Page 62 - Electrical Installation in Hazardous Area

38 Electrical installations in hazardous areas

that those who carry out the area classification procedures carry a personal
responsibility in addition to the corporate responsibility carried by their
organization. Decisions which introduce hazards which are considered to
be unacceptable, either because of a later incident or identification as such
during third-party inspections will be placed at the door of those who make
them in addition to their organizations.
Typical examples of methods of reducing or removing sources of hazard
are: blanketing a vessel vapour space with inert gas which reduces the
source of hazard produced by the liquid surface to secondary grade from
continuous grade as, provided the blanketing has sufficient integrity, its
failure is abnormal. (This does not affect the grade of release constituted
by any atmospheric vent.); the use of all-welded pipe instead of gaskets or
screwed connections which removes the possibility of leakage; and dilution
with copious quantities of air at the point of release so that the mixture of
released flammable material and air is below the lower explosive limit at
the point of release. (It is recognized that there must, in these circumstances,
be a very small area of explosive atmosphere at the actual point of release
but its volume will be negligible.)
What is the rate of release? Having optimized the types of release in order
to minimize both their number and severity it is then necessary to quantify
the amount of explosive atmosphere which will be formed by each release.
To do this it is necessary to quantify the amount of flammable material
which can be released and this is normally done by identifymg the rate
of release from each source. For outdoor circumstances an equilibrium will
be reached in its mixing with air and an extent of explosive atmosphere
resulting can be identified. This is not the primary objective, however, which
is to minimize the release by improvements to design of containment and
therefore to minimize the extent of any hazardous area and risk. Typical
of such improvements are: the use of spirally supported gaskets instead of
Standard CAF* gaskets or similar gaskets. The former will not break in a
transverse direction and the results of a piece of gasket being blown out
of the joint is removed. Retained '(0)' rings have a similar effect although
in this case transverse cracking not likely with spirally supported gaskets
must be considered; and throttle bushes on rotating seals which serve to
limit the amount of flammable material released on abnormal failure of the
seal, although not in the case of a failure which is considered as normal
operation and, therefore, a primary grade source of release. The application
of the above will result in a sensible area classification which is not unduly
onerous but, at the same time, is fully objective.
The activity of area classification involves the classic sources of release
which are, in general, fixed such as pipes, pumps and vessels but is not
limited to these. In many operations it is necessary to transport flammable
materials around process areas and working sites and the exercise must also
take account of such activities, particularly with respect to the transport

*Term normally used to describe an unsupported gasket historically made from compressed
Asbestos Fibre.