JP7638598B2 - Low-temperature liquid storage tank, its manufacturing method, and method for mitigating thermal shock - Google Patents

Description

The present invention relates to a low-temperature liquid storage tank that stores low-temperature liquid below 0°C, a manufacturing method thereof, and a thermal shock mitigation method for mitigating thermal shock to a low-temperature liquid storage tank.

A conventional cryogenic liquid storage tank is known that has an inner tank that stores cryogenic liquid inside and an outer tank that covers the inner tank from the outside, with the inner surface of the outer tank coated with a first foamed resin layer to prevent leakage of the cryogenic liquid and to mitigate thermal shock, and a mesh-structured reinforcing sheet on the surface (see, for example, Patent Document 1).

Patent No. 3044605 (paragraph [0002], Figure 4)

In the conventional low-temperature liquid storage tank described above, if the reinforcing sheet floats or peels off from the surface of the first foamed resin layer, the leaked low-temperature liquid may expose the first foamed resin layer to thermal shock and cause it to break, so there is a need to alleviate the thermal shock transmitted to the first foamed resin layer.

The low-temperature liquid storage tank of the present disclosure is a low-temperature liquid storage tank comprising an inner tank in which low-temperature liquid below 0°C is stored, an outer tank covering the outer tank, and a first foamed resin layer coated on the inner surface of the outer tank to prevent leakage of the low-temperature liquid and mitigate thermal shock, and the low-temperature liquid storage tank has a second foamed resin layer on the inner tank side surface of the first foamed resin layer, the second foamed resin layer having a bubble structure that communicates in the planar direction.

FIG. 1 is a cutaway front view of a cryogenic liquid tank according to an embodiment of the present disclosure. Enlarged cross-sectional view of the tank Cross-section of the relaxation layer Schematic diagram showing the flow of liquefied natural gas entering the inside of a diffusion layer made of low-density rigid polyurethane foam. A diagram showing the construction of a relief layer on the inner surface of the outer tank A diagram showing the flow of construction method for the relief layer

The cryogenic liquid storage tank 100 of the present disclosure will now be described with reference to Figures 1 to 4. As shown in Figure 1, the cryogenic liquid storage tank 100 of this embodiment comprises a hollow cylindrical tank section 40 equipped with an inner tank 20 and an outer tank 30, and a cylindrical liquid barrier 50 surrounding the tank section 40. The tank section 40 stores liquefied natural gas L inside the inner tank 20. The capacity of the cryogenic liquid storage tank 100 that is called large is generally 140,000 to 230,000 kL, and in the case of a 230,000 kL cryogenic liquid storage tank 100, the diameter of the liquid barrier 50 is approximately 90 m and its height is approximately 40 m.

The inner tank 20 and the outer tank 30 each have a ceiling 21, 31, and are structured so that their interiors are sealed off from the outside. The ceilings 21, 31 are dome-shaped with a bulging center, forming a space filled with vaporized liquefied natural gas L. Both the inner tank 20 and the outer tank 30 are made of metal, and from the standpoint of low-temperature toughness, for example, iron or steel is preferred. In particular, since the inner tank 20 is constantly exposed to extremely low temperatures, an alloy such as nickel, whose main component is iron and has excellent low-temperature toughness, is preferred.

The liquid barrier 50 is installed to prevent the diffusion of the liquefied natural gas L in the event of a leakage accident of the liquefied natural gas L, and in this embodiment, the inner surface of the liquid barrier 50 is overlapped with the outer surface of the outer tank 30. The liquid barrier 50 is made of prestressed concrete, which is resistant to cracking.

In the tank section 40, the space formed between the inner tank 20 and the outer tank 30 is provided with a cold insulation layer 60 for keeping the liquefied natural gas L at approximately -160°C and reducing the evaporation of the liquefied natural gas L. The cold insulation layer 60 is composed of a ceiling cold insulation layer 61, a side cold insulation layer 62, and a bottom cold insulation layer 63.

In detail, the spaces formed in the ceilings 21, 31 and the spaces formed in the sides 22, 32 of the inner tank 20 and the outer tank 30 are filled with granular perlite or the like having heat insulating properties as the ceiling cold insulation layer 61 and the side cold insulation layer 62. Also, the spaces formed in the bottoms 23, 33 of the inner tank 20 and the outer tank 30 are filled with perlite concrete, lightweight aerated concrete, or the like having load-bearing and heat-insulating properties as the bottom cold insulation layer 63.

Now, in the low-temperature liquid storage tank 100, the inner surface 30S of the outer tank 30 is coated with a buffer layer 10. This buffer layer 10 is formed to prevent the thermal shock of the leaked liquefied natural gas L from being transmitted suddenly to the liquid barrier 50.

As shown in FIG. 2, the relaxation layer 10 is composed of a side relaxation layer 10S that covers the entire inner side surface 30S of the outer tank 30, and an annular bottom relaxation layer 10T that covers the entire peripheral portion of the inner bottom surface 30T of the outer tank 30. The bottom relaxation layer 10T also covers the peripheral portion of the bottom cold insulation layer 63.

Figure 3 shows the cross-sectional structure of the relaxation layer 10 of this embodiment. The relaxation layer 10 is formed by laminating the under-blow layer 12, the first foamed resin layer 13 (13A, 13B), and the second foamed resin layer 14 on the inner surface (inner side surface 30S and inner bottom surface 30T) of the outer tank 30.

The first foamed resin layer 13 is laminated on the under-blow layer 12 and is made of a rigid urethane foam formed by foaming and hardening a urethane foam raw material. The first foamed resin layer 13 needs to mitigate the thermal shock of the liquefied natural gas L and suppress the thermal shock from affecting the liquid barrier 50. Therefore, the rigid urethane foam that is made of the first foamed resin layer 13 has excellent heat insulating performance and compressive strength, and is preferably thin in terms of efficient use of space. Specifically, the density is preferably 40 to 80 kg/m ³ , the air permeability is 1 ml/cm ² /s or less, the thermal conductivity is 0.040 W/mK or less, the thermal resistance is 1.00 m ² K/W or more, the compressive strength is preferably 360 kPa or more, and the thickness is preferably 40 mm or more and 60 mm or less.

In this embodiment, the first foamed resin layer 13 is composed of two layers (13A, 13B), but it may be composed of one layer or three or more layers. Here, the skin layer of the first foamed resin layer 13 is a high-density urethane layer, and since the ratio of urethane resin is higher than that of the core portion, the thermal conductivity is higher and the heat insulation performance is reduced. For this reason, it is preferable to have a small number of layers that make up the heat-insulating layer, and it is more preferable to have a composition of one or two layers.

The first foamed resin layer 13 in this embodiment has a density of 60 kg/ ^m3 , air permeability of 0.01 ml/ ^cm2 /s or less, porosity of 95%, thermal conductivity of 0.022 W/mK, thermal resistance value of 2.3 ^m2 K/W, compressive strength of 520 KPa, and thickness of 50 mm (25 mm for two layers).

The compressive strength required for the first foamed resin layer 13 is approximately 360 KPa, calculated from the Japan Gas Association's LNG aboveground storage tank guidelines (section 9.5.2.2 Load calculation), assuming a liquid barrier height of 40 m (assuming a 230,000 kL low-temperature liquid storage tank) and a safety factor of 2.0 (section 8.4.4 Cold and heat resistance mitigation material). Therefore, the compressive strength required for the first foamed resin layer 13 is 360 KPa or more.

The underblow layer 12 is a layer that is laminated directly on the inner surface of the outer tank 30, and acts as a primer to ensure the adhesion of the first foamed resin layer 13. The underblow layer 12 is made of the same hard urethane foam as the first foamed resin layer 13, and is formed by spraying the same urethane foam raw material as the first foamed resin layer 13 onto the inner surface of the outer tank 30 and hardening or foaming and hardening it. The thickness of the underblow layer 12 is preferably 0.1 to 5 mm.

The second foamed resin layer 14 is laminated on the inner surface of the first foamed resin layer 13, and constitutes the outermost surface of the buffer layer 10, serving as a protective layer that protects the first foamed resin layer 13 from thermal shock. The second foamed resin layer 14, like the first foamed resin layer 13, is formed by foaming and hardening a urethane foam raw material, but is made of a low-density rigid urethane foam that has a lower density than the first foamed resin layer 13.

Next, the rigid urethane foam that constitutes the second foamed resin layer 14 and the first foamed resin layer 13 will be described. As shown in FIG. 4, both rigid urethane foams are porous bodies having cells (air bubbles) with an independent air bubble structure in which each air bubble P is independent, and the gas enclosed in the air bubbles P is independent, making it difficult for temperature changes to be transmitted to the gas in adjacent air bubbles P, thereby providing excellent heat insulation performance.

The rigid urethane foam constituting the first foamed resin layer 13 has more bubbles P with a closed cell structure than the low-density rigid urethane foam constituting the second foamed resin layer 14, whereas the low-density rigid urethane foam constituting the second foamed resin layer 14 has more bubbles Q with an open cell structure in which some of the bubbles Q are connected than the bubbles P with a closed cell structure. Specifically, the closed cell ratio, which is the proportion of the bubbles P with a closed cell structure among the bubbles Q, P, is 65 to 100% (preferably 80% or more) for the first foamed resin layer 13, whereas it is 0 to 35% (preferably 20% or less) for the second foamed resin layer 14. Furthermore, most of the bubbles Q with an open cell structure in the second foamed resin layer 14 are connected in the surface direction. Here, the nature of the spraying method is that the adhesive surface and the surface of the urethane foam tend to have low foaming efficiency and become dense, and closed bubbles tend to increase, so the distribution is extreme in the vertical direction, but the core layer with the intended foaming efficiency spreads in the surface direction, that is, the open cell portion spreads. "Bubbles Q with an open cell structure are connected in the surface direction" refers to a state in which the open cell portion spreads in the surface direction, as described above.

If the closed cell ratio of the first foamed resin layer 13 falls below 65%, the thermal conductivity decreases and the thickness required to obtain the required thermal resistance value increases. If the thickness of the first foamed resin layer 13 increases, there is a risk that costs will increase, the working space will become narrower, and work efficiency will decrease, or the layer may be accidentally damaged and cracked.

As a result, the second foamed resin layer 14 has a larger porosity and is more flexible than the first foamed resin layer 13. The second foamed resin layer 14 also has higher breathability than the first foamed resin layer 13.

The low-density rigid urethane foam for the second foamed resin layer 14 preferably has a density of 7 to 40 kg/m ³ , air permeability in the thickness direction of 0.05 to 30 ml/cm ² /s, compressive strength of 15 to 150 KPa, thermal resistance of 0.30 m ² K/W or more, and porosity of 97.5% or more. The thickness is preferably 10 to 20 mm. The thermal resistance of the second foamed resin layer 14 is lower than that of the first foamed resin layer 13.

The second foamed resin layer 14 made of low-density rigid urethane foam used in this embodiment has a density of 11 kg/ ^m3 , a thermal conductivity of 0.036 W/mK, a porosity of 99.1%, a thermal resistance of 0.42 ^m2 K/W, an air permeability of 0.3 ml/ ^cm2 /s, a compressive strength of 30 KPa, and a thickness of 15 mm.

Now, if liquefied natural gas L leaks from inside the inner tank 20, the liquefied natural gas L will come into contact with the second foamed resin layer 14 before the first foamed resin layer 13. The surface (skin layer) of the second foamed resin layer 14 will be exposed to localized rapid cooling, causing it to shrink and break. At this time, since the second foamed resin layer 14 has a high proportion of bubbles Q with a continuous cell structure that are connected in the surface direction, the thermal shock of the liquefied natural gas L will spread more in the surface direction than in the thickness direction, and the first foamed resin layer 13 will be cooled slowly, mitigating the thermal shock transmitted to the first foamed resin layer 13.

In addition, since the second foamed resin layer 14 is more flexible than the first foamed resin layer 13, the surface (skin layer) is easily broken, but the break is unlikely to extend to the back (first foamed resin layer 13). In other words, the portion of the second foamed resin layer 14 on the side of the first foamed resin layer 13 is unlikely to break, so the first foamed resin layer 13 is not exposed to localized rapid cooling, but is cooled slowly by the liquefied natural gas L inside the second foamed resin layer 14, and the thermal shock transmitted to the first foamed resin layer 13 is mitigated. In addition, since the breakage range of the second foamed resin layer 14 is shallow, the first foamed resin layer 13 is unlikely to receive the energy of the breakage that occurs in the second foamed resin layer 14.

In addition, a temperature gradient is generated within the second foamed resin layer 14 such that the inside near the liquefied natural gas L is low temperature and the outside is high temperature. When the surface (skin layer) of the second foamed resin layer 14 is broken and the center of the second foamed resin layer 14 is exposed to the liquefied natural gas L, the center is at a higher temperature than the surface (skin layer), and it is believed that the liquefied natural gas L will boil. As a result, a gas layer is generated between the center of the second foamed resin layer 14 and the liquefied natural gas L until the center of the second foamed resin layer 14 is cooled, making it difficult for the liquefied natural gas L to penetrate into the inside of the second foamed resin layer 14 and creating an insulating effect. During this time, the first foamed resin layer 13 is gradually cooled, and it is believed that the cold-heat shock transmitted to the first foamed resin layer 13 is mitigated.

In addition, since the thermal conductivity of the second foamed resin layer 14 is slightly higher than that of the first foamed resin layer 13, the first foamed resin layer 13 is gradually cooled through the second foamed resin layer 14. It is considered that when the thermal resistance value of the second foamed resin layer 14 becomes 0.30 ^m2 K/W or less, the cooling rate of the first foamed resin layer 13 increases significantly.

For the first foamed resin layer 13 and the second foamed resin layer 14 of this embodiment, the density was measured based on JIS K 7222:2005/ISO 845:1988, the air permeability was measured in accordance with JIS K 6400-7 Method B:2012/ISO 7231:2010, the thermal conductivity was measured in accordance with JIS A 1412-2:1999/ISO 8301:1999, the compressive strength was measured in accordance with JIS K 7220:2006/ISO 844:2004, and the closed cell ratio was measured with reference to ASTM D 2856-87.

In detail, the following measurement samples were prepared based on JIS A9526:2015 and measurements were performed. The measurement samples were prepared by spraying an underspray layer 12 of about 3 mm using urethane foam raw material for the first foamed resin layer 13 onto an aluminum plate of 900 mm square x 5 mm thickness, and then laminating two heat insulating layers of about 25 mm to prepare a first foamed resin layer 13 of about 50 mm. As with the first foamed resin layer 13, a measurement sample was also prepared for the second foamed resin layer 14 using the urethane foam raw material for the second foamed resin layer 14.

The density was measured by cutting out a measurement sample into a size of 100 mm square by 30 mm thick (no skin layer on the entire surface) so as to include the skin layer of the outer first foamed resin layer 13A in the thickness direction. The thermal conductivity was measured by cutting out a measurement sample into a size of 200 mm square by 25 mm thick (no skin layer on the entire surface) so as to include the skin layer of the outer first foamed resin layer 13A in the thickness direction. The compressive strength was measured by cutting out a measurement sample into a size of 50 mm square by 30 mm thick (no skin layer on the entire surface) so as to include the skin layer of the outer first foamed resin layer 13A in the thickness direction. The air permeability was measured by cutting out a measurement sample into a size of 220 mm square by 10 mm thick (no skin layer on the entire surface) from the inner first foamed resin layer 13B. The air permeability was measured without including the skin layers of the outer first foamed resin layer 13A and the inner first foamed resin layer 13B in the thickness direction. The closed cell ratio was measured by creating a first foamed resin layer 13B and a second foamed resin layer 14 each measuring 30 mm square and 20 mm thick.

Next, the construction method of the relaxation layer 10 will be explained using Figures 5 and 6. The construction of the relaxation layer 10 is carried out when the inner tank 20, the outer tank 30 and the liquid barrier 50 are almost completed, and before granular perlite is filled between the sides 22, 32 of the inner tank 20 and the outer tank 30. Therefore, as shown in Figure 6, workers M, N, and O enter the narrow space between the side 22 of the inner tank 20 and the side 32 of the outer tank 30 to carry out the construction. At this time, since the bottom cold insulation layer 63 is arranged on the outer tank 30, and the inner tank 20 is arranged on top of that, workers usually enter and exit through an entrance provided in the ceiling (not shown). The width between the side 22 of the inner tank 20 and the side 32 of the outer tank 30 is 1000 mm to 2000 mm, and the height is about 45 m.

The construction of the side relief layer 10S, which is provided on the inner surface 30S of the outer tank 30, is performed by a worker M or N who gets into a gondola 70 attached to a trolley beam installed on the ceiling (not shown), as shown in Figure 5. The gondola 70 is suspended within the space K so that it can move up and down and horizontally along the inner surface 30S of the outer tank 30.

The construction of the relief layer 10 is performed for each of a plurality of construction areas W obtained by dividing the inner surface 30S and the inner bottom surface 30T of the outer tank 30 at a predetermined interval in the vertical direction. In the construction of the side relief layer 10S, the worker M or N who gets on the gondola 70 works on the construction area W in order from the upper end or the lower end. When the construction of a certain construction area W is completed, the worker M or N moves horizontally to the adjacent construction area W and repeats the construction from the upper end or the lower end in the same manner. When the construction of the construction area W is performed in order from the upper end or the lower end, the worker M or N does not work on the area that cannot be worked on from the gondola 70 and moves horizontally to the adjacent construction area W. The above-mentioned areas of the side relief layer 10S that cannot be worked on from the gondola 70 and the bottom relief layer 10T are performed by the worker O after the construction of the side relief layer 10S is completed or in parallel with the construction of the side relief layer 10S, as shown in FIG. 5. Alternatively, M or N may get off the gondola 70 each time and work continuously.

Figure 6 shows the flow of construction of the relief layer 10. As shown in the figure, the construction of the relief layer 10 begins with the first step S1 being performed by worker M. Then, the second step S2 is performed by worker N, following worker M.

In the first step S1, the urethane foam raw material is sprayed onto the inner surface of the outer tank 30 by a spray method, and is foamed and cured to form the first foamed resin layer 13. Before forming the first foamed resin layer 13, the under-blow layer 12 is formed by the same spray method.

In detail, in the first step S1, the worker M sprays the urethane foam raw material toward the inner surface of the outer tank 30 with the spray gun 90 carried by the worker M to form the underspray layer 12, and then sprays again to form the first foamed resin layer 13 to a predetermined thickness. In this embodiment, spraying is performed twice to form two first foamed resin layers 13A and 13B. This is because even if a predetermined thickness is to be formed with one spray, there is a risk that the sprayed urethane foam raw material will drip and the predetermined thickness cannot be secured. In this case, after the first spraying is completed, the second spraying is performed after the curing progresses and the tack (stickiness) of the surface disappears. The outer first foamed resin layer 13A and the inner first foamed resin layer 13B are formed to have approximately the same thickness.

In this embodiment, the under-blow layer 12 is formed by applying the same urethane foam raw material as the first foamed resin layer 13. The presence of the under-blow layer 12 can improve the adhesion of the outer first foamed resin layer 13A to the inner surface 30S of the outer tank 30. In this case, the under-blow layer 12 is sprayed after the under-blow layer 12 is sprayed, and the hardening progresses and the tackiness of the surface disappears. If the under-blow layer 12 is not provided and the first foamed resin layer 13 is formed directly on the inner surface of the outer tank 30, heat is taken away from the part attached to the inner surface of the outer tank 30, which is made of metal and has high thermal conductivity, and the degree of foaming may become insufficient or the adhesive strength between the outer tank 30 and the first foamed resin layer 13 may decrease, causing the first foamed resin layer 13 to peel off from the outer tank 30.

In the second step S2, the worker N sprays the urethane foam raw material onto the first foamed resin layer 13 with a spray gun 90 carried by the worker N, forming the second foamed resin layer 14 made of low-density rigid urethane foam to a predetermined thickness. At this time, the urethane foam raw material is sprayed to form a rigid urethane foam with a lower density than the first foamed resin layer 13. Specifically, the polyol (corresponding to an "active hydrogen compound") in the urethane foam raw material of the second foamed resin layer 14 contains polyols having a larger molecular weight (weight average molecular weight (Mw)) (i.e., high molecular weight) than the polyols in the urethane foam raw material of the first foamed resin layer 13. For example, the polyols in the urethane foam raw material of the first foamed resin layer 13 mainly have an average molecular weight of 1000 or less, and the polyols in the urethane foam raw material of the second foamed resin layer 14 contain 5 wt% or more of polyols having an average molecular weight of 1500 or more.

This concludes the explanation of the configuration of the buffer layer 10 and its construction method in this embodiment. Next, we will explain the effects of the buffer layer 10 and its construction method.

In the relaxation layer 10 of this embodiment, the first foamed resin layer 13 is covered with the second foamed resin layer 14, and the liquefied natural gas L leaking from inside the inner tank 20 comes into contact with the second foamed resin layer 14 before the first foamed resin layer 13. The surface (skin layer) of the second foamed resin layer 14 is exposed to localized rapid cooling and shrinks and breaks. At this time, since the second foamed resin layer 14 has a high proportion of bubbles Q with a continuous cell structure that are connected in the surface direction, the thermal shock of the liquefied natural gas L spreads in the surface direction rather than in the thickness direction, and the first foamed resin layer 13 is cooled slowly, and the thermal shock transmitted to the first foamed resin layer 13 is mitigated. In addition, since the second foamed resin layer 14 is softer and has a smaller compressive strength than the first foamed resin layer 13, the surface (skin layer) is easily broken, but the breakage is less likely to spread to the depth (first foamed resin layer 13). That is, the portion of the second foamed resin layer 14 on the side of the first foamed resin layer 13 is less likely to break, so the first foamed resin layer 13 is not exposed to localized rapid cooling, but is slowly cooled by the liquefied natural gas L on the inside of the second foamed resin layer 14 (inside the broken portion), mitigating the thermal shock transmitted to the first foamed resin layer 13. Also, because the breakage range of the second foamed resin layer 14 is shallow, the first foamed resin layer 13 is less likely to receive the energy of breakage that occurs in the second foamed resin layer 14.

In addition, since the second foamed resin layer 14 can protect the first foamed resin layer 13 from the thermal shock of the liquefied natural gas L, it is not necessary to provide a mesh-structured reinforcing sheet to reinforce the surface of the first foamed resin layer 13 as in the conventional case.

Specifically, in a configuration in which a reinforcing sheet is laminated on the surface of the first foamed resin layer 13, after the first step S1, the reinforcing sheet is attached to the surface of the first foamed resin layer 13 with an adhesive or the like. At this time, there is a risk that the reinforcing sheet may float or peel off from the surface of the first foamed resin layer 13 due to its rigidity. Therefore, a step of cutting the surface of the first foamed resin layer 13 to make it flat is required. This step is performed manually for all of the construction areas W, which requires a huge amount of labor and cost. Moreover, labor and cost are also required to remove this dust. Furthermore, the dust from the cutting chips generated during cutting not only worsens the working environment, but also creates a risk of dust explosion. In contrast, this step is not required in the present embodiment, so such problems do not occur and workability can be improved.

The cutting process is intended to flatten the surface, so it must be performed after the foaming and hardening of the first foamed resin layer 13 has progressed and sufficient strength has been achieved. If cutting or grinding is performed before sufficient strength is achieved, there is a risk that the surface may not be cut flat or may tear. The estimated time required for sufficient strength to be achieved is about 24 hours (one day), which requires additional days and increases costs. In contrast, in this embodiment, the next second step S2 can be performed after the hardening of the first step S1 has progressed and the tack on the surface has disappeared. This eliminates the need to wait for the foaming and hardening of the first foamed resin layer 13 in the first step S1. This avoids the above-mentioned problems and improves workability.

[Confirmation experiment]
It was confirmed by an experiment that the buffer layer 10 of the above embodiment can buffer a thermal shock when subjected to a thermal shock by protecting the first foamed resin layer 13 made of a rigid urethane foam with the second foamed resin layer 14 made of a low-density rigid urethane foam. In this experiment, the buffer layer 10 was made in a metal mold, liquid nitrogen was poured on top of it, and it was confirmed whether or not the first foamed resin layer 13 made of a rigid urethane foam cracked. The temperature of liquid nitrogen is -196°C, which is a more severe condition than liquefied natural gas L, which is about -160°C. Nitrogen is an inert gas and does not pose a risk of fire, so it was used as a substitute liquid for the experiment.

Specifically, a dismantled metal mold with inner dimensions of 1600 mm length x 700 mm width x 100 mm thickness and an open top is prepared. The metal mold is erected (the bottom faces are in the length and thickness directions), and the bottom face of the metal mold (opposite the open face) is treated as the outer tank 30. The urethane foam raw material for the first foamed resin layer 13 is sprayed to form a lower blown layer 12 of about 3 mm, and then a first foamed resin layer 13 made of hard urethane foam with a thickness of 50 mm (two-layer structure with each layer being 25 mm thick) is formed. Furthermore, the urethane foam raw material for the second foamed resin layer 14 is sprayed on top of the first foamed resin layer 13 to form a second foamed resin layer 14 made of low-density hard urethane foam with a thickness of 15 mm (single-layer structure) to prepare a test piece. Then, the test piece thus prepared was turned over (the length direction and width direction were the bottom surface), and liquid nitrogen was poured in from above (the second foamed resin layer 14 side) so that the liquid nitrogen level was 20 to 30 mm above the second foamed resin layer 14. After that, liquid nitrogen was added as needed so that the liquid nitrogen level was 20 to 30 mm, and two hours were allowed to pass. After two hours had passed, the liquid nitrogen was removed from the metal mold, and the presence or absence of cracks was visually confirmed. If cracks were present, a dye diluted with a solvent was dripped from the surface of the crack with a dropper, and the cracks were colored by leaving it for about one hour. Then, the metal mold was disassembled, the test piece was taken out, the test piece was cut, and the cut cross section was visually observed to confirm the presence or absence of cracks in the first foamed resin layer 13 made of hard urethane foam. For comparison, comparative sample 1 was made without the second foamed resin layer 14 made of low-density rigid urethane foam, and only had the first foamed resin layer (50 mm thick (2-layer structure, each layer is 25 mm thick)), comparative sample 2 was made by spraying the urethane foam raw material for the first foamed resin layer onto the first foamed resin layer (50 mm thick (2-layer structure, each layer is 25 mm thick) to laminate a 15 mm thick rigid urethane foam, and for reference, a reference sample (conventional configuration) was made in which a reinforcing sheet was fixed to the surface of the first foamed resin layer (50 mm thick (2-layer structure, each layer is 25 mm thick)) with adhesive.

As a result, no cracks were generated in the first foamed resin layer 13 of the buffer layer 10 having the second foamed resin layer 14 made of low-density rigid urethane foam, and in the heat insulating layer of the conventional buffer layer having a reinforcing sheet on the surface of the first foamed resin layer. On the other hand, many cracks were generated in the first foamed resin layer of Comparative Sample 1 and Comparative Sample 2. From this experiment, it was confirmed that by protecting the first foamed resin layer 13 made of rigid urethane foam with the second foamed resin layer 14 made of low-density rigid urethane foam, it is possible to suppress the occurrence of cracks in the rigid urethane foam of the first foamed resin layer 13 when subjected to thermal shock. It was also confirmed that the buffer layer of the present disclosure buffers thermal shock to the same extent as a conventional buffer layer having a reinforcing sheet on the surface of the first foamed resin layer.

[Other embodiments]
(1) In the above embodiment, the cryogenic liquid storage tank 100 stores liquefied natural gas L. However, the cryogenic liquid storage tank 100 may store other cryogenic liquids, such as liquefied propane gas.

(2) In the above embodiment, the tank portion 40 has ceiling portions 21, 31, but it may also have a lid and be open at the top.

(3) In the above embodiment, the second foamed resin layer 14 made of low-density rigid urethane foam is a single layer, but multiple layers may be laminated.

(4) In the above embodiment, a reinforcing sheet having a mesh structure may be laminated between the first foamed resin layer 13 and the second foamed resin layer 14.

At this time, between the first step S1 and the second step S2, a step S12 is performed in which the reinforcing sheet is laminated to the first foamed resin layer 13. In step S12, the reinforcing sheet may be laminated to the first foamed resin layer 13 and temporarily fixed with a tacker or the like before performing the second step S2, so that the reinforcing sheet is fixed to the first foamed resin layer 13 so as to be enclosed in the low-density rigid urethane foam as the second foamed resin layer 14. In this way, an adhesive is not required to attach the reinforcing sheet to the first foamed resin layer 13, and the step of flattening the first foamed resin layer 13 is also not required.

(5) In the above embodiment, the second foamed resin layer 14 is a low-density rigid urethane foam, but it may be another type of foamed resin, such as a flexible urethane foam. The first foamed resin layer 13 may also be another type of foamed resin.

(6) In the above embodiment, the second foamed resin layer 14 was formed by spraying urethane foam raw material onto the first foamed resin layer 13 and allowing it to foam and harden. However, it may also be formed by fixing a prefabricated panel made of low-density rigid urethane foam to the first foamed resin layer 13.

Note that although specific examples of the technology included in the scope of the claims are disclosed in this specification and drawings, the technology described in the claims is not limited to these specific examples, but includes various modifications and variations of the specific examples, as well as parts of the specific examples taken separately.

<Additional Notes>
Hereinafter, the first to eleventh aspects of the invention extracted from the above-described embodiment will be described, showing their effects and the like as necessary.

[First aspect]
A low-temperature liquid storage tank comprising an inner tank for storing a low-temperature liquid below 0°C, an outer tank for covering the outer tank, and a first foamed resin layer laminated on the inner surface of the outer tank to prevent leakage of the low-temperature liquid and to mitigate thermal shock,
A low-temperature liquid storage tank having a second foamed resin layer on the inner tank side surface of the first foamed resin layer, the second foamed resin layer having a bubble structure communicating in the planar direction.

According to the first aspect of the invention, the second foamed resin layer has a bubble structure that is interconnected in the surface direction, so that the thermal shock of the low-temperature liquid spreads more in the surface direction than in the thickness direction, the first foamed resin layer is cooled slowly, and the thermal shock transmitted to the first foamed resin layer is mitigated.

[Second aspect]
The low-temperature liquid storage tank according to the first aspect, wherein the second foamed resin layer has a higher porosity than the first foamed resin layer.

[Third aspect]
The low-temperature liquid storage tank according to the first or second aspect, wherein the porosity of the second foamed resin layer is 97.5% or more.

According to the second aspect of the invention, the second foamed resin layer has a larger porosity and is more flexible than the first foamed resin layer 13, so the breakage range of the second foamed resin layer is shallower and the first foamed resin layer is less susceptible to the energy of the breakage that occurs in the second foamed resin layer. In addition, the porosity of the second foamed resin layer is preferably 97.5% or more (third aspect).

[Fourth aspect]
The low-temperature liquid storage tank according to any one of the first to third aspects, wherein the second foamed resin layer has a thermal resistance value lower than that of the first foamed resin layer.

[Fifth aspect]
The low-temperature liquid storage tank according to the fourth aspect, wherein the thermal resistance value of the second foamed resin layer is 0.30 ^m2 ·K/W or more.

According to the fourth aspect of the invention, the second foamed resin layer transmits cold to the first foamed resin layer, and the first foamed resin layer is gradually cooled. When the thermal resistance value of the second foamed resin layer is 0.30 ^m2 ·K/W or more, the cooling speed of the first foamed resin layer is considered to be appropriate (fifth aspect).

[Sixth aspect]
The first foamed resin layer has a closed cell rate of 65% to 100%, and the second foamed resin layer has a closed cell rate of 0 to 35%.

According to the sixth aspect of the invention, the second foamed resin layer has more open air bubbles than the first foamed resin layer, so the thermal shock of the low-temperature liquid spreads into the second foamed resin layer, the first foamed resin layer is cooled slowly, and the thermal shock transmitted to the first foamed resin layer is mitigated.

[Seventh aspect]
The low-temperature liquid storage tank according to any one of claims 1 to 7, wherein the first foamed resin layer and the second foamed resin layer are made of urethane foam.

[Eighth aspect]
A method for manufacturing a cryogenic liquid storage tank according to any one of the first to seventh aspects,
A method for manufacturing a low-temperature liquid storage tank, wherein the foaming resin raw material used for the second foaming resin layer contains an active hydrogen compound (mainly a polyol) having a higher molecular weight than the active hydrogen compound (mainly a polyol) in the foaming resin raw material used for the first foaming resin layer.

[Ninth aspect]
A method for manufacturing a cryogenic liquid storage tank according to any one of the first to seventh aspects,
A first step of applying a raw material to an inner surface of the outer tank and foaming and curing the raw material to form the first foamed resin layer;
A second step of applying a raw material to a surface of the first foamed resin layer facing the inner tank and foaming and curing the raw material to form the second foamed resin layer;
A method for manufacturing a low-temperature liquid storage tank.

[Tenth Aspect]
A method for manufacturing a cryogenic liquid storage tank according to any one of the first to seventh aspects,
A method for manufacturing a low-temperature liquid storage tank, comprising a panel installation process for installing a resin panel having the first foamed resin layer and the second foamed resin layer laminated thereon on the inner surface of the outer tank.

[Eleventh aspect]
A method for mitigating thermal shock in a low-temperature liquid storage tank, the method comprising: an inner tank for storing a low-temperature liquid below 0° C.; an outer tank for covering the inner tank; and a first foamed resin layer laminated on the inner surface of the outer tank to prevent leakage of the low-temperature liquid and to mitigate thermal shock, the method comprising the steps of:
A method for mitigating thermal shock in a low-temperature liquid storage tank, comprising disposing a second foamed resin layer on the inner tank side surface of the first foamed resin layer, the second foamed resin layer having a bubble structure communicating in the planar direction, and boiling the low-temperature liquid leaked from the inner tank to generate a gas layer.

According to the eleventh aspect of the invention, a gas layer is generated between the center of the second foamed resin layer and the low-temperature liquid until the center of the second foamed resin layer is cooled, so that the first foamed resin layer is gradually cooled during that time, making the first foamed resin layer less likely to break.

10 Relaxation layer 13 First foamed resin layer 14 Second foamed resin layer 20 Inner tank 30 Outer tank 50 Liquid barrier 100 Low-temperature liquid storage tank L Liquefied natural gas (low-temperature liquid)

Claims (2)

A method for manufacturing a low-temperature liquid storage tank comprising an inner tank for storing a low-temperature liquid below 0°C, an outer tank for covering the inner tank, and a first foamed resin layer laminated on the inner surface of the outer tank to prevent leakage of the low-temperature liquid and to mitigate thermal shock, comprising:
a second foamed resin layer having a bubble structure communicating in a direction along the inner tank side surface of the first foamed resin layer is disposed on the inner tank side surface of the first foamed resin layer;
the first foamed resin layer and the second foamed resin layer are urethane foams,
The porosity of the second foamed resin layer is higher than the porosity of the first foamed resin layer and is 97.5% or more;
The thermal resistance value of the second foamed resin layer is lower than the thermal resistance value of the first foamed resin layer and is 0.30 m2 ·K/W or more;
The first foamed resin layer has a closed cell ratio of 65% to 100%, and the second foamed resin layer has a closed cell ratio of 0% to 35%,
A method for manufacturing a low-temperature liquid storage tank, wherein the foaming resin raw material used for the second foaming resin layer contains an active hydrogen compound with a higher molecular weight than the active hydrogen compound in the foaming resin raw material used for the first foaming resin layer. A method for manufacturing a low-temperature liquid storage tank comprising an inner tank for storing a low-temperature liquid below 0°C, an outer tank for covering the inner tank, and a first foamed resin layer laminated on the inner surface of the outer tank to prevent leakage of the low-temperature liquid and to mitigate thermal shock, comprising:
a second foamed resin layer having a bubble structure communicating in a direction along the inner tank side surface of the first foamed resin layer is disposed on the inner tank side surface of the first foamed resin layer;
a panel installation step of installing a resin panel, in which the first foamed resin layer and the second foamed resin layer are laminated, on an inner surface of the outer tank.

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