Method for operating liquefied gas storage tank and liquefied gas storage tank

Abstract

Provided is a method for operating a liquefied gas storage tank (1) comprising: an inner tank (4) for storing a liquefied gas; an outer tank (3) for accommodating the inner tank (4) with a cold-reserving layer (13) therebetween; and a powder heat-insulation material (15) with which the cold-reserving layer (13) is filled in a state in which a gap is generated with respect to a roof (33) of the outer tank (3). The method comprises: performing a hot-up in which the liquefied gas is removed from the inner tank (3) to bring the temperature of the inner tank (3) close to room temperature; and performing a powder fluidization in which a gas is supplied to the cold-reserving layer (13) during the hot-up, and the powder heat-insulation material (15) is fluidized by the supplied gas.

Description

Liquefied gas storage tank operation method and liquefied gas storage tank

[0001] The present disclosure relates to a liquefied gas storage tank in which a cold insulation layer between an outer tank and an inner tank is filled with powder insulation material.

[0002] The liquefied gas storage tank may be opened for maintenance or the like, as described in, for example, Patent Document 1 below. Specifically, Patent Document 1 describes the following steps before opening a liquefied gas storage tank (low-temperature tank): draining the remaining liquid from the inner tank; discharging combustible gas (boil-off gas) from the tank using an inert gas such as nitrogen gas supplied from outside after the remaining liquid is drained; heating the inner tank using the same inert gas; and introducing outside air into the inner tank after the inner tank has been heated. This creates an environment inside the tank that is suitable for workers, allowing maintenance of the tank.

[0003] As mentioned above, before opening a liquefied gas storage tank, it is necessary to warm up the inner tank. Hereinafter, this process will be referred to as "hot-up." When hot-up is performed, the temperature of the inner tank, which had dropped to about the same temperature as the liquefied gas during tank operation, rises to near room temperature. This causes the inner tank to expand, reducing the volume of the cold insulation layer between the inner and outer tanks. In cases where the volume of the cold insulation layer reduces relatively significantly, the powder insulation material filled in the cold insulation layer may be compacted, potentially applying a large amount of pressure to the inner tank that could buckle it.

[0004] JP 2015-25467 A

[0005] The present disclosure has been made in consideration of the above circumstances, and aims to prevent powder insulation from being compacted when a liquefied gas storage tank is heated up.

[0006] In order to solve the above-mentioned problems, one aspect of the present disclosure provides an operating method for a liquefied gas storage tank having an inner tank for storing liquefied gas, an outer tank that accommodates the inner tank via an insulation layer, and powder insulation filled in the insulation layer with a gap between the inner tank and the roof of the outer tank, the operating method including: performing a hot-up process by removing liquefied gas from the inner tank to bring the temperature of the inner tank closer to room temperature; and performing powder fluidization during the hot-up process by supplying gas to the insulation layer and fluidizing the powder insulation with the supplied gas.

[0007] In addition, a liquefied gas storage tank according to another aspect of the present disclosure includes an inner tank for storing liquefied gas, an outer tank for accommodating the inner tank via an insulation layer, powder insulation material filled in the insulation layer with a gap formed between the inner tank and the roof of the outer tank, and a gas supply device capable of supplying gas to the insulation layer to fluidize the powder insulation material.

[0008] According to the present disclosure, it is possible to prevent powder insulation from being compacted when a liquefied gas storage tank is heated up.

[0009] Fig. 1 is a cross-sectional view schematically showing a liquefied gas storage tank according to an embodiment of the present disclosure; Fig. 2 is a schematic plan view showing the structure of a supply ring; Fig. 3 is a cross-sectional view showing a procedure (part 1) for hotting up the tank body; Fig. 4 is a cross-sectional view showing a procedure (part 3) for hotting up the tank body; Fig. 5 is a cross-sectional view showing a procedure (part 4) for hotting up the tank body.

[0010] Hereinafter, embodiments of a liquefied gas storage tank according to the present disclosure will be described in detail with reference to the drawings. The liquefied gas storage tank according to the present disclosure is a tank for storing low-temperature liquefied gas. The stored liquefied gas may be, for example, liquefied hydrogen, liquid helium, liquid nitrogen, liquefied natural gas, or liquefied petroleum gas. In the following embodiments, a flat-bottomed cylindrical double-shell tank containing liquefied hydrogen as the liquefied gas storage tank will be exemplified. Of course, the present disclosure can also be applied to double-shell tanks other than flat-bottomed cylindrical tanks, or multi-shell tanks such as triple-shell tanks.

[0011] [Structure of Liquefied Gas Storage Tank] Figure 1 is a cross-sectional view showing a schematic diagram of a liquefied gas storage tank 1 according to one embodiment of the present disclosure. As shown in this figure, the liquefied gas storage tank 1 is a tank for storing liquefied hydrogen (LH2), which is a cryogenic liquefied gas, and includes a ground-mounted tank body 2 erected on a tank foundation 10, a gas supply device 5 that supplies gas to the tank body 2, and a gas replacement device 6 that replaces the gas in the tank body 2.

[0012] The tank body 2 is a flat-bottomed cylindrical double-shell tank, and includes an outer tank 3 and an inner tank 4 housed inside the outer tank 3. The outer tank 3 and the inner tank 4 are each formed in a circular shape when viewed from above, and are arranged concentrically. Liquefied hydrogen is stored in a storage space defined inside the inner tank 4.

[0013] The outer tank 3 is a sealed body made of metal such as carbon steel, and includes a disk-shaped outer tank bottom plate 31 laid on the upper surface of the tank foundation 10, a cylindrical outer tank side plate 32 erected from the periphery of the outer tank bottom plate 31, and an outer tank roof 33 that closes the upper opening of the outer tank side plate 32. The outer tank roof 33 is a dome-shaped roof with a convex spherical surface on the upper side.

[0014] The inner tank 4 is made of low-temperature steel such as austenitic stainless steel, nickel steel, or aluminum alloy, and is housed inside the outer tank 3. The inner tank 4 includes a disk-shaped inner tank bottom plate 41 having a smaller diameter than the outer tank bottom plate 31, a cylindrical inner tank side plate 42 standing on the periphery of the inner tank bottom plate 41, and an inner tank roof 43 that closes the upper opening of the inner tank side plate 42. The inner tank roof 43 is a dome-shaped roof with a convex spherical surface on the upper side.

[0015] The upper layer of the inner tank 4 is a gas phase section SG. The gas phase section SG is a space filled with hydrogen gas (GH2) evaporated from the liquefied hydrogen, and is formed between the inner tank roof 43 and the liquid surface S1 of the liquefied hydrogen (LH2).

[0016] A bottom cold insulation layer 11 is interposed between the inner tank bottom plate 41 and the tank foundation 10. The bottom cold insulation layer 11 is an insulating layer for suppressing heat input from the tank foundation 10 to the inner tank bottom plate 41. The bottom cold insulation layer 11 can be formed, for example, from a laminate of solid insulating materials such as foam glass or lightweight aerated concrete.

[0017] A cold insulation layer 13 is formed between the outer tub 3 and the inner tub 4. The cold insulation layer 13 is a gap (space) of a predetermined width formed between the inner tub 4 and the outer tub 3. That is, the cold insulation layer 13 includes a cylindrical ring-shaped side cold insulation layer 13a between the outer tub side plate 32 and the inner tub side plate 42, and a dome-shaped roof cold insulation layer 13b between the outer tub roof 33 and the inner tub roof 43. The cold insulation layer 13 has the function of suppressing heat input from the outside air to the inner tub side plate 42 and the inner tub roof 43.

[0018] The cold insulation layer 13 is filled with powder insulation material 15 to enhance the cold insulation performance. The powder insulation material 15 is a fluid insulation material made of an aggregate of particles such as perlite or glass bubbles. In other words, the outer vessel 3 accommodates the inner vessel 4 via the cold insulation layer 13 filled with the powder insulation material 15.

[0019] The powder insulation material 15 is filled so as to occupy most of the cold insulation layer 13, except for a portion of the roof cold insulation layer 13b that contacts the outer tank roof 33. That is, an unfilled space NS where no powder insulation material 15 is present is formed in the roof cold insulation layer 13b. In the example of Figure 1, the unfilled space NS is a ring-shaped space defined between the peripheral edge of the outer tank roof 33 and the upper surface of the powder insulation material 15. In other words, the powder insulation material 15 is filled in the cold insulation layer 13 with a gap (unfilled space NS) formed between the outer tank roof 33 and the powder insulation material 15.

[0020] The non-filled space NS is formed naturally during construction of the liquefied gas storage tank 1. That is, during construction of the liquefied gas storage tank 1, after the outer tank 3 and inner tank 4 are constructed, powder insulation material 15 is filled into the cold insulation layer 13 through a manhole or the like (not shown) formed in the outer tank 3. At this time, the outer tank 3 and inner tank 4 are at room temperature. Therefore, when liquefied hydrogen is subsequently introduced into the inner tank 4 and the inner tank 4 is cooled, the inner tank 4 contracts as it cools. This contraction of the inner tank 4 expands the volume of the cold insulation layer 13. Therefore, even if the powder insulation material 15 filled the cold insulation layer 13 almost completely before the inner tank 4 contracted, the non-filled space NS will naturally be formed in part of the cold insulation layer 13. Note that, because the powder insulation material 15 collects downward due to gravity, the non-filled space NS is formed only at the upper end of the cold insulation layer 13, i.e., in a portion of the roof portion cold insulation layer 13b that contacts the outer tank roof 33. As described above, the liquefied gas storage tank 1 necessarily has an unfilled space NS (gap) in the roof insulation layer 13b due to its structure or construction reasons.

[0021] The cold insulation layer 13 is filled with a seal gas. Specifically, the cold insulation layer 13 is filled with a gas corresponding to the liquid (liquefied hydrogen) in the inner tank 4, i.e., hydrogen gas, as the seal gas. In this embodiment, the seal gas is hydrogen gas supplied from the gas phase section SG of the inner tank 4. That is, the hydrogen gas introduced into the cold insulation layer 13 from the gas phase section SG through a communication pipe 45, which will be described later, is used as the seal gas for the cold insulation layer 13.

[0022] A resin-based insulating material 17 is disposed on the inner periphery of the side cold insulation layer 13a surrounding the inner tank side plate 42. The resin-based insulating material 17 is a cotton-like insulating material having a predetermined thickness in the radial direction of the tank body 2. Suitable examples of the resin-based insulating material 17 include glass wool and rock wool. The resin-based insulating material 17 contacts the inner tank side plate 42 and is disposed only on a portion of the radially inner side of the side cold insulation layer 13a. The outer region of the resin-based insulating material 17 in the side cold insulation layer 13a is occupied by powder insulating material 15.

[0023] The inner tank roof 43 is provided with a communication pipe 45 (communication portion) that connects the interior of the inner tank 4 with the cold insulation layer 13. The communication pipe 45 is a circular pipe with a relatively short axial length in the vertical direction, and is provided so as to penetrate the top of the dome-shaped inner tank roof 43. A breathable filter 46 is attached to the upper end of the communication pipe 45. The filter 46 is made of a material that can block the flow of the powder insulation material 15 while allowing the flow of gases such as hydrogen gas. Such a communication pipe 45 plays a role in balancing the pressure of the cold insulation layer 13 and the pressure of the gas phase portion SG of the inner tank 4.

[0024] The gas supply device 5 is a device that fluidizes the powder insulation material 15 by supplying gas to the cold insulation layer 13. The gas supply device 5 is used when hotting up the tank body 2 for maintenance or the like. Hotting up is a process in which liquefied hydrogen is removed from the inner tank 4 to bring the temperature of the inner tank 4 closer to room temperature. Details of hotting up will be described later.

[0025] The gas supply device 5 fluidizes the powder insulation material 15 by circulating hydrogen gas generated by the evaporation of liquefied hydrogen, i.e., boil-off gas, inside and outside the tank body 2 and supplying it to the cold insulation layer 13. Specifically, the gas supply device 5 includes a blower 51 provided outside the tank body 2, a plurality of supply rings 52 arranged in multiple tiers above and below the side cold insulation layer 13a, an upstream pipe 53 connecting the inside of the inner tank 4 with the blower 51, and a downstream pipe 54 connecting the blower 51 with each supply ring 52.

[0026] FIG. 2 is a schematic plan view showing the structure of the supply ring 52. As shown in FIGS. 1 and 2 , the supply ring 52 is a ring-shaped pipe disposed at the outermost periphery of the side cold insulation layer 13a. That is, the supply ring 52 is formed so as to extend along the inner surface of the outer tank side plate 32 at a position close to the inner surface. The supply ring 52 is formed with a number of nozzle holes h1 serving as outlets for hydrogen gas. The nozzle holes h1 are arranged in a line at predetermined intervals in the direction indicated by arrow A1 along the inner and outer peripheries of the supply ring 52, i.e., in the circumferential direction of the tank body 2. In other words, the nozzle holes h1 are dispersed at various locations on the circumferential surface of the supply ring 52 so that hydrogen gas is sprayed to multiple locations on the side cold insulation layer 13a that are spaced apart in the circumferential direction. The nozzle holes h1 are also dispersed in the circumferential direction of the circle when the supply ring 52 is viewed in cross section, i.e., in the direction perpendicular to arrow A1. Each nozzle hole h1 is covered with a breathable filter made of glass wool or the like to prevent the powder insulating material 15 from entering the inside of the supply ring 52 through the nozzle hole h1.

[0027] As shown in FIG. 1 , in this embodiment, five supply rings, namely, first to fifth supply rings 52A to 52E, are provided as the multiple supply rings 52. The first to fifth supply rings 52A to 52E are arranged in the side cold insulation layer 13a at equal vertical intervals. Specifically, the first supply ring 52A, the second supply ring 52B, and the fifth supply ring 52E are arranged in descending order of height. In other words, the first to fifth supply rings 52A to 52E are arranged at different heights within the side cold insulation layer 13a, with the first supply ring 52A being the tallest and the fifth supply ring 52E being the shortest. In the following description, when referring to the first to fifth supply rings 52A to 52E without distinction, they are simply referred to as supply rings 52.

[0028] The upstream piping 53 is a piping that penetrates the inner tank roof 43 and the outer tank roof 33. That is, the upstream piping 53 is led from the inside of the inner tank 4 to the outside of the outer tank 3 while penetrating the inner tank roof 43 and the outer tank roof 33, and is connected to the blower 51 outside the outer tank 3.

[0029] An upstream valve 55 is provided in the upstream pipe 53. The upstream valve 55 is a valve for opening and closing the upstream pipe 53, and is disposed outside the tank body 2. During normal operation when liquefied hydrogen is stored in the tank body 2, the upstream valve 55 is maintained in a closed state.

[0030] The downstream piping 54 includes a main pipe 54a extending downstream from the blower 51 and a plurality of branch pipes 54b branching from the main pipe 54a and reaching each supply ring 52. Each branch pipe 54b passes through the outer tank side plate 32 to connect the main pipe 54a to each supply ring 52. In this embodiment, five supply rings 52 (first to fifth supply rings 52A to 52E) are provided, and therefore five branch pipes 54b are provided correspondingly.

[0031] Each branch pipe 54b is provided with a downstream valve 56. That is, in this embodiment, the same number (five) of downstream valves 56 as the number of supply rings 52 are provided to open and close each branch pipe 54b connected to the five supply rings 52. In other words, the multiple downstream valves 56 include a first downstream valve 56A that opens and closes the branch pipe 54b for the first supply ring 52A, a second downstream valve 56B that opens and closes the branch pipe 54b for the second supply ring 52B, a third downstream valve 56C that opens and closes the branch pipe 54b for the third supply ring 52C, a fourth downstream valve 56D that opens and closes the branch pipe 54b for the fourth supply ring 52D, and a fifth downstream valve 56E that opens and closes the branch pipe 54b for the fifth supply ring 52E. In the following description, when the first to fifth downstream valves 56A to 56E are not to be distinguished from one another, they will simply be referred to as downstream valves 56.

[0032] The blower 51 is a device that pumps hydrogen gas and functions as a pressure source for circulating hydrogen gas inside and outside the tank body 2. As shown in FIG. 4 (described later), the blower 51 draws hydrogen gas from the inner tank 4 to the outside of the outer tank 3 via an upstream pipe 53 and pumps the drawn hydrogen gas to each supply ring 52 (first to fifth supply rings 52A to 52E) via a downstream pipe 54. Each supply ring 52 ejects hydrogen gas from its nozzle hole h1 into the side cold insulation layer 13a. The ejected hydrogen gas travels from the side cold insulation layer 13a to the roof cold insulation layer 13b, and then travels through the connecting pipe 45 from the roof cold insulation layer 13b into the inner tank 4. The hydrogen gas in the inner tank 4 is again sucked out of the outer tank 3 by the blower 51. In this way, the blower 51 generates a circulating flow of hydrogen gas that flows from the inside of the inner tank 4 to the outside of the outer tank 3 and returns to the inner tank 4.

[0033] The circulating flow of hydrogen gas as described above fluidizes the powder insulation material 15 in the cold insulation layer 13. That is, while the circulating flow is being generated, hydrogen gas is sprayed onto the cold insulation layer 13 from the numerous nozzle holes h1 in each supply ring 52, causing the powder insulation material 15 in the cold insulation layer 13 to become suspended in air and change into a state with liquid-like fluidity.

[0034] The gas replacement device 6 is a device for replacing the gas inside the tank body 2 with a safe gas during maintenance of the tank body 2. The gas replacement device 6 includes a replacement gas supply source 61 arranged outside the tank body 2, an inlet pipe 62 that connects the supply source 61 with the inside of the inner tank 4, an outlet pipe 63 that leads from the roof cold insulation layer 13b to the outside of the outer tank 3, an openable / closable first valve 64 provided on the inlet pipe 62, and an openable / closable second valve 65 provided on the outlet pipe 63. During normal operation when liquefied hydrogen is stored in the tank body 2, the first valve 64 and the second valve 65 are maintained in a closed state.

[0035] The replacement gas supplied from the supply source 61 may be any inert gas, but in this embodiment, nitrogen gas is used. The nitrogen gas supplied from the supply source 61 is introduced into the inner tank 4 through the inlet pipe 62. The introduced nitrogen gas drives the hydrogen gas (boil-off gas) remaining in the inner tank 4 into the cold insulation layer 13 through the communicating pipe 45, and then discharges it to the outside of the outer tank 3 through the outlet pipe 63. As a result, the gas in the inner tank 4 is replaced from hydrogen gas to nitrogen gas.

[0036] A discharge pipe 71 for discharging liquefied hydrogen from the tank body 2 is connected to the tank body 2. The discharge pipe 71 is led out from near the bottom of the inner tank 4 to the outside of the outer tank 3. A third valve 72 that can be opened and closed is provided on the discharge pipe 71. The third valve 72 is maintained in a closed state during normal operation of the tank body 2.

[0037] [Hot-up Procedure] Next, we will explain the procedure for hot-up of the tank body 2. Hot-up is a process for removing liquefied hydrogen from the inner tank 4 and bringing the inner tank 4 to room temperature for maintenance or the like, and is performed in the following procedure.

[0038] First, as shown in Fig. 3, liquefied hydrogen (LH2) is discharged from the tank body 2. That is, by opening the third valve 72, the liquefied hydrogen in the inner tank 4 is discharged to the outside of the tank body 2 through the discharge piping 71. As a result, the liquefied hydrogen disappears from the inner tank 4, and the inside of the inner tank 4 becomes occupied by boil-off gas, i.e., hydrogen gas (GH2) generated by vaporization of the liquefied hydrogen.

[0039] Next, as shown in FIG. 4 , hydrogen gas (GH2) is circulated inside and outside the tank body 2 using the gas supply device 5. That is, the upstream valve 55 is opened, and at least one of the first to fifth downstream valves 56A to 56E is opened. In this state, the blower 51 is operated. As a result, hydrogen gas is sucked from the inner tank 4 to the outside of the outer tank 3 through the upstream piping 53, and the sucked hydrogen gas is introduced into the cold insulation layer 13 through the downstream piping 54 and the supply ring 52. The hydrogen gas introduced into the cold insulation layer 13 is returned to the inside of the inner tank 4 through the communicating pipe 45. In this way, a circulating flow of hydrogen gas is generated, flowing from the inside of the inner tank 4 to the outside of the outer tank 3 and returning to the inner tank 4.

[0040] The above-described operation of circulating hydrogen gas fluidizes the powder insulation material 15 filled in the cold insulation layer 13. That is, hydrogen gas introduced into the supply ring 52 from the downstream piping 54 is sprayed onto the cold insulation layer 13 from the numerous nozzle holes h1 of the supply ring 52, thereby fluidizing (suspending) the powder insulation material 15. As will be described in detail later, this fluidization of the powder insulation material 15 serves to prevent consolidation of the powder insulation material 15 that may be caused by expansion of the inner vessel 4 due to heating up.

[0041] In this embodiment, hydrogen gas is sequentially ejected from multiple (five) supply rings 52 of different heights so that the powder insulation material 15 is fluidized from the top down. Specifically, the first downstream valve 56A, the second downstream valve 56B, ..., and the fifth downstream valve 56E are opened in this order so that hydrogen gas is supplied from the highest position to the lowest position in the side cold insulation layer 13a. At this time, when the second and subsequent valves are opened, the previously opened valves are maintained open so that the number of open valves increases over time. As a result, hydrogen gas is ejected from the first supply ring 52A, the second supply ring 52B, ..., and the fifth supply ring 52E in this order, and eventually, hydrogen gas is ejected from all supply rings 52A to 52E, as shown in FIG. 5 . By starting the ejection of hydrogen gas earlier for the supply rings 52 located higher, the powder insulation material 15 is fluidized from the top down, and ultimately the powder insulation material 15 is fluidized as a whole.

[0042] Furthermore, the circulation of hydrogen gas by the gas supply device 5 described above gradually increases the temperature of the hydrogen gas. That is, during the circulation process, the hydrogen gas is repeatedly compressed by the blower 51. Furthermore, the hydrogen gas receives heat input from the outside air as it passes through the piping outside the tank body 2, i.e., the downstream portion of the upstream piping 53 and the downstream piping 54. The temperature of the hydrogen gas gradually increases due to this compression by the blower 51 and heat input from the outside air. As the temperature of the hydrogen gas increases, the temperature of the inner tank 4 also increases, and the tank body 2 is heated up.

[0043] As described above, the circulation of hydrogen gas by the gas supply device 5 has the effect of raising the temperature of the inner vessel 4 while fluidizing the powder insulation material 15. The circulation of hydrogen gas continues until the temperature of the inner vessel 4 has risen sufficiently and approached room temperature, at which point it is stopped. For example, if the gas supply device 5 is equipped with a temperature sensor that detects the temperature of the circulating hydrogen gas, it is conceivable that the circulation of hydrogen gas will be stopped when the temperature of the hydrogen gas detected by this temperature sensor rises to a predetermined threshold value close to room temperature. Note that, to stop the circulation of hydrogen gas, the operation of the blower 51 is stopped and the upstream valve 55 and the first to fifth downstream valves 56A to 56E are all closed.

[0044] Once the circulation of hydrogen gas is stopped, the gas in the tank body 2 is replaced from hydrogen gas (GH2) to nitrogen gas (GN2) using a gas replacement device 6, as shown in Figure 6. That is, with the first valve 64 and the second valve 65 both open, nitrogen gas is supplied from a supply source 61 to the inner tank 4 through an inlet pipe 62. The supplied nitrogen gas pushes the hydrogen gas present in the inner tank 4 from the inner tank 4 through the connecting pipe 45 and into the cold insulation layer 13. The hydrogen gas in the cold insulation layer 13 is also pushed by the nitrogen gas into the outlet pipe 63 and discharged to the outside of the tank body 2 through the outlet pipe 63. As a result, the gas in the inner tank 4 and the cold insulation layer 13 is replaced from hydrogen gas to nitrogen gas.

[0045] By replacing the gas inside the tank body 2 with nitrogen gas in the above manner, it becomes possible to open the tank body 2 and perform maintenance. For example, a manhole provided in the outer tank 3 is opened and air is introduced into the tank body 2 through the manhole. This allows workers to enter the tank body 2 and perform maintenance on the tank body 2.

[0046] [Effects] As described above, in this embodiment, when the tank body 2 is heated up, hydrogen gas is supplied to the cold insulation layer 13 using the gas supply device 5, thereby fluidizing the powder insulation material 15 in the cold insulation layer 13. This configuration has the advantage of preventing the powder insulation material 15 from being compacted during the hot-up process.

[0047] During normal operation when liquefied hydrogen is stored in the tank body 2, the inner tank 4, which is in contact with the liquefied hydrogen, is in an extremely low temperature state close to the temperature of the liquefied hydrogen. When hot-up is performed from this state, the temperature of the inner tank 4 rises significantly to a temperature close to room temperature, causing the inner tank 4 to expand. On the other hand, the outer tank 3, which is in contact with the outside air, is in a state close to room temperature both before and after hot-up. Therefore, when the inner tank expands due to hot-up, the volume of the cold insulation layer 13 between the outer tank 3 and the inner tank 4 decreases accordingly, which may cause the powder insulation material 15 in the cold insulation layer 13 to be compacted. In contrast, in this embodiment, hydrogen gas is supplied to the cold insulation layer 13 during hot-up, fluidizing the powder insulation material 15. Therefore, even if the volume of the cold insulation layer 13 decreases due to the expansion of the inner tank 4, the powder insulation material 15 can escape from high-pressure areas to low-pressure areas within the cold insulation layer 13, preventing the powder insulation material 15 from being compacted. In addition, by preventing the powder insulation material 15 from being compressed, the large stresses that accompany the compression can be prevented from being applied to the inner tank 4, thereby preventing situations such as the stress causing the inner tank 4 to buckle.

[0048] In addition, in this embodiment, resin-based insulation material 17 is placed in the side cold insulation layer 13a, and the cushioning effect of this resin-based insulation material 17 relieves the pressure of the powder insulation material 15, making it possible to prevent compaction of the powder insulation material 15 with a higher probability.

[0049] In this embodiment, the gas supply device 5 is provided with a plurality of supply rings 52 arranged in multiple tiers in the cold insulation layer 13 (side cold insulation layer 13a), and a number of nozzle holes h1 for spraying hydrogen are formed in each supply ring 52. With this configuration, hydrogen gas can be supplied to the cold insulation layer 13 from a number of locations that are separated in the vertical and circumferential directions, and the powder insulation material 15 in the cold insulation layer 13 can be appropriately fluidized.

[0050] In particular, in this embodiment, a downstream valve 56 is provided on each branch pipe 54b connected to each supply ring 52, and each downstream valve 56 is controlled so that the injection of hydrogen gas begins in order, starting from the supply ring 52 that is located at the highest position. With this configuration, the powder insulation material 15 in the cold insulation layer 13 is fluidized in order from the top, thereby making it possible to uniformly fluidize the powder insulation material 15 over a wide range.

[0051] The powder insulation material 15 in the lower part of the cold insulation layer 13 is subjected to greater pressure due to its own weight than the powder insulation material 15 in the upper part of the cold insulation layer 13. Therefore, the powder insulation material 15 is more difficult to fluidize in the lower part than in the upper part of the cold insulation layer 13. On the other hand, the powder insulation material 15 in the upper part of the cold insulation layer 13 is subjected to less pressure due to its own weight and is closer to the unfilled space NS of the powder insulation material 15 formed near the outer tank roof 33, making it relatively easier to fluidize. In this situation, if hydrogen gas begins to be ejected simultaneously from multiple supply rings 52 at different heights, the powder insulation material 15 may fluidize only in the upper part of the cold insulation layer 13, but not in the lower part of the cold insulation layer 13. For example, in the lower part of the cold insulation layer 13, a phenomenon called channeling may occur, in which a gas passage like an ant's nest is formed within the powder insulation material 15. When channeling occurs, the gas path becomes fixed, making it impossible to fluidize the powder insulating material 15.

[0052] In contrast, in this embodiment, the hydrogen gas starts to be ejected in order from the supply rings 52 located at higher positions, so the powder insulation material 15 can be fluidized in order from the top of the cold insulation layer 13, where fluidization is more likely to occur. If the powder insulation material 15 is fluidized in the upper part of the cold insulation layer 13, the powder insulation material 15 adjacent to the lower side of this fluidized area also becomes more likely to fluidize. Therefore, by fluidizing the powder insulation material 15 in order from the top, as in this embodiment, it is possible to gradually expand the fluidized range of the powder insulation material 15 while preventing the occurrence of phenomena such as the channeling described above, and ultimately uniformly fluidize most of the powder insulation material 15.

[0053] In this embodiment, the gas supply device 5 for supplying hydrogen gas to the cold insulation layer 13 includes the supply ring 52 arranged in the cold insulation layer 13, a blower 51 arranged outside the tank body 2, an upstream pipe 53 connecting the blower 51 to the inside of the inner tank 4, and a downstream pipe 54 connecting the blower 51 to the supply ring 52. With this configuration, hydrogen gas sucked from the inner tank 4 to the outside of the outer tank 3 is supplied to the cold insulation layer 13, and the supplied hydrogen gas can be returned from the cold insulation layer 13 to the inner tank 4 via the connecting pipe 45 penetrating the inner tank 4. In this way, when a circulating flow of hydrogen gas is generated, escaping from the inside of the inner tank 4 to the outside of the outer tank 3 and returning to the inner tank 4, it is not necessary to separately supply gas from outside the tank body 2 to fluidize the powder insulation material 15. Furthermore, because the temperature of the hydrogen gas increases during the circulation process, the fluidization of the powder insulation material 15 and the warming up of the tank body 2 can be carried out in parallel.

[0054] [Modifications] Although preferred embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure.

[0055] For example, a heater for heating the hydrogen gas may be added to the upstream piping 53 or the downstream piping 54 of the gas supply device 5. Adding a heater can increase the temperature rise rate of the hydrogen gas circulating inside and outside the tank body 2, and can shorten the time required to heat up the tank body 2.

[0056] In the above embodiment, the temperature of the inner vessel 4 is raised (heated up) while fluidizing the powder insulation material 15 by generating a circulating flow of hydrogen gas that circulates inside and outside the tank body 2. However, the gas used for fluidization and heating up is not limited to hydrogen gas. For example, when the internal temperature of the inner vessel 4 reaches or exceeds the boiling point of nitrogen (approximately -196°C), nitrogen gas may be used to fluidize and heat up the inner vessel 4.

[0057] The resin-based insulation material 17 disposed in the cold insulation layer 13 (side cold insulation layer 13a) may be omitted. If the powder insulation material 15 can be sufficiently fluidized by gas supply, consolidation of the powder insulation material 15 can be prevented even without the resin-based insulation material 17.

[0058] In the above embodiment, an example in which the present disclosure is applied to a double-shelled liquefied gas storage tank 1 having an outer tank 3 and an inner tank 4 has been described, but the present disclosure can also be applied to a liquefied gas storage tank having an additional tank outside the outer tank, i.e., a triple-shelled tank having an inner tank, an outer tank that houses the inner tank, and an outermost tank that houses the outer tank. The present disclosure can also be applied to such a triple-shelled tank in order to prevent the powder insulation material in the cold insulation layer between the inner and outer tanks from being compacted during heating up.

[0059] [Summary] The above-described embodiment and its modifications include the following disclosures.

[0060] An operating method according to a first aspect of the present disclosure is a method of operating a liquefied gas storage tank having an inner tank for storing liquefied gas, an outer tank that accommodates the inner tank via an insulation layer, and powder insulation material filled in the insulation layer with a gap between the inner tank and the roof of the outer tank, and includes performing a hot-up process in which liquefied gas is removed from the inner tank to bring the temperature of the inner tank closer to room temperature, and performing powder fluidization during the hot-up process in which gas is supplied to the insulation layer and the supplied gas fluidizes the powder insulation material.

[0061] According to this first aspect, when the tank is heated up, gas is supplied to the cold insulation layer to fluidize the powder insulation material, so even if the volume of the cold insulation layer decreases due to the expansion of the inner tank caused by the hot-up, the powder insulation material can escape from high-pressure areas to low-pressure areas within the cold insulation layer, preventing the powder insulation material from being consolidated. Furthermore, by preventing the powder insulation material from being consolidated, it is possible to prevent the large stress that accompanies the consolidation from being applied to the inner tank, preventing situations such as the stress causing the inner tank to buckle.

[0062] In a second aspect of the operating method, in the first aspect, the gas is supplied to the cold insulation layer from a plurality of locations spaced apart in a circumferential direction during the powder fluidization.

[0063] According to the second aspect, the powder insulation material can be appropriately fluidized by utilizing gas supplied from a plurality of locations in the circumferential direction.

[0064] In a third aspect of the present invention, in the first or second aspect, the powder fluidization is performed by supplying the gas to the cold insulation layer from a plurality of locations spaced apart in a height direction.

[0065] According to the third aspect, the powder insulation material can be appropriately fluidized by utilizing gas supplied from a plurality of positions in the height direction.

[0066] The operating method according to the fourth aspect is the same as that of the third aspect, in which the timing at which the gas supply starts during the powder fluidization is varied depending on the height of the supply point, so that the lower the height of the gas supply point, the later the start of the gas supply.

[0067] The powder insulation material in the upper part of the cold insulation layer is more likely to fluidize than the powder insulation material in the lower part of the cold insulation layer. Furthermore, if the powder insulation material in the upper part of the cold insulation layer is fluidized, the powder insulation material adjacent to the fluidized area below this fluidized area is also more likely to fluidize. Therefore, when the powder insulation material is fluidized from the top down, as in the fourth aspect, the fluidized range of the powder insulation material can be gradually expanded, and ultimately, most of the powder insulation material can be uniformly fluidized.

[0068] The operating method according to the fifth aspect is the same as that according to the first to fourth aspects, in which, during the powder fluidization, gas sucked out from the inner tank to the outside of the outer tank is supplied to the cold insulation layer, and the supplied gas is returned from the cold insulation layer to the inner tank via a communication part that penetrates the inner tank.

[0069] In the fifth aspect, when a circulating flow of gas is generated that flows from the inside of the inner tank to the outside of the outer tank and then back to the inner tank, it is not necessary to separately supply gas from outside the tank to fluidize the powder insulation material. In addition, since the temperature of the gas increases during the gas circulation process, it is possible to fluidize the powder insulation material and heat up the tank simultaneously.

[0070] A liquefied gas storage tank according to a sixth aspect of the present disclosure comprises an inner tank for storing liquefied gas, an outer tank for accommodating the inner tank via an insulation layer, powder insulation material filled in the insulation layer with a gap formed between the inner tank and the roof of the outer tank, and a gas supply device capable of supplying gas to the insulation layer to fluidize the powder insulation material.

[0071] According to this sixth aspect, when the tank is heated up, the powder insulation material in the cold insulation layer can be fluidized using a gas supply device, thereby preventing compaction of the powder insulation material that may be caused by expansion of the inner tank due to heating up.

[0072] A seventh aspect of the present invention is a liquefied gas storage tank according to the sixth aspect, wherein the gas supply device includes a supply ring having a plurality of nozzle holes serving as outlets for the gas inside the cold insulation layer.

[0073] According to the seventh aspect, the powder insulating material can be appropriately fluidized by gas ejected from a plurality of nozzle holes in the supply ring arranged in the cold insulation layer.

[0074] An eighth aspect of the present invention is a liquefied gas storage tank according to the seventh aspect, wherein the gas supply device includes a plurality of supply rings disposed at different heights inside the cold insulation layer.

[0075] According to this eighth aspect, by starting the gas injection from the supply ring located at a higher position, for example, the powder insulation material can be fluidized from the top, and ultimately most of the powder insulation material can be fluidized uniformly.

[0076] A liquefied gas storage tank according to a ninth aspect is the liquefied gas storage tank according to the seventh or eighth aspect, wherein the inner tank includes a communication part that connects its interior with the cold insulation layer, and the gas supply device includes the supply ring, a blower that pressurizes the gas, an upstream pipe that connects the interior of the inner tank with the blower, and a downstream pipe that connects the blower with the supply ring.

[0077] According to the ninth aspect, by operating the blower, gas can be sucked from the inner tank to the outside of the outer tank and supplied to the cold insulation layer, and the supplied gas can be returned from the cold insulation layer to the inner tank via the communication part that penetrates the inner tank. In other words, a circulating flow of gas can be generated that flows from inside the inner tank to the outside of the outer tank and then returns to the inner tank.

[0078] REFERENCE SIGNS LIST 1 Liquefied gas storage tank 3 Outer tank 4 Inner tank 5 Gas supply device 13 Cold insulation layer 15 Powder insulation material 45 Connecting pipe (connecting portion) 51 Blower (pressure source) 52 Supply ring 53 Upstream piping 54 Downstream piping h1 Spray hole

Claims

1. A method for operating a liquefied gas storage tank comprising an inner tank for storing liquefied gas, an outer tank for accommodating the inner tank via a cold insulation layer, and powder insulation material filled in the cold insulation layer with a gap between the inner tank and the roof of the outer tank, the method including: performing a hot-up operation in which liquefied gas is removed from the inner tank to bring the temperature of the inner tank closer to room temperature; and performing powder fluidization in which gas is supplied to the cold insulation layer during the hot-up operation and the supplied gas fluidizes the powder insulation material.

2. A method for operating a liquefied gas storage tank as described in claim 1, wherein, in the powder fluidization, the gas is supplied to the cold insulation layer from a plurality of points spaced apart in the circumferential direction.

3. A method for operating a liquefied gas storage tank as described in claim 1, wherein, in the powder fluidization, the gas is supplied to the cold insulation layer from a plurality of points spaced apart in the vertical direction.

4. A method for operating a liquefied gas storage tank as described in claim 3, wherein, in the powder fluidization, the timing at which the gas supply starts is varied depending on the height of the gas supply point so that the lower the height of the gas supply point is, the later the start of the gas supply is.

5. A method for operating a liquefied gas storage tank as claimed in any one of claims 1 to 4, wherein, in the powder fluidization, gas sucked out from the inner tank to the outside of the outer tank is supplied to the cold insulation layer, and the supplied gas is returned from the cold insulation layer to the inner tank via a communication part penetrating the inner tank.

6. A liquefied gas storage tank comprising: an inner tank for storing liquefied gas; an outer tank for accommodating the inner tank via a cold insulation layer; powder insulation material filled in the cold insulation layer with a gap formed between the inner tank and the roof of the outer tank; and a gas supply device capable of supplying gas to the cold insulation layer for fluidizing the powder insulation material.

7. A liquefied gas storage tank as claimed in claim 6, wherein the gas supply device includes a supply ring having a plurality of nozzle holes serving as an outlet for the gas inside the cold insulation layer.

8. A liquefied gas storage tank according to claim 7, wherein the gas supply device includes a plurality of the supply rings disposed at different heights within the cold insulation layer.

9. A liquefied gas storage tank as described in claim 7 or 8, wherein the inner tank includes a communication portion that connects its interior with the cold insulation layer, and the gas supply device includes the supply ring, a blower that pressurizes the gas, an upstream piping that connects the interior of the inner tank with the blower, and a downstream piping that connects the blower with the supply ring.

Images