Double-Wall Underground Container for Storing Liquids and Leak Detecting Means Therefor

This invention relates generally to underground tanks, and more particularly to a glass fiber reinforced plastic double-wall underground tank with leak detecting means.

Environmental protection is becoming increasingly important. Single-wall underground tanks are advisedly tested periodically for leaks. Many times small leaks go undetected, and often by the time a large leak is detected, pollution of the environment has already occurred.

In accordance with the invention, a glass fiber reinforced plastic double-wall underground tank with leak detecting means is provided.

The invention is more fully explained hereinafter with reference to the accompanying drawings in which:

FIG. 1 is an elevational view, partly in section, of a double-wall tank constructed in accordance with the invention and installed underground, with leak detecting means aboveground;

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

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

FIG. 4 is a fragmentary perspective view of a molded plastic spacing strip of the type used to manufacture the double-wall tank of 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 pipe or conduit 14. Essentially, the tank 10 comprises inner and outer tanks 10a and 10b with space therebetween, the space being filled with a leak detecting liquid 15, preferably an anti-freeze solution, which also fills the pipe 14 and partially fills a reservoir or container 16, the pipe 14 being in communication with the space between the inner and outer tanks 10a and 10b and with the container 16. Besides the container 16, the leak detector 13 includes an electrical control box 18 mounted above the container 16 and having a pair of sensing probes 20 normally extending below the surface of the leak detecting liquid 15. The container 16 and control box 18 are mounted in a housing 22. A monitoring board 24 is shown mounted on the housing 22, but the board 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 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 this 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 halves such as would lie on opposite sides of a vertical plane represented by the line X--X in FIG. 1. Each half includes an inner cylindrical wall 11a and an inner end cap 12a. Preferably, the end cap 12a is shaped in accordance with the disclosure of U.S. Pat. No. 4,071,161, having a spherical central portion merging into a frusto-conical portion joined to the wall 11a. The wall 11a may be formed on a collapsible mandrel such as disclosed in U.S. Pat. No. 4,233,020, and preferably, the end cap 12a is formed integrally with wall 11a on an end cap mold mounted on the mandrel, somewhat as disclosed in U.S. Pat. No. 4,255,302. The procedure for forming the wall 11a may be somewhat as disclosed in U.S. Pat. No. 3,700,512. A surface mat, hardenable liquid resin, chopped glass strand, and a filler such as sand are applied to the mandrel surface and the resin is cured. The end cap 12a is also formed of surface mat, hardenable liquid resin, chopped glass strand, and a filler such as sand. A plurality of molded plastic spacing strips 30, best shown in FIG. 4, are then put in place one at a time around the wall 11a at spaced intervals. A carriage feeds dry continuous glass filament windings (not shown) around the mandrel and a spacing strip 30 is fed under the windings. The carriage stops at pre-programmed intervals along the mandrel. Each spacing strip 30 includes a sheet-like body portion 30a having cup-like projections 30b. At opposite logitudinal edges of a strip 30, the cup-like projections are cut in half when a strip 30 is severed from a larger sheet. The cup-like projections 30b engage the wall 11a and space the sheet-like body portion 30a of a spacing strip 30 away from the wall 11a. After the spacing strips 30 are in place, a relatively thin layer (not separately shown) of resin-impregnated continuous glass filament windings is applied over each strip, eventually becoming part of the wall 11b, and an annular connecting rib 32 formed of resin-impregnated continuous glass filament windings is applied on the wall 11a between each pair of adjacent strips 30. The ribs 32 eventually connect the cylindrical walls 11a and 11b of the tanks 10a and 10b. An annular chamber 33 is defined by each pair of adjacent ribs 32, the spacing strip 30 therebetween, and the corresponding portion of the cylindrical wall 11a. Before the outer tank 10b is formed, provision is made to interconnect all the annular chambers 33 and two end cap chambers 35, one of which is shown in FIG. 1, by cutting away portions of the ribs 32 to provide two sets of generally aligned passageways extending axially of the tank 10, one set being indicated by numeral 34 in FIG. 2. The cutting for each set of aligned passageways may be carried out by two passes of a rotary saw axially of the cylindrical wall 11a, which incidentally also cut away portions of the spacing strips 30. After the cut portions are removed, a strip 36 of tape is secured over the area of the cut to maintain the passageways when the wall portion 11b is laid up. In a finished tank 10, one set 34 of generally aligned passageways extending through the ribs 32 axially of the tank 10 is disposed essentially at the top of the tank and one is disposed essentially at the bottom.

A previously formed end cap skin (not separately shown) made of hardenable liquid resin and chopped glass strand and provided with a short cylindrical flange is then telescoped by its flange over the end of the wall 11a and the end spacing strip 30 with its thin layer of filament windings, in spaced relationship to the end cap 12a. The main portion of the end cap skin eventually becomes the inner surface of the end cap 12b.

The wall 11b is then formed over the spacing strips 30 and the connecting ribs 32 by application of hardenable liquid resin, chopped glass strand, and a filler such as sand. The end cap 12b is similarly formed over the end cap skin (not separately shown).

Annular forms 37 are then secured in place by dry windings around the wall 11b at spaced intervals and ribs 38 are formed thereover by glass mats or axial tape and resin-impregnated continuous glass filament windings, similarly to the formation of the ribs in U.S. Pat. No. 3,700,512.

Two tank halves are so formed and fastened together at a butt joint by alternate layers of glass mat and woven roving impregnated with hardenable liquid resin and applied on the interior of the tank 10a and on the exterior of the tank 10b, similarly to the way the single wall tank halves are fastened together at the exterior surface in U.S. Pat. No. 3,700,512.

When the leak detecting liquid 15 is added to the container 16, it fills all the annular spaces between the walls 11a and 11b and between each pair of connecting ribs 32 as well as the space between the end caps 12a and 12b at both ends of the tank 10, all these spaces being connected by the sets 34 of generally aligned passageways.

Various modifications may be made in the structure shown and described without departing from the scope of the invention as set forth in the appended claims.