Page 167 - Introduction to Marine Engineering

Fuel oils, lubricating oils and their treatment 153

The various types of engine and other equipment will have oils
developed to meet their particular duties.
Trunk piston engine lubricating oil must lubricate the cylinders as well
as the crankcase: some contamination from the products of combustion
will therefore occur, resulting in acidity and carbon deposits. The oil
must, in addition to lubricating, neutralise the acids and absorb the
deposits.
Turbine oil, while lubricating the moving parts, must also carry away
considerable quantities of heat from the bearings. This calls for a stable
oil which will not break down at high temperatures or form deposits.
Where gearbox lubrication is also required certain extreme pressure
(EP) additives will be needed to assist the lubricating film. Contact with
water in the form of steam will be inevitable so good demulsifying
properties will be essential.
Slow-speed diesel engines will have separate cylinder and crankcase
lubrication systems. The cylinder oil will have to neutralise the acidic
products of combustion and also have good detergent properties to keep
the metal, surfaces clean. Crankcase oils are either detergent type,
multi-purpose oils or rust and oxidation inhibited. Good demulsification
and anti-corrosive properties are required together with oxidation
resistance which is provided by the inhibited crankcase oil. The
detergent or multi-purpose oil is particularly useful where oil cooling of
pistons occurs or where contamination by combustion products is
possible.

Oil treatment

Both fuel oils and lubricating oils require treatment before passing to
the engine. This will involve storage and heating to allow separation of
water present, coarse and fine filtering to remove solid particles and also
centrifuging.
The centrifugal separator is used to separate two liquids, for example
oil and water, or a liquid and solids as in contaminated oil. Separation is
speeded up by the use of a centrifuge and can be arranged as a
continuous process. Where a centrifuge is arranged to separate two
liquids, it is known as a 'purifier'. Where a centrifuge is arranged to
separate impurities and small amounts of water from oil it is known as a
'clarifier'.
The separation of impurities and water from fuel oil is essential for
good combustion. The removal of contaminating impurities from
lubricating oil will reduce engine wear and possible breakdowns. The
centrifuging of all but the most pure clean oils is therefore an absolute
necessity.