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
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.