Gas Groups
Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Empirical testing is done to determine parameters such as the maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.
Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. The flash point determines if an area needs to be classified. A material may have a relatively low autoignition temperature yet if its flash-point is above the ambient temperature, then the area may not need to be classified. Conversely if the same material is heated and handled above its flash-point, the area must be classified.
Each chemical gas or vapour used in industry is classified into a gas group.
| Group | Representative Gases |
|---|---|
| I | All Underground Coal Mining. Firedamp (methane) |
| IIA | Industrial methane, propane, petrol and the majority of industrial |
| IIB | Ethylene, coke oven gas and other industrial gases |
| IIC | Hydrogen, acetylene, carbon disulphide |
Apparatus marked IIB can also be used for IIA gases. IIC marked equipment can be used for both IIA and IIB. If a piece of equipment has just II and no A, B, or C after then it is suitable for any gas group.
A list must be drawn up of every chemical gas or vapor that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how volatile the gas or vapor would be if it was ignited, IIC being the most volatile and IIA being the least. The groups also indicate how much energy is required to ignite the gas by spark ignition, Group IIA requiring the most energy and IIC the least.
Read more about this topic: Electrical Equipment In Hazardous Areas
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