Page 166 - Introduction to Marine Engineering

152 Fuel oils, lubricating oils and their treatment

attention to filters and general system cleanliness is essential. Various
additives are used to, for instance, remove lacquer from metal surfaces,
reduce wear and prevent rust.

Lubricating oils

Lubricating oils are a product of the crude oil refining process. The
various properties required of the oil are obtained as a result of blending
and the introduction of additives. The physical and chemical properties
of an oil are changed by additives which may act as oxidation inhibitors,
wear reducers, dispersants, detergents, etc. The important lubricant
properties will now be examined.
Viscosity has already been mentioned with respect to fuel oils, but it is
also an important property of lubricating oils. Viscosity index is also
used, which is the rate of change of viscosity with temperature.
The Total Base Number (TEN) is an indication of the quantity of alkali,
i.e. base, which is available in a lubricating oil to neutralise acids.
The acidity of an oil must be monitored to avoid machinery damage
and neutralisation number is used as the unit of measurement.
The oxidation resistance of a lubricant can also be measured by
neutralisation number. When excessively oxidised an oil must be
discarded.
The carbon-forming tendency of a lubricating oil must be known,
particularly for oils exposed to heat. A carbon residue test is usually
performed to obtain a percentage value.
The demulsibility of an oil refers to its ability to mix with water and then
release the water in a centrifuge. This property is also related to the
tendency to form sludge.
Corrosion inhibition relates to the oil's ability to protect a surface when
water is present in the oil. This is important where oils can be
contaminated by fresh or salt water leaks.
The modern lubricant must be capable of performing numerous
duties. This is achieved through blending and additives. It must prevent
metal-to-metal contact and reduce friction and wear at moving parts.
The oil must be stable and not break down or form carbon when
exposed to high temperatures, such as where oil cooling is used. Any
contaminants, such as acidic products of combustion, must be
neutralised by alkaline additives; any carbon build up on surfaces must
be washed away by detergent additives and held in suspension by a
dispersant additive. The oil must also be able to absorb water and then
release it during purification, but meanwhile still protect the metal parts
from corrosion.