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During the loaded passage, the inert gas present in the cargo tanks may be expelled and outside atmosphere pulled in to replace it due to:
Changes in tank pressure due to air temperature variations or changing temperature differences between the sea and air, or
Rolling and pitching in rough seas.
The cargo tank oxygen content and tank atmosphere pressure should be frequently checked during the voyage. If an increase in oxygen content is noted, the inert gas generator system must be placed in operation long enough to reduce the cargo tank oxygen level to an acceptable level. An IGS positive pressure of at least 100 mm water gauge must be maintained during the loaded passage.
Normally the tanker will have a small 'make-up' IGS blower for the purpose of topping up tanks during the loaded passage. The IGS system should be secured as soon as the oxygen level is below 5% in all tanks. Excessive running to the IGS system on loaded passage will increase vapour losses for the voyage.

4.10.1 IGS maintenance
At the end of the loaded passage, proper operation of the IGS system will be essential to a successful discharge operation. All tankers are required by charter party to discharge cargo as rapidly as possible. The vessel can only meet this requirement if the IG system is capable of providing IG as rapidly as the cargo pumps can discharge the cargo. During the loaded passage, the chief engineer should complete all necessary IGS maintenance checks and immediately investigate any suspicious or sub-standard performance.

A typical arrangement for an inert gas system.

Significant failures of the IGS system should be reported immediately to owners and charterers.
IGS system components to be checked include:
Scrubber unit.
Deck water seal.
Non-return valve.
Scrubber effluent line.
Oxygen sensor and meter.
Alarm systems.

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