Page 177 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 143

as the amount of air necessary if this were not so would be prohibitive. It is
worth noting, however, that vertical overlapping strip curtaining has been
used with success in this activity.

5.4.3 High integrity ventilation

All of the above is based upon the fact that forced ventilation can fail and
that failure is an abnormal effect and, therefore, it is only considered where
continuous or primary grade sources of release are involved. The types of
failure involved are normally mechanical or electrical failure of the venti-
lation itself. This problem may be overcome by duplicating the ventilation
system so that a failure particular to one will merely bring the standby
system into operation. Provided the separation between the two ventilation
systems is sufficient (e.g., different mechanical parts and different electrical
supplies) then failure of both systems can be neglected as it would require
two abnormal events (which need not be considered for area classification
purposes). In such cases it is possible to define limits of Zones within the
building as the ventilation can be relied upon and tests can be carried out
to show the extent of hazardous areas produced by particular releases in
cases where no experience exists.
In such situations there remains the possibility of total electrical failure
which is not likely to coincide with a release where secondary grade sources
of release are involved, and in cases where continuous grade and primary
grade sources of release are involved all electrical equipment will be de-
energized by the failure. In such circumstances, provided that residual hot
surfaces in equipment are addressed, then a pre-purge of the area before
energizing the electrical equipment will ensure that the situation is accept-
able. This can be achieved by introducing a delay between restoration of
the airflow and energization. The time for this delay will depend upon the
geometry of the building but if the building is properly designed to mini-
mize the risk of unventilated pockets, which should be the case, the time
should not need to exceed that required for an airflow equivalent to five
times the volume of the building to pass through it.
In such circumstances the extent of any hazardous area can be calculated
on the basis of the Equations in Chapter 4. The extents of the hazardous
areas produced should be based upon the volume of explosive atmosphere
expected.

Example

A volume of vapour of mass 2.5 x is released by a source of release
inside a building which has high integrity ventilation. The material released
is at 295K, has a molecular weight of 50, and a lower explosive limit of
2 per cent.