LNG Membrane Tank Insulation

With Reference to membrane tanks for the carriage of liquified gas at very low temperatures

Describe with a sketch one method of building up of insulation
Membrane tanks are not self supporting. They consist of very thin tank material supported by insulation which is in turn supported by ships hull. The system is not competitive for the carriage of LPG and is primarily for LNG.





The GAZ Transport System uses a 36 % Nickel Iron alloy called "Invar" for both the primary and secondary barriers. Invar has a vary low coefficient of thermal expansion which makes any corrugations in the tank structure unnecessary.

The invar sheet membrane used is only 0.5 mm to 0.7 mm thick which makes for a very light structure.


Insulation consists of plywood boxes filled with perlite.


State which alloy is used and the reason for the membrane.
The GAZ Transport membrane systems uses a 36 % Nickel-Iron Alloy called "INVAR" for both the primary and secondary barriers.
INVAR has a very low coefficient of thermal expansion. This makes any corrugations in the tank structure unnecssary.
The INVAR sheet membrane used is only 0.5mm to 0.7 mm thick which makes for a very light structure


Explain why a secondary barrier is installed.
Many gases are often liquified and carried in bulk. In order that the gases may remain liquid, they must be kept under high pressure or at low temperature.
This liquid must not normally be allowed to come into direct contact with the ship's hull or to take up heat from the surrounding air.
Hence a secondary containment system must be incorporated to protect the ships hull from the effects of low temperature in the event of cargo leakage.
This is known as "SECONDARY BARRIER".
LNG is carried at its boiling temerature of -164 deg. cel. at slightly higher than atmospheric pressure.
The 'secondary barrier' must be provided to contain any leakage from the tanks if the liquid is at temperatures of below -10 deg. cel.
For liquids between -10 deg. cel. to -55 deg. cel. the outer hull of the ship may serve as secondary barrier.
For liquids whose temperature is below -55 deg. cel., hull is not acceptable for this purpose and specially designed insulation, separated from tank by a space which is often filled with nitrogen gas.