4. With an inert gas system, a tank is protected from exploding by introducing inert gas into it to keep the oxygen content low and to reduce to safe proportions the hydrocarbon gas concentration of its atmosphere.
5. (1) A mixture of hydrocarbon gas and air cannot ignite, unless its composition lies within a range of gas-in-air concentrations known as the flammable range.
(2) The lower limit of the range, known as the "lower flammable limit", is any hydrocarbon concentration below which there is insufficient hydrocarbon gas to support combustion.
(3) The upper limit of the range, known as the "upper flammable limit", is any hydrocarbon concentration above which air is insufficient to support combustion.
(4) The flammable limits vary somewhat for different pure hydrocarbon gases and for the gas mixtures derived from different petroleum liquids; in practice, however, the lower and upper flammable limits of oil cargoes carried in tankers can be taken, for general purposes, to be 1 per cent and 10 per cent hydrocarbon by volume, respectively.
Effect of Inert Gas on Flammability
6. (1) When an inert gas is added to a hydrocarbon gas/air mixture, the result is an increase in the lower flammable limit concentration and a decrease in the upper flammable limit concentration. Figure 1 illustrates these effects which should be regarded only as a guide to the principles involved.
FIGURE 1 Hydrocarbon gas/air/inert gas mixtures effect on flammability ^
(2) Any point on the diagram represents a hydrocarbon gas/air/inert gas mixture, specified in terms of its hydrocarbon and oxygen content.
(3) Hydrocarbon/air mixtures, without inert gas, lie on the line AB, the slope of which shows the reduction in oxygen content as the hydrocarbon content increases.
(4) Points to the left of AB represent mixtures whose oxygen content is further reduced by the addition of inert gas.
(5) As indicated in Figure 1, as inert gas is added to hydrocarbon/air mixtures, the flammable range progressively decreases, until the oxygen content reaches a level generally taken to be about 11 per cent by volume, at which no mixture can burn.
(6) The figure of 8 per cent by volume, specified in this Standard for a safely inerted gas mixture, allows some margin beyond this value.
(7) The lower and upper flammability limit mixtures for hydrocarbon gas in air are represented by points C and D.
(8) As the inert gas content increases, the flammable limit mixtures change; lines CE and DE indicate this, finally converging at point E.
(9) Only those mixtures represented by points in the shaded area within the loop CED are capable of burning.
(10) Changes of composition, due to the addition of either air or inert gas, are represented by movements along straight lines; these lines are directed either towards point A (pure air), or towards a point on the oxygen content axis corresponding to the composition of the added inert gas; such lines are shown for the gas mixture represented by point F.
(11) When an inert mixture, such as that represented by point F, is diluted by air, its composition moves along line FA and enters the shaded area of flammable mixtures; this means that all inert mixtures in the region above line GA (critical dilution line) pass through a flammable condition as they are mixed with air (for example, during a gas-freeing operation); those below line GA, such as that represented by point H, do not become flammable on dilution.
(12) It will be noted that dilution with additional inert gas, i.e. purging, makes it possible to move from a mixture, such as that represented by F, to one such as that represented by H, by dilution with additional inert gas, i.e. purging.
7. Possible sources of inert gas on tankers including combination carriers are:
8. Good combustion control in ships’ boilers is necessary to achieve an oxygen content of 5 per cent by volume; to obtain this quality, it may be necessary to use automatic control.
Methods of Gas Replacement
9. (1) Three operations involve replacement of gas in cargo tanks, namely:
(2) In each of these replacement operations, one of two processes can predominate -
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 the gas replacement process actually taking place within the tank must be understood to correctly interpret the reading shown on the appropriate gas sampling instrument.
(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.
(4) 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.
(5) For complete gas replacement, it is important that the entry velocity of the incoming gas be 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 ^
FIGURE 3 ^
Dilution process of gas in cargo tanks
Figure 2 shows an inlet and outlet configuration of 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.
(6) Ideal replacement happens when a stable horizontal interface exists 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 achieve the required degree of gas replacement throughout the tank.
FIGURE 4 ^
Displacement process of gas in cargo tanks
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.
General Policy of Cargo Tank Atmosphere Control
10. (1) The cargo tanks in tankers fitted with an inert gas system should be kept in a non-flammable condition at all times (see Figure 1) and to meet this requirement shall comply with subsections (2) through (5).
(2) Tanks should be kept in the inert condition at all times except when entry is required.
(3) The oxygen content should be kept at 8 per cent or less by volume with a positive gas pressure in all the cargo tanks.
(4) The atmosphere within the tank should make the transition from the inert condition to the gas-free condition without passing through the flammable condition; in practice, this means that, before any tank is gas freed, it should be purged with inert gas until the hydrocarbon content of the tank atmosphere is below the critical dilution line (see Figure 1) .
(5) When a ship is in a gas-free condition prior to arrival at a loading port, tanks should be inerted before loading.
(6) To maintain cargo tanks in a non-flammable condition, the inert gas plant will be required to: