Page 154 - Electrical Installation in Hazardous Area

120 Electrical installations in hazardous areas

the extent of hazardous area using Equation 4.7. In all cases multiply the
result obtained by 1.5 if the release is within 1.5m of the ground. If the
release is at a pressure greater than 2 x 1@N/m2, but there is an obstruc-
tion within the distance calculated using Equation 4.6 then calculate the
amount of gas in the mixture using Equation 4.8. Then calculate the molec-
ular weight of the mixture using Equation 4.9 after which the total volume
of the gas/air mixture at the obstruction can be calculated using Equa-
tion 4.10.
The Lower explosive limits of the mixture when further mixed with air
can now be calculated using Equation 4.12, after which the wind dispersal
distance can be calculated using Equation 4.7 taking account of the ground
proximity. This should be added to the distance between the release and
the obstruction to give the extent of the hazardous area from the release
point in the direction of the obstruction.

4.4.2 Liquid releases below boiling point

The mass of liquid released is calculated using Equation 4.13. The distance
travelled by the jet of liquid is then calculated using Equation4.15 if
the release is above horizontal, or 4.16 if it is below horizontal. If the
release trajectory is not known use Equation 4.15. In both cases it is
necessary to calculate the release velocity using Equation4.17. Using
Equation 4.19 calculate the vaporization rate and then Equation 4.23 will
permit calculation of the pool area. Finally, using Equation 4.26, the extent
of the hazardous area above and beyond the pool edge can be calculated.
The hazardous area can then be normally defined as the horizontal
distance from the leak to the pool centre, plus the pool radius, plus the
extent beyond the pool and the vertical distance just the extent beyond the
pool. This takes no account of mists and if these are present they should be
assumed as vapour and treated using the procedure in section 4.4.1

4.4.3 Liquid releases above boiling point

It is first necessary to calculate the amount of liquid vaporizing in the
leak path using Equation 4.29 and, from this, the amount of vapour in
the emerging mixture using Equation4.27 and mixture density of the
gadliquid mixture emerging from the release using Equation4.30, after
which the mass of mixture released can be calculated using Equation 4.28.
The mass of vapour immediately outside the leak due to that released
and that flashing-off immediately can be calculated using Equation 4.27
again, but at atmospheric pressure, and the extent of the hazardous area
thus formed can be calculated using Equation 4.6 taking account of ground
proximity. This figure is multiplied by a further factor of 1.5 because of the
likelihood of mist formation.