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.
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.
Guidelines for the shipment of hazardous chemical cargoes
#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
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.
- 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.