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20 litre-sphere

In a standardised test apparatus with a contents of 20 litre, a small number of tests is performed over a wide range of concentrations (normally from 30 g/m3 to 2000 g/m3) to determine whether or not the dust is explosible.

Importance

Before embarking on more specific tests, it is sometimes considered desirable to perform an introductory test in order to determine whether the powder or dust in question can give a dust explosion at all.

Test equipment & procedure

The test is performed in a standardised test apparatus with a contents of 20 litre.

20 litre-sphere

The dust sample is dispersed into the explosive chamber with compressed air from a storage container via a special distribution system. The tests are performed with two pyrotechnic igniters of 1 kJ each as ignition source. The course of the explosion is recorded as a function of time (with two quartz pressure sensors), and from the pressure-time curve the explosion pressure and the rate of pressure rise are recorded.
The explosion behaviour in the 20 litre-sphere is evaluated by means of the explosion pressure. If the total explosion pressure exceeds 0.5 bar, an explosion is said to have occurred.
Correction of the measured explosion pressure is necessary to account for the influence of the igniters. Even without an explosive dust in the 20 litre-sphere an important overpressure is recorded, which is caused by the heat liberation from the chemical igniters. The influence of the igniters on the measured explosion pressure diminishes as the pressure effect of the explosion itself becomes larger.
A dust which cannot be induced to explode over a wide range of concentrations (normally from 30 g/m3 to 2000 g/m3) with an ignition energy of 2 x 1 kJ (pyrotechnic igniters) is classified as not explosible.
The test is performed on the sample fraction having a particle size less than 63 µm.

Further remarks

'Not explosible' means that most probably the dust cannot be exploded at all, except by application of even stronger ignition sources (ignition energy > 2 kJ).

Bibliography

  • EN 14034: Determination of explosion characteristics of dust clouds
  • VDI-Richtlinien 2263, Blatt 1: Untersuchungsmethoden zur Ermittlung von sicherheitstechnischen Kenngrössen von Stäuben (1990)
  • Kühner AG, Operating instructions for the 20 litre apparatus
  • W. Bartknecht, Staub Explosionen: Ablauf und Schutzmassnahmen (1987)
  • R.K. Eckhoff, Dust explosions in the process industries (1997)
  • Handboek explosiebeveiliging, Kluwer-Editorial