Page 102 - Electrical Installation in Hazardous Area

78 Electrical installations in hazardous areas

In such cases no Zone 2 exists visibly as its size is synonymous with the
Zone 1.
Figure 3.13 is for a ground level oil/water separator but if the ground
is at or about the base level of the separator then the Zone 2 is projected
vertically down the separator side.
Finally, should the oil/water separator overflow, a pool of mixed oil and
water would form. The Zone 2 associated with this pool can be derived
using Table 3.7 assuming the highest volatility oil is present. Because water
is present and may cause the mixture to be transmitted further than would
be the case if only oil were present, it is advisable to mechanically limit
the pool size with some sort of containment sized to contain the maximum
overflow possible. This containment size should then be used in all circum-
stances for determination (using Table 3.7) even where the equilibrium pool
size is smaller.

3.4.6 Other open vessels

Open vessels will behave in a very similar manner to oil/water separators,
except that usually only one flammable material is present. This will give
the following hazardous areas.
Zone 0 will be the interior of the vessel.
Zone 1 will be as for an oil/water separator but defined on the basis
of the boiling point of the single flammable liquid where appropriate. As
before, it is not recommended that open vessels be used where the liquid
contained has a boiling point of less than twice the ambient temperature
of the location (this effectively means boiling points of more than 65°C in
the UK).
The only situation giving rise to a Zone 2 is the overflow situation and
the Zone 2 resulting should be defined as for oil/water separators in this
circumstance.

3.4.7 Open drains

Open drainage gullies are often used to collect released flammable liquids so
that the dimensions of any pool can be accurately defined, and to provide
a route for collection and disposal of such liquids. They do not, unlike
oil/water separators, normally contain flammable liquids but may do so
in cases of spillage. The presence of such liquids within them is, therefore,
either in normal operation or only abnormal and, because they actually
drain away the flammable liquid, overflow is abnormal. This leads to the
following hazardous areas.
If the flow into the drain is in normal operation the interior of the drain is
Zone 0 because of the persistence effect which the containment of the drain