ATEX explosions manifest the dynamics of combustion of a combustible product with the oxygen in the ambient atmosphere. Explosivity parameters quantify these dynamics. However, one study identifies a lack of knowledge ofproduct explosivity as one of the main causes of recent explosions.
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WHAT ARE EXPLOSIVE PROPERTIES?
The explosive characteristics of products help employers to assess the risk of ATEX explosions in the workplace. They help define the :
- Probability of ATEX formation, ignition and explosion,
- ATEX hazardous area categories,
- Categories of ATEX equipment or protection systems to be installed.
The characterization of combustible products provides information on protection and safety measures for workers. It is therefore an employer’s obligation. To measure the safety of his product, the employer can consult databases on product explosivity (e.g. CarATEX database). These data must be used with discretion, particularly for dusts, which are highly sensitive to operating conditions. In case of doubt, the employer should have a sample taken in the work area analyzed (e.g. INERIS).
Explosive characteristics determine the product risk contributing to the ATEX explosion risk. This is followed by an analysis of process and organizational risks, which contribute to the risk of explosion. They are a prelude to measures to prevent andmitigate explosions by calibrating the explosive properties of the combustible product, in particular :
- theability of the combustible product to form ATEX, by diffusing into the surrounding atmosphere. The characteristics describing this property are used to eliminate or limit ATEX.
- theflammability of the combustible product in contact with a source of ignition. The resulting characteristics help to identify and eliminate ignition sources.
- the explosive force of the combustible product, when the explosion occurs. Explosive force characteristics are used to: reduce the effects of an explosion, select explosion-resistant equipment, eliminate the beginnings of explosions by detecting them.
HOW DOES THE AMBIENT ATMOSPHERE AFFECT THE EXPLOSIVE CHARACTERISTICS OF MY PRODUCT?
The risk of an ATEX explosion begins as soon as the explosive product diffuses into the surrounding atmosphere. In other words, as soon as the reagents of the violent combustion that is an explosion are simultaneously present:
- combustible product (fuel),
- and oxygen in the atmosphere (the oxidizing agent in most work atmospheres).
Oxygen content specifies the capacity of the ambient atmosphere to fuel the explosion. It influences the explosive potential of the ATEX, and hence the explosive characteristics of the product. A preventive measure would be to act on the quantity of oxidizer. For example, reducing the oxygen content can lower the explosive range of the fuel making up the ATEX.
The physical characteristics of the atmosphere also influence the explosive properties of the combustible product. Temperature, pressure and atmospheric humidity, particularly in the case of dusts, influence the explosive properties of combustible products. It should be noted that the explosive characteristics of combustible products apply for “normal atmospheric conditions”, i.e. :
- 21% oxygen content
- Temperature range between : -20°C and 60°C
- Pressure range: 80kpa (0.8bar) to 110kpa (1.1bar)
These standards are based on ATEX operating procedures, which determine the values of explosive parameters; data supplied by explosive product databases. However, depending on operating circumstances, the conditions of an oxidizing atmosphere may deviate from these values, particularly inside or near equipment in operation. Hence the need for safety margins in the use of available data.
WHAT PRODUCT CHARACTERISTICS SHOULD BE STUDIED TO ASSESS THE RISK OF EXPLOSION?
First of all, the physical state of the material determines its explosive parameters. Thus, among the products used or emitted into the ambient atmosphere, the employer will distinguish :
- Gases and flammable liquidsemitting vapours and mists
- Combustible solids: dust, powder.
Main explosion characteristics used to measure the risk of explosion and select preventive measures :
| Explosion hazard formation conditions | Combustible product properties | Characteristics of the combustible product as a gas or vapour | Characteristics of combustible product as dust or powder | Combustion atmosphere characteristics | Prevention and safety measures |
|---|---|---|---|---|---|
| Simultaneous presence of fuel and oxidizer. | Ability to form an ATEX | – Lower Explosive Limit: LEL – Flash Point: PE – Lower Explosion Point: PIE – Chemical incompatibility with other products | – Minimum Explosive Concentration: MEC – Particle size – Chemical incompatibility with other products | – Oxygen content – Temperature – Pressure – Humidity level | Eliminating or limiting ATEX |
| Additional presence of an ignition source | Flammability Explosion sensitivity | – Minimum ignition energy: EMI – Auto-ignition temperature: TAI | – Minimum ignition energy: EMI – Minimum ignition temperature: TMI | – Oxygen content – Temperature – Pressure – Humidity level | Eliminate ATEX ignition sources |
| Conjunction of explosive characteristics at explosive level favoured by containment | Explosive force | – Maximum pressure generated by explosion PMAX – Maximum speed of pressure rise – Explosion index Kst | – Maximum pressure generated by explosion PMAX – Maximum speed of pressure rise – Explosion index (Kg) | – Oxygen content – Temperature – Pressure – Humidity level | Reducing the effects of |
Each stage in the formation of the explosion hazard corresponds to one of the product’s explosive properties. A property is specified by a set of parameters which depend on both the physical nature of the product, and the conditions of the oxidizing atmosphere. However, certain explosive parameters influence each other, and can therefore contribute to more than one explosive property. Furthermore, the presence of several products in the oxidizing atmosphere can cause explosion parameters to evolve towards different thresholds: more explosive or less explosive, depending on the nature of the products present.
Parameter values quantify the probability of explosion risk. And hence the protection and safety measures to be adopted. Employers begin to implement ATEX regulations with parametric knowledge of the explosive potential of their products.
