Page 587 - Dust Explosions in the Process Industries

554 Dust Explosions in the Process Industries

Zone 20. A place in which an explosive atmosphere in the form of a cloud of com-
bustible dust in air is present continuously, for long periods, or frequently.
Zone 21. A place in which an explosive atmosphere in the form of a cloud of com-
bustible dust in air is likely to occur occasionally in normal operation.
Zone 22. A place in which an explosive atmosphere in the form of a cloud of com-
bustible dust in air is unlikely to occur in normal operation but, if it does occur, per-
sist for only a short period.

In accordance with the philosophy of the “Atex 100a” Directive (1994), layers or
deposits of combustible dusts are not considered hazardous in themselves. They are
taken into account only if they are considered possible sources of generation of explo-
sive dust clouds or if they can give rise to dust fires that can ignite dust clouds (see Section
8.1.3).
Beck (2002) summarized the main philosophy of the “Atex 118” and “Atex 100a”
Directives, with regard to dusts. Electrical apparatuses are classified in three categories.
Category 1apparatuses can be used in all three zones, category 2 apparatusescan be used
in zones 21 and 22 but not in 20, whereas category 3 apparatuses can be used only in
zone 22.

8.2.3
INTERNATIONAL STANDARDS

The CENELEC (2002) and draft IEC (2002b) standards for classification of areas con-
taining combustible dusts are practically identical, and both define the hazardous zones
20,21, and 22 in accordance with the definitions given in Section 8.2.2. The term source
of release for explosive atmosphere is central in both standards. The term has been
adopted from the corresponding gas standards, which to a large extent focus on explo-
sive gas clouds generated by intentional or accidentalreleases and leaks of combustible
gas into the atmosphere. In the case of dust clouds,point of dust cloud generation or area
of dust cloud generation would seem more appropriate.
The way in which the three zones are to be marked on hazardous area classification
drawings accordingto the CENELEC (2002) and draft IEC (2002b) standardsis shown
in Figure 8.1, and examples to illustrate the use in practice are given in Figures 8.2
and 8.3.
The interior of the hopper in Figure 8.2 is classifiedas zone 20, because explosive dust
clouds are generated there during the quite frequent emptying of bags into the hopper.
Because of the dust extraction system, explosive dust clouds are not expected to extend
outside the hopper in normal operation. However, abnormal situations may arise in
which this may occur, such as if a bag bursts during being emptied or the dust extrac-
tion system fails. Therefore, a zone 22 classificationis assigned to a limited areajust out-
side the hopper opening.
The interior of the cyclone in Figure 8.3 is classified as zone 20 because of the pres-
ence of an explosive dust cloud frequently, or even continuously, for long periods.
However, the dust concentration in the gas leaving the cyclone and entering the dusty
side of the filter is normally below the minimum explosiveconcentration,except for short