. . 'mixture with air, under atmospheric conditions, of flammable substances in the form of gas or vapour which, after ignition, permits self-sustaining flame propagation.'
The traditional ignition triangle shows the 3 parts which are required to be present1 for an explosion to occur. 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.
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.
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.
. . 'mixture with air, under atmospheric conditions, of flammable substances in the form of dust, fibres or flyings which, after ignition, permits self-sustaining propagation.'
'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'
Combustible Flyings solid particles, including fibres3, greater than 500 μm in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, can burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures
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 dust generally does not disperse as evenly as a gas cloud.
'Mixture of flammable substances in different physical states4.
Older equipment may be marked for GD, but unless it specifies conditions for a Hybrid atmosphere in the Type Certificate it must be interpreted as Gas or Dust not Gas and Dust.
Prior to the publishing of EN 60079-31 (2009) EX d Flameproof was the goto solution for dust certification and its use was often misunderstood5.
Newer equipment certified to standards released after 2014 should conform to the tighter restrictions on labelling implemented for this very reason. Equipment certified to current standards will 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.
To be pedantic if the fuel is Acetylene, then external Oxygen (Air) does not need to be present as Acetylene creates it own oxygen to mainatain combustion if ignited. ↩
Flameproof (Ex d) protection method allows the explosion to happen and contains it, preventing propagation outside the device ↩
Fibres or Flyings are log thin particles, examples of which include rayon, cotton (including cotton linters and cotton waste), sisal, jute, hemp, cocoa fibre, oakum, and baled waste kapok. ↩
Where Gas and Dust could be present e.g. Methane & Coal dust. ↩