What are explosive atmospheres (ATEX)?

Explosive atmosphere (ATEX) is defined as the mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapors, mists or powders, in which, after ignition, combustion spreads to the entire mixture not burned.

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

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

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

Two types of ATEX atmospheres can be distinguished:

  • Explosive gas atmospheres: mixture of a flammable substance in the state of gas or vapor with air, in which, in case of ignition, combustion spreads to the entire unburned 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, or when explosive mixtures are subjected to conditions not considered normal atmospheric conditions, such as mixtures under pressure, is not included in the ATEX definition. .

A potentially explosive atmosphere requires the combination of a flammable or combustible substance with an oxidant at a certain concentration, and an ignition source. The risk becomes greater and more complicated when we find ourselves in a confined space and with jobs handling these substances in many different industries and production processes.

ATEX characteristic parameters

  • Explosive range: For the atmosphere to become explosive, the concentration of the aforementioned elements must be within a range. Above or below it cannot be considered as such. The range is determined by the explosive limits:
  • Lower Explosive Limit (LEL):It is the minimum concentration of flammable gases, vapors or mists in air below which the mixture is not explosive.
  • Upper Explosive Limit (UEL): It is 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 sufficient for ignition to occur due to the contribution of energy from an external source.
  • Ignition or autoignition temperature: At this temperature the mixture spontaneously combusts. It does not require an external power source for ignition to occur.
  • Maximum surface temperature:Maximum temperature that a material can reach without becoming a source of ignition for the surrounding atmosphere.
  • Minimum ignition energy: It is the energy that we must contribute to an explosive atmosphere for ignition to occur.
  • Explosion group: Depending on their limit gap (the penetration capacity of an explosion flame through a given gap is determined in a standardized appliance) and their ignition energy, gases and vapors are subdivided into three groups: II A, II B, II C, with II C being the group with the smallest limit gap.

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

  • Minimum explosion concentration. Equivalent to the lower explosive limit of gases.
  • Minimum cloud ignition temperature (TIN). Equivalent to flash point.
  • Minimum layer ignition temperature (TIC). Equivalent to the maximum surface temperature.
  • Minimum Ignition Energy (EMI). Equivalent to the minimum energy of inflammation.
  • Maximum allowed oxygen concentration to prevent ignition. It is the maximum oxygen concentration at which combustible dust does not explode.
  • Maximum burst pressure. Maximum pressure that is reached during the explosion.
  • Maximum pressure gradient. Pressure growth rate. It gives us an idea of ​​the virulence of the explosion