The present invention relates to the detection, localization, and quantification of leaks in a liquid storage vessel. More particularly, the present invention relates to a method and apparatus for detecting leaks in above ground liquid storage tanks by detecting the presence of vapor or liquid from the liquid stored in the tank in the strata underneath the tank.
Environmental awareness and concern for the economic loss from lost product has resulted in a relatively recent emphasis on insuring the tightness of liquid storage vessels. Of primary concern is the potential for contamination of underground water supplies caused by leaking gasoline or other refined petroleum products. In response to political pressures exerted by environmental and other interest groups, governmental authorities have imposed strict controls on the operation of such facilities to prevent contamination and to held arrest the deteriorating state of the environment.
Enforcement of these regulations has created a new and significant demand for testing procedures and equipment capable of detecting ever smaller amounts of leaking gasoline. For instance, current United States governmental regulations specify that the maximum allowable leakage from above ground storage tanks (ASTs) be no greater than 5 gallons per hour and it is expected that future standards will be even lower.
Most of the equipment and procedures currently known in the art for testing above ground storage tanks are not sufficiently accurate, and lack the greater resolution and precision required, to test against the lower standards. Improved methods such as that described in U.S. Pat. No. 4,462,249 have made it possible to test the underground storage tanks for leaks even smaller than 0.1 gal/hr, but so far as is known, no methods capable of such precision are available for testing above ground tanks for such leaks.
The difficulty with the testing of above ground tanks has been recognized for a long time. The diameter of such tanks is such that the drop in liquid level from a leak is barely perceptible because it is such a small portion of the total volume of the liquid tank in the tank. As noted in U.S. Pat. No. 3,062,994, in a tank having a diameter of two hundred feet, 10,000 gallons of liquid must be lost for the level to drop by 0.5 inches. Even if the capability existed to measure to measure such volumes accurately, a 0.05 inch decrease in the liquid level (and assuming a linear correlation between liquid level and volume), would evidence the loss of 1,000 gallons, which is a leak which would be considered a major environmental incident by current criteria. The size of these tanks also requires that the leak be located for repair. The tanks are often so large and expensive that replacement in the event of a leak is cost prohibitive such that repairs are required. Of course, finding the leak in the tank is a prerequisite to an effective repair.
The patent literature evidences many attempts to overcome the difficulty of detecting the location of a leak in an AST. For instance, U.S. Pat. No. 5,052,215 describes a method by which fluid is injected in the base beneath an above ground storage tank to enhance the rate of leakage from the bottom of the tank; leakage is then detected with acoustic sensors placed around the tank. Another approach is to use volatile liquid tracers as described in U.S. Pat. No. 5,048,324. The patent literature also includes grids or arrays of electronic probes (U.S. Pat. No. 4,646,069) for detecting the presence of the liquid that has leaked from the tank, ducts for conveying the gases from an escaped liquid past a sensor (U.S. Pat. No. 4,618,855), conductive wires which undergo a change in electrical properties when contacted by escaped liquid (U.S. Pat. No. 4,404,516), and reference and sensing electrodes for measuring changes in electric potential in the substrate/soil under the tank caused by escaped liquid (U.S. Pat. No. 4,166,244).
The above-mentioned, non-volumetric method described in U.S. Pat. No. 4,646,069, with the disclosure of an array of electronic probes, would provide at least some information as to the source of the leak. However, unless those probes are located close together, it may not be possible (because of the size of the tank) to localize the leak to a location any more precise than an area of several square feet on the bottom of the tank. There is the additional consideration as to the expense of using enough probes to increase the ability of such a system to localize the leak such that cost and resolution capability is always a trade-off. Further, ASTs are constructed on a bed of sand such that liquid and vapor originating at one point under the AST may be detected elsewhere. U.S. Pat. No. 4,896,528 describes a grid of test conduits under the tank and a description of isolating a portion of the conduits to better localize the leak, but the description is less than explicit and the cost and complication of the many conduits are disadvantageous. It is, therefore, a principal object of the present invention to provide an improved method for localizing a leak in an above ground storage tank. Other objects, and the advantages, of the present invention will be made clear to those skilled in the art by the following description of a presently preferred embodiment thereof.