Page 92 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 65

Feed Chute
-A

of tkmpmetal

AIR INTAKE
TO MILL

Figure 1.71 A pneumatic separator can be
Figure 1.70 A permanent magnetic separa- used to remove most foreign bodies from the
tor fitted in the feed chute of a grinding mill feed stock: the air current induced by the mill
to remove magnetic tramp metal (From is adjosted to convey the feed stock and reject
Department of Employment and Productivity, heavier foreign bodies (From Department of
1970). Employment and Productivity, 1970).

1.4.2.7
Electric Sparks and Arcs and Electrostatic Discharges
The various types of electric sparks and arcs and electrostatic discharges are described
in Section 1.1.4.6. Sparks between two conducting electrodes are discussed in more
detail in Chapter 5. Sparks or arcs due to breakage of live circuits can occur when fuses
blow, in rotating electric machinery, and when live leads are accidentally broken. The
main rule for minimizing the risk of dust explosions due to such sparks and arcs is to
obey the regulations for electrical installations in areas containing combustible dust.
The electrostatic hazard is more complex and it has not always been straightforward to
specify clearly defined design guidelines. However, Glor (1 988) contributed substantially
to developing a unified approach. As a general guideline, he recommends the following
measures:
Use conductive materials or materials of low dielectric strength, including coatings,
(breakdown voltage across dielectric layer or wall, <4 kV) for all plant items that may
accumulate very high charge densities (pneumatic transport pipes, dust deflector
plates, and walls of large containers that may become charged due to ionization
during gravitational compaction of powders). This prevents propagating brush
discharges.