Experimental Parameter Study and Inherent Scattering of Safety Characteristics of Dusts

Safety characteristics are widely used in industrial processes to avoid explosive atmospheres (primary explosion protection) or to mitigate the consequences of an explosion (constructive explosion protection). Several laboratory parameters influence the determined values when performing the test series such as the beginning pressure and the pre-ignition pressure rise, the ignition source, the ignition energy, the burning duration and volume or the concentration of the combustible substance. In the different standards for the determination of safety characteristics of dusts there is no statement about the scattering or the deviation when parameters are chosen or occur on the borders of their allowed range. Thus, two laboratories might determine values that are hardly comparable for the same given substance. This article summarizes some of the influential factors that cause a deviation and shows the inherent scattering of dust tests when all other parameters are kept constant. It also provides some advice how to minimize the deviation and the scattering with little effort

Experimental investigation of the consequences of acetylene pressure cylinder failure under fire conditions

Acetylene pressure cylinders are widely used in the industrial sector for welding, flame cutting, or heating. Sometimes during work, not only with acetylene cylinders, fires occur and in this case the risk of destruction increases and the behavior of such an exposed cylinder is unpredictable. The purpose of this study is to identify those critical conditions when acetylene cylinders burst and explode in fires. In the present study, acetylene cylinders were exposed to fire conditions. For this purpose, a woodpile as a source of fire was chosen, tested, and evaluated. In addition to the fire condition, this option guaranteed reproducibility and similar conditions for all tests. The individual cylinders were equipped with thermocouples measuring the shell temperature, and half of them were prepared in order to measure the temperatures inside the cylinder. An important factor was the measurement of the amount of pressure that was achieved during the destruction of the cylinder. For this purpose, a pressure transducer was attached to the outlet of the cylinder valve. Exposed to direct fire, they can explode in 10 min, which was confirmed. The critical pressure of 40 bar has been reached in 6 min, followed by destruction after 7 min in fire. Cylinders with internal thermocouples were destroyed when lower pressure was achieved. This confirms the fact that any change of the pressure cylinder affects the original properties. After the tests, the fragments of the selected cylinders were subjected to material tests. The results obtained in these tests are the main source of information for understanding the behavior of acetylene cylinders in fire and the possi-bility of increasing the safety of intervening rescue services in an emergency.Web of Science80art. no. 10487

Some considerations when evaluating physico-chemical hazards of nanopowders

The regulatory classification of the physico-chemical hazards of powders is defined in two references, which describe the test methods to be applied, as well as the criteria and the classification thresholds. These two references are respectively the European CLP Regulation and the transport of dangerous goods UN regulation. These two reference systems enable to associate identified hazards with safety rules related to the handling, storage and production of the powders. Traditional microscale powders as well as innovative materials like nanopowders or very fluffy materials need to follow these classification rules. Typically, the properties of interest for nanopowders are related to the reactivity with air and water. The related hazards are defined by the CLP regulation in the following classes: flammability, pyrophoricity, self-heating, water reactivity and oxidizing capability. The hazardous properties of nanopowders can a priori be routinely determined with the regulatory experimental tests performed in an adequate platform like S-NANO at INERIS. However, several aspects were overlooked so far for innovative materials that show specific properties like very low density and high specific surface area. This presentation deals with a review of the limitations of the current tests as well as detailed considerations related to some tests (powder segregation issues, specific ignition issues, safety issues for the operators…). These aspects could perhaps be not thoroughly considered for microscale powders but in the case of nanopowders, these parameters are shown to be of importance in order to evaluate physico-chemical hazards. It is then of primary importance to assess carefully these new behaviors in order to prevent the production of misleading information in safety data sheets

Course of explosion behaviour of metallic powders – From micron to nanosize

International audienc

Requirements for a Hybrid Dust-Gas-Standard: Influence of the Mixing Procedure on Safety Characteristics of Hybrid Mixtures

While developing a standard for the determination of safety characteristics for hybrid mixtures the authors discovered, that, beside the ignition source, the mixing procedure is the main difference between the single-phase standards for dusts and gases. The preparation of hybrid mixtures containing a flammable gas and a flammable dust in the 20 L-sphere can be realized in different ways. Either the flammable gas is filled only in the sphere or only in the dust container or in both. In previous works, almost always the first method is applied, without giving any information on the accuracy of the gas mixtures. In this work the accuracy of the gas mixtures and the results of the tests applying two methods of mixing were studied. No significant influence of the mixing method itself on the safety characteristics explosion pressure pex and the normalized rate of pressure rise (K-value) was found. Obviously, homogenization of the gas mixtures can be obtained sufficiently by the turbulence that is caused during the injection from the dust container into the explosion chamber within a short time. However, the mixing procedure has a great influence on the accuracy of the gas amount of the mixtures obtained. Without modifying the 20 L-sphere by installing precise pressure sensors, assuring its tightness and performing gas analysis, it must be expected, that the accuracy of the gas mixtures is very low. This has a significant influence on the measured safety characteristics and may lead to unsafe facilities or unnecessary expensive safety measures

Evaluating the explosion severity of nanopowders: International standards versus reality

International audienceThe maximum explosion overpressure and the maximum rate of pressure rise, which characterize the dust explosion severity, are commonly measured in apparatuses and under specific conditions defined by international standards. However, those standards conditions, designed for micropowders, may not be fully adapted to nanoparticles. Investigations were conducted on different nanopowders (nanocellulose, carbon black, aluminum) to illustrate their specific behaviors and highlight the potential inadequacy of the standards. The influence of the sample preparation was explored. Various testing procedures were compared, focusing on the dust cloud turbulence and homogeneity. Dust dispersion experiments evidenced the importance of the characterization of the dust cloud after dispersion, due to the fragmentation of agglomerates, using metrics relevant with nanoparticles reactivity (e.g. surface diameter instead of volume diameter). Moreover, the overdriving phenomenon (when the experimental results become dependent of the ignition energy), already identified for micropowders, can be exacerbated for nanoparticles due to their low minimum ignition energy and to the high energy used under standard conditions. It was evidenced that for highly sensitive nanopowders, pre-ignition phenomenon can occur. Finally, during severe explosions and due to a too long opening delay of the ‘fast acting valve’, the flame can go back to the dust container

Vliv před-iniciačního nárůstu tlaku na bezpečnostní charakteristiky prachů a hybridních směsí

For the determination of the safety characteristics of dusts it is necessary to disperse the dust in the oxidating atmosphere (usually air). In the standard procedures for dusts this is realized by a partially evacuated explosion vessel (20L-sphere) in which the dust gets injected from a dust chamber pressurized with air. Shortly after that injection (60 ms) the dust cloud gets ignited under turbulent conditions, that are otherwise seen as almost ambient with 20 degrees C and about 1 bar (abs). While there has been a lot of research about the influence of the ignition delay time and the level of turbulence in the recent years little attention was paid to the pre-ignition pressure rise and the allowed variations in the standards. In the following work we showed that the allowed ranges for the pressures in the different dust standards influence the safety characteristics of dust alone severely. Even though hybrid mixtures are an emerging risk problem in an interconnected industry there is no standard for the determination of their safety characteristics. In this work it is shown that especially for the preparation of hybrid mixtures of flammable dust and gas the pressures after injection of the dust and the mixing procedure have a large influence on the composition of the tested mixtures and therefore on the safety characteristics. Considering both effects, wrong concentration of gas and wrong initial pressure, the discrepancy of safety characteristics from different facilities will be too big to applicable. The methods to overcome these weaknesses are also presented.Pro určení bezpečnostních charakteristik prachů je nezbytné rozprášit prach v oxidační atmosféře (obvykle vzduchu). Ve standardní proceduře pro prach je toto realizováno částečně evakuovanou výbuchovou nádobou (20L-koule), ve které je prach rozprášen z prachové komory natlakované vzduchem. Krátce po tomto rozvíření (60 ms) je oblak prachu iniciován za turbulentních podmínek, které jsou jinak považovány za téměř okolní s teplotou 20 °C a okolo 1 bar (abs). Zatímco bylo v současné době věnováno hodně pozornosti věnováno době zpoždění iniciace a stupni turbulence, jen málo pozornosti bylo věnováno před-iniciačnímu nárůstu tlaku a odchylkám povoleným normou. V následující práci je ukázáno, že povolené rozsahy tlaku v různých standardech ovlivňují bezpečnostní charakteristiky samotných prachů závažně. Přestože jsou hybridní směsi rizikem v propojeném průmyslu, neexistuje norma pro určení jejich bezpečnostních charakteristik. V této práci je ukázáno, že speciálně pro přípravu hybridních směsí hořlavého prachu a plynu má tlak po rozvíření prachu a proces mísení velký vliv na složení testované směsi a proto na bezpečnostní charakteristiky. Za předpokladu obou efektů, špatné koncentrace plynu a špatného počátečního tlaku, budou rozdíly v bezpečnostních charakteristikách z různých zařízení příliš velké pro aplikaci. Metody pro překonání těchto slabin jsou také uvedeny

Requirements for a Hybrid Dust-Gas-Standard: Influence of the Mixing Procedure on Safety Characteristics of Hybrid Mixtures

While developing a standard for the determination of safety characteristics for hybrid mixtures the authors discovered, that, beside the ignition source, the mixing procedure is the main difference between the single-phase standards for dusts and gases. The preparation of hybrid mixtures containing a flammable gas and a flammable dust in the 20 L-sphere can be realized in different ways. Either the flammable gas is filled only in the sphere or only in the dust container or in both. In previous works, almost always the first method is applied, without giving any information on the accuracy of the gas mixtures. In this work the accuracy of the gas mixtures and the results of the tests applying two methods of mixing were studied. No significant influence of the mixing method itself on the safety characteristics explosion pressure pex and the normalized rate of pressure rise (K-value) was found. Obviously, homogenization of the gas mixtures can be obtained sufficiently by the turbulence that is caused during the injection from the dust container into the explosion chamber within a short time. However, the mixing procedure has a great influence on the accuracy of the gas amount of the mixtures obtained. Without modifying the 20 L-sphere by installing precise pressure sensors, assuring its tightness and performing gas analysis, it must be expected, that the accuracy of the gas mixtures is very low. This has a significant influence on the measured safety characteristics and may lead to unsafe facilities or unnecessary expensive safety measures