progress. Minute gas particles are generated and transported away from the source by diffusion, air movement, and weak convection movement. Ignition The point of ignition is reached when the reaction becomes self-sustaining and does not require an external heat source. Growth (smoulder and flame) The smouldering stage is a region of fully developed pyrolysis that begins with ignition and includes the initial stage of combustion. Invisible aerosol and visible smoke particles are generated and transported away from the source by moderate convection patterns and background air movement. The flaming stage is a region of rapid reaction that covers the period of initial occurrence of flame to a fully developed fire. Heat transfer from the fire occurs predominantly from radiation and convection from the flame. Steady state The steady state, also called the free-burning phase, is generally when enough oxygen and fuel are available for fire growth, and open burning to a point where total involvement is possible. Whilst sufficient fuel and oxygen are present, this stage will continue. Decay In this (final) stage the combustion is coming to an end, the fire is reducing in heat in a constant manner, and ‘the fire is burning itself out’ . The decay stage does eventually occur naturally, however, fighting a fire is about getting to the decay stage as quickly as possible. It is also useful to be aware of the steady state phase. This is the phase when by various means or methods, the fire is coming under control and is characterised by a heat release rate, which is relatively unchanging. Transition from the growth phase to the steady state phase can occur when fuel or oxygen begins to be limited, or when suppression activity begins to impact the fire. Factors that influence fire growth rates and smoke movement Most fires start out quite small and because of the circumstances can spread and very quickly get out of control. Fire growth is influenced by the amount of oxygen in the atmosphere. A well-ventilated room will quickly cause a fire to grow at a very rapid rate. Oxygen-enriched atmospheres considerably increase the risk of fire and fire growth rates. Normally harmless sparks or materials which do not burn in the air, including fireproof material, can burn vigorously in this type of oxygen-rich environment. Oil and grease are particularly hazardous in the presence of oxygen as they can ignite spontaneously and burn with explosive violence. They should never be used to lubricate oxygen or enriched air equipment (special lubricants which are inert when used with oxygen should always be used).
©SHEilds – All rights reserved FCV5 JUN 2021
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