Page 93 - Electrical Installation in Hazardous Area

Area classification for gases, vapouts and mists 69

will exist normally and as the actual part of the vapour space cannot be
defined, the entire vapour space must be classified as Zone 0.
Zone 1 In addition to the drawing in of air in the emptying cycle, the
filling cycle will exhaust a mixture of flammable vapour and air from the
vent and, as this flammable vapour is almost certain to be heavier than air,
an explosive atmosphere will form around the vent. This will tend to travel
downwards and outwards along the tank top possibly overlapping the
edges of the tank. The liquid pool in the bund, in abnormal circumstances,
will give off vapour into the bund which, unless very large and shallow, will
form a containment impeding the dispersal of the vapour. It is likely that
the interior of the bund will also be Zone 1 due to this impeded dispersal.
Zone 2 If the tank is overfilled, flammable liquid will exit the vent,
travel down the sides of the tank and collect in the bund. There will
be vaporization from the outside of the tank and from the surface of
the pool which will create a Zone 2 around the tank and beyond the
bund wall. A Zone 2 will also be produced by leakage from the tank and
associated pipework but this will be contained within the Zone 2, caused
by overfilling.

Limitations and other considerations

In producing Fig. 3.9, a tank of some 250m3 capacity has been used and
no pumps or other pressurizing devices have been considered as present
within the bund. The water drainage facilities necessarily associated with
the bund have also not been considered as they are assumed not to transmit
flammable liquid or vapour from the bund. Their design should achieve this
in any properly designed installation but if it does not then such possibilities
must be considered in area classification.
In conditions where solar gain brings the contained flammable liquid near
to its boiling point, consideration should be given to providing a cover
to prevent direct sunlight from striking the tank. This has the effect of
removing solar gain and reducing internal tank maximum temperatures to
around 32 "C with corresponding reduction in the range of liquids consid-
ered as flammable liquids. Such action can, however, have an adverse effect
on the external hazardous areas created. If the cover is flat, then the area
under the cover will all become Zone 1 with a lm distance laterally from
the edge of the cover and a similar radial distance from any vents in the top
of the cover. A cover with sides is not recommended as this could severely
adversely affect the ventilation in the area between the tank top and the
cover with the result, at the worst, of creating a Zone 0 in that space, partic-
ularly if there are no top vents in the cover to create a chimney effect.

3.4.2 The f0ating roof tank
These tanks are different to fixed roof tanks in that the tank roof is supported
by the liquid within the tank and moves up and down with the level of