In a double-walled tank for low-temperature liquids, such as liquid gas, an inner tank forming a primary safety casing is enclosed by an outer tank forming a secondary safety casing. The outer tank is constructed of reinforced concrete. A layer of thermal insulation is provided between the inner and outer tanks. The inside surface of the outer tank at least on the side wall and base is covered first by a vapor-tight layer and then by a layer of a liquid-tight thermal insulation material, preferably polyurethane foam.

A storage vault includes an inner tank entombed within a concrete outer tank by first erecting a pair of sidewall forms on a base plate to create an open-top enclosure. The wall forms are securely clamped together. A layer of concrete is then poured onto the base plate and the inner tank is lowered onto the layer of concrete. Concrete is poured into the enclosure to entomb the inner tank while the bottom layer of concrete is still wet. The inner tank is restrained by a pair of reinforced hold-down bars extending over the top of the enclosure and having a pair of downwardly extending hold-down tubes which contact the top of the inner tank. A back hold-down bar is forced downwardly by a chain which passes over the hold-down bar and is fastened to the base plate. After the concrete has set sufficiently, the hold-down bars and tubes are removed and the holes created by the hold-down tubes are filled with concrete. After curing, the wall forms and base plate are removed from the vault created within the form assembly.

A method for making a storage vault includes a steel inner tank entombed within a reinforced concrete outer tank. The inner tank is covered with a thermal insulating spacer material and then wrapped with a sheet of liquid-impervious material to trap leaks from the inner tank. The spacer layer is made of a material which liquifies upon contact with the liquid stored within the inner tank. This permits the spacer layer to fill the space between the inner and outer tanks while permitting any leak to collect at a liquid collection region between the two. The leaked liquid collection region can be monitored through a leak detector tube. Since the covering of the inner tank with the spacer layer substantially fills the space between the inner and outer tanks, pouring the concrete outer tank is simplified and thermal insulation is maximized.

A method for making a steel/concrete tank is provided. A steel interior tank is pressurized during or after pouring of a concrete outer tank around a steel inner tank. Pressurization is commenced before the concrete has set, and is maintained until substantial setting of the concrete. The resulting structure is less susceptible than previous structures to cracking of the exterior concrete upon pressure-testing of the structure for leaks.

A dual containment, free-standing, earthquake resistant, above-ground, flammable liquid storage vault is made by first pouring a reinforced concrete outer tank having a flat bottom, for stability, and side walls. The inner surface of the concrete outer tank is covered with an epoxy resin, the epoxy resin acting as an outer liquid barrier to provide secondary containment. A layer of gravel is placed on the epoxy resin covering the bottom of the outer tank. A primary containment steel inner tank is then placed in the outer tank with a space between the walls of the inner and outer tanks. That space is then filled with concrete when a concrete cap is poured covering the inner tank. Leaks from the inner tank are monitored at the gravel layer. The concrete outer tank has integrally formed skids extending from its lower surface for ease of movement and for visual inspection beneath the vault for leaks. The vault can also be made by placing the inner tank within a freestanding, open-top shell; the combination is then encased in reinforced concrete.