Page 144 - Electrical Installation in Hazardous Area

1 10 Electrical installations in hazardous areas

These figures are derived from a monograph by Lippincott and Lyman6.
If the partial pressure is known, however, it should be used, as the simpli-
fication executed will give answers which are higher than the actual figure
in accordance with the estimation procedure used throughout this book.
The mass transfer coefficient kg can be found from the following formula:
kg = 3.6 Gm/B0.67 Ro.2 M (Equation 4.20)

where B = Schmidt number (kinematic viscosity of material
divided by diffisivity of vapour in air)
R = Reynolds number
G, = mass flow of air kg-mole/s

The classic formula for Reynolds number is:

R = v d a/r
where v = air velocity m/s
d = characteristic dimension m
a = density of air at ambient
temperature and pressure kg/m2
t = absolute viscosity of air Ns/m2

Using the kinematic viscosity this equation simplifies to:
R = v d/A

where A = kinematic viscosity m2/s

Taking the wind velocity as 2m/s and the kinematic viscosity of air as 1.5
x~O-~ this simplifies to:
R = 1.33 d lo5 (Equation 4.21)

As R is the only parameter subject to significant change with pool size then:

Kg = 2 x 10-3/R0.2
We can now calculate the mass of vapour released from the pool using
Equation 4.18 as follows:

G = z x A ~p M/RO.~ kgls
(Figure 4.3 gives values of R0.2 for various pool sizes.)
This must equal the release rate of liquid from Equation 4.13 and thus the
following relationship must be true if the pool size is not restricted physi-
cally:
=
rp
1.13 a [al(p - 10~)lO.~ 2 x ~o-~A M/RO.~ &Is
where A = pool area m2
a = leak orifice area m2