NZ240909A - Underground storage tank having thin, self-supporting corrosion-resistant semi-rigid liner - Google Patents

Description

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Cu.iijjiwtu Z- '.cific-lion F.'.cr1: ................ pJb=:^ . ./ 2 5 NOV 1993 P.O. J. l.Y*Mr Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION STORAGE TANK HAVING SECONDARY CONTAINMENT WE, OWENS-CORNING FIBERGLAS CORPORATION, a corporation under the state of Delaware, U.S.A. of Fiberglas Tower, Toledo, Ohio 43659, U.S.A. hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: (followed by page la) 24 0 9 0 9 This invention generally relates to storage 20 tanks and more particularly to underground storage tanks with secondary containment.

Environmental protection is becoming increasingly important. As understanding of 25 contamination of soil and water beneath the surface grows, efforts to prevent leaks have increased. Early efforts resulted in glass fiber reinforced plastic (FRP) single wall underground tanks. See U.S. Patent No. 3,661,294 issued in 1972. As ' awareness grew, efforts to protect 3Q 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.

Attempts were made — to encase or fit rigid storage tanks with secondary containment systems, often with flexible bladders or jackets. See U.S. Patent No. (followed by page 2) 9 h n q Cm 2 23150A c 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.

Basically, the invention is a standard single wall tank (SWT) on the inside of which is 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 permeable core such as a thin high density polyethylene (HDPE) fluid transmitting net.

The novelty of the 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 will be shown, carbon steel 20 liners and flexible bladders do not compare with the invention.

The thin liner employed , 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 of the primary tank, the FRP outer vail will permeate only negligible amounts of contents, e.g.: fuel, its /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 used has low permeability. This is important in that some leak sensors that are located two between the / walls will false alarm if the rate of 24 0 9 0 2315OA 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 used have provided corrosion resistance for the internal walls 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.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is an elevational view of a single wall tank containing an FRP liner in accordance with the 20 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.

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 30 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 35 tensile strength, they aloe absorb tensile stresses to icted. It is important to which the tank- 2Jo4nayT^" 29 JUL 1993 i-n q n °-|i' ij \J 23150A c 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 an annular space 4 20 between wall 40 and wall element 25. Space 4 3 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 2 5 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 4 0 including end cap, curing the FRP, placing another sheet of Mylar on top of liner 40 and then carrying out the 30 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 35 polyesters. "7!:~ \ 24 0 9 0 9 23150A 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 its handling 20 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 25 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 30 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 35 typically contain ah ethylenically unsaturated monomer, 24 0 9 23150A such as styrene, copolymerizable with the unsaturated polyester for curing.

The glass fibers preferably are "E" glass fibers which are non-conductive textile and reinforced.

See-U.S. Patent No. 2,334,961.

As stated above, permeable core material 4 4 may fill space 43. Examples of permeable 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 matt.ing, nylon matting and corrugated cardboard.

INDUSTRIAL APPLICABILITY The following table summarizes the advantages of my invention over other alternatives: TftPfcE THIN WALL FRP INNER TANKS VS OTHER ALTERNATIVES INVENTION CONTROL THIN FRP INNER WALL STAINLESS STEEL 1/10" CARBON STEEL 1/4" CARBON STEEL 1/10" FLEXIBLE RUBBER-LIKE BLADDER Salf-Si^porting Yes Yes Yes Yes No Low Permeability to Fuel* Yes Yes Yes Yes No Internal Wall of Primary Containment Corrosion-Resistant to Alcohol Bland, Fuels, Water Yes Yes No No No External Wall of Primary Containment Corrosion Resistant to Water Yes Yes No No Yes Independent (unconnected) Free Outer Wall Yas Yes Y«S Yes Yes Able to Determine the location of leeks Yes Yes Yes Yes No 24 0 9 0 23150A ....

An alternative embodiment comprises FRP inner wall 40 sections small enough to fit inside a tank through access 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 watting 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 20 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 25 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 access fitting 54 (Figure 1) communicating with the inside thereof, and a double-flanged extension 55 30 normally covered by a cover 56. Hand lay-up secures access 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. & 24 0 9 0 23150A The thin FRP is: * * * 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 tank. For these narrow widths, the inner liner is bonded to the I5 rigid primary tank, thus allowing easy manufacture and installation of tank accessories such as fittings and ports. access / Alternately, the inner liner can end near the top of the tank resulting in only one wall at the top of the tank. used inner liner that is / is unique in that it Self Supporting Corrosion resistant Offers lower permeability 29 JUL 1995

Claims (9)

WHAT WE CLAIM IS: 2<tGyQ9j

1. An underground storage tank comprising: a rigid tank particularly suited for use underground; and an inner liner located on the inside of the tank, the inner liner having sufficient flexibility so that it would substantially collapse if totally unconstrained, but having sufficient rigidity so that it would substantially conform to the shape of the tank when positioned within the tank.

2. The tank of Claim 1 wherein the inner liner is structurally independent of the tank.

3. The tank according to any one of the preceding claims wherein an annular space exists between the tank and inner liner.

4. The tank according to claim 3 including gas permeable material in the annular space between the tank and inner liner.

5. The tank according to claim 4 wherein the gas permeable material is a high density polyethylene net.

6. The tank according to any one of the preceding claims wherein the inner liner is glass fiber reinforced plastic.

7. The tank according to any one of the preceding claims wherein the inner liner is glass fiber reinforced plastic from 0.020" to 0.250" thick.

8. The tank according to any one of claims 1 to 5 wherein the inner liner is stainless steel or other metallic material from 0.010" to 0.125" thick. :'/ v G -\ ^ 'V ■C-soc,h 24 0 9 09 -10-

9. An underground tank substantially as herein described with reference to the accompanying drawings. OWENS-CORNING FIBERGLASS CORPORATION tttorneys JON & CAREY /« \ 20 SEP 1993 4