Page 172 - Introduction to Marine Engineering

158 Fuel oils, lubricating oils and their treatment

Lubricating oil centrifuging
Diesel engines
Lubricating oil in its passage through a diesel engine will become
contaminated by wear particles, combustion products and water. The
centrifuge, arranged as a purifier, is used to continuously remove these
impurities.
The large quantity of oil flowing through a system means that full flow
lubrication would be too costly. A bypass system, drawing dirty oil from
low down in the oil sump remote from the pump suction and returning
clean oil close to the pump suction, is therefore used. Since this is a
bypass system the aim should be to give the lowest degree of impurity for
the complete system, which may mean running the centrifuge somewhat
below the maximum throughput.
Water-washing during centrifuging can be adopted if the oil
manufacturer or supplier is in agreement; but some oils contain
water-soluble additives, which would of course be lost if water-washed.
The advantages of water-washing include the dissolving and removal of
acids, improved separation by wetting solid impuritiesj and the constant
renewal of the bowl liquid seal. The washing water is usually heated to a
slightly higher temperature than the oil.
Detergent-type oils are used for cleaning as well as lubricating and
find a particular application in trunk-type engines and some slow-speed
engines. Detergent-type oil additives are usually soluble and must not
therefore be water-washed.

Steam turbines
The lubricating oil in a steam turbine will become contaminated by
system impurities and water from condensed steam, so bypass
separation is used to clean the oil. The dirty oil is drawn from the bottom
of the sump and clean oil returned near the pump suction. Preheating of
the oil before centrifuging assists the separation process. Water washing
of the oil can be done where the manufacturer or supplier of the oil
permits it.

Homogenisers

A homogeniser is used to create a stable oil and water emulsion which
can be burnt in a boiler or diesel engine. Such an emulsion is considered
to bring about more efficient combustion and also reduce solid emissions
in the exhaust gas.