2Definition and characteristics2 Fire is a self-sustained exothermic chemical reaction involving a combustible and a combustive (very often the oxygen in the air). It can only break out if there is a minimum initial energy, the source of ignition. Fire is subsequently (more or less) self-sustained by the effect of the heat produced by the combustion.
In a fire, a gas is always burning: either directly by the combustible involved, by vapours generated by a liquid or the pyrolysis gases of a solid. The thermal power released by the fire, resulting from the energy balance of this chain of events, depends on the reactivity of the combustible and combustive, on the one hand, and on the conditions in which they are in contact: proportions, quality of exchanged surfaces etc.
If the combustible is not very dense (for example storage of empty pallets), the fire will easily propagate, especially if a continuous air supply is provided (outside or warehouse with open doors). Conversely, if the combustive is rare or the combustible very dense, the fire will be limited. The temperature will remain moderate and the unburned gases will accumulate. When this happens, the intervention of emergency services is highly hazardous as opening confined premises will suddenly provide the missing oxygen and the gases will suddenly catch fire (“backdraft” phenomenon).
2How fire starts2 Possible ignition sources are varied, the main ones being:
Ignition causes can be natural (lightning), human (carelessness, malice) or technological.
Substances are more or less subject to ignition in the presence of a combustive and source of energy. The following characteristics are commonly used (established based on standards):
2Fire effects2 Most of the heat produced by a fire is emitted by electromagnetic radiation. This radiation is expressed as a quantity of energy per time and surface unit, referred to as heat flow. A liquid hydrocarbon fire radiates approximately 100 kW/m2. The fireball of a BLEVE (boiling-liquid expanding-vapour explosion) radiates approximately 200 kW/m2 on its surface.
Heat flow is emitted in all directions. A target situated at a certain distance therefore only receives part of it. To put it simply, the heat flow received decreases as per the inverse square of the distance from the fire. Furthermore, smoke and air absorb part of the energy emitted. The fire of a hydrocarbon slick with a surface area of 100 m2 therefore produces a perceived radiation of 3 kW/m2 at a distance of 26 m from its edge.
Fire damage depends on the quantity of energy received by the target. As a fire is characterised by a certain power, the damage depends on the exposure time. This is not a proportional relationship: fire resistance is higher for short exposure times. This is why one can briefly put one’s hand in a flame without being burned. This is also true for structures.
Regulatory threshold values to evaluate heat effects on man or structures are as follows:
2How to prevent and fight fire2 The idea is to constantly maintain conditions in which the combustible, the combustive and the source of ignition are not brought together:
In case a fire starts, its extent should be limited, for example: