Page 89 - Dust Explosions in the Process Industries

62 Dust Explosions in the Process Industries

Use of electrical apparatus approved for use in the presence of combustible dust.
Use of equipment with a minimal risk of overheating.
Inspection and maintenance procedures that minimize the risk of overheating.

1.4.2.5
Smoldering Nests

Pinkwasser (1985,1986) studied the possibility of dust explosionsbeing initiated by smol-
dering lumps (“nests”) of powdered material conveyed through a process system. The
object of the first investigation(1985) was to disclosethe conditions under which smol-
dering material that had entered a pneumatic conveying line would be extinguished,that
is, cooled to a temperaturerange in which the risk of ignition in the downstreamequip-
ment was no longer present. In the case of >1 kg/m3pneumatic transport of screenings,
low-gradeflour and C3 patent flour,it was impossibleto transmit a 10 g smoldering nest
through the conveying line any significant distance. After only a few meters, the tem-
perature of the smoldering lump had dropped to a safe level. In the case of lower dust
concentrations,between 0.1 and 0.9 kg/m3,that is, within the most explosiblerange, the
smoldering nest could be conveyed for an appreciable distance, as shown in Figure 1.68,
but no ignition was ever observed in the conveying line.

Figure 1.68 The distance traveled in pneumatic
tranmort bv a smoldering nest before becoming
v
Y
0 0.2 0.4 0.6 0.8 1.0 extinguished, as a function of dust concentration in
the pipe. The air velocity in the pipe is 20 m/s (from
DUST CONCENTRATION [kg/m31 Pinkwasser, 7985).
In the second investigation,Pinkwasser (1986) allowed smoldering nests of 700°C to
fall freely through a 1m tall column containing dust clouds of 100-1000 g/m3of wheat
flour or wheat starch in air. Ignition was never observed during free fall. However, in
some tests with nests of at least 25 mm diameter and weight at least 15 g, ignition
occurred immediately after the nest had come to rest at the bottom of the test column.
This may indicate the possibility that a smoldering nest, falling freely through a dust cloud