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Safety Characteristics in Explosion Protection

Working Group 3.71


Main areas

  • Research and development on the influence of non-atmospheric conditions and mixture composition
  • Measurement of safety characteristic data for external customers
  • Provision of such data free of charge in the data base CHEMSAFE
  • Expert advice for politics, industry and supervising bodies


The explosion behaviour of combustible substances can be quantified by the safety characteristics of explosion protection. These data classify the substances with regard to various properties of hazardous potential. Safety characteristic data help to determine safe operation parameters as well as constructive measures of explosion protection.



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Autoignition temperature in large containers

To what extent is it justifiable to extrapolate safety-related autoignition temperatures determined in small volumes to larger volumes?
On the basis of the data obtained, it is possible to estimate and evaluate both the extent of the reduction in the autoignition temperature and the extent of the extension of the ignition delay time as a function of the vessel volume from the standard ignition temperature or the cold flame temperature according to the standard. For some substances that tend to undergo distinct cold flame reactions, the explosive atmosphere inside larger heated containers is ignited by hot walls at significantly lower temperatures (>100 K difference) than the standard ignition temperature of the fuel.

Influence of pressure and temperature on the explosion limits

with the aim of standardising a method of determination under non-atmospheric conditions











Influence of the oxidizing gas on ignition temperature

The autoignition temperature of various organic substances in dinitrogen monoxide/air-mixtures was determined. No ignitions up to 590°C could be observed in pure dinitrogen monoxide. The ignition temperature of all substances tested generally rises with increasing nitrous oxide content in air and reaches almost 600°C at a level of 80% by volume in the oxidizing gas at the latest. The ignition is initiated by the oxygen content and the nitrous oxide is subsequently stimulated to self-decomposition.

In addition, the dependence of the autoignition temperature on the oxygen/nitrogen ratio in the oxidizing gas was determined in a slightly modified standard apparatus for various organic substances. As expected, the ignition temperature decreases with increasing oxygen content. In most cases the decrease is greater than it would correspond to a linear course between the standard ignition temperature and that in pure oxygen. In individual cases, the ignition temperature already reaches the value found in pure oxygen at an oxygen content in the oxidizer of 30 percent by volume.

Influence of inert gases on the maximum experimental safe gap

including development of an estimation procedure











Lower explosion point under non-atmospheric conditions

The lower explosion point (UEP) is the temperature above which an explosive atmosphere is created above a flammable liquid. The vapour-air mixture above the liquid does not have to be ignited by means of an electric spark at this temperature. In modern processes, however, non-atmospheric conditions are increasingly used for process optimization. These can be increased or reduced pressures, changed oxygen concentrations or completely different oxidants than air. Lowering the pressurer would cause the UEP to sink and thus increase the risk potential. An increase in pressure would have the opposite effect.



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Measurement of safety characteristic data

at atmospheric and non-atmospheric conditions






Evaluation of explosive mixtures:

  • flashpoint
  • lower and upper explosion limit
  • limiting oxygen concentration
  • lower and upper explosion point

Evaluation of ignition sources: 

  • autoignition temperature
  • maximum experimental safe gap


Evaluation of an explosion effect

  • maximum pressure rise rate 
  • maximum explosion pressure

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Safety characteristics are no pure substance data but depend to a varying degree on the method used for their determination. These methods are, therefore, usually standardized. Safety characteristics depend on pressure and temperature. Suitable measuring devices are available to cover a pressure range from 10 mbar to 100 bar (initial pressure) and a temperature range from -20 to 200 °C.

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