5.21 CHANGING TANKS ON THE SHIP
The discharge sequence will normally be arranged to pump the tanks
in groups or sets. Discharge begins with the first set, usually between
1/3 and 1/2 of the total number of tanks carrying cargo. When the first
set of tanks has been drawn down to a low innage at or near the top
of the tank bottom framing, the cargo pump(s) speed must be reduced
and the pump discharge valve closed partially to prevent vortexing and
entrainment of air into the pump(s) (see section 5.26).
A little later, it may be prudent to open slightly the suction valve
to a full tank of the same grade of cargo. This action will provide
the pump with an adequate flow of cargo. The cargo watch officer must
closely observe the ullages of the low tanks to ensure that the cargo
is not gravitating in from the full tank.
The chief officer's cargo orders or night orders will indicate how far
he believes the main cargo pumps can continue to be used to discharge
low cargo tanks and the cargo watch officer should endeavour to achieve
that result. This part of the discharge is a balancing act, which can
only be done well by personnel who are experienced both with the ship
and the type of cargo being discharged. However if the main cargo pump
begins to cavitate or vibrate excessively, then it is time for the suction
to be changed to the next set of cargo tanks. The remaining cargo in
the first set of tanks will have to be discharged using the stripping
pump(s) or eductor. (See section 5.28.2)
The change of sets is carried out by opening the suction valves on one
or more tanks of the second set, while simultaneously closing the suction
valves in the first set.
When all valves on the second set have been opened, the cargo watch
officer and his watch team should complete the same checks of inactive
tanks and non-cargo spaces as were required at the start of discharge
(see section 5.19.2).