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ATEX – the most important explosion indicators at a glance

9. November 2022

HET-Filter

Reading time: 2 min.

How do explosions occur? What does it take? How to assess and ultimately avoid the danger? These questions arise wherever explosive mixtures can arise, for example, due to the presence of dusts.

In the following article, you will find out what the criteria for assessment are and what requirements must be met.

How do explosions occur?

When dealing with the topic of ATEX, the same terms are used again and again. To get an overview of this topic, these terms should be clarified.

A brief overview of the terms and their meaning:

  • ATEX: ATmosphères EXplosibles = explosive atmospheres
  • Explosive substances: solid, liquid, pasty, gelatinous substances or mixtures that can be explosive without the oxygen in the air.
  • Explosive mixture/explosive atmosphere: mixture of flammable gases, vapors, mists, or swirled dusts with air or other oxidant.
  • Explosion: Abrupt increase in pressure and/or temperature

Important explosion indicators on the subject of ATEX

The safety characteristics describe the fire and explosion properties of a substance. They depend on the environmental conditions, the determination method as well as the substance itself.

With the explosion indicators, suitable protective measures and necessary protective devices can be calculated and selected to suit the intended use. Furthermore, these key figures are the basis for the creation of risk assessments (explosion protection document) or also for the selection of Ex zones.

The most important key figures for dust can be found below as a practical overview:

Explosion indexMeaning
Burning number BNParameter for the fire behavior of solids. Experience has shown that the spread of fire increases with increasing temperature in the case of substances that show only weak burning behavior at room temperature, i.e. the burning rate increases.
1 -> no burning, e.g. salt
2 -> short burning, e.g. malt
3 -> longer localized smouldering/burning, e.g. lactose
4 -> Spread of a smouldering fire, e.g. tobacco
5 -> Spread of an open fire, e.g. sulphur
6 -> deflagration-like burning, e.g. black powder
Glow temperature [°C]Characteristic value for the ignition temperature of a dust layer: lowest temperature of a hot surface determined under test conditions at which a dust layer of 5 mm thickness ignites.
As the thickness of the dust layer increases, the glow temperature
glow temperature decreases.
Explosion pressure Pmax [bar]Highest pressure value of a dust/air mixture in a closed container in the event of an explosion.
Explosion group IIIClassification of dusts according to their specific
ignitability, differentiation according to properties:
IIIA (combustible fluff), e.g. flock
IIIB (non-conductive dust), e.g. powder paint
IIIC (conductive dust), e.g. metal dust
Minimum ignition energy MZE [mJ]Under prescribed test conditions
(atmospheric pressure, 20°C), the smallest energy
energy sufficient to ignite the most ignitable explosive atmosphere.
atmosphere. (EN 13237)
MIE > 10,000 mJ non-flammable
MIE 10 - 10,000 mJ normally flammable
MIE 3 - 10 mJ extremely flammable
MIE < 3 mJ extremely flammable
Explosion constantK/KST value [bar *m/s]Classification value that expresses the explosiveness of a combustion
expresses. It corresponds numerically to the value for the maximum rate of pressure rise in the event of an explosion of a dust/air mixture in a 1 m³ container.
Dust explosion classExpression of the explosive capability
St 1 KST - value: < 200
St 2 KST - value: 200 - 300
St 3 KST - value: > 300
Oxygen limit concentration SGK [Vol%]Value for the highest oxygen concentration in a mixture of air, inert gas and gas at which no explosion is possible. If the SGK is undershot, it is no longer possible to initiate an explosion (too little oxygen present)."
Spontaneous combustionIgnition of a dust heap when exposed to heat from all sides and in the presence of air after previous self-heating
Auto-ignition temperature [°C]Temperature at which self-ignition occurs in dust. occurs.This depends on the type of dust, the shape andsize of the bulk material and duration of heat exposure.
Particle size/grain size [μm]Size of the particles, information about the median (50% are larger and 50% are smaller than this value). Indicator for the ignitability of the dust. As the particle size decreases, the tendency of the dust to dusts to explode, i.e. the finer the dust, the easier it is easier it is to ignite and the more violent the reaction
Ignition temperature [°C]The lowest temperature of a hot surface, determined under test conditions, at which the ignition of a flammable substance occurs as a dust/air or vapor/air mixture. The ignition temperature of a dust layer (glow temperature) differs from the ignition temperature of a dust cloud. The lower value determines the maximum permissible surface temperature.

Prerequisites for an explosion

Gefahrenfuenfeck

Explosions can be prevented. This requires knowledge of the “hazard pentagon”: this pentagon puts the causes of explosions in context. Here, eliminating one of these conditions for an explosion is enough to stop an explosion.

RequirementExplanation
1Combustible substancee.g. dust generated during production
2sufficient degree of finessThe finer the dust, the easier it is to ignite. If the grain size is < 1 mm, there is no risk of explosion. The smaller the grain size, the more likely an explosion will occur (depending on the type of dust)
3mixtureThe greater the dust density in the air, the more explosive the mixture is. The concentration is decisive here: there is a minimum and a maximum concentration limit that restrict the explosive range (if there is too little or too much dust, no explosion can take place).
4effective source of ignitionDIN EN 1127 describes various ignition sources. Hot surfaces, static electricity or mechanically generated sparks, for example, should be mentioned for the design and application of dust extraction systems or filter systems.
5Oxidizing agentsOxidizing agents are, for example, the oxygen in the air. The proportion can be reduced by inerting, which reduces the risk of explosion.

Conclusion

Depending on the type of dust, there is a greater or lesser risk of explosions occurring in a production plant. This occurrence must be avoided to protect the personnel as well as the plant and, if necessary, subsequent plants. For this purpose, knowledge of the type of dust as well as the general conditions for the formation of an explosion are of elementary importance. If it is known what will happen in the process, the risk of an explosion can be effectively eliminated in advance.

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