How do you classify a hazardous area?
When dealing with explosive atmospheres a hazardous area is defined as an “area in which an explosive atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment” [AS/NZS 60079.0]. But how are these hazardous areas classified?
1. Identify Flammable Materials
The first step is to identify flammable substances (gases, liquids and dusts) which may be present. Key data for these materials can then be identified, including flashpoints, autoignition temperatures, and gas sub-group. This information can be found in the Material Safety Data Sheet (MSDS) or AS/NZS/IEC 60079.20.1.
Gas groups and temperature class of your hazardous zones can be assigned using this information.
2. Identify Sources of Release
A source of release can exist during normal operation of plant, or during abnormal situations such as equipment or process failures. After the sources of release are identified, they can be classified into three grades, with corresponding hazardous area zones:
- Continuous – a flammable atmosphere will be present continuously or for long periods. Commonly resulting in a Zone 0 or Zone 20, for gases and/or vapours or dust atmospheres respectively
- Primary – a flammable atmosphere is likely to be present occasionally and for a short duration during normal operation. Commonly resulting in a Zone 1 or Zone 21, for gases and/or vapours or dust atmospheres respectively
- Secondary – present in abnormal or failure condition. Commonly resulting in a Zone 2 or Zone 22, for gases and/or vapours or dust atmospheres respectively
3. Determine the size and shape
The extent of a hazardous area zone is determined by the following factors:
- Release rate which will depend on the geometry, velocity and concentration of the source of release.
- Ventilation which will influence the persistence and concentration of the release. (This may also impact the grade of the zone).
- Lower explosive limit (LEL) of the material.
- Density of the material in particular if it is heavier or lighter than air will influence the shape of the zone e.g. a heavier than air material will most times have a zone that extends to ground level from the source of release.
If you need help with hazardous are classification, design, inspection or dossier management contact us today we’d love to help.