Page 456 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 423

1 10 100 1000
ELECTRIC SPARK ENERGY lmll

Figure 5,.24 Effect of tangential "impact" velocity on ignition sensitivity of clouds of lycopodium in
air. Delay between "impact"and electric spark discharge is 0.3-7 0 ms. Envelopes embrace the exper-
imental points (From Pedersen and Eckhoff, 1987).

various types of steel and between steel and rusty steel or concrete are unable to ignite
clouds of grain and feed dust or flour, even if the dusts are dry. Impacts of standard qual-
ity aluminum against rusty steel will not generate even visible sparks. In the case of tita-
nium, the sparks produced can initiate explosions in clouds of dried corn starch but
not in. clouds of starch containing 10% moisture or more, not even in the case of ther-
mite Aashes. However, for net impact energies >>20 J the situation may be different.

5.5
lGNlTlON OF DUST CLOUDS BY HOT SURFACES

5.5.1
EXPERIMENTAL STUDY OF THE INFLUENCE
OF SIZE OF THE HOT SURFACE

The decrease of the minimum ignition temperatures of explosible gas mixtures with
increasing hot surface size has been known for a long time. A classic investigation of this
subject is by Silver (1937). A similar dependence of the minimum ignition temperature on
the area of the hot surface would be expected for dust clouds. Figure 5.25 gives some exper-
imental data from Pinkwasser (personal communication, 1989) confirming this expectation.
The three smallest surfaces were 10 mm long pieces of heated wire of thickness 0.7,
1.2, and 6.0 mm, respectively, bent as a U. The largest surface of 1000 mm2 was obtained
by coiling 50 mm of the 6.0 mm diameter wire. Figure 5.25 also gives the BAM furnace
minimum ignition temperatures of the three dusts, assuming a hot surface area of about
2000 mm2.