Page 147 - Electrical Installation in Hazardous Area

Calculation of release rates and extents 1 13

4.2.1 Example of liquid release below its atmospheric boiling point

Example 3

Acetone is transported in a pipe with normal flanged joints at 22°C and
is at a guage pressure of 105N/m2. A failure in one of the flanged joints
occurs and presents an orifice of 4 x m2. The pipe is some 3 m off the
ground.
Relevant parameters of acetone are:

Liquid density = 791kg/m3
Vapour density = 2.58kg/m3
Boiling point = 56°C
Partial pressure at 22 "C = 2.2 x 104N/m2 (0.22 Atm.)
Molecular weight = 58
Lower explosive limit = 2.1% in air

It is first necessary to calculate the rate of liquid release from the leak
and to do this Equation 4.13 is used giving:

G = 1.13 x 4 x 10-5(791 x 105)0-5 kg/s
G = 0.4 kgls
In calculating the extents of hazardous areas created by such a leak two
assumptions must be made and the first of these is that the liquid reaches
the ground without evaporation. To determine the distance from the leak
where the liquid reaches the ground Equation 4.15 is used, with velocity
being determined from Equation 4.17 assuming the leak to be horizontal.

Velocity of jet = 1.13(105/791)0.5 = 12.7 m/s
Distance travelled = (2 x 12.7' x 3/9.81)0.5 m
Distance = 9.93m (say 10m)

A pool will therefore form with its centre at a horizontal distance of
9.85m from the point at which a vertical line from the leak orifice strikes
the ground. This can be in any compass direction unless the direction of
the jet of liquid can be established with confidence. This pool will, unless
contained, expand until the vaporization from its surface equals the leakage
rate from the orifice. The area of the pool can be calculated from the Equa-
tion 4.23 as follows, initially using a median value of 16 for Ro.2:
A = (5.65 x 10' x 4 x 10-5[791 x 105]0.5 x 15}/0.22 x 58 m2

At this size of pool from Fig. 4.3 R0.' is around 19 and so the calculation
for pool area should be repeated using this figure and the result is:

A = 299 m2
The pool diameter is thus (d) = 22m