Page 99 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 75

to limit the Zone 1 produced to a distance of 1.5m above and around the
flexible coupling projected vertically down to the ground.
Zone 2 This will result due to abnormal circumstances, the first of which
will occur if the tanker is overfilled. In these circumstances the liquid will,
as in the case of a fixed tank, flow out of the vents and down the sides of
the tank, ultimately forming a pool on the ground. The Zone 2 produced
above and around the tank by this event will be no larger than the Zone 1
produced by vapour release and the resulting Zone 2 will be invisible in
the case of the loading of road/rail tankers but, as it is due to abnormal
circumstances, it cannot be presumed not to exist in the unloading scenario.
Therefore a 1.5m Zone 2 above and around the tank will be present in all
circumstances and will be projected vertically down to the ground. The
pool of flammable liquid formed on the ground will grow until it is either
contained or until its vaporization rate is equal to the release rate of liquid
from the tank (see Chapter 4). The hazardous area created by this pool will
be Zone 2 and will be sized as given in Table 3.7.
A second source of release in abnormal circumstances will be that
occurring when a flexible connecting pipe actually ruptures. In this
circumstance it is not expected that total rupture will take place due to the
sort of reinforcement provided in such connecting pipes and, therefore, a
similar release to that occurring with rupture of a CAF gasket is considered
to be a fair model. Again, the release of liquid will form a pool and the
dimensions of this pool can be determined from Table 3.3. The extent of the
hazardous area around the pool so formed can then be determined from
Table 3.7. As the pressure will be low in these cases it is unlikely that any
mist will form and the Zone 2 around the connecting pipe will be minimal.
This is academic as a Zone 1 of 1.5m has already been declared and this
will encompass any small Zone 2 around the connecting pipe.
Finally, it is highly unlikely that solar gain will occur in road/rail tankers
unless they are parked for long periods in direct sunlight immediately prior
to unloading. Provided care is taken in this regard only liquids with flash-
points below 32 "C need be considered.

Road or rail tankers containing gases under pressure or
liquefied gases and vapours

Road or rail tankers which contain gases or vapours under pressure, or
liquefied gases or vapours which will of course be also under pressure,
will behave as any other vessel in such circumstances. The mechanisms of
release will be principally as follows: liquid, gas or vapour leakages due
to coupling and uncoupling; liquid, gas or vapour leakages due to failure
of valve actuator failure; liquid, gas or vapour leakages due to failure of
fixed pipe joints; and liquid, gas or vapour leakage due to failure of flexible
couplings.