In collaboration with CWA International, the Swedish Club has produced cargo advice to assist operators in the daily operation of their vessels, in relation to hazardous chemical cargoes.
A large range of
chemical commodities is carried on board chemical tankers, totalling more than
5,000 different chemical products and grades. These chemical products often
have a high minimum purity due to their intended end uses.
The contamination of chemical cargoes can often affect
the suitability for their intended end use, and this can play a significant
role in the cargo’s value. As a result, special consideration should be given
towards these sensitive cargoes in order to ensure their proper carriage.
#1 Pre-loading: The large variety of different chemical cargoes
carried by chemical tankers means that there is a possibility of incompatible
products being carried consecutively. As a result, special attention must be
paid to ensuring cargo tanks are thoroughly cleaned. Very large industry
standard cleaning matrices are available covering the various grade changeover
combinations.
In order to ensure sufficient tank preparation has
been carried out, thorough pre-loading surveys of a vessel’s cargo
tank/pump/heating equipment/ lines will be performed, including visual
inspection of tanks together with wall-wash tests, when appropriate, prior to
the loading of a ‘first foot’ trial quantity of cargo.
In addition to the pre-loading surveys often carried
out by shippers’ surveyors, first foot sampling methods can give a good
indication of the cleanliness of cargo tanks and lines. This is because any
contaminants present in the shore or vessel lines or the cargo tanks will be
concentrated in the first foot samples.
However, tanks coated with epoxy type coating systems
can absorb light solvent species, such as aromatics, which may not immediately
be detected during pre-loading inspections.
As with residues of previous cargoes, water is
undesirable in high purity chemical cargoes. Moisture can enter a cargo via
insufficient tank/line ventilation and draining of wash water, although this
will typically be detected in the pre-loading survey and first foot samples.
#2 Risks associated with carriage: Many cargoes carried on board chemical tankers are
prone to undergoing chemical reactions, either by themselves or with impurities
or air. These reactions reduce the purity of the products and in some cases can
generate large amounts of heat which presents a safety risk.
Depending on the exact cargo involved, there are
several factors which must be considered when stowing cargoes with a limited
‘shelf-life’, such as:
- Oxygen content
- Inhibitor content
- Temperature
In addition, high temperatures generally increase the
rate of chemical reactions – an increase in 10°C is often said to double the
rate of many chemical processes, and therefore increased storage temperatures
lead to a reduced ‘shelf-life’ of the chemical.
Nevertheless, some cargoes require heating to ensure
fluidity during cargo operations to prevent short delivery. In order to prevent
overheating, strict control of temperatures is required to ensure a balance
between fluidity and chemical stability in order to prevent both shortage and
quality disputes.
Moreover, some chemical cargoes are particularly
reactive towards oxygen and, as such, control of tank atmosphere may also be
required by way of nitrogen blanketing for partial or total exclusion of oxygen
(for example with propylene oxide/hexamethylene diamine-HMD). Partial reduction
of oxygen is also warranted for a range for oxygen dependent inhibited cargoes,
such as styrene and isoprene monomers, to prevent selfpolymerisation.
Lessons learned
- Ensure that
the stowage plan is suitable. When
stowing cargo in adjacent tanks pay attention to cargo carriage
temperature restrictions. This is especially important when the vessel is
loading cargo at several ports.
- Comply with
charterers’ carriage instructions, paying particular attention to temperature and
oxygen content. If these are unclear, seek clarification.
- Monitor the
temperature of the cargo at upper, middle and lower levels through the cargo column at least once a
day during the voyage to ensure compliance with heating rates and carriage
temperatures, where appropriate.
- Recirculate
the cargo if required. This
is important for inhibited cargoes which depend on dissolved oxygen
content to maintain inhibition.
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