Page 95 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 71

Limitations and other considerations

As for the fixed roof tank, a tank of some 250m3 capacity has been consid-
ered with no pumps or similar equipment being contained within the bund.
Again, drainage devices are assumed not to transmit flammable liquid out
of the bund. The effects of a cover will also be similar to those which occur
with the fixed roof tank although in this case it is more likely that a cover
will be present in view of the construction of the tank.

3.4.3 Tanks containing gas, vapour or liquefied vapours

These tanks are normally sealed as the gas, vapour or liquid will be under
pressure. Leaks from these should be treated as for leaks from pipe and
similar joints as, even if it is liquid which is released, it will rapidly vaporize.
Such tanks may, however, be bunded to take care of massive releases which,
although outside the scope of area classification (e.g., catastrophic) may
need to be taken care of. If such bunds are used then, even if the releases
are secondary grade (abnormal) the bunds may create conditions where the
explosive atmosphere persists within them and their interiors, as for liquid
storage tanks, should be considered as Zone 1.
For low pressure storage, such as is the case with gasholders, water seals
are often used. Where this is the case the possibility of gas exiting through
the liquid must be considered if the pressure inside the holder is too high, or
venting at pressure relief valves where these are provided to prevent over-
pressurizing. In such circumstances the following area classification may be
appropriate and is shown diagrammatically in Fig. 3.11. There should be
no Zone 0 either inside or, in the proximity of, the storage holder as, if this
were so (particularly within the holder), it would be tantamount to storing
and delivering an explosive atmosphere to a process and, in any event, the
liquid sealing arrangements should be such as to achieve the same condi-
tions as in a floating roof tank. It is highly likely that release at pressure
relief valves to a limited extent will occur in normal operation. This limited
release will give rise to a Zone 1 but as the pressure will be very low, the
Zone 1 will be of limited size. A figure of 3m is suggested as adequate to
cover this eventuality.
In abnormal circumstances, overpressurizing due to overfilling the tank
will cause significantly more release at the pressure relief valve and, in
addition, releases at the water seal or seals. The area contaminated in these
circumstances will be larger and a hazardous area of 10m from both of
these leakage points is suggested as necessary to cover this eventuality.

Limitations and other considerations

In selecting the above hazardous areas, normal venting is assumed to be
from an orifice of around 2cm2 at 1@N/m2, the passage through the