Page 447 - Dust Explosions in the Process Industries

4 14 Dust Explosions in the Process Industries

As Figure 5.18 shows (Parker, 1985),there is a limit to the combination of spark dis-
charge duration on spark energy that can be realized in practice. This is because a stable
arc phase cannot exist unless the degree of ionization of the gas, which is determined by
the spark current, exceeds a certain minimum level.
In Chapter 7, the concept of electric spark ignition sensitivity profile is discussed in
connection with a standard test for ignition of dust layers by electric sparks (Figure
7.33). In fact, Parker’s results for the four dusts in cloud form, as presented in Figure
5.18, are electric spark ignition sensitive profiles. The influence of the spark discharge
duration on the minimum ignition energy is important for adequate use of test data in
practice. For example, very low minimum ignition energies determined by the stan-
dard discharge circuits of L 2 1 mH discussed in Chapter 7 may not be relevant for
assessing the electrostatic spark ignition hazard in an industrial plant. This is because
high inductance values are unlikely to occur in accidental electrostatic discharge cir-
cuits in industry.
As discussed in Section 1.1.4.6 in Chapter 1, several kinds of electrostatic dis-
charges in air do not occur across two well-defined, sharp electrodes and therefore
don’t have such a well-defined shape as the dischargein Figure 5.17. In such cases, which
will not be discussed any further in the present context, one could expand the con-
cept of an ignition sensitivity profile to that of an ignition sensitivity surface for a given
dust cloud, by adding a spark geometry dimension, as illustrated in Figure 5.19. The
definition of an appropriate geometric parameter would, however, require careful
consideration.

Figure 5.19 Schematic illustration of the possible concept of electric spark ignition sensitivity sur-
face of an explosible dust cloud.