418

NICHOLLS S SEAMANSHIP AND NAUTICAL KNOWLEDGE

Tanks cannot be completely filled with oil as allowance has to
be made for expansion due to increase of temperature. The main
tanks are kept quite full by keeping a reserve of oil in the trunk. The
space above the surface of the oil in the trunk is called the "ullage,"
and, obviously, any change in the ullage scale indicates the change in
the volume of the oil due to change of temperature. See Fig. 6, page
365. Heavy oils when heated expand about 1 per cent for every 25°
of temperature.

An increase of 10 degrees in the temperature of 5000 tons of oil
would increase its volume to the capacity equivalent to 20 tons weight
which means that, approximately, 6400 more gallons would be pumped
out at the higher temperature than was taken in at the lower
temperature.

Fig. 10—Stop Valves.

When heavy oil is too viscous, that is very thick or congealed, it
cannot be pumped out, so heating pipes are either laid along the bottom
of the tank or arranged in coils to rake the temperature of the cargo and
reduce its viscosity. Some liquid cargoes have to be kept above a
given temperature, but generally the heating up of oil is only necessary
for a few days before arrival in port.
Figure 10 illustrates stop valves to admit steam to the coils
in the port or starboard tanks separately or simultaneously, and the