Page 146 - Electrical Installation in Hazardous Area

1 12 Electrical installations in hazardous areas

Thus the total volume of mixture leaving the pool edge is given by Q,,,
(volume of air plus gas mixture):
m=Q,+Qg m3/s (Equation 4.25)

where Q, = volume of air leaving pool m3

The percentage gas in the mixture leaving the pool then is:
Q, x lOO/(Q, + Q,) %

Using Equation 4.12:
LEL, = LELg(Qg + Qa)/Qg %
The source length is half the perimeter of the pool and this equals rrd/2.
Expressed as area, this is 1.77 A0.5. The volume of air flowing over the pool
is assumed as given above (Qa). Substituting in Equation 4.24 gives the
following hazardous area beyond the pool edge:
0.5 1.14
X = WWQ, + Q,)Q,I/[LEL(Q, + Q,) x 1.VA 11 m
X = [353 x Q,/LEL x Ao.5]1.14 m
Substituting the Equation for Qg this becomes:

X = [353 x 1.64 x x A0.5x Ap x T/LEL, x Ro.2]1.14 m

X = [5.8 Ap T/ld LEL, Ro.2]1.14 m
where LEL, = lower explosive limit of vapour %
A = pool area m
Ap = partial pressure as a fraction of atmosphere
T = temperature K
R = reynolds number for airflow (Equation 4.26)

The above equation gives the horizontal extent of hazardous area from the
edge of the pool which should also be used for the vertical extent. There
remains the problem of mist and this is very difficult to quantify. Mists
have been shown to be formed at pressures as low as 3 x 1@N/m2 and
thus it cannot be assumed that no mist will be formed from any leak of
liquid above the critical pressure (2 x 1@N/m2). Thus the only method
is to assume the worst case unless precautions are taken to avoid mist
production. This means that the entire leak should be assumed to evaporate
and Equation 4.4 should be used to determine the vapour equivalent this
would have. Equations 4.6 and 4.7 can then be used to determine the extent
of the hazardous area which will occur around the point of leakage. This
will generally produce a result which is much more onerous than what will
actually occur but, unless clear evidence as to the extent of formation of a
flammable mist and its ignition limits is available in a particular case, it is
hard to see how any alternative action can be taken.