Pre-Cooling and First Filling Procedures for Cryogenic Liquid Storage Tanks

Cryogenic liquid storage tanks, such as those for LNG, liquid nitrogen, or liquid oxygen, require careful pre-cooling and controlled initial filling to prevent thermal shock, structural damage, and operational hazards. Adhering to proper procedures ensures safe operation and extends the service life of the tank.

1. Pre-Cooling Procedure

Purpose:
Pre-cooling reduces thermal gradients between ambient temperature and cryogenic liquid temperature, preventing cracking of the tank shell, insulation damage, or excessive thermal stress on welds and supports.

Methods:

Vapor Pre-Cooling: Introducing cold vapor from the cryogenic liquid to gradually reduce tank temperature. This method avoids direct contact of the liquid with the tank walls.

Liquid Pre-Cooling: Controlled introduction of small amounts of cryogenic liquid while carefully monitoring temperature and pressure to slowly cool the tank.

Monitoring:
Temperature sensors, pressure gauges, and liquid level indicators are continuously monitored to ensure uniform cooling and avoid localized overcooling.

Duration:
The pre-cooling process should continue until the tank walls and internal components reach a temperature close to the cryogenic liquid temperature, typically over several hours depending on tank size and design.

2. First Filling Procedure

Stepwise Liquid Introduction:
The initial filling should be performed gradually to minimize thermal shock. Start with a small quantity of liquid and allow the tank to stabilize before increasing the flow rate.

Pressure Control:
Tank pressure must be carefully regulated using pressure relief valves, venting systems, and control valves to prevent overpressure during filling.

Level Monitoring:
Continuous monitoring of liquid level ensures that the fill rate does not exceed design limits, preventing overflow or excessive liquid sloshing.

Venting and Purging:
Proper venting of displaced air or gas during filling prevents pressure build-up. Purging may also be required to remove contaminants from the tank before full operation.

Personnel Safety:
Operators should follow safety protocols, including protective equipment, safe distance from venting lines, and emergency shutdown procedures in case of abnormal conditions.

3. Post-Filling Checks

Inspect instrumentation, insulation, and external surfaces for anomalies such as frost buildup, leaks, or abnormal pressure fluctuations.

Verify that all safety valves, alarms, and monitoring systems are fully operational.

Document the pre-cooling and first filling process for operational records and future reference.

Conclusion
Pre-cooling and first filling of cryogenic liquid storage tanks are critical operations that require careful control of temperature, pressure, and liquid flow. Following standardized procedures ensures tank integrity, operational safety, and long-term reliability. Gradual thermal conditioning and continuous monitoring are key to preventing structural damage and operational hazards during the initial commissioning of cryogenic tanks.

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.