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How to assess the extent of the explosive range?
The Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL) serve as the external boundaries of the explosive range. They express the extent of the explosive range: UEL – LEL.
There is no explosion at:
- the Lower Explosive Limit (LEL) and all concentrations below this limit. The combustible is within the lower flammability range.
The atmosphere is too lean in fuel. The combustion engine stalls. In other words: the quantity of combustible product will not be sufficient to trigger the combustion of the gaseous mixture via an ignition source.
- the Upper Explosive Limit (UEL) and all concentrations above this limit: upper flammability range.
The atmosphere is too rich in fuel. The combustion engine floods. Therefore, the quantity of combustible product is too high compared to the amount of oxygen to initiate the combustion of the gaseous mixture.
LEL and UEL: non-explosive concentrations of the flammable product, constituting respectively the lower external boundary and the upper external boundary of the explosive range.
LEL < Explosive Concentration < UEL
How is the quantity of explosive substance measured in ATEX?
First, the physical states of matter determine the unit of measurement. Then, concentration values are measured in the laboratory under standardized conditions; i.e., at a temperature of 25°C and an atmospheric pressure of 1 bar.
- For flammable gases and vapors: the concentration is expressed as a percentage of the volume of combustible product in the volume of the ATEX (%).
Example: hydrogen. LEL: 4%. UEL: 75%.
- For combustible dusts: the concentration is expressed as the mass of combustible product in the volume of the ATEX (g/m3).
The lower limit of the explosive range, called the Minimum Explosive Concentration, is usually measured.
Example sugar dust: LEL = MEC: 25 g/m3.
A particular value of the explosive range, the stoichiometric concentration (Cst), is its optimal concentration, relative to that of the oxidizer (oxygen), allowing for complete combustion of both reactants. At this concentration, the explosivity of the ATEX is maximal.
Explosive Range and Oxygen Concentration
The formation of ATEX is determined by an adequate combination of the respective concentrations of the combustible and the oxidizer (oxygen). However, the employer primarily refers to the concentration of the combustible to identify the ATEX risk. Because reaching the explosive range generally depends on operating conditions. In this sense, it is more symptomatic of the risk of ATEX formation than the oxygen concentration. Because the majority of operational processes take place in an ambient atmosphere where oxygen is the oxidizer with a constant concentration of 21%.
However, precise knowledge of the oxygen concentration proves useful in certain industrial situations. For example, when several oxidizing solvents that emit vapors and mix with air are used in an operation. Or when a process uses high concentrations of explosive substances in equipment that then admits very little or no oxygen.
Changes in the extent of the explosive range in industrial situations
The explosive range is measured under standardized temperature and pressure conditions because these parameters cause it to vary. These laboratory conditions rarely correspond to operational conditions. When the temperature increases, the explosive range widens. Therefore, the risk increases if operations are carried out at higher temperatures than the standardized temperature. Similarly, when the pressure decreases below atmospheric pressure, the explosive range narrows until it disappears. Certain vacuum operations use this property to prevent the risk of explosion. Other factors influence the flammability range: particle size of the substance (for dusts), humidity…
