Page 36 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 9

or, in a rearranged form,

Lllx = (pp/c)1’3
For particles of density 1 g/cm3,that is, lo6g/m3,a dust concentrationof 50 g/m3cor-
responds to L/x = 27. For 500 g/m3,which is a typical worst-case explosible concentra-
tion, L/x = 13.The actual density shown in Figure 1.4,of L/x = 4, corresponds to a very
high dust concentration, 16 kg/m3,which is well above the maximum explosible con-
centration for organic dust (2-3 kg/m3).
It is important to note that the absolute interparticle distance correspondingto a given
dust concentrationdecreases proportionallywith the particle size. For example, at a dust
colncentrationof 500 g/m3and a particle density of 1g/cm3,L equals 1.3 mm for 100pm
particles, whereas it is only 13pm for 1pm particles.
Zehr (1965)quoted a rule of thumb by Intelmann,saying that, if a glowing 25-W light-
bulb is observed through 2 m of dust cloud, the bulb cannot be seen at dust concentra-
tictns exceeding40 g/m3.This is illustratedin Figure 1.5.It followsfrom this that the dust
clouds in which dust explosions are primarily initiated are normally found inside process
equipment, such as mills, mixers, screens,dryers, cyclones, filters,bucket elevators,hop-
pers, silos, aspiration ducts, and pipes for pneumatic transport of powders. Such explo-
sions,initiatedby some ignition source(see Section 1.1.4) are calledprimary explosions.

Figure 1.5 A cloud of40 g/m3of coal dust in air is so dense that a glowing 25-W lightbulb can hardly
be seen through a dust of cloud 2 m thick.

This reveals an important difference between primay dust and gas explosions.In the
case of gases, the process equipment normally contains fuel only, with no air, and under
such circumstances,gas explosionsinside process equipment are impossible.Therefore,
most primary gas explosions occur outside process equipment, where gas from accidental
leaks is mixed with air and explosible atmospheres generated.
An important objective of dust explosion control (see Section 1.4)is to limit primary
explosions in process equipment to the process units in which they initiate.A central con-
cern is then to avoid secondary explosions due to entrainment of dust layers by the blast
wave from the primary explosion. Figure 1.3 shows that there is a gap of two orders of
magnitudebetween the maximum explosible dust concentration and the bulk density of
dust layers and heaps. The consequence of this is illustrated in Figure 1.6. This figure