Page 149 - Electrical Installation in Hazardous Area

Calculation of release rates and extents 1 15

possible using Equation 4.23 (assuming Reo.2 = 15) to determine the pool
area and diameter as follows:
A = 5.65 x lo2 x 4 x lop5 x [680 x 2 x 102]0.5 x 15/0.71 x 68 m2

A = 81.2 (say 82) m2
The figure for Ro.2 at this size of pool is around 17 and thus the equation
should be solved using this figure which will give:-
A = 93 m2

From the normal circular relationship the diameter can be calculated:
d = 11 m

For this part of the exercise it remains only to calculate the extent of the
hazardous area from the edge of, and above, the pool using Equation 4.26:

X = [5.8 x lo-* x 9.6 x 0.71 x 295/1 x 17]'.14
x=9 m

Thus there will be, due to the pool and liquid jet, a hazardous area 9 m high
for a circle of radius 26m from a point on the ground vertically below the
leak orifice.
We now have to consider, in this case because of the pressure, the possi-
bility of a mist comprising all of the released liquid being formed at the
point of leakage. To decide if this release is diluted by wind action or by
its own energy it is necessary to determine the velocity of the liquid release
as this will give an indication of the effect which the wind will have on
dispersion. The velocity of release can, as already indicated, be determined
by using equation 4.13 but dividing by 01 to convert to volume, and by the
orifice area to give velocity. Thus:

Velocity = 13.7 m/s
This is a fairly low velocity and it has to be assumed that the wind will play
a large part in dispersion. The hazardous area around the leak orifice has
to be assumed to be that which would be produced by total vaporization
of the release diluted by wind action. (Equation 4.7). Thus:
X = 10.8[0.53 x 295/68 x l]o.55 m

X= 17 m
As the hazardous area touches the ground then this figure needs to be
multiplied by 1.5 as previously stated. Thus:
X = 26 m
The hazardous area produced by this example will therefore be a hori-
zontal radius of 26m to a height of 10m with a partial sphere above it