Explosive atmosphere equipment is specified in terms of the types of gases or dust present.
Group I – Underground mining applications
Group II – Industries other than mining where explosive atmospheres may exist
Group III – Industries having explosive dust or flyings
Different explosive atmospheres (gases, vapours and dusts) will have different properties including flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. These properties will determine the likelihood and severity of an explosion. In order to clearly identify these properties testing is performed on the atmosphere to determine critical properties like maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. These values help to classify the substance so that they can be grouped in order to select the right equipment for explosive areas.
Explosive gases are grouped into three main groups.
|TYPICAL GAS EXAMPLE
|Firedamp (Methane) and coal dust
|Petrol, Propane, Industrial Methane
|Coke Oven Gasses, Ethylene
|Acetylene, Carbon Disulphide, Hydrogen
A combined group of IIB+H2 is often used for equipment which is designed for Group IIB gasses but can also be used when Hydrogen is also available.
Group IIC is the most volatile group and the gasses in this group can be ignited quite easily. Any equipment which is marked as IIC can be used in atmospheres that contain Group IIB and Group IIA gasses. Conversely equipment marked IIA cannot be used in IIB or IIC explosive environments.
Like gas, dusts are grouped in order to effectively identify the risks of ignition and to identify the severity of explosion. The size of the dust particles is 500 micro meter or less. Dust can remain suspended in air and may settle due to its own weight.
The groupings are:
|Combustible flyings (not dust particles)
|Combustible but non-conductive dust
|Combustible & conductive dust
Just like equipment in gas, the equipment marked IIIC can be used for IIIB and IIIA atmospheres.