STORAGE TANK HAVING SECONDARY CONTAINMENT

Abstract: An underground storage tank having secondary containment comprising a self-supporting, semi-rigid thin liner (40) located on the inside of the tank (25). The thin inner liner (40) completely lines the inside of the tank and is structurally independent of the tank.

Description: STORAGE TANK HAVING SECONDARY CONTAINMENT TECHNICAL FIELD This invention generally relates to storage tanks and more particularly to underground storage tanks with secondary containment. BACKGROUND ART Environmental protection is becoming increasingly important. As our understanding of contamination of soil and water beneath the surface grows, our efforts to prevent leaks increases. Early efforts resulted in glass fiber reinforced plastic single wall underground tanks. See U.S. Patent No. 3,661,294 issued in 1972. As our awareness grew, our efforts to protect the environment matured into glass fiber reinforced plastic double-wall underground tanks, often equipped with leak detection systems. See U.S. Patent No. 4,561,292 issued in 1985. Others have attempted to encase or fit rigid storage tanks with secondary containment systems, often with flexible bladders or jackets. See U.S. Patent No. 4,524,609 issued in 1985. Flexible bladders have obvious problems. For example, flexible inner bladders or flexible outer jackets are very susceptible to damage from cutting, tearing, puncturing, etc. DISCLOSURE OF THE INVENTION Basically, my invention is a standard single wall tank (SWT) on the inside of which I have added an FRP smooth inner wall that is not attached to the outer wall. The annular space between the inner and outer wall may have a non-structural porous core such as a thin HDPE fluid transmitting net. The novelty of my invention is a self- supporting, semi-rigid, thin liner inside the SWT. The inner wall can be a thin FRP liner, thin stainless steel or an equivalent material. As I will show, carbon steel liners and flexible bladders do not compare with my discovery. The thin liner I employ is self-supporting. Flexible bladders on the other hand require either internal supports or the application of a vacuum between the inner bladder and the outer tank. This tank has many advantages. One is that the installer can field test the outer wall for leaks prior to installation. Jacketed tanks cannot be field soap tested. Also, in the event of a breach to the primary tank, the FRP outer wall will permeate only negligible amounts of fuel into the environment, as opposed to a jacket which, because of its low resistance to fuel permeation, could allow a significant fuel spill prior to detection. The thin liner I use has low permeability. This is important in that some leak sensors that are located between the 2 walls will false alarm if the rate of permeation is too great. This is particularly a challenge for any material that must contain alcohol or blends of fuel containing alcohol. The thin liners I use have provided corrosion resistance for the internal wall of primary tanks. Carbon steel often rusts due to water condensate at the bottom of the tank. This has been traditionally overcome by using thick steel (at least 1/4") . This is why thin carbon steel will not work i.e., an allowance for corrosion must be incorporated into carbon steel tanks. BRIEF DESCRIPTION OF THE DRAWINGS The invention is more fully explained with reference to the accompanying drawing in which: Figure 1 is an elevational view of a single wall tank containing an FRP liner in accordance with the present invention. Figure 2 is a sectional view taken generally along the line 2-2 of Figure 1; and Figure 3 is a fragmentary perspective of an FRP inner wall panel in accordance with this invention. BEST MODE OF CARRYING OUT INVENTION Figure 1 shows a tank 20 which employs the FRP inner wall structure of the present invention (not shown) . The tank 20 is made up of opposed frusto-conical tank halves 22, connected together by center joint 24. Wall 26 includes a wall element 25 in combination with a rib 28. Actually, a plurality of ribs 28 are axially spaced along the length of the tank 20. Ribs 28 extend peripherally of the tank 20 and act in the nature of strong hoops against radially inwardly crushing forces. Since they are of high tensile strength, they also absorb tensile stresses to which the tank 20 may be subjected. It is important to note that the ribs 28 add to the stiffness of the wall 25; also, they provide protective buffers during handling. The ribs 28 are spaced apart a sufficient distance so that fill and vent fittings 30 and 32 can be installed between the ribs. Optional positions 34 for fittings are thus provided all along the length of the tank 20. In an actual 6,000-gallon capacity tank of 8 feet nominal diameter, and approximately 20 feet length, a spacing of 16 1/2 inches between rib enters was employed and this provided adequate space for the installation of the fittings 30 and 32. U.S. Patent No. 3,661,394 fully describes ribbed, single wall tank construction. Figure 2 shows FRP inner wall 40 on the inside of wall element 25. Figure 2 also shows annular space 43 between wall 40 and wall element 25. Space 43 is partially filled with porous core 44. Figure 3 shows a panel of FRP inner wall 40 detached from tank 20. Typically, one can use any molding process or spray up equipment to make FRP inner wall 40. One can achieve this by placing mold release (Mylar) on a conventional SWT mold, spraying up thin FRP inner liner 40 including end cap, curing the FRP, placing another sheet of Mylar on top of liner 40 and then carrying out the conventional construction of SWT, for example, as described in U.S. Patent No. 3,661,394. FRP inner wall 40 preferably is made of unsaturated polyester compounds. The practice of this invention, however, is not restricted to unsaturated polyesters. These compositions, intended to polymerize when molded under heat and pressure, are generally combined with fillers and chopped glass, to produce products having appearance surfaces with a minimum of irregularities. The use of chopped glass as reinforcement in such molding compounds is well known. The chopped glass is produced in the form of individual strands which are sized, gathered into rovings, chopped to the desired length and incorporated into the resin composite prior to molding. The sizes generally comprise a lubricant, film formers and the like and are extremely important in imparting to the reinforcing glass its ability to be wetted out by the molding compound. These sizes are also important in that they protect the glass in it handling subsequent to being sized and are influential in minimizing the amount of fuzz and fly which is produced on the glass, the fuzz and fly having a decided affect upon the appearance surface of the molded product. The sized glass fibers generally are employed as reinforcement for sheet molding compounds (SMC) and bulk molding compounds (BMC) . Unsaturated polyesters useful in this invention typically contain a polyesterification product of one or more ethylenically unsaturated dicarboxylic acids or anhydrides such as maleic or fumaric with one or more glycols such as ethylene or propylene glycol and, sometimes, minor proportions of other aromatic or aliphatic mono- or dicarboxylic acids or anhydrides and/or other mono- or polyhydroxyl compounds. They also typically contain an ethylenically unsaturated monomer, such as styrene, copolymerizable with the unsaturated polyester for curing. The glass fibers preferably are "E" glass fibers, well known to those skilled in the art. See U.S. Patent No. 2,334,961. As I stated above, porous core material 44 may fill space 43. Examples of porous core materials 44, are mattings, nets, screens, and meshes. Specific examples are high density polyethylene (HDPE) net, jute, polyurethane foam, polyester foam, fiberglass matting, cotton matting, nylon matting and corrugated cardboard. INDUSTRIAL APPLICABILITY The following table summarizes the advantages of my invention over other alternatives: TABLE THIN WALL FRP INNER TANKS VS OTHER ALTERNATIVES INVENTION CONTROL THIN STAINLESS CARBON CARBON FLEXIBLE FRP INNER STEEL STEEL STEEL RUBBER-LIKE WALL 1/10" 1/4" 1/10" BLADDER Self-Supporting Yes Yes Yes Yes No Low Permeability Yes Yes Yes Yes No to Fuels Internal Wall of Primary Yes Yes No No No Containment Corrosion-Resistant to Alcohol Blend, Fuels, Water External Wall of Primary Yes Yes No No Yes Containment Corrosion Resistant to Water Independent (unconnected) Yes Yes Yes Yes Yes From Outer Wall Able to Determine the Yes Yes Yes Yes No location of leaks An alternative embodiment comprises FRP inner wall 40 sections small enough to fit inside a tank through manway openings. Typically the panels are up to 8 feet in length and range from 2 to 4 feet in width. After the panels are in place inside the tank, one uses a hand lay-up procedure on the seams of each panel to form FRP inner wall 40. Basically, the procedure involves building up a combination of chopped glass fibers and a hardenable liquid resin and, if desired, a sand filler. Complete wetting of the chopped glass fibers is desirable and can be accomplished, as is well known in the art, by rolling out the resin and glass and sand mixture. After the seams are fabricated, heat or the passage of time cures the resin. One can use any spray device or combination of spray devices to apply the resin and chopped glass fibers. Often the resin contains an accelerator or catalyst to speed up the curing process. As shown in Figure 3, the panels and FRP inner liner 40 preferably have the same curvature as wall element 25. Preferably inner FRP liner 40 is thin and typically is 1/8 to 1/4 of an inch thick. Access to the inside of the tank 20 is provided by a flanged manway fitting 54 (Figure 1) communicating with the inside thereof, and a double-flanged extension 55 normally covered by a cover 56. Hand lay-up secures manway fitting 54 to tank 20 by application of hardenable resin, chopped glass strand and filler such as sand. The hand lay-up procedure is much the same as that used to connect the panels of FRP inner wall 40. The thin FRP inner liner that I use is unique in that it is: * Self Supporting * Corrosion resistant * Offers lower permeability In one embodiment, the thin inner liner is structurally independent of the tank for the entire circumference except for a narrow width centered at the top of the bank. For these narrow widths, the inner liner is bonded to the rigid primary tank, thus allowing easy manufacture and installation of tank accessories such as fittings and manways. Alternately, the inner liner can end near the top of the tank resulting in only one wall at the top of the tank.

Claims: C AIMS 1. An underground storage tank comprising: a rigid tank particularly suited for use underground; and a semi-rigid thin inner liner located on the inside of the tank; the inner liner completely lining the inside of the tank, being structurally independent of the tank, thin but self-supporting, corrosion resistant and having low permeability. 2. The tank of Claim 1 wherein the inner liner is glass fiber reinforced plastic. 3. The tank of claim 1 wherein an annular space exists between the tank and inner liner. 4. The tank of Claim 3 including gas porous material in the annular space between the tank and inner liner. 5. The tank of Claim 4 wherein the gas pervious material is high density polyethylene net. 6. The tank of Claim 1 having a manway and/or fittings fitted into a wall portion of the tank and connecting with the inside of the tank. 7. The tank of Claim 1 wherein the glass fibers have an "E" glass composition. 8. The tank of Claim 7 wherein a portion of the glass fibers are chopped glass fibers. 9. The tank of Claim 7 wherein the plastic is made from unsaturated polyester resin, epoxy resin or vinyl ester resin. 10. The tank of Claim 7 wherein the walls are cylindrical and the inner liner has a curvature the same as the cylindrical walls. -lO- ll. The tank of Claim 1 wherein the thin, inner liner is glass fiber reinforced plastic from 0.020" to 0.250" thick. 12. The tank of Claim 1 wherein the thin, inner liner is stainless steel or other corrosion-resistant metallic material from 0.010" to 0.125" thick. 13. The tank of Claim 1 wherein the thin, inner liner is structurally independent of the tank for the entire circumference, except for a narrow width centered at the top of the tank. 14. The tank of Claim 1 wherein the inner liner ends near the top of the tank, resulting in only one wall at the top of the tank.

Inventors: BARTLOW DAVID HARVEY, BARTLOW, DAVID, HARVEY
Applicants: OWENS CORNING FIBERGLASS CORP, OWENS-CORNING FIBERGLAS CORPORATION
Application year: 1991
Main classification: B65D90
IPC codes: B65D90