Fire and explosion risks posed by carriage of scrap metal

NorthStandard P&I Club explains risks posed by carriage of scrap metal
NorthStandard P&I Club explains risks posed by carriage of scrap metal

The NorthStandard P&I Club presents the risk involved with the carriage of scrap metal, as explained by Dr Neil Sanders of Burgoynes Consulting Scientists and Engineers.

According to NorthStandard, bulk carriage of scrap metal (iron and steel) bring risks involving heating, flammable gas production and fires. Dr Neil Sanders of Burgoynes Consulting Scientists and Engineers shares the potential problems with all types of scrap ferrous metals in this article.

1 Self-heating, possibly to ignition.

In practice, self-heating has occurred in cargoes that were declared as scrap metal, Group C, as well as in ferrous turnings etc., Group B. Self-heating occurs because iron and steel oxidise (rust) by reacting with oxygen in air, or oxygen in water. Oxidation reactions produce heat, which tends to be retained due to the insulating effect of surrounding cargo. The oxidation reactions become exponentially faster at higher temperatures, so self-heating can worsen, sometimes to the point of ignition, depending on the circumstances and actions taken.

Freshly exposed iron and steel surfaces have a high tendency to oxidise, whereas ‘aged’ surfaces that are already oxidised react more slowly, or insignificantly. Fresh surfaces may occur because the metal has been recently shredded, or because it has been stored after shredding in a way that avoids exposure to air, e.g., within a large stockpile or coated in oil that drains off.

2 Hydrogen, produced by water oxidation of scrap iron and steel

Hydrogen is highly flammable over a wide range of concentrations of hydrogen and air/oxygen. It is also extremely easy to ignite, odourless and colourless. We have seen cargoes declared as scrap metal, Group C, producing problematic amounts of hydrogen which, if allowed to build up and ignite, would cause an explosion in ship’s holds.

Although the IMSBC Code entries mention trying to keep these cargoes dry, in practice scrap iron and steel cargoes are often stored in outside stockpiles in the rain, and terminals use water sprays to suppress dust. Therefore, iron and steel scrap may be wet, potentially producing hydrogen.

Seawater has a greater tendency than fresh water to oxidise iron and steel, producing hydrogen, but fresh water can also produce hydrogen, depending on the circumstances. The IMSBC Code does not indicate measuring gases in holds, so hydrogen production may go unnoticed.

3 Combustible materials in the cargo, such as cardboard, rags, plastics from recycled items, oils, and flammable gases from cylinders or aerosols.

Some of these combustible materials can be ignited at temperatures from roughly 250°C upwards, which is much lower than the ignition temperature of the scrap iron and steel. Fires in scrap metal (Group C) cargoes usually involve combustibles in the cargo and only rarely the scrap metal itself.

4 Ignition sources in the cargo, such as batteries and cylinders with flammable gases.

These items are of course contaminants, but they may be present due to the source of the scrap, e.g., household recycling. They may cause ignition during handling or shifting of cargo, or spontaneously. Burning material may also be loaded with the cargo.

5 Asphyxiating atmospheres due to oxidation of iron and steel cargo removing oxygen from the hold gases.

Oxygen may also be removed from adjacent spaces, if there are any gaps in bulkheads. In the case of self-heating or fire, carbon monoxide and carbon dioxide will be produced, which can also cause asphyxiation. Therefore, thorough risk assessments and precautions are needed before entering holds containing iron and steel scrap, and the same applies to adjacent spaces.

Dealing with scrap iron and steel cargoes and incidents

According to Dr Neil Sanders, the following general points may assist, although no two incidents are the same and each must be assessed to determine the appropriate actions.

1 Measuring hold gases daily should help to identify any reduced oxygen levels, hydrogen, other flammable gases, and carbon monoxide, although gas measurement is not mentioned in the IMSBC Code. Carbon monoxide is a good indicator of smouldering and burning. Gas samples should preferably be drawn from hold sample points, with the sampling tube inserted at least 1 metre into the hold ullage to avoid drawing fresh air into the gas monitor.

Typical ships’ gas monitors are often only effective at measuring flammable gases down to about 10% oxygen. Below that level, a different type of flammable gas sensor will be needed, or a splitter to mix the hold gas with fresh air. With a splitter, of course the true readings must be calculated appropriately, also using readings taken without a splitter.

Some ship’s gas monitors use infrared (IR) sensors for flammable gases, which do not register hydrogen. This could lead to missing a dangerous situation. Therefore, gas monitoring for scrap iron and steel cargoes requires correct equipment and correct usage, which will be difficult to arrange if the vessel is already at sea.

2 Ignition sources should be avoided such as smoking, hot work, paint chipping etc., due to the possibility of flammable gas production, although this is not mentioned in the IMSBC Code. This applies unless a ‘gas free’ situation has been confirmed and is re-checked appropriately over time.

3 In the case of self-heating and / or gas production, the correct approach will depend on the situation. For example, production of excessive amounts of flammable gases e.g., hydrogen will mean that natural ventilation is required to keep below the lowest gas level that can allow an explosion, plus a safety margin, such as a maximum of 20% or 50% of the lower explosive limit (LEL). On the other hand, allowing more air (oxygen) into a hold through ventilation will tend to worsen any self-heating or fire. The correct actions will depend on factors such as the gas readings and temperatures.

4 In the case of a fire, the correct approach again depends on the circumstances. Scrap iron and steel in relatively large pieces (tens of millimetres and above), without finely divided material such as turnings, will usually not ignite, except in extreme circumstances. In this situation fires tend to involve combustible materials within the scrap such as cardboard, plastics, oils etc.

Fires can often be controlled via the hold being closed and un-ventilated to limit oxygen. This is easier if the proportion of combustibles in the cargo is not very high.

However, flammable gases can build up in a hold in a fire situation, or they may exist already e.g., due to hydrogen production. Low oxygen levels in a hold, due to oxidation reactions of the cargo or fire, may prevent explosive mixtures of flammable gases while the hold remains closed. Even so, opening up will introduce air and may risk producing a flammable mixture when air mixes with hold gases, if the flammable gas concentration was high to start with. Therefore, gases should be measured with appropriate equipment, and an assessment made about the correct approach.

Boundary cooling has a limited benefit, if any, for fires in holds. For the cargo itself, the IMSBC Code indicates that water should not be used for firefighting at sea with ferrous metal turnings etc. (Group B), but copious quantities of water may be used in port, having proper regard to the ship’s strength and stability. The entry for scrap metal (Group C) does not mention water. Using water to fight fires in scrap iron and steel cargoes could produce hydrogen and/or its weight could cause strength and stability problems, so it should be considered properly beforehand.

If fuel tanks are adjacent to holds on fire, then consideration should be given to filling them completely (“pressing them up”), to exclude air and prevent fire or explosion in the fuel tank. In extreme cases it may be better to fill fuel tanks with water completely, rather than run the risk of fuel igniting, but usually filling with water can be avoided.

If a fire cannot be controlled without an unacceptable build-up of flammable gases in a hold, then the only alternative may be to discharge the cargo. Once discharged, it can be spread out in a layer ashore and doused with water to deal with fire and self-heating.

After controlling a fire in a hold, some smouldering may continue, even if the oxygen level remains low. Therefore, suitable precautions should be put in place before opening a hold, to deal with any flare-up that may occur.

If a fire involves finely divided iron or steel, such a turnings, then the approach may be similar to that above, but the fire can potentially burn very hot. The same potential issues of flammable gas build-up, explosion risks and ventilation apply, particularly if hydrogen is produced by oxidation with water.

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