Flame Transmission Research Articles

Revestimiento Anticondensante, Aislante Térmico e Intumescente

A formula of a paint for interior use for roofing sheet zinc was optimized. The properties developed and optimized were thermal insulation, anticondensation capacity and fire-retardant properties. A fire-retardant latex coating was used as the formulation base and three types of fillers were investigated: hollow glass microspheres, hollow ceramic microspheres and expanded perlite. The thermal transfer coefficient of the different coatings, the water absorption capacity and the thermal flame transmission were evaluated. From the results, it was determined that the formulation with the best performance, both in thermal insulation and in water absorption capacity, was the one that contained expanded perlite. Regarding the thermal transmission due to the action of the flame, no significant difference was observed between the different loads used.

Considerations regarding the main differences between flameproof electric motors, designed for explosion Group IIC, compared to explosion Group IIB, when testing in explosive mixtures

The flameproof enclosure type of protection is still one of the safest because it is based on a very simple and, therefore, unlikely fallible technology. If an explosive atmosphere penetrates an enclosure producing a trigger, the explosion occurs, but it remains confined within the enclosure. EN 60079-1 standard, states that flanged flame paths are allowed in the presence of gases and vapours of Group IIB, but cannot be used with gases and vapours of Group IIC. Also, flame transmission tests for Group IIB equipment are carried out at atmospheric pressure, whereas for Group IIC equipment they are conducted (depending on the testing method applied) at a higher initial pressure. The purpose of the paper is to perform a thorough study upon the differences between testing of flameproof electric motors of Group IIC, compared to electric motors of Group IIB gases and vapours, in terms of testing conditions required by the standard and how the explosion group influences the maximum explosion pressures and explosion pressure development, when tested in explosive mixtures of gases and vapours. The purpose of this paper is to assist manufacturers in designing flameproof electric motors that satisfy the requirements of the specific standard for either Group IIB or Group IIC applications in order to smooth the path for certification.

Determination of the performance limits of flame arresters at increased oxygen concentrations

This investigation shows how an increased oxygen concentration influences the performance limits of crimped ribbon deflagration flame arresters at elevated pressures. An evaluation of the maximum experimental safe gap (MESG) as reliable criterion for describing the performance limits under non-atmospheric conditions is given. Measurements of MESGs and flame arrester performance tests were performed. Various fuel/oxygen/air mixtures containing ethylene and propane were used as testing gases. Former studies on the pressure dependence and the influence of oxygen on the MESG were initially confirmed. Furthermore, performance tests using a commercial deflagration flame arrester revealed that such a flame arrester may prevent flame transmission also at non-atmospheric conditions within a limited range. For various oxygen concentrations the performance limits were reached at the same MESG. Hence, it can be assumed that a flame arrester possesses a device- and fuel-specific maximum experimental safe gap for a specific gas mixture in different concentrations and at different pressures. This performance-related maximum safe gap can be used as a parameter for estimating and describing the performance limits of a flame arrester. It offers an attempt to simplify the testing and qualification of deflagration flame arresters for non-atmospheric conditions.

Effect of Ignition Position on Methane Explosion in Spherical Vessel with a Pipe

Abstract An experimental system consisting of a spherical vessel with a horizontal pipe was established. By changing the pipe length, the effect that the ignition position has on methane explosion in an airtight spherical vessel with a pipe was studied, and the regularities of Pmax, (dP/dt)max and flame transmission were analyzed in detail. The results show that different ignition position leading to different propagation of flame and pressure wave. As the ignition position moved from the wall of vessel to the end of pipe, the Pmax in the apparatus first increased, then decreased when ignition occurs behind the center of vessel, and finally increased when ignition occurs behind the middle of pipe. The same trend was observed for (dP/dt)max. When ignited near the end of pipe, the Pmax and (dP/dt)max were large, and the explosion was the most dangerous. In addition, gas oscillation was observed during the explosion process in the pipe, and it was more serious when igniting at the wall of vessel, the center of vessel and the end of pipe. Furthermore, the influence that structure effect had on the gas explosion was determined. With ignition in spherical vessel, the longer the pipe is, the weaker the explosion intensity in vessel is, but the greater the explosion intensity in pipe is. With ignition in the pipe, the longer the pipe is, the greater the explosion intensities are in both the vessel and pipe. The influence that the ignition position has on a gas explosion was revealed, and the reason was analyzed. This research has great theoretical value and practical significance for preventing and controlling potentially disastrous accidents in complex units. The research results provide a theoretical foundation for antiknock and explosion venting in technical vessels and pipes.

Flame Transmission at Extremely Low Temperatures When Pressure Piling Is Present

The International Electrotechnical Commission (IEC) Standard 60079-1 has requirements for testing flameproof enclosures to ensure that there is no transmission of an internal explosion, but there are no special requirements for nontransmission testing for equipment intended for use in extremely low temperatures. The gas mixtures used for test are based on a measure called the maximum experimental safe gap (MESG). It is known that with initial pressures higher than standard atmospheric pressure, flamepaths are less effective and hence the MESG decreases. Conversely, when temperatures are lowered, the flamepaths become more effective and hence the MESG increases. But there is a lack of published literature on how the two effects interact in combination on MESG, in particular at extremely low temperatures. The situation is more complex when pressure piling is present as the pressures become even higher. An analysis of published data suggests that for some gases when the temperature drops from +20 to -60 °C, the impact of pressures may be more significant than the direct impact of temperature on MESG. Hence, transmission may be more likely at lower temperatures where high pressures are present than at ambient temperature. This paper reports on experiments investigating the practical likelihood of flame transmission at extremely low temperatures, in particular when pressure piling is present.

Detailed investigation of flame transmission from a vessel to a discharge duct

The present paper deals with a problem of explosion initiated in a vessel and vented through a duct. On the basis of numerical simulation (CFD) and visualization by means of high speed camera it completes and discusses the existing results and hypotheses (especially those presented in the work of Ponizy and Leyer [1]) concerning the process of enhancement of vessel pressure rise during such an explosion. In particular, numerical simulation indicates that a secondary explosion in the duct, known as a “burn-up”, which is responsible for higher reduced explosion pressures during ducted venting, has its source in a highly turbulent zone generated at the duct entrance by the flame itself independently of the shape of the vessel/duct passage. Camera images confirm the hypothesis that the flame penetrating into the duct is strongly torn in this turbulent zone and mixed with fresh gases. On the other hand, these images show that the reverse flow created by the burn-up returns back to the vessel some amount of rapidly burning gases, which contributes to the vessel pressure increase in the same degree that the blockage of the outflow from the vessel and the intensification of combustion of the unburned mixture previously left in the vessel.Experimental results evidence also a strong dependence of the flame behaviour in the vessel on the vessel length and the duct diameter. They demonstrate that the final effect of the burn-up action on maximum explosion pressures reached in the vessel depends on the whole process of flame propagation in the vessel, which in turn is closely related to the vessel and duct dimensions. In particular, in some configurations, an apparition of the “tulip flame” combined with inertia of the gas column in the duct may in a spectacular manner reduce the effect of the burn-up action.

The Use of Porous Structures in Flameproof Enclosures to Reduce the Maximum Explosion Pressure

Electrical and non-electrical equipment used in hazardous areas can be designed in the type of protection “flameproof enclosure”. Following this safety concept, potential ignition sources are enclosed by containments. If a combustible enters the enclosure and is ignited, flame transmission to the surrounding explosive atmosphere has to be definitely avoided. Additionally, the enclosure must be robust enough to withstand the emerging explosion pressure without rupturing or temporarily opening up any of the joints to a critical value. The design of these enclosures is, therefore, related to the maximum explosion pressure of an internal explosion and as a consequence, the enclosures must be constructed with an enormous material effort. To develop more efficient enclosure constructions, it is necessary to reduce the maximum explosion pressure.This paper presents a novel approach to reducing the explosion pressure within flameproof enclosures, which offers the possibility of improving the design of flameproof enclosures, leading to lower production costs without reducing any safety aspects. This approach is based on the integration of porous structures into the enclosure walls acting as venting and flame quenching elements. Different porous structures were investigated in terms of their ability to relieve pressure and to avoid flame transmissions. It is shown that proper use of these structures can enormously decrease the maximum explosion pressure inside the enclosure while safely avoiding flame transmissions, depending on the structure specifics. Based on these results it is possible to introduce a novel approach of flameless venting, which is named, regarding its application in flameproof enclosures, “flameproof explosion pressure relief”.

Influence of geometry shape on gas explosion propagation laws in bend roadways

Based on the experiment, the important influence of the pipe with abnormal structure on gas explosion propagation characteristics is analyzed. The results show that gas explosion propagation at the turning point is a process of interaction of pressure wave, flame and complicated flow field. The overpressure of pressure wave and flame transmission velocity are rapidly accelerated and the pipe wall at the turning point is seriously destroyed. The angle of bend has a great influence on the gas explosion propagation characteristics, i.e. there is a different degree of change in both flame velocity and pressure wave overpressure value when gas explosion propagates through bend with different angles. The bend not only increases the turbulent flow of combustion area that accelerates the reaction and velocity, but also enlarges the resistance and heat transmission to the pipe wall. At the same time, the expansion wave at the turning point restrains the gas explosion propagation. The influence of bend on gas explosion propagation characteristics is determined by the combined impact of inhibitor factors and motivation factor. Experiment research and theory analysis indicate that gas explosion propagation characteristics in bend pipe are obviously different from that in straight pipe. So in the mine exploitation, the bend roadways should be decreased as few as possible, and preventive measures should be taken in order to diminish the loss caused by gas explosion. The research results will play an important role in the guidance of preventing gas explosion from occurring and alleviating the damage in coal mine.

Terahertz time-domain spectroscopy of high-pressure flames

Laser spectroscopy in the visible and near infrared is widely used as a diagnostic tool for combustion devices, but this approach is difficult at high pressures within a sooty flame itself. High soot concentrations render flames opaque to visible light, but they remain transparent to far-infrared or terahertz (THz) radiation. The first far-infrared absorption spectra, to the best of our knowledge, of sooty, non-premixed, ethylene high-pressure flames covering the region of 0.2–2.5 THz is presented. A specially designed high-pressure burner which is optically accessible to THz radiation has been built allowing flame transmission measurements up to pressures of 1.6 MPa. Calculations of the theoretical combustion species absorption spectra in the 0.2–3 THz range have shown that almost all the observable features arise from H2O. A few OH (1.84 and 2.51 THz), CH (2.58 THz), and NH3 (1.77 and 2.95 THz) absorption lines are also observable in principle. A large number of H2O absorption lines are observed in the ground vibrational in a laminar non-premixed, sooty flame (ethylene) at pressures up to 1.6 MPa.

Endurance burning flame arresters using sintered metal fibres

Endurance burning flame arresters are used to protect the inner part of an apparatus or fuel tank against flame transmission from the outside. Based on the endurance burning test according to EN 12874, the temporal and local temperature distributions are measured using different flame arrester elements. Based on these results, porous structures made of sintered fibres seem to be a very attractive material for the design of endurance burning flame arrester elements.

Detailed investigation of ignition by hot gas jets

Experimental and numerical investigations of the ignition of hydrogen/air mixtures by jets of hot exhaust gases are reported. An experimental realisation of such an ignition process, where a jet of hot exhaust gas impinges through a narrow nozzle into a quiescent hydrogen/air mixture, possibly initiating ignition and combustion, is studied. High-speed laser-induced fluorescence (LIF) image sequences of the hydroxyl radical (OH) and laser Schlieren methods are used to gain information about the spatial and temporal evolution of the ignition process. Recording temporally resolved pressure traces yields information about ambient conditions for the process. Numerical experiments are performed that allow linking these observables to certain characteristic states of the gas mixture. The outcome of numerical modelling and experiments indicates the important influence of the hot jet temperature and speed of mixing between the hot and cold gases on the ignition process. The results show the quenching of the flame inside the nozzle and the subsequent ignition of the mixture by the hot exhaust jet. These detailed examinations of the ignition process improve the knowledge concerning flame transmission out of electrical equipment of the type of protection flameproof enclosure.

Investigation of deflagrations and detonations in pipes and flame arresters by high-speed framing

Abstract A widespread method to avoid damage from incidental gas explosions is to use flame arresters. The new European Standard EN 12874 provides guidance for testing such flame arresters. Nevertheless, there are still unsolved problems as regards the effects of flame speed and transient explosion pressure, tests of unstable detonations and statistics of the deflagration-to-detonation transition (DDT). To better understand these phenomena, a transparent test set-up (pipe and model arrester) was built and the flame propagation was directly recorded using high-speed cameras. Flame velocity and flame pressure were determined in each phase of the explosion. Time-resolved observations of the flame propagation inside the flame arrester provided direct insight into the quenching potential and the flame transmission process.

Investigation of deflagrations and detonations in pipes and flames arresters by high-speed framing

A widespread method to avoid damage from incidental gas explosions is to use flame arresters. The new European Standard EN 12874 provides guidance for testing such flame arresters. Nevertheless, there are still unsolved problems as regards the effects of flame speed and transient explosion pressure, tests of unstable detonations and statistics of the deflagration-to-detonation transition (DDT). To better understand these phenomena, a transparent test set-up (pipe and model arrester) was built and the flame propagation was directly recorded using high-speed cameras. Flame velocity and flame pressure were determined in each phase of the explosion. Time-resolved observations of the flame propagation inside the flame arrester provided direct insight into the quenching potential and the flame transmission process.

Crimped metal ribbon flame arrestors for the protection of gas measurement systems

Abstract This paper summarises the results of extensive research to determine the limit of safety against flame transmission for flame arrestors of relatively small size fitted with arrestor elements made of crimped metal ribbon. Depending on the reactivity of the fuel gas/air mixture and the tube geometry the running up to detonation and hence the stressing of a flame arrestor by a detonation is possible in longer tubes with relatively small diameters. Only with reactive gas phases of explosion group I this stressing case for a flame arrestor can be excluded. With detonative gas phases the stressing of the flame arrestor decisively depends on the place of installation with respect to the point of transition from deflagration to detonation in the system considered. Five different stressing cases with a probably very different limiting pressure of safety against flame transmission must be distinguished. The results of the investigations will help to evaluate the results from testing of flame arrestors carried out according to the requirements in national and international standards or regulations.

Critical dimensions of holes and slots for transmission of gas explosions

Abstract Critical hole diameters for explosion transmission from a primary virtually closed chamber into an ambient gas cloud were determined. Most of the present experiments were conducted with a 1-l primary chamber. The motivation for the study is two-fold. First, results from this kind of experiment are of direct practical use in further improvement of design and maintenance procedures of “flame-proof” electrical equipment. However, such experiments can also contribute to improvement of the general understanding of the mechanisms of flame propagation in turbulent, premixed gases. The preliminary experimental results presented confirm that the minimum tube diameter for flame transmission depends strongly on the location of the ignition point. The generally accepted limiting values are conservative, in the sense that they can only be approached if the ignition source is located in a narrow zone in the vicinity of the entrance to the transmission hole. This must be taken into account if flame-proof equipment is to be designed on the basis of risk analysis. An observation related to mechanisms of turbulent flame propagation in premixed gases in general was also made. In the case of ignition far away from the transmission hole, i.e. for high gas velocities through the hole at the moment of flame front arrival at the hole, the re-ignition probability for a given hole diameter was in fact higher for off-stoichiometric propane concentrations than for concentrations close to stoichiometry. The average chemical reaction rate in the primary chamber had its peak in the region of stoichiometry, and hence the pressure in the primary chamber at the moment of flame front arrival at the transmission hole entrance, was also at its maximum in that concentration range. Therefore, the average turbulence intensity in the potential re-ignition zone, and hence the rate of entrainment of cold unburnt gas by the hot jet was also at its maximum. Hence, a main reason for the observed effect may be that at stoichiometry more efficient cooling by cold-gas entrainment, compared with at leaner and richer mixtures, more than compensated for the faster chemical reaction.

Limit of safety against flame transmission for sintered metal flame arrester elements in the case of flashback in fuel gas/oxygen mixtures

In autogenous welding, cutting and allied processes, so-called dry safety devices are used to protect the outlet terminals of gas distribution lines. To prevent flame transmission, these units are fitted with sintered metal flame arrester elements. At the Federal Institute for Materials Research and Testing (BAM), extensive investigations have been carried out with an experimental flame arrester to determine the limits of safety against flame transmission for sintered metal flame arrester elements in the case of flashback in fuel gas/oxygen mixtures. On the basis of the results of these investigations, it is possible to estimate the pore sizes up to which a sintered metal element can prevent any flame transmission with a given fuel gas.

Safety devices for gas welding, cutting and allied processes

This paper describes the results of extensive research to determine the limit of safety against flame transmission for sintered metal flame arrester elements when stressed by a flashback in a fuel-gas/oxygen mixture and when stressed through a stabilized burning of a flowing mixture of fuel-gas and air and of fuel-gas and oxygen at the sintered metal element. On the basis of the results of these investigations, the limit of safety against flame transmission for sintered metal flame arrester elements can be estimated and the conditions for testing can be specified. An analysis of the protection of gas outlets on gas distribution lines for welding, cutting and allied processes has been carried out, resulting in recommendations for necessary changes to the regulations for testing of safety devices with sintered metal flame arrester elements, which should be made in the next revision of the corresponding technical regulations for acetylene installations and calcium carbide stores (TRAC), and the standards DIN 8521, EN 730 and ISO 5175.

Radiation properties of soot from diffusion flames

The spectral extinction coefficients k λ of soot from flame transmission data are obtained for small pool diffusion flames of various fuels including solid polystyrene and plexiglas; foam polystyrene and polyurethane; and liquid isooctane and toluene. Good agreement between experimental and predicted k λ from Mie theory and soot optical properties throughout the visible and infrared ranges substantiates the general applicibility of these optical properties. On the other hand, the determination of soot sizes and volume fractions from visible range data and optical properties requires the form of the size distribution be known. The effect of using different size distributions on these soot results is evaluated by using the three-parameter distribution for the particle number density N( r) = ar b exp (− br/ r m ), with b varied to create different functional dependence on radius r. The most probable radius r m is found to increase by about 80% as b is changed from 2 to 6, while the volume fraction changes by less than 10%. Hence, the most probable radius is rather sensitive to this functional dependence, while the volume fraction is only slightly affected.

Experimental and Theoretical Comparison of the Precision of Flame Atomic Absorption, Fluorescence, and Emission Measurements

Theoretical equations and experimental evaluation procedures for the determination of the precision of flame atomic absorption, emission, and fluorescence measurements are presented. These procedures and noise power spectra are used to evaluate the precision and noise characteristics of atomic copper measurements with all three techniques under the same experimental conditions in an H2-air flame. At the detection limit, emission and fluorescence measurements are limited by background emission shot and flicker noise whereas absorption measurements are limited by flame transmission lamp flicker noise. Analyte flicker noise limits precision at higher analyte concentrations for all three techniques. Fluctutations in self-absorption and the inner filter effect are shown to contribute to the noise in atomic emission and fluorescence measurements.

Optical constants of soot in hydrocarbon flames

A dispersion model for the optical constants of soot in hydrocarbon flames has been established on the basis of a more rigorous consideration of the electronic band structure and the dispersion constants. The dispersion constants of hydrocarbon soot have been determined from this model in conjunction with the in-situ flame transmission data in the visible and infrared ranges, thus truly reflecting soot in flames. It is shown that the soot optical properties are rather insensitive to temperature changes at elevated temperatures but the room temperature values representative of soot in smoke do exhibit appreciable deviation. The results also indicate these properties to be relatively independent of the fuel hydrogen/carbon ratio.