A boilover (or boil-over) is an extremely hazardous phenomenon in which a layer of water under a pool fire (e.g., an open-top tank fire) starts boiling, which results in a significant increase in fire intensity accompanied by violent expulsion of burning fluid to the surrounding areas.[1][2] Boilover can only occur if the liquid fluid is a mixture of different chemical species with sufficiently diverse boiling points, although a so-called thin-layer boilover – a far less hazardous phenomenon – can arise from any water-immiscible liquid fuel. Crude oil, kerosene and some diesel oils are examples of fuels giving rise to boilover.
Boilovers at industrial scale are rare but can lead to serious plant damage. Given the sudden and not easily predictable onset of the phenomenon, fatalities can occur, especially among firefighters and bystanders that have not been made to leave the area.
Slopover and frothover are phenomena similar to boilover but distinct from it. A slopover occurs when pouring water over a liquid pool fire, which may result in sudden expulsion of blazing fluid as well as considerable flame growth if the fire is small, as is the case when dousing water over a chip pan fire. A frothover is a situation occurring when there is a layer of water under a layer of a viscous fuel that, although not on fire, is at higher temperature than the boiling point of water.