Flammable limits, also known as explosive limits, define the range of concentrations of a flammable gas or vapor in air that can ignite or explode when an ignition source is present. This range is bounded by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). It's important to note that LFL and UFL values can be influenced by factors such as temperature, pressure, and the presence of inert gases. Therefore, these limits are typically determined through laboratory testing under specified conditions.
Lower Flammable Limit (LFL) or Lower Explosive Limit (LEL) - This is the "minimum concentration" of a combustible gas or vapor in air (expressed as a percentage by volume) below which the mixture is too "lean" (not enough fuel) to ignite and propagate a flame. Below the LFL, even with an ignition source, there isn't enough fuel to sustain combustion.
Upper Flammable Limit (UFL) or Upper Explosive Limit (UEL) - This is the "maximum concentration" of a combustible gas or vapor in air (expressed as a percentage by volume) above which the mixture is too "rich" (too much fuel, not enough oxygen) to ignite and propagate a flame. Above the UFL, there isn't enough oxygen to support the combustion process, even if there's plenty of fuel.
Flammable Range - The range between the LFL and UFL is called the flammable range. Any concentration of the flammable substance within this range, in the presence of an ignition source, can lead to a fire or explosion.
