Area classification is a method of analyzing and classifying the environment where explosive gas atmospheres may occur, to facilitate the proper selection, installation and operation of equipment to be used safely in that environment. The classification also considers the ignition characteristics of the gas or vapour such as ignition energy and ignition temperature. Area classification has two main objectives, the determination of the type of any hazardous zone, and the extent of the zone.
The hazardous area classification of sources will be determined using calculated hazard radii together with either the physical geometry (e.g. a pit) or the shape factors, a risk-based approach will be used. The risk-based approach methodology provides a means of adjusting release frequency and hence hazard radii, to fit specific process scenarios.
Generally, process plant will constitute a Zone 2 area inside plant boundaries, within which there may be local Zone 1 and, more rarely, Zone 0 areas. Refer Hazardous Area Classification Layouts for better understanding of boundaries with respect to Zones.
The intention is to reduce, to an acceptable level, the probability of coincident existence of a flammable / explosive atmosphere and sources of ignition. This approach is implemented through:
The methodology adopted for Hazardous Area classification approach is based on below mentioned technique.
For installations or processes (due to variability of temperature, pressure, equipment and the degree and type of ventilation) the extent of flammable release that would occur may vary greatly making individual assessment necessary. This is known as the 'point source approach’.
The Point source method can be used for all situations, where the release hole size is known, the hazard radius used from the Hazard radius table (Annex C, Table C4 from EI-15 standard).
Where ‘Release Rate’ is unknown (hole size and pressure), the ‘Risk-based approach’ shall be used as outlined in Annex C, Part-2 EI-15. The Risk based approach methodology provides a means of adjusting release frequency and hence hazard radii, to fit specific process scenarios. Examples include failure of pump / compressor seals, leaks from valves and flanges, or operational error.
Generic industrial equipment handling common flammable materials may be classified directly from typical examples (Annex- D from EI-15). Examples include drilling, work over and wellhead sites, tank storage for both upstream and downstream.