Explosive atmosphere
As explosive atmosphere , a is a mixture of combustible materials defined with air. An explosive atmosphere is a special case of explosive mixtures with air as oxidizing agent and in the temperature range −20 ° C ≤ T ≤ 40 ° C and under ambient pressure conditions 980 mbar ≤ p ≤ 1,100 mbar. Gases, vapors, mists or dusts mixed with air (oxygen) are possible explosive mixtures. Since the amendment of the Hazardous Substances Ordinance and the Industrial Safety Ordinance in 2015, explosion protection is no longer geared exclusively to the dangerous, explosive atmosphere, but comprehensively to explosive mixtures. This means that oxidizing agents other than air or other temperature and pressure conditions must also be taken into account.
In an explosive atmosphere, the explosiveness depends on the substance used, its flammability and the mixture with air or oxygen. If the mixture is made with an air-oxygen mixture which has a different composition than the air mixture normally present in the environment, the oxygen content must be carefully considered. If the oxygen content falls below a certain substance-dependent value, the so-called limit oxygen concentration , this mixture cannot be ignited. This effect is used in so-called inerting , in which oxygen is replaced by an inert gas - one that is not suitable for oxidation. Furthermore, the concentration of the combustible substance in the air mixture must be between the lower and the upper explosion limit .
An explosive atmosphere can be present intentionally or unintentionally. The former is found in many areas of the production process . Here, an explosive atmosphere occurs wherever the relevant substances are used in the presence of the ambient air. These can be areas in which adhesives or paints dry out or are applied, in the vicinity of fermentation vats in alcohol production , in areas where substances are comminuted, such as in grain mills or numerous other areas. Unwanted explosive atmosphere is present when an error occurs. This can be, for example, the bursting of a gas pipeline , the unintentional opening of an outlet valve or leaks in boilers that occur due to insufficient maintenance.
Notes on practice
An ignitable mixture can be caused to explode by an ignition source . For example, smoking at a petrol station can lead to a situation in which the vapors that escape from the filler neck during a refueling process mix with the air and thus possibly form an explosive atmosphere. When a smoker draws on a cigar or lights a lighter, the ignition temperature can be reached through the increased oxygen supply in the glowing area. If the cigar embers come into contact with the air / gas mixture, it can ignite. As a result, deflagration occurs, which can result in a fire. In the case of a gasoline bomb, the initially liquid gasoline evaporates and disperses in the air. This explosive atmosphere is ignited, causing a deflagration (explosion). Whether there is an explosion therefore depends crucially on the amount of combustible material and the respective framework conditions.
The presence of such an atmosphere can be limited in time and space. For example, if gasoline vapors escape from a crashed tanker, an explosive atmosphere will develop directly at the leak point together with the ambient air. Depending on the wind speed and direction, a cloud spreads, the concentration of which drops due to the dilution effect and falls below the lower explosion limit at a certain distance. In the event of a leak in a pressure vessel, this area can extend up to a distance of 200 m, depending on the wind direction and strength.
literature
- S. Bussenius: Scientific principles of fire and explosion protection. Kohlhammer, Stuttgart / Berlin / Cologne 1996, ISBN 3-17-013867-7 .
- H. Groh: Explosion Protection. Elsevier Butterworth-Heinemann, Oxford / Expert-Verlag, Renningen 2004, ISBN 0-7506-4777-9 .
- M. Hattwig, H. Steen: Handbook of Explosion Prevention and Protection. Wiley-VCH, Weinheim 2004, ISBN 3-527-30718-4 .
- E. Lienenklaus, K. Wettingfeld: Electrical explosion protection according to DIN VDE 0165. 2nd edition. VDE-Verlag, Berlin / Offenbach 2001, ISBN 3-8007-2410-3 .
- NN: Basics of explosion protection. (= Publication no. 1213/6 / 04.04 / SD ). Cooper Crouse-Hinds GmbH, Soest 2004.
- H. u. A. Olenik: Electrical installation and equipment in potentially explosive areas. Hüthig & Pflaum, Munich / Heidelberg / Berlin 2000, ISBN 3-8101-0130-3 .