Page 94 - Electrical Installation in Hazardous Area

70 Electrical installations in hazardous areas

Fig. 3.10 Floating roof stock tank. A = 1.5 m, B = 2.5 m

liquid. A seal is provided between the moving roof and the walls of the
tank and vapour can only be present in this situation where it, or small
quantities of liquid, pass that seal. This, of course, is either a situation of
'normal operation' or, in cases where the amount of liquid passing the
seal normally gives sufficient vapour to create an explosive atmosphere, a
continuous situation. There is also the possibility of the walls of the tank
which are above the roof restricting dispersion and causing the explosive
atmosphere to persist. This effect will be exacerbated where a cover is fitted
to protect the top of the tank roof from the environment. The result of this
is shown in Fig. 3.10 and is basically as follows:
Zone 0 does not exist within the tank, unlike the fixed roof case as the
roof is in intimate contact with the stored liquid. Provided the space in
the tank above the roof is unimpeded, the retention of flammable vapour
will not be sufficient to create a Zone 0 in this space and, therefore some
advantage over fixed roof tanks is apparent.
The interior of the tank above the tank roof will be Zone 1 and this will
extend for 1.5m above and laterally from the edges of the tank. As the
vapour is generally heavier than air the Zone 1 can be expected to extend
further down the sides of the tank accordingly. In abnormal circumstances
the behaviour of the tank will be similar to a fixed roof tank in that liquid
will overflow into the bund. As in the case of the fixed roof tank a pool
will form in the bund and the vapour space above that pool will contain an
explosive atmosphere. Because of the likely interference of the bund wall
with ventilation, there will be persistence within the bund and the interior
vapour space will be Zone 1.
As in the case of the fixed roof tank there is the abnormal possibility of
overflow of liquid into the bund and to allow for this a Zone 2 of 1.5m
from the top of the tank and around its sides will be present. This will not
show where the Zone 1 exists as it has a similar extent and is thus masked.
A Zone 2 will also exist from the bund limits for a vertical and horizontal
distance of 2.5m, as for the fixed roof tank.