BLEVE Fireball Analysis

1. BLEVE Mechanism

A Boiling Liquid Expanding Vapor Explosion (BLEVE) is the catastrophic failure of a vessel containing a pressurized liquid at a temperature significantly above its normal atmospheric boiling point. The term was coined by researchers at Factory Mutual in the late 1950s after investigating a series of devastating industrial incidents. While a BLEVE can occur with any superheated liquid, the most dangerous events involve flammable materials such as propane, butane, and LPG, which produce massive fireballs upon release.

Conditions Required for BLEVE

Three conditions must be present simultaneously for a BLEVE to occur. The liquid must be at a temperature above its atmospheric boiling point, meaning it is superheated relative to atmospheric pressure. The vessel must contain this liquid under pressure that prevents boiling. And the vessel must fail suddenly, allowing the liquid to depressurize and flash nearly instantaneously. The degree of superheat determines the violence of the event; higher superheat produces more rapid flashing and a larger fraction of the liquid converts to vapor.

The superheat limit theory (SLT) provides a thermodynamic threshold for BLEVE. If the liquid temperature exceeds the superheat limit temperature (approximately 89% of the critical temperature for many hydrocarbons), homogeneous nucleation occurs throughout the liquid bulk, producing an extremely rapid phase change that generates significant overpressure. Below the superheat limit, the flashing occurs more gradually through bubble nucleation at surfaces and impurities, producing a less violent event.

2. Fireball Correlations

Fireball dimensions are predicted using empirical correlations derived from experimental BLEVE tests ranging from small laboratory scale (a few kilograms) to large-scale tests involving several tonnes of propane and butane. The most widely used correlations relate the maximum fireball diameter and duration to the total mass of fuel involved.

Alternative Correlations

Several researchers have proposed different correlations based on varying datasets and assumptions. The Roberts (1981) correlation uses D = 5.8 * M^0.333, which is identical to the CCPS recommendation and is the most widely accepted. Moorhouse and Pritchard (1982) proposed D = 5.33 * M^0.327, which gives slightly smaller diameters. The Martinsen and Marx correlation, recommended for large inventories above 30 tonnes, adjusts the duration exponent to account for buoyancy effects that become dominant at large scales.

Fireball Dynamics

The fireball goes through three distinct phases. In the initial growth phase lasting approximately one-third of the total duration, the fireball expands to its maximum diameter near ground level. During the liftoff phase, buoyancy forces cause the hot combustion gases to rise, pulling the fireball upward and slightly reducing its diameter. In the final burnout phase, the remaining fuel is consumed as the fireball rises, and the luminous sphere gradually becomes an ascending cloud of hot combustion products. The maximum thermal radiation at ground level typically occurs during the early growth phase when the fireball is closest to the ground and at its largest diameter.

3. Thermal Radiation Modeling

Thermal radiation from a BLEVE fireball is modeled using the solid flame approach, which treats the fireball as a sphere with a uniform surface emissive power (SEP). The incident radiation flux at any receptor point depends on the SEP, the geometric view factor between the fireball and the receptor, and the atmospheric transmissivity along the radiation path.

Atmospheric Transmissivity

Thermal radiation is absorbed by water vapor and carbon dioxide in the atmosphere as it travels from the fireball to the receptor. The Wayne (1991) model accounts for this absorption as a function of the path length and the partial pressure of water vapor. At short distances and low humidity, transmissivity approaches unity. At long distances or high humidity, transmissivity can drop to 0.5-0.7, significantly reducing the incident radiation. This effect is particularly important when calculating safe distances, as it provides some natural attenuation of the thermal hazard.

Radiation Fraction

The radiation fraction represents the proportion of total combustion energy that is emitted as thermal radiation rather than carried upward by convection. For hydrocarbon fireballs, experimental data suggests radiation fractions between 0.25 and 0.40, with propane and butane typically around 0.30. Larger fireballs tend to have lower radiation fractions because soot in the outer shell absorbs some outgoing radiation and re-radiates it inward. The radiation fraction is one of the most significant sources of uncertainty in BLEVE consequence modeling.

4. Safe Separation Distances

Safe distances from a potential BLEVE event are determined by the thermal radiation flux thresholds that correspond to specific injury or damage levels. These thresholds are used in facility siting studies per API RP 752 and quantitative risk assessments to determine minimum separation distances between LPG storage and occupied buildings, property boundaries, and emergency response positions.

Thermal Dose and Probit Analysis

For transient events like BLEVE fireballs, the probability of injury or fatality is calculated using the thermal dose and probit functions. The thermal dose combines the radiation intensity and exposure duration into a single parameter that correlates with injury severity. The Eisenberg probit model, widely used in QRA, relates the probability of fatality to the thermal dose through a probit equation. At a thermal dose of approximately 1000 (kW/m^2)^(4/3) * s, the probability of fatality for unprotected personnel is approximately 1%. At 2000, it rises to approximately 50%.

5. BLEVE Prevention

BLEVE prevention focuses on two strategies: preventing the conditions that lead to vessel failure, and mitigating the consequences if a BLEVE does occur. The primary prevention measure is adequate fire protection to keep the vessel wall cool during an external fire, preventing the temperature rise that leads to wall weakening and rupture.

Key Design Measures

• Install fixed water spray (deluge) systems on all LPG vessels per NFPA 15 and API 2510
• Provide adequate drainage to prevent pool fire formation under vessels
• Install thermal relief valves sized for fire case per API 521
• Consider mounded or buried storage to eliminate fire impingement exposure
• Maintain adequate separation distances between vessels per API 2510 and NFPA 58
• Implement fire detection and automatic deluge activation
• Provide emergency depressuring capability for large vessels
• Apply passive fire protection (PFP) to critical structural supports and vessel saddles

6. Fragment Hazards

In addition to the fireball and thermal radiation, a BLEVE produces vessel fragments that are projected at high velocity. Fragment throw distance is a significant hazard that often extends beyond the thermal radiation hazard zone. Historical data shows that BLEVE fragments from railway tank cars have been found up to 800 meters from the failure point, and in some events fragments have been projected over 1 kilometer.

Fragment characteristics depend on the vessel material, wall thickness, internal pressure at rupture, and the mode of failure. Ductile materials like carbon steel tend to produce fewer, larger fragments, while brittle materials produce more numerous smaller fragments. The number of fragments from a cylindrical vessel BLEVE typically ranges from 2 to 6 pieces, with end caps often being projected furthest due to the rocket effect from escaping pressurized contents.

7. Historical Incidents and Lessons Learned

BLEVE incidents have provided critical data that shaped modern safety standards and prevention practices. The frequency of BLEVE events has decreased significantly since the 1970s and 1980s due to improved fire protection, better vessel inspection practices, and adoption of the standards discussed above. However, BLEVEs continue to occur when fire protection systems fail or are absent, reinforcing the importance of robust prevention and mitigation measures.

Key Lessons from Historical BLEVEs

• Water cooling is essential: Virtually every major BLEVE incident involved either absent or inadequate water deluge coverage on the vessel that failed. Water spray at the design application rate can prevent BLEVE indefinitely.
• Relief valves cannot prevent BLEVE: Pressure relief protects against overpressure but does not prevent the temperature-induced wall weakening that causes BLEVE. A vessel can BLEVE below its design pressure.
• Vapor space exposure is critical: The unwetted wall above the liquid level is the vulnerable zone. As liquid boils off, the wetted area decreases and the vulnerable zone increases, creating a race between boil-off rate and fire exposure.
• First responder positioning: Emergency responders must maintain separation distances based on the maximum credible BLEVE scenario. Approach should only be from the sides of a horizontal vessel, never from the ends (fragment throw direction).
• Domino effects: A single vessel BLEVE can impinge on adjacent vessels, creating a cascading failure scenario. Separation distances and fire protection must account for these escalation scenarios.

Key Takeaways