Page 135 - Electrical Installation in Hazardous Area

Calculation of release rates and extents 101

(LEL) in circumstances where the jet is at sonic velocity and is given by the
following equation:

X = [2 x lo4 d/EM] x [av/aa]0.5 m
where E = lower explosive limit (LEL) 70
a, = gas or vapour density at ambient pressure and kg/m3
temperature
D~ = density of air at ambient pressure and temperature kg/m3
d = diameter of release orifice corrected for pressure
drop to ambient at release m

Assuming, as previously discussed, sonic velocity, then the corrected orifice
diameter is given by:
d = [4G/n a, v,]O.~ m
Substituting for v, in Equation 4.5 and using 1.2kg/m3 as the density of
air at ambient pressure and temperature the distance to the LEL can be
expressed as:
X = 2.1 x 103[G/E2M'~5TO~5]05 m (Equation 4.6)

The above equation remains valid as long as the jet velocity on release
is sufficiently high in relation to the typical wind speed specified earlier
(0.5 - 2m/s). Where this is not so the flow of gas or vapour will cease
to be the dominant factor and its dispersal will become more and more
reliant on the atmospheric conditions local to the release. In the ultimate
circumstance where the release has virtually no velocity (i.e., no energy of
its own) it becomes more difficult to calculate dispersal and the derivation of
the extent of the hazardous area becomes based on the following empirical
formula which was produced by experimentation.

X = 10.8[GT/ME]o.55 m (Equation 4.7)

This equation can only be used where the upstream pressure is such that the
release velocity is not considerably higher than the wind velocity (say more
than 20 times) and will not be valid unless that pressure is significantly
less than the critical pressure, or the extent of hazardous area produced
will be excessively large. The changeover will not be at a specific point but
will be progressive. It is difficult to determine this progression as it will
vary from gas to gas but Fig. 4.1 provides a graphical progression which
will give an acceptable usable solution. This gives a smoothed changeover
between Equations 4.6 and 4.7. This figure can be simply used by calculation
of the mass release of gas or vapour at the critical pressure, and utilizing
the result of this to calculate the extent of the hazardous area using Equa-
tion 4.7. This result is then multiplied by a factor produced from Fig. 4.1
for the actual pressure and the gas or vapour in question. It is considered
best, although not essential, to utilize Equation 4.3 for calculation of mass