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- Explosionsschutz (8)
- Explosionsgrenzen (6)
- Tetrafluoroethylene (6)
- Explosion limits (5)
- Flammability (4)
- Self-ignition (4)
- Calculation method (3)
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- Explosion protection (3)
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Organisationseinheit der BAM
Messung und Simulation des Inertgaseinflusses auf Explosionsgrenzen bei erhöhten Anfangsdrücken
(2001)
At Juelich Research Center the prototype of an alkaline 120-bar electrolyser has been developed and built. Constructive and process-engineering measures must be taken to ensure the safe operation of such facilities. Potential hazards occur due to the high operating pressure in conjunction with the reactivity of the product gases and the electrolyte. First of all, the operating mode and technical features of the Juelich high-pressure electrolyser will be dealt with. Within the framework of a parametric study, the potential for weight reduction of the prototype while observing the rules for pressure vessel design will be shown. The Federal Institute for Materials Research and Testing in Berlin has performed measurements concerning the explosion limits of H2/O2 mixtures at different temperatures and pressures up to 200 bars. At an electrolysis test rig of IWV-3, which can also be operated up to 200 bars, investigations were carried out concerning the gas composition on the H2 and O2 path under different operating conditions. These measurement series were compared to the explosion limits determined and evaluated to derive safety measures required for the operation of high-pressure electrolysers.
Messung und Simulation des Inertgaseinflusses auf Explosionsgrenzen bei erhöhten Anfangsdrücken
(2001)
International and European dangerous substances and dangerous goods regulations refer to the standard ISO 10156 (1996). This standard includes a test method and a calculation procedure for the determination of the flammability of gases and gas mixtures in air. The substance indices for the calculation, the so called Tci values, which characterise the fire potential, are provided as well. These ISO Tci values are derived from explosion diagrams of older literature sources which do not take into account the test method and the test apparatus. However, since the explosion limits are influenced by apparatus parameters, the Tci values and lower explosion limits, given by the ISO tables, are inconsistent with those measured according to the test method of the same standard. In consequence, applying the ISO Tci values can result in wrong classifications. In this paper internationally accepted explosion limit test methods were evaluated and Tci values were derived from explosion diagrams. Therefore, an open vessel method with flame propagation criterion was favoured. These values were compared with the Tci values listed in ISO 10156. In most cases, significant deviations were found. A detailed study about the influence of inert gases on flammability is the objective of Part 2.
Ternary systems, which contain flammable gas, inert gas and air, were studied in order to give the user an evaluation of the ISO 10156 calculation method for the flammability of gas mixtures. While in Part 1 of this article the fire potential of flammable gases was the focal point, the influence of inert gases on the flammability of gas mixtures was studied in Part 2. The inerting capacity of an inert gas is expressed by the dimensionless K value, the so-called coefficient of nitrogen equivalency. The experimental determination of K values is demonstrated by using explosion diagrams. The objective of this study was to compare the estimated results, given by ISO 10156, with measurements of explosion ranges based on the German standard DIN 51649-1, given by CERN and CHEMSAFE. The comparison shows that ISO 10156, Table 1, supplies conservative K values, which can be regarded as safe in all cases. Nevertheless, in a number of cases ISO underestimates the inerting capacity, so that non-flammable gas mixtures are considered flammable.
According to international standards several safety characteristics of dusts are determined in the 20-Lsphere (also known as SIWEK Chamber). Dust cloud ignition is carried out using pyrotechnical igniters.
Due to various disadvantages of such igniters the need for alternative ignition sources arises again and again. An alternative ignition source could be the so called 'exploding wire' or 'fuse wire'. The paper presents test results of a comparative study between both ignition sources for the determination of the safety characteristics Maximum Explosion Pressure and Maximum Explosion Pressure Rise of five selected dusts in the 20-L-sphere. In addition to that the ignition mechanisms of both ignition sources were analysed by high speed camera recordings and the ignition energy was determined with electric and calorimetric recordings.
The Ignition Temperature (IT) of stoichiometric tetrafluoroethylene–air mixtures on hot walls was determined in a 3-dm³-reactor. Tests at elevated pressure conditions were performed, namely at 5, 15 and 25 bar(a), showing a decrease of the IT with the initial pressure. Furthermore, the measured ignition temperatures of stoichiometric tetrafluoroethylene–air mixtures were lower than the ignition temperatures required for the decomposition pure tetrafluoroethylene (Minimum Ignition Temperature of Decomposition, MITD) reported in previous works.
Equations from the Semenov thermal explosion theory on spontaneous ignition were used to identify approximate combustion kinetics of tetrafluoroethylene from the experimental results. The determined kinetics was used for the prediction of the IT of stoichiometric tetrafluoroethylene-air by simplified calculation methods. A very good agreement with the experimental results was observed.
There is a lack of data on the self-ignition behaviour of tetrafluoroethylene (TFE) in industrial sized
equipment. In order to assess the tendency of TFE for
decomposition in large scale vessels, , a facility was
designed and constructed. Tests were
carried out in a cylindrical reactor with a volume of 100 L with initial
pressures of 5 and 10 bar(a). The effect of the reacto
r adjustment (vertical or horizontal) was taken into
account. The current work describes the test set
up and summarizes the experimental results achieved.
Furthermore, this paper reports on a numerical model for
the prediction of the self
-heating of TFE in closed
vessels, which was previously developed for small scale reactors and has been here validated for larger
dimensions with the experiments performed in the new facility.
According to international Standards several safety characteristics of dusts are determined in the 20-L-sphere (also known as SIWEK-Chamber). Dust cloud ignition is carried out using pyrotechnical igniters. Due to various disadvantages of pyrotechnical igniters the need for alternative ignition sources arises again and again. An alternative ignition source could be the socalled “exploding wire” or “fuse wire". The paper presents test results of a comparative study between both ignition sources for the determination of the safety characteristics Maximum Explosion Pressure and Maximum Rate of Explosion Pressure Rise of selected dusts in the 20-Lsphere.
In addition to that the flame/arc propagation of both ignition sources was analysed by high speed and IR camera recordings. Tests in a windowed autoclave were performed to get information on the influence of turbulence generated due to dispersion of the dust on the shape of the generated flame and its propagation. Turbulence measurements in the windowed autoclave as well as in the 20-L-sphere with a LDA System allowed investigation if the igniter led to turbulence increase at the moment of ignition in comparison to tests without ignition.
Many industrial processes include a gas explosion hazard. If safety measures are not adequate to prevent a potentially explosive atmosphere or to avoid effective ignition sources in enclosures, at least the effects of an explosion can be limited e.g. by gas explosion venting systems.
For the design of gas explosion venting systems for confinements only little guidance is given when considering the constructional boundary conditions or process conditions. For this reason conservative assumptions are prevalent in practice and in many cases the protective systems become significantly oversized. From safety perspective such safety margins in venting areas can lead to a critical acceleration of the pressure rise. Moreover, a gas explosion venting at turbulent conditions caused by over sizing or by obstacles rather leads to an under-sized system. The present investigation was focused especially on the influence of certain obstacles as well as the influence of elevated initial pressures on explosion venting
behaviour of quiescent hydrogen, methane or ethylene in air.
The Minimum Ignition Temperature of Decomposition (MITD) of tetrafluoroethylene in a partially heated pipe was analyzed for different initial pressures (5, 10 and 15 bara). The pipe used had an internal length of 1 m, an internal diameter of 30 mm with a volume of about 0.7 dm³ and was vertically oriented. Pressure at the pipe top and temperature at four different locations along the pipe axis were measured. Tetrafluoroethylene was found to decompose at lower temperatures for increasing initial pressures, in agreement with previous tests with reactors with fully heated walls. A complete passive quenching in the non-heated part of the pipe was observed only for an initial pressure of 5 bara, while for higher initial pressures, the decomposition propagated completely along the test pipe. Moreover, the test results on the MITD were compared with data from previous experiments in fully heated 0.2 and 3 dm³ cylindrical reactors and showed a decrease of the MITD with the heated volume through heated surface ratio of the vessel. Furthermore, the prediction of the MITD of tetrafluoroethylene by simplified calculation methods was attempted, showing a good agreement with the experimental results.
This work investigates the ignition of tetrafluoroethylene induced by the adiabatic compression that can arise by activating a high speed valve separating two portions of a pipeline with a high pressure difference. In the tests performed the high pressure zone contained tetrafluoroethylene at pressures between 15 and 30 bar. For the low pressure zone, experiments with nitrogen, air and tetrafluoroethylene were carried out. The pressure range in the low pressure zone was comprised between 0.05 and 1 bar. The pipe diameters analyzed were 15 and 20 mm. For the analyzed geometries, special conditions were required in order to reach reproducible ignitions, namely air at temperatures of at least 105 °C had to be present in the compression pipe. Furthermore, a minimum length of the compression pipe had to be used. The current work describes the experimental setup employed for the tests and discusses the achieved results. Numerical simulations were performed in order to clarify unexpected findings.
Für die sicherheitstechnische Beurteilung von Sterilisationsprozessen wurden die Explosionsgrenzen von Gasgemischen aus Ethylenoxid, Inertgas und Luft experimentell bestimmt. Die Messungen sind mit Stickstoff, Kohlenstoffdioxid und Wasserdampf bei 20 und 100 °C sowie bei 0,4 und 1,0 bar durchgeführt worden. Mit Hilfe der Daten wurde ein halbempirisches Modell entwickelt, mit dem die Explosionsfähigkeit von Prozessgasgemischen berechnet werden kann.
Explosionsgrenzen der Zerfallsreaktion von Gemischen aus Ethylenoxid, Propylenoxid und Stickstoff
(2007)
In technischen Ozonolyseanlagen kommen gasförmige Oxidationsmittel mit brennbaren Flüssigkeiten in Kontakt. Dies geschieht in der Regel bei tiefen Temperaturen, weit unterhalb des Flammpunktes. Trotzdem ist es in einer Anlage zu einer Explosion gekommen, deren Ursache möglicherweise eine Flammenausbreitung auf der Oberfläche des Lösungsmittels war. In Laborversuchen ist dieses Phänomen in Abhängigkeit von der Gasphasenzusammensetzung, vom Lösungsmittel, vom Druck und von der Temperatur untersucht worden.
Es wurde ein Verfahren zur Berechnung von Explosionsgrenzen ethylenoxidhaltiger Gasphasen in Sterilisatoren entwickelt. Mit Hilfe der Software GasEq® und des neu entwickelten Makros SterEx für MS-Excel® lassen sich die Explosionsgrenzen für Gemische aus Ethylenoxid, Luft und Inertgasen bei Temperaturen zwischen 20°C und 100°C sowie 0,4 bar und 1,0 bar berechnen. Somit ist es schnell möglich, sichere Betriebsbedingungen für Sterilisationsprozesse mit Ethylenoxid festzulegen. Das halbempirische Modell basiert auf der Annahme konstanter Flammentemperaturprofile an den Explosionsgrenzen in Abhängigkeit der EO-Konzentration für verschiedene Gemische. Durch systematische Zündversuche wurden Explosionsgrenzen für Gemische aus Ethylenoxid, Stickstoff, Kohlenstoffdioxid, Wasserdampf und Luft zur Bestimmung von Modellparametern und zur Validierung des Verfahrens bestimmt. Um die Prozessbedingungen in Sterilisatoren möglichst genau zu simulieren, wurden die Versuche in einem geschlossenen Autoklav in Anlehnung an DIN EN 1839-B durchgeführt. Berechnungen der Explosionsgrenzen von Prozessgasgemischen mit SterEx ergeben eine gute Übereinstimmung mit experimentell ermittelten Werten.
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A calculation method for flammability limits of gas phases with ethylene oxide in sterilisers was developed. Using the Software GasEq® and the newly developed Makro SterEx for MS-Excel®, flammability limits of mixtures with ethylene oxide, air and inert gases at temperatures between 20°C and 100°C and pressures between 0.4 bar and 1.0 bar can be calculated. This method can be used to easily determine safe operating conditions. The used semi-empirical model is based upon the assumption of constant flame temperature profiles at the flammability limits subject to the EO-concentration for different mixtures. To collect model parameters and to validate the model, several experiments with mixtures of ethylene oxide, nitrogen, carbon dioxide, water vapour and air were carried out to determine flammability limits. To simulate the structural conditions of sterilisers, the experiments were conducted in accordance to DIN EN 1839-B in a closed autoclave with temperatures and pressures relevant for sterilisation processes. The calculation of flammability limits of process gas mixtures with SterEx provides good agreement with flammability limits that were determined in experiments.
There is a lack of data on the self-ignition behaviour of tetrafluoroethylene in industrial sized equipment. Therefore, a facility was designed and constructed for the determination of the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene in a cylindrical reactor with a volume of 100 dm3. Tests with initial pressures of 5 and 10 bar(a) were performed. The Minimum Ignition Temperature of Decomposition of tetrafluoroethylene was observed to decrease with the initial pressure, in agreement with previous experiments with small scale cylindrical vessels. This paper describes the test set-up und gives an overview of the achieved experimental results. In particular the effect of the reactor orientation (vertical or horizontal) is discussed. Furthermore, simplified equations from the Semenov thermal explosion theory are used to attempt extrapolations of previous and current data on the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene to other vessel volumes or initial pressures. Moreover, the experimental data are plotted together against the heated volume to heated surface ratio, which should provide a better extrapolation to other vessel dimensions by taking into account that the efficiency of the dispersion of the heat generated by the reaction is different for two reactors with the same volume but different diameter. Finally, simplified methods for predicting the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene presented previously by the authors are validated for large scale reactors with the experimental data collected within the current work.
In the industry there is a lack of data on the Minimum Ignition Temperature of decomposition of tetrafluoroethylene in industrial sized equipment. In order to determine the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene in large scale vessels, a facility was designed and constructed. Tests were carried out in a cylindrical reactor with a volume of 100 dm³ with initial pressures of 5 and 10 bar(a). In agreement with previous experiments with small scale cylindrical vessels, the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene was observed to decrease with the initial pressure. The current paper describes the test setup and summarizes the experimental results achieved. The effect of the reactor adjustment (vertical or horizontal) is discussed. Moreover, simplified equations from the Semenov thermal explosion theory are used to attempt extrapolations of previous and current data on the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene to other initial pressures and vessel volumes.
The effects of a gas explosion in enclosures like vessels can be limited e.g. by gas explosion venting
systems. The major design step of this constructive explosion protection method is to determine the
required vent area, which depends significantly on whether turbulent combustion exists. However,
current standards like NFPA 68 or EN 14994 are applicable only to limited boundary conditions and
as far as possible only to laminar flame propagation. Difficulties arise in the assessment or predictability
of gas explosion hazard when turbulence occurs.
In this research especially venting at elevated initial pressure has been shown to accelerated
flame propagations and therefore, to a considerably higher reduced pressure. Therefore, it is essential
to provide a broader data base of turbulent combustion and explosion behaviour to verify the
existing rules or to determine their safety-relevant parameters.
For a better safety assessment or design of protective systems the turbulent combustion and
accelerated gas explosion behaviour of quiescent methane in air were investigated at initial pressures
up to 8 bar using vessels up to 100 litres. In particular a systematic study was performed to
investigate the influence of turbulence on the overpressure development during accelerated gas
explosion. Moreover, the present study consider the position of the spark igniters, the burning velocity
and the maximum pressure rise for different concentration of fuel as well as the size of orifice
and/or vent area.
A choice of experimental tests showed under the investigated conditions that not only turbulence
inducing obstacles but also over sized vent areas could lead to an increased pressure development
and therefore to an inacceptable safety state. Due to the numerous influencing variables of
explosion behaviour the presented experimental results help to judge whether another more sophisticated
method should be applied than the one described in standards.
The present paper aims to re-examine the validity of the linear correlation found between AFTLOC, the adiabatic flame temperature at the apex of the flammability range of fuel-air-inert mixtures (where LOC, the Limiting Oxygen Concentration, is measured) and AFTLFL, the adiabatic flame temperature at the lower flammability limit of fuel-air mixtures (LFL). New sets of experimental measurements of LFL and LOC referring to fuel-air mixtures diluted with N2, CO2 and H2O(vap) from trusted literature sources form a comprehensive database for such evaluation. Both the slope and intercept of correlations AFTLOC = a + b*AFTLFL are dependent on the nature of inert gas and on initial temperature. Based on the linear correlation between AFTLOC and AFTLFL, a procedure for calculation of LOC and MIC (Minimum Inert Concentration) of fuel-air-inert mixtures is presented, using measured or calculated LFL of fuel-air mixtures and their corresponding AFT. The method predicts with reasonable accuracy LOC and MIC of fuel-air-inert mixtures (relative deviations ranging between -14 and +17% when calculated and measured LOC and MIC for fuelair-nitrogen and fuel-air-carbon dioxide at ambient initial conditions are examined).
For the determination of safety characteristics of gases, vapors and dusts different types of ignition sources are used in international standards and guidelines. The paper presents test results of a comparative calorimetric and visual study between four different types of ignition sources. The ignition procedures were analyzed visually with a high-speed camera and electric recordings. In addition to that, the influence of the electrode-orientation, -distance as well as ignition energy on the reproducibility of the exploding wire igniter was tested.
The exploding wire is already in use for standardized determination of safety characteristics of gases, first tests on the suitability of the exploding wire igniter for dust testing have been carried out but are not standardized yet. Using the exploding wire, the ignition energy can be varied from 2 J to 10 000 J (2 x 5000 J) and thus it could be used for gases, vapors, dusts and hybrid mixtures. Moreover it can be used at high initial pressures and it is the only ignition source with an easily measurable ignition energy release. Furthermore, it does not introduce another chemical reaction into the system.
Finally, a proposal for a standard ignition source for explosion tests on hybrid mixtures is derived from the test results.
Safety characteristics for explosion protection of natural gas/hydrogen mixtures relevant in connection with the Power2Gas technology were studied in this work. Lower explosion limits (LEL) and upper explosion limits (UEL), limiting oxygen concentrations (LOC), maximum experimental safety gaps (MESG), maximum explosion pressures (pmax) and maximum rates of pressure rise (dp/dt)max were determined experimentally in dependence of the hydrogen fraction. Adding hydrogen did mainly effect the UEL, LOC, MESG and (dp/dt)max. The mixtures become more "critical" concerning the explosion hazards with increasing hydrogen fraction. However, the dependency of the safety characteristics from the hydrogen fraction is mainly not linear. Adding up to 10% hydrogen to natural gas had nearly no effect on the safety characteristics. More significant effects on the safety characteristics were observed at hydrogen fractions of more than 25%. For example the explosion group changes from IIA to IIB. Considering the huge explosion region and very high (dp/dt)max of hydrogen compared to natural gas, even adding 50% hydrogen to natural gas has a rather small effect on these characteristics. Furthermore pmax of hydrogen/natural-gas mixtures can be calculated with good accuracy assuming ideal adiabatic conditions. EL and LOC of natural gas/hydrogen mixtures in ternary systems with inert gas and air were calculated in dependence of the type of inert gas with the so called “model of constant adiabatic flame temperature profiles”.
Many industrial processes are run at non-atmospheric conditions (elevated temperatures and pressures, other oxidizers than air). To judge whether and if yes to what extent explosive gas(vapor)/air mixtures will occur or may be generated during malfunction it is necessary to know the safety characteristic data at the respective conditions. Safety characteristic data like Explosion limits, are depending on pressure, temperature and the oxidizer. Most of the determination methods are standardized for ambient conditions.
In order to obtain determination methods for non-atmospheric conditions, particularly for higher initial pressures, reliable ignition criteria were investigated. Ignition tests at the explosion Limits were carried out for mixtures of methane, propane, n-butane, n-hexane, hydrogen, ammonia and acetone in air at initial pressures up to 20 bar. The tests have been evaluated according to different ignition criteria: visual flame propagation, temperature and pressure rising. It could be shown that flame propagation and occasionally self-sustained combustion for several seconds occurred together with remarkable temperature rise, although the pressure rise was below 3%. The results showed that the combination of a pressure rise criterion of 2% and a temperature rise criterion of 100 K seems to be a suitable ignition criterion for the determination of explosion limits and limiting oxidizer concentration at higher initial pressures and elevated temperatures. The tests were carried out within the framework of a R&D project founded by the German Ministry of Economics and Technology.
In der Prozesstechnik kommt es immer wieder zu Zündungen durch Druckstöße mit Gasen. Bekannt ist dieses Phänomen vom Umgang mit Sauerstoff im Kontakt mit nicht geeigneten Materialien. Aber auch andere Gase, z. B. Acetylen, können durch schnelle Kompression gezündet werden. In der vorliegenden Arbeit werden, in Fortführung früherer Untersuchungen in Rohren, solche Zündvorgänge mit Hilfe von Zündtemperaturen abgeschätzt.
Die Kenntnisse der Einflüsse von Druck, Temperatur und Oxidationsmittel auf sicherheitstechnische Kenngrößen wie Explosionsgrenzen sind eine notwendige Voraussetzung, um zu entscheiden, ob mit den bei atmosphärischen Bedingungen geltenden Kenngrößen eine ausreichende Sicherheit zu gewährleisten ist. Dies wird anhand von Beispielen erklärt und die Konsequenzen für die praktische Anwendung in der Industrie diskutiert.
The classification of flammable gas mixtures is based on either testing or calculation methods proposed by the revised international standard ISO 10156. This standard is used for classification of physical hazards in Chapters 2.2 and 2.4 of the UN Globally Harmonized System of Classification and Labelling of Chemicals (GHS) and in the UN Recommendations on Transport of Dangerous Goods (TDG). The test methods of flammability and oxidizing potential in this standard were developed by BAM. Earlier versions of this standard are not based on triangular diagrams and on the reference combustible substance 'ethane'. The old material characteristics, especially in case of oxidizing potential, are based mostly on practical experience without any quantifiable test results. First time it is possible to compare experimental results from the CHEMSAFE database with the newly developed calculation method. In this paper the basic principles of the calculation methods are presented and the methods are validated by examples. A comparison of experimental flammability data with classification results gained by the calculation methods of ISO 10156 is demonstrated.
In Gasphasen aus Ethylenoxid (EO) und Propylenoxid (PO), die bei technischen Alkoxylierungsreaktionen vorkommen, können auch ohne den Zutritt von Luft Zerfallsreaktionen stattfinden, die explosionsartig mit einer vielfachen Temperatur- und Drucksteigerung verlaufen. Zur Abschätzung der Auswirkungen solcher Explosionen wurden die Explosionsdrücke und die zeitlichen Druckanstiege von reinem EO und EO/PO-Gemischen bei Temperaturen von 100°C bis 200°C und Drücken von 1 - 10 bar in einem 3-dm³-Behälter und punktuell in einem 100-dm³-Behälter experimentell bestimmt.
The hazardous potential of accidental heavy gas releases, especially those involving flammable and toxic gases, is widely known. In order to predict the area in which these gases are in hazardous concentrations, an estimation of the dispersion of these gases must be carried out. While the hazardous area for flammable heavy gases is determined by the lower flammability limit (ca. >1 vol%), the release of toxic heavy gases can result in a much larger hazardous area. Toxic gases, even in very low concentrations (ca. <3,000 ppm), have the potential to be highly damaging. State-of-the-art dispersion models, such as the VDI Guideline 3783, can be used to estimate the dispersion of heavy gases. However, VDI 3783 gives no method for the prediction of the height and width of a heavy gas cloud, which are both required for quantitative risk analysis as well as for a possible coupling of a Lagrangian particle model with the VDI 3783 heavy gas dispersion model. Therefore, further calculation methods were used to describe these dimensions and were evaluated against experimental studies of the length, width, and height of the heavy and neutral gas field. In addition to that the influence of the source height on the heavy gas dispersion was also examined
The paper provides a summary of safety characteristics of hydrogen and hydrogen fuel gases. The inves-tigations focus on water electrolysis, the feeding of hydrogen into the natural gas grid and the use ofhydrogen for the fermentation process in biogas plants. The safety characteristics of hydrogen such as avery low minimum ignition energy, very large explosion range and high flame velocity with the resultingrapid pressure increase of hydrogen explosions are of particular importance for explosion protection andthey differ strongly from those of natural gas. Explosion ranges of hydrogen-methane-carbon dioxidemixtures have been measured for the use of hydrogen in biomethane production. The paper also showsand discusses explosion ranges of hydrogen and hydrogen-natural gas mixtures. Pressure and tempera-ture dependencies of the explosion limits of mixtures were investigated. Furthermore, pressure rise rates(KGvalues) were measured with regard to constructive explosion protection. The maximum experimen-tal safe gaps were determined for the classification of the mixtures and assignment to explosion groupsaccording to the European ATEX directives. It was found that admixture of 10% hydrogen to natural gashas only a minor influence on the safety characteristics of gas explosions.
Validation of the new ignition source 'exploding wire' for dust explosion testing in the 20-L-sphere
(2014)
The safety characteristics Maximum Explosion Pressure pniB, Maximum Explosion Pressure Rise (dp/dt)™* and Lower Explosion Limit LEL are determined in closed vessels such as the 20-L-sphere according to international Standards. Dust ignition is carried out using pyrotechnical igniters which are defined in the Standards. Due to various disadvantages of pyrotechnical igniters the need for alternative ignition sources arises again and again. Studies at the Federal Institute for Material Research and Testing (BAM) with ignition units which were able to generate ignition energies up to 2000 J showed that the so-called “exploding wire” or “fuse wire“ is suitable as an alternative ignition source. The paper presents further test results for the Validation of the exploding wire for the determination of Pm» and (dp/dt)mx in the 20-L-sphere. The tests were performed with a new ignition unit and improved electrodes which allowed ignition energies up to 10000 J. The paper also analyses propagation of flame and electrical arc on basis of high speed camera recordings. Turbulence measurements with a LDA System in the 20-L-sphere allowed investigation whether the activation of the ignition sources has an influence on the turbulence field generated during dust dispersion and whether the influence differs depending on the ignition source.
The safety characteristics of flammable gases and liquids are required when identifying potentially explosive mixtures and taking appropriate actions concerning explosion protection. Examples are given here of the safe handling and evaluation of hazards during the processing, storage, transport, and disposal of flammable liquids and gases. The CHEMSAFE database is presented as a reliable source of safety characteristic data, and its new open-access version is introduced. CHEMSAFE currently contains assessed properties for about 3000 liquids, gases and mixtures. The lack of a broad experimental foundation in the extensive field of non-atmospheric conditions shows the need for further investigation and standardization. This article summarizes experimental evidence and estimation methods for safety characteristic data under non-atmospheric conditions pointing out current limitations. Suggestions for pre-normative research on safety data under non-atmospheric conditions are given.
One of the fundamental principles of the UN-GHS (Globally Harmonized System of Classification and Labelling of Chemicals) is that all hazards of a chemical should be assigned and communicated. There is no general prioritization of hazards in the sense that certain hazard classes are not applicable if another one has been assigned. In contrast to health and environmental hazards, there are physical or chemical factors which preclude certain combinations of physical hazard classes. So far, there is no common understanding as to which combinations are relevant and which not. For example, should a pyrophoric liquid be classified as flammable liquid in addition, or is this redundant and unnecessary? In the course of the implementation of the GHS by countries or sectors and the actual application by industry all over the world, such questions become more and more important.
This publication systematically discusses all combinations of the UN-GHS physical hazard classes and assesses them with regard to the relevance of possible simultaneous assignment to a chemical. For many of the combinations an unambiguous decision based on theGHS alone is not possible, thus confirming that the question which physical hazard classes might be assigned simultaneously to a chemical is not trivial. As one more milestone on the path to a globally harmonized system for the classification of hazardous chemicals, this should be discussed and ultimately solved on a global basis. It is the hope that this publication might serve as an impetus for such discussions.
For evaluation of explosion scenarios in closed systems involving the mildly flammable refrigerants R1234yf, R1234ze and R32 dependent on the ignition energy, ignitions were carried out in a closed autoclave. A newly developed ignition system was used, which allows generating electric arcs with defined energies in a range between 3 J and 1000 J. The lower explosion limit of R32 decreases with increasing ignition energy. R32-explosions can be more severe than explosions involving highly flammable substances.
However, in case of R1234yf and R1234ze, the ignition energy had to be increased to more than 100 J and more than 500 J to detect explosions in the closed system at all, although flame Propagation phenomena can already be observed if these substances are ignited with much weaker ignition sources in open glass tubes. The explosions were very mild with these substances.
The presentation will discuss the difference between EU and US standards for the determination of explosion (flammability) limits and limiting oxygen concentration. Small differences observed in measured values can be traced back to the different test apparatuses and criteria. The discrepancies can be much greater in the case of limiting oxygen concentration because of the high amount of inert gases and the corresponding low laminar burning velocities. The paper describes some examples and the influence of the chosen criteria on the results. The European and US standards use the criteria of flame propagation in open test vessels and of pressure rise in closed ones. The examples discussed show that flame propagation is still possible at very small pressure rise values, as observed much below the pressure rise criterion of usual standards. However, flame propagation in a process plant can cause an accident or explosion and must be avoided. Therefore, the flame propagation criterion is recommended to be used in chemical safety engineering. The European safety database CHEMSAFE contains expertevaluated safety data for cases where the determination method and criteria are known. Flammability characteristics based on the pressure rise criterion may suffice in certain cases, e.g. for explosion protection in closed vessels without any connecting pipes.
The flammable hydrogen-blended methane–air and natural gas–air mixtures raise specific safety and environmental issues in the industry and transportation; therefore, their explosion characteristics such as the explosion limits, explosion pressures, and rates of pressure rise have significant importance from a safety point of view. At the same time, the laminar burning velocities are the most useful parameters for practical applications and in basic studies for the validation of reaction mechanisms and modeling turbulent combustion. In the present study, an experimental and numerical study of the effect of hydrogen addition on the laminar burning velocity (LBV) of methane–air and natural gas–air mixtures was conducted, using mixtures with equivalence ratios within 0.90 and 1.30 and various hydrogen fractions rH within 0.0 and 0.5. The experiments were performed in a 14 L spherical vessel with central ignition at ambient initial conditions. The LBVs were calculated from p(t) data, determined in accordance with EN 15967, by using only the early stage of flame propagation. The results show that hydrogen addition determines an increase in LBV for all examined binary flammable mixtures. The LBV variation versus the fraction of added hydrogen, rH, follows a linear trend only at moderate hydrogen fractions. The further increase in rH results in a stronger variation in LBV, as shown by both experimental and computed LBVs. Hydrogen addition significantly changes the thermal diffusivity of flammable CH4–air or NG–air mixtures, the rate of heat release, and the concentration of active radical species in the flame front and contribute, thus, to LBV variation.