Reasons for Ventilation:
A large number of cargo claims are made for goods transported by ships which have been damaged in transit. Much of this damage is caused by either ‘ships sweat’ or ‘cargo sweat’ and could be effectively reduced by prudent ventilation of cargo spaces.
Cargo or goods carried at sea are exposed to the atmosphere which is high in moisture content. Due to the very nature of the trade, cargo damage due to moisture continues to be a cause for concern to all parties in the trade. Very heavy cargo claims due to damage to cargo caused by moisture continue to be made.
Between 1996 and the year 2000 claims accounted for in excess of 2 million US dollars in only one P&I club.
Cargo ventilation, as statistics indicate, appears to be the most neglected or least understood aspects of cargo care & cargo operations. It is also one such activity that does not attract the attention it deserves and more often than not it is too late by the time the consequences are realised.
Formation of Sweat:
Sweat is caused by the condensation of moist saturated air on a cooler surface.
It is formed when water vapour in the air within the hold condenses out into water droplets once the air is cooled below its dew point. The water droplets may be deposited onto the ship’s structure or onto the cargo.
Some definitions are useful in understanding sweat.
Dewpoint: Is defined as the temperature at which air cannot absorb any more water vapour; i.e. it is saturated. In simple language if the air parcel falls below its dew point temperature, then the moisture within the air parcel will condense.
Saturation: Means that a parcel of air has absorbed its maximum amount of moisture for that temperature, however raising the temperature will allow for more moisture to be absorbed.
Relative Humidity: Is the ratio between the actual moisture content and the maximum moisture content that the parcel of air can contain.
Hygroscopic cargo: Is cargo, which can absorb or give off moisture. They are mainly of plant origin. Since they may retain or absorb water, excessive amounts of inherent moisture may lead to significant self-heating and moisture migration within the cargo. This will result in cargo getting caked or rotting.
Examples of Hygroscopic cargo: Rice, flour, grain, coffee, tea.
Non-hygroscopic cargo: Is cargo, which does not absorb or give off moisture. Cargoes like steel are non-hygroscopic but are liable to damage if the atmosphere is moist.
This results when water droplets are deposited on parts of the cargo when the surrounding air comes in contact with a cool cargo. For this to happen, the temperature of the cargo must be lower than the dewpoint of the surrounding air.
This can also be explained as:
The condensation which forms directly on the cargo, when the temperature of the cargo is less than the dew point of the air in the hold.
This is generally associated with incorrect ventilation. That is introducing warm moist air into the hold with cold cargo.
It usually occurs when the voyage is from a cold region to a warmer place and the outside air has a dew point above the temperature of the cargo.
Cold cargo cools the air in contact with it and condensation takes place.
Is the condensation which forms directly on the vessels structure when the temperature of the ship’s steelwork is lower than the dew point of the air inside the hold.
Condensation so formed on the structure will then trickle down to the cargo.
This can occur when the ship moves to cooler climates (from a relatively warm area) and the ship’s sweat so formed can trickle onto the cargo or even accumulate at the bottom of the hold.
Control of Sweat by Ventilation:
The underlying principle is to keep the dew point of the air within the hold below the temperature of the hold structure and the cargo to stop the formation of ship and cargo sweat.
Continuous monitoring of the condition of the atmosphere within the hold is essential if correct ventilation and subsequent prevention of cargo damage needs to be achieved.
The reason for this is the ship and cargo move through regions with different air temperatures, different sea temperatures and different moisture content of the atmosphere.
Following is an example to explain sweat.
Assuming a cargo ship is passing through seawater at a temperature of 15°C; the steel structure in the hold would then also be close to that temperature. Now imagine if, inside the hold, some cargo was loaded in a cold country at a temperature of about 10°C.
If air from outside the hold at a temperature of 18°C and a dewpoint temperature of 13°C is introduced into the hold, cargo sweat will occur on the surface of the cargo because the air would then be cooled below its dewpoint. However, there will be no ship sweat because the air will not be cooled below its dew point however if the temperature of the ship structure were 12°C, then ship sweat would result.
The following information is necessary before deciding whether to ventilate or not:
- Temperature and dewpoint of the outside air.
- Temperature and dewpoint of the air inside the hold.
- Temperature of the cargo surface.
- Temperature of the steel structure (Ships) inside the cargo compartment and
- The moisture content of hygroscopic cargoes if applicable.
Whether to Ventilate or not to ventilate?
The following basic Dew Point rule provides relevant guidance which is:
- If the dew point of the air within the hold is higher than the dew point of the OUTSIDE atmosphere, then ventilate.
- If the dew point of the air within the hold is lower than that of the OUTSIDE atmosphere, then do not ventilate.
ANOTHER WAY OF EXPRESSING THIS IS:
- If the Dewpoint of the outside air is lower or equal to that of the hold atmosphere…….continue ventilation.
- If the Dewpoint of the outside air is higher than the hold atmosphere dewpoint ………do not ventilate with outside air.
A further extension of the basic rule must be examined depending upon the two basic types of cargo.
Warm region to Cold region: As the ship proceeds from a warm region to a cold region it will experience a gradual drop in the temperature and a drop in the dew point. The atmosphere within the hold will receive large quantities of moisture from the cargo. Hence in a situation such as this, ventilate vigorously during the early stages but eventually the outside Dewpoint will be too low.
Warm region to Cold region:
Ship sweat is inevitable but cargo unaffected unless, condensation drips back onto the cargo.
Cold region to warm region:
Not critical, therefore ventilation not essential. In some cases on opening hatches at the discharge port immediate condensation may form on surface but will dry off as cargo is discharged.
Cold region to warm region:
No Ventilation. Cargo sweat would occur on the surface of the cargo if relatively warm moisture laden air was admitted.
If due to any reason it is not possible to take readings within the hold then ‘Three Degree Rule’ can be used;
- During loading regular cargo temperatures should be recorded. Whilst on passage if the atmosphere dry bulb temperature is at least 3ºC cooler than the average temperature of the cargo when loaded, then ventilate the hold, if the temperature of the atmosphere is less than 3ºC cooler than the average temperature of the cargo when loaded, the do not ventilate the hold.
The holds of most dry cargo ships are ventilated by a mechanical supply and natural exhaust system. Here the object is to reduce the hold temperatures if necessary and prevent large amounts of condensation accumulating on the hull and cargo.
Often the cargo hold fans which are of the axial type are located in the mast houses, although they have been positioned in derrick posts where these posts are used to ventilate the tweens and holds.
- Natural Ventilation method
In the natural ventilation method, cowls are trimmed into the wind to take in outside air, and trimmed back to wind to allow the air circulation an exit from the hold.
Fans may be incorporated into this cowl ventilator system especially for the lower hold regions where fans assist delivery and air extractors assist the exhaust system. Cowls may also be fitted with manually operated closure flaps.
Forced Ventilation method:
If the dew point temperature in the cargo compartment can be retained below the temperature of the ships structure, i.e. decks, sides, bulkheads and the cargo, there would be no risk of sweat forming.
Such a condition cannot always be achieved without some form of mechanical (forced) ventilation from fans or blowers. There are several excellent systems on the commercial market which have the ability to circulate and dry the air inside the cargo holds.
Systems vary but often employ ‘baffle’ plates fitted in the hold and tween decks so that air can be prevented from entering from the outside when conditions are unfavourable. Systems re-circulating the compartment’s air can also operate in conjunction with dehumidifying equipment to achieve satisfactory conditions pertinent to relevant cargo.
Recent developments in ventilating systems have led to air being pre-dried before entering the hold. In some cases the temperature of the air as well as its humidity may be controlled before entering the compartment .This artificial or forced ventilation, has become increasingly popular because, when properly used, it can almost prevent any sweat damage to cargo.
Understanding monitoring Dew point temperature:
The vessel should closely monitor on a regular basis, the atmosphere within the hold and outside the hold by ascertaining the dew point. It should also record the sea water temperature.
The dew point of the atmosphere can be ascertained by use of the wet and dry thermometers in the Stevenson Screen on the windward bridge wing and a dew point table.
The dew point of the hold air temperature can be found by use of a whirling psychrometer which is a handheld device containing both a wet and dry thermometer.
The psychrometer is spun by hand until the temperature of the wet bulb stops decreasing and is stable. Successive readings should be taken until the readings are consistent. Then the wet and dry temperatures are used to ascertain the dew point using a dew point table.
In some instances it may not be possible to take readings within the holds. In such a case the ‘Three Degree Rule’ can be applied or used; for this the following is required.
During loading regular cargo temperatures should be recorded. Whilst on passage if the atmosphere dry bulb temperature is at least 3ºC cooler than the average temperature of the cargo when loaded then ventilate the hold, if the temperature of the atmosphere is less than 3ºC cooler than the average temperature of the cargo when loaded, the do not ventilate the hold.
Cargoes requiring special Ventilation:
COAL is very liable to spontaneous heating. If there is sufficient oxygen available, combustion is liable to take place.
The amount of heating that takes place depends on the type of type coal and how much heat can be dispersed by ventilating the coal. Ventilation can be a double-edged weapon as although it takes heat from the coal it also allows unwanted oxygen into the coal. To keep the coal as cool as possible it should be stowed away from hot bulkheads. To keep oxygen away from the coal only surface ventilation should be allowed.
Freshly mined coal absorbs oxygen, which, with extrinsic moisture, forms peroxides. These in turn breakdown to form carbon monoxide and carbon dioxide.
Heat is produced by this exothermic reaction causing further oxidation and further heat. If this heat is not dissipated ignition will occur. This is called Spontaneous combustion.
As this is essentially a surface reaction the smaller the surface available for the absorption of oxygen the better. Every attempt should be made to prevent undue breakage of the coal whilst it is being loaded. It may be noted that 1 MT of coal in an unbroken cube has a surface area of about 3.72m2, whereas if it is broken up to pass through a 1.5mm mesh screen its surface area is nearly 4000m2.
If a large amount of breakage occurs the small coal with the large surface area is found in the centre of the hold, whilst the large coal will roll down the sides. This aggravates the situation, as the large coal gives a good path for air to flow to the smaller coal where the spontaneous heating is most liable to occur.
Most coal fires in cargo occur at about ‘tween deck level and this is the area where the greatest attention should be paid to temperature and the restriction of through ventilation.
The following are recommendations for the carriage of coal.
The ventilators to the lower holds should be so arranged that they might be opened or closed at will during the voyage.
As the critical temperature at which the process of spontaneous heating in coal becomes greatly accelerated is in some varieties of coal as low as 36˚C, and generally is not much higher, the need of keeping the exteriors surface of the hull, and thereby the interior of the ‘tween decks and holds, as cool as possible is manifest.
The iron decks of ships carrying coal in the tropics can be covered with dunnage to lessen heating.
Suitable means should be provided for ascertaining from time to time the temperature of the lower mass of coal, particularly below the hatchways, and this might be done by means of two pipes leading down to the bottom of the coal at each hatchway.
The temperature tubes should have closed ends to prevent admission of air into the cargo. The temperature of the coal at three heights should be taken daily.
Gas from the holds or ‘tween decks space may find its way into shaft, peaks, chain lockers or similar space unless the bulkheads and casings are maintained in gas tight conditions.
Naked lights should not be used in holds or other spaces in which gas may accumulate until the spaces have been well ventilated.
Full use should, when necessary, be made of the breathing apparatus or smoke helmet and the safety lamp, which form part of the ship’s statutory fire appliances.
The employment of the crew in chipping and painting below decks during the voyage should be avoided. The danger from smoking should be realized and no oily waste, wood, old rope, sacking etc. should be left below where it can become ignited by spontaneous heating
On arrival at the port of discharge the hold ventilators should be unplugged and the lower hold well ventilated before commencing to work cargo.
List of cargoes which generate heat, gases and moisture:
- Wood, paper pulp and agricultural products may result in rapid and severe oxygen depletion and formation of carbon dioxide.
- The IMSBC code lists the following cargoes as potentially oxygen depleting: coal, direct reduced iron, sponge iron, sulphide concentrates, ammonium nitrate based fertilisers, linted cotton seed. Various gaseous products are formed including carbon monoxide, carbon dioxide, hydrogen sulphide and hydro carbons.
- Some cargoes like coal and sulphur can cause severe damage due to corrosion.
- Common cargoes like coal, sulphur, cotton, and fishmeal are liable to spontaneous heating. Coal also emits methane which is a flammable gas. When mixed with air it can form an explosive mixture.
- Dust created by certain cargoes may constitute an explosion hazard. Sulphur dust can readily ignite causing an explosion.
- Friction between cotton bales can cause spontaneous combustion and produce heat.
- Ammonium nitrate-based fertilisers support combustion. If heated, contaminated or closely confined, they can explode or decompose to release toxic fumes and gases.
- Metal sulphide concentrates. Some sulphide concentrates are prone to oxidation and may have a tendency to self-heat, leading to oxygen depletion and emission of toxic fumes. Some metal sulphide concentrates may present corrosion problems.