1. Field of the Invention
This invention relates to the insulation of tanks adapted for storage of liquid cryogens, and more particularly to the sidewall insulation of dome-roofed cylindrical tanks in this service.
2. Description of the Prior Art
Storing non-condensing gases, such as natural gas, oxygen, ethylene gas, in liquid state at cryogenic temperature region has for long met industrial storage requirements. Normally, these gases are stored at nearly atmospheric pressure, and accordingly the storing temperatures are extremely low; hydrogen at -252.degree.C, oxygen at -183.degree.C, natural gas -162.degree.C, while the ambient temperature is usually around 0.degree.C or higher.
The difference between the storing temperature of a liquified gas and the ambient temperature usually results in vaporization of the stored liquified gas, and the heat loss is always disadvantageous.
Therefore, one of designers biggest interests is how to minimize the vaporization loss. Usually, a cryogenic tank is designed to be of doublewall type, consisting of a metallic inner tank, an outer tank with dome roof, and cold insulation therebetween.
The inner tank is usually made of nine percent nickel steel, austenitic stainless steel or aluminium alloy, and usually consists of a cylindrical wall, which has sufficient strength to bear at least liquid head pressure. Further, the tank has a flat bottom, and a dome roof or a flat deck suspended from the outer roof.
The outer tank is usually made of carbon steel or concrete, and usually consists of a cylindrical wall, a flat bottom and a dome roof.
The insulation of the cryogenic tank is divided into three parts; the roof insulation, the bottom insulation, and the sidewall insulation. The roof insulating material is often fibrous, cellular, or granular. The roof insulation is installed between the inner roof and the outer roof, or on the roof deck when a suspended deck is employed.
The bottom insulating material is usually perlite mortar blocks, cellular glass blocks, or wooden panels. The bottom insulation must have enough strength to bear the weight of the inner tank, cryogenic liquid, etc.
The sidewall insulating material is in most cases granular perlite, which is filled in the gap between the outer cylindrical wall and inner cylindrical wall, and, sometimes, instead of granular perlite, a fibrous material, such as glass wool is used.
Perlite insulation is thought to be the most economical because granular perlite is cheap in comparison with other insulating materials and because granular perlite filling is rather easy which results in short-time installation.
However, in a cryogenic tank, a well-known phenomenon of perlite compaction is caused by the falling-down of perlite due to the differences in thermal and mechanical behavior between the inner tank and the outer tank during cool-down, operation, and warm-up. The perlite compaction results in lateral pressures acting on the tank walls, that is, internal pressure on the outer tank wall and external pressure on the inner tank wall, the latter of which might cause a disastrous collapse of the tank.
In order to minimize the lateral pressures, some measures have been devised and applied with good success so far. For example, the resilient blankets are installed on the tank walls to absorb the behavior differences between the inner and outer tanks, and to prevent the falling-down of perlite to some degree.
However, it is still evident that there can be no fundamental countermeasures to the perlite compaction, so far as perlite is filled in all of the gap between the inner and outer tanks.
When a fibrous material, such as glass wool, is filled in the gap between the inner and outer tanks, or when a cellular material, such as urethane foam, is filled in as well, the sidewall insulation can be installed in such a way that there occur no heavy lateral pressures on the tank walls. Differing from the perlite insulation, the cellular insulating material does not fall down in spite of the behavior differences between the inner and outer tanks.
However, in this case, perfect installation of the insulation is hardly possible, and in almost all cases imperfections of insulation will occur. Voids in materials, channels between materials are typical of these imperfections.
Once the imperfections appear, a certain amount of heat is transferred from the outer tank wall to the inner tank wall through them, and accordingly spots of the outer tank wall might be cooled down to such a critical low temperature where the outer tank material breaks due to low temperature brittleness.