Page 197 - Electrical Installation in Hazardous Area

Area classification practice for dusts 161

6.3 Practical situations

6.3.1 Cydones and bag filters

Figure 6.1 shows a cyclone which is separating the product of a process
which is formed of granules from dust of the same material produced by
the process. The cyclone and bag filter are in separate small rooms with the
bag filter air exhausting to the outside of the building. The hazardous area
situation is as follows.

Zone 20

Both the interior of the cyclone and the bag filter on the powder side of
the filter will be Zone 20, because dust is always present in both. As the
process works by extracting air, the system is at a pressure slightly lower
than that of the atmosphere and leakage from the pipework and the two
vessels may be ignored.

Zone 21

As the system works at slightly lower than atmospheric pressure there is
no leakage of dust from the system in normal operation. The dust from the
filter is, however, discharged into a bag and release is possible in normal
operation around the discharge duct. The discharge nozzle is a primary
grade source of release and a Zone 21 will exist for a distance of 1 m above
and around it and down to floor level.

Zone 22

The release of dust from the discharge duct can produce layers of dust, but
in normal operation it is expected that housekeeping will limit the extent
of these layers to within the Zone 21. It must occasionally be expected,
however, that due to misalignment of the dust receptacle, failure of the
receptacle/duct sealing arrangements (usually an air-operated expanding
seal) significantly more dust will be released. While this will not greatly
affect the extent of the dust cloud for reasons already mentioned, it will
increase the amount of dust-forming layers and probably the housekeeping
will not be sufficient to limit the extent of the layers in this case. Therefore,
a Zone 22 will exist for a larger distance because of these circumstances
and, as it will be difficult to define its extent, it will be assumed to cover
the entire building. If there were no ledges, etc., in the building it may be
possible to assume the Zone 22 is limited in height to 3m or so.