Explosive Gas Atmosphere

EN 60079-0 define the standard ambient atmosphere as:

• Temperature: –20 °C to 60 °C;
• Pressure: 80 kPa (0,8 bar) to 110 kPa (1,1 bar); and
• air with normal oxygen content, typically 21 % v/v.

Outside these conditions the standards do not generally apply although they can be used for 'special' assessments or considerations. Note. This applies to the atmosphere, not the operating conditions of any equipment.

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 60079-0 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.

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Explosive Dust Atmosphere

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.

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Hybrid Explosive Atmosphere

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 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.

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Notes

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