LNG Storage & Transfer Fundamentals

1. Boil-off gas (BOG)

LNG is stored at about −162 °C, far below ambient, so heat continuously leaks through even the best insulation and boils off a fraction of the inventory. The boil-off rate is a simple energy balance:

where Q̇_leak is the heat ingress (W) and h_fg the latent heat of vaporization (~510 kJ/kg for LNG). BOG is usually quoted as a boil-off rate (BOR) — percent of tank inventory vaporized per day (typical ~0.05–0.15%/day for large land tanks, higher for small/ship tanks). Because Q̇_leak scales with surface area while inventory scales with volume, small tanks have a higher percentage BOR — a key reason the LNG-boiloff screening must scale heat leak with the actual tank, not assume one fixed value.

2. Tank pressure & weathering

BOG must be removed (to a re-liquefier, a consumer, or — last resort — a flare/vent) or the tank pressure rises. As the lighter components (N₂, then methane) boil off preferentially, the remaining liquid weathers — its density and heating value drift upward over time. BOG management (reliquefaction, pressure-building/holding, fuel-gas use) is therefore both a pressure-control and a product-quality issue.

3. Rollover

Rollover is a hazard unique to LNG storage: if two stratified layers of different density form (e.g. from filling with a different LNG, or from weathering), they can equalize density and suddenly mix, releasing a large slug of accumulated superheat as a rapid BOG surge that can over-pressure the tank. It is prevented by filling-line selection (top vs bottom fill to control stratification), density monitoring, and circulation — and it is a primary driver behind NFPA 59A operating requirements.

4. Transfer & displaced vapor

Filling a receiving tank is not just a liquid problem. As liquid enters, it displaces an equal volume of vapor that must be returned or managed, and the transfer also generates extra BOG from line cooldown and heat leak. The screening balance is:

Without an adequate vapor-return line the receiving-tank pressure rises and the transfer rate must be throttled or the tank vented — so vapor handling, not pump rate, often limits the operation.

5. Bunkering

LNG bunkering (fuelling ships) adds marine-specific safety: compatibility assessment between bunker source and receiving vessel, a hazard/risk assessment with safety and security zones, emergency shutdown (ESD) and emergency-release-coupling (ERC) systems, and managed simultaneous operations (SIMOPS). The governing guidance is ISO/TS 18683 and the SGMF bunkering guidelines, with the facility designed to NFPA 59A (and, for marine interfaces, the relevant IGF/IGC code requirements). The screening transfer balance frames the operation; the actual procedure requires the full compatibility & risk assessment.

6. References

• NFPA 59A (2023) — Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG).
• ISO/TS 18683 — Guidelines for the safety of LNG bunkering.
• SGMF — Society for Gas as a Marine Fuel, LNG bunkering guidelines.
• EN 1473 — Installation and equipment for LNG: design of onshore installations.
• GPSA Engineering Data Book — LNG / cryogenic sections (BOG, properties).