2.6 Methods of gas replacement
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2.6 Methods of gas replacement

  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.


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