Cargo Damage Prevention During Ocean Transportation
1. Types of cargo damage reasons
To prevent cargo damage, we must first know the reason, and we must know the process and details of the formation of these reasons, so that we can correctly and deeply understand the reason of cargo damage and take targeted measures. There are many reasons for cargo damage found on the ship, which can be summed up as follows:
① Damaged before shipment or there are latent damage factors;
②Damage during loading and unloading operations;
③The conditions of the loading site do not meet the requirements;
④Improper stowage on board;
⑤Improper storage after loading, during the voyage and before unloading;
⑦Other accidents are in danger;
2. The principle of cargo hold ventilation
1. How does the water vapor in the air condense into water droplets?
Ventilation is one of the main measures for the crew to keep the cargo. Some people think that ventilation is better than nowhere. This statement is not always correct. Cargo compartment ventilation is not simple. In different situations, there are differences between necessary, unnecessary, forbidden, impossible to ventilate, and even slow (permeable) ventilation and accelerated ventilation. To correctly understand and truly grasp the timing of ventilation, you have to spend some time first to understand the objective natural phenomenon of water vapor in the air that is invisible to the naked eye condensing into water droplets that can be seen by the naked eye (the folks call dew, the crew calls sweat). At a certain temperature, the water vapor content in one cubic meter of air (that is, the absolute humidity, from now on) reaches saturation when it reaches a certain level, and it condenses into water droplets at this time. In the above-mentioned saturated state, we call the relative humidity at that time 100%; if the water vapor content is only half of that at saturation, the relative humidity is 50%, so that different relative humidity can be derived. It should be noted that no matter how high (or low) the temperature is, the water vapor content in the air must be a little more (or less) in order to (or be able to) reach saturation and condense water. Therefore, when the cargo is loaded in a port with high temperature and high relative humidity, sailing will begin immediately. If the water is not ventilated along the way and the temperature is very low, the water vapor in the cabin air is likely to condense into small water droplets on the cold hull plate. . On the contrary, when the cargo is loaded in a port with very low temperature, sailing will start immediately. If there is no ventilation along the way and a port with high humidity and relatively high temperature, if the cabin is suddenly opened or strong ventilation, the water vapor in the air outside the cabin will be cold. Condensed into small droplets on the surface of the cargo. Since at a certain temperature, the water vapor content per unit volume of air will condense into water droplets when the relative humidity is 100% when it reaches a certain amount; conversely, when a certain content of water vapor per unit volume of air drops to a certain temperature, it will Condensed into water droplets, this temperature is called the dew point temperature. The dew point temperature can be read from the instrument, or it can be found in the table (omitted) with the dry and wet temperature as two parameters.
2. What is the purpose of cargo hold ventilation?
There are 3 purposes in total to prevent cargo damage and the safety of people, ships and cargo, namely:
①Dehumidification, specifically to reduce the relative humidity, that is, to avoid condensation or sweating, and some goods are damp and moldy, heat spontaneously burned, and accelerate corruption;
②Cooling, its effect is the same as ①;
③Exclude dangerous gases and supply fresh air to prevent explosion and poisoning accidents.
3. What are the reasons for the complexity of cargo hold ventilation?
Cargo hold ventilation generally has a good effect, but sometimes it has a bad effect. Some of these complex factors are difficult for shore personnel to appreciate, such as:
①Ventilation is best if it can cool down temperature and humidity, but sometimes it will heat up when the humidity drops, or increase the humidity when the temperature drops; and general goods are afraid of temperature and humidity, but most of them are afraid of humidity and not too afraid of heat, but excessive It doesn’t work if it’s hot. If you can’t take care of it, you can only dehumidify first. In addition, many goods will become hot after being wet, so you must consider dehumidifying first. Only a few goods are not afraid of heat and humidity, so they can be ventilated and cooled, or even sprayed with water. Therefore, different types of cargo, different climates and weather zones, and ports should be treated differently in terms of ventilation. This is one of the reasons for complexity.
②There are different temperatures everywhere in the cargo hold, which not only affect each other but also rise and fall due to different factors. For example, the temperature changes with the outside air temperature during ventilation and the heat emitted or absorbed by the cargo in the cabin; the surface temperature of the cargo changes with the air temperature, temperature, cargo characteristics and water content of the loading port; the temperature of the hull plate changes with the seawater temperature; the deck The temperature changes with temperature, sun, waves, snow, and icing; the temperature of the bulkhead and the bottom of the tank changes with the temperature of the fuel adjacent to the fuel tank, and the temperature of the space with the engine room changes. Therefore, when the warm and humid air enters the cabin, some places in the cabin will condense dew, and some places will not condense. This is the second reason for complexity.
③Theoretically, cargo hold ventilation is equivalent to two groups of air exchange places with different temperatures and humidity, but in fact the ventilation process is by no means that simple. The low-temperature and dry outside air enters the cabin and pushes the high-temperature and high-humidity air out of the cabin. It's a great thing, but if you enter too fast and too violently, and two groups of air with different temperatures and humidity meet in a narrow path, the nearby places will condense water drops instead, and good things turn into bad things. This is the third reason for complexity. In short, cargo that is generally afraid of moisture needs to be ventilated when the dew point temperature of the air outside the cabin is lower than the surface temperature of the cargo, which is also the basic condition for ventilation.
3. Ventilation of Sangu
Sangu is difficult or impossible to ventilate in the sea, even without ventilation, which is harmful. This is practical experience.
1. Difficult to ventilate
Considering that the movement of bulk grain in the cargo hold will cause the ship to tilt and lose its stability and cause a vicious shipwreck accident, SOLAS has strict regulations on bulk grain loading. The most important thing is to fill the bulk grain into the cargo hold to eliminate or minimize the free grain surface. . The pressure tanks and high hatch coamings on both sides of the bulk carrier reflect this design purpose. A funnel is also set up in the upper tank, if the scattered valley slides down, to fill the space of the lower tank caused by the difficulty of trimming or the sinking of the scattered valley on the way. If there are too many free grains, it is necessary to use bagged loose grain cover to press, or to cover with cloth and then press it with wooden strips and tie it with ropes to prevent loose grains from moving. Therefore, it is difficult to ventilate the cargo hold or its surface is sealed.
2. No ventilation required
Although scattered grains will sink on the way to form a small area of free grain, these small areas are too small compared to the large grain volume, and the heat of the bulky cargo is difficult to radiate from the surface of the tiny area. But this is just the "no ventilation" proposed in the helpless situation.
A bulk carrier that was just received from a foreign shipyard was used to transport wheat from Canada and Australia to China. The crew did not expect that the lower openings of the ventilators were installed in the box beams below the main deck and above the transverse bulkhead, instead of passing through. For cargo holds, unless the access door of the box beam is opened. The cargo hold ventilation in the past six months was actually in vain, but the bulk wheat transported for four voyages did not suffer any cargo damage. In addition, there are also liquid bulk carriers that carry bulk valleys internationally. Due to the structure of the ship, the tanks of these ships do not have a through tube. If the small hatches and thin air pipes of the small tanks are not used, it will not work. It is not suitable to open on the sea for ventilation, but this type of ship was often used to carry corn from Argentina to Europe in the 1960s and 1970s, but there was no damage to the cargo due to lack of ventilation. But this only means that the experience gained from practice does not require ventilation.
According to the research by experts of the UK Shipowners’ P&I Association, corn with moisture content higher than the limit for safe transportation will cause heat and then mold, regardless of ventilation or non-ventilation. The main cause of cargo damage is the original moisture content of the scattered grains rather than lack of ventilation. In developed countries, dust and moisture must be removed when the bulk grain is loaded into the silo, so it is more difficult for the bulk grain to cause cargo damage due to lack of ventilation during the shipping process. This shows that the dry scattered grain does not require ventilation. For sea transportation, the moisture content of each batch of loose grain in a cargo hold is below 14%, which is a good condition.
3. Ventilation is harmful
Then, does the wet scattered valley need ventilation? If the scattered valley is relatively humid, the air in the void below the most surface layer of the scattered valley is hot and humid, and the cooler fresh air outside the cabin enters the cabin, the water vapor will condense into water droplets on the surface scattered valley, and the surface scattered valley will become moldy. It even clumps into layers. This layer will further seal the heat and humidity of the lower valley, so ventilation cannot improve the situation and may be harmful. But it does not mean that ventilation should not be absolutely necessary. When the temperature inside the cabin is higher than 2.5°C outside the cabin and the relative humidity inside the cabin is much higher than outside the cabin, it can be ventilated if the weather permits. The best first step is to ventilate slowly. , Such as opening the access cover and access door of the cargo hold and adopting natural ventilation, and then speed up the ventilation.
4. Loss of bulk grain cargo caused by heat in the ship
In the past, the bulkheads and bottoms of most ships adjacent to the cargo hold and engine room or fuel oil tank were covered with wooden guards, but nowadays, wooden guards are rarely seen in the cabins, and it is difficult for a wooden board to be washed after washing. Dry; Second, it is prone to insects, and often because of this, the inspection cabin cannot be passed, and loose grain cannot be loaded. However, frequent bulk cargo damage is also caused by this. Although there are thick wood panels on the bulkheads adjacent to the daily fuel tanks in the cargo hold of a ship and the engine room, the sounding pipes of the double bottom tank underneath are on the bulkhead (welded with a horse pan) , The heat of the precipitation and the daily fuel tank is transmitted to the sounding tube through such small iron horse pans, which can actually make the scattered grain wheat around the sounding tube agglomerate and stick to the sounding tube during long-term heating. on. This is what happened on the author's ship. Fortunately, the amount of damaged wheat is very small, only five or six lead barrels in total. After opening the hole cover of the sounding tube on the deck, I felt that there was hot air coming out of the tube hole. As far as practical experience is concerned, it should be noted that the temperature of the fuel tank is higher than that of the engine room, because the fuel tank sometimes needs to be heated; the heating temperature and heating of the precipitation tank and the daily tank in the fuel tank The time is higher and longer than other oil tanks. Therefore, as far as possible, the required fuel should be transferred to the fuel tank not adjacent to the cargo tank before loading. If it is necessary to transfer the fuel in the fuel tank adjacent to the cargo tank after loading, it is also advisable to minimize the heating time and refueling.温温。 Temperature. If there are two daily use and settling tanks, only the one that is not adjacent to the cargo hold is used, because the daily use and settling tanks must be heated all the time.