Page 161 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 127

be sufficiently long to disrupt the source of release/classification of area
relationship, certainly in pockets in the room. While this ventilation may be
improved by the inclusion of openings in room walls, this is not necessarily
as good an option as is at first thought. Most such openings are fitted with
louvres which are necessary to maintain the integrity of the room in such
circumstances. Louvres and similar devices such as meshes have a severely
deleterious effect on ventilation. Experiments have shown, for example
that strategically placed openings in walls of buildings produce significant
airflows (up to 0.3m/s) up to 10m from the walls, but the inclusion of
louvres has a significant negative effect in that when a louvre is present in
an opening which would otherwise produce the airflow described above,
no significant airflow is detectable more than 3 m from the walls'.

5.1.4 Below-ground rooms

Below-ground rooms produce the most adverse situation possible because
only the entry is above ground there is no possibility of effectively
introducing openings, and crack ventilation is effectively absent. The time
for the totality of the air in the room to change, while not being infinite, is
very long (many hours). Any release in such rooms would be expected to
persist for long enough to cause the room to be classified as Zone 0, even
if the release was from a secondary source of release.

5.2 Effect of walls on hazardous areas

The effect of a wall close to an area which is classified as hazardous will be
the same as that of ground proximity, although density of the flammable
material will not have such a marked effect. Neglecting prevailing winds,
which has to be the case in general, the presence of a wall will adversely
affect mixing of gas and air, as will any other obstruction. The effects of the
presence of a single wall will have the effect of changing the geometry of a
hazardous area in the same way as ground proximity (see Figs. 3.4 and 3.5
for details). Where the wall is not of sufficient length or height to completely
obstruct the gas/air mixture then the hazardous area will flow around and
over it (See Figs. 5.1 and 5.2). The resultant wrap around hazardous area
has been based on the entire leak exiting the edge of the wall.
This approach may be extended to cases where the source of release is
contained in an area bounded by two walls or by three walls. Figures 5.3,
5.4 and 5.5 indicate how the hazardous area is affected. Identification of the
hazardous area will basically be as in the case of one wall, but it should be
noted that unless the walls are extremely long in the case of the two-wall
scenario, the hazardous area will always extend at least to the end of both
walls. In the three-wall scenario, the extention should be at least as far as the
outboard end of both wing walls, with all of the enclosed area forming the
hazardous area. Vertical extents should follow the single wall approach. In