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Eingeladener Vortrag
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Bestimmung der Sauerstoffgrenzkonzentration von Gasen und Dämpfen - Normungsvorhaben bei CEN/TC 305
(2002)
Band 2 beinhaltet experimentell (im Wesentlichen nach DIN 51 649 bzw. EN 1839) ermittelte Explosionsbereiche von 158 Brenngas/Inertgas/Oxidator-Gemischen. Die Messpunkte sind als Zahlenwerte tabellarisch zusammengestellt und die entsprechenden Explosionsbereiche grafisch in Dreiecksdiagrammen sowie in karthesischen Koordinatensystemen dargestellt. Mit Hilfe der Diagramme ist es möglich, für beliebige Zusammensetzungen der Gemische zu ermitteln, ob sie explosionsfähig sind bzw. welche Zusammensetzungen im Bereich der totalen oder partiellen Inertisierung liegen. Zusätzlich sind die aus den Explosionsbereichen abgeleiteten folgenden Grenzwerte tabellarisch angegeben: Explosionsgrenzen, Sauerstoffgrenzkonzentration, minimal notwendiger Inert-Gasanteil im Inertgas/Oxidator-Gemisch und maximal zulässiger Brenngasanteil im Brenngas/Inertgas-Gemisch.
International and European dangerous substances and dangerous goods regulations refer to the standard ISO 10156 (1996). This standard includes a test method and a calculation procedure for the determination of the flammability of gases and gas mixtures in air. The substance indices for the calculation, the so called Tci values, which characterise the fire potential, are provided as well. These ISO Tci values are derived from explosion diagrams of older literature sources which do not take into account the test method and the test apparatus. However, since the explosion limits are influenced by apparatus parameters, the Tci values and lower explosion limits, given by the ISO tables, are inconsistent with those measured according to the test method of the same standard. In consequence, applying the ISO Tci values can result in wrong classifications. In this paper internationally accepted explosion limit test methods were evaluated and Tci values were derived from explosion diagrams. Therefore, an open vessel method with flame propagation criterion was favoured. These values were compared with the Tci values listed in ISO 10156. In most cases, significant deviations were found. A detailed study about the influence of inert gases on flammability is the objective of Part 2.
Ternary systems, which contain flammable gas, inert gas and air, were studied in order to give the user an evaluation of the ISO 10156 calculation method for the flammability of gas mixtures. While in Part 1 of this article the fire potential of flammable gases was the focal point, the influence of inert gases on the flammability of gas mixtures was studied in Part 2. The inerting capacity of an inert gas is expressed by the dimensionless K value, the so-called coefficient of nitrogen equivalency. The experimental determination of K values is demonstrated by using explosion diagrams. The objective of this study was to compare the estimated results, given by ISO 10156, with measurements of explosion ranges based on the German standard DIN 51649-1, given by CERN and CHEMSAFE. The comparison shows that ISO 10156, Table 1, supplies conservative K values, which can be regarded as safe in all cases. Nevertheless, in a number of cases ISO underestimates the inerting capacity, so that non-flammable gas mixtures are considered flammable.
The knowledge of the explosion limits of biogases in air is necessary to define explosion protection measures. Biogases from agricultural plants vary however in their composition, so that for each gas composition the explosion limits would have to be measured. Frequently the explosion limits of biogas are therefore referred in the safety-relevant literature only to the methane fraction of the gas mixture, without indicating this additionally. This leads to the fact that the explosion ranges for the total mixture consists of methane, carbon dioxide and further residual gases are falsely specified.
For this reason a simple method is presented in the following, which show, how the explosion limits of a biogas can be determined from its methane content using an explosion diagram of methane/carbon dioxide/air system. The explosion diagram was measured according to the German Standard DIN 51649-1.
The presented calculation method supplies however correct results only if the nitrogen fraction in the biogas does not exceed approx. 5 % by volume.