Page 74 - Electrical Installation in Hazardous Area

50 Electrical installations in hazardous areas

source of hazard method it is necessary to identify if that source can behave
in more than one way and then to identify the hazardous area created by
each of the possible methods of operation.

3.3.1 Types of release

In any process there are typical releases which fall into categories and
to which most normally encountered releases will fall. The following are
typical of these.

Release of gas or vapour under pressure

Most processes put gases and vapours under pressure to transport them
from place to place, to maintain them in a stable state or for such reasons
as injection into process vessels or similar functions. The release velocity of
any gas or vapour under pressure can be shown to be high and normally
at the speed of sound in the gas or vapour in question. This velocity is so
high that the gas or vapour will mix with air due to its own turbulence and
prevailing wind conditions will have little or no effect in outdoor situations.
This will remain true unless the jet of gas or vapour meets an obstruction in
which case velocity is likely to dramatically reduce and the wind become the
prime dispersant. Indoors or in less well-ventilated situations, unless special
precautions are taken, dispersion is into a limited amount of air and there
is a cumulative effect which can contaminate the whole indoor or shielded
area and so special precautions are often necessary. These can include the
extensive use of local extract ventilation and it is often advantageous to
ensure that gases and vapours under pressure are excluded as far as possible
from areas where ventilation is restricted.

Release of liquids under pressure

Liquids contained under pressure will often release as a jet and hazardous
areas may be defined in part by the distance travelled by such jets. Vapor-
ization at the point of release will be minimal provided the liquid is below
its boiling point at atmospheric pressure. Therefore, the principal source of
vapour is evaporation from the surface of the pool which is created by the
jet of liquid. Due to the distances travelled it is often wise to fit any possible
leak source with a baffle to prevent the jet from forming which limits the
location of the pool. The size of the pool can also be limited by design of
its location which leads to bunding. At any significant pressure (it is hard
to be specific as both leak geometry and pressure play a part) there is also
the possibility of the formation of mists which are finely divided liquid
particles. These behave like a gas or vapour and, provided the liquid has
a flashpoint, ignition of mists of liquids below their flashpoints is possible.
Therefore, where mists can occur flashpoint ceases to be a dividing criterion.