Static Electricity 2000 Edition

8.12 Bulk Storage.
When powders are moved into bulk stor-
age (e.g., silos, rail cars, trucks, IBCs, or flexible IBCs), the
powder is compacted by the force of gravity. The compaction
process is accompanied by bulking brush discharge, as
explained in 8.5.4. In the compaction process, the energy of
the discharge increases as the particle size increases. There-
fore, the systems most at risk are pellets with an appreciable
fraction of fines (dust).
The exact conditions for ignition-capable bulking brush
discharge are not well understood. However, the following
general factors that are known to increase its probability
have been identified by Glor in Electrostatic Hazards in Power
Handling:
(1) An increase in the resistivity of the powder, greater than
10
10
ohm-m
(2) An increase in the particle size of the powder, greater
than 1 mm
(3) An increase in the charge density of the powder, greater
than 1 
µC/kg
(4) An increase in filling rate — for granules with a diameter
greater than 1 mm to 2 mm, greater than 2 
× 10
3
kg/hr,
and for granules with a diameter of about 0.8 mm,
greater than 20 to 30 
× 10
3
kg/hr
Chapter 9 Specific Applications
9.1 Intermediate Bulk Containers.
9.1.1
The discussion and precautions for powders and granu-
lar solids, as set forth in Chapter 8, also apply to operations
that involve handling these materials in intermediate bulk
containers (IBCs). Static electric charges are generated in
granular materials when filling and emptying IBCs. These
charges result from movement and rubbing of the granules
against process equipment or against each other. The amount
of charge that can be accumulated on a bed of a powdered
material in a grounded container depends on the resistivity of
the powder, not the resistivity of a block of the material. The
higher the resistivity, the lower the apparent conductivity and
the longer a charge will be retained. In cases of very large resis-
tivities, charges will relax slowly and can remain on beds of
material for appreciable periods. Thus, generation and relax-
ation occur simultaneously when granular materials are
moved about. When the rate of generation exceeds the rate of
relaxation, significant charges can accumulate.
9.1.2
When a static electric charge accumulates on bulk con-
tainers or associated process equipment, the following four
types of discharge can occur:
(1) Spark discharge
(2) Brush discharge
(3) Propagating brush discharge
(4) Bulking brush discharge
(See Section 4.3 for discussions of these four types of discharge.)
9.1.2.1
Spark discharges can take place between two conduc-
tors at different potentials and can release energy capable of
igniting atmospheres containing flammable gases or vapors or
combustible dusts, depending on process conditions. An
example of a situation in which such a discharge can occur is
a conductor that is isolated from ground and located in a bin
of material.
9.1.2.2
Brush discharge is usually not a concern in the normal
handling of granular materials. However, brush discharge can
be a source of ignition where flammable gases or vapors are
present, as in the handling of hybrid mixtures or very rapid
discharge of a granular material from a container. Such situa-
tions should be avoided if possible.
9.1.2.3
Propagating brush discharges typically contain ener-
gies of 1 J or greater, depending on process conditions. These
discharges can ignite most flammable atmospheres.
9.1.2.4
Bulking brush discharges contain energies on the
order of 10 mJ. In order to minimize the risks associated with
bulking brush discharge, powders that have minimum igni-
tion energies of 10 mJ or less should only be loaded into con-
tainers of 2 m
3
or less, unless the vessel is inerted.

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