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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
  • Participation in standardization committees


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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Hybrid mixtures: Standardisable determination methods for safety-related parameters of explosion protection

Explosive dust-vapour or dust-gas mixtures are also referred to as hybrid mixtures. The explosion effects of these mixtures can be more severe than those of the corresponding pure substances. To date, the determination of safety parameters for explosion protection for such hybrid mixtures has not been standardised.

Within a WIPANO research project (NEX-HYS) running from 2019 to 2021, determination methods based on the previous methods for pure substance mixtures are being developed, validated and a DIN specification drawn up with the support of DIN. The PTB conducts tests with vapor-air mixtures on the modified 20l sphere. Target parameters are explosion limits, oxygen limit concentration, maximum explosion pressure and maximum temporal pressure rise under the influence of turbulence and ignition source.



Influence of inert gases on the maximum experimental safe gap

The maximum experimental safe gap (MESG) is an important criterion to avoid the ignition source flames and hot gases. Explosion-protected devices of the flameproof enclosure type of protection and the suitability of flame arresters are assessed on the basis of the MESG. Accordingly, these devices are designed for the oxidizing agent "air" at slightly increased pressure. However, since typical real operating conditions often require different oxygen contents and higher pressures, the question arises as to the suitability of the MESG as a classification criterion. Previous findings suggest that a device-specific maximum gap width results from the combination of pressure and oxygen content.
The aim of the project is to determine the MESG of gas mixtures as a function of inert gas content and pressure in the overpressure range. It should be clarified whether the theoretical correlations known so far can be validated for the estimation of the MESG and how large the required safety margin is. The result will also show whether the classification of fuels into explosion groups can be maintained under non-atmospheric conditions. 

Lower explosion point under non-atmospheric conditions

The lower explosion point (UEP) is the temperature at which an explosive atmosphere is created above a flammable liquid. At this temperature, the vapour-air mixture above the liquid can be ignited by an electrical spark. In modern processes non-atmospheric conditions are increasingly being used for process optimization. These can be increased or reduced pressures, different oxygen concentrations or different oxidizing agents. Lowering the pressure would cause the UEP to drop and thus increase the potential danger. An increase in pressure would have the opposite effect. Within the framework of the research work, an apparatus will be constructed and verified to ensure compliance with the existing standard. The measurement results are checked for conformity with the calculation of the UEP from the vapour pressure curve and the temperature dependence of the lower explosion limit.

Influence of pressure and temperature on the explosion limits

Within the framework of standardization activities, investigations are carried out at elevated pressures and temperatures in order to provide verification values for apparatus.

Furthermore, the dependence of the lower Explosion limit on pressure was determined for various flammable vapours in the pressure range between 1 bar and 20 bar and at temperatures of 100°C and 150°C. Including literature values for the vacuum range, it was found that the lower explosion limits of these substances above approx. 0.6 bar are only weakly pressure-dependent. It cannot be generally assumed that the LEL falls with increasing pressure. For several substances, a flat maximum of the LEL appears to exist between 2 bar and 10 bar. Such a behaviour is already known from the literature. The approximation of the behaviour of the LEL with pressure increase by a linear regression is therefore not always meaningful. In individual cases, the construction of a compensation line would lead to an estimation of the LEL on the unsafe side. 

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.

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.

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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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