Page 94 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 67

I .4.3
PREVENTING EXPLQSIBLE DUST CLOUDS

1.4.3.1
lnerting by Adding Inert Gas to the Air

The influence of the oxygen content of a gas on the ignitability and explosibility of dust
clouds was discussed in Section 1.3.6. For a given dust and type of added inert gas, there
is a certain limiting oxygen content, below which the dust cloud is unable to propagate a
self-sustained flame. By keeping the oxygen contentbelow this limit throughoutthe process
system, dust explosions are excluded.As the oxygen content in the gas is gradually reduced
from that of air,the ignitability and explosibilityof the dust cloud is also gradually reduced,
until ultimately flame propagation becomes impossible. Figure 1.72 shows some of the
results from the experiments by Palmer and Tonkin (1973) in an industrial-scale experi-
mental facility. Solid lines separate the experiments that yielded no flame propagation at
all, flamepropagation in part of the tube, and flamepropagated the entire length of the tube.
22
- 20
$
0
J 18
VI
4
W
z 16
b-z
e 14
z
3
5 12
0
>- A FLAME PROPAGATION FULL TUBE LENGTH
X
O 10 0 FLAME PROPAGATION PART TUBE LENGTH
ot I I X NO FLAME PROPAGATION I I
I
I
I
10 20 50 100 200 500 1000 2000 5000
DUST CONCENTRATION[q/d
Figure 1.72 Concentration range of flammability of clouds of phenol formaldehyde (15 p is: CO,.
Experiments are in a vertical tube of diameter 0.25 m and length 5 m. Flame propagated upward (From
Palmer and Tonkin, 1973).
Schofield and Abbott (1988) and Wiemann (1989) have given useful overviews of the
possibilities and limitations for implementing gas inerting in industrial practice. Five types
of inert gases are in common use for this purpose:
Q Carbon dioxide.
@ Water vapor.
0 Flue gases.
@ Nitrogen.
Q Raregases.
Fischea:(1978) also included halogenated hydrocarbons (halons) in his list of possi-
ble gases for inerting. However, due to the environmental problems caused by these sub-
stances, they may no longer be permitted for protecting against explosions and fires.