Page 92 - Electrical Installation in Hazardous Area

68 Electrical installations in hazardous areas

exhibit a particular problem in that when in direct sunlight solar gain can
occur, particularly in their vapour space and temperatures higher than the
typical ambient maximum of 32 "C can occur. The extent of this solar gain is
nowhere directly specified but a figure of 47.5 "C for small tanks is specified
in one company code and the HSE Booklet, HS(G)504 defines Flammable
Liquids as those with flashpoints up to 55 "C. It is prudent to take the latter
for the basis of discussion of explosive atmospheres around stock tanks of
this type and not to store any flammable liquid with a boiling point below
55 "C in such tanks. These tanks are normally bunded to contain spillages of
the contained flammable liquids in cases of gross overfilling or leakage and
the bund-in effect determines the extent of the pool of flammable liquid.
This, of course, may not be true in cases where a bund encompasses several
tanks and normal pool dimension determination applies.
There are two normal situations which apply. These are the situation
within the tank and the situation outside the tank when normal filling and
emptying operations are carried out. In addition, there is always the possi-
bility that in abnormal circumstances the tank will be overfilled resulting in
flammable liquid being present in the bund. This abnormal situation also
takes account of any flange leakage, etc., in the proximity of the bund. To
ensure that the bund is effective its nearest point to the tank should always
be more than 1 m so that liquid leakages from the tank always fall into the
bund (leakages due to overfilling or flange leakage local to the tank will
normally be at a pressure close to the liquid head pressure of the tank - if
this is not so, the bund should be designed accordingly). The bund should
also be capable of containing the entire tank contents with a safety factor
(say 1.1). Making these assumptions, the area classification for the tank and
its surrounding areas is as shown in Fig. 3.9.
Zone 0 Because of the regular emptying action, air is constantly being
drawn into the tank vapour space and mixed with vapour. It is likely,
therefore, that somewhere in that vapour space an explosive atmosphere