Definition and characteristics

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).

How fire starts

Possible ignition sources are varied, the main ones being:

• naked flame or incandescent solid,
• hot spot caused by Joule effect or friction,
• lightning,
• electric or electrostatic spark,
• adiabatic compression of a gas (which subsequently heats up).

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):

• flash point: lowest temperature at which a liquid produces enough vapours to be ignited in the presence of a specified source of ignition (petrol: - 46°C),
• spontaneous ignition temperature: temperature at which a substance ignites spontaneously, the source of ignition being the heat of the substance (petrol: 280°C),
• lower flammable limit or LFL: minimum concentration of a gas in the air to form a mixture capable of being ignited by a specified source of ignition (hydrogen: 4%, methane: 5%); below LFL, the mixture is deemed “too lacking” in combustible to burn,
• upper flammable limit or UFL: maximum concentration of a gas in the air to form a mixture capable of being ignited by a specified source of ignition (hydrogen: 75%, methane: 15%); beyond UFL, the mixture is deemed “too rich” in combustible to burn.

Fire effects

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:

• 3 kW/m2 or 600 [kW/m2]4/3(30 second exposure): irreversible effects
• 5 kW/m2 or 1000 [kW/m2]4/3 (60 second exposure): first lethal effects
• 8 kW/m2 or 1800 [kW/m2]4/3: significant lethal effects

How to prevent and fight fire

The idea is to constantly maintain conditions in which the combustible, the combustive and the source of ignition are not brought together:

• by removing the combustive: remove the air using floating covers in hydrocarbon tanks or replace it with an inert gas ceiling for highly flammable liquids or heated flammable liquids,
• by removing the combustible: sufficient ventilation of premises likely to contain flammable gases (maintain the gas-air mixture below LFL), detection of gas leaks or flammable liquids,
• by removing the source of ignition: distancing or elimination of any heat source, use of insulated electrical equipment, adapted to potentially explosive atmospheres (ATEX), burning and hot work permit instructions.

In case a fire starts, its extent should be limited, for example:

• by confining or removing combustible capacities: trenched areas underneath flammable liquid receptacles, distancing of storage, separation by means of fire walls or water curtains activated by automatic detection,
• by fighting fire using safety systems: automatic detection combined with a fire extinguishing system (sprinkler system), smoke extraction by openings in the upper part of the roof (extraction of hot and partially unburned gases),
• by building fire resistant and non-combustible structures in order to prevent premature failure and the consecutive spread of the damage,
• by acquiring fire fighting resources (fire extinguishers, personal protection, water or foam canons, water and foam compound supply) and adopting an efficient organisation (emergency plan, external emergency services), regularly tested during real-life drills.