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LEL, UEL LOC, MESG, pmax and (dp/dt)max of hydrogen/natural gas-mixtures were determined systematically according to international standards.
Mixtures of natural gas and hydrogen mainly become more „critical“ with increasing hydrogen fraction.
The effect of up to 10 Vol% H2–admixture to natural gas on safety characteristics is very low, no major adaption of existing measures for explosion protection is necessary.
More substantial adjustment of measures for explosion protection is necessary at hydrogen fractions of more than 25 Vol%.
Some of the safety characteristics can be estimated with good accuracy by calculation methods.
In industrial applications carbon steel is often replaced by stainless steel. Stainless steel is supposed to be less problematic than carbon steel as a possible ignition source because of the decreasing oxidizability of stainless steel particles with increasing content of alloying elements such as chromium. However, knowledge about the ignition probabilities and about factors influencing ignitions are far from being sufficient. Better knowledge could well be used to decrease and minimize explosion hazards. Additionally, adequate risk assessment could lead to cost reduction by avoiding possible overestimation of hazards. In this contribution, we present results of grazing impact experiments using four different stainless steels in acetylene-, hydrogen-, ethylene- and propane-air mixtures. We investigated the influence of the chromium content and of some of the material properties of stainless steel on the ignition probabilities of these gas mixtures.
The experiments show an appreciable amount of ignitions in the acetylene-air and the hydrogen-air mixture only. The observed source of ignition depends on the chromium content of the steel. At impact energies of 126 J and more, acetylene-air mixtures were ignited by hot surfaces resulting from impacts between stainless steel components containing 22.5 % chromium whereas separated and subsequently oxidized particles acted as ignition source after impacts of stainless steel containing 17.7 % chromium and less. These observations are in accordance with the decreasing oxidizability of stainless steel with increasing chromium content. However, an exclusive correlation between chromium content and ignition probability was not found. Almost all ignitions of hydrogen-air were initiated by hot surfaces and impacts of steel containing the highest amount of chromium were most incendive. This observation shows that a low oxidizability of the steel due to its high chromium content does not necessarily lead to a lower ignition probability. Quite contrary, an increasing chromium content leads to a decreasing thermal conductivity and thus to a higher ignition probability. As a result the thermal con-ductivity was found to be a key parameter determining the ignition probability. The density, specific heat capacity and hardness of the steel were also taken into account; however, they did not affect the ignition probability.
In conclusion, the different sources of ignition can be distinguished with regard to the resulting ignition probability of flammable gas mixtures subjected to grazing impacts of stainless steel. In cases where the ignitions are initiated by hot surfaces, the thermal conductivity of the impacting steel is the most critical parameter. On the other hand, the temperature increase due to impacts and the propensity of the steel to be oxidized mainly influence the ignition probability in cases where separated particles act as ignition source. Consequently, stainless steel with a high thermal conductivity and a low oxidizability has to be utilized in order to minimize the ignition probability due to mechanical impacts.