The engineer must select the area classification from several categories and ensure that the instrumentation is compatible with safe operation. The appropriate local regulatory agency defines the categories, and the instrumentation manufacturer defines the set of specifications appropriate for each equipment. Thus there are different hazardous area or zones.
The hazardous area classifications differ from country to country. Hazardous locations per the Zone system are classified according to its Zone which can be gas or dust.
The Zone defines the probability of the hazardous material, gas or dust, being present in sufficient quantities to produce explosive or ignitable mixtures. Hazardous Zone categories are:
Ignitable concentrations of flammable gases or vapours are present continuously or present for long periods of time. Examples include,
- Interior of tanks
- Locations near vents
- Vapour space above closed process vessel
- Areas containing open tanks of volatile, flammable liquid
There may be ignitable concentrations during normal operating conditions or concentrations exist frequently from repair or maintenance of the equipment. The area below the surrounding elevation or grade is such vapour may accumulate therein. Failure of equipment can cause an electrical system failure with the release of flammable gas. Examples include,
- An area where the breakdown of equipment could lead to a release
- Remember that pumps and compressors can have small leaks
Ignitable concentrations of flammable gases or vapours which are not likely to occur under normal operating conditions and do so only for a short period of time Examples include,
- Storage where hazardous materials are in containers.
- Areas adjacent to Zone 1 with no hazards of its own
- Ventilation could prevent the hazard, but it could fail during a leak
An area where combustible dust or ignitable fibres and flyings are present continuously or for long periods of time.
An area where combustible dust or ignitable fibres and flyings are likely to occur under normal operating conditions.
An area where combustible dust or ignitable fibres and flyings are not likely to occur under normal operating conditions and do so only for a short period of time.