Page 520 - Dust Explosions in the Process Industries

Assessment of lgnitability 487

clear whether this figure refers to the total mass, including the moisture, as 100% or
whether the 100% is the dry mass only.

7.4.5
ELECTRICAL RES I STl VI TY

The significance of electrical resistivity of powders and dusts in the context of process
safety is dual. First, the possibility of accumulating hazardous electrostatic charges and
voltages in an industrial process increases with increasing electrical resistivity powder.
Second, the chance of dusts that penetrate into electric and electronic equipment giving
rise to short-circuits and equipment failure increases as the dust resistivity decreases. From
the point of view of the dust explosion hazard, both situations may lead to generation of
ignition sources.
A method for determining the electrical resistivity of powderddusts has been devel-
oped by the International Electrotechnical Commission ( 1993). The test cell, illustrated in
Figure 7.1 1, consists of two metal electrodes resting on a nonconducting base plate (glass
or PTFE).

Figure 7.1 1 Test cell for determination of the elec-
trical resistivity of dust layers being evaluated by
the International Electrotechnical Commission
( 1993)

The right-angled prismatic dust sample of length Wand cross section H-L fills the
gap between the electrodes. The actual dimensions are W = 10 cm, H = 1.4 cm, and
L = 1.25 cm. The recommended width of the electrodes is 3.3 cm and the thickness of
the base plate, 0.5-1 .O cm. Two glass bars of height 1.4 cm are placed across the ends
of the electrodes to keep the dust sample in place. The dust to be tested is conditioned
at a relative air humidity of 50 k 5% and 20-25°C and should normally pass a 71 pm
test sieve. The moisture content of the dust and any changes of it during the resistivity
test must be reported.
During a test, the resistance R, of the empty test cell is first determined with the two
glass bars in position across the electrode ends. Then, a weighed amount of dust is
poured into the cell and the excess dust scraped off and weighed, to determine the weight
of the test sample and its bulk density. The resistance R, of the dust-filled cell is then mea-
sured for a range of cell voltages from l 10 to 2000 V. In general, R,, > 10 R,, and the resis-
tivity p of the dust is then approximately equal to