The invention relates to a storage tank for environmentally hazardous liquids. More precisely, the invention relates to a double-shelled tank from which the emission of en¬ vironmentally hazardous liquids to the environment through tank leakages is prevented.

The storing of many environmentally hazardous liquids in above-ground or underground storage tanks constitutes a problem. If the tank develops a leak, for example because of corrosion, the contents of the tank will leak into the environment and will, for example, pollute the groundwater. Often, and particularly in the case of underground tanks, leakages are not observed until a long time has elapsed, and thus considerable amounts of hazardous substances may flow out and contaminate the environment and the groundwater.

It is known to use double-shelled tanks for the storing of, for example, fuels and chemi¬ cals, and in certain regions tanks of other types are not permitted. It is also known to fill the space between the shells with a mixture of ethylene glycol and water, the mixture being pressurized or being subjected to hydrostatic pressure by means of an expansion vessel. If the inner shell or the outer shell develops a leak, this is observed as a change in the pressure or as a change in the liquid level in the expansion vessel.

However, the use of glycol is as such hazardous. If the outer shell develops a leak, glycol will pass into the soil, from which it is difficult to remove. Glycol is water soluble and will therefore spread rapidly over a large area, from which it is cumbersome and expensive to remove.

If the inner shell develops a leak, water and glycol will pass into the liquid being stored, thus spoiling the contents of the tank. Even a slight amount of glycol in, for example, gasoline, diesel fuel, or heating fuel oil will cause severe problems of contamination, operation, and deposits in engines and oil burners. Furthermore, a mixture of glycol and water will cause corrosion of the tank shells. The corrosion inhibitors necessary for inhibiting corrosion will further increase the toxicity of the mixture.

Thus there is a need for double-shelled tanks in which the shield liquid used will not involve the disadvantages mentioned above. In the tank according to the invention, the problems mentioned above have been solved.

The invention thus relates to a storage tank for environmentally hazardous liquids, a tank which is at least partially double-shelled and in which the space between the shells contains a liquid. The tank according to the invention is characterized in that the liquid used in the space between the shells is a liquid having the following properties:

(a)-it remains liquid in the conditions of use of the tank;

(b) it is environmentally non-hazardous if it escapes from the tank; and

(c) it is harmless to the product stored in the tank.

Remaining liquid in the environmental conditions of use means that the liquid must not freeze in cold conditions and thereby cause breakage of the tank walls. The liquid used also must not have so low a boiling point that evaporation requiring continual adding of liquid occurs therein.

The liquid used also must be environmentally non-hazardous, so that no problems of contamination or destruction are produced in the event the outer shell possibly develops a leak. The most preferred liquids are those which decompose by themselves under the effect of natural decomposition, in which case no separate collecting and destroying are needed, at least in the case of small leaks.

The third requirement set on the liquid used is that, in the event that the inner shell develops a leak, the liquid used will not harm the liquid stored in the tank. This means that in moderately small amounts of the liquid used will not hamper the use, for its original purpose, of the product stored. For example, in the case of gasoline or diesel fuel this means that the liquid used will not in moderate amounts cause problems in engine operation.

Liquids fulfilling the above-mentioned requirements include certain biodegradable liquids which in nature decompose by themselves under bacterial action. Highly suitable are certain natural substances such as various vegetable oils and their esters. Also suitable are ordinary alcohols such as methanol and ethanol. Certain synthetic esters, such as biode- gradable esters used as lubricants, are also suitable.

The above-mentioned liquids also do not cause problems of poisoning or contamination in terms of the liquid stored in the tank.

Examples of liquids suitable for use in tanks according to the invention thus include vegetable oils or vegetable oil esters, oil esters, alcohols, and synthetic biodegradable lubricating oils. Especially suitable are, for example, rapeseed oil, colza oil, soybean oil, and sunflower oil, or esters of these, and tall oil ester.

For example, the pour point of rapeseed oil ester is approximately -30 °C, which is a sufficiently low value for underground tanks. The flash point of rapeseed oil or it's ester is above 100 °C, and thus its fire classification will not cause problems. Rapeseed oil ester passing into fuel is an advantage rather than detrimental. For example, the smoke and particle emissions from a diesel engine are even lowered when rapeseed oil ester is mixed with diesel fuel. Furthermore, rapeseed ester will not cause corrosion problems. Some of the other liquids listed above also have at least in part similar properties.

The tank according to the invention is at least partially double-shelled. Thus the double shell need not envelope the entire tank; the tank may have a single wall in those parts which are not in contact with the liquid being stored. Thus the upper section of the tank may be of normal construction, but below and on the sides the wall is double.

Methods and devices known per se can be used for detecting leakages. One way of detecting a leakage is to maintain overpressure in the space between the double walls. The system may contain an alarm when the pressure decreases in the space between the walls. Another, more common, method is to use an expansion vessel and a surface sensor.

The tank according to the invention may be applied both as an above-ground tank and as

an underground tank. Especially in the case of an underground tank, the advantages of the invention are evident, since expensive and laborious cleaning of soil is avoided, and there is no need to construct an expensive lining, such as a concrete trough, under and around the tank.

The tank according to the invention is suitable for the storing of all kinds of toxic and environmentally hazardous liquids. Such liquids include fuels such as gasoline, diesel fuel and heating fuel oil, solvents, lubricating oils, and other toxic chemicals.