Page 160 - Electrical Installation in Hazardous Area

1 26 Electrical installations in hazardous areas

5.1 Typical areas of restricted ventilation

The following are typical of the areas where ventilation can be expected to
be restricted. (restriction will, of course, also appear in other areas).

5.1.1 Open areas surrounded by walls

Where walls are present, containing otherwise open areas, then such walls
can restrict ventilation. BS 5345, Part 2l suggests that to have an open area,
a building should have no walls below a significant height (say 3m) to
be assumed to be freely ventilated but a more relaxed approach is used
in a Netherlands Directorate General of Labour Guide2 which presumes
that, provided 50 per cent of a building’s walls are open construction,
the building is considered as a freely ventilated area if the 50 per cent is
evenly distributed over at least three walls and openings are at least 2.5m
high. While differing in approach, both of these statements confirm the fact
that walls have an obstructing effect on airflow which must be taken into
account.

5.1.2 Covered areas (dutch-barn type)

In areas, such as tanker loading bays, drum storage areas, and the like, it is
often necessary to provide a cover to prevent rain penetration. These covers
take the form of either flat roofs (e.g., as those on garage forecourts) or
hipped roofs (e.g., as those in dutch-barn type construction), the latter often
having apex vents. Both of these types of cover have a possibly deleterious
effect on airflow, even though there are no walls in such cases. The degree
of effect depends upon the flammable gas or vapour being handled and
the height of the cover, even though the open wall requirements specified
above are met.

5.1.3 Above-ground rooms

Above-ground rooms constitute a room which has four walls and a roof
with no effective deliberate ventilation. In such cases there will be air
movement by natural ventilation via cracks in the structure or around
doors and windows, etc. Table 4.2 shows the level of ventilation which
may be expected in such cases and this will be seen as airflow equivalent
to the volume of the room in around one hour in most cases. This will
not mean an air change in the building in one hour as the internal air
currents will not be uniform and pockets of the room will require more
time to be purged of air. To completely change the air in the room it is
expected that several hours will need to elapse and, therefore, the retention
time of any flammable gas or vapour will be long. This retention time may