Analysis of Common Failures in Liquid Level Measurement of Cryogenic Storage Tanks

Accurate liquid level measurement is essential for the safe and efficient operation of cryogenic storage tanks containing liquids such as LNG, liquid nitrogen, or liquid oxygen. Malfunctions or inaccuracies in level measurement can lead to overfilling, underfilling, excessive boil-off, and safety hazards. Understanding common faults helps improve reliability and maintenance strategies.

1. Sensor Malfunctions

Thermowell Damage: Sensors installed in protective thermowells may suffer mechanical damage, corrosion, or cracking, leading to inaccurate readings.

Cryogenic Temperature Effects: Extreme low temperatures can cause some sensors, such as floats or capacitance probes, to fail due to material brittleness or contraction.

Electrical Failures: Short circuits, cable insulation degradation, or connector corrosion can interrupt signal transmission from the sensor to the control system.

2. Signal Interference and Calibration Issues

Calibration Drift: Over time, sensors may drift from their calibrated settings due to thermal cycling, mechanical stress, or electronic aging.

Electromagnetic Interference (EMI): Nearby high-power equipment or radio-frequency sources can distort signals from level transmitters.

Incorrect Installation: Misalignment or improper sensor positioning can result in systematic measurement errors.

3. Process-Related Factors

Foaming or Turbulence: Rapid filling, withdrawal, or agitation can create foam or turbulence on the liquid surface, causing erratic sensor readings.

Stratification: Temperature gradients within the tank can cause density variations, affecting differential pressure or capacitance-based level measurements.

Buildup on Sensors: Frost, ice, or solidified deposits can accumulate on sensors, particularly floats or probes, impairing their responsiveness.

4. Instrument-Specific Faults

Float Level Sensors: Sticking floats, mechanical jamming, or density variations can cause inaccurate readings.

Capacitance Sensors: Dielectric changes in the liquid due to impurities or temperature shifts may lead to false signals.

Ultrasonic or Radar Sensors: Surface disturbances, vapor layering, or condensation on the sensor face can reduce measurement accuracy.

5. Preventive Measures

Regular inspection and maintenance of sensors, thermowells, and wiring.

Periodic calibration under cryogenic conditions to ensure accuracy.

Installation of redundant measurement systems for critical tanks.

Use of vibration-resistant, low-temperature-compatible materials for sensor construction.

Minimizing turbulence and foam during filling and withdrawal operations.

Conclusion
Liquid level measurement in cryogenic tanks can be affected by mechanical, electronic, and process-related factors. Identifying and addressing common failures through proper installation, maintenance, and redundancy ensures accurate monitoring, safe operation, and efficient management of cryogenic liquids.

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.