Page 158 - Introduction to Marine Engineering

144 Auxiliaries

Maintenance
During the operation of evaporating plants, scale will form on the
heating surfaces. The rate of scale formation will depend upon the
operating temperature, the flow rate and density of the brine.
Scale formation will result in greater requirements for heating to
produce the rated quantities of distilled water or a fall-off in production
for a fixed heating supply.
Cold shocking, the alternate rapid heating and cooling of the tube
surfaces, for a boiling process type, can reduce scale build-up.
Ultimately, however, the plant must be shut down and the scale removed
either by chemical treatment or manual cleaning.

Oil/water separators

Oil/water separators are used to ensure that ships do not discharge oil
when pumping out bilges, oil tanks or any oil-contaminated space.
International legislation relating to oil pollution is becoming more and
more stringent in the limits set for oil discharge. Clean water suitable for
discharge is defined as that containing less than 15 parts per million of
oil. Oil/water separators using the gravity system can only achieve 100
parts per million and must therefore be used in conjunction with some
form of filter.
A complete oil/water separator and filter unit for 15 parts per million
purity is shown in Figure 7.8. The complete unit is first filled with clean
water; the oily water mixture is then pumped through the separator
inlet pipe into the coarse separating compartment. Here some oil, as a
result of its lower density, will separate and rise into the oil collection
space. The remaining oil/water mixture now flows down into the fine
separating compartment and moves slowly between the catch plates.
More oil will separate out onto the underside of these plates and travel
outwards until it is free to rise into the oil collecting space. The almost
oil-free water passes into the central pipe and leaves the separator unit.
The purity at this point will be 100 parts per million or less. An
automatically controlled valve releases the separated oil to a storage
tank. Air is released from the unit by a vent valve. Steam or electric
heating coils are provided in the upper and sometimes the lower parts of
the separator, depending upon the type of oil to be separated.
Where greater purity is required, the almost oil-free water passes to a
filter unit. The water flows in turn through two filter stages and the oil
removed passes to oil collecting spaces. The first-stage filter removes
physical impurities present and promotes some fine separation. The