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).