Displacing atmosphere with inert gas (Inerting) - Liquefied gas carrier operational guidance

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Displacing atmosphere with inert gas (Inerting) - Liquefied gas carrier operational guidance

The term 'inerting' generally refers to the replacement of air or cargo vapour by inert gas before loading or gasfreeing respectively, to prevent the formation of flammable mixtures.Inert gas may be generated on board by controlled combustion of fuel oil, but will contain some free oxygen, carbon monoxide, carbon dioxide, nitrogen oxides and nitrogen.

Atmospheric air contains about 21% oxygen and 78% nitrogen. To produce inert gas the oxygen content has to be removed or reduced considerably. It may not be sufficiently pure for use with certain cargoes. The inert gas has to be water-scrubbed to remove soot and other products of combustion, and then dried before use. The fuel used in the plant must be of a high quality to reduce impurities (e.g. sulphur) in the inert gas produced.

Alternatively, inert gas from shore (usually liquid nitrogen) may be stored on board in vacuum insulated vessels

A recent trend is for ships to be provided with the means to produce nitrogen gas on board by physical separation from the atmosphere, using the pressure-swing absorption method or the membrane method. Nitrogen produced this way remains gaseous throughout. Ship's personnel should be aware that the exhaust from the plant is oxygenrich, and be fully alert to the increased flammability hazard which this presents.

Vapours from the last cargo in the system are displaced by inert gas from the ship’s inert gas generator, or by pure nitrogen from shore. If the ship’s inert gas is used, the cargo piping system from the tank should be opened to the vent before the inert gas supply is connected as an additional precaution against the possible backflow of flammable vapour to the generator.

Regulations regarding venting of cargo vapour in port should be observed. Such regulations may require that vented cargo vapours should be led to a flare or vent stack ashore. Inerting is continued until the required dew point or concentration of cargo vapour or oxygen level has been reached.

When inerting a tank which has been ventilated with air, the oxygen content should be checked regularly. The oxygen content after inerting should never exceed 5% by volume, and should normally be in the order of 2% to allow for uneven distribution. Much lower levels may be required for oxygen reactive cargoes (e.g. butadiene). The dew point temperature for the particular cargo to be loaded must be achieved during this operation.

When inerting a tank which has contained cargo vapour, the process should be continued until the cargo vapour concentration is sufficiently low to prevent formation of flammable mixtures during subsequent ventilation with air.

If liquid nitrogen is used as an inert gas, it should not be allowed to come into contact with any metal (other than the dedicated nitrogen storage and piping system) with a service temperature above -196 deg C. If it does, brittle fracture will occur. If the storage bottles and transfer line are vacuum insulated, the vacuum should be carefully maintained to prevent excessive boil-off. It is rarely possible to replace lost nitrogen except in port.

To avoid possible back-flow of cargo vapour, the cargo lines should be opened to the vent system before the inert gas system is connected. It is advisable to use hot gas or some other suitable method to warm up tanks that have contained low temperature cargoes before inerting, so that the steel temperature is above the dew point temperature. Failure to do so means that much larger quantities of inert gas will be required and moisture or CO? will freeze out. Similarly, if cold nitrogen vapour is used for inerting, atmospheric moisture is likely to be deposited in tanks.

Inert Gas Systems

Inert gas has an important role in maintaining safety aboard a gas carrier and the inert gas system should be kept in good working order. Regardless of frequency of use it should be tested regularly to prevent deterioration and enable any faults to be detected and rectified.

The following precautions should be observed:

The whole system should be visually checked before starting up, in particular the deck nonreturn valves.
The piping system to the vent outlet should be opened to release any pressure and prevent back-flow, and the temporary connections to the cargo system fitted.
The scrubber water supply should be started before beginning combustion.
The gas produced should be vented to atmosphere until it is of sufficiently good quality for use.
The air supply should be adjusted to produce the best quality inert gas possible: oxygen, carbon dioxide, carbon monoxide and soot levels should be controlled (see paragraph 4.6.2). If the air supply is reduced in order to lower the oxygen concentration the gas produced may often have a high soot content which can clog driers, non-return valves etc.
The gas quality should be continually monitored while the plant is in use,
After use, the temporary connections to the cargo system must be disconnected and the flanges blanked securely.

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