COMPANYNAME SOLAS TRAINING MANUAL PART B – Section 1 Theory of Fire / Explosion
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EXPLOSIVE COMBUSTION OF GASES AND VAPOURS
Explosive limits: As regards the explosion hazards of liquid petroleum or other flammable products in bulk we are especially interested in the explosive reactions which may occur in gas mixtures consisting of vapour (gas) and air.
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When an inflammable gas (vapour) is mixed with air, an explosive mixture may develop. However, this is not always the case. At very low vapour concentrations no fast reaction will take place, although sufficient oxygen is present for this reaction. The quantity of heat, which is developed, is so small that it cannot ignite the next gas zone. This means that the reaction stops by itself and no explosion takes place: the mixture is too “poor”. When the vapour concentration is increased, there will be an explosion at a certain critical vapour/air ratio. This is the Lower Explosive Limit (LEL). The lowest concentration at which a gas (vapour) can still just be brought to explosion. At very high vapour concentrations (too “rich” mixtures) the quantity of oxygen will be too small to maintain the combustion; the reaction cannot propagate through the whole mixture.
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Consequently, there are two explosion limits as follows:
Upper Explosive Limit (UEL) The concentration of a flammable gas (e.g hydrocarbon) in air above which there is insufficient oxygen to support and propagate combustion. Sometimes referred to as upper flammable limit (UFL).
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Lower Explosive Limit (LEL) The concentration of a flammable gas (e.g hydrocarbon) in air below which there is insufficient flammable gas to support and propagate combustion. Sometimes referred to as lower flammable limit (LFL).
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The explosive limits are usually expressed in volume per cents (%) in air or in gr (of gas) per m3 air or in gr (of gas) per kg air. From the (LEL) explosion is getting more violent as the gas concentration increases, when the (UEL) is approached the violence of the explosion decreases.
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It is noted that if the initial pressure is one atmosphere (1 atm) during the explosion the pressure is 7-8 times more e.g. about 7-8 atm. Although at vapour concentrations above the Upper Explosive Limit no explosions can take place, special attention has to be paid to vapour/air mixtures of high concentrations. In the first place, these will behave as an inflammable gas while, secondly, on the supply of air a dilution develops, as a result of which the vapour concentration may lie again within the explosive range. Above a tank filled with an inflammable liquid, (e.g. crude oil), which is in open contact with its surroundings, a situation will arise. The Fig.3, 4 and 5 in the end of this section contain some diagrams referred to explosive limits, explosion areas, in relation to various gas concentrations. The explosive range in air of the various products is dependent on the chemical properties of them, and in particular of:
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