The influence of air flow on maximum explosion characteristics of dust–air mixtures
Highlights
► Study the influence of flow of air masses on dust–air mixtures. ► Measurements were done in a spherical explosion test apparatus VA-250. ► 5 different samples of industrial dusts had been selected. ► Smooth wheat flour, brown coal, powdered sugar, meat–bone powder, torula yeast. ► Increasing of the maximum explosion pressure and brisance.
Introduction
Dust explosions are often not considered a major problem, although even a small amount of dispersed dust, under suitable conditions can give a strong explosion. As an example an explosion of sugar dust and a subsequent fire at the refinery Imperial Sugar, Georgia – USA (7th Feb. 2008) caused 14 deaths and 38 injuries (CSB, 2009). Explosions of dust mixtures are not as frequent as gas–air mixtures. This is because flammable gases freely diffuse in air to give ignitable mixtures. However dusts need to be dispersed and of course have a tendency to settle, so it is more difficult to get both the required correct dispersion and also sufficiently strong ignition during the (often limited) time in which the mixture is in the explosive regime. The maximum explosion characteristics of a given dust are primarily dependent on optimum dispersion of the material. Previous measurements carried out in the Czech Republic as well as abroad have shown that the degree of turbulence of an explosive mixture before the initiation plays a large role. In the case of gas–air mixtures, this amounts to a several-fold rise in maximum explosion characteristics. For this reason, we decided to study the influence of air flow on dust–air mixtures as well. Measurements were done in a spherical explosion test apparatus VA-250. It is a 0.25 m3 multi-functional piece of equipment that is primarily intended for the determination of maximum explosion characteristics of gas–air, vapour–air, dust–air and hybrid mixtures.
Section snippets
Determination of maximum explosion characteristics
In the course of a confined explosion, an increase in temperature and a resulting rise in pressure occur. The time rate of pressure rise during an explosion in a spherical container is shown in Fig. 1; the so-called pressure–time curve. The two key features are the maximum pressure pmax and the maximum rate of pressure rise (dp/dt)max. The maximum pressure is independent of volume but the rate of pressure rise depends on the volume V of sphere used. A volume-independent quantity KSt, known as
Description of the test apparatus
The measurement was carried out using an apparatus VA 250 (for diagram see Fig. 3). The basis for the design of VA-250 is ČSN ISO 6184 and EN 14034, Explosion protection system. The explosion chamber is a stainless steel spherical vessel having a volume of 0.25 m3. It consists of two symmetrical hemispheres. The lower hemisphere is static and the upper one is attached to a movable frame enabling the opening and closing of the explosion chamber.
In the closed position, both the halves of the
Measurement results
The measurement of the influence of turbulence on maximum explosion characteristics was carried out for the set stirrer speeds, stated together with relevant values of Reynolds number. As starting concentrations for measuring the influence of turbulence on explosion characteristics, optimum concentrations presented in Table 2 were taken. The measurements show that with growing turbulence, both explosion pressure and rate of pressure rise increase. Under turbulent conditions, the effect of the
Conclusions
A modified explosion test apparatus VA 250 has demonstrated a laboratory method for determining pressure characteristics of dust explosions under conditions of some turbulence rather than quiescent ones, as is usually done.
It has been confirmed that the flow of air can have a major influence on explosion characteristics of combustible dusts. Increasing the air velocity can increase both the maximum pressure and the rate of pressure rise by a large amount.
For the substances tested, the rise is
Acknowledgements
The article was written in the framework of the project Institute of Clean Technologies for Mining and Utilization of Raw Materials for Energy Use, reg. No. CZ.1.05/2.1.00/03.0082 supported by the Research and Development for Innovations Operational Programme, financed from the EU Structural Funds and from the state budget of the Czech Republic.
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