Frosting or “Sweating” on the Exterior of Storage Tanks

Frosting or “sweating” on the outer surface of storage tanks is a common phenomenon observed in cryogenic tanks, refrigerated tanks, and other low-temperature liquid storage systems. Although often considered cosmetic, it can indicate thermal performance issues and may affect insulation efficiency and safety.

1. Mechanism of Frosting and Sweating

Frost Formation:
When the tank wall temperature falls below the dew point of the surrounding air and below freezing, water vapor condenses and freezes on the surface, forming frost. This commonly occurs in cryogenic tanks storing liquids like LNG, liquid nitrogen, or liquid oxygen. Frosting indicates that heat is being transferred from the ambient environment into the tank.

“Sweating” (Condensation):
If the tank wall temperature is above 0°C but below the dew point, water vapor condenses into liquid droplets, giving the appearance of “sweating.” This generally occurs in refrigerated tanks operating above freezing but at low temperatures.

2. Contributing Factors

Insulation Quality: Poor or damaged insulation allows more heat ingress, lowering the external surface temperature and promoting condensation or frost formation.

Ambient Humidity and Temperature: High relative humidity and cooler ambient air increase the likelihood of condensation.

Tank Surface Temperature: Variations in wall temperature due to thermal bridges, welds, or localized cooling can create spots where frosting or sweating occurs.

Operational Factors: Frequent filling, withdrawals, or vaporization events can alter wall temperatures, triggering temporary frosting or sweating.

3. Implications

Thermal Performance Indicator: Persistent frosting may indicate insulation degradation or vacuum loss in double-walled tanks, which can increase boil-off rates.

Safety Considerations: Ice accumulation can increase surface slipperiness and create maintenance hazards. In extreme cases, uneven frost can affect stress distribution on tank supports.

Maintenance and Monitoring: Visual inspection of frost or condensation patterns can provide early warning of insulation failure or other thermal issues.

4. Mitigation Measures

Insulation Maintenance: Ensure vacuum integrity, repair damaged insulation, and replace degraded materials.

Environmental Control: Reduce ambient humidity near the tank where feasible or use protective coatings to minimize condensation effects.

Operational Adjustments: Gradual filling and controlled vapor management can help maintain stable surface temperatures, reducing frosting incidents.

Conclusion
Frosting or “sweating” on the exterior of storage tanks is primarily a result of condensation of water vapor due to low tank wall temperatures. While often harmless, it serves as an indicator of thermal performance and insulation condition. Proper maintenance, insulation integrity checks, and environmental management help mitigate these effects and maintain tank efficiency and safety.

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.