A Boiling Liquid Expanding vapour Explosion (BLEVE) is caused by the catastrophic failure of a pressure vessel containing a liquid which is well above its boiling point at atmospheric pressure. Although rare events, the consequences of a BLEVE can be catastrophic, leading to the prominent position of the phenomenon in the safety analysis of LPG transport and storage.
Experimental observations show five distinct stages in the development of fireballs:
Stage 1: The vessel fails, missiles are generated and ejected and an initial overpressure wave, Ps1, is produced by the expanding vapour phase. The overpressure pulse is followed by a rarefaction.
Stage 2: The bursting vessel ejects a cloud of liquid droplets which flash adiabatically as the
pressure in the cloud drops. The mass, and therefore ultimately volume, of vapour flashed from the liquid droplets vastly exceeds that of the vapour initially released in Stage 1. There is little mixing with the surrounding air. The cloud pressure, eventually drops to the surrounding atmosphere and the volume of the cloud then becomes equal to the volume of flashed vapour at saturation temperature and pressure. If the outward radial velocity of the cloud exceeds the local speed of sound in the rarefaction following Ps1, then a blast wave can form as a result of the vapour expansion from the flashing liquid. As it separates from the outer cloud boundary, this blast wave, Ps2, leaves the cloud in a highly turbulent state at ambient pressure. It has been observed from experiments that the blast wave is only associated with the initial high fill ratio. At lower levels of fill the blast wave is unlikely to occur.
Stage 3: At this stage the overpressure peaks Ps1 and Ps2 leave the cloud. The cloud continues to expand due to outward radial momentum but the radial expansion velocity slows as the turbulent mixing entrains more and more air. Rapid expansion continues until the random velocities in the turbulent eddies overwhelm the radial expansion velocity, further expansion being due to the turbulence alone.
Stage 4: Ignition occurs near the centre of the cloud and an hemispherical fireball develops. The
expansion is suddenly arrested as the last visible part of the vapour cloud is consumed by flame. At this point the fireball is at its brightest. Since the cloud contains air, it is assumed that during this stage the flashed vapour is consumed, there being insufficient time for radiation to substantially heat and flash the cold liquid droplets. The expansion causes an overpressure pulse in the surrounding atmosphere which travels outwards from the cloud. The pulse is followed by a
rarefaction caused by the sudden arrest in cloud expansion. The expansion velocity of the fireball is equal to the flame propagation velocity through the turbulent vapour cloud. The heat radiation
pulse from the fireball peaks at the end of this stage.
Stage 5: The hemispherical fireball rises to become a sphere sitting on the ground. Combustion
continues but the fireball does not expand, indicating that the air required for combustion is already pre-mixed into the cloud. The fuel for combustion is supplied by the liquid droplets. The fireball then rises at approximately constant velocity and volume, and assumes a typical mushroom shape. The visible flame area decreases as the fireball becomes patched with sooty combustion products. Once combustion is substantially complete, the smoky torrid of hot combustion products rises, expands and dissipates with a flow pattern similar to a thermal. The heat radiation pulse decreases systematically to zero during this phase.
Stage 2: The bursting vessel ejects a cloud of liquid droplets which flash adiabatically as the
pressure in the cloud drops. The mass, and therefore ultimately volume, of vapour flashed from the liquid droplets vastly exceeds that of the vapour initially released in Stage 1. There is little mixing with the surrounding air. The cloud pressure, eventually drops to the surrounding atmosphere and the volume of the cloud then becomes equal to the volume of flashed vapour at saturation temperature and pressure. If the outward radial velocity of the cloud exceeds the local speed of sound in the rarefaction following Ps1, then a blast wave can form as a result of the vapour expansion from the flashing liquid. As it separates from the outer cloud boundary, this blast wave, Ps2, leaves the cloud in a highly turbulent state at ambient pressure. It has been observed from experiments that the blast wave is only associated with the initial high fill ratio. At lower levels of fill the blast wave is unlikely to occur.
Stage 3: At this stage the overpressure peaks Ps1 and Ps2 leave the cloud. The cloud continues to expand due to outward radial momentum but the radial expansion velocity slows as the turbulent mixing entrains more and more air. Rapid expansion continues until the random velocities in the turbulent eddies overwhelm the radial expansion velocity, further expansion being due to the turbulence alone.
Stage 4: Ignition occurs near the centre of the cloud and an hemispherical fireball develops. The
expansion is suddenly arrested as the last visible part of the vapour cloud is consumed by flame. At this point the fireball is at its brightest. Since the cloud contains air, it is assumed that during this stage the flashed vapour is consumed, there being insufficient time for radiation to substantially heat and flash the cold liquid droplets. The expansion causes an overpressure pulse in the surrounding atmosphere which travels outwards from the cloud. The pulse is followed by a
rarefaction caused by the sudden arrest in cloud expansion. The expansion velocity of the fireball is equal to the flame propagation velocity through the turbulent vapour cloud. The heat radiation
pulse from the fireball peaks at the end of this stage.
Stage 5: The hemispherical fireball rises to become a sphere sitting on the ground. Combustion
continues but the fireball does not expand, indicating that the air required for combustion is already pre-mixed into the cloud. The fuel for combustion is supplied by the liquid droplets. The fireball then rises at approximately constant velocity and volume, and assumes a typical mushroom shape. The visible flame area decreases as the fireball becomes patched with sooty combustion products. Once combustion is substantially complete, the smoky torrid of hot combustion products rises, expands and dissipates with a flow pattern similar to a thermal. The heat radiation pulse decreases systematically to zero during this phase.
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