Page 44 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 17

LRoad tanker 1000 50 1 200 450

Glor' (1988) has given some typical approximate capacitance-to-ground values for
objects encountered in the process industry. These have been incorporated in Table 1.2
arid used for estimating the maximum theoretical spark energy '/zCV2 when discharg-
ing an object of capacitance C at a voltage V to ground.
Minimum electric spark energies (MIE) for ignition of dust clouds vary, as already
mentioned, with dust type, particle size, and so forth, but many dusts have MIE values
well below the higher '/2CV2values in Table 1.2. However, it may not be appropriate
to apply MIE values from standard tests directly to the electrostatic spark problem (see
Chapter 5).
Turbulence in the dust cloud raises the effective MIE and therefore provides a safety
factor. For example, Yong Fan Yu (1985) was unable to ignite turbulent clouds of wheat
grain dust in a container at the exit of a pneumatic transport pipe, even with soft elec-
tric sparks of energies on the order of 1J.
Glor (1988) emphasized that, due to increasing use of nonconducting construction parts
in modern industrial plants, the chance of overloolung nongrounded conducting items
is high. Therefore, the effort to ensure proper grounding of all conducting parts must be
maintained, in particular in plants handling dusts of low MIE. According to Glor (1988)
adequate grounding is maintained as long as the leak resistance to ground does not
exceed lo6i2 for process equipment and IOs Q for personnel. However, in practice, one
aims for considerably lower resistances to ground.
Brush discharges occur between a single curved, grounded metal electxode (radius of
curvature 5-50 mm) and a charged nonconducting surface (plastic, rubber, dust). Brush
discharges can ignite explosible gas mixtures. However, according to Glor (1988), no
ignition of a dust cloud in air by a brush discharge has yet been demonstrated, not even
in sophisticated laboratory tests using very ignition sensitive dusts. Section 9.2.3.4 in
Chapter 9 gives further information. It must be emphasized, however, that this does not
apply if the powder or dust contains significant quantities of combustible solvents (see
Section 1.3.9).
(Coronadischarges occur under the same conditions as brush discharges but are
associated with grounded electrodes of much smaller radii of curvature, such as sharp