Section 9.4 Dealing with Cargo Sweat and Condensation
One of the most persistent challenges in ensuring cargo quality during a sea voyage is the management of moisture in the form of condensation, commonly referred to as “sweat.” Sweat can form either on the cargo itself (“cargo sweat”) or on the internal structure of the ship’s holds (“ship’s sweat” or “hull sweat”). Both types can cause significant damage to a wide variety of bulk cargoes, leading to claims, disputes, and financial losses. Understanding the science behind sweat formation, recognizing the conditions conducive to it, and implementing effective preventative measures – primarily through judicious ventilation – are crucial aspects of cargo care for the Master and officers of a bulk carrier.
1. Understanding Sweat and Condensation:
Sweat is simply the visible result of water vapor in the air condensing into liquid water when that air is cooled below its dew point, or when warm, moist air comes into contact with a surface that is colder than the air’s dew point.
A. Key Definitions:
Dew Point: The temperature to which air must be cooled, at constant pressure and water vapor content, for saturation to occur and condensation to begin. Air at its dew point has a relative humidity of 100%.
Relative Humidity (RH): The ratio of the amount of water vapor actually present in the air to the maximum amount of water vapor the air could hold at that same temperature, expressed as a percentage. Warm air can hold significantly more moisture than cold air.
Cargo Sweat: Condensation that forms directly on the surface of the cargo. This typically occurs when warm, moist outside air is introduced into a hold containing a relatively cold cargo. The air in contact with the cold cargo surface is cooled below its dew point, and moisture condenses onto the cargo. This is a common risk when sailing from a cold region to a warmer, more humid region.
Ship’s Sweat (Hull Sweat / Hold Sweat): Condensation that forms on the internal surfaces of the cargo hold, such as the underside of hatch covers, deck beams, shell plating, and bulkheads. This typically occurs when the ship’s structure becomes colder than the dew point of the air within the cargo hold. This is a common risk when:
Sailing from a warm region to a colder region (the ship’s steel cools down rapidly from the outside sea and air temperatures).
Carrying a cargo that was loaded warm and/or has a high moisture content, which then releases moisture vapor into the hold atmosphere, raising its dew point. If the ship’s structure is cool, this moisture will condense on it.
B. The Science Behind Sweat Formation: The formation of sweat is governed by the relationship between air temperature, its moisture content (absolute humidity), its relative humidity, and its dew point temperature.
When air containing a certain amount of water vapor is cooled, its relative humidity increases even if no moisture is added or removed. If cooled sufficiently to reach its dew point temperature, it becomes saturated (100% RH), and any further cooling will result in condensation.
Alternatively, if warm, moist air (with a high dew point) comes into contact with a surface (either cargo or ship’s structure) that is at or below that dew point temperature, condensation will form on that cooler surface.
2. Consequences of Sweat and Condensation:
The impact of sweat on bulk cargoes can be severe and varied:
Moisture Damage: Direct wetting of the cargo can lead to:
Spoilage and Mold Growth: Especially for organic cargoes like grains, seeds, flour, and some agricultural products.
Caking and Hardening: For hygroscopic powders or crystalline substances like sugar, salt, cement, and some fertilizers, making them difficult to handle and reducing their value.
Discoloration and Staining: For cargoes like paper, pulp, or light-colored minerals.
Corrosion of Metallic Cargoes: Steel products, machinery, or even metal content within ores/concentrates can corrode rapidly if wetted by sweat (especially if the sweat is acidic or contains salts).
Physical Damage: Repeated cycles of wetting and drying can cause some cargoes to break down or lose structural integrity.
Loss of Quality and Value: Many types of cargo damage from sweat can lead to a significant reduction in the commercial value of the product.
Safety Hazards:
Wet, slippery surfaces in holds or on deck from excessive condensation.
Reduced visibility if condensation is very heavy (“fog” in holds).
Damage to Ship’s Structure: Persistent ship’s sweat, especially if acidic (e.g., from sulphur-bearing cargoes or CO₂ from respiring cargo mixing with water), can accelerate corrosion of hold plating, frames, and underdeck structures.
Commercial Implications: Cargo damage due to sweat is a frequent cause of claims against the shipowner. Demonstrating that due diligence was exercised in preventing sweat (e.g., through proper ventilation and record-keeping) is crucial for defending against such claims.
3. Cargoes Prone to Sweat Damage or Causing Sweat:
Hygroscopic Cargoes: These cargoes readily absorb or release moisture to/from the atmosphere. They are particularly vulnerable to damage from sweat and can also contribute to high humidity in the hold.
Examples: Grains (wheat, corn, rice, barley), seeds, flour, wood products (timber, pulp, paper), tobacco, cocoa beans, coffee beans, some fertilizers, sugar, salt.
Non-Hygroscopic Cargoes (but susceptible to surface wetting):
Examples: Steel products (coils, plates, pipes – prone to rusting), machinery, canned goods (labels peel, cans rust), some minerals, bagged cement.
Cargoes Loaded Cold: If a cold cargo is loaded in a cold port and the vessel sails into warmer, humid conditions, cargo sweat is a high risk if outside air is introduced without control.
Cargoes Loaded Warm and/or Moist: These cargoes can release moisture into the hold atmosphere during the voyage, increasing the hold air dew point and leading to ship’s sweat if the vessel’s structure becomes cooler. Examples include some agricultural products, freshly cut timber, or cargoes loaded during rain.
4. Preventative Measures:
The primary method for controlling and preventing sweat is judicious ventilation, supplemented by other good practices.
A. Ventilation (The Dew Point Rule is Key): This has been discussed in Section 9.1 but is so central to sweat prevention that it warrants re-emphasis here.
To Prevent Cargo Sweat (Condensation on cargo when sailing cold-to-warm):
DO NOT VENTILATE IF: The Dew Point of the Ambient (Outside) Air is higher than the temperature of the cargo (or, if cargo temp is unknown, higher than the hold air dew point when the vessel was in the colder region). Introducing warmer, moister air will cause it to condense on the cold cargo.
CONSIDER VENTILATION CAREFULLY IF: The Dew Point of the Ambient Air is lower than the hold air dew point but potentially still higher than the deep-stow cargo temperature. Surface ventilation might be possible, but through-stow ventilation could still cool the outer air enough to cause condensation deeper in the stow.
The safest approach when sailing from cold to warm with a cold, non-hygroscopic cargo is often to keep the holds sealed to prevent entry of warm, moist air, provided the cargo was loaded dry.
To Prevent Ship’s Sweat (Condensation on ship’s structure when sailing warm-to-cold, or with warm/moist cargo):
VENTILATE IF: The Dew Point of the Ambient (Outside) Air is lower than the Dew Point of the Hold Air. This allows the drier outside air to remove moisture from the hold atmosphere, reducing the hold air dew point and preventing it from reaching saturation as the ship’s structure cools.
DO NOT VENTILATE IF: The Dew Point of the Ambient Air is higher than the Dew Point of the Hold Air, as this would add more moisture to the hold.
Practical Application: Regular (e.g., watch-by-watch) measurement of ambient and hold air dry/wet bulb temperatures, determination of dew points, and logging of ventilation decisions are crucial.
B. Controlling Hold Atmosphere (Limited on most bulkers):
Dehumidifiers: Some specialized vessels (e.g., those carrying high-value steel or paper) are equipped with dehumidification systems that can actively remove moisture from the hold air. This is not standard on typical bulk carriers.
Inert Gas (if used for safety, e.g., for coal): While primarily for fire prevention, an inert atmosphere (low oxygen) will also have very low humidity, preventing sweat. However, inerting is a specialized operation not used for general sweat prevention.
C. Proper Stowage:
Dunnage: For some cargoes susceptible to damage from contact with steel (e.g., bagged goods, steel products), laying dunnage (timber planks, bamboo) on the tank top and sometimes along the sides can create an air gap, improving ventilation at the boundaries and keeping the cargo off potentially cold/damp steel. Less common for full bulk stows but important for certain commodities or if carrying mixed cargo.
Avoiding Contact with Cold Steel: Try to avoid stowing highly moisture-sensitive cargoes directly against ship’s side plating or bulkheads that are exposed to cold sea temperatures, if stowage options permit (often difficult in fully laden bulkers).
Ventilation Channels: For some hygroscopic cargoes that respire (like grains), ensuring clear ullage space above the cargo and that any built-in surface ventilation channels (if the ship has them) are not blocked allows for better removal of moist air.
D. Loading Cargo in Good Condition:
Avoid loading cargo that is already wet or has an excessively high moisture content beyond its safe limits for carriage, if possible.
Avoid loading cargo that is significantly overheated.
Protect cargo on the quay from rain before and during loading. Stop loading during rain.
E. Maintaining Weathertight Integrity:
Ensure hatch covers, access hatches, and ventilator closures are fully weathertight to prevent ingress of rain or sea spray during the voyage, which would directly cause cargo wetting and exacerbate sweat problems.
5. Dealing with Sweat/Condensation if it Occurs:
Despite preventative measures, sweat can sometimes still occur.
Resume/Commence Ventilation (If Permissible): If sweat is observed and the Dew Point Rule indicates that ventilating with outside air is beneficial (i.e., ambient dew point is lower than hold dew point), then ventilate vigorously.
Surface Wiping/Drying (Limited Applicability): For accessible surfaces (e.g., underside of hatch covers if safe to access slightly, exposed parts of cargo if holds are slack), wiping away condensation can help. However, this is usually impractical for the bulk of the cargo in a fully loaded hold.
Use of Absorbent Materials (Dunnage Paper/Specialized Absorbents): For very high-value, sensitive bulk cargoes, sometimes layers of absorbent paper or specialized desiccant materials might be used within the stow or on top, but this is not common for most bulk trades due to cost and practicality.
Draining Bilges: Regularly sound and pump cargo hold bilges to remove any accumulated sweat water. This prevents water from rising and damaging the lower layers of cargo.
Record Keeping: Meticulously record:
When and where sweat was first observed.
Type of sweat (ship’s or cargo).
Extent and severity.
Atmospheric conditions (ambient and hold) at the time.
Actions taken (e.g., commenced ventilation, pumped bilges).
Photographs can be very useful evidence.
Inform Relevant Parties: Notify the company (DPA) and charterers if significant sweat or potential cargo damage is observed.
6. Master’s Role and Responsibilities:
Training and Procedures: Ensure deck officers are thoroughly trained in understanding psychrometric principles, the correct use of psychrometers/hygrometers, dew point determination, and the company’s ventilation policy (which should align with the Dew Point Rule and IMSBC/IGC guidance).
Equipment: Ensure the vessel is equipped with accurate and calibrated instruments for measuring temperature and humidity.
Decision Making: The Master has the ultimate responsibility for deciding on the ventilation strategy, based on all available information (cargo type, voyage route, weather forecasts, instrument readings). This often involves balancing conflicting advice or pressures.
Supervision of Record Keeping: Ensure that accurate and detailed logs of atmospheric conditions, dew point calculations, and ventilation actions are maintained throughout the voyage. These records are vital in the event of a cargo claim.
Prioritizing Cargo Care: While voyage efficiency is important, decisions regarding ventilation must prioritize the care and preservation of the cargo, especially when carrying high-value or sensitive commodities.
Dealing effectively with cargo sweat and condensation is a hallmark of a professional and experienced Master Mariner. It requires a good understanding of the underlying science, diligent monitoring of conditions, and the courage to make sometimes difficult decisions about ventilation, always prioritizing the safety and integrity of the cargo.