Question

If a flammable vapor-air mixture is currently above its Upper Flammability Limit (UFL), what must happen to the mixture for it to become ignitable?

Accepted Answer

A flammable vapor-air mixture is a combination of a combustible gas or vapor and air, which contains oxygen, capable of burning when ignited. For such a mixture to become ignitable, meaning it can sustain a flame after an ignition source is applied, its concentration of flammable vapor must fall within its flammable range. This range is defined by two critical points: the Lower Flammability Limit (LFL) and the Upper Flammability Limit (UFL). The LFL is the minimum concentration of flammable vapor in air, expressed as a percentage by volume, below which the mixture is too "lean" (has too little fuel) to burn. Conversely, the UFL is the maximum concentration of flammable vapor in air, also expressed as a percentage by volume, above which the mixture is too "rich" (has too much fuel and not enough oxygen) to burn. A mixture is ignitable only when its fuel vapor concentration is greater than or equal to the LFL and less than or equal to the UFL. If a flammable vapor-air mixture is currently above its Upper Flammability Limit (UFL), it means the concentration of the flammable vapor is too high, and there is insufficient oxygen in the mixture to sustain combustion. In this state, the mixture is considered "too rich" to ignite. For this "too rich" mixture to become ignitable, its concentration of flammable vapor must decrease. This decrease typically occurs by introducing more air into the mixture, a process known as dilution. As additional air is mixed in, the ratio of flammable vapor to air (and thus oxygen) changes. The mixture becomes progressively less rich until its flammable vapor concentration falls below the UFL and into the flammable range, specifically between the LFL and the UFL. Once the vapor concentration is within this ignitable range, the mixture can then be ignited by a suitable ignition source.

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If a flammable vapor-air mixture is currently above its Upper Flammability Limit (UFL), what must happen to the mixture for it to become ignitable?