Figure 4.18 Flow diagram of an inert gas generator
should be closely followed. A particular point to watch is that poorly maintained plant can produce significant quantities of carbon monoxide or soot such that, even after aerating, carbon monoxide levels in a tank may be unacceptable.
The mode of operation is shown in Figure 4.18. Here it will be seen that the inert gas generator has three main parts. These are as follows:
• A combustion chamber with scrubbing and cooling (the generator)
• A refrigerated drier — cooled normally by R22, and
• An absorption drier
Combustion chamber
Combustion-type generators must be located outside the cargo area and are usually installed in the ship's engine room. It is usual to find the inert gas main permanently piped into the cargo holds and temporary connections are provided between the inert gas main and the cargo system for tank inerting operations. When not in use, these must be disconnected and blanks fitted. Two non-return valves (or equivalent) are fitted in the inert gas main to prevent any back-flow of cargo vapours. When not being used for high capacity tank inerting operations the inert gas plant is used from time to time to top up hold and interbarrier spaces.
Within the combustion chamber, the burner is designed to ensure good combustion so producing a minimum of oxygen residue in the inert gas. Operationally, however, there is a fine balance to be achieved in generator adjustment as minimising oxygen output tends to increase the production of carbon monoxide: and further adjustment can result in the overproduction of soot. The combustion chamber itself is water-jacketed. After combustion, the inert gas enters the washing section of the generator at a very high temperature and is cooled and scrubbed by spraying with sea water. This is also carried out for the removal of soluble acid gases such as sulphur dioxide and the oxides of nitrogen. The inert gas is then filtered to remove solid particles. The gas leaves the generator at approximately five degrees Centigrade above sea water temperature and by this time it should be essentially free from sulphur oxides formed by burning the sulphur present in the fuel — but it is saturated with water vapour. Accordingly, it is then further cooled and dried (as covered below) and delivered to the cargo tanks.
The refrigerated drier
In the refrigerated drier, the inert gas is cooled to approximately four degrees Centigrade, resulting in the condensation of much of the water vapour. Figure 4.19 shows the content of water vapour in saturated inert gas as a function of temperature. From this diagram, the reduction in water vapour content can be seen as the temperature is reduced.
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Figure 4.19 Saturated water content of inert gas
The absorption drier
The absorption drier consists of two vessels filled with activated alumina or silica gel. One vessel is on drying duty while the other is being regenerated. Typically, the cycle time is six hours.
Figure 4.20 Drying of inert gas
Drying in the absorption drier reduces the dew point of the inert gas to -40°C or below. A layer of molecular sieves can be added to the bottom of the drying tower to improve the dew point. In order to ensure stable combustion in the generator, the pressure in the drying system must be kept constant and this is achieved by means of a pressure control valve as shown in Figure 4.20.
4.7.2 Nitrogen production on ships
The most common system utilised for the production of nitrogen on ships is an air separation process. This system works by separating air into its component gases by passing compressed air over hollow fibre membranes. The membranes divide the air into two streams — one is essentially nitrogen and the other contains oxygen, carbon dioxide plus some trace gases. This system can produce nitrogen of about 95 to 97 per cent purity. The capacity of these systems depends on the number of membrane modules fitted and is dependant on inlet air pressure, temperature and the required nitrogen purity. Figure 4.21 shows one such system.
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