Testing Methods for Daily Evaporation Rate (NGL) of Cryogenic Storage Tanks

The daily evaporation rate, or Natural Gas Liquids (NGL) boil-off rate, is a critical parameter in cryogenic storage tanks such as those used for LNG, liquid nitrogen, or liquid oxygen. Accurate measurement of NGL ensures efficient operation, safety, and inventory management.

1. Significance of NGL Testing

Operational Efficiency: Minimizing boil-off reduces product loss and improves overall storage efficiency.

Safety Considerations: Excessive evaporation can increase tank pressure, triggering relief valves and potential safety hazards.

Inventory Control: Accurate NGL measurement allows precise accounting of stored cryogenic liquids and planning for supply and distribution.

2. Testing Methods

Mass Balance Method:

Measure the initial and final mass of the tank over a 24-hour period.

The difference, adjusted for any withdrawals or additions, represents the daily evaporation rate.

Requires accurate load cells or weigh scales integrated with the tank or truck.

Volume and Level Measurement Method:

Monitor liquid levels using level gauges, radar, or ultrasonic sensors.

Convert the change in volume to mass based on liquid density at operating temperature.

Adjust for thermal contraction and expansion effects.

Flow Meter Method:

Use vapor flow meters installed on vent lines to measure the quantity of evaporated gas released from the tank.

Integrate the flow over 24 hours to determine total NGL.

Calibration and temperature compensation are essential for accuracy.

Combined Method:

Employ both level/volume measurement and vent flow measurement for cross-verification.

Particularly useful for large tanks or when continuous monitoring is required.

3. Testing Considerations

Temperature and Pressure Compensation:
Boil-off rates are influenced by ambient temperature, insulation performance, and tank pressure. Measurements should be corrected for these factors to obtain accurate NGL values.

Instrumentation Accuracy:
High-precision sensors, properly calibrated, are essential to minimize errors in mass or volume measurements.

Data Logging:
Continuous data acquisition ensures precise determination of evaporation trends and allows early detection of insulation degradation or abnormal tank behavior.

Conclusion
The daily evaporation rate (NGL) of cryogenic storage tanks can be accurately measured using mass balance, liquid level monitoring, or vent flow analysis. Correct instrumentation, calibration, and environmental compensation are essential to obtain reliable results. Regular NGL testing supports operational efficiency, safety, and effective inventory 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.