2.6.1 There are three operations which involve
replacement of gas in cargo tanks, namely:
-
.1 inerting;
-
.2 purging;
-
.3 gas-freeing.
2.6.2 In each of these replacement operations,
one of two processes can predominate:
-
.1 dilution, which is a mixing process (see 2.6.3);
-
.2 displacement, which is a layering process (see 2.6.4).
These two processes have a marked effect on the method of monitoring
the tank atmosphere and the interpretation of the results. Figures 3 and 5 show that
an understanding of the nature of the gas replacement process actually
taking place within the tank is necessary for the correct interpretation
of the reading shown on the appropriate gas sampling instrument.
2.6.3 The dilution theory assumes that the incoming
gas mixes with the original gases to form a homogeneous mixture throughout
the tank. The result is that the concentration of the original gas
decreases exponentially. In practice the actual rate of gas replacement
depends upon the volume flow of the incoming gas, its entry velocity,
and the dimensions of the tank. For complete gas replacement it is
important that the entry velocity of the incoming gas is high enough
for the jet to reach the bottom of the tank. It is therefore important
to confirm the ability of every installation using this principle
to achieve the required degree of gas replacement throughout the tank.
Figure 2 shows an inlet and outlet
configuration for the dilution process and illustrates the turbulent
nature of the gas flow within the tank.
Figure 3 shows typical curves of gas concentration against
time for three different sampling positions.
2.6.4 Ideal replacement requires a stable horizontal
interface between the lighter gas entering at the top of the tank
and the heavier gas being displaced from the bottom of the tank through
some suitable piping arrangement. This method requires a relatively
low entry velocity of gas and in practice more than one volume change
is necessary. It is therefore important to confirm the ability of
every installation using this principle to achieve the required degree
of gas replacement throughout the tank.
Figure 4 shows an inlet and outlet
configuration for the displacement process, and indicates the interface
between the incoming and outgoing gases.
Figure 5 shows typical curves of gas concentration against
time for three different sampling levels.