When inerting tanks which are gas free, as would be the case following dry-dock or a repair period, inert gas is to be introduced through the distribution system while venting the tanks to the atmosphere.
Inerting empty tanks can be carried out by supplying the inert gas via either the deck entry points or through the tank suctions. The advantage of the latter is that atmosphere separation in the tanks can be achieved and reduced inerting times result. This option will only be workable if the tanks are empty.
The first method described, inerts the tanks by dilution. This can be carried out irrespective of whether the tank suctions are covered, but will generally take longer, and thus cost more in expended fuel.
This operation is to be continued until the oxygen content is reduced below 7% by volume. When all tanks have been inerted they must be kept common with the inert gas main and the system pressurised with a minimum positive pressure of at least 100mm water gauge, or until the low pressure limit of the IG system is reached, whichever is higher.
The IG tank valves are to be kept open during loading, voyage(s) and discharge. Mechanical means are to be used to lock the tank valves in open position to prevent unauthorised operation.
(The only exception occurs if there is possibility of vapour contamination of multigrade cargoes or flammable vapour contamination of non flammable cargo ullage spaces on vessels with common IG deck piping). It must always be borne in mind that the vessel may be loading different grades requiring segregation which will mean that the inert gas distribution system on each grade must be isolated before loading commences. When the IG plant is not in use the inert gas main master valve or deck isolating valve must be closed.
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