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This section gives an explanation of the fuel treatment system including settling tanks and the fuel centrifuge.

Settling Tank

The effectiveness of a settling tank depends on a number of factors. At the design stage, the number of tanks, size and height will affect the effectiveness for the removal of either water or solids. Many installations only have one settling tank fitted and this factor reduces the residence time of the fuel within the tank. To an extent, the residence time can be increased by increasing the size of the tank.

Operationally, the effectiveness is determined by the viscosity of the fuel bunkered and the temperature at which the tank is maintained. It can be shown from Stokes’ Law that the greater the viscosity, the slower is the rate of settling for water or dirt within the fuel. By heating the tank, the viscosity is reduced and it is usual to maintain a temperature of 50°C or 10°C below the flash point.

Fuel Centrifuges

The correct sizing of the fuel centrifuges depends on the daily fuel consumption and on the design viscosity of the system. As a matter of prudence, many centrifuges are designed for a fuel of 700 cSt at 50°C. This fuel is the maximum specified in accepted fuel standards, such as ISO 8217. In actual service, the fuel used is more typically IF180 or IF380. The reality is that under these conditions it is oversized and theoretically operates more effectively.

Basically, there are two different types of centrifuge which are installed and found in engine plant. Those of what may be described as the “traditional” design have a density limit of 991 kg/m3 (0.991 kg/m3) at 15°C. In such instances, it is usual to fit three machines; two in use and one as a stand by unit. Over the years, the consistent recommendation has been that in normal circumstances, two machines are run in series, with one as a purifier and the other as a clarifier. The purifier, for the removal of water and solids, is situated before the clarifier which primarily removes solids.

High specific gravity fuels and fuels containing catalytic fines require separation at the highest temperature, 98°C, to reduce the viscosity as much as possible, and reduction of throughput as efficiency increases with decreasing flow rate.

Parallel operation of both the main and standby separators provides good pre-treatment of fuels but has proved to be slightly less efficient as series-operated separators when dealing with modern day fuels. Parallel operation in the clarifier mode with lowest possible throughput is the most effective method of treating fuel contaminated with Cat Fines and only traces of water content.

The other type of machine is able to operate effectively on fuels up to 1010 kg/m3 at 15°C. In this case, normally only two machines are fitted: one in operation with the other on standby.