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MHI Risk Engineers

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Hazardous Area Classification for Flammable Gases and Vapours

Area classification may be carried out by direct analogy with typical installations described in established codes, or by more quantitative methods that require a more detailed knowledge of the plant. The starting point is to identify sources of release of flammable gas or vapour. These may arise from constant activities; from time to time in normal operation; or as the result of some unplanned event. In addition, inside process equipment may be a hazardous area, if both gas/vapour and air are present, though there is no actual release.

Catastrophic failures, such as vessel or line rupture are not considered by an area classification study. A hazard identification process such as a Preliminary Hazard Analysis (PHA) or a Hazard and Operability Study (HAZOP) should consider these abnormal events.

Zoning

Hazardous areas are defined as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers". In this context, 'special precautions' is best taken as relating to the construction, installation and use of apparatus.

 

Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur. The main purpose is to facilitate the proper selection and installation of apparatus to be used safely in that environment, taking into account the properties of the flammable materials that will be present.

Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:

  • Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods;
  • Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;
  • Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.

Various sources have tried to place time limits on to these zones, but none have been officially adopted.

When the hazardous areas of a plant have been classified, the remainder will be defined as non-hazardous, sometimes referred to as 'safe areas'.

The zone definitions take no account of the consequences of a release. If this aspect is important, it may be addressed by upgrading the specification of equipment or controls over activities allowed within the zone.