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Many industrial processes include a gas explosion hazard. If safety measures are not adequate to prevent a potentially explosive atmosphere or to avoid effective ignition sources in enclosures, at least the effects of an explosion can be limited e.g. by gas explosion venting systems.
For the design of gas explosion venting systems for confinements only little guidance is given when considering the constructional boundary conditions or process conditions. For this reason conservative assumptions are prevalent in practice and in many cases the protective systems become significantly oversized. From safety perspective such safety margins in venting areas can lead to a critical acceleration of the pressure rise. Moreover, a gas explosion venting at turbulent conditions caused by over sizing or by obstacles rather leads to an under-sized system. The present investigation was focused especially on the influence of certain obstacles as well as the influence of elevated initial pressures on explosion venting
behaviour of quiescent hydrogen, methane or ethylene in air.
Propane/air deflagrations and CTA measurements of turbulence inducing elements in closed pipes
(2008)
The current paper presents experimental investigations on several influence parameters of deflagrations in long closed pipes. The experiments were carried out with different propane/air mixtures, various length-to-diameter ratios (up to L/D=143, D=0.159 m) and miscellaneous initial pressures. Maximum values of explosion pressures and flame speeds were found at stoichiometric propane/air compositions. With increasing L/D, the explosion pressures decreased while higher flame speeds were determined. In addition, elevated initial pressures led to rising explosion pressures. Furthermore, it is well known that turbulence inducing elements in pipes, ducts or vessels enhance the heat and mass transfer during reactive flows. This may lead to a significant acceleration of ongoing gaseous reactions and could finally cause a deflagration to detonation transition (DDT) aligned with much higher pressures and flame speeds. Therefore, constant temperature anemometry (CTA) measurements were carried out to describe the influence of frequently used flange assemblies and baffles on the velocity and turbulence distributions of air flows in steel pipes similar to those in the explosion measurements. The experiments showed that in contrast to flange assemblies, baffles caused a significant increase of the turbulence intensity in pipe flows.
There is a general lack of information on the effects of full-bore obstacles on combustion in the literature, these obstacles are prevalent in many applications and knowledge of their effects on phenomena including burning rate, flame acceleration and DDT is important for the correct placing of explosion safety devices such as flame arresters and venting devices. In this work methane, propane, ethylene and hydrogen-air explosions were investigated in an 18 m long DN150 closed pipe with a 90 degree bend and various baffle obstacles placed at a short distance from the ignition source. After carrying out multiple experiments with the same configuration it was found that a relatively large variance existed in the measured flame speeds and overpressures, this was attributed to a stochastic element in how flames evolved and also how they caused and interacted with turbulence to produce flame acceleration. This led to several experiments being carried out for one configuration in order to obtain a meaningful average. It was shown that a 90 degree bend in a long tube had the ability to enhance flame speeds and overpressures, and shorten the run-up distance to DDT to a varying degree for a number of gases. In terms of the qualitative effects on these parameters they were comparable to baffle type obstacles with a blockage ratios of between 10 and 20%.
The method described in this paper enabled reliable and accurate positioning of an overdriven detonation by calculation of shock wave velocities (detonation and retonation) for hydrogen explosions in a closed 18 m long horizontal DN150 pipe. This enabled an empirical correlation between the ignition position and the run-up distance to DDT to be determined. It was shown that the initial ability of the flame to expand unobstructed and the piston-like effect of burnt gas expanding against the closed end of the tube contributed to initial flame acceleration and hence were able to affect the run-up distance to overdriven detonation. Flame speeds and rates of initial pressure rise were also used to explain how these two competing effects were able to produce a minimum in the run-up distance to DDT. The shortest run-up distance to DDT, relative to the ignition position, for this pipe and gas configuration was found when the ignition position was placed 5.6 pipe diameters (or 0.9 m) from the closed pipe end. The shortest run-up distance to DDT relative to the end of the pipe was recorded when the ignition source was placed 4.4 pipe diameters or 0.7 m from the pipe end.
The effects of a gas explosion in enclosures like vessels can be limited e.g. by gas explosion venting
systems. The major design step of this constructive explosion protection method is to determine the
required vent area, which depends significantly on whether turbulent combustion exists. However,
current standards like NFPA 68 or EN 14994 are applicable only to limited boundary conditions and
as far as possible only to laminar flame propagation. Difficulties arise in the assessment or predictability
of gas explosion hazard when turbulence occurs.
In this research especially venting at elevated initial pressure has been shown to accelerated
flame propagations and therefore, to a considerably higher reduced pressure. Therefore, it is essential
to provide a broader data base of turbulent combustion and explosion behaviour to verify the
existing rules or to determine their safety-relevant parameters.
For a better safety assessment or design of protective systems the turbulent combustion and
accelerated gas explosion behaviour of quiescent methane in air were investigated at initial pressures
up to 8 bar using vessels up to 100 litres. In particular a systematic study was performed to
investigate the influence of turbulence on the overpressure development during accelerated gas
explosion. Moreover, the present study consider the position of the spark igniters, the burning velocity
and the maximum pressure rise for different concentration of fuel as well as the size of orifice
and/or vent area.
A choice of experimental tests showed under the investigated conditions that not only turbulence
inducing obstacles but also over sized vent areas could lead to an increased pressure development
and therefore to an inacceptable safety state. Due to the numerous influencing variables of
explosion behaviour the presented experimental results help to judge whether another more sophisticated
method should be applied than the one described in standards.
The effects of a gas explosion in enclosures like vessels can be limited e.g. by gas explosion venting systems. The major design step of this constructive explosion protection method is to determine the required vent area, which depends significantly on whether turbulent combustion exists. However, current standards like NFPA 68 or EN 14994 are applicable only to limited boundary conditions and as far as possible only to laminar flame propagation. Difficulties arise in the assessment or predictability of gas explosion hazard when turbulence occurs. In this research especially venting at elevated initial pressure has been shown to accelerated flame propagations and therefore, to a considerably higher reduced pressure. Therefore, it is essential to provide a broader data base of turbulent combustion and explosion behavior to verify the existing rules or to determine their safety-relevant parameters. For a better safety assessment or design of protective systems the turbulent combustion and accelerated gas explosion behaviour of quiescent methane and hydrogen in air were investigated at initial pressures up to 8 bar using vessels up to 100 litres. In particular a systematic study was performed to investigate the influence of turbulence on the overpressure development during accelerated gas explosion. Moreover, the present study consider the position of the spark igniters, the burning velocity and the maximum pressure rise for different concentration of fuel as well as the size of orifice and/or vent area. A choice of experimental tests showed under the investigated conditions that not only turbulence inducing obstacles but also over sized vent areas could lead to an increased pressure development and therefore to an inacceptable safety state.
The ignition probability of gaseous mixtures of acetylene, hydrogen and ethylene with air due to mechanical impacts between stainless steel components was examined for various impact energies. Additionally, the sources of ignitions were identified by infrared high-speed recordings. The stainless steel types used had different chemical compositions in order to investigate the influence of the chromium content on the ignition probability. The investigations reveal different ignition probabilities of the gas mixtures as well as different sources of ignitions depending on the steel type used and the impact energy applied. Impact energies below 126 J resulted in ignition of the gaseous mixture at the hot surfaces of the pin or the plate in most of the cases. At higher energies, initiation of ignition due to abraded particles was more probable when using stainless steel components with lower chromium content whereas the source of ignition was almost exclusively limited to the hot surfaces of pin and plate for the steel with the highest chromium content. However, as opposed to the source of ignition, the probability of ignition could not be correlated to the chromium content of the stainless steel.
For the design of gas explosion venting systems for confinements only little guidance is given when considering the constructional boundary conditions or process conditions. For this reason conservative assumptions are prevalent in practice and in many cases the protective Systems become significantly oversized. Such safety margins in venting areas can lead to a critical acceleration of the pressure rise. Finally, a gas explosion at turbulent conditions caused by oversizing of the venting area rather leads to an under-sized system and supports the deflagration to detonation transition (DDT). The present investigation was focused especially on the influence of certain obstacles as well as the effect of the initial pressures on the explosion venting behavior of methane-air-mixtures and of hydrogen-air-mixtures.
In industrial applications carbon steel is often replaced by stainless steel. Stainless steel is supposed to be less problematic than carbon steel as a possible ignition source because of the decreasing oxidizability of stainless steel particles with increasing content of alloying elements such as chromium. However, knowledge about the ignition probabilities and about factors influencing ignitions are far from being sufficient. Better knowledge could well be used to decrease and minimize explosion hazards. Additionally, adequate risk assessment could lead to cost reduction by avoiding possible overestimation of hazards. In this contribution, we present results of grazing impact experiments using four different stainless steels in acetylene-, hydrogen-, ethylene- and propane-air mixtures. We investigated the influence of the chromium content and of some of the material properties of stainless steel on the ignition probabilities of these gas mixtures.
The experiments show an appreciable amount of ignitions in the acetylene-air and the hydrogen-air mixture only. The observed source of ignition depends on the chromium content of the steel. At impact energies of 126 J and more, acetylene-air mixtures were ignited by hot surfaces resulting from impacts between stainless steel components containing 22.5 % chromium whereas separated and subsequently oxidized particles acted as ignition source after impacts of stainless steel containing 17.7 % chromium and less. These observations are in accordance with the decreasing oxidizability of stainless steel with increasing chromium content. However, an exclusive correlation between chromium content and ignition probability was not found. Almost all ignitions of hydrogen-air were initiated by hot surfaces and impacts of steel containing the highest amount of chromium were most incendive. This observation shows that a low oxidizability of the steel due to its high chromium content does not necessarily lead to a lower ignition probability. Quite contrary, an increasing chromium content leads to a decreasing thermal conductivity and thus to a higher ignition probability. As a result the thermal con-ductivity was found to be a key parameter determining the ignition probability. The density, specific heat capacity and hardness of the steel were also taken into account; however, they did not affect the ignition probability.
In conclusion, the different sources of ignition can be distinguished with regard to the resulting ignition probability of flammable gas mixtures subjected to grazing impacts of stainless steel. In cases where the ignitions are initiated by hot surfaces, the thermal conductivity of the impacting steel is the most critical parameter. On the other hand, the temperature increase due to impacts and the propensity of the steel to be oxidized mainly influence the ignition probability in cases where separated particles act as ignition source. Consequently, stainless steel with a high thermal conductivity and a low oxidizability has to be utilized in order to minimize the ignition probability due to mechanical impacts.
Grazing impact experiments of various types of stainless steels were performed in explosive atmospheres of hydrogen, acetylene, ethylene or propane with air. Depending on the gas mixture, kinetic energy of the impact, and applied stainless steel, the dominant ignition sources are either separated particles or hot friction surfaces. An influence of chromium content on the ignition probability was not found, although an increase in chromium content results in a reduction of the oxidizability of separated particles. Additionally, the influence of the material properties thermal conductivity, specific heat, density and hardness on the ignition probability of the hydrogen–air mixture was investigated. With increasing thermal conductivity a decreasing rate of ignitions was observed. In contrast, an influence of the physical properties specific heat, density and hardness on the ignitability was not found.
Effects of the "new" ATEX Directive 2014/34/EU - consequences for manufacturers - claims by users
(2016)
Directive 94/9/EC of the European Parliament and of the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres has been substantially amended. The new ATEX Di-rective 2014/34/EU on equipment and protective systems intended for use in potentially explosive atmospheres replaces Directive 94/9/EC. The new Directive was published on March 29, 2014, became enacted on April 19, 2014 and will become effective starting April 20, 2016. It was the aim of the revision to harmonize the requirements with the New Legal Framework (New Legislative Framework - NLF). The NLF consists of the following two documents: the Regulation (EC) No 765/2008 of the European Parliament and of the Council of 9 July 2008, which sets out the re-quirements for accreditation and market surveillance relating to the marketing of products, and the Decision No 768/2008/EC of the European Parliament and of the Council of 9 July 2008, which sets on a common framework for the marketing of products.
The harmonization with the NLF especially results in a lot of new definitions and a new structure compared with the “old” ATEX. Some content which was in the annexes of the “old” ATEX is now turned into distinct chapters or articles in the main part. New content was included which comprises things like presumption of conformity, principles of CE marking, notification of conformity as-sessment bodies, Union market surveillance, and Union safeguard procedure. What is more, the definitions of the economic operators like manufacturer, distributor and importer and the description of their duties are now in separate chapters. Thus, the “new” ATEX is much bigger than the “old” one. With the publication of the Directive 2014/34/EU a lot of questions were raised especially from manufacturers.
These questions include:
- Did everything change now?
- Does the new Directive affect essential safety and health requirements (ESHR) for equipment and protective systems intended for use in hazardous areas?
- Is the module system for conformity assessment changed?
- What happens with existing certificates?
- What happens with products which were already produced before April 20, 2016 but will be placed on the market later?
The presentation will deal with some of these questions. But it has to be stated that the essential health and safety requirements for equipment and protective systems intended for use in potentially explosive atmospheres have not changed. That means: A product which was safe under the Directive 94/9/EC is still a safe product in the sense of the Directive 2014/34/EU.
Effects of the "new" ATEX Directive 2014/34/EU - consequences for manufacturers - claims by users
(2016)
Directive 94/9/EC of the European Parliament and of the Council of 23 March 1994 on the approx-imation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres has been substantially amended. The new ATEX Di-rective 2014/34/EU on equipment and protective systems intended for use in potentially explosive atmospheres replaces Directive 94/9/EC. The new Directive was published on March 29, 2014, became enacted on April 19, 2014 and will become effective starting April 20, 2016. It was the aim of the revision to harmonize the requirements with the New Legal Framework (New Legislative Framework - NLF). The NLF consists of the following two documents: the Regulation (EC) No 765/2008 of the European Parliament and of the Council of 9 July 2008, which sets out the re-quirements for accreditation and market surveillance relating to the marketing of products, and the Decision No 768/2008/EC of the European Parliament and of the Council of 9 July 2008, which sets on a common framework for the marketing of products.
The harmonization with the NLF especially results in a lot of new definitions and a new structure compared with the “old” ATEX. Some content which was in the annexes of the “old” ATEX is now turned into distinct chapters or articles in the main part. New content was included which comprises things like presumption of conformity, principles of CE marking, notification of conformity as-sessment bodies, Union market surveillance, and Union safeguard procedure. What is more, the definitions of the economic operators like manufacturer, distributor and importer and the description of their duties are now in separate chapters. Thus, the “new” ATEX is much bigger than the “old” one. With the publication of the Directive 2014/34/EU a lot of questions were raised especially from manufacturers.
These questions include:
- Did everything change now?
- Does the new Directive affect essential safety and health requirements (ESHR) for equipment and protective systems intended for use in hazardous areas?
- Is the module system for conformity assessment changed?
- What happens with existing certificates?
- What happens with products which were already produced before April 20, 2016 but will be placed on the market later?
The presentation will deal with some of these questions. But it has to be stated that the essential health and safety requirements for equipment and protective systems intended for use in potentially explosive atmospheres have not changed. That means: A product which was safe under the Directive 94/9/EC is still a safe product in the sense of the Directive 2014/34/EU.