Page 112 - Electrical Installation in Hazardous Area

88 Electrical installations in hazardous areas

a reasonable level. confidence and which have since given no indication
of inadequacy. The latest international Code, BS/EN 6079-103 which is
expected to replace BS 5345, Part 2l in the relatively near future has, in
common with all of its predecessors, failed to effectively approach the deter-
mination of outdoor hazardous area extents by mathematical means indi-
cating that doubt still remains although it does contain a basic mathematical
relationship. Its subjective approach does, however, give some yardsticks
against which mathematically produced solutions can be measured and, as
this code has the confidence of most of the world’s developed countries, it
can serve to support the mathematical approaches included in this chapter.
This is helpful as, notwithstanding the difficulty in producing an approach
which will satisfy everyone, there remains the necessity to carry out the
business of area classification.
For the above reasons the mathematical approaches described in this
chapter are included in this book. The use of these mathematical relation-
ships must, however, be carefully approached and only by those sufficiently
expert to identify their limitations. There is no evidence that the calcula-
tions described do anything but define hazardous areas at least as large
as is necessary when expertly used. This is because they are largely based
on ideal releases from nozzles rather than the accidental leaks which occur
due to failure of such containment elements as glands and gaskets or, in
the case of those included in the new IEC documenp, because of the many
safety factors added.
The basis for this mathematical approach comes from sources such as
Perry4 and work done by Sutton and Katau based on a combination of
fluid dynamics, kinetic theory of gases and practical measurements. Most
of these latter formulae were reported in British Standards Institution Draft
Document 79/270135 in 1979 and constituted the proposal for a calculative
approach which was not adopted as earlier described.

4.1 Releases of gas and vapour

The release of gas or vapour from an orifice or nozzle is given by the
following equation which is widely accepted:
Mass release (G) = CdaP{ (SM/RT)(IZ/S + l)(b+i’s-i)}0.5 kg/s
where G = mass release kg/s
Cd = coefficient of discharge -
a = cross-sectional area of leak m2
P = upstream pressure N/m2
6 = ratio of specific heats Cp/Cv -
M = molecular weight -
R = gas constant (8312) m/kgMole/ OK
T = absolute temperature of released Gas “K