Page 157 - Electrical Installation in Hazardous Area

Calculation of release rates and extents 123

4.5.1 Example of gas release using BS/EN 10079-lo3
formulae

Example 6

The ethylene release discussed in Example 1 occurs indoors in a building
which is well ventilated (i.e., having a factor f of 1). From Example 1 we
know that the mass release rate is 0.55 kg/s.
Using equation 4.31 airflow required at leak:
Q, = 0.03 x 0.475 x 2952128 x 2.7 m3/s
Q, = 16.4 m3/s

This value can now be used in Equation 4.34 to give the maximum
volume of flammable atmosphere present in the enclosed space if it is venti-
lated at the same rate as an open space ventilated by the wind, as follows:
Q, = 16.410.16 x 0.5 m3
Q, = 205 m’

This volume will give a larger overall hazardous area than would actually
exist outdoors (estimated at between 20m and 45m in the direction of
ventilation flow from the leakage source) as the volume is not a sphere.
Thus this equation is not applicable to the outdoor situation but only to
enclosed finite spaces where the ventilation conditions are similar.
If we now apply Equation 4.35 to this situation we obtain the persistence
time after release ends as follows:
t = (1/0.16)Ln(0.5 x 2.71100) S
t = 27 S

Again, this is a very high figure for an outside location but more realistic
for an indoor situation where overall volume is more limited.

4.6 Conclusion

This chapter contains a relatively comprehensive set of calculations which
may be used for determination of extents of hazardous areas. Those
concerned with outdoor areas for vapours and for liquids have been
extensively used for several years and basically agree with the subjective
judgements found in several codes and thus have some pedigree. The
formulae in Section 4.4, however, are newer. Until they have been in use
for a considerable time and evidence ammassed, they should be used
with extreme care. It is suggested that initially, they are used as yardstick
calculations only (i.e., to determine if the persistence time in a particular
circumstance would cause a secondary source of release to produce a Zone 1
indoors where it occurs). It is stressed that there is no intent to suggest that