Page 98 - Electrical Installation in Hazardous Area

74 Electrical installations in hazardous areas

w
C
a = 1.5m
b = These depend upon the characteristics of the pool formed by
c = } any liquid release
q5 = Limit of Zone 1 for off-loading operation

Fig. 3.12 Loading and off-loading of roadrail tankers

to the lack of knowledge in regard to the amount of such a manually intro-
duced leak but it is not likely to be large and a Zone 1 of 1.5m above
and around the source of leakage projected down to the ground (as shown
in Fig. 3.12) should cover all eventualities. The downward projection takes
account of the fact that the vapour is likely to be heavier than air. In the
case of tanker off-loading the hazardous area defined due to the vents will
not exist, and in this case the extent of the Zone 1 due to the coupling
will be defined by the dotted lines in Fig. 3.12. This Zone 1 extends right
under the tanker to take account of the restricted ventilation there due to
the presence of the tanker.
A third primary grade source of release exists in the flexible pipes used
to couple tankers which are subjected to regular movement and failure in
normal operation can result. Such failure is likely to take the form of a small
crack and, as the pressure is low (being usually the tank head pressure),
release in these circumstances. For this reason it is considered as appropriate