Hazardous Area Classification: ATEX Zones

What Is Hazardous Area Classification?

Hazardous area classification is the engineering process used to identify and categorize locations where an explosive atmosphere may occur due to the presence of flammable gases, vapours, mists, or dusts mixed with air.
In Europe, this process is commonly referred to as ATEX zone classification.

The purpose of hazardous area classification is not to assess ignition sources or protective measures, but to define where an explosive atmosphere may realistically exist. This classification then forms the basis for equipment selection, operational controls, and further risk assessments.

In Europe, hazardous area classification is regulated under the ATEX framework, but the underlying principles are common to all international approaches to explosion risk management.

Formation of Explosive Atmospheres: Gases and Vapours

An explosive atmosphere may exist when a flammable gas, vapour, mist, or dust is released and mixes with air within its flammability limits.

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In industrial environments, this situation can occur during normal operation, start-up, shutdown, or foreseeable abnormal conditions, depending on the nature of the process, the substances handled, and the effectiveness of containment.

For gases and vapours, the key factor is the potential for release and dispersion into the surrounding air. Even relatively small leaks, if occurring in confined or poorly ventilated areas, can lead to the formation of a flammable mixture.

The same principle applies to combustible dusts. Fine particles, when dispersed in air, may form explosive atmospheres under certain operating or maintenance conditions, even if the process normally handles solids in a non-dispersed state.

These conditions illustrate how explosive atmospheres can realistically form in industrial environments under foreseeable operating situations.

Frequency and Duration of the Explosive Atmosphere

Once the potential for the formation of an explosive atmosphere has been established, hazardous area classification is based on how often such an atmosphere may occur and how long it is likely to persist.

These two parameters — frequency and duration — are evaluated considering foreseeable operating conditions, including normal operation, start-up, shutdown, and reasonably expected abnormal situations. The analysis is not limited to steady-state operation, but reflects the real way industrial processes are operated.

An atmosphere may be present continuously, occur repeatedly during normal operation, or appear only occasionally and for short periods. Each of these situations represents a different level of exposure to explosion risk and therefore requires a different classification outcome.

Hazardous Area Zones: Classification Based on Frequency and Duration

Hazardous areas are classified into zones by translating the frequency and duration of the explosive atmosphere into discrete categories. These zones provide a practical way to distinguish areas where an explosive atmosphere is present continuously, occurs during normal operation, or appears only occasionally.

For gases, vapours, and mists, three zones are defined:

• Zone 0
  An area in which an explosive atmosphere is present continuously, for long periods, or frequently.
• Zone 1
  An area in which an explosive atmosphere is likely to occur in normal operation.
• Zone 2
  An area in which an explosive atmosphere is not likely to occur in normal operation, and if it does occur, it will persist for a short period only.

For combustible dusts, a similar approach is used, with zones reflecting how often a dust cloud may be present:

• Zone 20
  An area in which a cloud of combustible dust is present continuously, for long periods, or frequently.
• Zone 21
  An area in which a cloud of combustible dust is likely to occur occasionally in normal operation.
• Zone 22
  An area in which a cloud of combustible dust is not likely to occur in normal operation, and if it does occur, it will persist for a short period only.

This zoning system allows hazardous areas to be classified consistently based on realistic process behaviour, providing a clear reference for subsequent equipment selection and safety measures.

Directive 1999/92/EC of the European Parliament and of the Council (ATEX Workplace Directive)

Implications of ATEX Area Classification

Once an area is classified as ATEX, the result of the classification becomes a mandatory technical reference for all equipment and instrumentation installed in that area.

The hazardous area classification is the responsibility of the employer, as defined by “Directive 1999/92/EC (ATEX Workplace Directive)”, who must identify where explosive atmospheres may occur based on process conditions, operating scenarios, and foreseeable modes of operation.

Equipment and instrumentation installed within the classified area must be suitable for the defined zone, in accordance with the applicable ATEX requirements. The responsibility for equipment conformity and certification lies with the equipment manufacturer or supplier, as defined by “Directive 2014/34/EU (ATEX Equipment Directive)”.

Where Hazardous Area Zoning Applies

Hazardous area classification applies to any location where an explosive atmosphere may realistically occur, regardless of whether the space is inside or outside equipment.

The zoning process is not limited to open plant areas or equipment surroundings. It also includes internal volumes where flammable substances and air may coexist under foreseeable conditions. Typical examples include vessels, pumps, piping systems, seals, and enclosed spaces that may be exposed to air during normal operation or specific operating phases.

Hazardous area zoning must also consider non-steady-state conditions, such as start-up, shutdown, venting, draining, and maintenance activities. During these phases, containment conditions may differ significantly from normal operation, allowing flammable atmospheres to form even if they are not present during steady operation.

The classification is therefore driven by process behaviour and operating scenarios, not by equipment type alone. An area is classified whenever the formation of an explosive atmosphere is credible under realistic and foreseeable conditions, independent of any protective measures that may later be applied.

Conclusion

Hazardous area classification is a structured engineering activity aimed at identifying where and under what operating conditions an explosive atmosphere may exist. By analysing the realistic formation of flammable atmospheres and translating their expected frequency and duration into zones, engineers can describe hazardous areas in a clear and consistent way.

Understanding how zoning is derived — and where it applies — is therefore essential for ensuring that explosion risks are addressed in a rational, systematic, and technically sound manner.

Ing. Ivet Miranda

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