Cryogenic liquid storage tank, its manufacturing method, and method for mitigating thermal shock

Abstract

To reduce thermal shock transmitted to a first foamed resin layer coated on an inside surface of an outer tank of a low-temperature liquid storage tank.SOLUTION: In a low-temperature liquid storage tank 100, an under-blown layer 12, a first foamed resin layer 13, and a second foamed resin layer 14 are laminated on the inner surface of an outer tank 30. Both the first foamed resin layer 13 and the second foamed resin layer 14 are hard urethane foam. The second foamed resin layer 14 has more cells Q having a continuous open cell structure than the first foamed resin layer 13 does. Most of the cells Q having an open cell structure in the second foamed resin layer 14 communicate in a plane direction.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a low-temperature liquid storage tank for storing low-temperature liquid at 0° C. or below, a manufacturing method thereof, and a thermal shock mitigation method for mitigating thermal shock to a low-temperature liquid storage tank. [Background technology]

[0002] A known conventional cryogenic liquid storage tank is equipped with an inner tank for storing cryogenic liquid therein and an outer tank covering the inner tank from the outside, with the inner surface of the outer tank coated with a first foamed resin layer for preventing leakage of the cryogenic liquid and mitigating thermal shock, and with a mesh-structured reinforcing sheet provided on the surface (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3044605 (paragraph

[0002] , Figure 4) Summary of the Invention [Problem to be solved by the invention]

[0004] 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 first foamed resin layer may be exposed to thermal shock by the leaked low-temperature liquid and may break, so there is a need to alleviate the thermal shock transmitted to the first foamed resin layer. [Means for solving the problem]

[0005] The low-temperature liquid storage tank of the present disclosure comprises an inner tank in which low-temperature liquid below 0°C is stored, an outer tank covering the outside of the inner 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. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a cutaway front view of a cryogenic liquid tank according to an embodiment of the present disclosure. [Diagram 2] Enlarged cross-sectional view of the tank [Diagram 3] Cross-section of the relaxation layer [Figure 4] Schematic diagram showing the flow of liquefied natural gas entering the inside of a diffusion layer made of low-density rigid polyurethane foam. [Diagram 5] A diagram showing the construction of a relief layer on the inner surface of the outer tank [Figure 6] A diagram showing the flow of construction method for the relief layer DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Hereinafter, a cryogenic liquid storage tank 100 according to the present disclosure will be described with reference to Figs. 1 to 4. As shown in Fig. 1, the cryogenic liquid storage tank 100 according to the present embodiment is composed of a hollow cylindrical tank section 40 having 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, which is called large, is generally 140,000 to 230,000 kL, and in the case of a cryogenic liquid storage tank 100 with a capacity of 230,000 kL, the diameter of the liquid barrier 50 is about 90 m, and its height is about 40 m.

[0008] The inner tank 20 and the outer tank 30 each have a ceiling portion 21, 31, and are structured so that their interiors are sealed off from the outside. The ceiling portions 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 viewpoint of low-temperature toughness, for example, iron or steel is preferable. In particular, since the inner tank 20 is constantly exposed to extremely low temperatures, an alloy such as nickel, which is mainly composed of iron and has excellent low-temperature toughness, is preferable.

[0009] The liquid retaining dike 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 retaining dike 50 is overlapped with the outer surface of the outer tank 30. The liquid retaining dike 50 is made of prestressed concrete that is resistant to cracking.

[0010] In the tank section 40, a 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 about −160° C. and reducing vaporization 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.

[0011] In detail, the spaces formed in the ceiling parts 21, 31 and the spaces formed in the side parts 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 bottom parts 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 properties and heat insulating properties as the bottom cold insulation layer 63.

[0012] In the low-temperature liquid storage tank 100, the inner surface 30S of the outer tank 30 is coated with a mitigation layer 10. This mitigation layer 10 is formed to prevent the thermal shock of the leaked liquefied natural gas L from being transmitted to the liquid barrier 50 suddenly.

[0013] 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 periphery of the inner bottom surface 30T of the outer tank 30. The bottom relaxation layer 10T also covers the periphery of the bottom cold insulation layer 63.

[0014] 3 shows a cross-sectional structure of the relaxation layer 10 of this embodiment. The relaxation layer 10 is formed by laminating an under-blow layer 12, a first foamed resin layer 13 (13A, 13B), and a second foamed resin layer 14 on the inner surface (inner side surface 30S and inner bottom surface 30T) of the outer tub 30.

[0015] The first foamed resin layer 13 is laminated on the under-blow layer 12, and is composed 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 retaining dike 50. For this reason, the rigid urethane foam that is composed of it has excellent heat insulating performance and compressive strength, and is preferably thin from the viewpoint of efficient use of space. Specifically, it has a density of 40 to 80 kg / m 3 , breathability is 1ml / cm 2 / s or less, thermal conductivity is 0.040W / mK or less, and thermal resistance is 1.00m 2 The compressive strength is preferably 360 kPa or more, and the thickness is preferably 40 mm or more and 60 mm or less.

[0016] 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 part, the thermal conductivity is higher and the heat insulation performance is reduced. For this reason, it is preferable that the number of layers constituting the heat-insulating layer is small, and it is more preferable to configure it with one or two layers.

[0017] The first foamed resin layer 13 of this embodiment has a density of 60 kg / m 3 , breathability 0.01ml / cm 2 / s or less, porosity 95%, thermal conductivity 0.022W / mK, thermal resistance 2.3m 2 K / W compressive strength is 520KPa, thickness is 50mm (2 layers of 25mm).

[0018] The compressive strength required for the first foamed resin layer 13 is calculated as approximately 360 KPa, based on the Japan Gas Association's LNG aboveground storage tank guidelines (9.5.2.2 Load calculation), assuming a liquid retaining dike height of 40 m (assuming a 230,000 kL low-temperature liquid storage tank) and a safety factor of 2.0 (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.

[0019] The under-blown layer 12 is a layer that is laminated directly on the inner surface of the outer tub 30, and serves as a primer to ensure the adhesion of the first foamed resin layer 13. The under-blown 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 tub 30 and curing or foaming and curing it. The thickness of the under-blown layer 12 is preferably 0.1 to 5 mm.

[0020] 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 relaxation 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 curing a urethane foam raw material, but is made of a low-density rigid urethane foam having a lower density than the first foamed resin layer 13.

[0021] Next, the rigid urethane foams constituting 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.

[0022] The rigid urethane foam constituting the first foamed resin layer 13 has more bubbles P having 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 having an open cell structure in which some of the bubbles Q are interconnected than the bubbles P having a closed cell structure. Specifically, the closed cell ratio, which is the ratio of the bubbles P having 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. Moreover, most of the bubbles Q having an open cell structure in the second foamed resin layer 14 are interconnected in the surface direction. Here, as a characteristic of the spraying method, 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 part spreads. "The bubbles Q having an open cell structure are connected in the surface direction" refers to the state in which the open cell part spreads in the surface direction, as mentioned above.

[0023] 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 narrow, and the working efficiency will decrease, or the layer may be accidentally broken and cracked.

[0024] As a result, the second foamed resin layer 14 has a larger porosity and is more flexible than the first foamed resin layer 13. In addition, the second foamed resin layer 14 has higher breathability than the first foamed resin layer 13.

[0025] The density of the low-density rigid urethane foam as the second foamed resin layer 14 is 7 to 40 kg / m 3 , air permeability in the thickness direction is 0.05~30ml / cm 2 / s, compressive strength 15-150KPa, thermal resistance 0.30m 2The second foamed resin layer 14 has a thermal resistance lower than that of the first foamed resin layer 13. ...

[0026] The second foamed resin layer 14 made of low-density rigid urethane foam used in this embodiment has a density of 11 kg / m 3 , thermal conductivity 0.036W / mK, porosity 99.1%, thermal resistance 0.42m 2 K / W, breathability 0.3ml / cm 2 / s, compressive strength 30KPa, and thickness 15mm.

[0027] Now, in the case where the liquefied natural gas L leaks from inside the inner tank 20, the liquefied natural gas L 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, causing it to shrink and break. At this time, since the second foamed resin layer 14 has a high proportion of bubbles Q having an open cell structure that communicates in the surface direction, the thermal shock of the liquefied natural gas L spreads more in the surface direction than in the thickness direction, and the first foamed resin layer 13 is cooled slowly, thereby mitigating the thermal shock transmitted to the first foamed resin layer 13.

[0028] 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 breakage is unlikely to extend to the back (first foamed resin layer 13). In other words, since the portion of the second foamed resin layer 14 on the first foamed resin layer 13 side is unlikely to break, the first foamed resin layer 13 is not exposed to localized rapid cooling, but is slowly cooled 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.

[0029] In addition, a temperature gradient is generated in 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 considered that the liquefied natural gas L boils. 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, and it is considered that the liquefied natural gas L is less likely to penetrate into the inside of the second foamed resin layer 14 and a heat insulating effect is generated. During that time, the first foamed resin layer 13 is gradually cooled, and it is considered that the thermal shock transmitted to the first foamed resin layer 13 is mitigated.

[0030] 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. The thermal resistance value of the second foamed resin layer 14 is 0.30 m 2 K / W or less, the cooling rate of the first foamed resin layer 13 is thought to increase significantly.

[0031] With regard to the first foamed resin layer 13 and the second foamed resin layer 14 in 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.

[0032] 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 under-blow layer 12 of about 3 mm on an aluminum plate of 900 mm square and 5 mm thickness using urethane foam raw material for the first foamed resin layer 13, 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.

[0033] The density was measured by cutting out a measurement sample to 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 to 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 to 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 to 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 for the core part without including any of 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 preparing the first foamed resin layer 13B and the second foamed resin layer 14 each having a size of 30 mm square and a thickness of 20 mm.

[0034] Next, the construction method of the relaxation layer 10 will be described with reference to Figs. 5 and 6. The construction of the relaxation layer 10 is performed when the inner tank 20, the outer tank 30, and the dike 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 Fig. 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 perform 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 it, 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 1000mm to 2000mm, and the height is about 45m.

[0035] The construction of the side relief layer 10S provided on the inner surface 30S of the outer tub 30 out of the relief layers 10 is performed by a worker M or N who gets on a gondola 70 attached to a trolley beam installed on a ceiling (not shown), as shown in Fig. 5. The gondola 70 is suspended in a space K along the inner surface 30S of the outer tub 30 so as to be able to ascend and descend and to move horizontally.

[0036] 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 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 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 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.

[0037] Fig. 6 shows the flow of construction of the relieving layer 10. As shown in the figure, in the construction of the relieving layer 10, the first step S1 is first performed by worker M. Then, the second step S2 is performed by worker N following worker M.

[0038] In the first step S1, a 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.

[0039] 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 under-spray 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 layers of the first foamed resin layers 13A and 13B. This is because, even if a predetermined thickness is to be formed by one spraying, there is a risk that the sprayed urethane foam raw material will drip and the predetermined thickness cannot be ensured. 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.

[0040] 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 tub 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 tub 30, heat is taken from the part attached to the inner surface of the outer tub 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 tub 30 and the first foamed resin layer 13 may decrease, causing the first foamed resin layer 13 to peel off from the outer tub 30.

[0041] In the second step S2, the worker N sprays 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 having a lower density than the first foamed resin layer 13. Specifically, the polyol (corresponding to "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.

[0042] The configuration of the relaxing layer 10 and the construction method thereof according to the present embodiment have been described above. Next, the effects of the relaxing layer 10 and the construction method thereof will be described.

[0043] 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, shrinking and breaking. At this time, since the second foamed resin layer 14 has a high ratio of bubbles Q having a continuous cell structure communicating 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 more flexible and has a smaller compressive strength than the first foamed resin layer 13, the surface (skin layer) is easily broken, but the breakage is unlikely 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 that 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. In addition, 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.

[0044] Furthermore, 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.

[0045] Specifically, in the 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, 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 the application areas W, which requires a huge amount of labor and cost. Moreover, labor and cost are 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.

[0046] In addition, since the cutting process is intended to flatten the surface, it is necessary to perform the cutting process after the foaming and curing of the first foamed resin layer 13 has progressed and the layer has developed sufficient strength. If cutting or grinding is performed before the layer has developed sufficient strength, the layer may not be cut flat or may tear. The estimated time required for the layer to develop sufficient strength 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 first step S1 has progressed in curing and the surface has lost its tack. This eliminates the need to wait for the foaming and curing of the first foamed resin layer 13 in the first step S1. This avoids the above-mentioned problems and improves workability.

[0047] [Confirmatory experiment] It was confirmed by an experiment that the relaxation layer 10 of the above embodiment can mitigate a thermal shock when subjected to a thermal shock by protecting the first foamed resin layer 13 made of a hard urethane foam with the second foamed resin layer 14 made of a low-density hard urethane foam. In this experiment, the relaxation 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 hard urethane foam cracked. The temperature of liquid nitrogen is -196°C, which is a more severe condition than liquefied natural gas L at 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.

[0048] Specifically, a dismantled metal mold with inner dimensions of 1600 mm length x 700 mm width x 100 mm thickness and open at the top was prepared. The metal mold was stood upright (the bottom was in the length direction and thickness direction), and the bottom surface of the metal mold (opposite the open surface) was treated as the outer tank 30. The urethane foam raw material for the first foamed resin layer 13 was sprayed to form an under-blow layer 12 of about 3 mm, and then the first foamed resin layer 13 made of hard urethane foam with a thickness of 50 mm (two-layer structure, each layer is 25 mm thick) was formed. Furthermore, the urethane foam raw material for the second foamed resin layer 14 was 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 produce a test piece. Then, the test piece thus prepared was turned upside down (the length direction and width direction were the bottom surfaces), 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 the dye 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 prepared which had only the first foamed resin layer (50 mm thick (two-layer structure, each layer 25 mm thick)) without the second foamed resin layer 14 made of low-density rigid polyurethane foam; comparative sample 2 was prepared in which the urethane foam raw material for the first foamed resin layer was sprayed onto the first foamed resin layer (50 mm thick (two-layer structure, each layer 25 mm thick)) to laminate a 15 mm thick rigid polyurethane foam; and for reference, a reference sample (conventional configuration) was prepared in which a reinforcing sheet was fixed to the surface of the first foamed resin layer (50 mm thick (two-layer structure, each layer 25 mm thick)) with adhesive.

[0049] As a result, no cracks were generated in the first foamed resin layer 13 of the relaxation 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 relaxation 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 the comparative sample 1 and the comparative sample 2. From this experiment, it was confirmed that the occurrence of cracks in the rigid urethane foam of the first foamed resin layer 13 can be suppressed when subjected to a thermal shock 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 was also confirmed that the relaxation layer of the present disclosure can mitigate thermal shock to the same extent as a relaxation layer having a reinforcing sheet on the surface of the conventional first foamed resin layer.

[0050] [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.

[0051] (2) In the above embodiment, the tank portion 40 includes the ceiling portions 21, 31. However, the tank portion 40 may include a lid and have an open top.

[0052] (3) In the above embodiment, the second foamed resin layer 14 made of low-density rigid urethane foam is a single layer, but a plurality of layers may be laminated.

[0053] (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.

[0054] At this time, between the first step S1 and the second step S2, a step S12 is performed in which a reinforcing sheet is laminated on the first foamed resin layer 13. In step S12, the reinforcing sheet may be laminated on 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 for attaching the reinforcing sheet to the first foamed resin layer 13 is not required, and the step of flattening the first foamed resin layer 13 is also not required.

[0055] (5) In the above embodiment, the second foamed resin layer 14 is a low-density rigid urethane foam, but 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.

[0056] (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 foaming and hardening it, but it may also be formed by fixing a pre-manufactured panel made of low-density rigid urethane foam to the first foamed resin layer 13.

[0057] In addition, although this specification and drawings disclose specific examples of the technology included in the scope of the claims, 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 alone.

[0058] <Additional Notes> Hereinafter, the first to eleventh aspects of the invention extracted from the above-mentioned embodiment will be described, showing effects and the like as necessary.

[0059] [First aspect] A low-temperature liquid storage tank comprising an inner tank for storing a low-temperature liquid below 0°C, an outer tank 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 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 communicating in the planar direction.

[0060] According to the first aspect of the invention, the second foamed resin layer has a bubble structure that is interconnected in the planar direction, so that the thermal shock of the low-temperature liquid spreads more in the planar direction than in the thickness direction, the first foamed resin layer is cooled more slowly, and the thermal shock transmitted to the first foamed resin layer is mitigated.

[0061] [Second aspect] The cryogenic liquid storage tank according to the first aspect, wherein the second foamed resin layer has a higher porosity than the first foamed resin layer.

[0062] [Third aspect] The cryogenic liquid storage tank according to the first or second aspect, wherein the second foamed resin layer has a porosity of 97.5% or more.

[0063] 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 that the breakage range of the second foamed resin layer is shallower and the first foamed resin layer is less susceptible to the breakage energy that occurs in the second foamed resin layer. The porosity of the second foamed resin layer is preferably 97.5% or more (third aspect).

[0064] [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.

[0065] [Fifth aspect] The thermal resistance value of the second foamed resin layer is 0.30 m 2 The low temperature liquid storage tank according to the fourth aspect, wherein the low temperature liquid storage tank has a melting point of 1.0 K / W or more.

[0066] According to the fourth aspect of the present invention, the second foamed resin layer transmits cold to the first foamed resin layer, and the first foamed resin layer is gradually cooled. 2 When the cooling rate is .K / W or more, it is considered that the cooling rate of the first foamed resin layer becomes appropriate (fifth embodiment).

[0067] [Sixth aspect] The low-temperature liquid storage tank according to any one of the first to fifth aspects, wherein 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%.

[0068] 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 that 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.

[0069] [Seventh aspect] 8. The cryogenic liquid storage tank according to claim 1, wherein the first foamed resin layer and the second foamed resin layer are made of urethane foam.

[0070] [Eighth aspect] A method for producing 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.

[0071] [Ninth aspect] A method for producing 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 the inner tank side surface of the first foamed resin layer and foaming and curing the raw material to form the second foamed resin layer; A manufacturing method of a low-temperature liquid storage tank.

[0072] [Tenth aspect] A method for producing 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 step of installing a resin panel, in which the first foamed resin layer and the second foamed resin layer are laminated, on the inner surface of the outer tank.

[0073] [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 for preventing leakage of the low-temperature liquid and mitigating thermal shock, the first foamed resin layer being laminated on the inner surface of the outer tank ... 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.

[0074] 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. [Explanation of symbols]

[0075] 10 relaxation layer 13 First foamed resin layer 14 Second foamed resin layer 20 Inner tank 30 Outer tank 50 Liquid dike 100 Low temperature liquid storage tank L Liquefied natural gas (low temperature liquid)

Claims

1. 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 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.

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 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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