In small-scale LNG and other cryogenic installations, BOG (Boil-Off Gas) is not a supporting secondary system, but an integral part of the overall design. The way in which BOG is managed has a direct impact on the availability of the installation, safety (ESD and SIS), and the total costs over the lifetime of the plant.
Insight into the actual BOG profile
BOG is not only caused by constant heat leaks. In practice, the profile is determined by several factors, such as heat ingress via tanks and pipes, operational peaks during loading and unloading, and abnormal situations such as trips, low consumption, or aging insulation. Particularly in small-scale installations, these transients often determine the dimensioning of compressors and blow-off systems.
Recovery or discharge: designing for practice
When designing a BOG strategy, a choice is usually made between recovery, controlled discharge, or a combination of both. Full recovery maximizes efficiency and minimizes emissions, while thermal oxidation or flaring provides a robust safety layer. In many cases, a hybrid solution proves to be the most realistic: normal BOG is recovered, while peaks can be safely vented. This provides a good balance between efficiency, safety, and operational reliability.
Compression or re-liquefaction
BOG compression is an effective solution when a stable gas buyer or export option is available. Re-liquefaction becomes more relevant when emissions must be strictly limited and there is no reliable gas outlet. In both cases, controlling compressor trips is essential, as a sudden failure can lead to rapid pressure build-up in the tank.
Safety and ESD philosophy
The BOG concept largely determines the ESD philosophy of the installation. A robust design has a clear hierarchy: normal pressure control, a recovery override, ESD actions such as stopping transfers and isolation, and ultimately pressure protection via safety valves to a safe discharge. A BOG strategy is only valid if a safe situation is guaranteed even in the event of disruptions, power failures, or compressor unavailability.
Economics: availability over yield
The economic considerations go beyond the value of recovered LNG. In small-scale installations, the impact of downtime is often greater than product losses. The loss of a single loading or production day can outweigh months of additional BOG recovery.
Conclusion
BOG management is at the intersection of process design, control, safety, and operational economics. A well-designed system increases the availability of the installation, improves safety, and contributes to sustainable and predictable operations. This is precisely what Cryonorm's engineering approach focuses on: reliable cryogenic installations with a strong focus on safety and long-term performance.
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