Boilover

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.

The extreme violence of boilovers is due to the expansion of water from liquid to steam, which is by a factor of 1500 or more.

An intermediate fuel layer, called the hot zone or heat wave, is formed, which becomes progressively richer in higher-boiling-point species.

Its lower boundary moves downwards towards the fuel–water interface at a speed higher than the overall level of fuel decreases due to the fire burning it.

[5] In these conditions water may be superheated, in which case part of it goes through an explosive boiling with homogeneous nucleation of steam.

[16][17] Ejected blazing fluids can travel at speeds up to 32 kilometres per hour (20 mph)[14] and attain distances well in excess of the limits of secondary containment bunding, often hundreds of meters or in the order of ten tank diameters downwind.

[18] Moreover, since boilover inception is sometimes unpredictable —either in terms of time to onset or whether it will occur at all (because the presence of water in the tank bottom may not be a known factor)— the impact on the firefighters that have intervened to control the fire can be deadly.

[19] Failure to appreciate the hazards posed by a water layer underneath the fuel has been a significant contributing cause to the aftermath of boilover accidents, in terms of human and material losses.

Uncertainty surrounding the time to boilover onset adds unpredictability that further complicates the efforts of the firefighting services.

[citation needed] The study of thin-layer boilover is of interest in the context of in-situ burning of oil spills over water.

[33] Serious burn incidents have also occurred during Mid-autumn Festival celebrations, where boiling candlewax and pouring water on it for entertainment has become a habit.

Although nothing may happen at first, water may eventually superheat and later start to boil violently, resulting in overflow.

[17] Open-top crude oil tank fires can be tackled using firefighting foam at rates of 10–12 L/(min × m2).

However, it is not clear if these rates are adequate to minimize the potential for a boilover event, especially in cases where foam attack is initiated long after the inception of the tank fire.

An estimation can be made a priori from the distillation curve and the properties of the fuel, with the aid of mathematical formulas, including the ones given above.

However, uncertainty regarding the presence and depth of a water or a water–fuel emulsion layer remains, and unpredictability about boilover onset cannot be completely dispelled.