Abstract: A method is disclosed for manufacturing from a rigid single wall tank a rigid double wall tank for storage of liquids. The method includes the steps of applying to at least a portion of the outer surface of the inner tank a flexible sheetlike spacing material having a substantially continuous and generally planar surface with a plurality of projections extending outwardly of the planar surface and engaging the outer surface of the inner tank and then applying over the inner tank outer surface and the spacing material a sheath of a rigid and liquid-tight material. From this method there results a rigid double wall storage tank having an outer sheath spaced from the outer surface of the inner tank.

Abstract: A double wall tank for the storage of liquids is manufactured from a rigid, cylindrical single wall inner tank by applying a primary spacing material to at least a portion of the exterior surface of the inner tank, applying over that inner tank exterior surface and primary spacing material a substantially rigid outer sheath of material, forming an opening through that sheath extending the full length of the sheath, introducing the secondary spacing material between the sheath and the inner tank exterior surface, and closing the sheath opening. This method provides a double wall storage tank in which at least a portion of the liquid-tight outer sheath is spaced from the inner tank.

Abstract: A double wall tank for the storage of liquids is manufactured from a rigid single wall inner tank by applying a spacing material to at least a portion of the exterior surface of the inner tank and applying over that inner tank exterior surface and the spacing material a substantially rigid outer sheath of a material that is substantially liquid-tight.