How do explosions occur? What is required? How do you assess and ultimately avoid danger? These questions arise, for example, wherever explosive mixtures can occur due to the presence of dusts.
The following article explains the criteria for assessment and the requirements that must be met.
When dealing with the subject of ATEX, the same terms are used time and again. To get an overview on this subject, the following terms should be clarified.
Terms and their meaning in short:
The safety-technical coding describes the fire and explosion properties of a substance. They depend on the environmental conditions, the determination method as well as the substance itself.
The explosion codes are used for calculating suitable protection measures and necessary protection devices, and selected to suit the intended use. Furthermore, these codes are the basis for the creation of hazard assessments (explosion protection document) or also for the selection of Ex zones.
The most important codes for dust are listed below in the overview:
|Combustion factor CF||Parameter for the fire behavior of solids. Based on experience, the fire spread of substances that show low combustion behavior at room temperature increases as the temperature rises, meaning, the combustion factor increases.
1 -> No combustion, e.g. salt
2 -> Brief combustion, e.g. malt
3 -> Prolonged local smouldering/burning, e.g. lactose
4 -> Spreading of a smouldering fire, e.g. tobacco
5 -> Spreading of an open fire, e.g. sulfur
6 -> Deflagrative combustion, e.g. black powder
|Smoulder-temperature [°C]||Lowest temperature of a hot surface, determined under experimental conditions, at which a dust layer of 5 mm thickness ignites.
With increasing layer thickness of the dust, the smoulder-temperature decreases.
|Explosion pressure Pmax [bar]||Highest pressure value of a dust/air mixture in a closed container during an explosion.|
|Explosion group III||Classification of dusts based on their specific ignitability, differentiation according to properties:
IIIA (combustible lint), e.g. flock
IIIB (non-conductive dust), e.g. powder paint
IIIC (conductive dust), e.g. metal dust.
|Minimum ignition energy
|The lowest energy determined under prescribed test conditions (atmospheric pressure, 20°C) that is sufficient to ignite the most ignitable explosive atmosphere. (EN 13237)
MZE > 10.000 mJ not inflammable
MZE 10 - 10.000 mJ normal inflammable
MZE 3 - 10 mJ very inflammable
MZE < 3 mJ extremely inflammable
K/KST value [bar *m/s]
|Classification value that expresses the explosiveness of a combustion. It corresponds numerically to the value for the maximum speed of pressure increase in the event of an explosion of a dust/air mixture in a 1 m3 container.|
|Dust explosion class||Indication of explosiveness
St 1 KST value: < 200
St 2 KST value: 200 - 300
St 3 KST value: > 300
|Limiting oxygen concentration LOC [vol%]||Value for the highest oxygen concentration in a mixture of air, inert gas, and gas at which no explosion is possible. If the LOC is fallen below, it is no longer possible to initiate an explosion (too little oxygen present).|
|Spontaneous ignition||Ignition of a dust fill when heat is applied from all sides and air is present after previous self-heating.|
|Spontaneous-ignition temperature [°C]||Temperature at which spontaneous ignition occurs with dust. Dependent on the type of dust, the shape and size of the fill, and the duration of heat exposure.|
|Particle size/grain size [μm]||Size of particles, indicated by the median (50% are larger and 50% are smaller than this value). Parameter for the ignitability of the dust.
With decreasing grain size, the tendency of dusts to explode increases, i.e. the finer the dust, the easier it is to ignite and the more severe 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 (smoulder temperature) differs from the ignition temperature of a dust cloud. The lower value determines the max. permissible surface temperature.|
Explosions can be prevented. Knowledge of the "hazard pentagon" is a precondition for this: This pentagon puts the causes of explosions in a context. Here, eliminating one of these conditions for an explosion is enough to stop an explosion.
|1||Combustible substance||e.g. dust occurring from a production process|
|2||Sufficient degree of fineness||The more fine the dust, the easier it is to ignite. If the grain size is < 1mm, there is no risk of explosion. The smaller the grain size becomes, the more likely an explosion will occur (depending on the type of dust).|
|3||Mixture||The greater the dust density in the air, the more explosive the mixture. Here, the concentration is decisive: There is a minimum and a maximum concentration limit which defines the range of explosive capability (if there is too little or too much dust, an explosion cannot occur).|
|4||Effective source of ignition||The German standard DIN EN 1127 describes various ignition sources. For the design and application of dedusting or filter systems, examples include hot surfaces, static electricity, or mechanically generated sparks.|
|5||Oxidizing agents||An oxidizing agent is, for example, is the oxygen in air. The level can be lowered by inertization, which will contain the danger of explosion.|
Depending on the type of dust, there is a more or less high risk of explosions occurring in a production plant. Such occurrence must be avoided to protect the personnel as well as the plant and, if necessary, downstream equipment and machinery. For this, knowledge of the type of dust as well as the general conditions for the formation of an explosion are of elementary importance. If what happens in the process is known, then the risk of an explosion can be effectively eliminated beforehand.