Page 71 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 47

A Zone 0 will also occur in the vapour space of an open-topped tank
containing a flammable liquid. All other places where flammable gas or
vapours normally released will also be Zone 0.

Zone 1

Zone 1 is any area containing vessels, pumps, compressors, pipes fittings
and similar items of equipment which may be considered to leak in normal
operation (e.g., with some degree of regularity) and any area containing
such things as relief valves, vents and similar devices which are designed
to release flammable gases, vapours and liquids in normal operation will
be classified as Zone 1.
Areas containing sample points which are not specially designed to
prevent release in normal operation are in Zone 1. In the case of sample
points the Zone 1 can, however, be limited to the immediate location of the
sample point particularly if sampling is manual.
Finally, areas where releases are so rare as not to be assumed to occur in
normal operation will also be identified as Zone 1 if ventilation is restricted.
Such areas incIude both indoor areas and other areas where, as a result of
any form of containment or airflow restriction, ventilation is considered to
be restricted.

Zone 2

An area containing vessels, pumps, compressors and similar equipment
which are so well maintained that leaks can be assumed only to occur
very rarely (in abnormal operation) and relief valves which only operate
very rarely (abnormally) will be classified as Zone 2. The vents associated
with such things as bursting discs will also normally give rise to Zone 2 in
areas in which they occur. In all these cases the Zone 2 classification is only
acceptable provided that the areas in question are freely ventilated, that is,
the equivalent of a normal outdoor situation, so that the released vapour is
rapidly dispersed. Less well-ventilated zones in these Zones 2, such as pits
and trenches, will be defined as Zone 1.
As a result of the danger to personnel carrying out sampling, sampling
points are normally designed to ensure that any area into which personnel
access for sampling purposes are Zone 2.

3.2.2 Generalized extents of zones
The extents of the zones produced by this approach tend to be arbitrary
and large. The interiors of vessels are well defined but when the less
hazardous areas - such as Zone 1 and particularly Zone 2 - are considered,
entire locations have to be classified, particularly in the case of Zone 2 where
it is not unknown for an entire plant to be classified.