2.7.1 Tankers fitted with an inert gas system
should have their cargo tanks kept in a nonflammable condition at
all times (see figure 1). It follows that:
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.1 tanks should be kept in the inert condition
whenever they contain cargo residues or ballast. The oxygen content
should be kept at 8% or less by volume with a positive gas pressure
in all the cargo tanks;
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.2 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 would be purged with inert
gas until the hydrocarbon content of the tank atmosphere is below
the critical dilution line (see figure 1);
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.3 when a ship is in a gas-free condition before
arrival at a loading port, tanks should be inerted prior to loading.
2.7.2 In order to maintain cargo tanks in a nonflammable
condition the inert gas plant will be required to:
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.1 inert empty cargo tanks (see 5.1);
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.2 be operated during cargo discharge, deballasting
and necessary in-tank operations (see 5.2, 5.5, 5.6, 5.8 and 5.9);
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.3 purge tanks prior to gas-freeing (see 5.10);
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.4 top up pressure in the cargo tanks when necessary,
during other stages of the voyage (see 5.4 and 5.8).