5.30 MULTIPLE CARGO DISCHARGE SEQUENCES
The chief officer's discharge plan for multiple grades of cargo has
been described in section 5.22. In many cases,
the grades will not fit neatly into the natural segregation of the ship's
cargo systems and there may be more than one grade within a system.
The procedure for changing grades within a system must be carefully
planned and carried out to prevent cargo contamination. The main rule
for discharging multiple grades of cargo is that protection of cargo
quality is more important than cargo outturn or time in port. Every
reasonable effort (and occasionally some unreasonable ones), must be
made to avoid cargo contamination.
5.30.1 Changing tanks
When the first grade has been discharged as far as possible with the
main cargo pump, the cargo watch officer must confirm (by referring
to the cargo orders), if the stripping pump is to be used for final
stripping of the tank. It may be that the stripping pump has been used
for another purpose and cannot safely discharge the tank bottoms without
risk of contamination. If the tank must be stripped ashore and the condition
of the stripping system is uncertain, then the stripping pump, suction
and discharge line should first be flushed to the slop tank before stripping
the cargo ashore. Note the amount flushed to the slop tank in the logbook.
5.30.2 Draining lines between grades
The discharge of multiple grades would normally be scheduled so that
the most critical (most easily contaminated), cargo grade is discharged
first and any residue from that cargo is down-graded into the following,
compatible but 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 the pipeline contents to the slop tank,
or to a commingled stripping tank. The lines must not be drained into
the pumproom bilges!
In some cases it may be necessary, before discharging the second grade
of cargo, to flush the piping. The cargo piping is filled by gravity
flow or by counted pump strokes and allowed to soak for 30 minutes,
then re-strip it to the slop (or accumulation), tank. It can then 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.
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 than to contaminate
the entire parcel or shore tank by a hurried or careless discharge programme.
5.30.3 Changing shore connections
In many cases, the terminal must change the receiving hose or arm between
grades of cargo discharged on the same system. The need for hose/arm
changes should have been identified during the pre-transfer conference
and incorporated in the chief officer's discharge orders or night orders.
If the dock operator indicates any change to the agreed hose/arm change
procedure, the chief officer must be advised. It is prudent to insist
that the procedure change be confirmed by the receiving terminal superintendent
or manager before agreeing to discharge the next grade of cargo.