section of a bottom knuckle expansion joint or base joint formed by the bottom flanges of curved wall panels of the lowermost ring module and the rim section of the base of the tank shown in FIG. 5.

FIG. 13 is a perspective view in section of a circumferential or horizontal joint between two ring modules.

The modular-construction tank shown in FIG. 5 comprises a base 20, a lower ring module 25a, an upper ring module 25b, a base joint 28, a circumferential joint 29, and a top 35. The base joint 28 and the circumferential joint 29 are each horizontally disposed and permit movement of upper and lower members. The base 20 is shown as formed of four pie-shaped elements 21. When these are assembled, the base 20 comprises a raised central section 22, a rim section 24, and four recesses 23. The rim section 24 and the central section 22 are connected with a recurved sidewall 175 having bends 173 and 177. The central section 22 may be laid over a concrete pad or may be hollow, as shown in FIG. 12, with a bottom 181 and sides 183 that enclose a filler 185 such as wood, high-density foam, and the like. The base joint 28 has a sealant 62 between ring module 25a and rim section 24.

The ring modules 25a, 25b comprise four curved panels 26 and four compressed butt joints 27 which are vertically disposed. The lower ring module 25a also has a discharge port or valve 32 and an inspection hatch or opening 33. Its butt joints 27 fit within the recesses 23 when base joint 28 is formed. The top 35 comprises four wedge-shaped elements 36, four wedge joints 37 therebetween, and a central hatch cover 38. Within practical and utilitary limitations almost any number of panels 26 may form a ring module and any number of ring modules may be vertically stacked upon a base 20, due regard being accorded the strength of materials limitations. For purposes of description herein ring modules will be confined to lower and upper ring modules 25a and 25b.

Each curved wall panel 26, as shown in FIGS. 6, 7, 10, and 11, comprise a main wall section 39, a strap assembly 80, and a recurved wall section 40 along each vertical side of section 39. Each recurved wall section 40 is formed by a first partial bend 43, a diagonal 45, a second partial bend 47, a strip 49, a right-angled bend 51, and a flange 53 having a tip 55. Each curved wall panel 26, as shown in FIGS. 6, 12, 13, further forms along its lower end at a right-angle thereto and outwardly extending therefrom a flat brim 63 having a downwardly extending, at a right-angle thereto, lip 67. On the inner side of the juncture of brim 63 with the lower end of the main wall section 39 of all panels 26, except those of lowermost ring module 25a, each wall panel 26 has a semi-recess 69. As shown in FIG. 13, each wall panel 26 also has along its top side a flat rim 73, which extends from the main wall section 39 at a right-angle thereto, and another semi-recess 79 in the inner side of the juncture of flat rim 73 with the upper end of main wall section 39.

When a plurality, n, of wall panels 26 are conjoined by means of compressed butt joints 27, they form a ring module, for example, ring modules 25a or 25b. When two or more ring modules are stacked with the top ring panel 73 of a lower module beneath the bottom ring panel 63 of an upper module, a circumferential joint 29 is formed. Within this joint, a sealant 62 of any suitable type known to the art is placed for adhesively conjoining the flat surfaces.

Butt joint 27 comprises a pair of the recurved wall sections 40 of two adjacent curved wall panels 26, a sealant 62 therebetween, a pair of the strap assemblies 80, and an interconnecting means 90. As shown in FIG. 8, the strap assembly 80 comprises an embedded strap 81, a junction plate 83, a point strap 85, and a pair of lugs or eyelets 87 on either side of strap 85, all being rigidly attached, as by welding. The junction plate 83 may alternatively be omitted, so that the embedded strap 81 and the joint strap 85 will be a single continuous strap, designated as joint strap 85. In order to insure strong bonding between the strap and panel the junction plate 83 may be preforated or serrated, and likewise if a continuous strap is used it may be perforated or serrated. The important feature is to provide the strongest possible bond between the strap 81 and panel 26 to prevent delamination and failure of the strap and panel to form a composite having a modulus different from either the panel or strap.

An embedded strap 81 can selectively extend from one side edge to the other side edge of each main wall section 39 as a single, continuous strap. For the lowermost ring module 25a, it is indeed desirable to use such a continuous embedded strap 81 because it must withstand the highest hydrostatic pressure. For higher ring modules, however, the embedded straps 81 suitably extend inwardly for merely a portion of the width of the main wall section 39 from each side thereof and are not necessarily continuous. Near the top of the topmost ring module, it is satisfactory for the embedded straps 81 to extend but a short distance away from the bends 43. The straps must be tangentially attached to and embedded within the main wall sections 39 because non-tangential attachment tends to cause delamination of a wall panel 26 when it is subjected to hydrostatic pressure.

The interconnecting means 90 comprises, in combination with the eyelets 87, a bolt 91, a washer 93, and a nut 95, in pairs for each pair of coaxially aligned eyelets 87. Each bolt 91 is inserted into one of the opposed eyelets 87, and a nut 95 is turned thereon to obtain a desired tightness. Each pair of bolts 91 astraddle a joint strap 85 must be tightened equally, as with a torque wrench. Numerous methods may be utilized in interconnecting the straps between conjoint panels 26. For example, the straps could be welded, bolted, or fixed with turnbuckles, etc. Such means are used in such manner as to maintain the butt joints 27 free of tension.

Referring to FIGS. 10 and 11, hydrostatic pressure 31 within the tank is exerted perpendicularly to the walls thereof and tends to straighten the pair of inwardly recurved sections 40 of each butt joint 27. The partial bends 43, 47, which are 45.degree. .+-. 15.degree. bends, thus tend to be diminished by the pressure so that the diagonal 45 and the strip 49 are put in edgewise compression. This pressure is transmitted to the bends 51 and the flanges 53, whereby the sealant 62 is likewise compressed.

Depending upon the amount of bend possessed by the partial bends 43, 47, the flanges 53 tend to move outwardly in direction 34. When a compressed butt joint 27 is initially assembled, a space 59 exists between the flange tips 55 and the sides of the bolts 91, as clearly shown in FIG. 10. When the butt joint 27 is placed under sufficient pressure, however, the flanges 53 move outwardly until the tips 55 contact the sides of the bolts 91, as shown in FIG. 11. Thereafter, as pressure increases, the recurved sections 40 receive support from the bolts 91, whereby compression forces created by hydrostatic pressure 31 within the recurved sections 40 are simply transmitted to and accepted by the interconnecting means 90 so that the compressed butt joint 27, with respect to the potential passageway between flanges 53, can be justly described as an "unloaded joint", i.e., no tensile forces exist in the entire joint area of composite material.

FIG. 8 shows a lamination method for constructing a laminated panel 120 which comprises an outer sheet 121 having an edge 125, a middle sheet 122 having an edge 135 and a slot 137 for accommodating each joint strap 85, and an inner sheet 123 having a first partial bend 143, a diagonal 145, a second partial bend 147, a strip 149, a right-angled bend 151, and a flange 153 with tip 155. Such a laminated panel 120 can be field assembled from flexible sheets of fiberglass-polyester resin which can be cut along edges 125, 135, 155 and slots 137 and glued together with polyester resin gel so that the strap assembly 80 is between the middle sheet 122 and the inner sheet 123.

If desired and particularly if time is pressing, the inner sheet 123 can be molded in the field to the desired dimensions or can be made by postforming a fully cured, C-stage thermoset laminate by heating it over a mold to obtain the desired bends 143, 147, 151, separated by desired diagonal 145 and strip 149 and ending in desired flange 153. The sheets 121, 122, 123 can thus be formed from different polymers and have different reinforcing materials therein.

Moreover, an additional reinforcing layer can be laminated to the inner surface of sheet 123 to cover the inwardly recurved zone from beyond bend 143 to tip 155, whereby this zone can be as strong as the inwardly recurved section 40 of panel 26. Alternatively or in addition thereto, a reinforcing layer can be laminated to the outer surface of the sheet 123 so that it covers the edge 135, beneath the straps 85, and extends to the tip 155.

The preferred laminate construction is shown, however, in FIG. 9. This construction is highly suitable for factory manufacture to exact specifications in order to produce the curved wall panels 26, with or without pads or junction plate 83 and with the embedded straps 81 of the strap assembly 80 extending partially inwardly from each bend 43 or the entire width of main wall section 39 of the panels 26.

The laminate 126, as shown in FIG. 9, comprises a gel coat 161, a strap 81, an optional foam layer 163 which may contain some chopped fiberglass, a fiberglass-resin mixture 165, and another gel coat 167. The resin in each of these layers is preferably a polyester but may be any resin that is suitable for the liquid or flowable solid to be stored or processed and that forms a strong bond with the reinforcing agent which is preferably chopped fiberglass. The polyester resin may be combined with a monomer such as styrene, vinyl monomer, alphamethyl styrene, and mixtures thereof. Other resins having possible utility are suitably catalyzed diallyl phthalate polymers, epoxy resins, phenolic resins, silicone resins, vinyl ester and other chemically resistant resins. The polyester resins may be applied to the chopped fiberglass to be bonded according to any manner known to the art, such as spraying, painting or flowing the resins onto a fiber mat to make the mixture 165 and optionally extruding a foam to make the foam layer 163. The laminate 126, as shown in FIG. 9, may utilize fibrous reinforcements other than the chopped type E fiberglass, which is the preferred material. Other suitable reinforcements include, but are not limited to, continuous random mat, specialty classes, such as types S, C and others, and other fibers such as nylons, aromids, graphite or carbon and boron, all used either as randomly oriented short or continuous filaments. Also, naturally occurring fibers such as jute and ramie, and whisker type materials such as aluminas, titanates, sapphires and similar reinforcements may be used.

In constructing a panel 26 having a fitting such as discharge port or valve 32 or inspection hatch or opening 33 therein, embedded straps 81 should pass relatively close above and beneath the fitting or opening in order to receive the full tensile stress delivered thereto by hydrostatic pressure. The stiff materials in the interconnecting system of the compressed butt joints 27, comprising the strap assembly 80 and the interconnecting means 90 must be combined with the flexible materials, such as fiberglass-polyester resin laminate, in such a way that the stiff materials carry the hoop stress concentrically with the circumference of the tank. The flexible materials are thus disengaged from the stiff material at the butt joints 27 and keep the contents of the tank from leaking out but do not carry any hoop-stress (i.e., tensile) load across the butt joints 27.

The butt joints 27 illustrated in FIGS. 6 and 7 are formed by recurved wall sections 40 which project inboard of the nominal circumference of the wall panels 26 and ring modules 25a and 25b, and the straps 81 of strap assemblies 80 are on the nominal circumference thereof. These butt joints 27 could be made with recurved wall sections 40 projecting outboard of the nominal circumference of the wall panels 26 and ring modules 25a and 25b. The important feature is that the straps 81 are on the nominal circumference of the wall panels 26 and ring modules 25a and 25b.

Because it will be readily apparent to those skilled in the art that innumerable variations, modifications, applications, and extension of these embodiments and principles can be made without departing from the spirit and scope of the invention, what is herein defined as such scope and is desired to be protected should be measured, and the invention should be limited, only by the following claims.