Page 198 - Electrical Installation in Hazardous Area

162 Electrical installations in hazardous areas

6.3.2 Loadjng hoppers within buildings
It is quite common to use hoppers as a means of loading powders into
process and quite often these hoppers will be on upper floors with effective
sealing arrangements to prevent spilled dust from accessing the process
floors. In these circumstances the following is the Zonal classification of the
loading area. (See Fig. 6.2.)

Zone 20

The hopper is normally filled, or partially filled, with feedstock which is
loaded into it by discharging containers, such as sacks of raw material.
There will normally be thick layers of feedstock and possibly small clouds
of the material within the confines of the hopper. For this reason the interior
of the hopper will be considered as Zone 20.

Zone 21

The area around the manhole will be contaminated with dust clouds during
loading of sacks if extreme care is not exercised and, as it is so likely that this
situation will occur, the clouds produced by emptying of sacks, etc., into
the hopper will be considered as a primary grade source of release giving a
Zone 21 around the charging manhole, extending to 1 m above and around
the manhole and down to the top of the manhole or the floor as appropriate.
The Zone 21 can effectively be removed by extracting room air into the
hopper so that any dust released is effectively sucked into the hopper. In this
case, there will be no Zone 21 around the manhole. The minimum airflow
considered necessary to achieve this will produce a velocity of 1 m/s at the
manhole mouth.

Zone 22
Dust will settle from the cloud but will normally be taken care of by house-
keeping. This, however, will not always be so and occasionally there will
be spillage from the bags in the room. These situations will constitute
secondary grade sources of release, which will give rise to Zone 22 in the
room. As before, unless the room is large the Zone 22 will be assumed to
extend for the whole area of the room and its height, unless there can be
guaranteed to be no ledges at high level in which case a height restriction
of 3 m may be appropriate.
If the housekeeping is of such a standard that spillages are removed
at the time which they occur and the hopper is arranged so that air is
extracted from the room into it, then the Zone 22 can be limited to around
2m surrounding the hopper manhole, as dust will only be released from
the loading procedure abnormally when the air extract system fails.