What are explosive atmospheres (ATEX)?

An explosive atmosphere (ATEX) is defined as the mixing with air, under atmospheric conditions, of flammable substances in the form of gases, vapors, mists or dusts, in which, upon ignition, the combustion spreads to the whole of the mixture Not burned.

For an explosion to occur, the explosive atmosphere and a source of ignition must coincide. This requires three simultaneous conditions:

  • 1st CONDITION: existence of a combustible substance (gases, vapors, dust or mist)
  • 2 nd CONDITION: existence of an oxidizer (air oxygen) in a given concentration range
  • 3rd CONDITION: presence of an energy source capable of initiating the reaction

Removing one or more of the above conditions means avoiding an explosion.

You can differentiate between two types of ATEX atmospheres:

  • Explosive gas atmospheres: mixing of a flammable substance in the gas or vapor state with air, in which, in case of ignition, the combustion spreads to the whole unburnt mixture
  • Atmosphere with explosive dust: mixture of air, under atmospheric conditions, with flammable substances in the form of dust or fibers, in which, in case of ignition, the combustion spreads to the rest of the unburned mixture

The risk of explosion of unstable substances such as explosives, pyrotechnic material and organic peroxides is not included in the definition of ATEX, or where explosive mixtures are subject to conditions not considered to be normal atmospheric conditions, such as mixtures subject to pressure .

To give a potentially explosive atmosphere requires the combination of the mixture of a flammable or combustible substance with an oxidant at a given concentration and a source of ignition. The risk becomes greater and more complicated when we are in a confined space and with work of manipulation of these substances in very diverse industries and productive processes.

ATEX Characteristic Parameters

  • Explosivity range: For the atmosphere to become explosive, the concentration of the aforementioned elements must be within a range. Above or below it can not be considered as such. The range is determined by the explosive limits:
  • Lower Explosivity Limit (LEL): The minimum concentration of flammable gases, vapors or mists in air below which the mixture is not explosive.
  • Upper Explosive Limit (LSE): The maximum concentration of flammable gases, vapors or mists in air above which the mixture is not explosive.
  • Flash point or flash point: It is the temperature at which the release of vapors is enough to produce the inflammation by the energy supply of an external focus.
  • Ignition or auto-ignition temperature: At this temperature the mixture enters spontaneous combustion. You do not need an external power source for ignition.
  • Maximum surface temperature: Maximum temperature that a material can reach without becoming a source of ignition for the surrounding atmosphere.
  • Minimal ignition energy: It is the energy that we must contribute to an explosive atmosphere so that ignition takes place.
  • Explosion group: Depending on its boundary gap (the penetration capacity of an explosion flame through a given gap is determined in standard equipment) and its ignition energy, the gases and vapors are subdivided into three groups: II A , II B, II C, where II C is the group with the smallest boundary gap.

Basic parameters on explosive atmospheres due to the presence of combustible dust

  • Minimum concentration of explosion. Equals the lower limit of explosive gases.
  • Minimum cloud ignition temperature (TIN). Equivalent to the flash point.
  • Minimum layer ignition temperature (TIC). It is equivalent to the maximum surface temperature.
  • Minimum ignition energy (EMI). Equals the minimum energy of inflammation.
  • Maximum permissible oxygen concentration to prevent ignition. It is the maximum concentration of oxygen at which there is no explosion of combustible dust.
  • Maximum explosion pressure. Maximum pressure that is reached during the explosion.
  • Maximum pressure gradient. Pressure growth rate. It gives us idea of ​​the virulence of the explosion