Following three operations involve replacement of existing gas inside cargo tanks:

(a) Inerting

(b) Purging

(c) Gas-freeing

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

Note :

· 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

Note :

· 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.


· 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