Explosive Atmosphere

Explosive Gas Atmosphere

'mixture with air, under atmospheric conditions, of flammable substances in the form of gas or vapour which, after ignition, permits self sustaining flame propagation.'
Gas Triangle The traditional ignition triangle shows the 3 parts which are required to be present1 for an explosion to occur and in the protection methods detailed in the EN 60079 standards are designed to remove at least one of those parts, the exception being Ex d flameproof2.

  1. Fuel can be Dust or Gas/vapour, Although the purists would argue Gas and vapour are different, within the context of Hazardous Areas the terms generally are interchangeable. For convenience the term Gas is generally used to mean both Gas and Vapour.

  2. The ignition source can be a hot surface or energy from a spark, they are different and need to be considered separately.

  3. Air (Oxygen): The 60079 standards refer to Normal Temperature and Pressure (NTP), it is assumed that Air/Oxygen is always present (unless of course removed by a protection technique). Locations where enriched or depleted Oxygen exists e.g. Waste digesters or extremes of pressure scenarios are special cases and strictly speaking outwith the scope of any conventional protection methods, more detailed risk assessments need to be conducted and standard product certification becomes invalid.

Explosive Dust Atmosphere

'mixture with air, under atmospheric conditions, of flammable substances in the form of dust, fibres or flyings which, after ignition, permits self-sustaining propagation.'

Combustible Dust::
'Finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, may burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures'

Dust Pentagon This includes dust and grit as defined in ISO 4225 and the term solid particles is intended to address particles in the solid phase and not the gaseous or liquid phase, but does not exclude a hollow particle.

Although the ignition triangle is widely used and is a perfectly good representation, perhaps a better depiction for dust would be the ignition pentagon.
Dust lying around might be a fire or health hazard but is generally not considered an explosive atmosphere, unless disturbed. For an explosive atmosphere to form there must be Dispersion but also some form of confinement to achieve explosive concentrations. However a dust cloud can be its own confinement with varying concentrations as it generally does not disperse as evenly as a gas cloud.


Hybrid Explosive Atmosphere

'Mixture of flammable substances in different physical states   e.g. Methane & Coal dust.
Thankfully this is not a common occurrence but occasionally exists in the chemical industry when areas are zoned for gas and dust.
As virtually no equipment is actually certified for a hybrid mixture this can create problems and additional risk assessments will generally need to be carried out. Older equipment may be marked for GD but unless it specifies conditions in the Type Certificate it must be interpreted as Gas or Dust not Gas and Dust see EX d Flameproof for more details and examples.

Newer equipment certified to standards released after 2014 should conform to the tighter restrictions on labelling implemented for this very reason and should have separate lines in the certification label for Gas (G) and Dust (D) with a third (GD) specifically for Hybrid mixture if covered by the certification.

If both gas and dust are present in a 'Hybrid' Atmosphere assessment, particularly of equipment protection methods, gets much more complicated. Very little equipment certification covers a Hybrid environment and specialist assistance should be sought regarding equipment choice.


Notes


  1. To be pedantic if the fuel is Acetylene, then external Oxygen does not need to be present as it creates it own oxygen to burn if ignited. 

  2. Flameproof (Ex d) protection method allows the explosion to happen and contains it, preventing propagation outside the device 

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