Page 100 - Electrical Installation in Hazardous Area

76 Electrical installations in hazardous areas

These leaks normally give rise to the following hazardous areas. The
interior of the tank should be considered as Zone 0 as, even though it is
less likely that air will enter, there is always the possibility that the tank will
regularly be empty and, rather than try to define and control such periods,
it is better to assume Zone 0. Such an assumption will not normally be
unnecessarily restrictive.
Zone 1 will result from one of three scenarios which give rise to primary
grade sources of release, two due to failure and one due to manual inter-
vention. Of the two due to failure, the first is the valve stem or actuator
and the Zone 1 deriving from this source should be based upon the normal
Zone 1 which results from a regularly operated valve and which is appro-
priate in this case. Therefore, a Zone 1 will be assumed to exist above and
around the valve for 1 m. The second scenario will be a failure in the flex-
ible coupling pipe and here the orifice is expected to be small and so total
vaporization can be assumed at the point of release. It is considered that
such a release will be similar to that occurring when a screwed joint leaks
as, because personnel are involved, it is almost certain that anythrng larger
would be very noticeable and evoke immediate rectification. The Zone 1
produced should be determined from Table 3.9 and that at ground level
from Table 3.10. The third primary grade source of release is that which
could occm due to manual intervention during coupling or uncoupling but
this is expected to be so small that it is covered by the Zone 1 due to flex-
ible pipe failure. If, however, the coupling point is remote from the flexible
pipe then it should be surround by a Zone 1 of similar proportions to that
produced for the flexible pipe and Tables 3.9 and 3.10 apply.
The Zone 2 areas present will be due to the flexible pipe, couplings and
other joints and the valve actuating stem. The valve stem seal can be consid-
ered as a spirally-supported gasket for Zone 2 definition purposes and the
flexible joint as a CAF gasket rupture and the extent of Zone 2 at joints,
couplings and the flexible joint can be derived from Tables 3.9 and 3.10.
It must again be emphasized that the above takes account only of the
hazardous areas created by the tank itself and its immediate environs. The
hazardous area created by fixed plant surrounding the tank must be addi-
tionally dealt with.

3.4.5 OiVwater separators

Oil/water separators are open vessels into which a mixture of water and
(usually spilled) oils are placed and permitted to dwell for a sigruficant time.
The objective of this is to allow the water to separate from the oil so that
the one can be disposed of and the other allowed into the normal drains.
Theoretically, the vessel will normally contain primarily lower volatility
oils as those with higher volatilities will have been subject to siphcant
vaporization before the mixture reaches the separator. Where the majority
of the oils used at a particular site are of high volatility this type of oil/water
separation is not appropriate and another method should be chosen as the