Page 40 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 13

Figure 1.1 0 A hidden dust deposit inside a
duct can be brought to ignition by heat sup-
plied to the duct wall from the outside.

It should be mentioned that van Laar (1981) found that burning cigarettes and cigars
may give rise to smoldering fires in tapioca and soybean meal. Further information is
given in Sections 1.4.2.5,and 9.2.3.3 and 9.3.5.2 in Chapter 9.

1.'I .4.3
Open Flames

The flames of welding and cutting burners are sufficiently powerful to initiate explosions
in anydust cloud able to propagate a self-sustainedflame. The cutting burner flame is par-
ticularlyhazardous,because it supplies excess oxygen to the working zone. If combustible
dusts are dispersed in atmospheres containing more oxygen than in air, both ignition sen-
sitiLvityand explosion violence increases compared with clouds in air (see Section 1.3.6).
All codes and regulations for preventing dust explosionscontain strict requirementsto the
safety precautions that have to be taken when performing hot work in areas containing dust.
Smoking should be prohibited in areas where combustible dusts exist. A burning
wooden match develops about 100J of thermal energy per second. This is more than suf-
ficient for initiating explosions in most combustible dust clouds. Further information is
given in Section 1.4.2.3.

I .I .4.4
Hot Surfaces

In ;additionto igniting dust layers, hot surfaces can initiate dust explosions by direct con-
tact between the dust cloud and the hot surface. However, the minimum hot surface tem-
peratures needed forthis are generallyconsiderablyhigher (typically40@-500°Cfor organic
dusts) than for ignition of dust layers. Further details are given in Section 1.4.2.4, and in
Chapters 5,7, and 9.

1.I .4.5
Heat from Mechanical Impact

The literature on dust explosions is sometimes confusing when discussing the ignition
of dust clouds by heat from mechanical impact. This is reflected in the use of terms such
asfriction orfriction sparks when categorizingignition sources.To clarify the situation,
it seems useful to distinguish betweenfriction and impact.