Page 100 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 73

Figure 1.77 Calibration data for light attenuation
loo *O0 300 400 500 6oo 700 dust concentration probe in Figure 1.76for native
ACTUAL DUST CONCENTRATION lg/rn31 maize starch in air (From Eckhoff et al., 1985).
plastic dust from pellets. By monitoring dust concentrations and controlling airflows, the
desired level of dust concentrationcan be maintained. However, if the airvelocities are too
low to prevent dust deposits on the internal walls of the ducting over time, dust explosions
may neverthelessbe able to propagate through the ducts (see Section 1.3.4and Chapter 4).
Possible dust entrainmentand formation of explosible dust clouds by the air blast pre-
ceding a propagating dust explosion may also occur in mixers, conveyors, and the like,
where sufficient quantities of fine dust are deposited. This means that, in many cases,
dust concentration control is feasible for preventing only the primary explosion initia-
tion, not propagationof secondary explosions.References to more recent works on rnin-
imum explosion dust concentrations are reviewed in Section 9.2.4.3 in Chapter 9.

1.4.3.3
Adding Inert Dust

This principle is used in coal mines, by providing sufficient quantities of stone dust either
as a layer on the mine gallery floor or on shelves and the like. The blast that always pre-
cedes the flamein a dust explosion then entrains the stonedust and coal dust simultaneously
and foms a mixture that is incombustiblein air;and the flame,when arriving, is quenched.
In industries other than mining, adding inert dust is seldom applicable, due to con-
tamination and other problems. It is neverthelessinteresting to note the special wartime
applicationfor protecting flour mills against dust explosionsinitiated by high-explosive
bombs, suggested by Burgoyne and Rashbash (1948). Table A.3 in the Appendix con-
tains some data for the percentage inert dust required for producing inert dust clouds with
various combustible materials.

1.4.4
PREVENTING EXPLOSION TRANSFER BETWEEN PROCESS
UNITS VIA PIPES AND DUCTS: EXPLOSION ISOLATION

1.4.4.1
Background

There are three main reasons for trying to prevent a dust explosion in one process unit
from spreading to others via pipes and ducts. First, there is always a desire to limit the
extent of the explosion as far as possible.
Second, a dust flame propagating in a duct between two process units tends to accel-
erate due to flow-inducedturbulencein the dust cloud ahead of the flame.For a sufficiently