Analysis of Factors Affecting Daily Evaporation Rate of Cryogenic Storage Tanks

The daily evaporation rate, also referred to as the boil-off rate, is a key performance parameter for cryogenic storage tanks containing liquids such as LNG, liquid nitrogen, or liquid oxygen. Understanding the factors that influence this rate is essential for operational efficiency, safety, and long-term storage optimization.

1. Tank Insulation Performance

Thermal Insulation Quality: High-performance vacuum insulation or multilayer insulation reduces heat ingress from the ambient environment. Degraded insulation or gaps increase thermal transfer, accelerating evaporation.

Insulation Thickness: Thicker insulation layers provide better thermal resistance, lowering the boil-off rate.

2. Ambient Temperature and Environmental Conditions

Temperature Fluctuations: Higher ambient temperatures increase heat transfer into the tank, raising evaporation rates.

Solar Radiation: Exposure to direct sunlight or hot environments can elevate tank surface temperature, contributing to increased boil-off.

Wind and Convection: Strong airflow around the tank can enhance heat transfer, marginally increasing the evaporation rate.

3. Tank Design and Structural Factors

Tank Geometry: Spherical tanks typically have lower surface area-to-volume ratios compared to cylindrical tanks, resulting in lower boil-off rates.

Tank Material: Material thermal conductivity influences heat ingress; stainless steel or low-conductivity alloys help reduce evaporation.

Tank Orientation: Horizontal versus vertical positioning affects surface area exposure and convective heat transfer.

4. Liquid Level and Fill Ratio

Low Fill Levels: Increased ullage (vapor space) can allow more heat absorption in the gas phase, increasing boil-off.

High Fill Levels: Reduced vapor space limits heat absorption but may increase pressure if venting is insufficient.

5. Operational Conditions

Pressure Control: Tank operating pressure influences the saturation temperature of the liquid. Higher pressures increase the boiling point, reducing evaporation.

Venting and Relief Operations: Frequent venting to control pressure may lead to higher measured boil-off rates.

Liquid Transfer: Filling, withdrawal, or recirculation operations can introduce heat and turbulence, temporarily increasing evaporation.

6. Maintenance and Age of the Tank

Insulation Degradation: Over time, insulation materials may deteriorate, leading to increased heat ingress.

Leakage or Imperfections: Micro-leaks or poor seals allow heat and air intrusion, elevating boil-off.

Conclusion
The daily evaporation rate of cryogenic storage tanks is influenced by a combination of thermal, structural, operational, and environmental factors. Optimizing insulation, tank design, pressure control, and operational procedures are key to minimizing boil-off, improving safety, and enhancing storage efficiency. Continuous monitoring and maintenance are essential to manage these influencing factors effectively.

References

EN 14620 – Design and Manufacture of Cryogenic Vessels.

NFPA 55 – Compressed Gases and Cryogenic Fluids Code.

Barron, R.F. (1999). Cryogenic Systems, 2nd Edition. CRC Press.

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

ISO 21014 – Cryogenic Vessels – Temperature and Pressure Measurement Guidelines.