Page 596 - Dust Explosions in the Process Industries

Electrical Apparatuses for Areas Containing Combustible Dusts 563

8.4.2.2
Ignition of Dust Layers by Hot Surfaces

The topic of self-heating and self-ignition in dust layers is discussed in Section 5.2
of Chapter 5 and Section 7.7 of Chapter 7. The specific test method prescribed by
CENELEC and IEC to determine minimum ignition temperatures of a dust layer on
a hot surface of constant temperature is illustrated in Figure 7.17 in Chapter 7 and
described in detail in CENELEC (1998~)and IEC (1994b). Tests with a given dust,
varying the thickness of the dust layer, show that the minimum ignition temperature
decreases systematically with increasing layer thickness. Both CENELEC (I998b) and
IEC (2002~)require that the maximum temperature of the enclosure surface be at least
75°C lower than the minimum ignition temperature determined in the test. Figure 8.5
indicates how the maximum permissible enclosure surface temperature decreases sys-
tematically with increasing dust layer thickness, for three different dusts having min-
imum hot surface ignition temperatures of 25OoC,320°C, and 400°C, respectively, for
5 mrn layer thickness. However, if a large part of the hot surface is covered by a com-
paratively thick dust layer, the surface temperature of the enclosure may increase to
a value significantly higher than that attained in the absence of dust, as discussed in
Section 8.3.3. In that case, Figure 8.5 does not apply and special assessmentis required.
This may imply both special tests and mathematical model simulations, as also dis-
cussed in Section 8.3.3.

0 10 20 30 40 50
fhlckness of layer rnrn

Figure 8.5 Graph of maximum permissible temperatures of enclosure surfaces as functions of the
thickness of a layer of combustible dust on the surface. It is assumed that the dust layer on the enclo-
sure surface does not significantly increase the temperature of the surface over that attained without
a dust layer (From CENELEC, 1998b).