Source of release
Where can the substance escape or be generated? Examples include vents, seals, fill points, flanges, open containers, spray operations, dust collection and powder transfer points.
Zones 0, 1, 2 • Zones 20, 21, 22
Hazardous Area Classification
Identification and classification of places where explosive atmospheres may occur, documenting the basis of classification and the zone extent needed to support ignition-source control and equipment decisions.
Purpose
Hazardous area classification is about likelihood and location.
DSEAR requires areas where explosive atmospheres may occur in quantities requiring special precautions to be identified and classified. Classification is not a label applied to an entire building because flammable material is present. It is based on where a release can occur, how often or how long an explosive atmosphere may exist, and how the substance disperses or accumulates.
For gases, vapours and mists, zones are commonly designated 0, 1 or 2. For combustible dusts, zones are 20, 21 or 22. The classification then informs control of ignition sources and selection or verification of equipment suitable for the relevant hazardous area.
Classification logic
What determines a zone
Frequency & duration
How often is release expected and for how long? This is central to the grade of release and resulting zone classification.
Ventilation & dispersion
Open-air dispersion, enclosure, natural ventilation, mechanical extraction and ventilation availability can materially affect the extent and persistence of an explosive atmosphere.
Substance properties
Gas/vapour density, flash point, volatility, lower explosive limit and dust characteristics influence how a release behaves.
Geometry & obstructions
Pits, trenches, enclosed spaces, roofs, walls, vessels, ducts and nearby structures can prevent dilution or create accumulation pockets.
Operating conditions
Pressure, temperature, flow, cleaning, start-up, shutdown and maintenance modes can produce different release conditions from normal steady operation.
Zone overview
Gas/vapour and dust zones
| Atmosphere | Zone | General meaning |
|---|---|---|
| Gas, vapour or mist | Zone 0 | An explosive gas atmosphere is present continuously, for long periods or frequently. |
| Gas, vapour or mist | Zone 1 | An explosive gas atmosphere is likely to occur occasionally in normal operation. |
| Gas, vapour or mist | Zone 2 | An explosive gas atmosphere is not likely in normal operation and, if it occurs, will exist only for a short period. |
| Combustible dust | Zone 20 | An explosive dust atmosphere is present continuously, for long periods or frequently. |
| Combustible dust | Zone 21 | An explosive dust atmosphere is likely to occur occasionally in normal operation. |
| Combustible dust | Zone 22 | An explosive dust atmosphere is not likely in normal operation and, if it occurs, will persist only for a short period. |
These are simplified descriptions. Actual classification must be based on the specific release, substance and operating conditions.
Deliverable
What a classification package can contain
- Basis of classification and assumptions
- Release-source schedule
- Zone designation and extent
- Marked-up plans/elevations where required
- Relevant ventilation assumptions
- Notes on ignition-source control
- Interface with DSEAR action plan
- Limitations and information gaps requiring confirmation
Classification is not the same as equipment certification.
The zone tells you the hazardous-area environment. Equipment selection and verification then need to consider the required equipment category/EPL, gas or dust group, temperature classification, installation and condition. Where specialist electrical inspection is required, that should be undertaken by an appropriately competent Ex professional.
Authoritative basis
HSE states that DSEAR requires areas where accidental releases of dangerous substances could occur to be identified and classified according to the likelihood of a flammable atmosphere forming, with ignition sources controlled in classified areas.
Need zoning or a DSEAR review?
Send available drawings, process description, substances/SDS information and photographs. Vulcan can confirm whether a DSEAR assessment, hazardous area classification, or both are appropriate.