Page 151 - Electrical Installation in Hazardous Area
P. 151
Calculation of release rates and extents 1 17
the leak path. In these circumstances the basic equation remains as Equa-
tion 4.13 but atmospheric pressure is replaced by P, where P, is 0.55 x the
saturated vapour pressure of the contained liquid at operating tempera-
ture of the process. This may not, of course, be the same as the actual
containment pressure, which may be higher if additional external means of
pressurization are used but such is not normally the case - the maintenance
of the material as liquid being secured by its own vapour pressure acting
upon the containment. In addition, the density of the released mixture of
combined liquid and vapour is the composite of the two elements of the
mixture. Thus:
G = 0.8A[2am(P1 - P,)]o.5 kg/s (Equation 4.28)
where A = cross-sectional area of leak
am = density of released mixture
P1 = containment pressure
P, = 0.55 x vapour pressure
In order to calculate a m it is necessary to investigate the conditions relative
to a pressure of P, and to do this it is necessary to have the vapour pressure
curve for the material in question. From this a value can be obtained for
the temperature at which a vapour pressure of P, would be exerted (T,).
This allows the calculation of the fraction of the release which emerges as
vapour as follows:
where M, = fraction of mass release which is vapour
TI = process temperature K
T, = temperature giving P, K
CI = heat capacity of liquid kj / kg "C
6 = latent heat of vaporization kj/kg
This allows the density of the mixture to be calculated as follows:
where am = density of released mixture
a, = density of vapour at TI
01 = density of liquid at T1
The results of these equations allow the velocity of release to be calcu-
lated, together with the amount of liquid in the jet. This jet can then be
treated using Equations 4.14, 4.15 or 4.16 as appropriate. The amount of
liquid reaching the ground is then assumed to evaporate instantly and
the extent of the hazardous area due to this calculated using Equation 4.7,
corrected for a low-level point source.
