Page 155 - Electrical Installation in Hazardous Area

Calculation of release rates and extents 121

The remaining liquid is assumed to reach the ground and the point at
which it does is calculated as for liquids below their boiling points, using
Equations 4.15 or 4.16 as appropriate.
Vaporization is expected immediately contact is made with the ground
and the extent of the hazardous area is calculated as for gas/vapour using
Equation 4.7 with the result multiplied by 1.5 because of ground proximity.
The hazardous areas are thus around the leak due to vaporization at
the leak, and at ground level with an extent from the leak of the distance
travelled by the jet plus the radius of the area created by vaporization at
ground level. The height of this will be the radius created by ground level
vaporization.

4.5 Releases in areas which are not well ventilated

All of the foregoing calculations depend upon wind reaching all parts at
around 2m/s. In poorly ventilated areas, such as indoors, this wind does
not exist in the same form and is replaced by artificial ventilation in most
circumstances. In addition, the enclosed volume inside the containment
which limits the natural ventilation is not unlimited. Thus all of the
foregoing calculations must be considered suspect unless the ventilation in
all nooks and crannies of the enclosure can be considered as equivalent
to natural ventilation. In BS/EN 60079-lo3 which will replace BS 5345*
there is an attempt to define more globally the extents of and persistence
of explosive atmospheres in a more general set of circumstances and the
following equations are based upon that.
In general, therefore, the only way to deal with indoor areas is to install
ventilation taking account of the leaks present. The minimum flow of air
to dilute a given release of flammable material to below its lower explosive
limit is as follows:

Q, = 0.12 x Gf x Tt/To x M x LEL m3/s (Equation 4.31)

This simplifies to:
Q, = 0.03 x Gf x T:/M x LEL m3/s

where Q, = theoretical minimum air flowrate required m3/s
Gf = maximum mass release rate of flammable material kg/s
T, = ambient temperature K
M = molecular weight of flammable material
LEL = lower explosive limit % v/v

If Q, can be related to the number of air changes around the source of
release (i.e., the airflow velocity) then a further calculation can be carried
out to determine the size of the explosive atmosphere around the gas cloud
as follows:
Vm = Qa/C m3