ATEX - Explosive Atmospheres

To prevent the risks associated with the presence of explosive atmospheres (ATEX) in the workplace, the French Labor Code for instance defines the ATEX regulations initially resulting from two European directives:

• Directive 1999/92/EC of 16 December 1999 for worker safety and;
• Directive 2014/34/EU of February 26, 2014 for equipment intended for use in explosive areas has been implemented.

Directive 1999/92/EC requires the employer to:

• Assess the risks of explosion of an ATEX on its site,
• Classify hazardous locations into zones,
• Take technical and organizational explosion protection measures,
• Ensure the compliance of the installation of new equipment in its industrial environment according to Directive 2014/34/EU,
• Coordinate the various stakeholders working on its site
• Have updated the Explosion Protection Documen. 

Directive 2014/34/EU requires manufacturers to only market devices or equipment that comply with the requirements of this same directive if they are intended for use in explosive atmospheres (ATEX). The essential safety requirements of this directive are to use integrated safety against explosions from the design stage, to affix a CE marking to the product, to draw up a written CE declaration of conformity and to produce an instruction manual.

As mentioned above, the regulations require the employer to identify the areas of the workplace where explosive atmospheres (ATEX) can form. These result from a mixture in the air of a combustible substance in the form of gas/vapours or dust, the ignition of which instantly propagates combustion to the entire unburned flammable mixture. For this, it is necessary that the conditions of the explosion hexagon are met.

Indeed, for an explosion to take place, six conditions must be met simultaneously, the first three being those of the fire triangle:

• combustible products,
• fuel,
• source of ignition,
• suspended products,
• explosive range,
• confinement.

The methodology for classifying ATEX zones is described in standards EN 60079-10-1 for gaseous ATEX and EN 60079-10-2 for dusty ATEX. The general methodology consists in describing the activities and premises presenting an ATEX risk by identifying the products at the origin of an ATEX zone; potential sources of leaks (flanges, valves, valves, etc.), leak conditions (infrequent, frequent, permanent, etc.), dispersion conditions (leak in an open or closed environment, natural or forced ventilation, reliable ventilation system or not…), then to characterize the zone generated (type and extent of the zone).  At this stage, a table of ATEX sources must be produced containing all the information needed to define and characterize all the ATEX zones.

There are three types of ATEX zones:

• Zone 0 (gas/vapours) or zone 20 (dust): Explosive atmosphere present continuously or for long periods of time during normal operation = permanent, long-lasting or frequent danger,
• Zone 1 (gas/vapours) or zone 21 (dust): Explosive atmosphere present occasionally, during normal operation = occasional hazard,
• Zone 2 (gas/vapours) or zone 22 (dust): Explosive atmosphere present accidentally, in the event of a malfunction or for short periods = rare or short-lived hazard.

Depending on the product in question, the ATEX zone must also be characterized by the temperature class and the gas group (for gases).

The temperature class is dependent on the autoignition temperature of the product at the origin of the ATEX zoning. The temperature classes are defined as follows:

Note that the characterization of the area will have a direct impact on the type of equipment to be installed (see next chapter)

The extent of an area can be set in 2 ways:

• Qualitative method
• Quantitative method

The qualitative method consists in basing oneself on extents of area already known or defined in guides or standards. 

The quantitative method consists of using zone extents calculated according to the method described in particular in standard EN 60079-10-1 for gaseous ATEX.

Once the sources of ATEX zoning have been identified and the ATEX zones characterized, they should be identified on the site. First of all, it is necessary to represent them on a plan of the site. Depending on the size of the site, several plans can be made. The idea is to have a precise view of all areas.

Then, in order to identify the areas at risk of explosion called ATEX zone on site, it is necessary to delimit them by a sign (on the ground for example) accompanied by the following pictogram:

After defining the ATEX zones, it is necessary to identify ignition sources. Indeed, an explosion will occur in case of presence of an ignition source. The main sources of ignition observed in industrial environment are :

• Electrostatic
• Thermal
• Electrical installation
• Mechanical friction
• Flame
• Hot surfaces
• Hot works
• Lightning

 13 sources of ignition are known and listed in the standard NF 1127-1. Once these ignition sources identified, one of the main measure to prevent explosion is to eliminate them when it is feasible: moving the source, installation of suitable electrical equipment, hot works forbidden…

When the analysis identifies a risk of gas or dust explosion, it is essential for the protection of people and facilities:

• To minimize this risk by limiting as reasonable as possible the volume of potential explosive mixture
• To eliminate this risk by preventing the explosion to occur and thus by eliminating the source of ignition 
• To limit the consequences of explosion by means of prevention and protection measures

In this frame, the implementation of technical and organizational measures is necessary. For instance :

• Selection of suitable electrical material to be used in the zone where it is installed. 
• Installation of non-electrical materials satisfying norms EN 1127-1 for hot surfaces, EN 13463-1 for electrostatic loads and EN14986 for air ventilation systems
• Replacement of products by non flammable products 
• Equipment grounding 
• Individual Protection Equipement and work rules
• Procedure and rules (cleaning, work permit, no smoking…)
• Hazardous areas signage
• Personnel training and information

The overall procedure (assessment and prevention of explosion risk) has to formalized into a document called “Explosion Protection Document. Its contents is driven for instance in France by article R.4227-52 of labor code. This document has to be included in the Occupational Risk Assessment of the company and updated every years specifically when :

• Changes of products used, process facilities or work organization
• Information about risk evaluation on a specific unit is obtained

It is recommended to consult any key people to draw up this document. The document is finalized under the responsibility of employer and submitted for comment to personnel representation entity.

The marking of ATEX equipment indicating the conformity of equipment was introduced with directive 94/9/EC and evolved with directive 2014/34/EU which requires manufacturers to market only compliant devices or equipment. the requirements of the directive, if they are intended for use in explosive atmospheres (ATEX) due to gases, vapors or dust.

Devices falling within the scope of the European directive and meeting the essential health and safety requirements are identified by a marking plate.

This is broken down into several parts:

• The logo CE for compliance,
• The number of the notified agency,
• The specific explosion protection mark (ATEX directive),
• The place of use: I for mining, II for surface industries such as chemicals and petrochemicals,
• The ATEX zone concerned: 1 for zones 0 and 20, 2 for zones 1 and 21, 3 for zones 2 and 22,
• The type of area: G for gas or steam zones, D for dusty areas,
• The standard to which the equipment meets: E for CENELEC, Ex for IEC (international),
• The type of security:  for explosion proof, e for increased safety, ai Where ib for intrinsic safety,
• The reference gas (for the gas zones): I for methane, IIA for propane, IIB for ethylene, CII for hydrogen and acetylene,
• Maximum surface temperature: T1 = 450 °C, T2 = 300 °C, T3 = 200 °C, T4 = 135 °C, T5 = 100 °C, T6 = 85 °C

Figure 1 - Illustration du marquage ATEX