Boiling Liquid Expanding Vapour Cloud Explosions (BLEVE) A BLEVE occurs when a vessel containing a pressurised liquid and vapour (such as propane or butane) catastrophically fails, usually because of an external fire affecting the vessel's walls. The heat is initially absorbed by the liquid but causes the pressure within the vessel to increase. This causes the relief valve to open, which allows the pressurised vapour to escape. As the liquid level in the vessel decreases, the flames impinge on the vessel wall above the liquid level. This part of the wall rapidly heats up because it is not protected by the cold liquid inside the tank. The wall weakens and then tears, resulting in a sudden catastrophic failure of the vessel. This causes a rapid boiling of the liquid because of the pressure drop, which then releases large amounts of vapour. On mixing with the air, and finding contact with an ignition source, the vapour cloud will explode violently, generating a significant fireball, and causing large fragments of the vessel to be projected significant distances. In addition, other major consequences include massive thermal radiation and blast waves. The debris and thermal radiation can cause a chain reaction if other storage tanks are nearby. Examples of major BLEVE incidents include Mexico City 1984 and Feyzin, France 1966.
Dust explosions A dust explosion involves the rapid combustion of flammable/combustible dust particles that release energy and usually generate gaseous reaction products. A mass of solid combustible material as a heap or pile will burn relatively slowly owing to the limited surface area exposed to the oxygen of the air. However, If the same solid in the form of a fine powder is suspended in air as a dust cloud the result will be quite different. In this case, the surface area exposed to the air is much larger, and if ignition occurs, the whole of the cloud may burn very rapidly. This results in a rapid release of heat and gaseous products and in the case of a contained dust cloud will cause the pressure to rise to levels which most industrial plant is not designed to withstand. For a fire to start and burn, three things are needed - fuel, an ignition source, and oxygen. If any of these elements are not present, a fire cannot start. In fire safety, this principle (known as the ‘Fire Triangle’) is commonly used to help to avoid industrial fires. For a dust explosion to occur two more elements are required. They are Confinement and Dispersion . These elements are created when the fuel, combustible dust, is dispersed into the atmosphere as a dust cloud within a confined area, such as a room or vessel. Similar to the fire triangle, removing just one of these elements can remove the risk of a dust explosion.
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