Page 156 - Electrical Installation in Hazardous Area

122 Electrical installations in hazardous areas

Because of the degree of uncertainty in this two safety factors need to be
added, one for the uncertainty in LEL and another for the uncertainty in
ideal airflow. This gives:

V, = Q,f/C k m3/s (Equation 4.32)
where V, = volume of explosive atmosphere m3
Q, = airflow at leak source m3/s
f = Effectiveness of ventilation
(1 = ideal airflow)
(2-4 = intermediate grades of airflow)
(5 = most obstructed airflow)
C = number of air changes per second based on
airflow around source of release
k = safety factor applied to LEL
(0.25 for continuous and primary grades of release)
(0.5 for secondary grades of release)

In the case of an enclosed (defined) area, C can be calculated as follows:
m3 (Equation 4.33)
c = Q/VO
Vo = volume of enclosed space
The above equation relates to defined (enclosed) spaces only. For the
open-air situation a further equation is derived from Equation 4.32 and is:
Vo = Qa/C k m3

If one takes a volume of air in an unrestricted area (outdoors) of around
1000m3 (assumed to be circular as that is the worst case) then it will be
swept with air in around 6 seconds (C = 116 = 0.16). Thus:

V, = QJO.16 k m3 (Equation 4.34)

Due to the nature of airflow in outdoor conditions this latter equation is
likely to give a pessimistic (overlarge) volume of explosive atmosphere.
The equations in this series also include the following equation which is
intended to allow the calculation of dispersion time after the release has
stopped:
t = (f/C)G, (LEL kUL) s YO v/v (Equation 4.35)
where LEL = lower explosive limit Yo v/v
UL = maximum concentration

To assist in the use of this equation where ventilation is poor (indoors
where no extra ventilation is added) Table 4.1 may be used. This is derived
from a table produced from the guide book of the Building Services
Institute7 which was used to calculate the heating necessary for any building
to take account of losses due to natural ventilation occurring.