Page 196 - Electrical Installation in Hazardous Area

160 Electrical installations in hazardous areas

Fig. 6.3 Manually loaded hopper without air purge inwards through manhole outside
a building. X = 1.5m, y = 2.5m. Notes: (1) The area below the loading
floor is not addressed. (2) Where air is drawn into the hopper through the
manhole, the dust cloud producing the Zone 21 will not exist provided the
airflow at the manhole mouth exceeds 1 m/s

Layers of dust are formed wherever dust is released, even if that release is
not sufficient to produce an explosive dust cloud. The extent and thickness
of such layers is, to a degree, dependent upon the level of housekeeping in
the area of release, particularly if this is indoors. While the level of house-
keeping will always need to be sufficient to ensure that extensive, thick
dust layers will not form from releases which occur in normal operation
(from primary grade sources of release), only a very high level of such
housekeeping can be assumed to have a signhcant limiting effect upon the
extent of hazardous areas from dust-layer formation from secondary grade
sources of release. These are usually from rare failures of process equip-
ment containment and are less predictable and usually larger (for example,
from the rupture of a bag being brought to the discharge point). For this
reason it is not unusual to find the entire area of a building containing dust
processing equipment classified as Zone 22. In addition, it is quite normal
to classlfy the entire height of such a building as Zone 22, even though the
dust layer will only be at floor level. Dust layers can form on beams, stan-
chions and similar items which produce ledges at higher levels and these
must be taken into account, which is why a height limit is often not used.
Where dust is stored rather than processed the situation may be different
depending upon the method of containment. Where drums and similar
containers are used, the likelihood of leakage is small, and coupled with
good housekeeping it is not difficult to ensure that dangerous layers of dust
do not occur. Where the material is stored in such things as paper sacks this
ceases to be true and it is likely that the storage area should be classified
as Zone 22.