Comprehensive Structural Analysis of LNG Storage Tanks

Comprehensive Structural Analysis of LNG Storage Tanks

LNG storage tanks are engineered to store cryogenic liquids under significant pressure. These tanks typically feature a double-layer structure designed to withstand both the pressure of the stored medium and the extreme low temperatures. The inner tank, which directly contacts the cryogenic liquid, is constructed from low-temperature resistant alloy steel, specifically 0Crl8Ni9. This material ensures durability and resistance to the thermal stresses associated with cryogenic storage.

Between the inner tank and the outer protective layer, there is a carefully maintained space that serves multiple purposes. This insulation space is crucial for bearing the gravitational load of the tank and its contents, as well as maintaining the vacuum negative pressure within the insulation layer. The outer shell, made from standard vessel steel, does not come into contact with the low temperatures, thereby protecting it from thermal degradation. The insulation layer is predominantly filled with pearlescent sand, which is ideal for high-vacuum pumping, ensuring minimal heat transfer and a low evaporation rate, typically less than 0.2%.

Operational Dynamics of LNG Storage Tanks

The outlet of a LNG storage tank operates under the self-pressure of the stored liquid. As the liquid is dispensed, the liquid level decreases, increasing the gas phase space within the tank. This results in a pressure drop, necessitating the continuous addition of gas to maintain the required pressure for operational stability and process efficiency.

To manage this, a pressurized vaporizer and a booster valve are installed beneath the storage tank. The pressurized vaporizer is an air-heated device designed to convert the liquid natural gas into a gaseous state. Its installation height must be lower than the minimum liquid level of the storage tank to ensure proper functioning.

The booster valve operates in opposition to a pressure-reducing valve. When the outlet pressure falls below the set value, the booster valve opens, allowing gas to flow and increase the pressure. Conversely, when the pressure exceeds the set value, the valve closes. This mechanism ensures that the tank pressure remains within the desired range.

Pressurization Process

The pressurization process involves several steps:

1. Pressure Drop Detection: When the pressure within the tank falls below the set value of the booster valve, the valve opens.

2. Liquid Flow: The liquid natural gas flows from the tank into the pressurized vaporizer due to the liquid level difference.

3. Gasification: The liquid gasifies within the vaporizer, converting into a gaseous state.

4. Gas Supplementation: The generated gas flows through the booster valve and the gas phase pipe, supplementing the tank and increasing the pressure.

5. Pressure Stabilization: Once the pressure rises above the set value of the booster valve, the valve closes, preventing further liquid flow into the vaporizer. The pressure within the vaporizer then acts as a barrier, preventing additional liquid from entering the vaporization process.

This continuous cycle of pressure monitoring and adjustment ensures that the LNG storage tank operates efficiently, maintaining the necessary pressure for optimal performance and safety.