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Paper des Monats
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Bond monitoring in temperature dependent applications using Brillouin optical time domain analyser
(2016)
Adhesive bond has to be evaluated for its integrity over a range of temperature. Adhesive is being used to bond the sensors with structures. There is no validated technique to test its performance. In this paper, we propose a Brillouin Optical Time Domain Analyzer (BOTDA) based methodology to detect temperature-induced adhesive bond failure below room temperature using distributed fiber optic sensor. The differential coefficient of thermal expansion of the structure and fiber sensor can lead to bond failure at low temperature. Optical fiber impregnated in the structure will experience differential temperature/strain due to debond of the adhesive. This leads to the frequency and amplitude decomposition of the Brillouin spectra. This is a good indication for real-time monitoring of the integrity of a bond.
Determination of deflagration venting requirements in chemical/process plants is usually carried out using well established standards employing an empirically based formula. However, this formula is shown to have severe shortcomings, especially in the range of low KG-values, where either negative or inconceivably large venting areas can be predicted. Due to these shortcomings a method has been developed using the efflux function for gases as a basis to predict the mass flow through a vent opening in a vessel during an internal explosion. The simulated rise in pressure due to the internal explosion is quantitatively determined from the KG-value, with the mass flow through the vent opening in the vessel resulting from the pressure difference between the vessel and its surroundings. This enables the maximum overpressure as a function of the pressure relief surface area to be predicted. The method takes into account the temperature of the efflux gases and turbulence enhancement brought about by the venting process. In the following paper explosion pressure relief experiments are described and the results from these experiments are compared to predictions from the efflux method. It is shown that by adjusting the assumed turbulence which evolves during the venting process, the reduced explosion pressure can be reasonably well reproduced.
Tetrafluorethen wird von der Polymerindustrie seit Jahrzenten als monomeres Ausgangsmaterial sowohl für die Herstellung von Polymeren (PTFE) als auch für Kopolymere (PCTFE) eingesetzt.
Aufgrund seiner Eigenschaft als chemisch instabiles Gas kann TFE auch ohne Luftsauerstoff oder einen anderen Oxydator explosionsartig zerfallen. Nach der Initiierung des Zerfalls kann dieser unter bestimmten Bedingungen aufgrund des exothermen Reaktionsverhaltens sich selbstständig in Apparaten und Rohrleitungen ausbreiten. Dies geht aufgrund der freigesetzten Reaktionsenthalpie mit einem schlagartigen Anstieg von Druck und Temperatur einher, was zu erheblichen Belastungen der Materialien bis hin zum Versagen und Bruch und möglichen Folgeschäden einschließlich Personenschäden führen kann und in der Vergangenheit bereits mehrfach geführt hat.
Besonders nach Wartungsarbeiten besteht die Gefahr, dass Teilabschnitte im Rohrleitungssystem mit TFE, Stickstoff oder Luft gefüllt sind mit Drücken in einem Bereich zwischen technischem Vakuum und atmosphärischem Druck wohingegen angrenzende Rohrabschnitte oder Behälter immer noch TFE bei Betriebsdrücken bis 32 bar enthalten können. Dabei sind die Abschnitte in der Praxis häufig durch Kugelhähne voneinander getrennt, die aufgrund ihrer Öffnungscharakteristik bereits bei geringen Betätigungswinkeln eine große Querschnittsfreigabe für die Strömung im Rohr ermöglichen. Dadurch können schlagartige Kompressionsvorgänge des Gases im Niederdruckbereich ermöglicht werden, die allein aufgrund der thermodynamischen Zustandsänderung zu einer erheblichen Temperaturerhöhung führen und im schlimmsten Fall zur Initiierung der Zerfallsreaktion führen können.
Es wird erstmalig ein Versuchsaufbau im Industriemaßstab, der einer explosionsartigen Zerfallsreaktion von TFE standhalten kann. Zahlreiche Sicherheitskonzepte einschließlich diverser Berstscheibenkonfigurationen als auch zeitgesteuerte Schnellschlussventile wurden eingehend untersucht und bewertet, um die optimale Versuchskonfiguration für bestmögliche Reproduzierbarkeit festzulegen. Es fand eine systematische Untersuchung der schlagartigen Kompression der Systeme Luft/Luft, TFE/Luft, TFE/TFE und TFE/N2 statt.
In der Hochdrucksektion wurden Drücke bis 30 bar realisiert und im Niederdrucksektor konnten Anfangsdrücke im Bereich weniger Millibar bis hin zu Atmosphärendruck eingestellt werden.
Als Hauptergebnis wurde ein „Hazard diagram“ erstellt, mit dessen Hilfe die Zündwahrscheinlichkeit in Abhängigkeit vom Hochdruck und Niederdruck abgeschätzt werden kann. Gefährliche Bedingungen in Rohrleitungen können dadurch auf einfachem Weg identifiziert werden.
Als Referenzsystem zur Beurteilung der maximal erreichbaren nicht reaktiven Kompressions-temperaturen wurde Luft/Luft verwendet. Die damit ermittelten Daten dienten zur Bewertung von zusätzlichen exothermen Effekten, wie sie etwa bei Vorreaktion des TFE im Falle einer Dimerisierung auftreten können.
Entgegen der ursprünglichen Annahme konnten die Systeme TFE/Stickstoff und TFE/TFE im verwendeten Aufbau nicht durch Kompressionsvorgänge gezündet werden.
The polymeric industry handles Tetrafluoroethylene (TFE) as basic material for polymer (PTFE) and co-polymer (PCTFE) production. As a chemically unstable gas, it can react in an explosive way, without the presence of any other gases. Once initiated such an exothermic reaction can propagate through the pipe system of a plant and might lead to massive damages and/or fatalities. Especially after maintenance parts of the pipe systems can be filled with TFE, nitrogen or air at pressures up to atmospheric conditions whereas connected parts of pipes might still contain TFE at operating pressure state. Many of the regarding pipes are separated by ball valves, which allow a fast opening procedure. Thereby fast compression of the gas can occur and lead to a massive temperature increase which might induce unwanted reactions. Former tests in laboratory scale described by Meyer (2009) allowed an ignition of a TFE/air system by rapid compression only for a set of sharp defined boundary conditions. First tests in the lower industrial scale were done by Ferrero et al. (2013), where an ignition at typical industrial operating conditions was initiated. The results of the tests indicated that the critical achievable compression temperatures strongly depend on the setup and therefore on the pipe diameter as well. Therefore the necessity of further tests has been pointed out. The original setup presented by Ferrero (2013), which represents the smallest typical industrial size with an inner diameter of 1.125”, was modified to withstand an explosive decomposition reaction and to avoid a deflagration to detonation transition. Different safety concepts as burst discs and time controlled cut-off valves had been tested and evaluated to optimize the experimental setup for reproducible test conditions. This allowed the systematic investigation of the rapid compression of TFE–systems for the first time in the described scale without serious damages after an ignition. In the donor pipe always TFE at high pressure and in the receiving pipe TFE, nitrogen or air were present at an absolute pressure ranging from 500 Pa to atmospheric pressure.
The scope was to generate a “hazard diagram” in which the ignition probability in dependence of donor (high) pressure and the receiving (low) pressure is shown. Hazardous conditions can easily be determined. A reference method for the maximum achievable temperatures of non-reacting gas systems was created using an air/air-system. Thus reactive TFE-systems could be evaluated regarding additional exothermic effects. The final hazard diagram demonstrates that there is no sharp limit between a “safe” state and an “ignition” for a TFE/air-system. Rather a transition range exists, which decreases with rising donor pressure. An increased temperature in this range, sometimes combined with small pressure peaks in the profile, indicates first partial restricted reactions near the end flange. The more it gets closer to the “ignition” transition the more traces like soot or undefined solid fractions were found. A TFE/nitrogen- and a TFE/TFE-system could not be ignited at all. A description of the experimental tests as well as a detailed explanation of the hazard diagram will be presented.
In North America certain hazardous materials are transported in rail tank cars that must be able to survive an engulfing liquid hydrocarbon pool fire for 100 minutes without rupture. To meet this requirement these tanks are normally equipped with pressure relief valves (PRV) and some form of thermal insulation or thermal protection (TP).
These tanks sometimes have non-accident releases (NAR) due to unwanted activation of, or leakage from the pressure relief valves (PRV). These NARs are a nuisance for Industry and for this reason, the industry now wants to remove the PRVs from certain tanks. This is known as total containment and is common practice in Europe. However, Europe does not have a 100 minute fire survival requirement. This paper is about a series of fire tests of 1/3 rd linear scale US DOT 111 Tanks cars. The 2.4 m3 vessels were subjected to fully engulfing fires generated by liquid propane fueled burners.
A new technique for optical fiber defect detection using Brillouin distributed fiber optic sensor (DFOS) has been proposed and experimentally demonstrated in this paper. This technique is based on stimulated Brillouin scattering (SBS), which offers three wave interaction in single mode optical fiber (SMF -10 μm/125 μm acrylic coated fiber). The nonlinear effect of SBS is manipulated to locate the defect in optical fiber using distributed sensing technology. Various kind of defects may be present in optical fibers. This paper details a case study on observation of a defect, which manifests its presence in certain temperature values. The detail of defect detection through distributed fiber sensor using the SBS has been brought out. SBS is sensitive to temperature and strain. In order to study the effect of defect in distributed fiber sensor as function of temperature and strain, the distributed pre-strained and unstrained optical fiber is subjected to temperature variation and corresponding measurements are obtained with Brillouin optical time domain analyser (BOTDA). This technique enables the utilization of Brillouin parameters, such as decreased amplitude, frequency and increased linewidth in the defect region of the fiber length. The fiber defect location can be determined with spatial resolution accuracy of less than 50 cm of using BOTDA technique.
This article addresses the imperfections caused by the weld assembly in I-shape sections made of two structural steel grades. Load influencing imperfections are assumed as deviations from the ideal shape (e.g. bending distortion) and longitudinal residual stresses. The quality of a numerically aided design of components exposed to either compression and/or bending is significantly affected, depending on these parameters. The Eurocode (EC3) provides robust simplified models. As a result, the Ultimate Limit State (ULS) is approached on a conservative basis. The following investigations are aimed at providing further guidance on these values in component-like specimens. The long term goal is an improved understanding of the load-bearing capacity of such sections. As a first step in this process, the experimental and corresponding numerical studies are presented.
Neues aus der Lagerung
(2016)
Self-ignition of coal dust deposits poses a higher risk of fires in oxygen-enriched oxy-fuel combustion systems. In this work, we develop a numerical method, using the commercial software COMSOL Multiphysics, to investigate self-ignition behaviour of coal dust accumulations with a main emphasis on the roles of oxygen, diluent gas and dust volume. A one-step 2nd-order reaction kinetic model considering both coal density and oxygen density is used to estimate reaction rate using the kinetic parameters from previously conducted hot-oven tests. This model is validated to predict the transient temperature and concentration profiles of South African coal dusts until ignition. The computed self-ignition temperatures of dust volumes show a good agreement with experimental results. In addition, it is found that the inhibiting effect of carbon dioxide is comparatively small and oxygen consumption increases dramatically after ignition. Parameter analysis shows that the heating value and kinetic parameters have a comparatively pronounced effect on self-ignition temperature. The model provides a satisfactory explanation for the dependence of self-ignition behaviour on gas atmospheres, thus helping to further understand the fire risk of self-ignition in oxy-fuel combustion systems.
The self-ignition of coal dust deposits and its subsequent smoldering combustion pose a high fire hazard to oxy-fuel power systems which burn fuels using pure oxygen for the sake of carbon capture and storage. The increasing risk of explosion in the gas-phase and self-ignition in the solid-phase for an oxygen enhanced combustion environment has not been well studied yet. In this work, the heterogeneous reactions of a bituminous coal dust are investigated by using a novel hot-basket apparatus with an emphasis on the roles of O2 and diluent gas in chemisorption and smoldering. Experiments show that increasing O2 mole fraction accelerates both self-ignition and the following smoldering combustion. On the other hand, the presence of CO2 increases the ignition temperature and reduces the maximum smoldering temperature. However, the promotion in the fire and explosion risk by elevating O2 mole fraction is substantially stronger than the retardation effected by presence of CO2. The emission-gas measurements show that the CO to CO2 ratio increases significantly after self-ignition, and CH4 counts for 1 % to 8 % of the total carbon emission. This research may help improve the understanding of heterogeneous coal combustion and the fire safety in oxy-fuel power systems.