Page 150 - Electrical Installation in Hazardous Area

1 16 Electrical installations in hazardous areas

described by a radius of 26m from the source of release. This is a very
onerous result but, in the absence of knowledge of the release orifice geom-
etry, a necessary one.

4.3 Release of liquid above its atmospheric boiling point

This is the final one of the three 'outdoor' scenarios and addresses a situa-
tion where the flammable material would be a vapour in normal ambient
circumstances but is maintained as a liquid by the containment pressure.
In these circumstances some of the liquid will evaporate on release and the
energy necessary to cause this evaporation will so lower the temperature of
the remainder that further evaporation will only occur as it gains heat from
its environment. It is assumed that this gain will be insignificant during
its 'jet' life but rapid as soon as it comes into contact with the ground,
due to the short duration of its travel between the point of release and the
ground. Once on the ground, however, it is presumed that the assumption
of additional energy is so rapid that instant evaporation will take place.
The first necessity therefore is to calculate the amount of liquid which will
flash off as vapour at the point of release. This is not normally necessary
for calculation of the hazardous area at the point of release.
There is a problem in that mist may be formed by the nature of the release
and that the hazardous area created by the release will be multiplied by 1.5
to allow for this, unless this would vaporize the whole of the release when
this will be assumed to be the case. The fraction of the flammable material
reaching the ground is based upon the heat capacity of the material which
can be used in the vaporization process before the temperature lowers to
boiling point, the latent heat of vaporization of the liquid which identifies
the heat required to vaporize it, and the difference between the boiling point
of the liquid and its actual temperature, which also identifies the amount
of heat available. Thus:

Fraction of liquid vaporizing = [(TI - Tb)/S]cl (Equation 4.27)

where TI = release temperature of liquid
Tb = boiling point of liquid
6 = latent heat of vaporization
C1= liquid heat capacity

The release of the liquid is also more complicated as it is possible for
vaporization to take place in the leak path if it is long, thus reducing the
amount of the release. This can be said to occur where the length of the
leak path is greater than 10 x the effective diameter of the leak path (given
by the relationship, Diameter = [4 x area/^]^.^ for a non-circular orifice).
At and above this figure the release equation is modified as the amount of
liquid/vapour released will total significantly less than would be derived
from Equation 4.13 which is used where vaporization does not occur in