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The introduction of hydrogen as a safe energy carrier needs a robust knowledge base, tools for the design and safety assessment of hydrogen technologies built on it, and an internationally harmonized set of standards and regulations. Many of the innovative technologies imply hydrogen at high pressures and/or cryogenic temperatures, with which private users come into contact for the first time in distributed applications. In order to avoid over-conservative, expensive safety solutions, while at the same time demonstrating the usability and safety of hydrogen applications and maintaining acceptance for the technology, safety research must also keep pace with, or better yet anticipate, trends in technological development. Thus, this overview article describes not only the current state of knowledge and technology regarding hydrogen safety, but also its further development.
The hybrid mixture of combustible dusts and flammable gases/vapours widely exist in various industries, including mining, petrochemical, metallurgical, textile and pharmaceutical. It may pose a higher explosion risk than gas/vapor or dust/mist explosions since the hybrid explosions can still be initiated even though both the gas and the dust concentration are lower than their lower explosion limit (LEL) values. Understanding the explosion threat of hybrid mixtures not only contributes to the inherent safety and sustainability of industrial process design, but promotes the efficiency of loss prevention and mitigation. To date, however, there is no test standard with reliable explosion criteria available to determine the safety parameters of all types of hybrid mixture explosions, nor the flame propagation and quenching mechanism or theoretical explanation behind these parameters. This review presents a state-of-the-art overview of the comprehensive understanding of hybrid mixture explosions mainly in an experimental study level; thereby, the main limitations and challenges to be faced are explored. The discussed main contents include the experimental measurement for the safety parameters of hybrid mixtures (i.e., explosion sensitivity and severity parameters) via typical test apparatuses, explosion regime and criterion of hybrid mixtures, the detailed flame propagation/quenching characteristics behind the explosion severities/sensitivities of hybrid mixtures. This work aims to summarize the essential basics of experimental studies, and to provide the perspectives based on the current research gaps to understand the explosion hazards of hybrid mixtures in-depth.
Minimum explosion concentration (MEC) of three agriculture dusts and two coal dusts was studied via a 20-L explosion chamber to reveal the role of gaseous inhibitors. Both active method (CO2 diluting air) and passive method (CO2 replacing O2) were used. The TG and DTG thermal analysis tests were conducted to study the pyrolysis and combustion characteristics of dust samples. An alternative explosion criterion based on combustion duration time was used to determine MEC, and compared with the standardized overpressure method. Under 10-kJ ignition condition, as oxygen mole fraction (XO2) decreased from 21% to 10%, MEC of agriculture dusts and coal dusts respectively increased by around 5 times and 2 times. The active inerting method with a lower N2/CO2 ratio was found to have a better suppression effect on the explosion of the five carbonaceous dusts because the blend has a higher specific heat and a lower oxygen diffusion rate.
There is no applicable existing standard for the determination of safety characteristics for hybrid mixtures. While developing a new standard in a joint research project in Germany first results from parameter studies led to a standard procedure that can be adopted by laboratories that are already testing dusts in the so called 20L-sphere with as little additional effort as necessary. In fact, one of the main objectives of this research project was to keep modifications and adjustments from the generally accepted dust testing procedures as easy and minimal as possible so as to limit potential deviations from one laboratory to another.
In this first round robin test on hybrid mixtures ever, with methane as gas component and a specific corn starch as dust sample, the practicality of the whole procedure, the scattering of the results and the deviation between the testing apparatuses is investigated. This paper summarizes the experimental procedure adopted and objectives of the first round-robin phase involving three of the four original German companies, plus volunteering laboratories from Australia, Belgium, Czech Republic, France, Poland and P.R. China. The results will have an impact on the new standard and may lead to robust data for later simulation purposes.
Quasi-static dispersion of dusts for the determination of lower explosion limits of hybrid mixtures
(2022)
Knowledge of explosion limiting concentration of explosible materials is necessary forthe design of explosion protection measures. Currently employed methods of testing MEC of a dust cloud or LEL of a hybrid mixture are based on arbitrary assumptionsand possess technical limitations that often lead to values of MEC/LEL, which are unrealistically low or poorly reproducible. This contribution presents an improved method for experimental determination of MEC of a combustible dust cloud or LEL of a flammable gas or hybrid mixture. The new setup operates under laminar conditions and allows a uniform suspension of dust particles in an open top acrylic glass tube. Dust concentration is measured with the help of infrared sensors installed a few centimeters above and below the ignition source. In order to evaluate the dependence of MEC on flow front velocity, MEC of lycopodium was determined at four flow velocities. The results show that the flow field intensity does not significantly influence the MEC of lycopodium for the flow ranges tested in this work. Moreover, LEL of hybrid mixtures of lycopodium and methane was also tested at flow velocities of 4.7 cm/s, 5.8 cm/s, 7 cm/s and 11 cm/s and compared with the values obtained from other sources. The results suggest that the requirement of high energy pyrotechnical igniter may be relinquished, provided that a truly homogeneous suspension of dust particles could be achieved. Moreover, the effect of relative amount of dust and gas, on the course of ignition and flame propagation in hybrid mixtures at their LEL, was studied by the help of high speed videos. For hybrid mixtures of carbonaceous dusts (like lycopodium) at their LEL, ignition occurs in the gas phase, however, flame propagation is only possible through a two-way interaction of dust and gas during the course of combustion.
MIKE III apparatus tests were conducted to investigate the minimum ignition energy (MIE) of coal dusts in air and O2/CO2 atmospheres with and without small amount of CH4/H2. The O2 mole fraction (XO2) in the gas mixtures varied from 21% to 50% with the CH4/H2 mole fraction from 0 to 2%. Experimental result showed that MIE of coal dusts significantly decreases even by three orders of magnitude in mJ with increasing XO2 and the addition of CH4/H2. Compared with CH4, H2 had a relatively strong promotion effect on the spark ignition of coal dusts. The inhibiting effect of CO2 was found to be much stronger than N2, but this inhibiting effect of CO2 could be eliminated by 9% increment of XO2. The effect on MIE of coal dusts thus followed by the order: 9% increment of XO2 > CO2 replacing N2 > 2% CH4 or H2 addition. Moreover, two empirical models were used to estimate the MIE of hybrid dust-gas mixture (HMIE), and the results showed that calculated data can well reflect the promoting effect of elevated XO2 and flammable gas addition, and the inhibiting effect of inert gas
Major accidents in the process industry often lead to the release of light or dense gases, which can mean a thread to employees, local residents or to the environment. Possible scenarios are therefore analyzed and evaluated in advance for approval issues. There is a trend, where simple empirical models are being replaced with more complex numerical models. Gaussian dispersion models or models based on dimensional analysis approaches are for example, increasingly replaced by CFD simulations. The main reason for this is the potentially higher accuracy. However, usually scenarios using sharp parameter values are calculated, since comprehensive consideration of parameter distributions via Monte Carlo or Latin Hypercube Sampling fails due to the numerical effort. This includes the risk that the influence of uncertainties on the simulation results is not taken into account. Response surface methods offer an alternative, with which the CFD problem can be mapped onto an algebraic surrogate model. If this is sufficiently precise, parameter sampling can also be carried out with the surrogate as well, as shown in some publications. Previous investigations only demonstrated the basic principle using trivial dispersion models. In this paper two realistic CFD simulations from the plant safety area are considered: VOC emissions from a storage tank and near-ground dense gas emissions. The entire procedure of response surface determination and parameter studies was automated and parallelized for high-performance-computing, and is carried out on the underlying CFD grids. For the CFD simulations as well as for all visualizations, the commercial software ANSYS CFX and the open source software OpenFOAM were used. The aim of this paper is to demonstrate the method using industry-relevant applications as well as to show how this can be used in practical engineering applications. The quality of surrogate modeling, the numerical effort and advantages that can result from the procedure are discussed as well as advantages which may result from taking parameter uncertainties into account in safety studies.
The explosion characteristics of anthracite coal dust with/without small amount of CH4 (1.14 vol %) were investigated by using a 20 L spherical explosion apparatus with an emphasis on the roles of oxygen mole fraction and inert gas. Two methods based on overpressure and combustion duration time were used to determine the minimum explosion concentration (MEC) or the lower explosion limit (LEL) of the pure anthracite coal dust and the hybrid coal-methane mixtures, respectively. The experiment results showed that increasing oxygen mole fraction increases the explosion risk of coal dust: with increasing oxygen mole fraction, the explosion pressure (Pex) and the rate of explosion pressure rise ((dp/dt)ex)) increase, while MEC decreases. The explosion risk of anthracite dust was found to be lower after replacing N2 with CO2, suggesting that CO2 has a better inhibition effect on explosion mainly due to its higher specific heat. However, the addition of 1.14% CH4 moderates the inhibition effect of CO2 and the promotion effect of O2 on anthracite dust explosion for some extent, increasing explosion severity and reducing the MEC of anthracite dust. For hybrid anthracite/CH4 mixture explosions, Barknecht’s curve was found to be more accurate and conservative than Chatelier’s line, but neither are sufficient from the safety considerations. The experimental results provide a certain help for the explosion prevention and suppression in carbonaceous dust industries.
The production of materials with dimensions in the nanometre range has continued to increase in recent years. In order to ensure safety when handling these products, the hazard potential of such innovative materials must be known. While several studies have already investigated the effects of explosions (such as maximum explosion pressure and maximum pressure rise) of powders with primary particles in the nanometre range, little is known about the ignition temperatures and flame velocities. Therefore, the minimum ignition temperature (MIT) of metallic nano powders (aluminium, iron, copper and zinc) was determined experimentally in a so called Godbert-Greenwald (GG) oven. Furthermore, the flame velocities were determined in a vertical tube. In order to better classify the test results, the tested samples were characterised in detail and the lower explosion limits of the tested dust samples were determined. Values for the burning velocity of aluminium nano powders are higher compared to values of micrometre powd
ers (from literature). While MIT of nanometre aluminium powders is within the range of micrometre samples, MIT of zinc and copper nano powders is lower than values reported in literature for respective micrometre samples.
Lithium ion batteries (LIBs) are prone to spontaneous and subsequent fire or explosion resulting from thermal runaway. The vented gases are not only toxic and flammable, their emission can also raise the surrounding pressure rapidly. In this study, characteristic variations of explosion pressure rise, rate of explosion pressure rise and 𝐾(sT)-value have been evaluated. The characteristic 𝐾(𝑠𝑇)-values were determined to evaluate the explosive behavior of LIBs during thermal runaway. The estimated values were compared to that of other explosive substances.
In diesem Verbundvorhaben werden standardisierte Messverfahren für hybride Gemische erarbeitet, die der Bestimmung sicherheitstechnischer Kenngrößen des Explosionsschutzes dienen. Unter einem hybriden Gemisch wird dabei ein mehrphasiges System aus Brenngas oder brennbarem Dampf, sowie Luft und brennbarem Staub verstanden. Die Veröffentlichung der Ergebnisse erfolgt in einem Abschlussbericht und als DIN-Spezifikation (DIN SPEC). Diese DIN SPEC versetzt Prüfinstitute und Industrie in die Lage, Explosionsgefahren beim Betrieb technischer Anlagen mit hybriden Gemischen einzuschätzen und damit Prozesse sowohl sicherer als auch effizienter zu steuern. Dieses Projekt wird im Rahmen des WIPANOProgramms (Wissens- und Technologietransfer durch Patente und Normen) vom BMWi gefördert.
Brandlasten stellen eine Gefahr für den Menschen sowie für das industrielle Gebäude dar, in dem sie gelagert sind. Das Brandverhalten der Brandlasten bestimmt dabei maßgeblich die Schwere der Gefahr. Brandschutzmaßnahmen dienen dazu, im Brandfall das Schadensausmaß zu minimieren. Um geeignete Brandschutzmaßnahmen ableiten zu können, ist es erforderlich, das Brandverhalten der Materialien zu bewerten. Eine international angewandte Möglichkeit zur Bewertung des Brandverhaltens von Feststoffen und Flüssigkeiten ist die Verbrennungseffizienz χ. Sie ist das Verhältnis der während einer Verbrennung effektiv freigesetzten Wärme (effektive Verbrennungswärme) zu der maximal möglichen Wärmemenge (Heizwert). Die Grundlage zur Bestimmung von Verbrennungseffizienzen stellen die Wärmefreisetzungsrate (HRR) und die Massenverlustrate (MLR) dar. Ein standardisiertes Bestimmungsverfahren existiert nicht. So liegt es in dem Ermessen des Anwenders, in welcher Prüfapparatur die Experimente durchgeführt werden und welcher Zeitbereich der HRR und der MLR zur Berechnung der Verbrennungseffizienz verwendet wird. Im Rahmen dieser Arbeit wurden Versuche im Cone Calorimeter und im Single Burning Item test anhand von hölzernen Materialien durchgeführt. Die Ergebnisse zeigen auf, dass sich in Abhängigkeit von der verwendeten Prüfapparatur und in Abhängigkeit des für die Berechnung betrachten Versuchszeitraumes verschiedene Werte der Verbrennungseffizienz ergeben. Auf Grundlage dieser Erkenntnisse wurde eine Methode entwickelt, mit der reproduzierbare Werte berechnet werden können. Dabei erfolgt die Berechnung lediglich für die Vollbrandphase. Diese Brandphase stellt die Hauptbrandphase dar und ist durch eine gleichmäßige Verbrennung mit wenigen Änderungen in der Branddynamik gekennzeichnet.
Vor dem Hintergrund der Zurückziehung der DIN 18230 Teil 2 zur Bestimmung des m-Faktors von Materialien für die Brandlastbewertung im Industriebau müssen neue Wege gefunden werden, wie das Abbrandverhalten alternativ quantifiziert werden kann. Der Beitrag fasst die Entstehung und die Entwicklung der Bestimmungsweise von Abbrandfaktoren zusammen und gibt Ausblick auf eine neue Möglichkeit zur Bewertung von Brandlasten, die Verbrennungseffizienz.
CFD modeling approach of smoke toxicity and opacity for flaming and non-flaming combustion processes
(2016)
Current engineer’s methods of fire safety design include various approaches to calculate the fire Propagation and smoke spread in buildings by means of computational fluid dynamics (CFD). Because of the increased computational capacity, CFD is commonly used for prediction of time-dependent safety parameters such as critical temperature, smoke layer height, rescue times, distributions of chemical products, and smoke toxicity and visibility. The analysis of smoke components with CFD is particularly complex, because the composition of the fire gases and also the smoke quantities depends on material properties and also on ambient and burning conditions. Oxygen concentrations and the temperature distribution in the compartment affect smoke production and smoke gas toxicity qualitatively and quantitatively. For safety designs,
it can be necessary to take these influences into account. Current smoke models in CFD often use a constant smoke yield that does not vary with different fire conditions. If smoke gas toxicity is considered, a simple approach with the focus on carbon monoxide is often used. On the basis of a large set of experimental data, a numerical smoke model has been developed. The developed numerical smoke model includes optical properties, production, and toxic potential of smoke under different conditions. For the setup of the numerical model, experimental data were used for calculation of chemical components and evaluation of smoke toxicity under different combustion conditions. Therefore, averaged reaction equations were developed from experimental measurements and implemented in ANSYS CFX 14.0.
Bei der Erstellung von brandschutztechnischen Nachweisen für Industriebauten werden häufig Ingenieurmethoden des Brandschutzes genutzt. Dazu zählen die Modellierung und Berechnung von Brandszenarien mittels numerischer Strömungsmechanik (computational fluid dynamics, CFD). In dieser Arbeit wurde ein auf numerischer Strömungs- und Strukturmechanik basierendes Verfahren zur brandschutztechnischen Bemessung von Bauteilen am Beispiel einer Industriehalle angewendet. Instationäre Temperaturverläufe aus der Heißgasschicht dienten dabei als Eingangswerte für die Bauteilberechnung. Aus den Ergebnissen wurde die äquivalente Branddauer berechnet, mit der die rechnerisch erforderlichen Feuerwiderstandsdauern der Bauteile bestimmt werden konnte.
This paper describes experiences and results of experiments with several metallic dusts within the nanometer range. The nano dusts (aluminium, iron, zinc, titanium and copper) were tested in a modified experimental setup for the test apparatus 20 L-sphere (also known as 20-L Siwek Chamber), that enables the test samples to be kept under inert atmospheric conditions nearly until ignition. This setup was already introduced in earlier papers by the authors. It was designed to allow the determination of safety characteristics of nano powders under most critical circumstances (e.g. minimisation of the influence of oxidation before the test itself). Furthermore the influence of passivation on explosion behaviour is investigated and additional tests with deposited dust were carried out to describe the burning behaviour of all dusts. For a better characterisation all samples were tested with a simultaneous thermal analysis (STA). To minimise the influence of oxidation all samples were handled at inert conditions until shortly before ignition or start of the test respectively.
Behavior of dust/air mixtures is very complex and difficult to predict since it depends on material properties as well as boundary conditions. Without other influences airborne particles deposit due to gravity but the time it takes for total deposition as well as easiness of resurrection depends very much on the specific dust sample and the boundary conditions. It still lacks a complete understanding of all interacting reasons and one approach is using experimentally determined characteristics, one is named dustiness.
Dustiness is the tendency of dust to form clouds and to stay airborne. Dustiness is determined with two basic principles, which are light attenuation and ratio of filled-in and measured mass. Assessment of dustiness of industrial powders has been done for a long time regarding work place safety. Dustiness is used there to determine inhalable fraction and to evaluate health risks. Lately it became interesting in dust explosion protection as well. Dustiness could be used to optimize determination of zones, adaption of venting area and/or for positioning of suppression systems.
Dustiness can be useful in many ways but is not a physical property of dusts, therefore it depends on material properties such as density, particle size distribution, shape and water content as well as boundary conditions or determination method. This makes it very difficult to compare dustiness for different techniques and apparatuses and determination method as well as results should be considered carefully. This work gives an overview of existing standards, recent research and suggests improvements to the new dustiness as proposed for dust explosion protection.
The amount of heat, which is released by fire loads during the combustion
process, depends on the material characteristics, the ventilation
conditions, the storage density and the distribution. To evaluate the
structural stability of buildings in case of fire, the fire load has to be
quantified. In Germany, the fire load is quantified by using the combustion
factor m, while internationally the combustion efficiency χ is
applied. Both factors assess the burning behavior of materials, but the
determination is carried out in different ways. Since the testing facility
was abolished fifteen years ago, it is not possible to determine the combustion
factor m anymore. So, it should be found out, if the combustion
efficiency χ is a convenient method to quantify the fire load under the
consideration of the German standards. As a part of the research, combustion
efficiencies χ were determined for eight materials in the cone
calorimeter and the single burning item test at different heat fluxes.
The values of the combustion efficiencies χ as well as the corresponding
combustion factors m were discussed and compared to the values of the
literature. The results show an influence of the testing facility on the
combustion efficiency. The values of the combustion efficiency determined
in the single burning item test were higher than the values from
the cone calorimeter.
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.
Um den Anforderungen der Bauordnung an den Brandschutz für Gebäude gerecht zu werden, sind bauliche Anlagen so zu errichten und zu betreiben, dass der Entstehung eines Brandes und der Ausbreitung von Feuer und Rauch vorgebeugt, die Rettung von Menschen und Tieren sowie wirksame Löscharbeiten ermöglicht werden.
Hierbei besteht, einer steigenden Komplexität sowie einem differenzierten Nutzungskonzept öffentlicher Gebäude geschuldet, häufig die Notwendigkeit von normativen Vorgaben abzuweichen und dennoch ein gleichwertiges Sicherheitsniveau nachzuweisen.
Es werden Personensimulationsmodelle eingesetzt, die einerseits das Vorhandensein eines Schutzzielniveaus durch die Abbildungen von Personenbewegungen nachweisen können und andererseits durch Parametervariation eine szenarienabhängige Betrachtung ermöglichen.
Der vorliegende Beitrag vergleicht die Ergebnisse einer kleinskaligen Evakuierungsübung mit drei mikroskopischen Personensimulationsmodellen und geht der Frage nach der Aussagekraft dieser Bewertungsmethode nach. Es zeigt sich, dass die alleinige Betrachtung absoluter Entleerungszeiten keine zuverlässige Beurteilungsgrundlage für das komplexe Schutzziel Personensicherheit darstellt.
Gegenstand des Aufsatzes ist die experimentelle Untersuchung der Auswirkungen verschiedener Zündquellen sowie der Systembeschaffenheit auf das Brandverhalten EPS-basierter Wärmedämm-Verbundsysteme (WDVS). Dazu wurden Brände ausgewertet, die mit EPS-basierten WDVS in Verbindung stehen. Ergebnis dieser Auswertung war, dass nicht wie bisher angenommen der Brand in dem Gebäude (z. B. der Wohnungsbrand), sondern vielmehr der Brand außerhalb des Gebäudes, insbesondere der des Abfallbehälters, das häufigste und kritischste Brandszenario für EPS-basierte WDVS darstellt. Daraufhin wurden an der BAM Bundesanstalt für Materialforschung und -prüfung Brandversuche in der Single Burning Item-Prüfeinrichtung (SBI) an nicht intakten EPS-basierten WDVS-Ausschnitten durchgeführt. Mit zunehmender Beschädigungsgröße und -tiefe stieg die vom System freigesetzte Wärmefreisetzungsrate an. Zusätzlich werden Brandversuche eines Sockelbrandszenarios mit einer abfallbehälteräquivalenten Brandlast vorgestellt. Aus den Brandversuchen werden Empfehlungen zur Verbesserung der Systemsicherheit abgeleitet.-------------------------------------------------------------------------------------------------------------------------------------------------
The influence of different ignition sources and conditions of the systems on the fire behaviour of external thermal insulating systems (ETICS) have been investigated experimentally. As a basis of the investigations real cases with EPS based ETICS have been analysed. The analysis showed that the fire in front of the building is the most common and critical fire scenario for these systems. These fires are often caused by waste containers. This result contradicts the former common assumption that the fire inside the building is the most critical fire scenario for ETICS. Intermediate scale experiments in the Single Burning Item (SBI) test apparatus have been performed at BAM with ETICS specimens. The render of the systems was partially damaged. The measured heat release rate of the specimens increased significantly with size and depths of the damages. Additionally results of large scale experiments with a waste container equivalent ignition source are presented and recommendations to enhance fire safety of these systems are given.
Computer simulation tools were used to evaluate pedestrian movement. This study compares the reliability of three simulation tools and the calculation method by Predtetschenski und Milinski with a controlled evacuation exercise of a lecturer room. The comparison shows that each simulation tool used underestimate the total evacuation time and that an indicator for a low level of fitness does not necessarily lead to a long evacuation time. Results revealed that calibration of simulation tools is required to model heterogeneous population properly. Therefore, further experiments with a realistic, heterogeneous composition of participants have to be made.
Filter criteria in the Frame of Fire PSA identify compartments in a first qualitative analysis for which the contribution to the overall core damage frequency of the NPP is negligible. The aim of the filter criteria is to reduce the number of compartments to be analysed precisely in Fire PSA. One example for filter criteria is the 'fire load criterion'. By the fire load criterion compartments with a fire load density of less than 90 MJ/m² are 'screened out' which means to exclude them from a precise analysis in Fire PSA. Neither the justification of the particular value of 90 MJ/m² is well documented nor does this criterion take into account varying compartment configurations such as ventilation conditions, physical and chemical properties of the fire load as well as compartment characteristics.
A probabilistic set of filter criteria was developed to overcome the restrictions of the fire load criterion. In line with the 'fire load criterion', the probabilistic set of filter criteria assumes that a compartment can be screened out if a fire is not able to cause any damage to other components within the compartment. Therefore, the electrical failure of an electrical cable conservatively represents the damages of all components. It is assumed that the electrical cable failure occurs when the maximum cable temperature exceeds an experimentally determined failure temperature. The maximum cable temperature that can occur in a compartment fire is mainly influenced by the four significant factors: 1. inlet air stream of the mechanical ventilation, 2. the fire growth rate, 3. the compartment floor area and 4. the compartment height. A parameter study revealed how the significant factors affect the maximum cable temperature in fictitious compartment fires. The results of the parameter study are transferred on true Nuclear Power Plant compartments. However, it is not possible to determine precisely the occurrence of an electrical cable failure because of uncertainties in the maximum cable temperature and the failure temperature. The probabilistic set of filter criteria considers these uncertainties and determines the probability of cable failure for true compartments to be screened in Fire PSA. Finally, a compartment can be screened out in Fire PSA if the failure probability exceeds a predefined accepted threshold value for the failure probability. The theoretical application of the methodology is shown at the end of the paper.
Measurement and numerical simulation of local dust concentrations over time in a vessel can only be done with some uncertainty due to the complexity of the behavior of dust/air mixtures. Dust concentration was measured in a 50 m³ vessel and compared to simulations with a commercial CFD code. A 50 m³ silo was used with two different filling methods. In one setup dust was conveyed with pneumatic filling at the top. The other filling was done with pressurized air and a homogenous injection via eight nozzles. Experiments were repeated three to four times with two kinds of dusts and the results were used to evaluate reproducibility of dust concentration measurements over time in a vessel depending on the filling method. Dust concentrations over time varied up to 30% from the average for homogenous injection and even more for pneumatic filling.
Numerical investigations were done with maize starch. Measured concentrations were compared to simulated ones with the commercial CFD code ANSYS CFX R14 using an Euler/Lagrange approach. Drag force, turbulent dispersion force, particle size distribution, particle surface area and particle/particle interaction were modeled. A general agreement of measurement and simulation was achieved. Numerical simulations of filling processes were used to predict parts of the vessel where the lower explosion limit is reached and exceeded. This could help to improve dust explosion protection, if it is used to find configurations where the dust concentration exceeds the lower explosion limit only in small parts of the vessel during filling, e.g. using different injection points or injection angles. The volume where LEL is reached or exceeded in a 50 m³ silo is shown for pneumatic and homogenous filling. Volume of combustible atmosphere in the vessel over time is compared for two pneumatic filling configurations and one worst-case homogenous injection configuration.
This paper describes experiences and results of experiments with several metallic dusts within the nanometer range. The nano dusts (aluminium, iron, zinc, titanium and copper) were tested in a modified experimental setup for the test apparatus 20 L-sphere (also known as 20-L Siwek Chamber), that enables the test samples to be kept under inert atmospheric conditions nearly until ignition. This setup was already introduced in earlier papers by the authors. It was designed to allow the determination of safety characteristics of nano powders under most critical circumstances (e.g. minimisation of the influence of oxidation before the test itself). Furthermore the influence of passivation on explosion behaviour is investigated and additional tests with deposited dust were carried out to describe the burning behaviour of all dusts. For a better characterisation all samples were tested with a simultaneous thermal analysis (STA). To minimise the influence of oxidation all samples were handled at inert conditions until shortly before ignition or start of the test respectively.
Nanotechnologie gilt als die Wachstumsbranche der nächsten Jahrzehnte. Nicht zuletzt ist diese Entwicklung den veränderten Eigenschaften der Materialien im Nanometerbereich zuzuschreiben. Das anders geartete Verhalten solcher Stoffe und die daraus ableitbare Unkenntnis über die Interaktion dieser Materialien mit ihrer Umgebung erfordert zur Risikoabschätzung eine intensive Auseinandersetzung mit der Thematik. Aufgrund von Erfahrungen mit Stäuben mit Partikelgrößen im Mikrometerbereich ist bekannt, dass Stoffe mit abnehmender Partikelgröße kritischer einzustufen sind, da ihre Explosionsheftigkeit und Zündempfindlichkeit durch die Zunahme der reaktiven Oberfläche des Staubs zunimmt. Nachfolgend werden Ergebnisse von Versuchen mit nanoskalig hergestellten Metallstäuben präsentiert. Der Fokus dieser Studie lag darauf, möglichst das kritischste Brand- und Explosionsverhalten solcher Stäube zu erfassen.
The standard cone calorimeter according to ASTM E 1354 and ISO 5660 enables reaction-to-fire tests to be performed in ambient atmospheric conditions. A controlled-atmosphere chamber modifies the standard apparatus in a way that allows tests to be performed in nonambient conditions as well. The enclosed chamber is placed underneath the standard exhaust hood and does not have a closed connection to the hood. With this open arrangement, the exhaust gases are diluted by excess air drawn in from the laboratory surroundings. Heat-induced changes in the consequential dilution ratio affect the calculation of fire quantities and, when neglected, lead to deviations of up to 30% in heat release rate. The paper introduces a test protocol and equations to calculate the heat release rate taking dilution effects into account. A mathematical correction is shown that compensates for the dilution effects while avoiding extensive mechanical changes in the equipment.
This paper describes a modified experimental setup for the test apparatus 20-L-Sphere (also known as 20-L Siwek Chamber), that enables the test samples to be kept under inert atmospheric conditions nearly until ignition. This setup was designed to allow the determination of safety characteristics of nano powders under most critical circumstances (e.g. minimisation of the influence of oxidation before the test itself). The aim of this modification was to determine, whether or not the current setup and procedures underestimate the explosion violence and ignitability of nano powders. For this purpose, two different methods, the so called inerting method and the modified method (using a special nozzle called mushroom nozzle) are used. Both methods are described in the paper. The work includes experimental results of micrometer dusts to validate the modified method. Moreover first results of nanometer iron and aluminium dusts are presented, which were kept at inert conditions until shortly before the ignition. The tested nano iron was found to react pyrophoric, as soon as it gets in contact with air, while the tested nano aluminium did not generally show such behaviour. Tests with nano aluminium using the inerting method revealed a higher pressure rise in comparison to the standard test procedure. This could suggest a different extent of passivation. To investigate this effect more closely, further testes with more nano powders are required.
Modified setup of 20-L-sphere for the determination of safety characteristics of nano powders
(2013)
This paper describes a modified experimental setup for the test apparatus 20-L-Sphere (also known as 20-L Siwek Chamber), that enables the test samples to be kept under inert atmospheric conditions nearly until ignition. This setup was designed to allow the determination of safety characteristics of nanopowders under most critical circumstances (e.g. minimisation of the influence of oxidation before the test itself). The aim of this modification was to determine, whether or not the current setup and procedures underestimate the explosion violence and ignitability of nanopowders. The work includes experimental results of micrometer dusts to validate the modified setup. Moreover first results of nanometer iron and Aluminium dusts are presented, which were kept at inert conditions until shortly before the ignition. The tested nano iron was found to react pyrophoric, as soon as it gets in contact with air, while the tested nano Aluminium did not generally show such behaviour.
In practice, occurring explosive dust/air mixtures are usually not homogeneous and in many cases do not spread over the whole enclosure. For the safety measure explosion venting, a smaller venting area might be sufficient as calculated according to venting standards (EN 14491 'Dust Explosion Venting Protective Systems'). The tendency of bulk materials to form dust clouds is not taken into account so far. To optimize the design of explosion protection measures, a new safety characteristic the so-called 'dustiness' (see VDI 2263 part 9 'Determination of Dustiness of Bulk Materials'.) could be useful. Dustiness means the tendency of dust to form dust clouds. There are six dustiness groups (DG) from one to six (one means little tendency to stay airborne, six the opposite). The paper describes first results on the reasons for different dustiness and on dust explosion venting for several dusts of different dustiness. A 75 L vertical tube apparatus for vented dust explosion experiments was created. Pressure, pressure rises and flame speeds were measured to evaluate the course of the dust explosions. In addition the paper includes CFD simulations (ANSYS CFX) about the possibilities to model the dust/air mixtures with the Euler/Lagrange approach. In order to simulate the settling of dust clouds more realistic the particle surface area was adjusted and the particle size distribution was taken into account. Results were compared to experimentally determined terminal velocities.
Observed autoignition events and extinguishing the resulting smouldering fires in an underground storage system of a coal-fired power plant have provided insight into the array of contributing variables, and some experience on quantifying the risk with alternative scenarios of event initiation, progress and potential mitigation. Although the first attempts to quantify the risk suggest high sensitivity to the sequence of action taken after fire alarm, and no similar storage sites really exist, some recommended preventive, corrective and other mitigating activities can be at least partly defined and improved by using the cumulative experience and parallel efforts in other closed or underground storage sites. However, there are also so-called black (or at least grey) swans: unexpected events for which the facility may be poorly prepared for. In the case of the underground storage silos, such an event was experienced when incoming cold coal during a harsh winter season froze the sewer system that normally protects the stored coal from seepage water. With blocked normal bypass, the seepage water found its way to the coal silos and created large clumps of icy coal that blocked the coal conveyors. Although freezing weather is not unusual at high-latitude power plants, the common methods to combat freezing of coal are mainly useful for open storage sites and above-ground transport. Options for mitigation are discussed, as well as the event chain leading to an event that had never previously occurred. The case is discussed from the point of view of options to prepare for rare or unforeseen events.
A new safety characteristic the 'dustiness' according to VDI 2263 part 9 (Verein Deutscher Ingenieure, 2008) is investigated. Dustiness means the tendency of a dust to form clouds. The paper deals with the influence of the dustiness on vented dust explosions. In order to look into the effects of the dustiness on dust cloud formation and explosion properties experiments and simulations in a vertical dust dispersion glass tube apparatus were carried out.
Preliminary explosion experiments showed that the dustiness has an influence on the reduced explosion pressure in a vented 75 L test apparatus. Dusts with comparable pmax and KSt values and different dustiness were tested. Dusts with higher dustiness produced higher overpressures, despite comparable safety characteristics. In order to verify the results for applications in the process industries further tests with different settings are planned as well as industrial scale experiments. Characteristics of the dust such as particle size, density, specific surface area and particle shape, which influence the dispersibility, have been determined experimentally.
The Euler/Lagrange and the Euler/Euler approaches are compared for simulating an exemplary dust/air mixture. Especially sedimentation and the ability of the approaches to simulate the tendency of dust to stay airborne were investigated. The Euler/Lagrange approach is better suited for simulating local dust concentrations, particle size distributions and particle forces. It could be used to point out regions of high dust concentrations in a vessel. With the Euler/Euler method it is possible to achieve fast solutions for one specified diameter, but the simulated dust/air mixtures are always more homogenous than in reality. ANSYS CFX version 13 was used in all simulations.
Determination of measurement uncertainties in adiabatic hot-storage experiments for reactive dusts
(2013)
The formal kinetics of self-ignition of solid bulk materials theoretically can be derived from just one single adiabatic hot-storage test. The question arises how uncertainties in the measurements can be quantified and how these uncertainties affect the results of the subsequent predictions. Adiabatic and isoperibolic hot-storage basket tests were performed for samples of lignite coal, black coal, cork dust, a polymer dust, and wax-coated silicid acid. In the adiabatic experiments, the starting temperature and the temperature control of the oven were varied systematically to study the uncertainty margin of adiabatic tests. The apparent activation energy and the pre-exponential factor of the lumped reaction were derived from the adiabatic tests including average values and standard deviations and compared to isoperibolic experiments performed according to the European standard EN 15188. It could be shown that with a precise temperature control of the adiabatic oven combined with an automated computation of the maximum rate of temperature rise the uncertainty of the apparent activation energy can be limited to less than 10%.
The Fractional Effective Dose model was used to predict the fire smoke toxicity numerically. In this context fire tests were carried out for three different building materials: polyurethane, flame retardant polyurethane and polyvinyl chloride. The fire tests were performed for flaming and smoldering combustion. The aim of the fire tests was to determine the light extinction, the smoke density and the combustion products at varying oxygen concentrations, different temperatures and different irradiance levels. The fire tests were performed in the German DIN-tube and also in the Cone Calorimeter. Stoichiometric coefficients at varying oxygen concentrations and temperatures were determined from measurements. With these stoichiometric coefficients reaction equations were defined and implemented in the Computational Fluid Dynamics (CFD) program, ANSYS CFX. The implemented reaction equations were used to account for different ventilation and temperature conditions in the simulation. The fire and smoke propagation was calculated numerically with CFD in the room corner test geometry. Equations to account for toxicity and light extinction were also implemented and were used to analyse the toxicity and the optical properties of fire smoke with CFD.
Die Nanotechnologie gilt als die Wachstumsbranche der nächsten Jahrzehnte. Nicht zuletzt ist diese Entwicklung der veränderten Eigenschaften der Materialien im Nanometerbereich zuzuschreiben. Das andersgeartete Verhalten solcher Stoffe und die daraus ableitbare Unkenntnis über die Interaktion dieser Materialien mit ihrer Umgebung erfordert eine intensive Auseinandersetzung mit der Thematik zur Risikoabschätzung. Deshalb wurden und werden z.B. die gesundheitlichen Wirkungen von nanoskaligen Materialien auf den menschlichen Organismus bereits in zahlreichen Studien und Forschungsvorhaben untersucht.
Fundierte Erfahrungen und Erkenntnisse über Brand- und Explosionsgefahren bei der Herstellung und Handhabung von nanoskalig hergestellten brennbaren Materialien bzw. von Stoffen mit Partikelgrößen < µlpm liegen hingegen bisher kaum vor. Aufgrund von Erfahrungen mit Stäuben mit Partikelgrößen im Mikrometerbereich ist jedoch bekannt, dass Stoffe mit abnehmender Partikelgröße kritischer einzustufen sind, da beispielsweise ihre Explosionsheftigkeit und Zündempfmdlichkeit zunehmen.
Die Explosionseigenschaften brennbarer Stäube werden durch sicherheitstechnische Kenngrößen beschrieben. Die Bestimmung sicherheitstechnischer Kenngrößen von Nanostäuben - welche die Grundlage für eine Gefährdungsbeurteilung und das Auslegen von Schutzmaßnahmen für Anlagen bilden - erfolgte bislang nur in geringem Umfang. Diese Untersuchungen wurden zudem großteils in Analogie zu den genormten Verfahren für Mikrostäube durchgeführt. Es wurde dabei jedoch nicht überprüft, ob die für Mikrostäube etablierten Prüfverfahren für eine sichere Bewertung des Brenn- und Explosionsverhaltens geeignet sind. Darüber hinaus kann es durch aus den Prüfapparaturen freigesetzten Staub zu einer Gesundheitsgefährdung der Mitarbeiter kommen.
Da bisherige Untersuchungen zum Brand- und Explosionsverhalten von Nanostäuben den Oxidationsgrad der Proben nicht berücksichtigt haben, bestand die Vermutung, dass das Gefahrenpotential von Nanostäuben bislang unterschätzt werden könnte. Deshalb wurde ein modifizierter Prüfaufbau der Prüfapparatur 20-L Kugel entwickelt, um Nanostäube und deren Gefahrenpotential richtig einschätzen und der Kernfrage im Titel dieses Beitrages näher auf den Grund gehen zu können.
Insbesondere metallische Nanostäube wurden bei den ersten Tests zur Bestimmung des Brand- und Explosionsverhaltens untersucht. Bei entsprechenden Stäuben ist zu erwarten, dass sich eine mögliche Oberflächenoxidation bzw. Passivierung besonders stark auf das Explosionsverhalten auswirken könnte. Darüber hinaus ist bekannt, dass metallische Stäube pyrophor reagieren oder sich aufgrund der hohen Scherkräfte bereites beim Eindüsen in die Versuchsapparatur entzünden können. Dies kommt gelegentlich bei Metallstäuben mit Medianwerten von wenigen Mikrometern vor. Da bei nanoskaligen Stäuben theoretisch wesentlich größere Oberflächen auftreten, ist davon auszugehen, dass sich dieses Verhalten bei solchen Stäuben verstärken könnte. Dies wurde von Wu et al. in für nanoskalige Eisen und Titanpartikel nachgewiesen.
Die Brandlastberechnung für Industriebauten erfolgt in Deutschland entsprechend der DIN 18230-1. Brandlasten bestehen in den meisten Fällen aus verschiedenen brennbaren Feststoffen und Flüssigkeiten, die in Abhängigkeit ihrer Lagerungsart, ihren stofflichen Eigenschaften und den Ventilationsbedingungen unterschiedliche Wärmeleistungen zur Folge haben. Als Eingangsgröße zur Brandlastberechnung berücksichtigt der sogenannte Abbrandfaktor m (m-Faktor) das individuelle Abbrandverhalten von Feststoffen und
Flüssigkeiten. Sein Wert ergibt sich aus dem Vergleich der zeitlichen Temperaturentwicklung beim Abbrand einer Probe mit der Referenzprobe. Bei der Referenzprobe handelt es sich um sägerauhes Fichtenholz, dem ein m-Faktor von m = 1,0 zugeordnet wurde. Die Bestimmung des m-Faktors erfolgte bis vor einigen Jahren entsprechend der DIN 18230-2 in dem m-Faktor-Ofen. Bis zu der Verschrottung des letzten in Deutschland existierenden m-Faktor-Ofens wurden mehr als 100 m-Faktoren von verschiedenen Stoffen ermittelt. Zurzeit ist es nicht möglich zur Brandlastermittlung m-Faktoren experimentell zu bestimmen.
Da infolge industrieller Entwicklungen zunehmend neue Stoffe auf den Markt kommen, deren Abbrandverhalten zu bestimmen.
Im Rahmen der schutzzielorientierten Brandschutzbemessung müssen, neben dem Nachweis der Wirksamkeit von Schutzmaßnahmen, Nachweise zur Einhaltung von in Abhängigkeit des Sicherheitsniveaus festgelegter Grenzwerte für die zulässige Wärmestrahlung, die Temperatur der oberen und unteren Rauchschicht, die toxische Konzentration von Rauchgasen und die Mindestsichtweite zum Schutz von Personen im Bauwerk erbracht werden.
Dieser Beitrag beschreibt einen modifizierten Versuchsaufbau für die Prüfapparatur 20-L Kugel, der es ermöglicht, die zu untersuchenden Staubproben bis kurz vor deren Entzünden unter Ausschluss von Sauerstoff zu handhaben. Dieser modifizierte Aufbau wurde entwickelt, um die Bestimmung Sicherheitstechnischer Kenngrößen von möglichst unpassivierten Nanostäuben zu gewährleisten. Das Ziel dieser Modifikationen ist es, festzustellen, ob es bei der Standardapparatur und -prüfverfahren zu einer Unterschätzung der Explosionsheftigkeit und Zündempfindlichkeit durch Passivierung von Nanonstäuben vor der eigentlichen Untersuchung kommen könnte.
Zu diesem Zweck wurden zwei verschiedene Methoden (das sogenannte Inertisierungsverfahren und das modifizierte Verfahren mit Verteilerschale) verwendet. Beide Methoden sind in diesem Artikel beschrieben. Er umfasst experimentelle Ergebnisse von Mikrostäuben, welche zur Validierung verwendet worden und darüber hinaus erste Ergebnisse mit Eisen und Aluminium Nanostäuben, die bis kurz vor dem Entzünden unter inerten Bedingungen gehandhabt worden. Das getestete Nano Eisen reagiert pyrophor, sobald es mit Luft in Kontakt kam, während bei dem untersuchten Nano Aluminium solches Verhalten nicht generell beobachtet werden konnte. Die Versuche mit Nano Aluminium und dem Inertisierungsverfahren zeigten einen höheren Druckanstieg im Vergleich zum Standard-Prüfverfahren. Dies könnte auf einen unterschiedlichen Grad der Passivierung hindeuten. Um diese Beobachtung näher zu untersuchen, sind weitere Versuche mit einer größeren Anzahl an Stäuben notwendig.
Anwendung und Vergleich von Verfahren zur Berechnung der äquivalenten Branddauer im Industriebau
(2013)
Die äquivalente Branddauer tä wird zur Bestimmung der Feuerwiderstandsdauer erftp von Bauteilen in Industriebauten verwendet. Die Berechnung erfolgt bislang auf der Grundlage des normativenAnsatzes nach DIN 18230 (Verfahren I) und gilt im Zusammenhang mit der Muster-Industriebaurichtlinie (M IndBauRL). Im Rahmen von brandschutztechnischen Nachweisen für Industriebauten ist es ebenfalls möglich, die äquivalente Branddauer unter Anwendung numerischer Methoden zu berechnen (Verfahren II). Die Anwendung solcher Ingenieurmethoden des Brandschutzes ermöglichen die Berücksichtigung komplexer Gebäudegeometrien und Ventialtionsbedingungen sowie lokaler Brandwirkungen auf Bauteile infolge inhomogener Brandlastverteilungen.
Am Beispiel einer Industriehalle werden die Verfahren I und II angewendet und die Ergebnisse gegenübergestellt.
Es werden die Untersuchungsergebnisse für ein geeignetes Zündkriterium zur Bestimmung von Explosionsgrenzen bei nichtatmosphärischen Bedingungen vorgestellt. Dazu wurden für Anfangsdrücke bis 20 bar Zündversuche an den Explosionsgrenzen von CHL/Luft-, CaHg/Luft-, H2/Luft- und NHs/Luft-Gemischen in einem Autoklav mit Sichtscheibe durchgeführt. In der Auswertung wurden visuelle Zündkriterien und ein Druckschwellenkriterium miteinander verglichen. Weiterhin wurden Untersuchungen zum erforderlichen Mindesvolumen des Explosionsgefäßes in Abhängigkeit des Anfangsdrucks durchgeführt. Dazu wurden jeweils Zündversuche mit demselben Gemisch aus CH4 und Luft bei Anfangsdrücken bis 50 bar in Volumina zwischen 0,2 dm3 und 6,0 dm3 durchgeführt.
Sicherheitstechnischen Kenngrößen stellen die Grundlage für die Bewertung von Explosionsrisiken in der chemischen Industrie dar, sowie für die Klassifizierung von gefährlichen Stoffen und Gütern. Sicherheitstechnische Kenngrößen sind, wie Stoff konstanten (z. B. Dichte, Siedepunkt) abhängig von Druck und Temperatur. Im Gegensatz zu Stoffkonstanten sind sicherheitstechnische Kenngrößen jedoch in unterschiedlichem Maße vom Bestimmungsverfahren abhängig. Einfluss können sowohl die Prüfapparatur (z. B. Zündgefäß, Zündquelle) als auch das Bestimmungsverfahren (vor allem das Kriterium) haben. Für die Anwendung im Explosionsschutz ist es erforderlich einheitliche und zuverlässige Werte für die Kenngrößen zu erhalten. Daher sind die Bestimmungsverfahren für Explosionskenngrößen in den meisten Fällen genormt. Sie sind in der Regel dem Geltungsbereich der europäischen Richtlinien 94/9/EG und 1999/92/EG zugeordnet. Wegen des Anwendungsbereiches der Richtlinien gelten bislang die meisten dieser genormten Bestimmungsverfahren (z. B. Explosionsgrenzen (EN 1839), Zündtemperatur (EN 14522)) nur für atmosphärische Bedingungen. Jedoch werden in der chemischen Industrie viele Prozesse mit anderen Oxidationsmitteln als Luft (z. B. reiner Sauerstoff, Distickstoffmonoxid) sowie bei höheren Drücken und Temperaturen durchgeführt.
A controlled-atmosphere cone calorimeter that is built by modifying the Standard apparatus with the addition of a controlled-atmosphere chamber offers a time- and cost-saving approach for reaction-to-fire testing in vitiation- and ventilation-controlled atmospheres. Due to the design of the added enclosure no mechanical changes on cone calorimeter’s Standard ductwork are required. This offers some advantages but also important limitations. The design has an open connection between the outlet of the added enclosure and the cone calorimeter’s exhaust hood. Therefore, sufficient ambient air from the laboratory surroundings is drawn into the System to introduce effects which have the potential to affect test results significantly.
A procedure which is is suitable to consider physical effects of the ambient air on the calculation of the heat release rate is presented as well as initial results of an application towards toxic potency assessment. Signs for Chemical effects of the ambient air such as post reactions are shown but subject of an ongoing work.