Underground tank with leak detection mechanism
BACKGROUND OF THE INVENTION 
     (Field of the Invention) 
     The present invention relates to an underground tank having a leak detection mechanism, and particularly relates to an underground tank having a mechanism for detecting the leak of a contained substance such as gasoline. 
     (Description of the Prior Art) 
     Since a conventional underground tank for oil such as gasoline at an oil handling facility such as a filling station is made of steel in accordance with the Fire Services Act, the underground tank is likely to corrode and the tank portion right under an oil pouring port is likely to be holed (eroded). 
     Even if the outside surface of the conventional underground tank is coated with a bituminous substance such as coal tar for rust prevention, the coating layer of the bituminous substance is dissolved by oil permeating through the ground from the surface thereof, so that the outside surface of the tank is likely to corrode. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an underground tank which can eliminate the abovementioned drawbacks of the conventional underground tank. 
     It is another object of the present invention to provide an underground tank having a leak detection mechanism and comprising an inner tank in which a fluid is to be contained; an outer tank covering said inner tank; a thin sheet wound on the outside surface of said inner tank so that a very narrow gap is defined between said inner tank and said outer tank; a conduit opened into the gap between said inner tank and said outer tank, a means for applying prescribed pressure of gas to said gap through said conduit, and a means for detecting the change in the pressure of gas in the conduit. 
     According to the present invention, the leak out of the underground tank can be easily and accurately detected by using a very small quantity of gas. 
     The other object and feature of the present invention are hereinafter described with reference to the drawings attached hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of an underground tank having a leak detection mechanism of the present invention; 
     FIG. 2 shows a longitudinally sectional view of the underground tank shown in FIG. 1; and 
     FIG. 3 shows an enlarged cross-sectional view of a portion of the tank of another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 show the embodiment of the present invention in which an inner cylindrical tank 1 made of steel and having an inside diameter of about 2 m, a length of about 6 m and a thickness of about 6 mm is covered with an outer tank 3 made of fiberglass-reinforced plastic and having a thickness of about 3 mm. A thin sheet 6, such as a reticulate polyvinylidene chloride sheet having a thickness of about 0.2 mm or the like is wound intermittently or densely around the outside surface of the inner tank 1, so that a gap 2 is defined between both the tanks. A steel conduit 4 having an outer diameter of about 80 mm and a thickness of about 6 mm extends down hermetically through the inner and the outer tanks 1 and 3 and is opened into the gap 2 formed between the interior surface 11 of the outer tank 3 and the outside surface 10 of the inner tank 1 at the bottoms of both the tanks. Gas pressure is applied to the gap 2 through the conduit 4 from a conventional source of gas pressure 7 through the conduit 4. A means for detecting the change in the pressure in the conduit 4 and a means for detecting the change in the pressure in the inner tank 1 are provided. A socket 5 having an oil pouring port, an air hole and so forth is provided on the upper portion of the inner tank 1 through the outer tanks 1 and 3. 
     A method of defining the gap 2 between the inner and the outer tanks 1 and 3 is that a plurality of projections each having a height of about 0.2 mm are provided on the outside surface of the inner tank 1, and the outer tank 3 split in two parts is then provided around the inner tank. 
     Another method of defining the gap 2 between the inner and the outer tanks 1 and 3 is that a polyvinylidene chloride sheet 6 having a thickness of about 0.2 mm and being reticulate or the like is intermittently or densely wound on the outside surface of the inner tank 1, a fiberglass fabric is densely wound on the sheet, and an unsaturated polyester is applied to the fiberglass fabric. In this method, the unsaturated polyester permeates into the fiberglass fabric so that they spontaneously harden to constitute the outer tank 3. The polyvinylidene chloride sheet has openings which have a thickness of about 0.2 mm and constitute the gap 2. 
     If a leak occurs in the inner tank 1, the gas in the conduit 4 and the gap 2 flows into the inner tank 1 so that the pressure of the gas in the conduit 4 drops. The leak in the inner tank 1 is detected by finding out the drop in the pressure of gas in the conduit 4. 
     The pressure in the gas in the conduit 4 drops at the time of a leak out of the outer tank 3 as well. If it is found out whether the pressure of the gas in the inner tank 1 goes up or not, it can be judged whether the leak has occurred out of the inner tank 1 or out of the outer tank 3.