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The maximum rate of pressure rise (dp/dt)𝑚𝑎𝑥 and the corresponding K-value of hybrid mixtures containing flammable gases and dusts are important for constructive explosion protection measures. Since the safety characteristics of dusts and gases are determined under different conditions, there has been considerable confusion about the influence of flammable gas on the (dp/dt) of dusts and vice versa. While some investigations showed comparably higher values for hybrid mixtures, others stated that the highest value for the gas component alone is the worst case.
The first part of this paper focuses on the confusion around the different statements about (dp/dt)𝑚𝑎𝑥 of hybrid mixtures and where they come from. In the second part of this paper experimental results are presented that illustrate how to clarify the different findings of past research and show what to expect as a real worst-case-value for hybrid mixtures.
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.
Many industrial processes include a gas explosion hazard. If safety measures are not adequate to prevent a potentially explosive atmosphere or to avoid effective ignition sources in enclosures, at least the effects of an explosion can be limited e.g. by gas explosion venting systems.
For the design of gas explosion venting systems for confinements only little guidance is given when considering the constructional boundary conditions or process conditions. For this reason conservative assumptions are prevalent in practice and in many cases the protective systems become significantly oversized. From safety perspective such safety margins in venting areas can lead to a critical acceleration of the pressure rise. Moreover, a gas explosion venting at turbulent conditions caused by over sizing or by obstacles rather leads to an under-sized system. The present investigation was focused especially on the influence of certain obstacles as well as the influence of elevated initial pressures on explosion venting
behaviour of quiescent hydrogen, methane or ethylene in air.
The 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.
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.