Dilution by repeated pressurisation
In the case of Type 'C' tanks, inerting by dilution can be achieved through a process of repeated pressurisation. In this case, inert gas is pressurised into the tank using a cargo compressor. This is followed by release of the compressed gases to atmosphere. Each repetition brings the tank nearer and nearer to the oxygen concentration of the inert gas. Thus, for example, to bring the tank contents to a level of five per cent oxygen within a reasonable number of repetitions, inert gas quality of better than five per cent oxygen is required.
It has been found that quicker results will be achieved by more numerous repetitions, each at low pressurisation, than by fewer repetitions at higher pressurisation.
Dilution by repeated vacuum
Type 'C' tanks are usually capable of operating under considerable vacuum and, depending on tank design, vacuum-breaking valves are set to permit vacuums in the range from 30 per cent up to 70 per cent. Inerting by successive dilutions may be carried out by repeatedly drawing a vacuum on the tank. This is achieved by using the cargo compressor and then, breaking the vacuum with inert gas. If, for instance, a 50 per cent vacuum can be drawn, then, on each vacuum cycle, half the oxygen content of the tank is removed. Of course, some of the withdrawn oxygen will be replaced by the oxygen content of the inert gas.
Of all the dilution processes, this method can be the most economical as only the minimum quantity of inert gas is used to achieve the desired inerting level. The overall time taken, however, may be longer than with the pressurisation method because of reduced compressor capacity when working on vacuum and a slow rate of vacuum-breaking due to limited output of the inert gas generator.
Continuous dilution
Inerting by dilution can be carried out as a continuous process. Indeed, this is the only diluting process available for Type 'A' tanks which have very small over-pressure or vacuum capabilities. For a true dilution process, (as opposed to one aiming at displacement) it is relatively unimportant where the inert gas inlet or the tank efflux are located, provided that good mixing is achieved. Accordingly, it is usually found satisfactory to introduce the inert gas at high speed through the vapour connections and to discharge the gas mixture via the bottom loading lines.
When using the continuous dilution method on ships with Type 'C' tanks, increased inert gas flow (and thereby better mixing and reduced overall time) may be achieved by maintaining the tank under vacuum. This is accomplished by drawing the vented gas through the cargo vapour compressor. Under these circumstances care should be taken to ensure good quality inert gas under the increased flow conditions.
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Where a number of tanks are to be inerted, it may be possible to achieve a reduction in the total volume of inert gas used, and the overall time taken, by inerting tanks one after the other in series. This procedure also inerts pipelines and equipment at the same time. (On some ships, cargo and vapour pipeline arrangements may prevent more than two tanks being linked in series.) The extra flow resistance of a series arrangement will decrease the inert gas flow rate below that achievable when inerting tanks singly.
As can be seen from the foregoing discussion, the optimum arrangement for inerting by dilution will differ from ship to ship and may be a matter of experience.
Inert gas — general considerations
It can be seen from the preceding paragraphs that inert gas can be used in different ways to achieve inerted cargo tanks. No one method can be identified as the best since the choice will vary with ship design and gas density differences. Generally, each individual ship should establish its favoured procedure from experience. As already indicated, the displacement method of inerting is the best but its efficiency depends upon good stratification between the inert gas and the air or vapours to be expelled. Unless the inert gas entry arrangements and the gas density differences are appropriate to stratification, it may be better to opt for a dilution method. This requires fast and turbulent entry of the inert gas, upon which the efficiency of dilution depends.
Whichever method is used, it is important to monitor the oxygen concentration in each tank from time to time, from suitable locations, using the vapour sampling connections provided. In this way, the progress of inerting can be assessed and, eventually, assurance can be given that the whole cargo system is adequately inerted.
While the above discussion on inerting has centered on using an inert gas generator, the same principles apply to the use of nitrogen. The use of nitrogen may be required when preparing tanks for the carriage of chemical gases such as vinyl chloride, ethy-lene or butadiene. Because of the high cost of nitrogen, the chosen inerting method should be consistent with minimum nitrogen consumption.
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