PARTICLE SIZE DISTRIBUTION
The explosion characteristics of the powders depend on the particle size.
As a rule, the situation is more critical for smaller particles. Therefore, testing standards require that the particle size of powders must be characterized for explosion tests.
The particle size distribution is measured by laser diffraction of raised dust in air. The median value is the particle size, which is exceeded by 50% (referring to the mass or volume) of the particles.
Depending on the test method and on the standard applied, the recommended particle size for dust explosion tests is limited to:
Median value < 63 micron (e.g. VDI-recommendation Vol. 134)
Particle size 100% < 63 micron (e.g., for VDI 2263, EN 13821)
Particle size 100% < 71 micron (e.g., for IEC 1241)
Particle size 95% < 75 micron (e.g., for ASTM E2019-99, ASTM E1491-97)
Unless stated otherwise by the requestor of the test, powders are prepared by the test laboratory (grinded or sieved and dried in vacuum, 1h, at 50°C to fulfill these requirements).
Independent of the requirements and recommendations above, the particle size distribution must be determined and reported for dust explosion tests.
For the assessment of electrostatic cone discharges, which become more dangerous with increasing particle size, the particle size distribution representative for the material handled in the plant must be used.