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Difference between UEL and LEL

Understanding Flammability and Flammable gases:

 

Mixtures of dispersed combustible or flammable materials (such as gaseous or vaporized fuels, and some dusts) and air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to as flammability limits or explosive limits. Combustion can range in violence from deflagration, through detonation, to explosion.

Flammability limits vary with temperature and pressure, but these are normally expressed in terms of volume percentage at 25 °C and atmospheric pressure. These limits are relevant both to producing and optimising explosion or combustion, as in an engine, or to preventing it, as in uncontrolled explosions of build-ups of combustible gas or dust. Attaining the best combustible or explosive mixture of a fuel and air (the stoichiometric proportion) is important in internal combustion engines such as gasoline or diesel engines.

Lower explosive limit (LEL)

Lower explosive limit (LEL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Many safety professionals consider this terms same as the lower flammable limit (LFL). At a concentration in air lower than the LEL, gas mixtures are “too lean” to burn. Methane gas has an LEL of 5.0%. If the atmosphere has less than 5.0% methane, an explosion cannot occur even if a source of ignition is present.

Ignition Curve of a flammable gas with LEL, UEL plotting
Ignition Curve of a flammable gas with LEL, UEL plotting

Upper explosive limit (UEL)

Upper explosive limit (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentrations higher than UFL or UEL are “too rich” to burn.

Hence, safety systems and gas monitoring systems are designed to monitor and keep concentration of flammable gases below their respective LEL levels, or below 100% LEL. Therefore, gas detectors or system for flammable gases are designed for monitoring 0% LEL to 100% LEL.