Page 180 - Electrical Installation in Hazardous Area

146 Electrical installations in hazardous areas

The ideal solution is to design the process or installation so that all walls,
etc., which abut hazardous areas are effectively sealed and this should be
possible in the majority of circumstances.

5.7 Particular circumstances

Quite often, rooms are employed for particular circumstances where
flammable materials are used. Typical of these are the paint-spray booth
(and similar rooms where personnel work) and the paint-drying oven.

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5.7. The paint spray booth
In a paint-spray booth an operative actually releases a flammable material
(the solvent in the paint) as both a vapour and a mist in their work. The
booth will obviously become a hazardous area and, as spraying is going
on for most of the time, would normally be a Zone 0. The gun is not in
the same place all the time and moves during the spraying process which
means that no part of the booth will contain an explosive atmosphere for
more time than is appropriate to Zone 1 classification. There is also a risk to
the operative and both of these problems give rise to a very careful design
whereby airflow is carefully controlled to ensure that solvents are swept
to the back of the booth and not spread around. This is done by keeping
the ventilation velocity relatively high and ventilating by extract via a grid
(or similar) at the rear of the booth to even-out airflow. Also, the spraying
activity is normally interlocked with the ventilation to ensure that spraying
cannot take place unless the ventilation is on. This is normally achieved
by creating curtain flow across the aperture of the booth and to arrange
activities so that the spraying activity is normally directed to the back of
the booth. This coupled with the velocity of airflow which should be more
than 0.5m/s creates a scenario where any Zone 1 is within the booth and
for a small distance in front of it. In these circumstances, even with the
airflow, it is prudent to classlfy an area to the front of the booth as Zone 1
for a small distance in front of the booth (say 1 m) and to take account of
misdirection of spray, to define a Zone 2 for 3m in front of the booth.
Figure 5.15 shows a typical classification for a booth of this type. It
should, however, be remembered that other precautions will need to be
taken for reasons of toxicity or situations where other sources of release
occur (e.g., vehicle petrol tanks). HSE Note I'M 254 should be consulted in
this regard).

5.7.2 The paint drying oven

Painted equipment is often dried in an oven giving a situation where
solvents in the paint are evaporated in an enclosed space. What is worse