Page 109 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 85

3.4.70 Walls and appettures
Where extents of hazardous areas are defined by a wall which contains
a door then the door will become a source of release insofar as the non-
hazardous area side of the wall is concerned and will result in a hazardous
area as shown in Fig. 3.17. Likewise where a wall is short the hazardous
area will extend around it as shown in Fig. 3.18. In both cases the extension
is based upon the original extent of the hazardous area and the distance
between the source of hazard and the wall.

3.4.11 Vents

Vents normally only operate in abnormal circumstances, such as the opening
of a relief valve, or rupture of a bursting disc. In such cases, relatively large
amounts of flammable gas or vapour are released via a vent or flare stack if
released directly to the surrounding atmosphere. This section only refers to
such direct situations as, in other cases, secondary containment is usually
the norm and that can be dealt with in accordance with earlier sections of
this chapter. The flare is assumed not to be ignited as this possibility exists.
In the case of a bursting disc there will be no release of flammable mate-
rial in normal operation but in the case of a relief valve a slight leakage
through the seat must be considered possible (this does not include the
valve-operating stem which should be dealt with separately). In the one
case a Zone 1 may be present at the vent exit and in the other not. When
the valve opens or the bursting disc ruptures a significant flow of flammable
material will occur in both cases but this will only be in abnormal opera-
tion. The small amount of release due to valve-seat leakage will drift from
the vent outlet but the release due to operation of the valve or rupture of
the bursting disc will be a rapid flow. This means that the hazardous area
due to the former will be a small sphere as a result of the drift being wind
dispersed, while the latter will be a plume because the jet turbulence will
be mixing. This gives a rather unusual hazardous area distribution which is
shown in Fig. 3.19, with the Zone 2 shown allowing for the wind distortion
of the plume and this is as follows.
Zone 0 is within the vent stack but only if a relief valve is used.
Zone 1 a sphere of lm around the vent outlet in cases where the relief
valve is used.
Zone 2 a hemisphere superimposed upon the top of the vent in all cases
and the interior of the vent stack where a bursting disc is used.

References

1 BS 5345 Selection, Installation and Maintenance of Elec-
trical Apparatus for use in Potentially Explosive
Atmospheres (other than mining operations or