Page 42 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 15

1.I .4.6
El'ectric Sparks and Arcs: Electrostatic Discharges

It has been known since the beginning of this century that electric sparks and arcs can
initiate dust explosions. The minimum spark energy required for ignition varies with the
type of dust, the effective particle size distribution in the dust cloud, the dust concen-
tration and turbulence, and the spatial and temporal distribution of the energy in the elec-
tric discharge or arc.
It was long thought that the electric spark energies needed for igniting dust clouds in
air were generally much higher, by one or two orders of magnitude, than the minimum
ignition energies for gases and vapors in air. However, it is now generally accepted that
many dusts can be ignited by spark energies in the range 1-10 mJ, that is, close to the
rainge of gases and vapors. Some dusts may ignite at even lower energies.
It may be useful to distinguish between discharges caused by release of accumulated
electrostatic charge and sparks or arcs generated when live electric circuits are broken,
either accidentallly or intentionally (switches). In the latter case, if the points of rupture
are separated at high speed, transient inductive sparks are formed across the gap, as illus-
trated in Figure 1.12.If the current in the circuit prior to rupture is i and the circuit induc-
tance k,the theoretical spark energy, neglecting external circuit losses, will be '/z Li2.
As, an example, a current of 10A and L equal to H gives a theoretical spark energy
of 0.5 mT. This is too low for igniting most dust clouds in air. However, larger currents
or inductances can easily produce incendiary sparks. Sometimes, rupture results in only
a small gap of permanent distance. This may result in a hazardous stationary arc if the
circuit is still live.

FAST SEPARATION
t----l-

SPARK ENERGY = + Liz IJI

Figure 1.12 inductive spark or "break flash" generated when a live electric circuit is suddenly
broken and the points ofrupture are separated at high speed.

Over the years, the question of whether electrostatic discharges can initiate dust explo-
sions has been discussed repeatedly. The basic mechanism causing accumulation of
electrostatic charges in industrial plants is the transfer of charge between objects during
rubbing. This occurs easily during the handling and transport of powders and dusts,
where charge is exchanged between the powder or dust and the process equipment. The
charge accumulated on process equipment or bulk powder can be released in various