Boilover

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. 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.

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. 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. Since the upper fuel layers, including the hot zone, are at or near their boiling temperature, it is necessary for the boiling point of the fuel to be high enough, such that the hot zone temperature is higher than the water boiling temperature. Both the effect of the static head of fuel above the water and the fact that the hot zone composition is different from that of the initial fuel have to be considered. In general, boilover is possible if the fuel mean boiling point (calculated as a geometric mean of its lower and upper boiling points, i.e. the temperatures at which the mixture, respectively, starts to boil and is completely vaporized) is higher than : {\bar{T}}_{\text{boil}}={{({T}_{\text{boil,min}}\ {T}_{\text{boil,max}})}^{0.5}}>{\text{120 °C}} As mentioned above, the composition of the fuel mixture must be sufficiently varied. It has been observed that the gap between and the higher value between and the boiling point of water at the fuel–water interface has to be higher than 60 °C (108 °F): {{T}_{\text{boil,max}}-\max{({T}_{\text{boil,min}},{{T}_{\text{boil,water}}})}}>{\text{60 °C}} Some sources indicate that the upper range of the boiling temperature has to be above : {{T}_{\text{boil,max}}}>{\text{149 °C}} The fuel viscosity must be sufficiently high to oppose the upwards movement of the steam bubbles. Otherwise, these may flow through the fuel without projecting it out of the blazing tank. Low viscosity may also make it difficult for a stable heavy-components hot zone to form, thanks to more efficient natural convection. Thus, experiments on gasoline (dynamic viscosity ≈ 0.37 cSt) pool fires have shown that boilover does not occur. In general, fuel dynamic viscosity has to be higher at least 0.73 cSt, which is the viscosity of kerosene. The hazards posed by a hot-zone boilover are significant for several reasons. At industrial scale, hydrocarbon tanks can contain up to hundreds of thousands of barrels of fluid. If a boilover occurs, the amount of blazing liquid erupting from the tank can therefore be huge. Ejected blazing fluids can travel at speeds up to 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. Bunding, however, remains an important measure to reduce fire spread. 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. In some cases, simple bystanders were caught in the blaze and perished. Tank fires that appear to be relatively stable may burst into massive boilovers several hours after the fire starts, as it occurred in the Tacoa disaster. 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. Mathematical models for boilover have been developed that predict the time necessary for boilover to initiate, among other things. ==See also== Phreatic eruption, a similar concept in volcanic eruption. ==References== ==External links==

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