Page 140 - Electrical Installation in Hazardous Area

1 06 Electrical installations in hazardous areas

The volume release rate of gas at the point of leakage will be:
V = 0.082GT/M m3/s (Equation 4.4)

V = 0.082 x 0.03 x 295/28 m3/s
V = 0.026 m3/s

The volume of the gas/air mixture at the point of obstruction will be:
V, = V x (100/Y0 gas in mixture) m3/s (Equation 4.10)

V, = 0.026 x 100/57.7 m3/s
Vm = 0.04m3/s

The molecular weight of the mixture at the point of obstruction needs now
to be calculated and is given by the following:
M, = [Mgas x (%gas/100)] + [M& x (%air/100)] (Equation 4.9)
M, = [28 x (64/100)] + [27 x (36/100)]

Mm = 27.6

Rearranging Equation 4.4 to give Equation 4.10, the mass of mixture at the
point of obstruction is as follows:

G, = 0.04 x 12.19 x 27.6/295
Gm = 0.04 kg/s

In addition, the new LEL will need to be calculated as follows:

LEL, = LEL x (100/%gas in mixture) YO (Equation 4.12)
LEL, = 2.7 x (100 + 57.7) YO

LEL = 4.68 Yo
The foregoing figures can now be inserted into Equation 4.7 to give the
extent of the hazardous area beyond the obstruction as follows:

X, = 10.8 (0.046 x 295127.6 x 4.21)0.55 m
Xm = 3.3 m

The hazardous area in the direction of the obstruction will, therefore,
extend to 4.3m from the source of leakage and vertically upwards and
downwards from the obstruction for 3.3m if this is larger than the vertical
extent derived from the basic release ignoring the effect of the obstruction
(see Equation 4.7). No adjustment for relative density is necessary but any
effects due to ground proximity need to be taken into account.