Electrostatic spark ignition of sensitive dust clouds of MIE<1 mJ
Journal of Loss Prevention in the Process Industries, 20 (4-6):
396--401 (2007)DOI: http://dx.doi.org/10.1016/j.jlp.2007.03.011
Abstract
Accidental electrostatic sparks in industrial plant producing/handling
powders/dusts occur whenever a non-earthed electrically conducting
object has been charged tribo-electrically to a high voltage and
suddenly discharges its energy to earth via an air gap of appropriate
length. When assessing the electrostatic spark ignition hazard in
an industrial plant, the parameters of prime concern are the capacitances
C of electrically conducting plant items that may become charged
tribo-electrically, the voltages U to which they may become charged,
and the minimum electric spark ignition energies (MIE) of the dust
clouds of concern. Whenever , there is a possibility of accidental
electrostatic spark ignition. Current standard apparatuses for determining
MIE of dust clouds have a lower spark energy limit of 2-3 mJ. In
an investigation by the present authors, discussed in detail elsewhere,
a new spark generator capable of producing synchronized capacitive
sparks of energies down to the order of 0.01 mJ was developed and
used for testing a selection of ignition-sensitive powders for MIE.
Several of the MIEs found were 1-2 orders of magnitude lower than
the lower energy limit of current standard test apparatus. Other
experiments by the present authors, also reported elsewhere, have
shown that quite low MIEs can be found for some dusts even with a
less optimal synchronization mechanism, which may occur accidentally
in practice. The main object of the present paper is to discuss possible
practical concerns arising from the finding that clouds in air of
some dusts can have very low MIEs. In such cases, one may have to
pay attention to even minor C values, i.e. minor plant items. Alternatively,
with larger C values, even quite low voltages may give rise to hazardous
spark discharges. However, some types of fine metal powders of low
MIEs will quite readily form electrically conductive layers on the
solid surfaces with which they make contact. Hence, electrostatic
spark ignition inside process equipment containing such dusts may
be less probable than in the case of process equipment containing
non-conducting dusts of correspondingly low MIEs. There may be a
need for a new standard test method for determination of MIEs of
dust clouds in the <1 mJ range.