Spaces within tanks or lines can be properly insulated against penetration of heat by one or more layers of foamed material on the inner surface of the walls. By foamed material is here meant a material having a cellular structure, the cells of which being open or closed. The material is preferably rigid. Suitable materials are organic foams which may be applied to the wall by spraying, such as polyurethane foam. The insulating lining is composed of adjacent strips and also often of several layers on top of each other. Other materials such as impermeable membranes may be used beneath and between the layers.
During use, cryogenic linings come into contact with cold liquid. As the result of cooling there is substantial shrinkage, with the result that cracks may occur in the foamed material. This problem becomes more acute as the liquid is colder, such as for example, with liquefied methane, natural gas or nitrogen. The occurrence of cracks as the result of shrinkage is promoted by hair cracks already present or by incomplete adhesion of adjacent strips of the foamed material, and it is of great importance to detect this imperfection in order to repair it before the space is taken into use. The present invention provides a method of so doing.