Page 93 - Dust Explosions in the Process Industries

66 Dust Explosions in the Process Industries

0 Ground all conductive parts of equipment that may become charged. This prevents
capacitive spark discharges from equipment.
Ground personnel if powders of minimum ignition energies <lo0 mJ are handled. This
prevents capacitive spark discharges from humans.
Ground electrically conductive powders (metals, etc.) by using grounded conductive
equipment without nonconductivecoatings.This prevents capacitivedischargesfrom
conductive powder.
If highly insulating material (resistivity of powder in bulk >loloQm) in the form of
coarse particles (particle diameter >1 mm) is accumulated in large volumes in silos,
containers, hoppers, or the like, electrostatic discharges from the material in bulk
may occur. These dischargescan be hazardous when a fine combustible dust fraction
of minimum ignition energy <10-100 mJ is present simultaneously. So far, no reli-
able measure is known to avoid this type of discharge in all cases,but a grounded metal-
lic rod introduced into the bulk powder will most probably drain away the charges
safely. It is, however, not yet clear whether this measure is always successful.
Therefore, the use of explosionventing, suppression,or inerting should be considered
under these circumstances.
If highly insulating,fine powders (resistivity of powder in bulk >1O1O Qm) with a min-
imum ignition energy 110 mJ, as determined with a low-inductance capacitive dis-
charge circuit, is accumulated in large volumes in silos, containers, hoppers, or the
like, measures of explosion protection should be considered. There is no experimen-
tal evidence that fine powders with no coarse particles generate discharges from
powder heaps, but several explosions have been reported with such powders in situ-
ations where all possible ignition sources,with the exceptionof electrostatic,have been
effectively eliminated.

If combustible powders are handled or processed in the presence of a flammable gas
or vapor (hybrid mixtures), the use of electrically conductive and grounded equipment
is absolutely essential. Insulating coatings on grounded metallic surfaces may be toler-
ated, provided the thickness is less than 2 mm, the breakdown voltage is less than 4 kV
at locations where high surface charge densities have to be expected, and the conduc-
tive powder cannot become isolated from the grounding by the coating. If the powder
is nonconducting (resistivity of the powder in bulk >lo6Qm), measures of explosion pre-
vention (e.g., inert gas blanketing) are strongly recommended. If the resistivity of the
powder in bulk is less than lo6Qm, brush discharges, which would be incendiary for
flammable gases or vapors, can also be excluded.
Glor pointed out, however, that experience has shown that, even in the case of pow-
ders of resistivities in bulk <IO6 Rm it is very difficult in practice to exclude all kinds
of effectiveignition sources when flammable gases or vapors are present. In such cases,
large amounts of powders therefore should be handled and processed only in closed sys-
tems blanketed with an inert gas.
Further details, including a systematicstep-by-stepapproach for eliminatingthe elec-
trostatic discharge ignition hazard, were provided by Glor (1988). He also considered
the specific hazards and preventive measures for different categories of process equip-
ment and operations, such as mechanicaland pneumatic conveying systems, sieving oper-
ations, and grinding, mixing, and dust collecting systems. Sections 9.2.3.4 and 9.3.5.4
in Chapter 9 give references to more recent works.