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Eingeladener Vortrag
- nein (55)
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.
Für den Explosionsschutz in Biogasanlagen ist es erforderlich, die Explosionsgrenzen des Gases in Luft zu kennen. Biogase aus landwirtschaftlichen Anlagen haben jedoch eine schwankende Zusammensetzung, sodass für jede Gemischzusammensetzung die Explosionsgrenzen experimentell bestimmt werden müssten. Dies ist i. d. R. mit erheblichem Aufwand verbunden. In diesem Beitrag wird eine Methode vorgestellt, bei der mithilfe von zwei Explosionsdiagrammen die Explosionsgrenzen von Biogas mit bekannter Zusammensetzung abgeschätzt werden können. Dabei werden neben dem Methan- und Kohlendioxidanteil des Biogases auch mögliche Anteile von Wasserdampf berücksichtigt.
The presentation will discuss the difference between EU and US standards for the determination of explosion (flammability) limits and limiting oxygen concentration. Small differences observed in measured values can be traced back to the different test apparatuses and criteria. The discrepancies can be much greater in the case of limiting oxygen concentration because of the high amount of inert gases and the corresponding low laminar burning velocities. The paper describes some examples and the influence of the chosen criteria on the results. The European and US standards use the criteria of flame propagation in open test vessels and of pressure rise in closed ones. The examples discussed show that flame propagation is still possible at very small pressure rise values, as observed much below the pressure rise criterion of usual standards. However, flame propagation in a process plant can cause an accident or explosion and must be avoided. Therefore, the flame propagation criterion is recommended to be used in chemical safety engineering. The European safety database CHEMSAFE contains expertevaluated safety data for cases where the determination method and criteria are known. Flammability characteristics based on the pressure rise criterion may suffice in certain cases, e.g. for explosion protection in closed vessels without any connecting pipes.
The limiting values of fuel concentration in a flammable fuelair mixture are the LEL (lower explosion limit) and UEL (upper explosion limit). The addition of an inert component to fuel/air mixtures determines the increase of LEL and decrease of UEL, until these values finally merge at the inerting point. The maximum oxygen amount of a non-flammable fuelairinert mixture is the LOC (limiting oxygen concentration), an important safety characteristics.
The investigation of a comprehensive set of flammability data at elevated temperatures and ambient pressure taken from literature sources was made for systems containing nitrogen, carbon dioxide and water(vapour) as inert components, at 100°, 200° and 250 °C. The adiabatic flame temperatures at LEL (CAFTLEL) and LOC (CAFTLOC) were calculated by taking into account the dissociation of gases within the flame. A linear correlation of CAFTLOC versus CAFTLEL was empirically derived for the examined systems. The slope and intercept of the correlation are dependent on temperature and on nature of the inert gas. The correlation allows the development of a simple procedure for estimating LOC, when the LEL of fuelair and the equivalence ratio of the fuelairinert mixture at the inerting point are known.
Knowing the scarce information concerning the flammability of fuelairinert mixtures at temperatures higher than ambient, the proposed procedure brings about an useful tool for estimation of LOC.
The limiting oxygen concentration (LOC) of fuel / air / inert premixed gaseous systems are usually determined from measurements of explosion limits at progressive dilution with inert gas of fuel / air mixtures, which is a long and cumbersome procedure. An alternative procedure to evaluate LOC would be of great interest for all fields of activity involving the use of flammable mixtures, especially when less characterized fuels are used. The paper describes a new procedure (algorithm) meant to estimate the LOC of fuel / air / inert premixed systems, using the values of lower explosion limit (LEL) of the fuel / air mixture and the calculated adiabatic flame temperature (CAFT) both at LOC and LEL when nitrogen is used as an inert gas. It is based on an empirical correlation established between the CAFT computed for fuel / air / nitrogen mixtures at LOC and CAFT at LEL, for a large number of flammable gases and vapors. This requires only the measurement of LEL. The correlation was derived from flammability data taken from literature sources (German and American recommended values). The method is based upon the assumption that mixtures at LOC have an equivalence ratio 8 / 1.250, which is close to the equivalence ratio of the most reactive fuel / air systems.
Inverse calculations made with this new algorithm for nine fuel / air / nitrogen mixtures allowed the determination of LOC with a relative deviation of 2/ 22%.
Summary
Substances like alcohols, ethers and esters have recently gained in importance as solvents for paints and varnishes, for surface degreasing and cleaning. Furthermore, fluorinated hydrocarbons are used as refrigerants, replacing chlorofluorocarbons. Because reliable data were not available for most of these substances, measurements of explosion limits and limiting oxygen concentration (MOC) have been carried out by PTB (flammable liquids) and BAM (flammable gases).
The poster presents the results of the most recent measurements of MOC for ethanol, i-propanol, ethyl acetate, propyl formate, hexane and for gases as e.g. difluoroethane and methane. Explosion limits and limiting oxygen concentrations have been measured in mixtures with air under ambient conditions and at elevated temperatures according to the German standard DIN 51649.