Line Tank Units For Transporting Corrosive Materials

Field of the Invention The present invention relates to containers for transporting highly corrosive fluids, and more particularly, the present invention relates to fluorocarbon polymer-lined transport containers and methods of fabricating the same.

The present invention relates to container units comprising a plurality of relatively small diameter structurally strong tanks, each having a seamless liner of a chemically resistant thermoplastic polymer, ganged together in parallel relation on a support such as a truck chassis or pallet. Such container units are designed to transport chemicals of all types, including highly cor¬ rosive fluids, and products such as food, where the purity of the products must be retained in transit, and the like. The tanks are easily cleaned, and are much lighter in weight than commonly used stainless steel tanks. Background of the Invention

In the manufacture of various electronic products, such as silicone wafers, high purity acids such as nitric and sulfuric acid are required. Although it is possible to produce such acids with high levels of purity, problems have arisen in maintaining the quality of the acids during their transport from the place of manufacture to the place of use. Normally, such an acid is transported in a tank truck having a chemically inert liner; however, such liners have been known to fail, enabling the acid to attack the metal shell surrounding the liner, and con¬ sequently introducing deleterious reaction products into the acid.

Various attempts have been made to provide tanks

with inert liners. Some tank trucks in which corrosive chemicals have been transported have a single, relatively large (both in diameter and in length) tank lined with an inert material. Glass has been used as an inert liner material,- however, glass is brittle and subject to cracking due to the flexure of the surrounding metal shell.

In order to overcome the limitations of glass liners, tanks have been lined with fluorocarbon polymer sheet material, the marginal edges of the sheets being overlapped and welded together to form a continuous liner. One of the problems of such liners, which are relatively expensive to install, is that such welding weakens the liner in the zone of the overlap, where the liners tend to crack and tear. Moreover, since the tanks are generally not completely filled with liquid in order to provide headspace for expansion, the liquid sloshes back and forth in the tank in response to starting and stopping of the vehicle on which it is mounted. This fluid motion causes a vacuum to be applied to the liner, and the vaccuia tends to tear the liner from the inside of the shell, as a result of which the shell is subject to attack by the acid.

Still other techniques have been tried in securing a liner to the inside surface of a protective shell. For instance, one side of a fiberglass cloth liner has been coated with a fluorocarbon polymer to provide a liner surface, and the opposite side coated with ^" an adhesive which adheres the cloth to the inner surface of a rigid shell. This technique has not been entirely satisfactory because of the difficulty in making fiberglass cloth completely impervious to fluids.

Another problem with tanks comprising a steel shell lined with a fluoropolymer is that vapors of some chemicals have the ability to permeate the liner and then condense between the liner and shell, where they attack

- _B Ull^;

the steel shell and the adhesive used to bond the liner to the shell, resulting in structural damage to the shell and separation of the liner from the shell.

Object of the Invention With the foregoing in mind, a primary object of the present invention is to provide improved containers for transporting various types of chemicals, including corrosive liquids, bulk food stuffs of all types, and other cargoes where maintenance of purity of the product in transport is essential.

Another object of the present invention is to provide a novel transport tank unit composed of a plurality of tanks, each having a chemically resistant seamless thermoplastic liner which is firmly secured within an outer shell formed of fiberglass-reinforced thermosetting resin which provides each tank with the necessary structural rigidity and strength.

Still another object of this invention is to provide transport tank units comprising a plurality of tanks which are not subject to attack by chemical vapors, even if such vapors are capable to permeating the liner material.

Summary of the Invention

The present invention provides a transport tank unit particularly suited for hauling chemicals of all types, including highly corrosive liquids. The tank unit comprises a support and a plurality of tanks mounted in parallel relation on the support. Each tank includes a rigid outer shell of fiberglass-reinforced thermosetting resin, and a seamless tubular liner of chemically resistant thermoplastic material, such as a copolymer of polytetrafluoroethylene and hexafluoropropylene, securely fastened to the inner surface of the shell. The liner and shell are joined by means of an intermediate' layer in

the form of a glass fiber fabric, which may be woven, knit, braided, etc.

More particularly, in the manufacture of the tanks which make up the transport tank units of this invention, the process by which the tanks are made being more fully described hereinafter, the exterior surface of the seamless thermoplastic liner is rendered in a molten state and caused to expand into the interstices of the glass fabric which surrounds the liner as a tight-fitting sleeve. Upon cooling, a permanent mechanical bond is achieved between the liner and fabric. A layer of con¬ tinuous fibers, preferably of glass, impregnated with a thermosetting resin, such as polyester resin, is applied to the outer surface of the glass fabric-reinforced liner and the resin is cured, whereby the outer layer is bonded to the glass fabric. This outer layer provides each tank with the desired structural rigidity and strength. Each tank may be provided with a pressure equalizer tube con¬ necting upper portions of the opposite ends of the tank to control vacuum induced by fluid sloshing back and forth in the tank.

Brief Description of the Drawings

The foregoing and other objects, features, and advantages of the present invention should become apparent from the following description when taken in conjuction with the accompanying drawings, in which:

FIG. 1 is a side elevational view of one embodiment of a tank transport unit embodying the present invention; FIG. 2 is an enlarged rear end view taken on line 2-2 of FIG. 1;

FIG. 3 is a side elevational view of another embodiment of a tank transport unit;

FIG. 4 is a rear end view of the -unit illustrated in FIG. 3;

FIG. 5 is a greatly enlarged fragmentary sectional view illustrating one preferred tank structure; and

FIG. 6 is a modified version of one of the tanks of the tank transport unit illustrated in FIG. 1, which tank is provided with an equalizer tube for equalizing pressures at the ends of the tank.

Description of Preferred Embodiments Referring now to the drawings, FIG. 1 illustrates a tank transport unit 10 fabricated in accordance with the present invention. As seen therein, the transport unit 10 comprises a trailer chassis 11 which supports a series of tanks 12 to 15 extending in spaced parallel relation lengthwise of the chassis 11. The tanks 12 to 15 are fastened together and to the chassis 11 by any one of various means, such as yoke assemblies 16, 17, and 18. In the illustrated embodiments, a pair of upper tanks 12 and 15 are superimposed vertically above a lower pair of tanks 13 and 14. It should be understood, however, that although four tanks are illustrated, six or more tanks can be similarly mounted, depending upon the overall capacity desired for the transport unit. Moreover, odd or even numbers of tanks may be arranged in the manner illustrated in FIGS. 3 and 4 where tanks 21, 22, and 23 are mounted in staggered relation on a support pallet 24. This embodiment provides a transport tank or container unit 25 which is particularly suited for shipment in oceangoing vessels as containerized cargo. Each of the tanks, such as the tank 12, is designed to carry corrosive liquids, such as acid or caustic solutions, or food products, the non-contamination of which must be maintained in transit, and the like. As noted heretofore, one of the problems encountered in transporting corrosive liquids in relatively large diameter

transport tanks resides in the proclivity- for the lining of the tank to crack due to flexure (if fabricated of glass) or to separate from the inside surface of the tank (if fabricated from chemically resistant plastic) due to vacuum applied to the inside of the liner by sudden stopping or acceleration of the vehicle on which the tank is mounted. Another problem is the permeation of the liner by corrosive vapors, which attack the adhesive bond between the liner and metal shell, as well as the shell itself.

Referring now to FIG. 5, which is an enlarged fragmentary view of the upper left-hand corner of tank 12 illustrated in FIG. 1, it may be seen that the tank 12 comprises a rigid tubular shell 12a having a pair of radially outwardly extending annular flanges 12b at opposite ends, only a portion of one such flange being shown in FIG. 5. Preferably, the flange members and tubular shell are fabricated separately according to the methods hereinafter described, and the^flanges have a circular opening just slightly larger than the outside diameter of the tank shell so as to be able to receive the end of the shell and be ad¬ hesively bonded thereto. A circular cover plate 12c is mounted across opposite ends of the tank 12 and is secured to the flanges 12b by bolts 12d. The cover plate 12c has an outlet fitting 12e and a vent 12f (see FIG. 2), both of which preferably have a liner or are made of corrosion- resistant material, e.g. a fluoropolymer. The inner periphery of the shell 12a is covered by a liner 19 of chemically-resistant thermoplastic material which, in the present instance, is preferably a copolymer of polytetra- fluoroethylene and hexafluoropropylene. A layer 21 of the same material is also provided on the inside surface of the cover plate 12c.

The liner 19 is preferably an extruded seamless tube of cylindrical cross-section. The length of the liner

should be such that the opposite ends of the liner 19a extend beyond flanges 12b. Heat and pressure are applied in a direction perpendicular to the axis of the liner 19 to deform the outer faces so as to extend radially outward on the exterior surfaces of the flanges 12b. The end covers 12c, on which similar liner material is adhesively secured, are disposed across the ends of the shell 12a and fastened to the flanges 12b by means of bolts 12d. The thus fabricated tank 12 may then be assembled with similarly fabricated tanks, such as the tanks 13, 14, and 15, in the manner illustrated in FIG. 2 to provide the transport units 10 or 25 _. , which are illustrated in FIGS. 1 and 3, respectively.

By way of example, and not by way of limitation, the tank shell preferably has an inside diameter of about two feet or somewhat less, and the length of the tank may vary, depending upon the intended use. In the embodiment of FIG. 1, the length is about thirty feet, providing a length to diameter ratio of about 15:1. The liner 19 is extruded so as to have a wall thickness of 100 mils or somewhat greater.

In order to further reduce the stress on the liner 19 caused by sloshing of liquid axially of the tank, each tank may be provided with means for equalizing the pressure in the headspace above the liquid. To this end, an equalizer tube 30 (FIG. 6) is provided to connect fit¬ tings 31 and 32 located at the top positions of the ends of tank 12. The equalizer tube 30 communicates with the headspace above the liquid in the tank at opposite ends of the tank 12 when the tank is level, but functions, when the tank is carrying fluid and is subjected to sudden ^• axial movement, to afford the flow of fluid into the tube 30, such as illustrated in FIG. 6, when the tank 12 is subjected to lefward acceleration. This fluid motion relieves vacuum on the liner 19 at the rear end of the

tank 12 and helps to prevent the liner 19 from being torn away from the shell. A vent 33 is provided in the equalizer tube 30; and the tube 30, fittings 31 and 32, and vent preferably have the same construction as the tanks themselves so as to be resistant to corrosive chemicals.

The tanks for the transport tank units of this invention preferably are made according to the process described in copending application Serial No. 06/200,263 filed October 24, 1980 for "Process for Forming Laminated Pipe."

According to the process of that application, a sleeve of fabric, which may be seamless, is applied about a seamless tube of chemically-resistant thermoplastic polymer. A plug is inserted into each end of the liner to seal the interior of the liner from the surrounding atmosphere. One such plug has a pressure relief valve to prevent pressure within the liner from exceeding a given value upon heating the liner and sleeve to elevated temperatures. A liquid which has a boiling point substantially below the melting point of the polymer of which the liner is formed, and which does not chemically attack the polymer, is introduced to the liner. The liner with surrounding sleeve is then placed on a pair of spaced parallel rolls, which extend the length of the liner, and are designed to support and rotate the liner and sleeve about the axis of the liner. Heat is applied to an arcuate portion of the outer surface of the liner through the sleeve while the liner and sleeve are rotated. The heat causes the polymer on the external surface of the liner to melt, and also causes volatilization of a portion of the liquid within the liner so that the pressure within the liner exceeds the pressure of the atmosphere external of the liner. The liner expands radially, whereby the molten surface thereof enters the interstices of the surrounding fabric. Expansion of

the liner into the fabric also results to a large degree at the nip between the liner and sleeve and the supporting rolls due to the weight of the liner and sleeve and liquid it contains. The liner is then cooled to solidify the molten polymer, whereby a permanent mechanical bond is formed between the liner and sleeve.

The supporting shell is then applied to the exterior of the sleeve. This shell may be formed by passing bundles of continuous filaments, e.g. glass filaments, through a bath of a liquid thermosetting resin, such as a polyester resin, and then wrapping the resin- impregnated fibers about the sleeve. This can be accom¬ plished by placing the liner and sleeve on a mandrel and rotating same. The outer shell is thus built up by applying layer after layer of resin-impregnated fibers until the shell has a thickness sufficient to provide the ultimate tank with the desired rigidity and strength, e.g. from about 1.5 to about 12.7 millimeters. Thereafter, the resulting composition is allowed to stand at room temperature to cure the thermosetting resin.

The liner 19 may be formed of any thermoplastic material which has those properties, particularly resistance to chemical attack by the material, required to maintain the purity of the material to be transported. Examples of such thermoplastic polymers are poloylefins, such as polyethylene and polypropylene, and fluorinated polymers, the latter being particularly preferred for liners for tank units designed to transport hightly corrosive chemicals. Typical of the fluoropolymers are polyvinylidene fluoride and melt-fabricatable perhalofluoroethylene copolymers , which include chlorotrifluoroethylene and tetrafluoro- ethylene copolymerized with fluoroolefins such as hexa- fluoropropylene (FEP), or with perfluoroalkyl vinyl ether, or with nonfluorinated monomers such as alkylenes, e.g. ethylene, including the tetrafluoroethylene/ethylen binary

polymers and terpolymers, copolymers of tetrafluoroethylene and hexafluoropropylene (FEP) being especially useful. FEP copolymers, such as those containing 5-20% by weight of hexafluoropropylene and 80-95% by weight of tetrafluoro- ethylene are particularly preferred as the liner material for the tanks of the transport tank units of this invention.

The wall thickness of the liner may vary, but generally will be on the order of about 100 mils or some¬ what greater. As to the fabric sleeve which surrounds the liner it can be woven, knit, or braided from any fiber which does not undergo degradation at the elevated temperatures used in laminating the liner to the sleeve. Suitable fibers include natural or synthetic fibers, such as glass fibers, metal fibers, graphite fibers, ceramic fibers, asbestos fibers, ara id fibers, and the like. Because of the desirable physical properties, such as chemical inertness, strength, etc., glass fibers are preferred.

The sleeve may be applied to the liner in the form of a woven fabric which is helically wrapped about the liner. On the other hand, the sleeve may be a seamless braid or tubular knit which Is applied over the liner. The end covers or caps for the tanks can be fabricated by taking a section of the tubular liner to which a fabric sleeve has been mechanically bonded by the process described in the above-identified copending application, and slitting it lengthwise to obtain a flat sheet. Layers of glass cloth impregnated with thermo¬ setting resin may then be superimposed on that side of the fabric opposite to that to which the liner material is mechanically bonded, and the resulting multi-layer laminate is allowed to stand at room temperature to cure the resin and bond the layers together to form an integral cover. Prior to assembly as herinabove described, the layers may be cut into circular sections having the

diameter of the cover plate. After curing of the resin, the end covers may be provided with holes about their peripheries for receiving bolts 12d for bolting the end covers to the flanges 12b. In view of the foregoing, it should be apparent that the present invention now provides improved lined transport tank units particularly suited for transporting corrosive fluids. nd other materials safely, without adversely affecting their quality or purity. While preferred transport tank unit structures have been described in detail, various modifications, alterations, and changes may be made without departing from the spirit and scope of the present invention as defined in the appended claims.