Root Cause Analysis of Overpressure Risks in Cryogenic Storage Tanks

Cryogenic storage tanks, which store liquids such as LNG, liquid nitrogen, or liquid oxygen, are designed to operate under low temperatures and controlled pressures. Overpressure in these tanks poses significant safety hazards, including the risk of tank rupture, venting incidents, and operational disruption. Identifying the root causes of overpressure is crucial for implementing preventive measures.

1. Thermal Expansion of Stored Liquid

Cryogenic liquids expand rapidly when exposed to heat ingress from the environment or process operations. Even small increases in temperature can significantly raise vapor pressure inside the tank.

Insufficient pre-cooling, rapid filling, or solar heating of the tank surface can exacerbate pressure buildup.

2. Insulation or Vacuum Loss

Damage or degradation of thermal insulation or loss of vacuum in double-walled tanks increases heat transfer from the environment.

Enhanced heat ingress accelerates liquid vaporization, increasing internal pressure.

3. Malfunctioning Pressure Relief Systems

Pressure relief valves, rupture disks, or venting devices may fail due to mechanical blockage, improper calibration, or freezing of moving parts.

If relief devices do not operate as intended, overpressure can develop rapidly.

4. Operational Errors

Overfilling the tank reduces ullage space, limiting vapor expansion capacity and leading to higher internal pressures.

Improper venting procedures during filling or withdrawal can create temporary pressure spikes.

Rapid withdrawal or transfer of liquid can create pressure fluctuations due to vapor generation.

5. Cryogenic Pump and Transfer Equipment Issues

Malfunctioning pumps or flow restrictions can cause localized pressure increases.

Blockages in piping or valves can prevent vapor escape, contributing to overpressure events.

6. Environmental Factors

Exposure to high ambient temperatures, direct sunlight, or fire can raise tank wall temperature, accelerating liquid vaporization.

Accidental heat exposure from nearby operations may lead to rapid pressure rise.

Conclusion
Overpressure in cryogenic storage tanks is typically caused by a combination of thermal expansion, insulation loss, malfunctioning pressure relief systems, operational errors, and environmental influences. A systematic root cause analysis—including inspection of insulation integrity, pressure relief devices, operational procedures, and environmental conditions—is essential to prevent overpressure incidents. Regular maintenance, proper tank operation, and monitoring systems are critical for mitigating overpressure risks.

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.