Explosive atmosphere
Definition of the explosive atmosphere
Explosive atmosphere is such mixture of the flammable matter (in the shape of dust, gas, fog or mist) which is able to explode in the mixture with air and some kind of ignition source. The effect of ignition is extremelly fast burning (oxidation) of the whole volume of the atmosphere - explosion.
Types of explosive atmospheres
Because of different states of matter have different potential of mixing with air and shaping homogenic mixtures, from the point of view of the explosion and its consequences, there are 2 types of explosive atmosphere - dust-air mixtures and gas-air mixtures.
Dust air atmospheres
Dust-air mixtures (atmospheres) are such mixtures, where there is one or more types of the dust sparsed in the air (or generally with mixture of gases where the oxygen is present too). It means that explosive dust-air mixture are not those, where only non-explosive dust is present (like cement), on the other side, there can be dust-air mixtures with matters which are imposible or very hardly ignitable, but easily explosible in the dust-air mixture (like metals - zinc, alluminium, magnesium, but during some special circumstances also steel, especially carbon steel or other metals).
Dust-air mixtures are typical by their inhomogenity in time and place. In practical conditions the ammount of dust particles in the testing sample can vary - the dust settles down and the the concentration decreases with the distnace from the source of the dust-air mixture. These chcarcteristics and changes usually are not linear and sometimes they're even hard to predict, so the biggest risk of dust-air mixtures is in their unsteadiness.
Dust-air mixtures are also typical by dependence of medium particle size ("softness" of the dust) on the explosion characteristics. The same matter in the same concentration in the air reaches more dangerous values when it has lower medium particle size. The reason is there are more particles taking part in the explosion, with bigger surface area - burning is happening on more places in the same time and the mixture burns down faster.
As dust-air mixtures are also considered those atmospheres made of dust and gas (or gases), flammable or non-flammable, where it is clear that the dust is the matter, which causes the main risk of explosion. On the other side, the risk caused by flammable or explosive gas in the mixture must be also considered, even if its concentration is so low that would be insufficient to make explosive atmosphere during normal circumstances. The presence of explosive gas in the mixture will mainly effect minimum ignition energy (ignition source of small energy can ignite small amount of gas which will continue igniting the dust).
It can be a surprise how many solids can be source of an explosive dust-air mixture. Generally we can say that there is more explosive than non-explosive dusts. Un-explosive are only those inorganic products which aren't flammable (salt, ciment). Among the explosive dusts belong for eaxmple:
- flammable minerals (fosil fuels) - coal (brown and black - brown has usually worse explosion characteristics due higher volume of sulphur), lignite
- biomass from all kinds of plants
- organic dusts generally - grain and products from grain, sugar, dry derivates from animal bodies (bone and flesh flours), feeding mixtures, agriculture dusts
- oil derivates - plastics, pharmaceuticals
- matals - especially non-ferrous metals (aluminium, magnesium, zinc and their alloys), iron, steel (especially carbon steel)
- chemicals and in basic chemistry used matters - sulphur, carbon, carbon black, resins,...
- others - glass, toner, colours, ...
Definition of the dust
Dust are, from the point of view of a dust explosion, particles of the diameter lower than 0,5 mm. It must be also considered that in the mixture of particles of lower and higher size, also bigger particles will take part in the process of explosion. The lower particles will burn faster than the bigger ones.
Fibers and pulps (of cotton, paper and other such materials) are also considered as dust in the point of explosion if it is fine enough to be ignited.
Gaseous atmospheres
Gaseous explosive atmospheres consists of at least one explosive gas and/or vapours of flammable liquids (including aerosols and mists). Their main advangate is their homogenity in closed vessels (where there is no strong airlow) which means expectable and calculable concentration which will be constant in the whole volume.
On the other side, gaseous atmospheres are much easier ignitable (with low concentration and by source of small energy, comparing to dust atmospheres).