Page 595 - Dust Explosions in the Process Industries

562 Dust Explosions in the Process Industries

r Valve

b
Glass window

1 Circulation pump or Other suitable means to maintain
the talcum powder in suspension

Figure 8.4 Apparatus used to test the ability of enclosures to prevent ingress ofdust (From IEC, 200 IC).

8.4.2
DESIGN OF APPARATUS TO PREVENT IGNITION OF DUST
CLOUDS AND DUST LAYERS BY HOT ENCLOSURE SURFACES

8.4.2.1
Ignition of Dust Clouds by Hot Surfaces

This topic is discussed in Section 5.5 of Chapter 5 and Section 7.8 of Chapter 7. Although
it is known that the minimumignition temperatureof a given dust cloud is not a true inher-
ent constant of the cloud,results from the laboratory-scaletests described in Section 7.8.2
of Chapter 7 are regarded as representative of the dust tested. The specific test method
prescribed by CENELEC and IEC is illustrated in Figure 7.24 in Chapter 7 and described
in detail in CENELEC (1998~)and IEC (1994b).The variation in the minimum ignition
temperature with variations in the practical conditions of ignition is accounted for by
requiring a substantial safety margin. Hence, both CENELEC (1998b) and IEC (2002~)
require that the maximum temperature of the dust-free enclosure surface does not exceed
two-thirds the minimum ignition temperature for dust clouds, in "C, measured in the stan-
dard test.