Top 10 Possible Causes for Sudden Increase in BOG Generation in LNG Storage Tanks

A notable rise in BOG (boil-off gas) production in an LNG storage tank typically indicates abnormal heat ingress, operational deviations, or equipment malfunction. Understanding the root causes is essential for preventing pressure buildup, product loss, and safety risks. Below are ten common causes.

1. Excessive Heat Ingress into the Tank

Any increase in heat leakage through tank insulation accelerates LNG vaporization. Causes include insulation aging, structural damage, gaps at penetrations, or inadequate perlite/vacuum integrity in double-wall tanks.

2. Loss of Insulation Vacuum

For double-wall LNG tanks and cryogenic vessels, vacuum degradation significantly increases thermal conductivity. Moisture ingress, seal failure, or long-term vacuum pump malfunction can cause rapid BOG growth.

3. Increased LNG Pump Circulation or Mixing

High pump flow rates, unnecessary recirculation, or tank stratification mixing operations can bring warmer upper-layer LNG downward, raising the overall heat load and generating more vapor.

4. LNG Stratification and Rollover Phenomenon

When LNG layers of different densities or temperatures form, sudden mixing (rollover) causes rapid heat release and a sharp rise in BOG. This is common when receiving cargo with different compositions.

5. Abnormal Ambient Temperature or Solar Radiation

Seasonal high temperatures, extreme sunlight exposure on above-ground tanks, or sudden weather changes can elevate tank shell temperature and increase heat ingress.

6. Malfunction of BOG Compressors or Recondenser Units

If BOG compressors trip, recondensers fail, or control valves malfunction, BOG cannot be processed normally. The tank’s internal pressure rises and forces more liquid to vaporize.

7. Excessively High Liquid Level

A nearly full LNG tank reduces vapor space and increases the frequency of liquid-vapor interactions. Small heat inputs can then cause disproportionately high BOG generation.

8. Warm LNG Inflow During Unloading

If unloading temperature is higher than expected, or LNG warming occurs in pipelines or pumps, the incoming volume contributes significantly to tank vaporization.

9. Valve or Pipeline Leakage Introducing Heat

Even small leaks of ambient-temperature air into the vapor header, top-fill lines, or safety relief systems introduce heat and lead to additional LNG evaporation.

10. Abnormal Operation or Control System Failure

Faulty tank pressure controls, incorrect setpoints, control logic failure, or malfunctioning instrumentation (temperature, pressure, or level transmitters) can cause unstable tank conditions and excess BOG.

References

GIIGNL. LNG Custody Transfer Handbook. International Group of Liquefied Natural Gas Importers.

API Standard 625. Tank Systems for Refrigerated Liquefied Gas Storage. American Petroleum Institute.

EN 1473. Installation and Equipment for LNG — Design of Onshore Installations.

SIGTTO. Liquefied Gas Handling Principles on Ships and Terminals.

Woodward, J. & Pitbaldo, R. LNG Safety and Risk Analysis.