Page 159 - Electrical Installation in Hazardous Area

5

Area classification practice for

gases, vapours and mists in areas
which are not freely ventilated

In Chapter 3 the effects of releases of flammable materials in freely venti-
lated areas was considered in much detail. It will be remembered that in
such areas there was, theoretically, an unlimited amount of diluting air
and much turbulence was produced by normal winds and breezes. While
this is useful in the majority of oil-refining installations and other process
plants, there are cases where it is necessary to enclose the sources of possible
release for operational or other safety reasons. Paint Spraying, for instance,
needs a carefully controlled environment and where such things as cyanide
are processed it is often necessary to enclose that process as leakage could
have catastrophic effects if this were not done. Once a source of release
of flammable material is enclosed, however, it is denied the large quantity
of diluting air and the natural turbulent conditions present in an outside
area, and its dilution and dispersion becomes a more significant problem.
Not only is it extremely difficult to introduce sufficient diluting air into
an enclosed space to accurately mimic the external situation in all parts of
that enclosed space, but it can cause adverse personnel reaction. Typically,
in external situations air movement in excess of 1m/s occurs for much
of the time, but in normal indoor conditions an air movement of more
than 0.2m/s in the general space is likely to cause discomfort to those
normally working indoors. The problem is, therefore, a complex one to
approach.
While places where ventilation is limited tend to be equated with build-
ings, many places which are not buildings are included in this consideration.
Often, such places as tanker loading bays are covered to protect operations
from the environment. Where this is done the enclosure is usually just a roof
and without walls and in these circumstances, although the ground-level air
movement is similar to that which would be expected in a freely ventilated
situation, there will be areas near the roof where ventilation is restricted.
There are four scenarios in which natural ventilation must be considered as
restricted. In such circumstances the important objective is to ensure that
such ventilation as exists is used to best effect, and this means that sources
of release need to be identified and ventilation arrangements need to take
account of their location so that ventilation is best at points where releases
can occur.