Abstract
The Nex-Hys Project, funded by the German Federal Ministry of Economy as part of the WIPANO frame program, aimed to develop standards for safety characteristics of hybrid mixtures. These should be designed in a way that the standards are extended and the existing values in the experimental setup still hold true.
As part of the project, the Otto-von-Guericke University focused on the Minimum Ignition Temperature. Starting from a comparison of apparatuses for liquids (EN 14522) and dusts (IEC 80079-20-2) the decision to use the Godbert Greenwald Oven was made, as only there the possibility to introduce substances in gaseous and solid phase is given in principle.
The standardized setup was extended by a heated chamber to evaporate the liquids to be tested. Burnable gases were introduced into the system by means of mixing according to the partial pressure method in the pressurized air chamber or the evaporation chamber. Thus, it was possible to determine the MIT of the pure gases and vapors in the GG oven. Series of test for different substances under variation of the concentration show comparable values to the standard methods and to values found in literature. A trend to slightly higher temperatures (for gases and vapors) with a deviation close to the measurement uncertainty can be found. All in all, the deviation by different operators seems to be in the same range and cannot be neglected.
Following various combinations of dust, vapors and gases were tested. Up to now, no combinatory effect was detected. This seems to be in contradiction to former own publications, but there the setup was different. This could be an explanation for these nonreproducible result.
In all combinations, the MIT of the substances that has the lower MIT is dominating the final value. The setup-up generally proved to be able to test for the MIT of hybrid mixtures without violating existing values.
Part of the tests were made in a GG oven of double length showing a clear influence on the MIT for some substances, due to the longer residence time.
Apart from some rather practical weaknesses of the standardized setup there are general disadvantages that limit its use for a further development. First there is the totally unknow concentration and distribution of fuels in this open setup. Secondly, the subjective detection of the ignition by the operator. Third, the temperature distribution and heat transfer conditions that are not well defined.
The original intention behind the experiment is not in line with the scientific intention to create a versatile instrument to determine the ignition temperature for all phases and their mixtures. Therefore, a completely new approach would be necessary. The existing MIT standards for dust and vapor/gases can further exist unchanged and fulfil their purpose.
