Claims can arise when cargo is not carried
at the recommended temperature. Handling of cargo during transit is the
responsibility of the vessel crew and owner. In this article, we look at two
cargo types and the issues that can arise.
Introduction
It is sometimes essential to apply heat during
transit to maintain or raise the temperature of cargo.
Over or under heating of
cargo can adversely affect the property of the cargo and, on many occasions,
this change in the cargo quality is irreversible. Improper temperature control
can lead to deterioration in the cargo quality or poor pumping performance
leading to excess cargo remaining on board (ROB).
Petroleum cargo
Highly viscous petroleum cargo or cargoes
with a high pour point or wax content require heating during transit because
heating reduces the viscosity of the cargo and enables it to flow better.
Heating also leads to a constant
circulation of cargo within the tank. This leads to a reduction in wax formation
and sedimentation, and helps in the outturn at the discharge port.
Usually, it is wise to heat early to maintain
the temperature during the voyage, rather than to be forced to raise the
temperature of the cargo significantly at the end of the voyage.
The rule of thumb is that the cargo temperature
should be at least 10°C above the pour point during the entire voyage. Heating
should not be stopped during the voyage and should be adjusted to gradually
change the temperature of the cargo to the desired level and then to maintain
it.
Heating instructions are often too vague,
with the charterers relying on
the experience of the master.
This is especially important if the cargo
is loaded at a temperature higher than that requested as per the charterparty.
In this case, it is recommended to wait for
the temperature to fall to the desired level and then start the heating to
maintain this temperature. It is important to note that if the pour point of
the cargo is high and the required temperature as per the charterparty is less
than 10°C above the pour point, then the charterer should be consulted and the
agreed temperature should be maintained to minimise any shortage claims. When
crudes with high wax content requiring heating are carried, it is important
that the charterer provides clear instructions for heating both on the voyage
and throughout discharge. The temperature of the cargo at any stage should not
be higher than the ship is designed to carry.
At the discharge port, the ship staff should
ensure that the cargo is stripped immediately after it reaches the level of the
heating coils in the cargo tanks.
Edible oil cargo
Edible oils such as the various categories
of palm oil and vegetable oils are highly viscous in nature and require special
attention during discharge. Each cargo of this type will have specific
transportation and
handling requirements. Shipowners should
ensure that the supplier or
charterer provides all the necessary information
about the cargo such
as specific gravity, colour, moisture content,
impurities, melting point and free fatty acid (FFA) content.
Heating instructions should also be
detailed and these should state the in-transit temperature, loading/discharging
temperature and the allowed daily rate of increase of temperature. Rapid
heating of certain grades can cause scorching of the cargo, and overheating the
cargo can promote oxidation, hydrolysis, scorching and discolouration of the
cargo. Rapid heating can also cause an increase in FFA content, which causes
the cargo to turn sour, and is used as a measure of cargo quality, especially
for palm oil.
It is therefore important to monitor the temperature
of the cargo carefully and, if increasing, raise the temperature of the cargo
at a gradual pace. Proper planning is required for heating these cargoes,
especially when a significant rise in the temperature is required over
a short voyage prompting a higher rate of
heating. To avoid overheating
or scorching, small parcels should be stowed
in a tank of suitable size and the use of larger tanks, with a large heating coil
surface area, should be avoided.
Crew should remember that carriage of
heated edible oil cargoes into cold climates may cause the cargo vapours to
solidify inside the cargo tank ventilation piping, effectively preventing the
pressure in the cargo tank to equalise. This could potentially result in a
catastrophic failure of the cargo tank.
The Food and Agriculture Organization of
the United Nations (FAO) recommends the practice for storage and transport of
edible fats and oils (CAC/RCP
36-1987). It
gives guidance on the maximum temperature required for the various grades
during transit and while loading and discharging.
These are as per below table:
Conclusion
1. The desired temperature of the cargo
should be maintained throughout the loading/discharging operation and during
transit.
2. Cargo tanks should be stripped as soon
as the liquid is below the level of the heating coils.
3. Clear heating instructions should be given
to the master. Any doubts should be addressed sooner rather than later.
4. The cargo plan should include instructions
for stripping heated cargo. Sufficient trim and temperature of the cargo should
be maintained. Shippers should be aware that heat loss increases as the level
of the cargo drops.
5. Heat loss is also accelerated if the
cargo tank is in contact with the ballast in the adjoining ballast tank.
Ballasting should be deferred until the stripping is completed, if it is safe
to do so.
6. Cargo temperature should always be
raised gradually.
7. Overheating of the cargo should be
avoided.
8. Cargo should not be loaded at a
temperature higher than the ship is designed to carry.
9. Cargo temperatures should be checked
daily at different levels and the temperature log must be maintained.
10. Over-reliance on remote temperature
reading equipment should be avoided and temperature should be compared with
other means to check accuracy.
Δεν υπάρχουν σχόλια:
Δημοσίευση σχολίου