What are the special requirements for LNG cryogenic storage tanks?

Large atmospheric low temperature storage tank is used to store liquefied natural gas (LNG). What are the special requirements for it?

1. Low temperature resistance

Under normal pressure, the boiling point of liquefied natural gas (LNG) is - 160 ℃. The storage method of LNG at low temperature and normal pressure is to reduce the temperature of natural gas below the boiling point and make the operating pressure of the storage tank slightly higher than the normal pressure. Compared with the storage method at high pressure and normal temperature, it can greatly reduce the thickness of the tank wall and improve the safety.

Therefore, LNG requires the liquid storage tank to have excellent low temperature resistance and excellent cold insulation function.

2. High safety requirements

Because cryogenic liquid is stored in the tank, once the tank leaks, a large amount of liquid will evaporate, and the gasification capacity is about 300 times that of the original refrigerated state, forming an air mass that will detonate actively in the atmosphere.

Therefore, API, BS and other specifications require the storage tank to adopt double-layer wall structure and apply the blocking concept. When the first layer of tank leaks, the second layer of tank can completely block the leaked liquid and evaporated gas to ensure storage safety.

3. Special material

The inner tank wall is required to be resistant to low temperature, and 9Ni steel or aluminum alloy and other materials are generally selected.

4. Strict thermal insulation measures

Because the maximum temperature difference between the surface and inside of the tank can reach 200 ℃, in order to maintain the temperature in the tank at - 160 ℃, the tank body must have excellent cold insulation function, and the cold insulation data shall be filled between the inner tank and the outer tank. The cold insulation data at the tank bottom shall also have sufficient pressure bearing function.

5. Good seismic function

The seismic requirement of general buildings is to crack without falling under regular seismic load. In order to ensure the safety of storage tank under accidental load, it is necessary for storage tank to have excellent seismic function. For LNG storage tanks, it is required not to fall or crack under regular seismic load.

Therefore, the selected construction site shall generally avoid the seismic fracture zone. Before construction, the seismic test shall be conducted on the storage tank to analyze the structural function of the storage tank under dynamic conditions, so as to ensure that the tank will not be damaged under a given seismic intensity.

6. Strict construction requirements

It is necessary to carry out 100% magnetic particle testing (MT) and 100% vacuum air tightness testing (VBT) for tank welds. The cold insulation materials shall be strictly selected, and the regular procedures shall be followed in the construction. In order to avoid cracks in concrete, post tensioned prestressed construction is selected, and the verticality of tank wall is strictly controlled.

The concrete outer tank top shall have high compressive and tensile capacity and be able to resist the impact of general falling objects. Because the concrete at the tank bottom is thick, the hydration temperature shall be controlled during pouring to avoid cracking Doer to temperature stress.