Ventilation of bulk grains and oilseeds
The ventilation of bulk cargoes, especially agricultural commodities, is a recurring problem . But how does a crew know when to ventilate a cargo, and why should they do it?
What is ventilation?
Agricultural bulk cargoes are usually hygroscopic; they possess a natural moisture content, and they may absorb, retain or release moisture into the environment depending on the surrounding atmosphere.
This can lead to condensation issues particularly if the loading port was in a relatively warm area.
Ventilation can be summarised as the act of replacing warm and humid air in the headspace with relatively cooler and dryer ambient air. The most commonly used ventilation on bulk carriers is natural ventilation (as opposed to mechanical ventilation). The hatch covers are usually equipped with a total of four ventilation windows with two windows forward and aft or two windows port and starboard.
Why do we ventilate?
The main goal of ventilation of a bulk grain or oilseed cargo is to reduce the incidence of cargo damage due to condensation of water vapour in the holds.
Some routes/ environments are more likely to lead to condensation than others.
For example, condensation is rarely seen during a short voyage in the same temperature range. On the other hand, for example, if we look at a soya bean cargo loaded in in a warm climate that will be discharged in Northern China at freezing temperatures, it is likely that condensation would occur as the warm and humid air in the headspace would condense on the steelwork cooled by the external temperature.
Ship’s sweat and cargo sweat
Ship’s sweat is the condensation that forms on interior steelwork, which often drips down onto the cargo surface. The condensation forms upon contact of the warm humid air in the headspace with the relatively cooler steelwork, such as the hatch covers. This drips back onto the cargo and runs down the sides of the holds, creating a characteristic pattern of damage ‘drip lines’.
It is essential, no matter the ventilation rule, to keep a detailed record of the ventilation details as this will be crucial evidence in case of a cargo damage at the discharge port.
Cargo sweat occurs under the same principle as ship’s sweat: but in this case however, the cooler surface is not the steelwork but the upper layer of the cargo itself. This will occur if a cargo loaded in a cold climate is exposed to warmer air, either due to the hatch covers being opened or incorrect ventilation of the holds.
Both cases can lead to mould growth and localised heating in the cargo.
How to ventilate
A proper ventilation regime helps by minimising the risk of ship’s sweat related damage and prevents the formation of cargo sweat.
There are two rules that the crew can use to determine if ventilation is necessary:
Dew Point Rule This is the most commonly seen rule when dealing with cargo claims. It requires that a cargo hold should be ventilated when the dew point of the outside ambient air is lower than the dew point of the air inside the headspace of the hold.
Three Degree Rule This rule states that a cargo hold should be ventilated when the outside ambient dry bulb temperature is at least 3°C lower than the cargo temperature.
Ventilation record keeping
It is essential, no matter the ventilation rule, to keep a detailed record of the ventilation details as this will be crucial evidence in case of a claim at the discharge port. The charterparty or voyage orders may contain instructions as to what information and items should be recorded.
Proper ventilation is the only cargo care that a Master and crew can provide during voyage to limit condensation damage. Unfortunately, despite their best efforts, claims can still arise at discharge port from condensation related damage.
