Page 499 - Dust Explosions in the Process Industries

466 Dust Explosions in the Process Industries

The enclosure considered is equipped with a vent opening. The expected maximum
pressure P,,, generated in vented explosions in the enclosure will, by and large,
decrease with increasing vent size. This is illustrated in Figure 6.26. If the vent area
is unnecessarily large, like AI, the distribution of expected explosion pressures will be
well below the maximum permissible pressure Pred. On the other hand, if the vent is
very small, like A,, a considerable fraction of all explosions will generate pressures
exceeding the maximum permissible one. (Note that the A, and A, cases in Figure 6.26
illustrate the pressures that would have been generatedhad the enclosurebeen sufficiently
strong to withstand even P,,, > Pred.)

Explosion
frequency
Over- I

0

max
Pred
Figure 6.26 Distribution of maximum explosion pressures generated in a given process unit, fitted with
vents of different sizes, by the same 1-million-yearpopulation explosions. The unit of explosion frequency
is number of explosions per million years per unit of pressure. The areas under the frequency curves
then give the total number of explosions in 1 million years and are therefore the same for the three cases.

In the case of A,, the vent size is capable of keeping a clear majority of all explosion
pressures below Pred.If the fraction of the explosions that generates P,, >Pred represents
a reasonable risk, A, constitutes an adequate vent size for the case in question. However,
the decision as to whether the fraction of expected destructive explosions is acceptable
depends on several considerations. The first is the expected total number of incidents of
ignition of a dust cloud in the enclosure in the 1-million-year period. This number is
strongly influenced both by the standard obtained with respect to elimination of potential
ignition sources and the standard of housekeeping. If these standards are comparatively
low, the overall chance of cloud ignitions is comparatively high. Consequently,it is nec-
essary to require that the fraction of all expected explosions that will not be taken care of