Page 158 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 13 1

of ignition source generation. Furthermore, the open construction provides generous
venting should an explosion nevertheless occur.
In Table 1.12,Noha also indicated that adding inert dust to the explosible dust is a means
of preventing dust explosions in crushers and mills, in principle. However, as pointed
out in Section 1.4.3.3, most often this method is not feasible in practice, due to con-
tamination of the product by the inert additive.

1.5.2.3
Dryers for Powders and Granular Materials

In the case of dryers (Table 1.13), the special protective method “intrinsic inerting” can
be a good solution in some cases, particularly for spray dryers. This method implies that
the required quantity of inert gas is generated in the plant itself by controlled combus-
tion in the hot-gas generator and recirculation of the gas. Such hot gases consist mainly
of nitrogen, carbon dioxide, and water vapor. The residual concentration of oxygen is
kept at a sufficiently low level to ensure inert conditions. Intrinsic inerting of dryers was
discussed specifically by Hammer (1978) and Klais (1989).
Spray dryers normally operate at dust concentrations significantly below the lower
explssible limit, which clearly adds to safety. However, dust deposits are often gener-
ated on walls and the like and smoldering nests may develop, depending on the local tem-
perature and oxygen concentration. Klais (1989) emphasized that oxygen concentrations
as low as 4 vol%, which exclude dust explosions for most organic powders, may still be
too high to prevent certain autooxidation processes in the dried powder when deposited
as a layer or a heap (see also Figure 1.67 in Section 1.4.2.2). If a smoldering nest loosens
and gets carried away with the product stream, it can initiate a dust explosion in down-
stream cyclones and silos.
In the case of disc type spray dryers, one cannot fully exclude the possibility that a
disk that flies away and impacts the walls of the dryer generates a hot spot of sufficient
size and temperature to initiate a dust explosion.
The powder and dust in belt and plate dryers are mostly in deposited form. In paddle
dryers, the dust Concentration normally is expected to exceed the upper explosible limit
in the areas where occurrence of an ignition source might be envisaged.
Gibson, Harper, and Rogers (1985) evaluated the fire and explosion risk in powder
dryers with particular emphasis on the detection of exothermic decomposition. Their con-
clusion was that existing methods for evaluating dust explosion risk, combined with an
adequate method for characterizing the exothermic decomposition properties of the
powder, provide a means of specifying safe drying conditions. Most often, control of the
atmosphere to keep the dust cloud nonexplosive or the use of venting, automatic sup-
pression, or explosion- (shock-)resistant equipment is required. However, in certain
cases, safety can be based solely on avoidance of decomposition and ignition.

1.5.2.4
Powder and Dust Mixers

In mixing equipment (Table 1.14), ignition sources may be avoided as long as there are
no fast-moving mixing tools. Inerting by adding, for example, nitrogen or another inert