Pre-cooling of LNG Storage Tanks for Safe LNG Storage

The pre-cooling of LNG storage tanks is an indispensable step in the liquefied natural gas storage system, designed to ensure that tanks and their associated pipelines can safely and effectively handle the storage requirements of extremely low-temperature liquids. Before LNG is introduced into the tank, it is imperative to gradually reduce the temperature of the tank and pipelines through pre-cooling operations to avoid material damage, excessive thermal stress, and other potential failures caused by sudden cooling. The pre-cooling of storage tanks not only affects the material properties and service life of the tanks but is also closely related to the overall operational safety of the system. This article will delve into the necessity, process, material characteristics, and common pre-cooling methods of LNG storage tanks to help gain a deeper understanding of the design and operational requirements of LNG storage tank systems.

The Importance of LNG Pre-cooling

 
Before LNG is officially introduced into the storage tank, the cryogenic liquid should first cool the tank and its associated pipelines, gradually bringing them to the operating temperature and releasing the deformation caused by cooling. Injecting LNG directly without pre-cooling may lead to thermal stress in the tank and pipelines due to sudden cooling, which can cause cracks, material damage, or even equipment failure. Therefore, the pre-cooling process can prevent damage caused by sudden temperature drops, ensuring the long-term stability and safety of the equipment.
 
Moreover, the pre-cooling process also provides a comprehensive system performance test, including the inspection of material performance at low temperatures, the examination of welding quality, and the sealing test of cryogenic valves, ensuring that LNG storage tanks and pipelines can operate normally under extreme low temperatures. It can be said that pre-cooling is not only a preparatory step before operation but also a safeguard for the safety and reliability of system operation.

Material Characteristics of LNG Storage Tanks and Pipelines

 
LNG storage tanks and pipelines are typically made of austenitic stainless steel. Austenitic stainless steel has excellent low-temperature performance, especially at extremely low temperatures of -162°C, where it still maintains good strength and toughness. However, due to the large coefficient of linear expansion of austenitic stainless steel, the effect of cold shrinkage is significant. In the ultra-low temperature environment of LNG, the cold shrinkage rate of austenitic stainless steel is about 0.5%. Taking 304L stainless steel as an example, a 100-meter-long pipeline will shorten by about 300 millimeters at a working temperature of -162°C. This significant cold shrinkage poses strict requirements for pipeline structural design.

Pipeline Cold Shrinkage and Compensation Design

 
The issue of pipeline cold shrinkage in LNG pipelines is a key consideration in design. Between two fixed points, the stress generated by cold shrinkage may far exceed the yield point of the material, causing the pipeline to be overstressed and damaged. Especially in the pipelines within LNG storage tanks, the design requirements are even more stringent, as any issues could lead to serious consequences such as leaks or explosions.
 
To address this issue, the design typically uses a bend compensation method, which absorbs the stress generated by cold shrinkage through the bending of the pipeline, reducing the pressure on fixed points and pipeline supports. Although this design can partially alleviate the cold shrinkage issue, the pipeline may still face the risk of excessive thermal stress in situations where ambient temperatures change dramatically. Therefore, pre-cooling operations are crucial.

Specific Objectives of LNG Pre-cooling

 
Pre-cooling is not only to prevent equipment damage due to excessive temperature differences but also to ensure that LNG storage tanks and pipelines can smoothly enter the working state. The specific objectives of pre-cooling operations include the following aspects.
 
Testing the performance of low-temperature materials: Through the pre-cooling process, inspect whether the low-temperature materials of the storage tank and pipeline meet the use standards, especially their ductility and crack resistance at extremely low temperatures.
 
Checking welding quality: Pre-cooling can effectively test the quality of the welding points of the storage tank and pipeline, ensuring that there will be no cracks or leaks due to weld issues in low-temperature environments.
 
Assessing pipeline cold shrinkage and support changes: Through the pre-cooling process, observe the cold shrinkage of the pipeline at low temperatures to verify whether the pipeline support system is reasonably designed to cope with stress changes caused by cold shrinkage.
 
Testing the sealing performance of cryogenic valves: Cryogenic valves are crucial in the LNG system, and pre-cooling operations can test their sealing performance to ensure that there will be no leaks or failures during operation.
 
Measuring the vacuum performance of the storage tank: During the pre-cooling process, the performance of the vacuum layer of the storage tank can also be tested to ensure its good insulation effect and reduce cold loss during operation.

LNG Storage Tank Pre-cooling Methods

 
Depending on the cooling medium, there are mainly two methods for pre-cooling LNG storage tanks: the liquid nitrogen plus LNG pre-cooling method and the LNG and BOG pre-cooling method.

1. Liquid Nitrogen plus LNG Pre-cooling Method

 
This method uses a combination of liquid nitrogen and LNG for pre-cooling operations. Initially, liquid nitrogen is used to cool the unloading pipelines and storage tanks, and when the temperature drops to between -120°C and -110°C, LNG is switched to continue cooling the pipelines and storage tanks. Liquid nitrogen is less expensive than LNG as a pre-cooling medium, and its non-flammable characteristic makes the pre-cooling process safer. This method not only saves costs but also improves the safety and efficiency of operations. It is typically suitable for large-scale LNG storage facilities and can significantly reduce the pre-cooling costs of LNG.

2. LNG and BOG Pre-cooling Method

 
This method directly uses LNG vapor (BOG) for pre-cooling operations. After the temperature of the pipelines and storage tanks drops to -120°C, LNG is injected, and the tank body is further cooled through the spray atomization equipment on the top of the storage tank. This method fully utilizes the existing LNG resources at the receiving station but requires a large consumption of LNG products. Therefore, this method is not as economically superior as the liquid nitrogen pre-cooling method and is suitable for occasions with abundant resources or lower cost control.

Conclusion

 
The LNG pre-cooling process is a crucial part of the liquefied natural gas storage and transportation system, effectively preventing equipment damage due to cold shrinkage, thermal stress, and other issues. At the same time, pre-cooling can also comprehensively test the low-temperature performance of tanks, pipelines, and related equipment, ensuring their safe and stable operation in extreme low-temperature environments. By choosing the appropriate pre-cooling method, such as the liquid nitrogen plus LNG pre-cooling method or the LNG and BOG pre-cooling method, the LNG system can be fully prepared before operation, ensuring the reliability and safety of long-term operation. In the future application of LNG, the continuous development and optimization of pre-cooling technology will further enhance the efficiency and safety of LNG storage and transportation systems.