Page 80 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 53

Figure 1.61 The influence of methane content in
the air on the minimum electric spark ignition energy
of a coal dust of 3 1 % volatile content. Average
I particle size is 40 pm. Note: This uses presumably
0 1 2 3 4 short-duration sparks from a low-inductance, low-
resistance capacitive discharge circuit (From Foniok,
CONTENT OF METHANE IN AIR [vo~%I 1985).

-
8
0 >
I
0.25 ‘L
Q
z
w
0.50
F
0.75 u_
0
1.00 2 Figure 1.62 Nomograph for minimum ignition
w
1.50 ki energy of hybrid mixtures of dust and methane in
2.00 :: air as a function of the methane content in the air
3.00
4.00 and the minimum ignition energy of the dust in air
0 2 4 6 8 1 0 only. Note: This uses a presumably short-duration
MIE OF DUST IN PURE AIR [JI capacitive circuit (From Foniok, 7 985).
It has been suggested that hybrid mixtures involving dusts that are very easy to ignite
even without combustible gas in the air (MIE <10 mJ) may be ignited by electrostatic
brush discharges, but definite proof of this has not been traced.
Figure 1.63illustrateshow the content of combustiblegas in the air influences the per-
centage of inert dust required for inerting coal dust clouds.
One of the first systematicinvestigations of the influence of combustiblegas in the air
on the explosion violence of dust clouds was conducted by Nagy and Portman (1961).
Their results are shown in Figure 1.64.The dust dispersion pressure is a combined arbi-
trary measure of the extent to which the dust is raised into suspension and dispersed and
of the turbulence in the dust cloud at the moment of ignition. As can be seen, the maxi-
inum explosionpressure,withand without methanein the air,firstrose, as the dust dispersion