My World, My Life: Hazardous Area

Hazardous areas are classified into zones according to the nature of the gases present in the potentially explosive atmosphere, and the likelihood of that atmosphere being present. The nature of the atmosphere is characterized by the chemical composition of the gas and its ignition temperature, and the notions of gas grouping and temperature classification have been developed in order to formalize this.

A useful concept in the consideration of how explosions occur is the hazard triangle. The sides of the triangle represent fuel, oxygen and a source of ignition, all of which are required in order to create an explosion. For the purposes of this chapter, a fuel is considered as a flammable gas, vapor or liquid, although dust may also be a potential fuel. Oxygen is of course present in air at a concentration of about 21 per cent. The ignition source could be a spark or a high temperature. Given that a hazardous area may contain fuel and oxygen, the basis for preventing explosion is to ensure that any ignition source is either eliminated or prevented from coming into contact with the fuel-oxygen mixture.

Zone classification

The zone classification defined in IEC 79 is used in Europe and most other parts of the world; it is summarized in Table 16.1. Various types of explosion protection are available, and their suitability for the different zones is shown in the table.

Zone	Suitable protection
Zone 0 Areas in which hazardous explosive gas atmospheres are present constantly or for long periods, for example in pipes or containers	Ex ‘ia’ Ex ‘s’ (where specially certified Zone 0)
Zone 1 Areas in which hazardous explosive gas atmospheres are occasionally present, for example in areas close to pipes or draining stations	Ex ‘d‘; Ex ‘ib’; Ex ‘p’; Ex ‘e’; Ex ‘s’; Ex ‘o’; Ex ‘q’; Ex ‘m’; Equipment suitable for Zone 0
Zone 2 Areas in which hazardous explosive gas atmospheres are rare or only exist for a short time, for example areas close to Zones 0 and 1	Ex ‘N’/Ex ‘n’; Equipment suitable for Zones 1 & 0

In the USA hazardous areas are classified in a slightly different way, according to the National Electrical Code. In brief, hazardous areas are classified either as Division 1, where ignitable concentrations of flammable gases or vapors may be present during normal operation, or as Division 2, where flammable gases or vapors occur in ignitable concentrations only in the event of an accident or a failure of a ventilation system.

Gas grouping and temperature classification

The energy required for ignition differs from gas to gas, and the grouping of gases together with their classification by temperature is used in Europe to describe the suitability of a piece of electrical equipment for use with explosive atmospheres of particular gases.

Table below lists common industrial gases within their appropriate groups. Gas group I is reserved for the classification of equipment suitable for use in coal mines. Gas group I1 contains those gases found in other industrial applications, and it is subdivided according to the relative flammability of the most explosive mixture of the gas with air.

Group	Representative gases
I	Methane
IIA	Acetone, ethane, ethyl acetate, ammonia, benzol, acetic acid, carbon monoxide, methanol, propane, toluene, ethyl alcohol, I-amyl acetate, N-hexane, N-butane, N-butyl alcohol, petrol, diesel, aviation fuel, heating oils, acetaldehyde, ethyle ether
IIB	Town gas, ethylene (ethene)
IIC	Hydrogen, acetylene (ethyne), hydrogen disulphide

Temperatures are classified from T1 to T6, as shown in Table below 3. The levels show the maximum surface temperature permitted for equipment which has been assigned that temperature class, and the common gases for which each class is appropriate are also shown.

North American practice is to define hazardous materials in classes. Flammable gases and vapors are Class 1 materials, combustible dusts are Class 2 materials and ‘flyings’, such as sawdust, are Class 3 materials. Class 1 is subdivided into four groups depending on their flammability: A (including acetylene), B (including hydrogen), C (including ethylene) and D (including propane and methane). The subgroup letters are in the opposite order of flammability to the IEC groupings shown in Table 2. The North American temperature classification is similar to the IEC system shown in Table 3, but the classes are further subdivided to give more specific temperature data.

Class	Highest permissible surface temperature (^oC)	Representative gases
T1	450	Acetone, ethane, ethyle acetate, ammonia, benzol, acetic acid, carbon monoxide, methanol, propane, toulene, town gas, hydrogen
T2	300	Ethyl alcohol, (-amyl acetate, N hexane, N-butane, N-butyl alcohol, ethylene
T3	200	Petrol, diesel, aviation fuel, heating oils
T4	135	Acetaldehyde, ethyl ether
T5	100
T6	85	Hydrogen disulphide