Page 500 - Dust Explosions in the Process Industries

Sizing of Dust Explosion Vents 467

by a vent be comparatively small to ensure that the expected number of destructive explo-
sions is kept at an acceptable level. On the other hand, if the probability of dust cloud igni-
tion is low, one can rely on a smaller vent than if the standard of housekeeping and the
efforts to eliminate ignition sources were inadequate. This is illustrated in Figure 6.27.

Explosion
frequency
Bad housekeeping.
Inadequate ignition
source prevention

1 N explosions

Proper housekeeping. I
I
Adequate ignition I I
source prevention. I
I
I
I
I
I
I

Figure 6.27 The reduction of necessary vent area resulting from reduction ofthe overallprobabiiity of
dust cloud ignitions: N is the maximum acceptable number of destructive explosionsper I million years.

Risk is often defined as the product of the expected number of a specific type of unde-
sired event in a given reference period and the consequence per event. When specifying
the maximum acceptable number N of destructive explosions in the 1-million-yearperiod
@e., the maximum acceptable number of explosions of P,, > Pred),it is therefore neces-
sary to take into account the expected consequences of the destructive explosions. This
comprises both possible threats to human life and health and possible damage to property.
In principle, the standard of explosion prevention can be so high that the total number
of expected explosions in the 1-million-year period is on the same order as the accept-
able number of destructive explosions. In such cases, it is questionable whether installing
a vent would be advisable at all.