What are the special requirements for LNG cryogenic storage tanks?

1. Low temperature resistance

The boiling point of liquefied natural gas at normal pressure is -160°C. LNG chooses low-temperature and normal-pressure storage method, which reduces the temperature of natural gas below the boiling point, so that the operating pressure of the liquid storage tank is slightly higher than normal pressure. Compared with the high-pressure normal-temperature storage method, it can greatly reduce the thickness of the tank wall and improve safety performance.

Therefore, LNG requires liquid storage tanks to have good low temperature resistance and excellent cold retention performance.

2. High safety requirements

As the tank stores low-temperature liquids, once an accident occurs in the storage tank, the refrigerated liquid will evaporate in a large amount, and the amount of vaporization is about 300 times that of the original refrigeration state, forming an air mass that will automatically detonate in the atmosphere.

Therefore, API, BS and other specifications all require storage tanks to adopt a double-wall structure. Using the sealing concept, when the first layer of tank leaks, the second layer of tank can completely seal the leaking liquid and vapor to ensure storage. Safety.

3. Special materials

The inner tank wall requires low temperature resistance, and 9Ni steel or aluminum alloy is generally used, and the outer tank wall is prestressed reinforced concrete.

4. Strict insulation measures

Since the temperature difference between the inside and outside of the tank can be as high as 200℃, to keep the temperature inside the tank at -160℃, the tank body must have good cold insulation performance, and high-performance cold insulation materials are filled between the inner tank and the outer tank. The cold insulation material at the bottom of the tank must have sufficient pressure-bearing performance.

5. Good seismic performance

Generally, the seismic requirements of buildings are to crack without falling under the specified earthquake load. To ensure the safety of storage tanks under accidental loads, storage tanks must have good seismic performance. For LNG storage tanks, it is required to not collapse or crack under the specified seismic load. For this reason, the selected construction site should generally avoid the earthquake fracture zone. Before construction, the storage tank should be subjected to seismic tests to analyze the structural performance of the storage tank under dynamic conditions to ensure that the tank body is not damaged under a given seismic intensity.

6. Strict construction requirements

Welds of storage tanks must be subjected to 100% magnetic particle testing (MT) and 100% vacuum tightness testing (VBT). It is necessary to strictly select cold insulation materials and follow the prescribed procedures during construction. In order to prevent the concrete from cracking, post-tensioned pre-stress construction is adopted, and the verticality of the tank wall is very strict.

The top of the concrete outer tank should have high compressive and tensile capacity, and be able to resist the impact of falling objects. Due to the thick concrete at the bottom of the tank, the hydration temperature must be controlled during pouring to prevent cracking due to temperature stress.