Moisture damage is the source of a significant number of cargo claims. Claimants allege that this is brought about by the ship’s failure to ventilate correctly, resulting in the development of condensation (known as “sweat”). This sweat can lead to the deterioration of a number of bulk cargoes such as grain, seedcake and steel surfaces. All modern bulk carriers are fitted with some form of ventilation, either natural or mechanical, which can be used to minimise the formation of sweat. The London P&I Club has launched new publication LP Focus to address all issues related to bulk carrier ventilation.
Ventilation can also be used for the removal of taint, fumigate gases, or other gases as directed by the shipper. These forms of ventilation only provide surface ventilation, i.e. lacking the ability to penetrate a bulk stow, and therefore cannot continue to maintain an equilibrium in conditioning a cargo such as grain within a silo.
Cargo sweat
Cargo sweat is condensation forming on the surface of a “cold” cargo as warm, moist air enters the cargo hold. For example, if a cargo of steel is loaded in winter in North China for discharge in Singapore, the temperature of the cargo will be low. If warm moist air is later introduced in the cargo hold, condensation takes place as soon as it comes into contact with “cold” cargo. To avoid the possibility of cargo sweat, ventilators should be closed and no ventilation carried out. However, if the moisture content of the cargo is high, extraction of the moist air from within the cargo holds may be required.
Ship Sweat
Ship’s sweat is the result of condensation forming on the ship’s structure as the ship becomes colder moving from a hot to a cold climate. The warm moist air within the cargo compartment condenses as it comes into contact with the cold structure of the ship. For ship sweat to occur, the dew point in the cargo hold must exceed the temperature of the ship’s structure. To eliminate ship sweat, the cargo should be ventilated if the ship is moving from a warm to a cold climate.
Cargoes can be:
- Hygroscopic: mainly agricultural products containing natural moisture. They may absorb, retain or release moisture, depending upon the surrounding atmosphere. Examples include grains. On a voyage from cold to hot regions, these type of cargoes need no ventilation, but surface ventilation is needed from hot to cold regions.
- Non-Hygroscopic: solid cargoes which contain no moisture such as steel; however, these cargoes are very likely to be damaged by cargo sweating. To avoid cargo damage no ventilation is required.
Any shipper’s instructions should be complied with and the following factors considered:
1. Ensure that the shipper’s declaration contains sufficient information about the cargo, particularly moisture content (MC), transportable moisture limit (TML) and ventilation requirements, and/or if the commodity is not normally carried or the areas of trade are uncommon.
2. Claims for moisture damage to cargo can only be defended if supported by properly maintained documentation. It is critical that records of hold temperatures, humidity and durations of ventilation are maintained.
3. The dew point temperature of the cargo hold and outside air should be compared. If the outside dew point temperature is lower or equal to that within the cargo hold, then ventilation should be continued.
4. Ventilation not only serves to control sweating, it can control the gases or odours emitted from cargo.
5. If there is a risk of seawater spray or rain entering the cargo holds, all ventilation should be stopped and times noted until conditions change to allow resumption of ventilation.
6. Ventilation should be continued even at night if required.
7. If circumstances allow, there should be regular inspections of the cargo space for any signs of condensation, e.g. on the underside of the hatch access covers. If condensation is found, ventilation should be continued.
8. A complete record of cargo hold temperatures and ventilation should be kept.
Learn more about bulk carrier ventilation by reading LP Focus issued by the London P&I Club:
Source: The London P&I Club
