Design of Safety Relief Systems for Cryogenic Liquid Storage Tanks

Cryogenic liquid storage tanks, used for substances such as LNG, LPG, and liquid nitrogen, operate under extremely low temperatures and moderate to high pressures. Safety relief systems are critical to prevent overpressure, protect structural integrity, and mitigate hazards in case of accidental heating, filling errors, or operational malfunctions.

1. Principles of Safety Relief System Design

Overpressure Protection:
Relief systems are designed to automatically release liquid or vapor if the internal pressure exceeds the tank’s design limit. The system must account for maximum expected filling rates, solar heating, and potential fire exposure.

Redundancy and Reliability:
Multiple relief valves or devices are often installed to ensure functionality in case one device fails. Redundant systems provide higher safety assurance, particularly for large-scale cryogenic tanks.

Cryogenic Compatibility:
Relief devices, piping, and venting components must withstand cryogenic temperatures without embrittlement or deformation. Materials like austenitic stainless steel or nickel alloys are typically used.

2. Components of the Safety Relief System

Pressure Relief Valves (PRVs):
PRVs open automatically when pressure exceeds a set threshold, venting vapor to maintain safe internal pressure. They are sized based on maximum flow rates, tank volume, and allowable overpressure.

Rupture Discs:
Rupture discs provide fail-safe protection against extreme overpressure scenarios, complementing PRVs. They burst at a predefined pressure to prevent catastrophic tank failure.

Venting Piping:
Proper venting directs released gas away from personnel and equipment to a safe location. Piping must be insulated and sized to minimize backpressure and prevent freezing blockages.

Level and Temperature Monitoring:
Sensors detect abnormal liquid levels or temperatures, triggering alarms or controlling emergency venting to prevent overpressure.

3. Design Considerations

Flow Capacity Calculation:
Relief devices must be sized to handle the maximum possible vapor generation rate during normal and abnormal conditions, including sudden heat input or tank filling errors.

Thermal Expansion Accommodation:
Tanks can experience thermal expansion due to environmental heat or operational factors. Relief systems must account for increased vapor pressure resulting from thermal effects.

Regulatory Compliance:
Designs must comply with standards such as API 521 (Pressure-Relieving and Depressuring Systems), ASME Section VIII (Pressure Vessels), and EN 14620 (Cryogenic Vessels).

4. Operational and Maintenance Measures

Regular inspection and testing of relief valves and rupture discs ensure reliable operation.

Monitoring systems should continuously check for pressure, temperature, and tank integrity.

Emergency response procedures must be established to manage vented cryogenic gases safely.

Conclusion
A properly designed safety relief system for cryogenic liquid storage tanks ensures that overpressure scenarios are managed safely, protecting personnel, equipment, and the environment. Integration of relief valves, rupture discs, venting systems, and monitoring devices provides comprehensive protection against potential hazards associated with low-temperature liquids.

References

API 521 – Guide for Pressure-Relieving and Depressuring Systems.

EN 14620 – Design and Manufacture of Cryogenic Vessels.

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

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

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