Britannia: Tips for proper cargo ventilation

According to Britannia, proper ventilation is instrumental in preventing the deterioration of cargo, by removing excessive moisture and minimising the formation of sweat. Ventilation is also required for some cargoes to disperse hazardous gases emitted by the cargo, such as carbon dioxide, carbon monoxide and methane.

When to ventilate?

The following should be considered prior to deciding whether to ventilate:

• Cargo properties and requirements for the safe carriage
• Temperature, DP and RH of the ambient air
• Temperature and moisture content of the cargo, and/or
• Temperature, DP and RH of the air inside the cargo hold (see further comments regarding ventilation rules below)
• Sea water temperature, particularly in cargo holds of open frame type where ship sweat can easily form
• Ballast water temperature where a filled tank is adjacent to the cargo
• Exposure of the ventilation system to rain, waves and spray.

As ventilation directly impacts the safe carriage of the cargo, the Safety Management System (SMS) should provide robust guidance to the crew. Whilst it may reiterate general industry practice, the SMS and operational procedures should also reflect the requirements of the company, considering its trading and operational profile (for example, steel trade in winter areas).

Ventilation rules

There are two basic ventilation rules which assist mariners with the decision whether ambient air is suitable for safe ventilation of the cargo:

• Dew point rule
• Three degree rule

Decisions to start or stop ventilation should be carefully recorded in the ventilation log.

Dew point rule

“Ventilate if the dew point of the outside air is lower than the air inside the cargo hold.”

Dew point rule refers directly to measurements at the time of decision: if the DP of the ambient air is lower, it is less humid than the air inside the hold. It is easy to understand but may be more difficult to apply.

Firstly, accurate DP measurements are required in spaces where access is restricted due to fumigation, safety reasons, or subject to the enclosed space entry protocol. Taking DP measurements in cargo holds filled to coaming height may also be physically impossible, with the headspace between the cargo and hatch cover sealed by the coaming. In such cases, air pockets in hold ends (accessible by manholes) are not connected with the main headspace above the cargo and cannot provide representative DP measurements.

Secondly, there may be insufficient personnel to measure as often as required (recommended at least every four hours or once per watch), with measurements to be obtained in each cargo hold (good practice recommends obtaining two measurements at each point to validate the reading). Furthermore, ventilation should be stopped for some time, or the measurement will be affected by outside air.

The above-mentioned difficulties are also often the reason for inaccuracies or apparent errors seen in ventilation logs when retrospectively reviewed in case of cargo claims.

Three degree rule

“Ventilate if the temperature of the outside air is at least 3°C lower than the temperature of the cargo.”

The three degree rule for use with hygroscopic cargoes can be applied without the need to repeatedly access cargo spaces or to stop ventilation for measurements. The cargo temperature is assumed to remain constant during the voyage due to its large mass and thermal inertia. The ambient air temperature can be measured frequently, easily, and with good accuracy.

However, it is important to understand the requirements and limitations of this rule:

• The rule applies to hygroscopic, mostly agricultural cargoes such as grain. The rule assumptions do not fully apply to non-hygroscopic cargoes.
• It requires a reliable temperature measurement of the cargo upon loading. For this, a surveyor may need to be appointed – the ship may not have sufficient access, or the appropriate equipment. Different parcels may have different temperatures, so accurate sampling is required.
• If the cargo temperature reduces during the voyage, for example, due to intensive ventilation with very cold air, or a voyage delay in cold weather, the original rule assumptions for the cargo may become invalid. Performing another cargo temperature measurement may not be practical or safe.

Explanation of the three degree rule

Agricultural cargoes are dried prior to loading to a transportable moisture content to slow down biological processes and prevent spoilage. Once loaded, the cargo will affect the air adjacent to it inside the hold, until an equilibrium is reached. Generally, the air will achieve a temperature close to the cargo temperature and relative humidity (RH) of around 70%. The DP of air at RH of 70% will be about 6°C below the dry bulb (air) temperature.

The RH of outside air at sea level is typically around 80%. In most temperatures, at RH of 80%, the DP will be at least 3°C below the dry bulb (air) temperature.

The combination of the above approximations means that if the ambient air is at least 3°C colder than the temperature of the cargo, the DP of the ambient air will be lower than the DP in the cargo space.

Practical verification of the three degree rule

Using RH tables and the rule assumptions regarding the equilibrium RH, the rule can be verified step-by-step for specific temperatures:

• Air inside the cargo hold at 20°C (same as cargo) at 70% RH has DP of 14.36°C.
• Outside air at 17°C (three degrees less) at 80% RH has DP of 13.52°C, lower than in the cargo hold. Ventilation is allowed.