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There are several standardized ignition sources in use for the determination of explosion characteristics of gases, vapors and dusts. In the standards the ignition source is stated, but they vary in stating other characteristics like burning duration, energy or burning volume. Since heat is not visible under normal circumstances a Schlieren-Technique was used to make the full heating volume and not just the flames visible.
While an earlier paper of the authors focused on the ignition energy and burning duration of four standardized ignition sources, this paper focuses on the initial igniting volume, the ratio between initial igniting volume and the test-vessel size and other phenomena that were observed with the Schlieren-Technique-.
There are several standardized ignition sources in use for the determination of explosion characteristics of gases, vapors and dusts. In the standards the ignition source is stated, but they vary in stating other characteristics like burning duration, energy or burning volume. Since heat is not visible under normal circumstances a Schlieren-Technique was used to make the full heating volume and not just the flames visible. While an earlier paper of the authors focused on the ignition energy and burning duration of four standardized ignition sources, this paper focuses on the initial igniting volume, the ratio between initial igniting volume and the test-vessel size and other phenomena that were observed with the Schlieren-Technique.
The maximum rate of pressure rise (dp/dt)𝑚𝑎𝑥 and the corresponding K-value of hybrid mixtures containing flammable gases and dusts are important for constructive explosion protection measures. Since the safety characteristics of dusts and gases are determined under different conditions, there has been considerable confusion about the influence of flammable gas on the (dp/dt) of dusts and vice versa. While some investigations showed comparably higher values for hybrid mixtures, others stated that the highest value for the gas component alone is the worst case.
The first part of this paper focuses on the confusion around the different statements about (dp/dt)𝑚𝑎𝑥 of hybrid mixtures and where they come from. In the second part of this paper experimental results are presented that illustrate how to clarify the different findings of past research and show what to expect as a real worst-case-value for hybrid mixtures.
Safety characteristics like the lower explosion limit, the highest concentration for a given substance mixed with air that does not result in a self-propagating flame, or the maximum explosion pressure, the highest pressure that can be reached after ignition for a given combustible substance mixed with air at any concentration, are widely used in the industry to either prevent an explosion or to mitigate the effects of it. Safety characteristics are not physical constants, are determined experimentally and depend on the chosen experimental parameters such as the ignition energy or turbulence.
For the determination of the safety characteristics lower explosion limit (LEL), limiting oxygen concentration (LOC), maximum explosion pressure (pmax) and maximum rate of pressure rise ((dp/dt)max) of gases and vapors the gas is filled via partial pressures into a test vessel and then ignited under quiescent conditions with a weak (2 J - 20 J) ignition source. For dusts, the same safety characteristics are determined under turbulent conditions to elevate
the dust homogeneously and it is ignited with two chemical igniters with an ignition energy of 1000 J each (LEL and LOC) or 5000 J each (pmax and (dp/dt)max). For the determination of safety characteristics of hybrid mixtures (a mixture containing a combustible dust and a flammable gas) there is no existing standard.
In the last 40 years most of the research on their safety characteristics was performed with dust testing equipment that was modified for the addition of flammable gas. Because of the different mixing procedures of gases and dusts with air and because of different ignition energies and sources that are normally used for the standard tests of gases or dusts, the results were hardly reproducible. Statements about the different safety characteristics were contradictory and left the reader or the person responsible for designing safety measures for a process plant or a facility behind with no clear suggestion about the behavior of hybrid
mixtures.
This thesis is aimed on determining the influence of the different ignition sources and energies on the safety characteristics pmax, (dp/dt)max, lower explosion limit, and limiting oxygen concentration of hybrid mixtures.
Several test series were conducted to characterize different standardized ignition sources, that are already in use for the determination of safety characteristics of single-phase substances (gases, liquids, dusts). The burning duration, the igniting volume and the net energy were investigated.
It was shown, that the chemical igniters and the exploding wire are suitable ignition sources for the determination of safety characteristics of hybrid mixtures in general. Their burning duration was long enough to ignite dusts and quick enough, that the decay of the turbulence or sedimentation of the dusts did not occur. Both ignition sources produced comparable results for the determination of pmax, LEL and (dp/dt)max of dusts when they had the same ignition energy. A reduced ignition energy of the exploding wire and the chemical igniters did not affect the pmax and (dp/dt)max. The LEL changed with lower energies. However, for the determination of the LOC of hybrid mixtures the exploding wires with an overall energy of 2 kJ produced the best results.
The influence of the different mixing procedures that have already been used for hybrid mixtures and the requirements for them were also investigated experimentally for the gas concentration and the determined safety characteristics and compared to each other. It was shown, that the partial pressure method works for mixing hybrid mixtures but the pressures should be measured very accurately and the gas concentration should be validated.
Furthermore, the influence of the turbulence that is inevitable when testing dusts, on the safety characteristics of gases was determined. It was found that the chemical igniters and exploding wires produced comparable results for the determination of pmax, LEL and (dp/dt)max under turbulence. For the LOC only exploding wires with two times 1 kJ worked.
Finally, the safety characteristics of hybrid mixtures were determined with different ignition energies and sources and the data were compared. It was discovered, that the pmax of hybrid mixtures was the same value than the higher determined one of the single substances while (dp/dt)max of hybrid mixtures was about 10 % to 25 % higher than the value of the stoichiometric gas mixture under turbulence. The point was found at the stoichiometric gas concentration with very little amounts of dust.
To prove the key findings of this work and for the establishment of a standardized procedure for the determination of safety characteristics of hybrid mixtures, an international round robin test was conducted with eleven participating facilities in seven countries. The results were comparable within a reasonable range and are presented in this dissertation in an extra chapter.
Based upon the observations in this work a reliable solution for a new standardizable ignition source to determine the safety characteristics of hybrid dust-gas-mixtures is proposed.