Dust explosion

A dust explosion is the variant of an explosion ignited in the air finely divided solid particles. The result is a sudden running power development, which is based on the expansion efforts of suddenly heated gases and vapors.

Conditions

Mixtures of dust and air are explosive when the dust of combustible material such as coal, flour, timber, cocoa, coffee, starch or cellulose. And inorganic materials and elements such as magnesium, aluminum, iron and steel, and even in this form explosive (or at least flammable). In addition to the flammability ( the ability to react exothermically with oxygen in the air ) is the small particle size of the dust decisive, ie the explosive effects increase with decreasing size. Through the process of crushing produces very large surfaces which reduces the dust particles absorb heat very well and thus by fire and then can oxidize very quickly. Through these effects, it is possible that even materials that are in solid form as non-combustible, may burn in this finely divided form. Fine steel wool, for example, burns with a bright flame, while a solid steel block could never be ignited under normal circumstances. Decisive for the ignitability are also the percentage of oxygen in the air and the physical state ( eg tiefstgekühlt liquid and thus highly concentrated).

For food and feed dusts the essential conditions of a dust explosion are given if meet 60-2000 grams of dust per cubic meter of air to a suitable ignition source depending on the fineness and substance.

The ignition source, various electrical or mechanical effects at the proper temperature and energy density can be used. A spark may be enough, caused by pulling an electrical plug or malfunctions in electrical equipment eg. But also in proper condition occur when operating switches and the like, under certain circumstances, to high-energy spark. Also a significant source of risk is the electrostatic electricity, in the small, for example, by electrostatic effective clothing, much more but also in funding ( conveyor belts made of rubber or the like. ), Through their ongoing friction and movement very significant electrostatic voltages and charges can generate ( baseband generator ). Other sources of ignition hot surfaces ( food production ), grinding or friction sparks, burning embers, etc.

Also a large amount of house dust that is often 80 percent exfoliated skin cells of humans and further organic material and collects in a suspended, but not airtight ceiling, for example, can ignite in case of fire and by the pressure wave, the other house dust stirred up, trigger a dust explosion in an entire floor with insufficient separation of suspended ceilings between rooms.

Photo sequence of a dust explosion

Prevention

Measures for the prevention of dust explosions are described in the field of dust explosion protection. These measures are designed to minimize the hazard potential and flowed as results of the current research in current standardization projects and legislative initiatives. We distinguish between provisions which are aimed at the operators of hazardous installations and which are necessary for the production of explosion protected apparatus. In principle, the operator has to assess the hazard potential of its investment and to define the resulting conditional measures.

One of the biggest projects for the prevention of dust explosions were triggered by the professional association of food and restaurants research, in which more than 700 dust explosions have been evaluated primarily in silos, dust collectors, mills and conveyors in the food industry in Germany. Then 1200 products and product mixtures of the food and feed industry was examined for its explosion characteristics and made available via a database of the industry in a laboratory dust.

Suitable preventive measures consist in the regular monitoring of entire systems such as regular inspections of electrical equipment, the grounding statically chargeable items and respecting the necessary cleanliness. This is especially important since even a few millimeters thick layer of dust with appropriate turbulence and the simultaneous occurrence of a spark may cause a dust explosion. In addition, all measures must be observed, which are enshrined in the explosion protection document for each facility.

Danger and possible sequence of dust explosions are difficult to assess. This is explained by way of example with the aid of the following scenarios: Insufficient cleaning action in a mill building, a flour layer of dust accumulated on an electric motor. As a result of the good thermal insulation properties of the dusts it comes to engine overheating, which ignites the dust layer. This creates embers that glow without visible flame. By opening a door, an air flow is created, which results in a swirling of the layer of dust near the glowing embers. It comes to a deflagration, which initially inflicts no damage. However, it is spun by this explosion of flour dust in the air, so now created an explosive atmosphere with large volume. The remaining embers ignite the dust-air mixture; only this explosion depends on the brunt of the damage, including the destruction of the entire plant.

In the coal mining of coal dust explosions are feared. Such dust explosions occur in the mine frequently by explosion, the first kicking the ubiquitous coal dust and then ignite. Prevent settle beat the coal dust with water underground coal dust explosions by water templates are located on the lug cap and bursting when passing through a firedamp explosion.

To determine the explosiveness of dusts frequently the modified Hartmann apparatus used. This consists of a cylindrical glass vessel in which the dust sample whirled through a defined air puff and is ignited by a spark or a filament. If visual flame propagation is observed, the dust sample is to be classified as a dust explosion.

Fire-fighting

Case of fire fighting no dust can be stirred up by the water jet, as this could form an explosive dust - air mixture. By firing the subsequent explosion could lead to a further spread of the fire. For this reason, the formation of dust in the vicinity of the fire area can be observed. Furthermore, overlook the possibility of a gas explosion in the assessment of the risks of fires in dust atmospheres caused by incomplete combustion of the dust (eg carbon monoxide ), which can then be ignited as air -gas mixtures.

An example is the explosion on December 14, 1970 at the Kiel northern harbor with 6 dead and 17 wounded. In the Bremen Roland mill a small fire on February 6, 1979 sparked the biggest flour dust explosion in Germany. For the severity of the damage a chain reaction with partially violent explosions was responsible. The loss figures reported 14 dead and 17 injured and damage to property in excess of 100 million marks, today about 112 million euros.