Structural Design of Low-Temperature Liquid Storage Tanks

Low-temperature liquid storage tanks, commonly used for liquefied gases such as LNG, LPG, and liquid nitrogen, require specialized structural design to withstand cryogenic temperatures while maintaining mechanical integrity, safety, and operational reliability. The design must account for material behavior at low temperatures, thermal contraction, pressure loads, and environmental factors.

1. Material Selection
Materials for low-temperature tanks must retain toughness, ductility, and strength at cryogenic temperatures to prevent brittle fracture. Typical materials include:

Austenitic Stainless Steels (e.g., 304L, 316L): Excellent low-temperature toughness, corrosion resistance, and weldability.

Nickel Steels (e.g., 9% Ni steel): High strength and toughness at cryogenic temperatures, commonly used for large-scale LNG storage.

Aluminum Alloys: Used in specific applications where lightweight construction and corrosion resistance are required.

2. Structural Configuration

Tank Shape: Cylindrical tanks with domed or ellipsoidal roofs are preferred for uniform stress distribution. Spherical tanks may be used for very large volumes, minimizing surface area-to-volume ratio and stress concentrations.

Support Systems: Tanks are typically supported on skirt structures, piles, or concrete foundations that accommodate thermal contraction and prevent settlement. Expansion joints may be incorporated to handle differential thermal movements.

Insulation Systems: Effective thermal insulation (perlite, vacuum insulation, foam glass) minimizes heat ingress, reducing boil-off rates and limiting thermal stresses.

3. Design Considerations

Thermal Contraction: Materials contract significantly at cryogenic temperatures; design must account for differential contraction between tank walls, roof, and piping.

Internal and External Pressure: Tanks must withstand hydrostatic pressure from stored liquids and potential vacuum conditions during emptying. Finite element analysis is often used to assess stress distributions.

Seismic and Wind Loads: Structural design must comply with local building codes to resist seismic activity and wind pressure, ensuring tank stability.

Welding and Fabrication: Cryogenic steels require precise welding procedures and post-weld inspections to maintain toughness and avoid micro-cracks.

4. Safety and Monitoring

Leak Detection: Sensors and alarms monitor for cryogenic leaks.

Overpressure Protection: Relief valves and venting systems prevent overpressurization.

Inspection and Maintenance: Regular NDT inspections, including ultrasonic and radiographic testing, ensure structural integrity throughout the tank’s service life.

Conclusion
The structural design of low-temperature liquid storage tanks integrates material selection, thermal and pressure load considerations, and safety measures to ensure reliable and long-term operation under extreme cryogenic conditions. Optimized design reduces risk, maintains mechanical integrity, and ensures operational efficiency.

References

API 620 – Design and Construction of Large, Welded, Low-Pressure Storage Tanks.

EN 14620 – Design and Manufacture of Cryogenic Vessels.

ASME Boiler and Pressure Vessel Code, Section VIII – Rules for Construction of Pressure Vessels.

Totten, G.E. (2006). Steel Heat Treatment: Metallurgy and Technologies. CRC Press.

Bratt, R., & Mort, P. (2015). Cryogenic Engineering: Fifty Years of Progress. Springer.