As part of their series, American P&I has published an animation video which provides a summary of how condensation forms as a result of ship sweat and cargo sweat.
“Moisture damage” is the source of a significant number of cargo claims, often involving bagged or bulk agricultural products. Claimants typically allege that failure by the ship to ventilate correctly resulted in the development of condensation (commonly known as “sweat”), causing the cargo to deteriorate.
As explained, rust formation on cargo is greatly accelerated when relative humidity is greater that 60%. Best practice is to keep relative humidity below 40%.
However, it is important to recognise that some commodities may have inherent moisture levels which exceed acceptable limits at the time of loading, making them biologically unstable. Such details may not be known to the ship, and prudent ventilation measures may be insufficient to prevent the cargo from deteriorating on passage.
Recommended rules for ventilating steel cargoes.
- The 3° Celsius rule: Ventilate when the dew-point of the outside air is least 3oC below the temperature of the cargo taken during loading.
- New point rule: Ventilate when the dew-point of the outside air is lower that the dew-point of the air in the hold.
According to American Club, all reading must be recorded in the ships cargo record book. Furthermore, both wet and dry bulb reading must be obtained at each position and the dew point temperatures carefully determined. The effectiveness of dehumidification is dependent upon provision of a constant airflow throughout the hold.
