Design and Frost Protection of Cryogenic Storage Tanks

 
Cryogenic storage tanks are crucial for storing liquefied natural gas (LNG) and other cryogenic liquids. The design and construction of these tanks must thoroughly address the impact of low temperatures on the structure and the need for foundation frost protection. In cold environments, the stored medium can cause frost heave in the foundation soil, potentially leading to soil uplift and foundation damage, which affects the tank's stability and safety. Therefore, effective foundation design and frost protection measures are essential for ensuring the reliable operation of low-temperature storage tanks. Here is a detailed analysis of the design of cryogenic storage tanks and their foundation frost protection measures.

Foundation Frost Protection Measures

 
The design of low-temperature storage tanks must consider the conductive effect of the low-temperature medium on the foundation soil, which can lead to frost heave and soil uplift, resulting in foundation damage. To address these issues, the following two frost protection measures are commonly used.

1. Electric Heating System

 
An electric heating system is an effective measure to prevent foundation frost heave. This design involves installing an electric heating system or other heating devices within the foundation slab, typically as part of a raft foundation with a circulating heating system. Although this method effectively prevents soil freezing, it is relatively costly and, therefore, not widely used in practical applications.

2. Elevated Foundation

 
An elevated foundation is another common frost protection measure. It involves constructing a support structure beneath the foundation slab to create an air layer, thereby separating the foundation slab from the foundation soil. This method is widely adopted in the construction of low-temperature storage tanks. The net height of the elevated layer is usually determined through temperature conduction calculations based on process pipework, equipment layout requirements, and the temperature of the stored medium.

Elevated raft foundations can be categorized into the following two forms:
 
Single Raft (Bearing Platform): Suitable for conditions with good geological properties. For high foundation settlement requirements, particularly on soft soil foundations with poor geological conditions, single raft pile foundations are also commonly used.
 
Double Raft (Bearing Platform): Can be used in good geological conditions, but double raft  pile foundations are also employed in poor geological conditions to ensure foundation stability and reliability.

Low-Temperature Storage Tank Structure

 
Low-temperature storage tanks typically consist of an inner tank and an outer tank, with insulating material filling the space between them to maintain the low-temperature environment inside the tank. The main structural components and their characteristics are as follows.

1. Inner Tank

 
The inner tank, often referred to as the "membrane tank," is made from thin low-temperature steel plates and features liquid tightness and flexibility. The primary function of the inner tank is to transfer the hydrostatic head to the insulation layer. The materials used for the membrane must remain non-brittle under low temperatures while possessing good toughness and processability. Common materials include nickel steel, stainless steel, and aluminum alloy.

2. Outer Tank

 
The design of the outer tank must be robust enough to withstand various loads. Depending on the materials used, outer tanks can be classified as follows:
 
Frost Soil Wall: Forms an airtight enclosure with the insulation cover, referred to as a pit storage cavity. During construction, cooling pipes freeze the soil around the inner tank. Once operational, the low-temperature liquid will continue to keep the soil frozen, and the frost layer will expand annually, reducing evaporation loss. The construction of a pit storage cavity requires a high groundwater level and a bottom layer of impervious rock or clay.
 
Steel Wall: Includes alloy steel and aluminum steel, mainly used for above-ground low-temperature storage tanks. Above-ground tanks are typically divided into ground-level and elevated types to address the risk of soil freezing and expansion:
 
Ground-Level: Uses perlite concrete insulation at the base, with heat wind or hot water supplied through embedded pipes or electric heaters inside the foundation to prevent soil freezing.
 
Elevated: Supported by columns, keeping the tank body separated from the ground, allowing air circulation between the tank and the ground to prevent LNG from absorbing ground heat and thus avoiding soil freezing.
 
Reinforced Concrete Wall: Suitable for underground storage tanks, reinforced concrete outer tanks have good low-temperature adaptability. Even if the membrane is damaged, contact between the low-temperature liquid and the reinforced concrete wall does not compromise the outer wall. Reinforced concrete outer tanks are durable, resistant to groundwater corrosion, less prone to brittleness, and offer good liquid tightness and seismic performance.
 
Prestressed Concrete Wall: Also a major choice for underground tanks, prestressed concrete walls offer similar advantages to reinforced concrete walls and apply prestress to further enhance strength and stability. Prestressed concrete walls effectively resist low temperatures and seismic impacts, ensuring long-term stability and safety of the tank.

In the design and construction of low-temperature storage tanks, effective foundation frost protection measures and a well-designed tank structure are crucial for ensuring the tank’s stability and safety. By adopting appropriate foundation frost protection measures, such as electric heating systems or elevated foundations, and selecting suitable outer tank materials, such as frost soil walls, steel walls, reinforced concrete walls, or prestressed concrete walls, potential issues during the operation of low-temperature storage tanks can be effectively addressed, ensuring their long-term safe operation.