5.22 DISCHARGING MULTIPLE GRADES

For multiple grade discharges, the chief officer will have determined the grade or series of grades which will take the longest to discharge. This grade (or grades), determines the 'critical path' of the discharge port activities. It is important that the discharge of the first grade on the critical path is started before any other cargo. When that cargo is moving ashore, the remaining grades can be started in turn.
The chief officer's discharge programme will indicate a pumping and ballasting sequence which maintains vessel stress and trim within acceptable limits. If there is a departure from the planned sequence, the discharge of the other cargoes and the addition of ballast, should be manipulated as necessary to maintain acceptable stress and trim, meanwhile keeping the critical path cargo(es) on schedule.

5.22.1 Two valve separation
Two valve separation should be maintained throughout the discharge of multiple grade cargoes. If possible, the separation valves should be sealed closed, with numbered seals, prior to discharge (if they were not already sealed in the loading port) and the seal numbers recorded by the independent petroleum inspector and in the ship's logbook. The crew must clearly understand that the chief officer is the only person authorised to break the separation seals. If possible, the separation seals should be left in place until the discharge is completed, and the petroleum inspector again requested to witness and record that the seals are intact.

5.22.2 Draining lines between grades
The discharge of multiple grades would normally be scheduled so that the most critical cargo grade is discharged first and any residue from that cargo is down-graded into the following, compatible or less critical grade. When the following grade cannot accept such contamination, it is necessary to drain the cargo lines thoroughly after the completion of the first grade. This is done using the stripping pump, discharging to the slop tank, or to a commingled stripping tank.
In some cases it may be necessary to flush the piping with the second grade. Allow it to soak for 30 minutes and then re-strip it to the slop (or accumulation), tank before it can be considered clean enough to handle the second grade of cargo. If there is any doubt about the efficiency of the line flushing, a sample of the second cargo should be drawn from the pumproom piping and tested ashore for the critical specification, before beginning discharge.
Flushing and re-stripping may seem like a waste of cargo, but the most important consideration is that the product be delivered within specification. It is preferable to lose a small fraction of the cargo through downgrading than to contaminate the entire parcel or shore tank by a hurried or careless discharge programme.

5.22.3 Heated cargo precautions
Cargo must be heated to the indicated discharge temperature to ensure an efficient discharge and acceptable outturn (minimum ROB). The cargo will not flow efficiently to the pumps unless the correct temperature has been achieved and is maintained throughout the discharge. (See section 4.2.1.) On motor ships, there may not be enough steam to operate the cargo pumps and do all the necessary heating simultaneously. If this is the case, then reduce steam to the centre tanks heating, concentrating on wing tank heating until they have been discharged. Alternatively, one or two of the cargo pumps can be stopped to ensure sufficient steam to maintain the required cargo discharge temperature.
On those ships fitted with re-circulation cargo heaters on deck, cargo cannot be heated once discharge has begun. On such ships, before discharge is started, it is necessary to raise the cargo temperature a few degrees above that required. If problems develop while discharging, the discharge must be stopped and tanks returned to recirculating heating mode.
During the discharge, the heating coils must also be adjusted as required to avoid overheating cargo in tanks being discharged. This is particularly important where vapour pressure can create loss of suction pumping problems. Cargo can be heated faster in port than at sea, so if the cargo is at discharge temperature and on maintenance heating before arrival, then the heating rate should be reduced after arrival. As heat is secured on empty tanks, more steam is available to heat the remaining tanks at a faster rate.
Use one cargo pump on each system, beginning the discharge with the forwardmost tank(s) on each system. Work from forward to aft on each system, discharging each tank as quickly as possible.
Maximum acceptable stern trim should be maintained to assist draining of tanks toward the main and stripping suctions.
Re-strip each tank two or three times at intervals of 30 minutes.
Keep the heating on until the cargo is at the level of the heating coils. Heating after that point will cause excessive vapour and contribute little to the discharge.
One tank of cargo should be retained to the end with sufficient head (innage), to prime a cargo pump if suction is lost due to vapour problems.
After completing a discharge of high-pour point cargo, all pipeline systems should be drained to a single tank and then flushed through with hot water to prevent blockage by the congealing cargo residue.