3.5.1 The size or volume of the gas or vapour
phase in the cargo tank system (usually a common system on a crude
oil tanker due to the interconnection through the Inert Gas pipeline
system) is an important criterion to establish the pressure within
the system. Again separate consideration should be given to the two
differing types of gases to be found in the vapour phase and how volume
may impact these component gases.
3.5.2 Saturated vapours from the crude oil liquid
phase, as described above in paragraph 3.2.2, under theoretical conditions
the pressure generated by saturated vapours will not be affected by
a change in the volume space occupied by the vapours. However, due
to the numerous species of hydrocarbon types to be found in evolved
vapour from a crude oil it has been found that a volumetric change
of the vapour phase from a 2% volume (V:L ratio of 0.02) to a 20%
volume (V:L ratio 0.2) will impact the saturated vapour pressure of
a crude oil at a constant temperature. For vapour volumes greater
than 20% of the total volume the pressure behaves similar to that
expected of a Saturated Vapour; namely nearly isobaric. These circumstances
can be seen in Figure 3.1 below for a selection of crude oil types.
Figure 3.1
3.5.3 The change in pressure with respect to volume,
for a vapour percent volume from 2% to 20%, for complexed vapour phases
evolved from crude oils, is due to the influence of the individual
volatile hydrocarbon types and their varying proportions in both the
liquid and vapour phase that separately contribute to the final saturated
vapour pressure under equilibrium conditions. The ratio of concentration
of the individual hydrocarbon compounds in the vapour phase is due
to the Partition Coefficients for each hydrocarbon type
in relation to another type. This will cause a differing distribution
of hydrocarbon species to that in the liquid phase when the vapour
phase volume is smaller.
3.5.4 Unsaturated gases (Inert Gas) in the vapour
phase system – this type of gas behaves in a manner simulated
by the Ideal Gas Law equationfootnote. Therefore
any reduction in the volume occupied by this gas will cause an increase
in the pressure exerted by the gas at a known temperature.