LNG/NGL Tank Heat Ingress & BOG Generation
Typical: Perlite 24-48", PU Foam 4-8", Vacuum varies
Auto-set from product: LNG=-260, Ethane=-128, Propane=-44, Butane=31
1.0 = outdoor, 0.0 = indoor/shaded
LNG=219, Ethane=210, Propane=184, Butane=166, NGL Mix=195
Typical 5-10% of total for piping and nozzle penetrations
Understand BOG generation, tank insulation systems, heat transfer mechanisms, and BOG recovery design
Boil-off gas (BOG) is the vapor generated from cryogenic liquids stored at or near their boiling point when heat enters the storage tank from the surroundings. For LNG stored at approximately -260°F (-162°C), any heat ingress through the tank walls, roof, bottom, or piping penetrations causes a small fraction of the liquid to evaporate. Typical BOG rates for well-insulated LNG tanks are 0.03-0.05% of inventory per day.
BOG rate is calculated by first determining total heat ingress into the tank (Q_total = Q_wall + Q_roof + Q_bottom + Q_misc), then dividing by the product's latent heat of vaporization: m_BOG = Q_total / h_fg. The heat ingress through each surface depends on the overall heat transfer coefficient (U-value), surface area, and temperature difference between ambient conditions and the stored product.
Typical BOG rates vary by tank construction: Full containment LNG tanks achieve 0.03-0.05%/day; double wall LNG tanks 0.05-0.08%/day; single containment 0.08-0.15%/day. Pressurized NGL bullets typically have lower BOG rates due to the product being stored at higher pressures. Tank insulation type (perlite, polyurethane foam, or vacuum) significantly impacts the BOG rate.
BOG management is critical because uncontrolled BOG causes tank pressure rise, potentially triggering relief valve operation and product loss. BOG must either be recovered (compressed and returned to the process or used as fuel gas), reliquefied using a BOG recondenser, or flared. Proper BOG management directly impacts terminal economics, safety, and environmental compliance.