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Explosive limits |
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| There is only a limited band of gas/air concentration
which will produce a combustible mixture. This band is specific for each
gas and vapour and is bounded by an upper level, know and the Upper
Explosive Limit (UEL) and a lower level, called the Lower
Explosive Limit (LEL). |
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| At levels below the LEL, there is insufficient gas to produce a explosion (i.e. the mixture is too "lean"), whilst above the UEL, the mixture has insufficient oxygen, (i.e. the mixture is too "rich"). The flammable range therefore falls between the limits of the LEL and UEL for each individual gas or mixture of gases. Outside the limits, the mixture is not capable of combustion. Table 1 indicates the limiting values for some of the better-know combustible gases and compounds. The data is given for gases and vapours at normal conditions of pressure and temperature. An increase in pressure, temperature or oxygen content will generally broaden the flammability range. | |
| In the average industrial plant, there would be normally be no gases leaking into the surrounding area or, at worst, only a low background level of gas present. Therefore the detecting and early warning system will only be required to detect levels from zero percent of gas up to the lower explosive limit. By the time this concentration is reached, shut-down procedures or site clearance should have been put into operation. In fact this will typically take place at a concentration equivalent to 50 percent of LEL value, so that an adequate safety margin is provided. |
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However, it should be always be remembered that in enclosed or unventilated areas, a concentration in excess of the UEL can sometimes occur. At times of inspection, therefore, special care needs to be taken when operating hatches or doors, since the ingress of air from outside can dilute the gases to a hazardous, combustible mixture. |
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