Page 169 - Introduction to Marine Engineering

Fuel oils, lubricating oils and their treatment 155

Figure 8.2 Clarifying bowl arrangement

clarifier bowl has only one outlet (Figure 8.2). No gravity disc is
necessary since no interface is formed; the bowl therefore operates at
maximum separating efficiency since the oil is subjected to the
maximum centrifugal force,

The bowl discs

Purifier and clarifier bowls each contain a stack of conical discs. The
discs may number up to 150 and are separated from one another by a
small gap. Separation of impurities and water from the oil takes place
between these discs. A series of aligned holes near the outside edge
permits entry of the dirty oil. The action of centrifugal force causes the
lighter components (the clean oil) to flow inwards and the water and
impurities flow outwards. The water and impurities form a sludge which
moves outwards along the undersides of the discs to the periphery of the
bowl.

Non-continuous operation

Certain designs of centrifuges are arranged for a short period of
operation and are then shut down for cleaning. After cleaning and
removal of the sludge from the bowl, the machine is returned to service.
Two different designs are used for this method of operation; a long
narrow bowl and a short wide bowl. The narrow-bowl machine has to be
cleaned after a shorter running period and requires dismantling in
order to clean the bowl. Cleaning of the bowl is, however, much simpler
since it does not contain a stack of discs. The wide-bowl machine can be
cleaned in place, although there is the added complication of the stack of
conical discs which must be cleaned.