Boil-Off Gas (BOG) Fundamentals

1. BOG Generation Physics

Boil-off gas (BOG) is the vapor that forms when heat enters a cryogenic storage tank containing a liquefied gas stored at or near its normal boiling point. Because the liquid is at its saturation temperature, any heat ingress from the warmer surroundings causes a portion of the liquid to evaporate. This evaporation is the fundamental mechanism that generates BOG, and it occurs continuously as long as a temperature difference exists between the ambient environment and the stored product.

Thermodynamic Basis

The rate of BOG generation is governed by the energy balance:

This equation assumes that all heat entering the tank goes to evaporating liquid rather than raising the liquid temperature, which is valid because the liquid is maintained at its boiling point by the continuous evaporation process.

BOG Rate Convention

In the LNG and NGL industry, the BOG rate is expressed as a percentage of the total liquid inventory that boils off per day. A BOG rate of 0.05%/day means that 0.05% of the stored liquid mass evaporates every 24 hours. This metric allows direct comparison between tanks of different sizes and products. For a 160,000 m³ LNG tank, a BOG rate of 0.05%/day represents approximately 3,500 kg/hr of methane vapor.

Product Properties Affecting BOG

The boiling temperature and latent heat of vaporization of the stored product directly affect both the heat ingress rate and the mass of BOG generated per unit of heat. Products stored at lower temperatures have larger temperature differences with the ambient, driving more heat ingress but also requiring more heat per pound of liquid evaporated.

2. BOG Types & Mechanisms

BOG generation is not a single phenomenon. Several distinct mechanisms produce vapor in cryogenic storage and transfer systems. Understanding each type is essential for accurate BOG prediction and proper sizing of vapor handling equipment.

Heat Ingress BOG (Steady-State)

Heat ingress BOG is the continuous vapor generation caused by ambient heat leaking through the tank insulation. This is the dominant source of BOG during normal holding or storage operations when no liquid is being added or removed. The rate depends on insulation quality, ambient temperature, wind speed, solar radiation, and tank geometry. Heat ingress BOG typically represents 0.03–0.15%/day of the stored inventory.

Flash BOG

Flash BOG occurs when warm liquid enters a tank that is at a lower pressure than the liquid's saturation pressure. The pressure drop across the fill valve causes instantaneous partial vaporization of the incoming liquid. The flash vapor fraction is determined by the enthalpy difference between the incoming liquid and the tank conditions.

Flash BOG can be very large during ship unloading when LNG from a pressurized ship tank (typically 1–3 psig) is let down to an atmospheric storage tank. The flash fraction for a 5°F temperature difference is approximately 1–2% of the incoming liquid mass, which for a ship unloading rate of 12,000 m³/hr represents 50,000–100,000 kg/hr of flash vapor.

Displacement BOG

When liquid is pumped into a storage tank, the rising liquid level compresses the vapor space and displaces the existing vapor. This displaced vapor must be removed from the tank to prevent overpressure. Displacement BOG is proportional to the volumetric liquid fill rate and is independent of the product's thermodynamic properties.

During ship unloading at high fill rates, displacement BOG can exceed heat ingress BOG by an order of magnitude. At a fill rate of 12,000 m³/hr into a tank at atmospheric pressure, the displacement vapor rate is approximately 12,000 m³/hr of methane vapor, which must be returned to the ship or processed by the BOG handling system.

Weathering

Weathering is the progressive change in composition of a multi-component liquefied gas due to preferential evaporation of the lighter (more volatile) components. In LNG, which is primarily methane but also contains ethane, propane, and heavier hydrocarbons, the BOG is enriched in methane relative to the bulk liquid. Over time, the remaining liquid becomes progressively richer in heavier components, and its boiling point increases.

The practical effects of weathering include a gradual decrease in the BOG rate (because the remaining liquid requires more energy to evaporate), an increase in the liquid density and boiling point, and a change in the heating value of the stored product. For LNG stored for extended periods without turnover, weathering can increase the liquid density by 1–3% and raise the boiling point by 5–15°F over several months.

3. Heat Transfer Pathways

Heat enters a cryogenic storage tank through multiple pathways. Understanding each mechanism is essential for accurate BOG prediction and for identifying the most effective insulation improvements. The total heat ingress is the sum of contributions from the tank walls, roof, bottom slab, and miscellaneous penetrations.

Wall Heat Ingress

The cylindrical shell of the tank is typically the largest single contributor to heat ingress, accounting for 50–65% of the total in most designs. Heat flows radially inward through the following resistances in series:

1. External surface film: Convection from ambient air to the outer tank wall. The film coefficient depends on wind speed, with higher winds reducing the resistance and increasing heat flow. A typical value is 1.5–3.0 Btu/hr·ft²·°F.
2. Outer shell conduction: Through the carbon steel or concrete outer wall. This resistance is negligible compared to the insulation.
3. Insulation: The dominant thermal resistance. Perlite, polyurethane foam, or vacuum insulation provides the bulk of the thermal barrier.
4. Inner shell conduction: Through the 9% nickel steel or aluminum alloy inner tank. Also negligible resistance.
5. Internal surface film: Natural convection from the inner wall to the liquid. Cryogenic liquids have relatively high natural convection coefficients (30–80 Btu/hr·ft²·°F).

The overall heat transfer coefficient for the wall, U_wall, is calculated from the series resistance model:

Roof Heat Ingress

The tank roof contributes 10–20% of total heat ingress. Roof heat transfer includes both conductive heat flow through the insulation and direct solar radiation absorbed by the roof surface. The vapor space above the liquid has a much lower film coefficient than the liquid contact areas, making the roof insulation somewhat less effective than wall insulation of the same thickness. Solar radiation adds a direct thermal load that depends on geographic latitude, roof coating reflectivity, time of year, and cloud cover. A bright aluminum or white-painted roof can reduce solar absorption to 15–25% of incident radiation.

Bottom Slab Heat Ingress

Heat transfer through the tank bottom typically accounts for 15–25% of the total. The heat source is the ground rather than the atmosphere. The bottom heat transfer path includes ground soil (acting as a semi-infinite thermal resistance), the concrete ring wall and slab, bottom insulation (typically foam glass blocks that can support the hydrostatic load), and in many LNG tanks, a foundation heating system to prevent frost heave. The ground temperature at foundation depth is typically 45–65°F, significantly lower than the ambient air temperature.

Miscellaneous Heat Ingress

Piping penetrations, nozzle connections, manways, instrumentation conduits, and structural supports all create thermal bridges through the insulation system. These penetrations account for 5–10% of total heat ingress in a well-designed tank. Thermal break hardware and extended-length nozzle necks are used to minimize this contribution. In older tanks with poorly designed penetrations, the miscellaneous heat ingress can reach 15–20% of the total.

Heat Ingress Distribution Summary

Seasonal and Diurnal Variation

Ambient conditions cause the BOG rate to fluctuate by 20–40% between winter and summer. Diurnal variations of 5–15% occur due to temperature swings between day and night. Solar radiation creates the largest diurnal variation on the roof contribution. Wind speed affects the external film coefficient, with high-wind events temporarily increasing heat ingress by 10–20% compared to calm conditions. Design BOG rates should always use worst-case summer conditions with appropriate solar and wind loads.

4. Tank Insulation Systems

The insulation system is the critical engineering component that determines the BOG rate. Selection of the appropriate insulation type and thickness involves balancing thermal performance, cost, reliability, and service life.

Expanded Perlite

Expanded perlite is the most widely used insulation material for large LNG and NGL storage tanks. It is a volcanic glass that is heated to approximately 1,600°F, causing it to expand to 4–20 times its original volume. The resulting lightweight granular material is poured into the annular space between the inner and outer tank shells. Perlite insulation has a thermal conductivity of approximately 0.02–0.03 Btu/hr·ft·°F at cryogenic temperatures. Typical perlite thickness for LNG tanks ranges from 24 to 48 inches. The annular space is filled with dry nitrogen or natural gas vapor to prevent moisture condensation. Main disadvantages include settling over time (1–3% per year, creating voids at the top) and susceptibility to moisture contamination.

Polyurethane Foam

Closed-cell polyurethane (PU) foam provides roughly twice the thermal resistance per inch compared to perlite, with a thermal conductivity of approximately 0.010–0.015 Btu/hr·ft·°F. This allows much thinner insulation layers (typically 4–8 inches) to achieve comparable thermal performance. PU foam adheres to the tank surface, eliminating settling issues. However, PU foam is combustible and requires fire protection measures. It also has a limited temperature range and can become brittle below -320°F. PU foam is more commonly used for pressurized NGL bullets and smaller storage vessels.

Vacuum Insulation

Multi-layer vacuum insulation (MLI) provides the highest thermal resistance of any insulation system. It consists of multiple layers of reflective foil (aluminum or aluminized Mylar) separated by low-conductivity spacers, all enclosed in a vacuum environment (below 10^-4 torr). Vacuum insulation achieves apparent thermal conductivities of 0.003–0.008 Btu/hr·ft·°F. It is used primarily for small cryogenic vessels, satellite LNG storage, and laboratory dewars. For large storage tanks, maintaining the required vacuum over the operational lifetime is challenging and expensive.

Insulation Performance Comparison

Insulation Aging and Degradation

All insulation systems experience performance degradation over time. Perlite settles 1–3% per year, creating gaps that increase heat transfer. PU foam can absorb moisture over decades, reducing its insulating capability. Vacuum systems experience gradual vacuum decay from outgassing and micro-leaks. Design engineers should account for end-of-life insulation performance when sizing BOG recovery equipment, typically adding a 15–25% degradation factor to new-condition U-values.

5. Tank Construction Types

The construction type of a cryogenic storage tank affects the safety, containment performance, thermal performance, and resulting BOG rate. The three main construction types defined in API 625 and EN 1473 have progressively better thermal characteristics.

Single Containment Tanks

A single containment tank has a liquid-tight inner tank (9% nickel steel or aluminum) surrounded by insulation and a non-liquid-tight outer shell (carbon steel) that serves primarily as a weather barrier and insulation container. In the event of an inner tank failure, a surrounding earthen dike or bund wall provides secondary containment. Single containment tanks are the simplest and least expensive to construct but typically have the highest BOG rates (0.08–0.15%/day) because the outer shell provides minimal additional thermal resistance.

Double Wall Tanks

Double wall tanks have both a liquid-tight inner tank and a liquid-tight outer tank, both designed to contain the full liquid inventory. The outer tank is typically constructed from prestressed concrete or 9% nickel steel. The concrete outer wall adds significant thermal resistance compared to a thin steel shell. The sealed annular space provides better moisture protection for the insulation. Typical BOG rates are 0.05–0.08%/day.

Full Containment Tanks

Full containment tanks extend the double wall concept by designing the outer concrete wall to contain both liquid and vapor in the event of inner tank failure. The thick prestressed concrete outer wall (typically 24–36 inches) provides substantial additional thermal resistance. Full containment tanks consistently achieve the lowest BOG rates of 0.03–0.05%/day and are the preferred construction type for modern LNG import and export terminals.

BOG Rates by Tank Construction

6. BOG Recovery & Handling

Effective BOG management is a critical operational and economic requirement for any cryogenic storage facility. If BOG is not removed from the tank, the vapor accumulates in the vapor space, causing the tank pressure to rise until the relief valve opens. This represents both product loss and a potential safety and environmental concern.

BOG Compression

The most common BOG management approach is to compress the boil-off gas using a dedicated BOG compressor and return it to the process. The BOG compressor takes suction at or near atmospheric pressure and compresses the gas to pipeline pressure (typically 200–1200 psig) or to a pressure suitable for use as fuel gas or feedstock. BOG compressors must be designed for continuous operation and high reliability, as a compressor failure will cause tank pressure to rise toward the relief set point. Redundant compressors (one operating, one spare) are standard practice.

For atmospheric LNG tanks compressing to 250 psig pipeline pressure, the compression ratio is approximately 18:1, typically requiring two-stage compression with intercooling. Compressor power is calculated from standard polytropic compression equations using the BOG mass flow rate, compression ratio, and gas properties (primarily methane for LNG BOG).

Reliquefaction

BOG reliquefaction systems use refrigeration to cool and condense the boil-off vapor back into liquid, which is returned to the storage tank. Reliquefaction is particularly common at LNG carrier ships where there is no pipeline to receive compressed gas. Shore-based reliquefaction plants are used at small-scale LNG facilities and satellite storage sites. The energy required for reliquefaction depends on the BOG temperature and the refrigeration cycle efficiency; typical specific energy consumption is 10–15 kWh per ton of BOG reliquefied.

BOG Recondenser

A BOG recondenser is a heat exchanger that cools and reliquefies the boil-off gas by mixing it with a stream of subcooled liquid from the tank or a sendout pump. The recondenser operates at an intermediate pressure between tank pressure and sendout pressure. This approach avoids the need for compression and is more energy-efficient, but it requires a subcooled liquid stream and is primarily used at LNG import terminals with continuous sendout operations.

Fuel Gas Utilization

BOG can be used directly as fuel gas for power generation, boilers, or gas turbine drivers at the storage facility. The heating value of methane BOG is approximately 1,010 Btu/SCF (HHV), making it an excellent fuel. However, the variable BOG rate requires supplementary fuel supply or buffer storage to maintain consistent fuel gas availability.

Flaring

Flaring is the last-resort method for BOG disposal when the recovery compressor is unavailable and the tank pressure approaches the relief valve set point. While flaring safely destroys the methane, it represents direct product loss and generates greenhouse gas emissions. Environmental regulations increasingly restrict routine flaring, making reliable BOG recovery systems an operational necessity.

BOG Handling Method Comparison

7. Ship Loading & Unloading

LNG ship loading and unloading operations generate BOG at rates far exceeding normal tank holding BOG rates. The combination of flash BOG, displacement BOG, and heat ingress from warm transfer piping creates peak vapor loads that govern the design capacity of the terminal BOG handling system.

Ship Unloading (Import Terminal)

During ship unloading, LNG is pumped from the carrier's cargo tanks to the onshore storage tanks. The primary BOG sources during unloading are:

• Displacement BOG in shore tank: As liquid enters the shore tank, vapor is displaced from the tank's vapor space. At typical unloading rates of 10,000–14,000 m³/hr, the displacement vapor rate is substantial.
• Flash BOG: The pressure drop across the unloading manifold and control valves causes the incoming LNG to flash partially. Even a small temperature difference of 3–5°F between the ship tank and shore tank conditions generates significant flash vapor.
• Line cooldown BOG: At the start of unloading, the transfer lines and loading arms are warm and must be cooled with LNG. The sensible heat in the piping steel and insulation is absorbed by evaporating LNG, producing a surge of BOG. Line cooldown typically takes 30–60 minutes.
• Ship tank heat ingress: The ship's cargo tanks continue to generate BOG due to their own heat ingress, which adds to the total BOG that must be managed.

Ship Loading (Export Terminal)

During ship loading, LNG is pumped from the onshore storage tanks to the carrier. The BOG dynamics are essentially reversed compared to unloading:

• Shore tank BOG reduction: As liquid is removed from the shore tank, the vapor space expands. If the tank pressure drops below the minimum set point, inert gas or atmospheric air bleed may be required, though this is rare with properly sized BOG systems.
• Ship tank displacement BOG: As the ship's cargo tanks fill with liquid, the existing vapor (typically inert gas from gassing-up or methane from a previous cargo) must be displaced. This vapor is typically sent to the terminal's BOG recovery system or flare.
• Ship tank cooldown: Before loading, the ship's cargo tanks may require cooling from ambient temperature to LNG temperature. This cooldown generates massive amounts of BOG as LNG spray evaporates upon contact with the warm tank surfaces. Ship cooldown can take 12–24 hours and generates the largest single BOG load during the entire ship-terminal interface.

Ship Loading/Unloading BOG Summary

8. Design Considerations

Designing a cryogenic storage facility requires careful integration of tank design, insulation specification, BOG recovery systems, and operational procedures.

Design BOG Rate

The design BOG rate should be based on worst-case conditions to ensure that the BOG recovery system can handle peak vapor generation without relief valve operation. Design conditions typically include maximum ambient temperature (design summer temperature plus 10°F margin), maximum solar radiation, maximum wind speed, and degraded insulation performance (end-of-life U-values at 115–125% of new-condition values).

Tank Size Effect

The surface-to-volume ratio of a cylindrical tank decreases as the tank size increases. Larger tanks inherently have lower BOG rates as a percentage of inventory, even with identical insulation systems. This economy of scale is one reason why modern LNG terminals favor fewer, larger tanks. A 160,000 m³ tank will have a significantly lower %/day BOG rate than a 40,000 m³ tank of the same construction type.

Holding Time

Holding time is the duration a fully insulated tank can maintain product without any BOG removal before the tank pressure reaches the relief valve set point. For atmospheric LNG tanks with relief valves set at 1–2 psig, the holding time can be as short as 2–5 days. For pressurized NGL vessels with higher relief pressures, holding times can extend to weeks. Holding time is a critical factor in determining BOG compressor reliability requirements and the need for redundant equipment.

Operational Best Practices

References

1. API 625 — Tank Systems for Refrigerated Liquefied Gas Storage
2. EN 1473 — Installation and Equipment for Liquefied Natural Gas — Design of Onshore Installations
3. GPSA Engineering Data Book, Chapter 11 — Hydrocarbon Recovery
4. NFPA 59A — Standard for the Production, Storage, and Handling of Liquefied Natural Gas
5. BS 7777 — Flat-Bottomed, Vertical, Cylindrical Storage Tanks for Low Temperature Service
6. IGC Code — International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk
7. SIGTTO — Ship/Shore Interface: Safe LNG Transfer Operations

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