What is boil-off gas (BOG) in LNG storage?

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.

How is BOG rate calculated for cryogenic storage tanks?

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.

What are typical BOG rates for different tank types?

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.

Why is BOG management important in LNG terminals?

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.

Boil-Off Gas (BOG) Calculator | LNG/NGL Tank Heat Ingress

Tank Type & Product

Tank Type

Product Stored

Tank Construction

Tank Dimensions

Dimension Input

Tank Diameter

Tank Height

Tank Fill Level

Insulation

Insulation Type

Insulation Thickness

Typical: Perlite 24-48", PU Foam 4-8", Vacuum varies

Environmental Conditions

Ambient Temperature

°F

Product Temperature

°F

Auto-set from product: LNG=-260, Ethane=-128, Propane=-44, Butane=31

Wind Speed

mph

Solar Radiation Factor

1.0 = outdoor, 0.0 = indoor/shaded

Additional Parameters

Latent Heat of Vaporization

Btu/lb

LNG=219, Ethane=210, Propane=184, Butane=166, NGL Mix=195

Misc. Heat (piping, penetrations)

Typical 5-10% of total for piping and nozzle penetrations

Natural Gas Price (for fuel value)

$/MMBtu

Understanding Boil-Off Gas

What is BOG?
Boil-off gas is vapor generated when heat enters a cryogenic storage tank. Since the product is stored at its boiling point, any heat ingress causes evaporation. The BOG rate depends on insulation quality, ambient conditions, and tank geometry.

Typical BOG Rates:
Full containment LNG: 0.03-0.05%/day
Double wall LNG: 0.05-0.08%/day
Single containment: 0.08-0.15%/day
Pressurized NGL: 0.01-0.03%/day

BOG Management Options:
BOG recovery compressor (most common), BOG recondenser, fuel gas system, or flare (last resort). Proper BOG management is critical for tank pressure control, safety, economics, and environmental compliance.

📚 Learn the Theory

Understand BOG generation, tank insulation systems, heat transfer mechanisms, and BOG recovery design

Read Engineering Guide →

Results

BOG Rate

% of inventory per day

Total Heat Ingress -

Heat Ingress (MMBtu/day) -

BOG Mass Rate -

BOG Volume Rate (SCFH) -

BOG Volume Rate (MMSCFD) -

Holding Time (no recovery) -

BOG Compressor HP -

BOG Fuel Value -

Heat Ingress Breakdown
-
-
-
-

Tank Surface Areas
-
-
-
-

Formula

m_BOG = Q_total / h_fg

m_BOG = Boil-off gas mass rate (lb/hr)

Q_total = Total heat ingress (Btu/hr)

h_fg = Latent heat of vaporization (Btu/lb)

Q_wall = U_wall × A_wall × ΔT

Q_roof = U_roof × A_roof × ΔT + Q_solar

Q_bottom = U_bottom × A_bottom × (T_ground - T_product)

Standards & References

API 625
Tank Systems for Refrigerated Liquefied Gas Storage
EN 1473
Installation and Equipment for LNG — Design of Onshore Installations
GPSA Engineering Data Book
Chapter 11: Hydrocarbon Recovery
NFPA 59A
Standard for LNG Production, Storage, and Handling
BS 7777
Flat-Bottomed, Vertical, Cylindrical Storage Tanks for Low Temperature Service

Engineering Notes

Design BOG: Size recovery systems for worst-case summer ambient plus solar load
U-values: Overall heat transfer coefficients include insulation, tank wall, and surface film resistances
Solar load: Can add 10-20% to total heat ingress for outdoor unshaded tanks
Wind effect: Higher wind increases the external film coefficient, raising U-values 5-15%
Aging: Insulation performance degrades 0.5-1% per year; design for end-of-life conditions
Hold time: Relief valve set pressure determines maximum hold time without BOG recovery

Quick Reference — Product Properties

LNG: -260°F, h_fg=219 Btu/lb, ρ=26.4 lb/ft³
Ethane: -128°F, h_fg=210 Btu/lb, ρ=35.5 lb/ft³
Propane: -44°F, h_fg=184 Btu/lb, ρ=31.6 lb/ft³
Butane: 31°F, h_fg=166 Btu/lb, ρ=36.1 lb/ft³
NGL Mix: -80°F, h_fg=195 Btu/lb, ρ=33.0 lb/ft³

Boil-Off Gas (BOG) Calculator