Page 168 - Introduction to Marine Engineering

154 Fuel oils, lubricating oils and their treatment

Centrifuging
A centrifuge consists of an electric motor drive to a vertical shaft on the
top of which is mounted the bowl assembly. An outer framework
surrounds the assembly and carries the various feed and discharge
connections. The bowl can be a solid assembly which retains the
separated sludge and operates non-continuously, or the bowl can be

1-feed
2—purified
3—separated water
4-sludge
Figure 8.1 Purifying bowl arrangement

arranged so that the upper and lower parts separate and the sludge can
be discharged while the centrifuge operates continuously. The dirty oil
is admitted into the centre of the bowl, passes up through a stack of discs
and out through the top (Figure 8.1).

The purifying process
The centrifugal separation of two liquids, such as oil and water, results
in the formation of a cylindrical interface between the two. The
positioning of this interface within the centrifuge is very important for
correct operation. The setting or positioning of the interface is achieved
by the use of dam rings or gravity discs at the oudet of the centriiuge.
Various diameter rings are available for each machine when different
densities of oil are used. As a general rule, the largest diameter ring
which does not break the 'seal' should be used.

The clarifying process
Cleaning oil which contains little or no water is achieved in a clarifier
bowl where the impurities and water collect at the bowl periphery. A