The reaction between aluminium phosphide tablets and moisture in the air is exothermic, meaning it releases heat, and disperses colourless phosphine gas into the atmosphere. Water moisture trapped in cargo spaces, either within a moist cargo or with high humidity within the hold, may accelerate this process, especially when fumigants are applied to the cargo surface. Ventilation flaps should be closed, and hatch covers sealed to prevent any water ingress during precipitation or shipping seas, and to prevent the escape of toxic gas.
If necessary, additional prophylactic measures, such as the application of expanding foam to hatch cover joints, can be considered to achieve gas- tight holds.
It is essential for fumigant tablets/ pellets to be spread evenly on the cargo surface and to ensure cargo is dry prior to application. When piled on a wet cargo surface, a rapid exothermic reaction could be sufficient to set fire to the cargo.
The molecular weights for both phosphine and methyl bromide gases are heavier than air and therefore the gas will slowly sink down through the cargo to achieve a good penetration within the stow depending on depth and exposure period. However, when the aluminium phosphide tablets or pellets do not fully react with moisture in the hold, an inert residual by-product, often a light, powdery, grey, ash-like substance, may be left behind. This residue creates an exposure risk if it is inhaled or comes into contact with the eyes. Exposure to the partially reacted residue may occur when the tablets/ pellets are used in low humidity, low temperature environments or opening of the holds prematurely before the full exposure period is completed. Care should be taken when disposing of the aluminium phosphide residues due to the risk of toxic gas. There have been previous cases where residues, when collected in buckets or similar, have ignited spontaneously.
Methyl bromide supplied in gaseous form can achieve an effective fumigation coverage of the cargo within 24 to 48 hours, whereas a typical aluminium phosphide tablet may take more than two days to breakdown. With aluminium phosphide tablets, the required exposure period can take approximately five to twenty days depending on the coverage, temperature, and relative humidity of the air. Colder temperatures and drier atmospheric conditions reduce the rate of the fumigant’s reaction and gas dissipation. In order to mitigate these effects, sub-surface probes are sometimes used to introduce the fumigant into the cargoes to speed up the process.
There are various methods for applying aluminium phosphide tablets and pellets to cargo:
• Standard fumigation by surface application of tablets in sleeves or blankets
• Sub-surface fumigation via trench application by placing fumigants in trenches dug into the cargo
• Fumigation with probes inserted into the cargo from around 0.3m to several metres
• Application via tubing with a safe fan circulating the fumigation gas within the cargo spaces.
The main method for applying methyl bromide gas is from cylinders connected to the pipework via a vaporiser. Crew will need to disembark whilst the operation is carried out.
FUMIGATION RISKS
Both methyl bromide and aluminium phosphide, chemicals used for fumigation, are toxic to humans and to the intended targets of insects and rodents. Unfortunately, by the time fumigant gas is detected, it is sometimes too late to prevent poisoning, since the concentration of gas may be above the safe working limit.
- The fumigator should conduct inspections of all access points to the cargo holds to ensure they are suitably gas-tight for the fumigation process. This should be done when the hold is empty and in accordance with the requirements in the countries of loading and discharging. The master should sign off the pre- fumigation inspection report.
When phosphine gas, without the presence of any impurities, is released, it is colourless and odourless. However, when contaminants exist, phosphine gas may give off white-coloured smoke and/or the smell of decaying fish, garlic or carbide.
Methyl bromide is colourless but comes with a fruity or musty odour at high concentrations, Britannia adds.
These odours serve as good warning signs of gas leakage to people working in or near the cargo spaces. However, these indicators should never be solely relied upon as a means of determining whether a space is safe.
Below are some of the practical preventive measures recommended by Britannia P&I Club that should be undertaken during or before the voyage:
- Before reaching the loading port, maintenance should be carried out on the weathertight integrity of the cargo hold, such as hatch cover seals, hatch cleats, ventilators, cement ports and booby hatches. It is recommended that, as the hatch covers and other access points have to be gas tight to ensure their integrity. Prior to arrival, any necessary repairs should be conducted in good time and their effectiveness verified.
- Records of inspection and maintenance should be kept on board for a suitable period prior to fumigation.
94 | ISSUE 106 | DEC 2023 | THE REPORT
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