Page 75 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 51

Release of liquefied gases and vapours
Where gases and vapours are maintained as liquid by pressure or a combi-
nation of pressure and temperature, release will usually result in instant
vaporization of some of the release. The remainder remains liquid until it
can obtain sufficient heat energy to transform itself in to a gas or vapour.
There will be a hazardous area due to the immediately formed gas or vapour
and, in addition the liquid pool scenario will hold. In addition, the vapor-
ization will create some mist due to the turbulence created and the resulting
situation is very complex indeed.

Unpressurized releases
In many containers, such as stock tanks, road tankers, paint vats, etc. there
will be an explosive atmosphere in the vapour space caused by the vapour
released by liquids above their flashpoints and below their boiling points.
In normal circumstances these vapours will be relatively heavy and will not
exit the vapour space of the container in any significant quantity while equi-
librium is maintained. Such containments are, however, frequently filled
and that operation expels the flammable material into the surrounding area
causing a hazardous area.

Communication between physical locations
A hazardous area may often be contained by a wall or similar blocking
device which effectively defines a zonal limit. In these circumstances it is
necessary to consider any opening in the blocking wall, such as a door or
cable trunk, as these can act as conduits to transfer the explosive atmosphere
from one area to another. The only effective way to do this is to consider any
such opening as a source of hazard in the remote area and act accordingly.

Pressurized gases and reagents

In many cases gases or reagents used above their flashpoints will be present
in or taken into particular areas for purposes such as analysis. While having
no direct relation to the process, these flammable materials can equally
be identified as sources of hazard and must be treated accordingly. The
problem here is that if they have not been identified in the construction of
the plant, they may be more significantly sensitive than the process mate-
rials and the equipment chosen for the plant may not be suitable for them.
While the source of hazard method of area classification requires each
source of hazard and its resulting hazardous area to be identified there are
some generalizations which may be used to simpllfy the area classification
activity, and although these often give excessive hazardous areas they are