Page 97 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 73

The results of this approach is that it is not generally considered as neces-
sary to area classify such places as parking areas where general public access
is available, but it becomes necessary in the case of parking areas on sites
where the materials are used and are designated specifically for such tanks.
Typical of these are railway sidings dedicated to the marshalling of tanks,
specific areas of dockyards again dedicated to location of tanks prior to
loading or unloading, and similar locations in process plant areas. This
generalization is not, however, always valid as there may be places where
many tankers regularly park and in such cases the division described above
becomes harder to jusbfy. Due to the uncontrolled nature of such parking
places it is not possible to use classic area classification and so the pres-
ence of the driver becomes critical. Routes and parking procedures need
to be determined to minimize the number of tankers parked in any one
location for significant periods, to reduce the need for overnight parking
and to ensure that when the vehicle is left there is no evidence of leakage,
all of which will reduce the risk to an acceptable level. There are many
government regulations in this area, typical of which are The Dangerous
Substances (Conveyance by Road in Road Tankers and Tank Containers)
Regulations 19815 which should be consulted in this regard.
Wherever tankers are loaded, off-loaded/discharged or parked when
loaded in sigTuficant numbers there is little justification for not applying
the normal area classification procedures, and the tank and its contents
form a source of release in these circumstances unless particular sources
of release are prevented (i.e., the source of release from tank vents may be
removed by an extraction system which will prevent any hazardous area
being formed apart from in the immediate vicinity of the vent, but in such
circumstances the vent exhaust needs consideration). In addition tanker
bays used for unloading only will be different from those used for loading
since there is little or no release from vents.

Tankers containing unpressurized flammable liquids
below their boiling point

These tankers are normally of mostly welded construction and thus the only
sources of release which generally need to be considered are the following:
releases from any joints for pipework connection, etc.; releases from vents
due to loading; and releases from flexible pipes used to connect the tanker
to fixed plant.
The area classification around the tanker resulting from the tanker itself
and its immediate coupling (not the fixed plant in the loading area which
must be dealt with additionally) will be as follows.
Zone 0 The interior of the tanker.
Zone 2 When a tanker is loaded or unloaded there is always the danger
of a small leak of liquid during coupling or uncoupling. To allow for this,
which is a primary grade source of release a Zone 1 needs to be defined
around the coupling points. The size of this is, to a degree, arbitrary due