Dust explosion

A dust explosion is the variant of an explosion in which finely divided solid particles in the air are ignited. The result is a sudden development of force, which is based on the tendency of suddenly heated gases and vapors to expand .

Pyrotechnicians use controlled dust explosions from bear moss spores to create fire effects and deflagrations on stages.

conditions

Mixtures of dust and air are explosive if the dust consists of combustible material such as B. coal , flour , wood , cocoa , coffee , starch or cellulose . Inorganic substances and elements such as magnesium , aluminum and even iron and steel are also explosive (or at least flammable) in this form. In addition to the flammability (the ability to react exothermically with the oxygen in the air ), the small particle size of the dust is decisive, ie the explosive effects increase with decreasing size. In general, dust is only explosive if the particle size is less than 0.5 mm. As a result of the shredding, large surfaces are created, which means that the dust particles absorb heat well so that they can ignite rapidly and then oxidize extremely quickly . These effects make it possible for materials that are considered non-flammable in compact form to burn in a finely divided form. Fine steel wool z. B. burns with a bright flame, while a solid block of steel could never be ignited under normal circumstances. The oxygen content in the air and the physical state (e.g. deeply frozen liquid and therefore highly concentrated) are also decisive for ignitability . Another decisive factor for the occurrence of dust explosions is the dustiness of bulk and loose materials. In risk analysis is used when other parameters do not allow clear assessment.

In general, dust explosions are not to be feared with dust concentrations of less than 20 grams per cubic meter of air. In the case of food and feed dust, the essential prerequisites for a dust explosion are met if, depending on the fineness and substance, 60 to 2000 grams of dust per cubic meter of air come into contact with a suitable ignition source.

Various electrical or mechanical effects with sufficient temperature and energy density can serve as ignition sources . One spark can be enough to B. by pulling an electrical plug or malfunctions in electrical appliances. However, under certain circumstances, when switches and the like are operated, high-energy sparks occur even in the correct state. Another important source of danger is electrostatic charge (through charge separation). On a small scale, for example, through electrostatically effective clothing, much more often in conveyors (conveyor belts made of rubber or similar), which can generate considerable electrostatic voltages and charges through their constant friction and movement ( belt generator principle). Other ignition sources are hot surfaces (food production), grinding or friction sparks, glowing nests and the like.

Also a large amount of house dust , which often consists of 80 percent of flaked skin cells from humans and other organic material and which is e.g. B. collects in a suspended, but not airtight ceiling, can ignite in the event of a fire and trigger a dust explosion in an entire floor if the suspended ceilings are insufficiently partitioned between the rooms due to the pressure wave that stirs up further house dust.

Cubic law

The explosion constant is the parameter of the explosiveness of a dust. The " cubic law " (not to be confused with a cubic function ) describes the dependence of the maximum pressure increase over time on the volume of the container. The maximum explosion constant characterizes the maximum increase in explosion pressure over time with an optimal fuel concentration in a container with the volume : ${\ displaystyle K}$ ${\ displaystyle K _ {\ text {max}}}$ ${\ displaystyle P}$ ${\ displaystyle \ left (dP / dt \ right) _ {\ text {max}}}$ ${\ displaystyle V}$

{\ displaystyle K _ {\ mathrm {max}} = V ^ {\ frac {1} {3}} \ cdot \ left ({\ frac {dP} {dt}} \ right) _ {\ mathrm {max}} }

It corresponds to the maximum pressure increase in a 1 m³ container under the test conditions defined in VDI guideline 3673 (pressure relief from dust explosions) and is a. depending on the surface structure and the particle size distribution of the dust.

Photo sequence of a dust explosion

Dust explosion (experimental setup)
Experimental setup (1/6)
Pressure surge to stir up the flour dust (2/6)
The dust is ignited (3/6)
Dust explosion (4/6)
Dust explosion (5/6)
Dust explosion (6/6)

Prevention

Measures to prevent dust explosions are described in the field of dust explosion protection . These measures serve to minimize the risk potential and have been incorporated into current standardization projects and legislative initiatives as the results of current research. A distinction is made between provisions that are aimed at the operator of potentially explosive systems and that are necessary for the manufacture of explosion-protected equipment. In principle, the operator has to assess the hazard potential of his system and define the resulting measures.

One of the largest projects to prevent dust explosions was the research initiated by the professional association for food and restaurants, in which over 700 dust explosions in Germany were evaluated, mainly in silos , dust extraction systems , mills and conveyor systems in the food industry . Then 1200 products and product mixtures from the food and feed industry were examined for their explosion-related parameters in a dust laboratory and made available to the industry by means of a database.

Appropriate precautionary measures consist of regular monitoring of the entire equipment such. B. the regular checks of the electrical devices, the grounding of statically chargeable objects and the maintenance of the necessary cleanliness . This is particularly important because a dust layer just a few millimeters thick can lead to a dust explosion if there is a corresponding turbulence and an ignition spark occurs at the same time. In addition, all measures that are specified in the explosion protection document for the respective system must be observed .

The risk and possible sequence of dust explosions are difficult to assess. This can be explained by way of example with the help of the following scenario: Due to insufficient cleaning measures in a mill building, a flour dust layer collects on an electric motor . As a consequence of the good thermal insulation properties of the dust, the motor overheats, which ignites the dust layer. This creates embers that glow without a visible flame. Opening a door creates a flow of air that swirls the dust layer near the embers. There is a deflagration, which initially does not cause any damage. However, this deflagration causes flour dust to be whirled into the air, creating an explosive atmosphere with a large volume. The remaining embers ignite the dust-air mixture; Only this explosion causes the main damage, up to and including the destruction of the entire system.

In hard coal mining , coal dust explosions are feared. Dust explosions of this kind often arise in the mine as a result of firedamp explosions , which first stir up the ubiquitous coal dust and then ignite it. Underground coal dust explosions can be prevented by means of water reservoirs that are located on the top of the tunnel and that break down the coal dust with water when a firedamp explosion occurs.

The modified Hartmann apparatus is often used to determine the explosiveness of dusts . This consists of a cylindrical glass vessel in which the dust sample is whirled up by a defined blast of air and ignited by a spark or a filament. If a flame spread is observed visually, the dust sample is classified as capable of dust explosion.

Fire fighting

During extinguishing work , no dust may be whirled up by the water jet, as this could result in an ignitable dust-air mixture. If ignited, the subsequent explosion could further spread the fire. For this reason, the formation of dust in the vicinity of the fire area can be observed. Furthermore, when assessing the dangers of fires in dust-endangered areas, the possibility of a gas explosion is overlooked, which is caused by incomplete combustion of the dusts (e.g. carbon monoxide) and can then be ignited as an air-gas mixture.

Historical damage events

The first dust explosion to be recognized and documented as such is believed to have occurred in a storage building in Turin on December 14, 1785.

Silos at Kiel's north port after the dust explosion in 1970

An example of a dust explosion in German-speaking countries is the explosion on December 14, 1970 at Kiel's northern port with 6 dead and 17 injured.

In the Roland mill in Bremen , a small fire on February 6, 1979 triggered the largest flour dust explosion in Germany to date. A chain reaction with sometimes more violent explosions was responsible for the severity of the damage. The damage balance recorded 14 deaths and 17 injured as well as property damage of over 100 million marks, today around 116 million euros.

14 workers were killed and 36 others were injured, some seriously injured, in the explosion of the Imperial Sugar sugar refinery , one of the largest sugar refineries in the United States in Port Wentworth, Georgia, on February 7, 2008 at around 7:15 pm.

literature

Wolfgang Bartknecht: Explosions. Procedure and protective measures. 2nd, revised and expanded edition. Springer, Berlin et al. 2013, ISBN 978-3-662-07158-8 .
John Barton (Ed.): Dust Explosion Prevention and Protection. (A Practical Guide). Institution of Chemical Engineers, Rugby 2002, ISBN 0-85295-410-7 .
Siegfried Bussenius: Scientific principles of fire and explosion protection (= fire and explosion protection. Vol. 1). Kohlhammer, Stuttgart et al. 1996, ISBN 3-17-013867-7 .
Rolf K. Eckhoff: Dust Explosions in the Process Industries. 2nd Edition. Butterworth-Heinemann, Oxford 1997, ISBN 0-7506-3270-4 .
Ute Hesener: A knowledge-based system for the safety assessment of dust-processing plants (= progress reports VDI. Series 3: Process engineering. No. 508). VDI-Verlag, Düsseldorf 1997, ISBN 3-18-350803-6 .
Erich Lienenklaus, Klaus Wettingfeld: Electrical explosion protection according to DIN VDE 0165. A practical introduction to the regulations, standards and guidelines to be observed (= VDE series of standards understandable. Vol. 65). 2nd, revised edition. VDE-Verlag, Berlin et al. 2001, ISBN 3-8007-2410-3 .
Heinz Olenik, Wolf-Dieter Dose, Herbert F. Rading: Electrical installation and equipment in potentially explosive areas. Hüthig and Pflaum, Munich et al. 2000, ISBN 3-8101-0130-3 .

Individual evidence

↑ ^a ^b VDI 2263 sheet 9: 2008-05 Dust fires and dust explosions; Hazards - assessments - protective measures; Determination of dustiness of bulk materials (Dust fires and dust explosions; Hazards - assessment - protective measures; Determination of dustiness of bulk materials). Beuth Verlag, Berlin, pp. 2–3.
↑ VDI 2263: 1992-05 Dust fires and dust explosions; Dangers, assessments, protective measures (Dust fires and dust explosions; hazards, assessment, protective measures). Beuth Verlag, Berlin, p. 8.
↑ Manual of explosion protection, p.102 .
↑ Intensive filter Himenviro: Definition of explosion parameters .
↑ Hartmut Beck, Arno Jeske: Reports on dust explosions - individual results and documentation. In: Commission for keeping the air clean in the VDI and DIN: Safe handling of combustible dusts. Conference Nuremberg, 23-25. October 1996 (= VDI reports. 1272). VDI-Verlag, Düsseldorf 1996, ISBN 3-18-091272-3 , pp. 365-387.
↑ Hartmut Beck: Dust explosion protection through the ages. In: Dust - cleanliness. Air . 46, No. 4, 1986, pp. 206-210.
↑ Wilhelm Jach: Grain Dust Explosions - A New Risk of Damage?
↑ Imke Molkewehrum: Heaviest detonation since the war shakes Bremen , on weser-kurier.de
↑ WDR: Quarks & Co. - Caution, risk of explosion! , P. 7, damage: over 100 million marks ( Memento from July 5, 2010 in the Internet Archive ) (PDF; 379 kB), requested on February 5, 2011.
↑ This figure was based on the template: Inflation determined, rounded to a full million EUR and applies to the previous January.

Web links

Commons : Dust Explosion - Collection of Pictures, Videos and Audio Files