The explosive properties of aluminum dust are in the literature, including the National Fire Protection
Association (NFPA) fire prevention standards. In NFPA 68, Guide for Venting Deflagrations
(2002a), aluminum is ranked among the most explosive of metal dusts. Preliminary data collected by the
CSB indicate that nearly one fourth of all dust explosions in the United States in the last 25 years
involved metal dusts, and that aluminum accounted for the majority of these events.25 Metals account for about 19 % of dust explosions worldwide.26 This data indicates that metal dusts are particularly
hazardous and all appropriate precautions need to be taken to prevent dust explosions.
Dust explosivity is typically expressed using the deflagration constant, KST.27 This constant is determined
experimentally by measuring how fast the pressure rises when dust of a known concentration is ignited in
a container of a specific volume (20 liters). The higher the KST, the more severe a dust explosion can be.
Three “dust hazard classes” indicate relative explosiveness. For comparison, Table 1 shows KST values
for a few known explosive dusts and a sample collected from the scene of the incident. Pure aluminum
has a high KST and is rated as a Class ST-328 dust.
Actual explosivity values vary significantly with the size and shape of dust particles, the concentration of
dust in the air, and the degree of surface oxidation (which, for metals, reduces flammability.)
CSB determined the aluminum dust that accumulated on surfaces in the building was explosive—though
lower in explosivity than reported for pure aluminum powder. CSB samples were primarily of
accumulations outside equipment, and included chips and dust with a wide distribution of particle sizes.
These samples29 generated pressure up to 155 pounds per square inch gage in the test chamber; sufficient
to have caused the damage observed at Hayes. The dust in the dust collector was likely comprised mainly
of finer particles,30 therefore exhibiting a higher explosivity. The explosion and fire completely
consumed this material.