Packaging problems in the container transportation

  • Detail

With the continuous deepening of China's reform and opening-up policy and the continuous development of economic construction, the acceleration of the process of global economic integration, especially China's formal accession to the WTO, the development of railway container transport of dangerous goods has accelerated significantly. From the perspective of the external environment of China's railway, compared with the world's advanced countries, China's railway gap is obvious. For example, Japan's original railway has eliminated the transportation of less than carload goods, including dangerous spare parts, in 1974. In terms of the current situation, China's container transportation is in an excellent period of development, and the container production has ranked first in the world [1]. At the same time, the extensive attention and investment in the railway container transportation of dangerous goods from all aspects will inevitably promote the rapid development of the railway container transportation of dangerous goods. However, China's railway container transportation of dangerous goods started late, with insufficient experience, backward transportation organization methods and loading and unloading equipment. In addition, the vast majority of China's railway dangerous goods are transported in ordinary containers, which have poor heat insulation and are greatly affected by temperature changes. Every year, due to the improper selection of packaging materials and packaging methods, container transportation accidents of dangerous goods occur frequently [2]. Although the railway has taken a series of measures and methods, it has not fundamentally prevented the occurrence of accidents. Therefore, this paper makes a detailed study on the packaging materials and packaging methods of the goods in the box

1packaging materials

there are many kinds of packaging materials used for dangerous goods, of which plastic is the most widely used. Such as various forms of plastic barrels, bags, bottles, etc. The main component of plastic is resin. Resin is a kind of polymer compound, which presents three physical states with different ambient temperatures: glass state, high elastic state and viscous flow state, as shown in Figure 1

plastics can only be used below the glass transition temperature and above the embrittlement temperature. When it is higher than the glass transition temperature, the plastic deforms greatly after being stressed (the elongation at break reaches 100% - 1000%), and the elastic modulus is very small (stiffness), so the glass transition temperature is the highest temperature for plastic use. See Table 1 for the use temperature range of several plastics commonly used in general dangerous goods packaging

when the ambient temperature is lower than the embrittlement temperature, the plastic becomes hard and brittle, which is the lowest temperature for plastic use. Therefore, when transporting dangerous goods in containers, it is necessary to put forward specific material requirements for the plastic packaging containers, gaskets, strapping belts, etc., and consider alternative materials if they do not meet the requirements [3]. Other packaging materials, such as wood, steel, paper, etc., do not have this problem

Table 1 applicable temperature range of several main plastics

ta B. 1 s e ve RA l Su ITA B Le TEM p e ra Tu re SCOP e of m a in p la s t ics

name applicable temperature range/℃

low density polyethylene (LDPE) - 50 ~ 65

medium density polyethylene (MDPE) 10 Turn off the power - 50 ~ 80

high density polyethylene (HDPE) - 50 ~ 120

polyvinyl chloride (PVS) - 50 ~ 75

polystyrene (PS) - 20 ~ 80

polypropylene (PP) 0 ~ 120

polyvinyl alcohol (PVAL) 65 ~ 85

2. The problem of cargo packaging method

for dangerous goods, the magnitude of its danger is closely related to the packaging method of goods in addition to the nature of the goods itself [4]. Therefore, the packaging method of dangerous goods must also be taken into account in determining the conditions for the entry of dangerous goods. In view of this, when determining the loading conditions of dangerous goods mixed with two materials, a lot of investigation and research have been done on the packaging of goods, especially absorbing the test and research results of the members of the research group on the packaging of railway dangerous goods for many years, and the relevant packaging tests have also been carried out in the laboratory

2. The practice of railway transportation shows that the danger of cargo transportation is closely related to the size of its packaging. For some dangerous goods, when their packaging volume is small, they can even be transported according to the conditions of ordinary goods. Therefore, for dangerous goods with small packages, when they are transported in containers. There should also be relatively loose requirements

generally, the packaging method with a unit quantity of no more than 1kg (or 1L) and a package of no more than 20kg is called small packaging. The packaging methods specified in the package numbers 18, 19, 20, 21, 22, 23, 24 and 25 in the packaging table of dangerous goods in Annex II of the dangerous goods regulations basically belong to small packages [5]. Dangerous goods with these packages (except explosives, compressed gases, liquefied gases, toxic substances, etc., which need special containers for dangerous goods or special containers for dangerous goods) can enter general containers for dangerous goods in principle

for small packages of dangerous goods, it can directly enter the general container of dangerous goods based on the following five reasons

1) the sealing performance of the package is good. Because the opening of the small packaging container is small, and the manufacturing process of the small packaging container is relatively fine, its liquid tightness and air tightness are also larger, and the packaging is better, so it can effectively prevent the leakage of the contents

2) packaging has a certain heat insulation effect. In addition to the inner container (package), the small package goods also have outer packaging or liner materials, and some also have medium packaging or absorbent materials. Because of the heat insulation effect of the outer packaging, the temperature of the goods in the inner container is always lower than the maximum temperature in the container. The measured data show that in the container, even the top-level goods that are most affected by temperature, their internal temperature is 1 ~ 2 ℃ lower than the upper space temperature of the container, that is, the temperature of small package goods is always lower than the inner wall temperature of the container, so they are relatively less affected by temperature and their safety is relatively increased

3) the outer packaging can absorb certain spills. The absorption of the outer package (or medium package) and the liner of small package goods makes the leaked liquid goods may be fully or partially absorbed in case of leakage of the inner container. The test shows that for ordinary corrugated paper, its water absorption can reach 1. 5% of its own quality 5~2. 5 times, while the usual corrugated box quality is 0 5~1. 5kg, its water absorption can reach 1 ~ 3L liquid. When 1 ~ 2 bottles of containers in the package are broken, basically all or most of the leakage can be absorbed. For wooden boxes or calcium plastic boxes, although their absorption of leakage is much smaller than that of corrugated boxes, they can still partially absorb leakage; Therefore, the harm caused by the leakage of small packaging is still much smaller than that caused by the large packaging of the goods

4) the outer packaging has a certain mechanical cushioning effect. Due to the cushioning effect of the outer packaging and liner materials, when the dangerous goods in small packaging are subjected to external forces (collision, friction or fall, etc.) during transportation, the external forces will not directly act on the inner container, so the leakage is also large, and the packaging will be much reduced; And only part of the inner container is broken, and usually the inner packaging will not be all broken. It can be seen that small packaged goods have a greater buffer effect on mechanical impact, and packaged goods should be safe

5) small packages have large internal pressure bearing capacity. The saturated vapor pressure inside the container will produce an outward vertical force on the inner wall of the container. For small packaging containers, the internal stress borne by the container wall is relatively small for larger packaging, so the safety of small packaging containers is relatively increased

since the steam pressure inside the container is evenly distributed on the whole surface of the container, the force on the inner surface is uniform. Compared with the container wall, the neck, shoulder and bottom of the container suffer less torque because of their short linear distance. Therefore, the stress condition of the container can be simplified to the stress condition of the vessel wall per unit width, as shown in Figure 2. The steam pressure perpendicular to the wall causes a certain bending deformation of the wall, that is, a torque converter on the cross section. Generally speaking, the bending moment varies with the position of the section, and the section where the maximum bending moment is located is the dangerous section. Obtain the bending moment equation according to figure 2

set point o as the coordinate origin, and X as the distance from any section on the bottle wall to point O, and then establish the equation. The bending moment equation is:

m = q, making China's polyaminoic acid at the international advanced level 2 (LX - x2) (0 x L) (1) when x = 0, M = 0; When x = L2, M = ql28; When x = L, M = 0

draw the bending moment diagram according to the bending moment equation. Figure 3 shows that the bending moment at both ends of the bottle wall is the smallest and the middle position is the largest. Therefore, the middle position of the vessel wall is a dangerous section. Because the size of the uniform load Q on the vessel wall is equal to its internal vapor pressure, and because a certain substance, at a certain temperature, its saturated vapor pressure is fixed, so the value is also certain. According to the formula of the maximum bending moment (i.e. the bending moment of the dangerous section), when q is constant, the maximum bending moment is proportional to the square (L2) of the wall height. According to this conclusion, because the height of the wall of the small packaging container is less than that of the large packaging, the bending moment is significantly reduced, so the damage probability of the small packaging container is reduced and the safety is increased

2. 2 sealing of packaging

the risk of transportation of dangerous goods is closely related to the sealing of goods. Some dangerous goods are easy to volatilize or emit strong smell or harmful steam. When the sealing of packaging is not good, its steam will continue to emit. If such goods enter the container, due to the poor ventilation performance of the container, these vapors will gather in the container, and the concentration of vapors will increase during transportation, which is very prone to danger. Therefore, if these dangerous goods want to enter the container, the packaging method must be changed and the packaging with good sealing performance must be adopted

liquid goods contained in sealed packaging containers absorb the heat transferred from the container wall, evaporate into gas, fill the entire space of the container and generate pressure on the container wall. The pressure changes with the temperature after the goods absorb heat, which can be calculated according to formula (2): P = NKT (2), where: P is the pressure on the container (PA); N is the molecular number density (the number of molecules per unit volume); K is Boltzmann constant; T is the temperature of the goods in the container

the design pressure of the packaging container is determined based on the calculation of formula (2) and other circulation environmental factors, such as the impact in loading and unloading and shunting, the safety factor and the practical experience of transportation for many years

according to the reference data provided by the Maritime Association dangerous code and the international dangerous code: the design parameter temperature is 65 ℃, the vertical falling impact value of mechanical loading and unloading is 2 (G is the gravitational acceleration m/S2) [6], the longitudinal horizontal impact value is 2, and the safety factor is 1 5。 Using containers to transport dangerous goods, the maximum temperature inside the container in some parts of China in summer can reach 85 ℃ [7]. As the temperature inside the container increases, the internal pressure of the container wall under steam increases. Therefore, the compressive strength and sealing performance of the original packaging container are studied

3 conclusion

the container is a closed container, and the maximum possible temperature inside the railway dangerous goods container in some regions in summer is 85 ℃. In order to ensure the safety of railway transportation, the maximum possible temperature inside the container should be determined by season. If plastic is used as the packaging material for the goods in the container, it should be optimized with reference to figure 1 and table 1; Through the experiment of railway dangerous goods transportation packaging, field investigation and calculation analysis, five advantages of small packaging are obtained. On the other hand, small packaging can reduce packaging costs, save resources, protect the environment, facilitate loading and unloading, and increase the loading capacity of goods. It can be boldly imagined that with the improvement of the organization and management level, the automation degree of loading and unloading equipment and the quality of the box, the small quantity of packaging, the reduction of packaging, the reuse of containers, the combination of packaging and products, and the selection and optimization of packaging materials are just around the corner

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