The Operation of the Centrifuges
There are several types of centrifuges used to remove water and particles from the fuel and lube oil; the most popular being the disc type as made by DeLaval rotating at 7000 revs and, the tubular type by Sharples which rotates at 15000 revs. However we shall look at the disc bowl type which is suitable for marine engine applications.
The centrifuges can be located close to where they are needed or kept together in a centrifuge room. I prefer them to be located in their own room which allows room for maintenance tools, a bench and cleaning troughs, although it can get pretty warm in the centrifuge room as the oil heaters are contained there as well.
Anyway the dirty heavy fuel oil is drawn from the bunkers, passing through several screens and a heater. It then enters centrifuge inlet pipe located at the centre of the top of the upper circular cover. Fresh water is also be supplied through a similar inlet and used to form a seal inside the centrifuge bowl. If a seal is not formed, oil will flow out of the water/sludge outlet.
The centrifuge operates by centrifugal force, the disc assembly spinning round at high revolutions and throwing the water and particles to the outside where they gather in a layer on the inside of the bowl, liquid sludge and water draining away from the lower centrifuge outlet to the sludge tank. The clean fuel oil is forced up the centre bore of the discs exiting at the top ring. It drains from here into the upper centrifuge outlet from where the cleaned oil is piped to the settling or day tanks ready to be supplied via the main fuel pump and filters, up to the main engine fuel pumps and injectors.
A sketch of the fuel oil system is shown below.
The lube oil is subjected to the same treatment except that it is drawn from the main engine sump and heated before entering the top of the centrifuge. The heater thins the oil making it more amenable to centrifuging. After cleaning it is returned to the main engine sump, in a closed circuit 24 hour constant operation.
A sketch of a typical lube oil system is shown below.