|Course of self-heating processes in feedstuffs of vegetable and animal origin containing residual oil||[German version]|
– by Capt. R. Becker, Hamburg 1996 –
|10° to 35°C
Product comes on board at port outside air temperature or at a higher temperature because it has not been cooled to the outside air temperature after oil and starch extraction.
In tropical ports, temperatures of 25° to 55°C may occur in the products to be loaded. Where the internal moisture content of the product exceeds the equilibrium moisture content of the product at an external relative humidity of 75%, heat- and moisture-producing biotic activities (nutrition and multiplication) of microorganisms must be anticipated. Mold formation may be visible on the surfaces.
In all organic goods, a relative moisture content of 75% promotes the onset of microbial biotic activity, because the nutrients are then available in sufficiently dissolved form in water for absorption through the cell walls of the microorganisms.
Microbial biotic activity in the temperature range of from 15° to 45°C may lead to spoilage of the product by rotting, fermentation or decomposition. At these temperatures, spoilage is not caused by the heat generated, but it is promoted by the water released by the microbial activity.
|30° to 40°C
The incipient biotic activity of the microorganisms is at its most vigorous (biological activity with release of moisture).
|from approx. 40°C
In addition to microbial biotic activity, the heat released in conjunction with microbial activity promotes oxidation of the unsaturated fatty acids in feedstuffs containing residual oil, which, like any oxidation process, is associated with evolution of heat (chemical oxidation).
|from approx. 45°C
Marked temperature increase resulting from a combination of the biotic processes of the microorganisms and incipient chemical oxidation processes of the unsaturated fatty acids.
Still no great decline in product quality. First of all, the nutritional value of the proteins is impaired. Incipient brownish discoloration of the product.
Critical temperature. Continuous temperature monitoring required. If the temperature remains constant, there is no danger of further spoilage due to heating. In a fully packed ship’s hold, there is no possibility of cooling the cargo in transit, other than by surface ventilation. In order to avoid moisture damage on the surface of the cargo, ventilation must not be performed with cold external air. The ventilation system must be switched to return air.
However, if the temperature of the cargo continues to rise, there is a risk that it will rise to the high self-heating temperatures of 75°, 80° or at most 90°C.
The cargo can no longer be left to its own devices.
The heated hold must now be sealed to stop air entering from outside.
The self-heating process is manifested externally by the emergence of white steam from the heated hold. Later, darker discoloration occurs together with an acetic „burning smell“.
If the hatches and other openings allowing ingress of air into the hold containing self-heated cargo are sealed, it is not necessary to call at a port of refuge to unload the heated cargo.
The ship can call at the destination ports for the cargo in the intended sequence, even when the heated cargo is destined for the last unloading port.
In such cases of self-heating, individual hot spots will generally have formed in the stowage. All vegetable and animal products containing residual oil are poor heat conductors with poor upward heat dissipation. Any heating of an organic product is associated with the release of water and a reduction in the moisture content of the commodity. Moisture released in the form of water vapor rises upwards through the cargo. The water vapor condenses in the air space between hatch cover/weather deck and cargo surface and forms a fire-extinguishing buffer above the cargo in the form of steam or water.
|55° to 85°C
Critical cargo heating range
|from approx. 75°C
After the preceding overlap between biological and chemical evolution of heat, these temperatures result in death of the microorganisms and the end of their biotic activity. Microbial spoilage of the cargo stops. No further temperature increases occur above 90°C (longish temporary plateau).
Previous substantial evaporation from the dried-out cargo temporarily inhibits the course of the already incipient chemical oxidation.
Once the microorganisms have died through the action of heat, the heat potential with its thermal energy is retained and promotes further change in the product. The cargo assumes a dark brown color.
After the temporary plateau, the chemical reactions in the cargo are resumed with renewed vigor, resulting in further heating.
Visible release of white steam from the cargo through any openings remaining in the hatch, ventilation and access areas. The increasingly obvious effects of cargo heating may easily give those on board the incorrect impression that there is a fire in the hold, an impression which is reinforced by the unpleasant „smell of burning“ likewise arising.
The increasing release of steam from the cargo is accompanied by a temporary, longish plateau in temperature increase. The cargo itself becomes drier owing to the release of moisture. Where air is excluded from the hold or the supply of oxygen is reduced, dry distillation (or pyrolysis = chemical decomposition of solid substances by heat) occurs in the cargo and any wooden dunnage which may be present.
Organic goods (wood, jute bags) turn black (carbonization) without any fire in the cargo.
If no spoilage has previously occurred on the surface of the cargo due to excessive moisture levels (mold formation), goods healthy in odor and color may be observed in the surface area of the cargo when the hatch is opened.
|from 100° to approx. 150°C
An unpleasant pungent smell penetrates to the outside as a result of protein decomposition. The steam starts to become discolored. Once organic goods reach a temperature of approx. 80°C, colorless volatile gases with an ignition temperature of as low as 60°C are released from the surface. These gases, which have a very low autoignition temperature, ignite when present in sufficient concentration and burn on the surface of the cargo with small bluish flames. However, they lack the energy to ignite the cargo. The small flames occur when the hatch is opened and the oxygen supply is resumed. The gases arising are a product of the dry distillation process (or pyrolysis = chemical decomposition of solid substances by heat) in the cargo. Small particles at the surface of the cargo may start to glow, but even then the ignition energy of the „glowing fire“ is insufficient to set the cargo fully on fire, even where the cargo contains crude fibers. If small flames or glowing occur during unloading of the cargo, a grab load is thrown onto the glowing cargo, in particular in the case of a cargo containing crude fibers, so as to smother the flames.
|from 150°C to approx. 200°C
Substantial release of gases, increased formation of smoke as consequence of continuing dry distillation. Rapid progression of chemical processes in cargo. Considerable losses in nutritional value.
|from approx. 230°C
Frequent localized glowing at cargo surface when exposed to oxygen from the air. The autoignition temperature of crude fibers and of dust particles on the cargo surface is reached. The glowing areas may be extinguished either by throwing material back down on them from the grab or by spraying them with water using a hose with a spray nozzle.
Where water is used as the extinguishing agent, it is best to avoid water from the harbor if a fire hydrant is available, since harbor water is frequently contaminated with salmonella and coliform bacteria.
The cargo and other organic components, such as wooden dunnage, may be further carbonized by continuing but slowing dry distillation (or pyrolysis = chemical decomposition of solid substances by heat).
Added to the existing foul smell of smoke, which gets into clothing, is a smell of charring.
|from approx. 280°C
Once temperatures exceed 250°C and especially 280°C, it must be anticipated that the autoignition temperature of the cargo dust will be reached. However, this is seldom the case when organic feedstuffs are transported by sea, since the heating plateau which occurs at approx. 90°C continues for a relatively long time.
|from approx. 330°C
It must now be anticipated that the autoignition temperature of the cargo will be reached. However, this is seldom the case when organic feedstuffs are transported by sea, since the heating plateau which occurs at approx. 90°C continues for a relatively long time.
Once a cargo has spontaneously ignited, it burns with flames. If the hold is opened, open fire may break out. In practice, this very seldom occurs at sea and is highly unlikely if the supply air openings into the holds are properly closed.
A truly burning cargo leaves behind ash, which is the sole unmistakable proof that a cargo or other items has burnt.
Upon opening previously well sealed holds which have reached temperatures higher than 150°C, a thick, white-colored cloud of steam escapes.
Capt. Becker has measured temperatures of up to 160°C in heated cargoes of feedstuffs. As a rule, however, temperatures of up to approx. 90°C are found in heated feedstuffs containing residual oil.
The highest flame temperatures measured by Capt. Becker at the cargo surface on board were below 230°C. Capt. Becker has not so far been able to measure the temperatures of small flames produced by burning volatile gases, as these small flames are transient and do not burn constantly.
Such small flames are light in color and their appearance on the surface of the cargo indicates flaring off of the gases.
Self-heated feedstuffs with temperatures of around 90°C in an as far as possible airtight hold should not be considered critical. There are no hot spots in the cargo, which have to be sprayed. Intensive extinguishing methods using water should only be used for feedstuff cargoes which, exceptionally, have undergone true spontaneous combustion at temperatures of over 280°C.
Before opening the hatch cover of a hold in which self-heating phenomena have been observed, the temperatures in the areas of the bulkheads of adjoining holds must be carefully measured. The temperatures of the steel structures in the deck area of a hold with a heated cargo must be monitored. In order to measure temperatures in a hold located beneath a heated part of the weather deck, holes suited to the introduction of a thermometer must be drilled in the weather deck plates. Once the temperature has been measured, these holes must be sealed with wooden stoppers. Where possible, temperatures must be measured in hold access openings. Temperature measurements must be performed carefully and repeatedly and recorded.
The weather deck hatch cover must only be opened if
|pressurized fire-extinguishing hoses are ready on deck,|
|foam extinguishers are at the ready,|
|an overview of the distribution of temperatures in the hold area has been obtained by temperature measurement.|
Holds containing self-heated cargo must never be flooded from beneath, since large amounts of water in the hold impair the stability of the ship. Never use a high-pressure extinguishing jet on a heated feedstuffs cargo, since the heated cargo must not be made to swirl around.
Never jump down from the deck onto a self-heated cargo to measure temperatures, since cavities underlying the surface may cause the cargo to collapse.
Heating of cargo containing residual oil without previous self-heating
An engine room bulkhead which is not or is only poorly insulated on the hold side and whose engine room side constitutes part of a heavy oil tank heated to 60°C during a voyage, for instance, may heat feedstuffs containing residual oil to over 60°C, with the consequent onset of chemical self-heating processes in the feedstuff.
Cargo hold lighting left on during a voyage may have the same effect if the cargo is stowed too close to the light source.
Equipment for unloading and removing heated feedstuffs
Chain- or cable-operated steel grabs may be used to unload the cargo. Hydraulically operated grabs are unsuitable, since hydraulic lines are not capable of withstanding the relatively high temperatures of self-heated cargo without sustaining damage. The heated cargo may be unloaded from the ship in:
|steel lighters without wooden floors because of the risk of carbonization,|
|trucks with steel dumping bodies,|
|Once unloaded, cool heated cargo on open ground at the port. The heated cargo does not require spraying to cool it.|
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