TECHNICAL FIELD This invention relates generally to underground storage tanks, and more particularly to a double-wall 15 underground storage tank having leak-detecting liquid between walls.

BACKGROUND ART In a prior double-wall underground tank each of the annular ribs bonded to and connecting the inner and 20 outer walls is formed of resin-impregnated glass filament windings wound to provide a rib of solid cross section.

DISCLOSURE OF INVENTION In accordance with the invention, spaced hollow annular ribs are provided on and bonded to the exterior of 5 the inner wall, a base sheet material is wound over the ribs to straddle the spaces therebetween, and the outer wall is formed over the ribs and base material, becoming bonded to the ribs. Leak-detecting liquid fills the spaces between ribs and the space within each rib. 0 BRIEF DESCRIPTION OF DRAWINGS

The invention is more fully described hereinafter, with reference to the accompanying drawings wherein:

FIG. 1 is an elevational view, partly in section, 5 of a double-wall underground tank constructed in accordance with the invention;

1 FIG. 2 is a cross-sectional view taken generally along the line 2-2 of FIG. 1;

FIG. 3 is a fragmentary enlarged longitudinal sectional view taken generally along the line 3-3 of FIG. 5 2;

FIG. 4 is a fragmentary longitudinal sectional view similar to a portion of FIG. 3 but illustrating an earlier step in the manufacture of the tank before formation of the outer wall; 10. FIG. 5 is fragmentary longitudinal view illustrating another step in the manufacture of the tank; and

FIG. 6 is a fragmentary longitudinal sectional view of the tank at the juncture of two halves. 15 BEST MODE OF CARRYING OUT THE INVENTION

With reference to the drawings and in accordance with the invention, FIG. 1 shows a double-wall tank 10 installed underground and connected to above-ground leak detecting means 13 by a conduit 14. Essentially, the tank 0 10 comprises an inner tank 10a and an outer tank 10b surrounding the inner tank 10a with space therebetween, the space being filled with a leak detecting liquid 15 which also fills the pipe 14 and partially fills a container or reservoir 16, the pipe 14 being in communication with the 5 space between the inner and outer tanks and with the reservoir. The leak detector 13 also includes an electrical control box 18 mounted above the reservoir 16 and having a pair of sensing probes 20 normally extending below the surface of the leak detecting liquid 15. The 0 reservoir 16 and control box 18 are mounted in a housing 22. A monitoring board 24 is shown mounted on the housing 22, but could be remotely located inside a building. A bell or buzzer 26 and a plurality of lights 28 are ^' mounted on the board 24 and electrically connected to the control 5 box 18. Regardless of whether the inner tank 10a is full or empty and regardless of whether the tank 10 is installed in dry ground or below the water table, if a leak develops

in either the inner tank 10a or the outer tank 10b, the ^• level of the leak detecting liquid 15 in the container 16 will decrease. When the level drops below the sensing probes 20, the bell or buzzer 26 will emit an audible signal and the lights 28 will give a visual indication that a leak has developed.

The double-wall tank 10 is made in two substantially identical halves each including an inner cylindrical wall 11a, an inner end cap 12a, an outer cylindrical wall lib, an outer end cap 12b, and a plurality of axially spaced, annular, generally hollow ribs 30 between and bonded to the walls 11a and lib. The outer end cap 12b includes a preform 12c and a lay-up 12d over the preform. Additional details are shown in FIGS. 2-6. The outer wall lib includes a base sheet lie covered by a lay-up lid. The ribs 30 include a preform 30a covered by a lay-up 30b. Each side of a rib 30 has four holes 32 extending therethrough respectively at the top, bottom, and opposite sides of the tank 10. A manway 34 is provided in one of the tank halves.

The inner cylindrical wall 11a and the inner end cap 12a of a tank half are formed first. Preferably the wall 11a is formed on a collapsible mandrel such as disclosed in U.S. Patent No. 4,233,020, and the end cap 12a is preferably formed integrally with the wall 11a on an end cap mold mounted on the mandrel in a manner similar to that disclosed in U.S. Patent No., but having a shape such as disclosed in U.S. Patent No. 4,071,161, wherein a spherical central portion merges with a frusto-conical peripheral portion. The procedure for forming the wall 11a may be somewhat as disclosed in U.S. Patent No. 3,700,512. A surface mat and a relatively thin layer of hardenable liquid resin and chopped glass strand are applied to the rotating mandrel surface, followed by a relatively thick layer of hardenable liquid resin, chopped glass strand, and sand, and an overlapping of thin strips of filament

1 windings axially spaced at intervals. The filament windings retain the rest of the material on the mandrel until the resin begins to set. While the wall 11a is being formed, an operator may spray-up the end cap 12a after 5 applying a surface mat and hardenable liquid resin to the mold, using a spray gun spraying hardenable liquid resin, chopped glass strand, and sand.

The axially spaced annular ribs 30 are then applied to the wall 11a. For each rib 30, a cardboard form

10 _. 30a similar to that shown in U.S. Patent No. 3,412,891 is wrapped around the wall 11a. The form 30a is in the cross-sectional shape of a top and two sides of a trapezoid, the sides extending generally at 45° angles and being slotted to allow bending into an overall circular

15 shape around the wall 11a. The trapezoidal shape is retained by correspondingly shaped foam blocks 30c spaced at intervals, the cardboard being stapled to the foam blocks. The form may be fed between the wall 11a and filament windings which hold it in place initially. A 0 glass fiber reinforcement mat such as glass cloth, woven roving, or axial tape is then placed over the form 30a and resin-impregnated glass filament windings are wound thereover to form the lay-up 30b. The filament windings may be applied circumferentially, with no helix angle, as 5 in U.S. Patent No. 3,818,950, but are preferably cross-wound as in U.S. Patent No. 3,661,294. Preferably the reinforcing mat is axial tape, a knitted undirectional mat of glass strands applied with the strands extending axially of the wall 11a and crosswise of the rib form 30a. _Q Before the lay-up 30b is cured, holes 32 are punched therein with punching tools 36 (FIG. 4) left in place during the curing and thereafter removed. The punching tools 36 may each conveniently be a rod pointed at one end 36a, long enough to extend through both sides of a rib 30, 5 and bent 90° at the other end to provide a handle 36b. The holes 32 are formed 90° apart circumferentially of a rib

and located adjacent the top, bottom, and opposite sides of the tank 10.

Base sheets lie are then placed over the ribs 30 as shown schematically in FIG. 5. Any suitable material may be used for the base sheets so long as the lay-up lid is not prevented from bonding to the ribs 30. Therefore, if the base sheet is not porous, a separate sheet must be used to straddle the space between each pair of adjacent ribs and extend less than to the center of each of the two ribs, leaving a bonding area for the lay-up lid open at the center of each rib. Preferably a porous scrim lie is used, wide enough to cover three ribs 30 and initially held in place with a few filament windings at least at the two outer ribs of the three. The mesh size of the scrim lie is large enough for resin to pass therethrough at the ribs 30 but small enough so resin applied to the scrim between ribs will bridge the holes to form a solid sheet. Therefore, resin is applied to the scrim lie and allowed to cure enough to make the scrim stiff between ribs. Preferably a small amount of chopped glass strand is applied to the resin impregnated scrim. Adjacent scrim strips lie, each wide enough to cover three ribs 30, are placed so that edge portions of two scrim strips lie overlap at a commonly covered rib 30, as indicated in FIG. 5. The scrim is preferably glass fiber scrim.

After the resin on the scrim has cured enough to stiffen the scrim, the lay-up lid of the outer wall is applied and consists of resin, chopped glass strand, and sand. At the free end of the mandrel, the preform 12c is telescoped over the corresponding end rib 30 and the lay-up 12d, of resin, chopped glass strand, and sand, is sprayed up thereover. After curing, the tank half is removed from the mandrel.

Two such tank halves are formed, each having a sacrificial rib 30 adjacent its open end. The tank halves are each sawed through to remove the sacrificial rib 30 and thereby provide an overhang of the outer wall lib flush

1 with the end of the inner wall 11a. One of the tank halves is provided with the manway 34, and the two tank halves are then secured together by an internal lay-up 37 and an external lay-up 38 (FIG. 6). The foam blocks 30c are not 5 in liquid-tight relationship with the forms 30a, and the leak-detecting liquid 15 fills the hollow ribs 30, the spaces between ribs 30 and between the walls 11a and lib, and the spaces between the end caps 12a and 12b.

Various modifications may be made in the 10. structure shown and described without departing from the spirit and scope of the invention.

INDUSTRIAL APPLICABILITY The double-wall underground tank of the invention, with the means for detecting a leak in either 15 the inner or the outer wall, is useful for storing liquids such as hydrocarbons, when it is desired to protect the environment from pollution. Upon a signal indicating a leak, the stored liquid may be pumped into another tank to minimize leakage into the environment. 0

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