01 FEB 2004
TYPES OF SWEAT AND HOW THEY FORM
Cargo sweat – Cargo sweat is condensation which forms on the cargo. It occurs when the temperature of the cargo is less than the dew point of the air in the hold. The cargo cools the air in contact with it and condensation forms on the cargo. The cargo may therefore be damaged by direct contact with moisture. Cargo sweat is most associated with the incorrect ventilation of warm moist air into a hold with cold cargo. It is akin to the condensation forming on a cold drinks can when taking it out of the fridge on a summer’s day.
Case example: A full cargo of refined sugar in bags was rejected due to serious caking. The cargo was sold for salvage, but at a significant loss. The claim against the vessel owner was settled for in excess of USD 1 million. The caking was largely attributed to cargo sweat, which had resulted from improper ventilation. The voyage was from Europe (a cool climate) to the Middle East (a hot climate) in May and ventilation was not necessary. Ventilation during humid conditions in the Red Sea, with the aid of mechanical fans, and with the stow arranged in channels is thought to have greatly exacerbated the damage.
Ship’s sweat – Ship’s sweat is condensation which forms on the internal ship’s steelwork. It occurs when the temperature of the ship’s steelwork is lower than the dew point of the air in the hold. Condensation will form on the deckhead and/or on the inside of the shell plating. This moisture can then drop down from the deckhead on to the surface of the stow or come into contact with the cargo stowed directly against the sides of the hold. Ship’s sweat is most associated with a voyage to a colder climate and the failure to ventilate the hold, replacing moist air from the load port or air made moist by the cargo, with drier air. It is akin to the condensation forming on the window when taking a warm shower on a cold winter’s day.
Case example: A bagged cargo of parboiled rice was discharged in West Africa severely caked and stained resulting in a depreciation in value. The loss claimed against the vessel owner necessitated a settlement of tens of thousands of US dollars. The caking and staining was largely attributed to ship’s sweat – direct contact of the bags with the internal steel of the vessel. The ship’s sweat had largely resulted from a failure of the vessel to ventilate the cargo on the early part of the voyage – from India to the Cape of Good Hope in October.
PROPER VENTILATION PRACTICE TO AVOID SWEAT
The nature of the cargo and its packing – There are two basic groups of cargo, which pose significantly different risks of sweat: hygroscopic and non-hygroscopic.
Hygroscopic cargoes contain moisture, mostly in natural form; for instance, agricultural, fish and forest cargoes. For example rice has a moisture content of around 14 per cent. It is important to note that these cargoes can absorb and release moisture. It is more likely that damage is caused when moisture is absorbed. A hygroscopic cargo with a moisture content and temperature such that water vapour will leave the cargo and enter the hold air will result in the build up of moist air and increase the risk of sweat.
Non-hygroscopic cargoes contain no moisture; for example, steel. Whilst non-hygroscopic cargoes may be wet before shipment, e.g., because they have been affected by rain before loading, it is better to avoid shipment of wet cargo rather than face the difficult task of deciding whether or not to ventilate. Non-hygroscopic cargoes do not give off moisture, but may absorb or be damaged by it.
Packing – Packing is an important factor and, for the purposes of ventilation, it can essentially make a non-hygroscopic cargo a hygroscopic cargo and vice-versa. For example, wood is hygroscopic and is often used as packing. Again however, the emphasis should be on avoiding shipment of wetted cargo. A starker example is sugar. Raw sugar is usually shipped in bulk and has a moisture content of around 0.25-1.1 per cent. However, refined sugar usually has a very low moisture content (0.03-0.05 per cent) and is commonly bagged within inner polythene plastic bags and outer woven polypropylene bags. The low permeability of the liner to moisture in vapour form essentially means therefore that refined sugar, bagged in this way, can be considered non-hygroscopic in nature.
Caked sugar as a result of cargo sweat.
Climatic changes on the voyage – Consideration needs to be given to the temperature and humidity conditions expected from the place of loading through the voyage to the place of discharge, as is relevant to the time of year. The term climate includes here the sea temperature, as that has particular relevance insofar as ship’s sweat is concerned. The sea temperature will have the greatest effect on the temperature of the steelwork in the hold which is adjacent to or in contact with the sea.
Putting together all of these factors, the risk of sweat can be predicted and therefore, broadly speaking, it can be decided whether ventilation is necessary, unnecessary or to be prevented. Some examples:
Hygroscopic cargo from warm to cold climate – There is a danger of heavy ship’s sweat and vigorous ventilation will therefore be necessary to replace moist air with drier air. The air in the hold may already be moist from ambient air at the place of loading, but is also likely to receive large volumes of moisture from the cargo itself. The aim is to lower the dew point temperature, thus making it unlikely that the steel structure of the vessel can cool air in contact with it sufficiently to reach that temperature.
Hygroscopic cargo from cold to warm climate – The risk of sweat is generally low and ventilation is therefore largely unnecessary. There is a risk of cargo sweat if warm moist air is introduced into the hold.
Non-hygroscopic cargo from warm to cold climate – There may be a risk of ship’s sweat if the hold air is moist from the load port. Ventilation to improve the dew point of the hold air is therefore advantageous.
Non-hygroscopic cargo from cold to warm climate – Cargo sweat is likely if warm moist air is introduced into the hold. Ventilation is not therefore necessary and the holds should be sealed.
As an aide mémoire, mariners have often relied on the following very general rule:
“Warm to cold: ventilate hold
It is important to remember, however, that the vessel will often pass through different climatic regions. In the example given for ship’s sweat above, whilst ventilation was necessary in the first part of the voyage, it was not necessary in the second part from the Cape of Good Hope to West Africa. It may also be very dangerous to ventilate, as has been shown in the example given above for cargo sweat. In that case the vessel ventilated whilst transiting humid conditions in the Red Sea, with the result that moister air was introduced into the holds. With all this in mind, a rule is required to ensure that, when the vessel does ventilate, it improves the air conditions in the hold, and thereby reduces the risk of sweat developing.
The Dew Point Rule – The Dew Point Rule is probably the most common rule followed to ensure best ventilation practice.3 It is simply:
The sea conditions – If, in adverse sea conditions, there is a risk of sea spray entering the ventilation openings to the cargo spaces, no ventilation should take place and the ventilator openings should be closed and sealed.
Inspections of the cargo spaces – The cargo spaces should be inspected regularly to check for signs of sweat (providing it is safe to enter).
Hours of darkness – Providing ventilation can and should be carried out, based on the above considerations, ventilation should continue to take place night and day. A failure to ventilate at night will probably be viewed unfavourably by a court or arbitration tribunal, unless
Expert advice – If there is any doubt about ventilation, expert advice ought to be sought.
Natural ventilation simply describes a system whereby ambient air is allowed to enter and leave the hold naturally, via trunking connecting the hold to and above deck level.
Mechanical ventilation describes a system whereby air movement is forced, usually by electrical fans that are fitted in the ventilation trunking. These can usually be operated so as to either draw air into or eject air out of the hold.
System features – In the hold there is usually two or more ventilation openings in the deckhead, one set forward, one set aft. This arrangement therefore only provides for surface ventilation of the cargo. It is less common for the trunking to extend down the bulkheads, so as to provide through ventilation (not normally required for bulk cargoes). Above deck the trunking may stand alone or may be built into part of a deck structure, e.g., the hatch coaming. Whatever the system, every ventilator should be provided with a means of closing, e.g., a screw-down mushroom cover, a flap or door. If the on-deck trunking is on a swivel arrangement, the vent opening can be turned to face forward when safe to ventilate (so as to maximise air intake), or aft when there is a risk of sea spray. The trunking arrangement may serve holds on an individual or common basis. Where cargoes with different ventilation requirements are stowed in different holds, care will need to be taken to arrange stowage and ventilators correctly.
Ventilation in practice – With natural ventilation, the air will usually move from forward to aft, with the movement of the vessel. Mechanical ventilation can aid this movement by setting fans to draw in forward and eject aft. It is important to avoid short-circuiting the air movement, e.g., via a hold opening between the ventilators. Fans can usually be operated at various speeds, so for example when vigorous ventilation is necessary high speeds can be used. If, despite ventilation, ship’s sweat is still occurring, the hatch covers can always be opened. Opening the hatch covers may also be necessary where there is an air pocket in the hatch square, caused for example by a high cargo stow.
System checks – Some points to check with ventilation systems:
De-humidifiers – On some ships fixed or portable de-humidifying units may be fitted in the holds. These act to remove moisture from the hold air and therefore control the dew point.
Charterparty – The ventilation system should be properly described in the charterparty and care should be taken to avoid accepting ventilation requirements, which may be difficult or even impossible to comply with. It is also a good idea to obtain written acknowledgment from the shipper where the vessel is only fitted with natural ventilation, particularly on voyages where ship’s sweat is expected. The shippers are then on notice of the vessel’s ventilation limitations.
TEMPERATURE MEASUREMENTS AND VENTILATION RECORDS
Records – As mentioned in the introduction, records are essential to the carrier’s defence of cargo claims. It may be that, on some voyages, sweat can not be prevented. For example, ship’s sweat resulting from ventilation being restricted due to say fog or heavy seas on deck. In those circumstances it may be necessary to evidence that the vessel did ventilate as much as it reasonably could, when it could. On the other hand, moisture damage to the cargo may be due to the inherent nature of the cargo itself, e.g., pre-shipment moisture content, and the carrier may need to evidence that when the cargo was ventilated it was ventilated correctly. It is important to bear in mind that, even if poor ventilation practice has only contributed to cargo damage, with the predominant cause being, for example, inherent vice of the cargo, the carrier faces the difficult, if not impossible, task of having to prove the extent of damage caused by the vessel’s wrong doing so as to avoid liability in full.
It is strongly recommended that a separate ventilation log be drawn up where the observations for each hold are recorded. A suggested proforma is set out below.
In addition to the ventilation record, it is also important to take and record the bilge soundings, as these too will be evidence of moisture within the hold. Obviously, the bilges should be dry before loading.
VENTILATION LOG – HOLD 1
Increasing the risk of sweat – Improper stowage can increase the risk of sweat. For example, stowage of a wet cargo in the same hold as dry cargo. Careful stowage planning is therefore important, but so too is the inspection of cargo before and during loading. A normally dry cargo may be wetted ashore, e.g., due to rain or snow, and it is therefore important to stop such cargoes from being loaded, either to be replaced or satisfactorily dried. Materials used in the stow, e.g., dunnage and shoring, should also be dry.
Moisture migration – Moisture migration, which is the movement of moisture within the cargo stow, is also worthy of mention. For example, moisture will migrate within a bulk cargo of grain to the outer parts of stow when these parts are cooled, e.g., because the ship’s steelwork is cooled by falling air and sea temperatures. The physics at work in this phenomenon are beyond the scope of this article, and it is considered sufficient to appreciate the importance of good stowage to remove excess moisture at the extremities of the stow.
Aiding ventilation – Stowage can go a long way to aid ventilation so as to avoid sweat. A very good example is when ventilation channels are employed in the stow. These can be very effective in preventing ship’s sweat where hygroscopic cargoes are carried from warm to cold climate. Ventilation is necessary to replace air in the hold made moist by the cargo, with drier air from outside. For this reason, stowage channels are often employed with bagged rice cargoes, but the effectiveness of these channels very much depends on the actual arrangement. Some arrangements are better than others and expert advice should be sought.
Protecting the cargo from sweat/moisture – As already discussed, some sweat (notably ship’s sweat) may be unavoidable, particularly at the sides and bottom of the hold where ventilating air can not reach or reach in sufficient quantity. Whilst this is not relevant for bulk cargoes, it is highly relevant for bagged and other packaged/unitised cargoes. In terms of trying to prevent cargo being damaged by sweat, dunnage is key. As already mentioned, it is important that the dunnage is dry, otherwise it defeats one of the very objects for which it is used. Probably the most common dunnage is kraft paper, but this has its limitations. For example, it provides only a thin barrier between the cargo and any moisture and can also easily be torn. A good example is when paper is used to cover the sides of a hold with internal frames (or a hold with corrugated bulkheads). The weight of the cargo often pushes through the paper spanning the frames, with the result that cargo comes into direct contact with the side of the hold between the frames. This also adversely affects the productiveness of the ventilation channels provided between the frames. In view of these problems it is recommended that kraft paper be used with other (sturdier) dunnage material or cargo battens.
Bamboo sticks and mats have commonly been used for shipments of bagged rice from places such as Thailand. The experience of some operators is that this material is less effective than kraft paper, used with, for example, plastic. This may be because the bamboo already contains some moisture and/or because, compared to paper, it more readily gives off moisture that is absorbed from sweat. It may also be that bamboo provides less of a barrier to cold air contacting the extremities of the stow, and which can be relevant to moisture migration.
On the tank top, dunnage should be aligned so that moisture can flow easily to the bilges. The dunnage should also be strong enough to withstand compression forces. For example, the weight of a steel cargo is often so great that the timber floor is compressed. As a result, the barrier between the cargo and any moisture on the tank top is greatly reduced.
Charterparty terms – It is fair to say that, in today’s world, shippers generally make it more difficult for the ship in terms of preventing damage caused by sweat. The cheaper stowage option is often chosen and from time to time cargoes with excessive moisture contents are hipped. Owners should therefore be alive to the risks of sweat when arranging fixtures and try to ensure that the charterers are made responsible for stowage. As already mentioned, onerous ventilation requirements should also be avoided.
VENTILATION FOR OTHER PURPOSES
This article has been produced with the kind assistance of John J Banister Ltd, Marine Surveyors and Consultants.