Purging method for triple-shell tank

Abstract

To provide a purging method for a triple-shell tank, the method enabling shortening of purging time.SOLUTION: There is provided a purging method for a triple-shell tank 1 which comprises an inner tank 4, an intermediate tank 3 and an outer tank 2. an inner cold insulation layer 11 between the inner tank 2 and the intermediate tank 3, and an outer cold insulation layer 12 between the intermediate tank 3 and the outer tank 2, and in which the inside of the inner tank 4 and the inner cold insulation layer 11 are communicated with each other by a communication pipe 44. The purging method for the triple-shell tank includes replacing a gaseous matter present in the outer cold insulation layer 12 with replacement gas using a second purge route different from the first purge route, while replacing a gaseous matter present in the inner tank 4 and the inner cold insulation layer 11 with a replacement gas using a predetermined first purge route that is, purging of the inner tank 4 and the inner cold insulation layer 11 is performed in parallel with purging of the outer cold insulation layer 12 simultaneously.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a method for purging a triple-shell tank having an inner vessel, an intermediate vessel, and an outer vessel. [Background technology]

[0002] Known tanks for storing cryogenic liquefied gases such as liquefied natural gas and liquefied hydrogen include double-shell tanks having an inner tank and an outer tank, and multi-shell tanks such as triple-shell tanks with an intermediate tank between the inner tank and the outer tank.

[0003] When a multi-shell tank is first used or undergoes maintenance, it may be necessary to purge the tank to replace the gas present inside the tank or in the cold insulation layer between the tanks with the required gas. Patent Document 1 discloses a purging method for a double-shell tank that stores liquefied natural gas, in which a purge gas is supplied into the tank from a BOG (boil-off gas) extraction pipe, and the purged gas is discharged from a liquefied natural gas discharge pipe. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2012-207710 A Summary of the Invention [Problem to be solved by the invention]

[0005] In general, in a double-shell tank, the inside of the inner tank and the cold insulation layer between the inner tank and the outer tank are purged at different times. Usually, the cold insulation layer is purged after the inner tank is purged. If such a purging method is applied to a triple-shell tank, the purging process may take a long time.

[0006] That is, in a triple-shell tank, it is necessary to purge the inner tank that stores the liquefied gas, the inner cold insulation layer between the inner tank and the intermediate tank, and the outer cold insulation layer between the intermediate tank and the outer tank. If a method is adopted in which these purging targets are purged in stages, the purging process time will be long. In particular, when high purging accuracy is required, the purging time at each stage will be long, and the purging process will take a significant amount of time.

[0007] An object of the present disclosure is to provide a method for purging a triple-shell tank that can shorten the purging processing time. [Means for solving the problem]

[0008] A method for purging a triple-shell tank according to one aspect of the present disclosure is a method for purging a triple-shell tank comprising an inner tank, an intermediate tank, and an outer tank, an inner cold-insulation layer between the inner tank and the intermediate tank, and an outer cold-insulation layer between the intermediate tank and the outer tank, the interior of the inner tank and the inner cold-insulation layer being connected by a connecting pipe, wherein gas present in the inner tank and the inner cold-insulation layer is replaced with a replacement gas using a predetermined first purge path, while gas present in the outer cold-insulation layer is replaced with a replacement gas using a second purge path different from the first purge path. Effect of the Invention

[0009] According to the present disclosure, a method for purging a triple-shell tank can be provided that can shorten the purging processing time. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view that shows a schematic example of a triple-shell tank to which the purging method of the present disclosure is applicable. [Diagram 2] FIG. 2 is a diagram showing a simplified structure of the purge ring. [Diagram 3] FIG. 3 is a process chart illustrating an example of a method for purging a triple-shell tank according to an embodiment of the present disclosure. [Figure 4] FIG. 4 is a diagram showing the open / closed states of the valves and the purging state in step 1 of the purging method of this embodiment. [Diagram 5] FIG. 5 is a diagram showing the open / closed state of the valves and the purging state in step 2. [Figure 6] FIG. 6 is a diagram showing the open / closed state of the valves and the purging state in step 3. [Figure 7] FIG. 7 is a diagram showing the open / closed state of the valves and the purging state in step 4. [Figure 8] FIG. 8 is a schematic diagram showing the purging process for the inner bottom cold insulation layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of a purging method for a triple-shell tank according to the present disclosure will be described in detail with reference to the drawings. The triple-shell tank to which the purging method of the present disclosure is applied is a ground-mounted tank that stores low-temperature liquefied gas. The stored low-temperature liquefied gas is, for example, liquefied hydrogen, liquid helium, liquid nitrogen, liquefied natural gas, or liquefied petroleum gas. In the following embodiment, as an example, a purging method for a ground-mounted triple-shell flat-bottom cylindrical tank in which the fluid contained in the tank is liquefied hydrogen is illustrated. Of course, the purging method of the present disclosure can also be applied to triple-shell tanks other than flat-bottom cylindrical tanks.

[0012] [Structure of triple-shell tank] First, a structural example of a triple-shell tank to which the purging method of the present embodiment is applied will be described. FIG. 1 is a cross-sectional view showing a triple-shell tank 1 according to one embodiment. The triple-shell tank 1 includes an outer tank 2, an intermediate tank 3, and an inner tank 4 erected on a tank foundation (not shown), and is equipped with a tank body 1T for storing liquefied hydrogen LH. The tank body 1T is in the form of a flat-bottom cylinder, and the outer tank 2, the intermediate tank 3, and the inner tank 4 all have a circular shape when viewed from above and are arranged concentrically. The inner tank 4 is a tank having a storage space 4A inside for actually storing liquefied hydrogen LH. The intermediate tank 3 surrounds the inner tank 4 via an inner cold insulation layer 11. The outer tank 2 surrounds the intermediate tank 3 via an outer cold insulation layer 12.

[0013] The outer tank 2 is a sealed body made of metal such as carbon steel, and includes an outer tank bottom plate 21, an outer tank side plate 22, and an outer tank roof 23. The outer tank bottom plate 21 is laid directly on the tank foundation and has a disk-shaped shape. The outer tank side plate 22 is erected from the periphery of the outer tank bottom plate 21 and has a cylindrical shape. The outer tank roof 23 is attached to the upper end of the cylindrical outer tank side plate 22 so as to close the upper opening of the outer tank side plate 22, and has a dome-shaped shape. The outer tank roof 23 has manholes 24 at appropriate positions for workers to enter and exit the tank main body 1T.

[0014] The intermediate tank 3 is a sealed body and includes an intermediate tank bottom plate 31, an intermediate tank side plate 32, and an intermediate tank roof 33. The intermediate tank bottom plate 31 has a disk-like shape with a smaller diameter than the outer tank bottom plate 21. The intermediate tank side plate 32 stands upright from the periphery of the intermediate tank bottom plate 31 and has a cylindrical shape. The intermediate tank roof 33 is attached to the upper end of the intermediate tank side plate 32 and has a dome-like shape.

[0015] The inner tank 4 is a sealed body made of low-temperature steel such as austenitic stainless steel, nickel steel, or aluminum alloy, and includes an inner tank bottom plate 41, an inner tank side plate 42, and an inner tank roof 43. The inner tank bottom plate 41 has a disk-like shape with a smaller diameter than the intermediate tank bottom plate 31. The inner tank side plate 42 stands upright from the periphery of the inner tank bottom plate 41 and has a cylindrical shape. The inner tank roof 43 is attached to the upper end of the inner tank side plate 42 and has a dome-like shape. Liquefied hydrogen LH is stored in the storage space 4A inside the inner tank 4. The upper layer of the inner tank 4 is a gas phase portion LA where hydrogen gas vaporized from the liquefied hydrogen LH accumulates.

[0016] Between the inner tank 4 and the intermediate tank 3, and between the intermediate tank 3 and the outer tank 2, there is formed a gap of a predetermined width that is used as an insulating space. The gap between the inner tank 4 and the intermediate tank 3 is used as an inner cold insulation layer 11, and the gap between the intermediate tank 3 and the outer tank 2 is used as an outer cold insulation layer 12. The inner cold insulation layer 11 and the outer cold insulation layer 12 are filled with a powder insulation material to improve the cold insulation. For example, granular perlite can be used as the powder insulation material. A heat insulation material 15 such as glass wool is disposed in the area surrounding the side periphery of the stored liquefied hydrogen LH between the intermediate tank side plate 32 and the inner tank side plate 42. The heat insulation material 15 may also be disposed between the outer tank side plate 22 and the intermediate tank side plate 32.

[0017] At the bottom side of the tank body 1T, the inner cold insulation layer 11 has an inner bottom cold insulation layer 13, and the outer cold insulation layer 12 has an outer bottom cold insulation layer 14. The inner bottom cold insulation layer 13 is interposed between the intermediate tank bottom plate 31 and the inner tank bottom plate 41 to insulate the lower surface of the inner tank bottom plate 41. The outer bottom cold insulation layer 14 is interposed between the outer tank bottom plate 21 and the intermediate tank bottom plate 31 to insulate the lower surface of the intermediate tank bottom plate 31. The inner bottom cold insulation layer 13 and the outer bottom cold insulation layer 14 can be composed of a stack of solid insulation materials. For example, foam glass blocks can be used as the solid insulation material. The inner bottom cold insulation layer 13 can be composed of, for example, ALC (lightweight aerated concrete) blocks laid on the stack of foam glass blocks. The outer bottom cold insulation layer 14 can also be composed of a similar solid insulation material.

[0018] The inner cold insulation layer 11 and the outer cold insulation layer 12 are filled with a predetermined seal gas. As the seal gas for the inner cold insulation layer 11, it is desirable to use hydrogen gas or helium gas. In this embodiment, a communication pipe 44 is disposed so as to penetrate the inner tank roof 43, which communicates the storage space 4A of the inner tank 4 with the space of the inner cold insulation layer 11. Therefore, the hydrogen gas in the gas phase part LA of the liquefied hydrogen LH stored in the inner tank 4 can be used as the seal gas. In addition, by installing the communication pipe 44, the pressure of the inner cold insulation layer 11 and the gas phase pressure of the inner tank 4 can be set to be substantially the same. This makes it possible to suppress the liquefaction or solidification of the seal gas of the inner cold insulation layer 11 due to the cold heat of the liquefied hydrogen LH stored in the inner tank 4. The outer cold insulation layer 12 is filled with an inert gas having a boiling point higher than that of hydrogen gas, such as nitrogen gas, as the seal gas. Filling the outer cold insulation layer 12 with the seal gas improves the insulation performance.

[0019] The tank body 1T includes a plurality of pipes extending from the inside to the outside of the tank body 1T. In Fig. 1, the plurality of pipes are shown as a BOG pipe 51, a discharge pipe 52, three roof pipes 53, an inner cold insulation layer pipe 54, a bottom cold insulation layer pipe 55, and an outer cold insulation layer pipe 56. A first valve 61, a second valve 62, a third valve 63, a fourth valve 64, a fifth valve 65, and a sixth valve 66 for opening and closing each pipe are attached to each of the pipes 51 to 56.

[0020] The BOG piping 51 is a piping for extracting the BOG of the liquefied hydrogen LH generated in the storage space 4A of the inner tank 4 to the outside of the tank body 1T. The lower end of the BOG piping 51 opens in the gas phase section LA, and the upper end side vertically penetrates the inner tank roof 43, the intermediate tank roof 33, and the outer tank roof 23 and is drawn out from the roof part of the tank body 1T. The BOG extracted from the BOG piping 51 is used for re-liquefying into liquefied hydrogen in a liquefaction facility, for sending as hydrogen gas to a consumer, for sending to a diffusion facility, etc. In the purging method of this embodiment, the BOG piping 51 is used as a piping for supplying a replacement gas (purge gas) for purging the storage space 4A of the inner tank 4 and the inner cold insulation layer 11.

[0021] The discharge pipe 52 is a pipe for discharging the liquefied hydrogen LH from the tank body 1T. One end of the discharge pipe 52 opens near the lower end of the inner tank 4, and the other end is drawn out to the outside of the tank body 1T. The discharge pipe 52 may be a pump barrel provided with a discharge port and installed so as to penetrate the tank body 1T in the vertical direction. In the purging method of this embodiment, the discharge pipe 52 is used as a pipe for discharging the gas present in the inner tank 4. Note that, instead of the discharge pipe 52, a drain pipe used for draining water during a water filling test of the inner tank 4 or a spare nozzle may be used as an exhaust pipe for the gas in the inner tank 4.

[0022] The roof piping 53 is a piping for discharging gas present in the roof portion 11A between the inner tank roof 43 and the intermediate tank roof 33 in the inner cold insulation layer 11 during the purging process. The lower end of the roof piping 53 opens to the roof portion 11A of the inner cold insulation layer 11. The upper end side of the roof piping 53 penetrates the intermediate tank roof 33 and the outer tank roof 23 vertically and is drawn out from the roof portion of the tank body 1T.

[0023] The inner cold insulation layer piping 54 is a piping that connects the area near the inner bottom cold insulation layer 13 in the side portion 11B between the inner tank side plate 42 and the intermediate tank side plate 32 of the inner cold insulation layer 11 with the outside of the tank body 1T. One end of the inner cold insulation layer piping 54 is connected to a first purge ring 71 arranged near the bottom of the side portion 11B of the inner cold insulation layer 11 so as to surround the periphery of the inner bottom cold insulation layer 13, and the other end is drawn out to the outside of the tank body 1T. In the purging method of this embodiment, the inner cold insulation layer piping 54 is used as a piping that supplies purge gas to the side portion 11B of the inner cold insulation layer 11, or as a piping that exhausts gas present in the side portion 11B.

[0024] FIG. 2 is a diagram showing a simplified structure of the first purge ring 71. The first purge ring 71 is a structure in which a plurality of short pipes 73 are connected in an annular or arc shape by a gas-permeable connecting portion 74. One end of a base pipe 75 is connected to one of the short pipes 73. The other end of the base pipe 75 is connected to a purge gas supply source, or to a recovery device or diffusion device for a post-purge gas containing a purged gas and a purge gas. When the first purge ring 71 is used in a supply system, when the purge gas is supplied to the base pipe 75 as indicated by an arrow F11, the purge gas is discharged from the connecting portion 74 as indicated by an arrow F12. On the other hand, when the first purge ring 71 is used in an exhaust system, when a negative pressure acts on the base pipe 75 as indicated by an arrow F21, the post-purge gas is recovered from the connecting portion 74.

[0025] Returning to Fig. 1, the bottom cold insulation layer piping 55 is a piping that connects the bottom 11C between the inner tank bottom plate 41 and the intermediate tank bottom plate 31 in the inner cold insulation layer 11 with the outside of the tank body 1T. An exhaust opening 551 at one end of the bottom cold insulation layer piping 55 is disposed near the radial center of the inner bottom cold insulation layer 13. The other end of the bottom cold insulation layer piping 55 is drawn out to the outside of the tank body 1T. In the purging method of this embodiment, the bottom cold insulation layer piping 55 is used as a piping that exhausts gas present in the bottom 11C.

[0026] The outer cold insulation layer piping 56 is a piping that connects the area near the outer bottom cold insulation layer 14 at the side bottom of the outer cold insulation layer 12 with the outside of the tank body 1T. One end of the outer cold insulation layer piping 56 is connected to a second purge ring 72 arranged at the side bottom of the outer cold insulation layer 12 so as to surround the periphery of the outer bottom cold insulation layer 14, and the other end is drawn out to the outside of the tank body 1T. In the purging method of this embodiment, the outer cold insulation layer piping 56 is used as a piping that supplies purge gas to the side bottom of the outer cold insulation layer 12. The second purge ring 72 has a configuration similar to that of the first purge ring 71 shown in FIG. 2.

[0027] [Purge process overview] In the purging method of the triple-shell tank 1 of this embodiment, the gas present in the inner tank 4 and the inner cold insulation layer 11 is replaced with a replacement gas (purged gas) using a predetermined first purge path, while the gas present in the outer cold insulation layer 12 is replaced with a purge gas using a second purge path different from the first purge path. In short, the purging of the inner tank 4 and the inner cold insulation layer 11 and the purging of the outer cold insulation layer 12 are carried out simultaneously in parallel.

[0028] The first purge path in this embodiment is a path through which purge gas is supplied from the BOG piping 51 and purged gas is exhausted from the discharge piping 52 for the inner tank 4 and from the roof piping 53, the inner cold insulation layer piping 54, or the bottom cold insulation layer piping 55 for the inner cold insulation layer 11. The second purge path is a path through which purge gas is supplied from the outer cold insulation layer piping 56 and purged gas is exhausted from the manhole 24 of the outer tank roof 23.

[0029] Fig. 3 is a process chart showing an example of the purging method of this embodiment. In Fig. 3, it is assumed that the inner tank 4, the inner cold insulation layer 11, and the outer cold insulation layer 12 are in an air environment exposed to air at time T0 when purging is started. The outer cold insulation layer 12 is purged to replace the air with an inert gas, here nitrogen gas. During this purging of the outer cold insulation layer 12, the inner tank 4 and the inner cold insulation layer 11 are purged in a first stage to replace the air present in both spaces with nitrogen gas, and in a second stage to replace the nitrogen gas filled in both spaces in the first stage purging with hydrogen gas.

[0030] In FIG. 3, the purge for replacing air with nitrogen gas is described as "air purge", and the purge for replacing nitrogen gas with hydrogen gas is described as "N2 purge". "Air purge" is performed on the inner tank 4 from time T0, and "air purge" is performed on the inner cold insulation layer 11 from time T1. As will be described in detail later, the inner cold insulation layer 11 is purged with purge gas introduced from the inner tank 4 through the connecting pipe 44. "Air purge" is also performed continuously on the outer cold insulation layer 12 from time T0. "N2 purge" is performed on the inner cold insulation layer 11 from time T3 after the "air purge" of the inner tank 4 and the inner cold insulation layer 11 is completed, and "N2 purge" is performed on the inner cold insulation layer 11 from the following time T4. "Air purge" of the outer cold insulation layer 12 is also performed during this "N2 purge".

[0031] According to the above-mentioned purging method, the purging of the inner tank 4 and the inner cold insulation layer 11 and the purging of the outer cold insulation layer 12 are performed in parallel, so that the time required for the purging process of the triple-shell tank 1 can be shortened. That is, the "Air purge" and "N2 purge" of the inner tank 4 and the inner cold insulation layer 11 are performed using the first purge path, while the "Air purge" of the outer cold insulation layer 12 is performed using the second purge path. In this way, since the two purge targets are purged simultaneously, the purging process time can be shortened compared to the case where each purging target is purged stepwise. In addition, during the "Air purge" of the outer cold insulation layer 12, the two-stage purging of the "Air purge" and "N2 purge" of the inner tank 4 and the inner cold insulation layer 11 is performed sequentially. When two-stage purging is required, the purging process time is inevitably long, but according to this embodiment, the purging process time of the triple-shell tank 1 as a whole can be shortened.

[0032] [Example of purging process] The purging method of this embodiment is divided into the following steps 1 to 4, which will be described in sequence with reference to Figs. 4 to 7. Figs. 4 to 7 show an example in which the purging process from time T0 to T2 in Fig. 3, i.e., "Air purging" of the inner tank 4 and the inner cold insulation layer 11 and "Air purging" of the outer cold insulation layer 12, are performed in parallel. In Figs. 4 to 7, "Air" is a symbol indicating air, and "N2" is a symbol indicating nitrogen gas. In Figs. 4 to 7, the first valve 61 to the sixth valve 66 that are "closed" are shown in black, and the valves that are "open" are shown in white. Step 1: A step in which the storage space 4A of the inner tank 4 and the outer cold insulation layer 12 are purged in parallel (FIG. 4). Step 2: A step in which the roof portion 11A of the inner cold insulation layer 11 and the outer cold insulation layer 12 are purged in parallel (FIG. 5). Step 3: A step in which purging of the side portion 11B of the inner cold insulation layer 11 and purging of the outer cold insulation layer 12 are carried out in parallel (FIG. 6). Step 4: A step in which the bottom 11C of the inner cold insulation layer 11 and the outer cold insulation layer 12 are purged in parallel (FIG. 7).

[0033] <Process 1> FIG. 4 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging state in the above step 1. The valves that are "opened" in step 1 are the first valve 61 of the BOG pipe 51 and the second valve 62 of the discharge pipe 52 for purging the storage space 4A of the inner tank 4, and the sixth valve 66 of the outer cold insulation layer pipe 56 for purging the outer cold insulation layer 12. The sealing cover of the manhole 24 is opened, and the space of the outer cold insulation layer 12 is in communication with the outside air through the manhole 24. Meanwhile, the third valve 63, the fourth valve 64, and the fifth valve 65 of the roof pipe 53, the inner cold insulation layer pipe 54, and the bottom cold insulation layer pipe 55 that communicate with the inner cold insulation layer 11 are "closed."

[0034] As shown by arrow A1, nitrogen gas is supplied to the BOG pipe 51 as a purge gas, and as shown by arrow A2, the nitrogen gas is introduced into the storage space 4A of the inner tank 4. Instead of the BOG pipe 51, other pipes that communicate the upper part of the storage space 4A with the outside air may be used as an introduction path for introducing the nitrogen gas into the inner tank 4. In this embodiment, an interface IF is created between the two gases based on the density difference between the two gases, and purging is performed by a piston flow method in which one gas is gradually driven in and discharged. Specifically, nitrogen gas heated to about 60°C is introduced into the storage space 4A to make the density difference apparent. Since room temperature air exists in the storage space 4A before purging, an interface IF is created between the nitrogen gas and the air due to the density difference between the two.

[0035] Since the storage space 4A of the inner tank 4 and the inner cold insulation layer 11 are connected by the communication pipe 44, it is necessary to narrow the outlet of the purged air to the discharge pipe 52 when implementing the piston flow method. The third valve 63, the fourth valve 64, and the fifth valve 65, which communicate the inner cold insulation layer 11 with the outside air, are "closed" for the above-mentioned reason, and the interface IF is maintained by this valve operation. In other words, air and nitrogen gas do not mix in the storage space 4A. As the introduction of nitrogen gas into the storage space 4A progresses, the interface IF gradually moves downward in the storage space 4A, and the air is dissipated from the discharge pipe 52 to the outside air in a manner that presses against the nitrogen gas layer, as shown by the arrow A3.

[0036] As shown by arrow A4, nitrogen gas is also supplied to the outer cold insulation layer piping 56 as a purge gas. The supplied nitrogen gas is introduced into the outer cold insulation layer 12 from the second purge ring 72. Air exists in the outer cold insulation layer 12 before purging. This air is expelled by the flow of the supplied nitrogen gas and dissipates into the outside air from the opened manhole 24 as shown by arrow A5. The purging operation for the outer cold insulation layer 12 is the same in the subsequent steps 2 to 4, so a description thereof will be omitted below.

[0037] <Process 2> 5 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging state in the above step 2. The valves that are opened in step 2 are the first valve 61 of the BOG piping 51 and the third valve 63 of the roof piping 53. For example, the oxygen concentration of the gas flowing through the discharge piping 52 is measured during the execution of the previous step 1, and when the oxygen concentration falls below a predetermined control value, it is assumed that the atmosphere in the inner tank 4 has been replaced from air to nitrogen gas, and the second valve 62 is closed while the third valve 63 is opened. By operating the valves in this way, the process moves from step 1 to step 2.

[0038] The purge path in step 2 is a path that passes through the BOG pipe 51 (introduction path), the inner tank 4, the connecting pipe 44, the roof portion 11A of the inner cold insulation layer 11, and the roof pipe 53 (exhaust path) in this order. At the time of transition to step 2, the storage space 4A of the inner tank 4 is replaced with nitrogen gas. From such an inner tank 4, as shown by the arrow A6, nitrogen gas is introduced into the roof portion 11A of the inner cold insulation layer 11 through the connecting pipe 44. Note that the other pipes that communicate with the inner cold insulation layer 11, i.e., the inner cold insulation layer pipe 54 and the bottom cold insulation layer pipe 55, are closed in order to efficiently replace the air in the roof portion 11A with nitrogen gas. The air that was present in the roof portion 11A is pushed out by the introduced nitrogen gas and dissipates from the roof pipe 53 to the outside air as shown by the arrow A7.

[0039] In step 2, after completing the purging of the inner tank 4, nitrogen gas is sent as a purge gas from the inner tank 4 side through the communication pipe 44 to the roof part 11A of the inner cold insulation layer 11, and air is discharged from the roof part 11A, thereby performing purging. The introduction path of the nitrogen gas may be set at an appropriate position in the inner cold insulation layer 11. In this case, however, it is necessary to provide an opening and closing valve in the communication pipe 44. In addition, when the introduction pressure of the nitrogen gas is applied to the inner cold insulation layer 11, a pressure (negative pressure) in a direction to contract the inner tank 4 is generated, and there is a concern that the inner tank 4 may buckle. In contrast, if a path is used to send the purge gas from the inner tank 4 side to the inner cold insulation layer 11, at least a positive pressure can be applied to the inner tank 4, and the above-mentioned concern can be eliminated.

[0040] <Process 3> 6 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging state in the above-mentioned step 3. The valves that are "open" in step 3 are the first valve 61 of the BOG piping 51 and the fourth valve 64 of the inner cold insulation layer piping 54. In step 3 as well, the introduction path of nitrogen gas as a purge gas is the path from the inner tank 4 through the communicating pipe 44 to the roof portion 11A of the inner cold insulation layer 11, as shown by arrows A2 and A6.

[0041] Since the roof piping 53 communicating with the roof portion 11A is closed, the flow of nitrogen gas is directed downward from the side portion 11B of the inner cold insulation layer 11, that is, toward the first purge ring 71 which serves as the inlet of the inner cold insulation layer piping 54 which is open. This flow of nitrogen gas causes the air present in the side portion 11B to be dissipated to the outside air through the inner cold insulation layer piping 54 as shown by the arrow A8. Since the first purge ring 71 is disposed so as to go around near the bottom of the side portion 11B, it is possible to evenly collect the air remaining in the annular side portion 11B, and to dissipate the collected air from the inner cold insulation layer piping 54.

[0042] <Step 4> FIG. 7 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging state in the above step 4. The valves that are "open" in step 4 are the fourth valve 64 of the inner cold insulation layer piping 54 and the fifth valve 65 of the bottom cold insulation layer piping 55. In step 4, the introduction path of nitrogen gas as a purge gas is changed from the BOG piping 51 to the inner cold insulation layer piping 54. The inner bottom cold insulation layer 13 is mostly occupied by solid blocks, etc., and the amount of air to be replaced is small. Therefore, the amount of nitrogen gas supplied is small, so the negative pressure burden on the inner tank 4 is small, and purging is possible without adopting a nitrogen gas introduction path via the inner tank 4.

[0043] As shown by arrow A9, nitrogen gas is supplied to the inner cold insulation layer piping 54 and introduced into the periphery of the bottom 11C of the inner cold insulation layer 11 through the first purge ring 71. That is, the first purge ring 71, which played a role in recovering the replaced air in step 3, plays a role in supplying purge gas in step 4. The introduced nitrogen gas enters the inner bottom cold insulation layer 13. The air remaining in the inner bottom cold insulation layer 13 is recovered from the exhaust opening 551 of the bottom cold insulation layer piping 55 and is dissipated to the outside air through the bottom cold insulation layer piping 55 as shown by arrow A10.

[0044] 8 is a top view of the tank bottom, showing the purging process for the inner bottom cold insulation layer 13 in step 4. Nitrogen gas is introduced from a first purge ring 71 arranged so as to surround the inner bottom cold insulation layer 13 on the radial outer side of the inner tank bottom plate 41. As shown by an arrow A9 in FIG. 8, when nitrogen gas is supplied to the inner cold insulation layer piping 54, the nitrogen gas is sent from a connection part 74 (FIG. 2) of the first purge ring 71 toward the vicinity of the periphery of the inner tank bottom plate 41.

[0045] Then, the nitrogen gas enters the radial center of the inner tank bottom plate 41 while pushing out the air present in the gaps between the blocks of the inner bottom cold insulation layer 13. The exhaust opening 551 of the bottom cold insulation layer piping 55 is disposed near the radial center of the inner tank bottom plate 41. The air pushed by the nitrogen gas is taken into the bottom cold insulation layer piping 55 from the exhaust opening 551 and exhausted as shown by the arrow A10. In this way, the air to be purged is driven from the radial outside to the inside of the inner tank bottom plate 41 and taken out from the exhaust opening 551 near the radial center, so that the purging process can be performed more efficiently.

[0046] <N2パージ> After replacing the nitrogen gas in the inner vessel 4 and the inner cold insulation layer 11 with nitrogen gas, N2 purging to replace the nitrogen gas with hydrogen gas can be performed in the same manner as described above in steps 1 to 4. However, during the piston flow in step 1, heating of the hydrogen gas introduced as a purge gas into the inner vessel 4 is not necessary because the density difference between the hydrogen gas and the nitrogen gas is sufficiently large.

[0047] [Variations] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to these embodiments. For example, the following modified embodiments are possible.

[0048] (1) In the above step 1, an example is shown in which the purge gas is introduced into the inner tank 4 from the BOG piping 51 and purged by the piston flow method. A method of purging by a gas flow of a purge gas may be adopted without adopting the piston flow method. In this case, a purge path may be adopted in which the purge gas is introduced into the inner tank 4 from the delivery piping 52 or the like and exhausted from the BOG piping 51 or the like.

[0049] (2) In step 4, an example has been shown in which the purge gas is introduced from the inner cold insulation layer piping 54 and the first purge ring 71 to the inner cold insulation layer 11. Alternatively, in step 4, similarly to steps 2 and 3, the purge gas may be introduced to the bottom 11C of the inner cold insulation layer 11 through the BOG piping 51 and the communicating pipe 44.

[0050] (3) In purging the inner cold insulation layer 11 in steps 2 to 4, a purge path may be adopted in which the purge gas is introduced from the inner cold insulation layer piping 54 and exhausted from the roof piping 53, or exhausted from both the roof piping 53 and the bottom cold insulation layer piping 55. In this case, it is preferable to adopt a configuration in which a part of the communication pipe 44 is exposed to the outside of the tank body 1T and an opening and closing valve is provided on the exposed part. After completing the purging of the inner tank 4, when purging the inner cold insulation layer 11, the communication pipe 44 is closed with the opening and closing valve to isolate the storage space 4A from the inner cold insulation layer 11, and then the purge gas is introduced from the inner cold insulation layer piping 54.

[0051] [Summary of this disclosure] The specific embodiments described above include disclosures having the following configurations.

[0052] A method for purging a triple-shell tank according to a first aspect of the present disclosure is a method for purging a triple-shell tank comprising an inner tank, an intermediate tank, and an outer tank, an inner cold-insulation layer between the inner tank and the intermediate tank, and an outer cold-insulation layer between the intermediate tank and the outer tank, the interior of the inner tank and the inner cold-insulation layer being connected by a connecting pipe, wherein gas present in the inner tank and the inner cold-insulation layer is replaced with a replacement gas using a predetermined first purge path, while gas present in the outer cold-insulation layer is replaced with a replacement gas using a second purge path different from the first purge path.

[0053] According to the above purging method, the purging of the inner tank and the inner cold insulation layer and the purging of the outer cold insulation layer are performed in parallel, so that the time required for the purging process of the triple-shell tank can be shortened. That is, the first purge path is used to purge the inner tank and the inner cold insulation layer by replacing them with the required replacement gas, while the second purge path is used to purge the outer cold insulation layer by replacing them with the required replacement gas. In this way, the two purging targets are purged simultaneously, so that the purging process time can be shortened compared to the case where each purging target is purged in stages.

[0054] The second aspect of the purging method for a triple-shell tank is the purging method of the first aspect, except that, while replacing the gas in the outer cold insulation layer with the replacement gas, a first stage purging is performed to replace the gas in the inner tank and the inner cold insulation layer with an inert gas, and a second stage purging is performed to replace the inert gas in the inner tank and the inner cold insulation layer with a storage gas stored in the inner tank.

[0055] According to the second aspect, the first and second stages of purging are sequentially performed on the inner tank and the inner cold insulation layer during the purging of the outer cold insulation layer. When two stages of purging are required, the purging time is inevitably long, but according to the second aspect, the purging time of the triple-shell tank as a whole can be shortened.

[0056] A third aspect of the purging method for a triple-shell tank is the purging method of the first or second aspect, wherein the first purge path includes an inlet path for introducing the replacement gas into the inner tank and an outlet path for discharging the replacement gas from the inner cold insulation layer to the outside, and after the gas in the inner tank is replaced with the replacement gas, the replacement gas is introduced from the inner tank to the inner cold insulation layer through the communicating pipe.

[0057] According to the third aspect, after completing the purging of the inner tank, the replacement gas is sent from the inner tank side through a connecting pipe to the inner cold insulation layer, and the replacement gas is discharged from the inner cold insulation layer, so that the purging is performed using the first purge path. If the replacement gas is sent directly to the inner cold insulation layer, a pressure (negative pressure) in a direction that causes the inner tank to contract may be generated, which may cause the inner tank to buckle. In contrast, if the replacement gas is sent from the inner tank side to the inner cold insulation layer, at least a positive pressure may be applied to the inner tank, which may eliminate the above-mentioned concerns.

[0058] A fourth aspect of the purging method for a triple-shell tank is the purging method of the third aspect, wherein the inner tank includes an inner tank roof and an inner tank side plate, the intermediate tank includes an intermediate tank roof and an intermediate tank side plate, the communicating pipe is arranged to penetrate the inner tank roof, and purging of the inner cold insulation layer is carried out in the following steps: replacing the gas in the roof portion between the inner tank roof and the intermediate tank roof with the replacement gas, and then replacing the gas in the side portion between the inner tank side plate and the intermediate tank side plate with the replacement gas.

[0059] According to the fourth aspect, the roof part of the inner cold insulation layer close to the position where the communicating pipe is arranged is purged first, and then the side part is purged. Therefore, the inner cold insulation layer can be subjected to a purging process according to the tank structure having the communicating pipe.

[0060] A fifth aspect of the purging method for a triple-shell tank is the purging method of the fourth aspect, wherein the inner tank has a flat-bottom inner tank bottom plate and the intermediate tank has a flat-bottom intermediate tank bottom plate, and purging the inner cold insulation layer, following purging of the sides, further includes a step of replacing the gas at the bottom between the inner tank bottom plate and the intermediate tank bottom plate with the replacement gas.

[0061] According to the fifth aspect, following the purging of the side portion of the inner cold insulation layer, the bottom portion of the inner cold insulation layer can also be purged, so that the inner cold insulation layer can be purged without leakage.

[0062] A sixth aspect of the purging method for a triple-shell tank is the purging method of the fifth aspect, wherein the bottom of the inner cold insulation layer is purged using a purge path that draws out the replacement gas from near the center of the inner tank bottom plate.

[0063] According to the sixth aspect, the replacement gas is introduced directly into the bottom of the inner cold insulation layer and discharged from the bottom, thereby improving the efficiency of the purging process. Alternatively, the replacement gas may be introduced from the periphery of the inner tank bottom plate and discharged from the center of the inner tank bottom plate. In this case, the gas in the bottom to be purged is driven from the radial outside to the radial inside of the inner tank bottom plate and discharged from the radial center, thereby making it possible to perform the purging process more efficiently. [Explanation of symbols]

[0064] 1 Triple-shell tank 11 Inner cold insulation layer 11A, 11B, 11C Roof, side, bottom 12 Outer cold layer 2 Outer tank 21, 22, 23 Outer tank bottom plate, outer tank side plate, outer tank roof 24 Manhole (Second purge route) 3 Intermediate tank 31, 32, 33 Intermediate tank bottom plate, intermediate tank side plate, intermediate tank roof 4 Inner tank 4A Storage space (inside the inner tank) 41, 42, 43 Inner tank bottom plate, inner tank side plate, inner tank roof 44 Communication pipe 51 BOG piping (inlet path / first purge path) 52 Discharge piping (exhaust path / first purge path) 53 Roof piping (exit route / first purge route) 54 Inner cold insulation layer piping (exhaust path / inlet path / first purge path) 55 Bottom insulation layer piping (Outlet route / First purge route) 56 Outer cold insulation layer piping (second purge path)

Claims

1. A method for purging a triple-shell tank comprising an inner tank, an intermediate tank, and an outer tank, an inner cold insulation layer between the inner tank and the intermediate tank, and an outer cold insulation layer between the intermediate tank and the outer tank, the inside of the inner tank and the inner cold insulation layer being in communication with each other through a communication pipe, comprising: The gas present in the inner tank and the inner cold insulation layer is replaced with a replacement gas through a predetermined first purge path, A method for purging a triple-shell tank, comprising: replacing the gas present in the outer cold insulation layer with a replacement gas using a second purge path different from the first purge path.

2. 2. The method for purging a triple-shell tank according to claim 1, A method for purging a triple-shell tank, comprising: performing a first stage purge in which the gas in the inner tank and the inner cold insulation layer is replaced with an inert gas while the gas in the outer cold insulation layer is replaced with the replacement gas; and performing a second stage purge in which the inert gas in the inner tank and the inner cold insulation layer is replaced with a storage gas stored in the inner tank.

3. The method for purging a triple-shell tank according to claim 1 or 2, The first purge path includes an introduction path for introducing the replacement gas into the inner tank and an outlet path for discharging the replacement gas from the inner cold insulation layer to the outside, A method for purging a triple-shell tank, comprising: replacing the gas in the inner tank with the replacement gas; and then introducing the replacement gas from the inner tank to the inner cold insulation layer through the communicating pipe.

4. The method for purging a triple-shell tank according to claim 3, the inner tank includes an inner tank roof and an inner tank side plate, the intermediate tank includes an intermediate tank roof and an intermediate tank side plate, and the communication pipe is disposed so as to penetrate the inner tank roof, A method for purging the inner cold insulation layer, comprising the steps of replacing the gas in the roof portion between the inner tank roof and the intermediate tank roof with the replacement gas, and then replacing the gas in the side portion between the inner tank side plate and the intermediate tank side plate with the replacement gas.

5. The method for purging a triple-shell tank according to claim 4, The inner tank has a flat-bottom inner tank bottom plate, and the intermediate tank has a flat-bottom intermediate tank bottom plate, The purging method for a triple-shell tank, wherein purging of the inner cold insulation layer further includes, following the purging of the side, replacing gas at the bottom between the inner tank bottom plate and the intermediate tank bottom plate with the replacement gas.

6. The method for purging a triple-shell tank according to claim 5, A method for purging a triple-shell tank, wherein the bottom of the inner cold insulation layer is purged using a purge path that leads out the replacement gas from near the center of the inner tank bottom plate.

Images