Following three operations involve replacement of existing gas inside cargo tanks:
In each of these replacement operations, one of two processes can predominate:
(a) Dilution, which is a mixing process
(b) Displacement, which is a layering process
· The dilution theory assumes that the incoming gas mixes with the original gases to form a homogeneous mixture throughout the tank.
· As a result, the concentration of the original gas decreases progressively.
· The conditions required for such a process to happen ideally are high incoming velocity of the gas so that the jet could reach the bottom most portions of the tank.
· Thus minimum numbers of tanks are used simultaneously while using dilution.
· Thus for dilution method, the points for entry and exit of the gas are both located towards the top of the tank.
Figure 1 Figure 2
· Figure 1 shows an inlet and outlet configuration of the dilution process and illustrates the turbulent nature of the gas flow within the tank.
· Figure 2 shows typical curves of gas concentration against time for three different sampling positions.
· Displacement method requires the presence of a stable horizontal interface between the top of the tank so that the light gas entering from the top can push the heavier gas out from bottom of the tank.
· Hence, this method requires a low velocity of gas entry.
· This method also requires that outlet pipe suction must be located somewhere towards the bottom of the tank.
· Thus several numbers of tanks can be used simultaneously while using Displacement method.
Figure 3 Figure 4
· Figure 3 shows an inlet and outlet configuration for the displacement process, and indicates the interface between the incoming and outgoing gases.
· Figure 4 shows typical curves of gas concentration against time for three different sampling levels.
PRINCIPAL ARRANGEMENTS ON BOARD TANKERS:
· One feature that should be common to all is the location of the inlet and outlet points such that efficient gas replacement can take place throughout the tank.
· Several arrangements are possible. A particular ship design may incorporate more than one arrangement also.
· However, there are following three principal arrangements :
Arrangement Inlet point Outlet point Method used
I top top dilution
II bottom top Displacement
III top bottom Displacement