BLEVE fireball

What is a BLEVE Fireball?

A fireball can be defined as a fire burning sufficiently rapidly for the burning mass to rise into the air as a cloud or ball. A BLEVE Fireball specifically arises from a sudden instantaneous release and ignition of flammable mass (cryogenic or pressurised liquefied gas) that is stored under pressure. Due to the extensive destruction force of the Fireball, this phenomenon is rarely caught on video. To provide you with a better idea of how such a typical rising Fireball can look, see the video from Gexcon’s Fire and Explosion testing facility showcasing an example of a hot BLEVE on an example of liquefied hydrogen.

How does a BLEVE Fireball arise?

A BLEVE Fireball occurs due to the catastrophic rupture of a vessel that contains a flammable liquid stored at a temperature above its normal boiling point temperature. These flammable liquids are at atmospheric pressure in a gaseous state but liquified for transportation and storage purposes (see phase diagram below). This means that most of the material is in a liquid state, and the vapours fill the space above the liquid creating pressure.

The main condition for the formation of a Fireball is that the material stored in the device is flammable. Due to the high burst pressure, its formation is completely independent of meteorological conditions. The Fireball is significant for a very short duration.

The most common causes of a BLEVE (Boiling Liquid Expanding Vapour Explosion) are typically attributed to the following:

• Equipment failure is caused by an external heat source heating the surface of the equipment, such as a vessel or truck. This leads to a rise in temperature and pressure inside the vessel above its burst pressure.

• Equipment failure due to different reasons, including:

  • Material fatigue: Equipment that has been in service for an extended period may experience repeated stress or mechanical damage, such as corrosion or erosion.

  • Mechanical failure: This can result from welding defects, material degradation, or inadequate pressure relief systems.

  • External hazards: Events like vehicular collisions during the transport of LPG, industrial accidents, or natural disasters such as earthquakes or extreme weather events can also cause structural damage, compromising the integrity of the equipment.

The origin of the BLEVE Fireball affects the formation mechanism. Based on the two main known mechanisms, we can differentiate two types of BLEVE Fireball: Cold BLEVE and Hot BLEVE (Bosch, 2005).

Characteristics of a BLEVE Fireball

When a liquefied gas is rapidly depressurised due to a sudden loss of tank integrity, it undergoes partial flash vaporisation, releasing a two-phased vapour mixture. This creates a cloud that burns at its edges since the concentration inside the cloud may not fall within flammability limits. Initially, the cloud has a semi-spherical shape above the ground or vessel, which transitions to a spherical shape as liquid droplets vaporise and the cloud’s density decreases, causing it to rise and increase its volume. If the material in the tank is cooled, then a Fireball with a large terrestrial diameter can be expected.

The Fireball radiates an intense thermal flux. The calculation and the heat radiation at certain distances involve applying mathematical models. As mentioned in the combustion characteristics chapter the SEP, view factor, and atmospheric transmissivity are part of the heat radiation calculation. Although Fireballs are considered fire events, along with the dominating thermal effects they produce also significant overpressure effects, which will be further discussed in the explosion chapter.

Hot BLEVE - Result of an external heat source

A Hot BLEVE occurs when the burst pressure exceeds the operation pressure (saturated vapour pressure). This can be caused by prolonged exposure of the vessel surface to an external heat source (e.g., a fire on the premises), causing the tank’s content temperature to rise. The liquid boils and rapidly evaporates, which increases the vapour pressure to the point of compromising the tank’s structural integrity, leading to catastrophic failure. If the tank ruptures, the hot liquid rapidly flashes into vapour, resulting in damage due to overpressure from the rapid expansion of vapours, known as a ‘‘blast wave’’. A BLEVE Fireball only occurs if the stored pressurised liquefied gas is flammable. Since the external heat source is typically from a nearby fire, the vapour cloud ignites immediately upon release, leading to the occurrence of a BLEVE Fireball.

Cold BLEVE - Result of equipment failure

A Cold BLEVE occurs when the burst pressure is equal to the operating pressure. The scenario begins with the failure of the pressurised process equipment, followed by a pressure drop inside the tank, causing the liquid's boiling point to decrease below the storage temperature in the tank. As a result, the liquid boils and rapidly evaporates, causing the tank to rupture. If the vapours are ignited directly upon release, it leads to the occurrence of a BLEVE Fireball.

References

Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.