The invention relates to a heat-insulated container for the storage or transport of cold liquids, such as liquefied gases, comprising a rigid wall and a heat-insulating lining of a rigid foam material, preferably of polyurethane foam, arranged on the rigid wall and more in particular to means and a method for detecting cracks in the heat-insulating lining of this container.
Such a container is adapted for use, respectively as a land storage tank or as a ship's tank, respectively for storage or transport of liquefied gases, such as, for example, liquefied natural gas, liquefied methane, liquefied propane or liquefied butane.
Liquefied natural gas is stored or transported in such a container at a temperature of about -160.degree. C. When a rigid wall of the container is normal steel and the heat-insulating lining is arranged on the inner surface of the rigid wall, the danger exists that, in case of failure of the lining, the steel wall will cool down to such a degree that it would become brittle and lose its strength.
Since the heat-insulating lining is applied at a temperature different from its temperature in use, cracks can develop in the lining as a result of changes in temperature. It is also possible for hairline cracks in the rigid wall to start a crack in the lining since the lining is generally bonded to the rigid wall.
It has already been proposed to arrange a barrier in the heat-insulating lining to act as a crack arrester (see U.S. patent application Ser. No. 523,641, filed Nov. 14, 1974, now abandoned). Preferably this barrier consists of a combination of woven glass-fibre material and an epoxy resin. It is possible to locate the barrier in such a manner that an incipient crack in the lining is arrested before it attains its "critical crack length".
By the term "critical crack length" is understood the maximum length a crack can attain before its length increases in an unstable manner.
Several leak detection systems based on the change in temperature of the heat-insulating lining as a result of the penetration of liquid into cracks in the lining, have been proposed already, such as for example a system in which thermocouples are used.
A drawback of these known leak detection systems is that they involve a delay in response; a crack that has attained the critical length can be so narrow that little or none of the cold liquid penetrates the lining although a critical situation has arisen and a warning should be given. It is also possible for the penetrating liquid to boil and produce so much vapour that the crack becomes thermally insulated and that there is too little cooling to trigger the alarm. The use of thermocouples, for example, is moreover limited to measurements in the vicinity of the individual thermocouples. In view of the large dimensions of the containers in practice, this means that a large number of thermocouples is needed. The distance between a crack and the nearest thermocouple acts as a delaying factor. It is also difficult to obtain a selective detection of cracks of critical length and to avoid triggering of the alarm in the case of cracks of smaller length.