Inert gas must be delivered in the gas main with a maximum oxygen content of 5%. The oxygen content of inert gas in the cargo tanks must not exceed 8%.
When using flue gas from a main or auxiliary boiler, an oxygen level of less than 5% can generally be obtained, depending on the quality of combustion control and the load on the boiler.
When an independent inert gas generator or a gas turbine plant with afterburner is fitted, the oxygen content can be automatically controlled within finer limits, usually within the range 1.5% to 2.5% by volume, and not normally exceeding 5%.
In certain ports, the maximum oxygen content of inert gas in the cargo tanks may be set at 5% to meet particular safety requirements, such as the operation of a vapor emission control system. Where such a limitation is in place, the vessel is usually advised of the requirements in the pre-arrival information exchange.
Efficient scrubbing of the inert gas is essential, particularly for the reduction of the sulphur dioxide content. High levels of sulphur dioxide increase the acidic characteristic of the inert gas, which is harmful for personnel and may cause accelerated corrosion to the structure of a vessel.
The table below provides an indication of the typical composition of inert gas generated from boiler flue gas, expressed as a percentage by volume.
|Sulphur dioxide||SO2||50 ppm|
|Water vapor||H2O||Trace (high if not dried)|
|Ash and soot||(C)||Traces|
Table 7.1 Typical composition of inert gas at the scrubber outlet