Purging method for triple-shell tank

Abstract

The present disclosure is a purging method for a triple-shell tank comprising an inner tank, an intermediate tank, 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, wherein the inside of the inner tank and the inner cold insulation layer are in communication with each other through a communication pipe. The purging method replaces gases existing in the inner tank and the inner cold insulation layer with a replacement gas using a predetermined first purge passage, and replaces a gas existing in the outer cold insulation layer with a replacement gas using a second purge passage different from the first purge passage.

Description

How to purge a triple-shell tank

[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.

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

[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 insulation layers between the tanks with a 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 boil-off gas (BOG) extraction pipe and the purged gas is discharged from a liquefied natural gas delivery pipe.

[0004] In general, in a double-shell tank, the inside of the inner tank and the cold insulation layer between the inner and outer tanks are purged at different times. Usually, the cold insulation layer is purged after the inner tank is purged. If this purging method is applied to a triple-shell tank, the purging process may take a long time.

[0005] That is, in a triple-shell tank, it is necessary to purge the inner tank that stores liquefied gas, the inner cold insulation layer between the inner tank and the middle tank, and the outer cold insulation layer between the middle tank and the outer tank. If a method of purging these purging targets is adopted, 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.

[0006] JP 2012-207710 A

[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.

[0008] A method for purging a triple-shelled tank according to one aspect of the present disclosure is a method for purging a triple-shelled 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 in communication with each other via 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.

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

[0010] FIG. 1 is a cross-sectional view schematically illustrating an example of a triple-shell tank to which the purging method of the present disclosure is applicable. FIG. 2 is a diagram schematically illustrating the structure of a purge ring. FIG. 3 is a process chart illustrating an example of a purging method for a triple-shell tank according to an embodiment of the present disclosure. FIG. 4 is a diagram illustrating the open / closed state of a valve and the purging status in step 1 of the purging method of this embodiment. FIG. 5 is a diagram illustrating the open / closed state of a valve and the purging status in step 2. FIG. 6 is a diagram illustrating the open / closed state of a valve and the purging status in step 3. FIG. 7 is a diagram illustrating the open / closed state of a valve and the purging status in step 4. FIG. 8 is a schematic diagram illustrating the purging process for the inner bottom cold insulation layer.

[0011] Hereinafter, with reference to the drawings, an embodiment of a triple-shell tank purging method according to the present disclosure will be described in detail. The triple-shell tank to which the purging method of the present disclosure is applied is an aboveground tank that stores low-temperature liquefied gas. The stored low-temperature liquefied gas may be, 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 an aboveground triple-shell flat-bottom cylindrical tank in which the fluid contained therein 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] [Structural Description of Triple-Shell Tank] First, an example of the structure of a triple-shell tank to which the purging method of this embodiment is applied will be described. FIG. 1 is a cross-sectional view schematically illustrating 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, which are erected on a tank base (not shown), and is equipped with a tank body 1T for storing liquefied hydrogen LH. The tank body 1T is cylindrical with a flat bottom, and the outer tank 2, intermediate tank 3, and inner tank 4 all have a circular shape in top view and are arranged concentrically. The inner tank 4 is a tank equipped with 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, outer tank side plates 22, and an outer tank roof 23. The outer tank bottom plate 21 is laid directly on the tank foundation and has a circular plate shape. The outer tank side plates 22 are erected from the periphery of the outer tank bottom plate 21 and have 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 cover the upper opening of the outer tank side plate 22 and has a dome shape. Manholes 24 are opened in appropriate positions on the outer tank roof 23 to allow 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, intermediate tank side plates 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 plates 32 are erected from the periphery of the intermediate tank bottom plate 31 and have a cylindrical shape. The intermediate tank roof 33 is attached to the upper end of the intermediate tank side plates 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, inner tank side plates 42, and an inner tank roof 43. The inner tank bottom plate 41 has a circular plate shape with a smaller diameter than the intermediate tank bottom plate 31. The inner tank side plates 42 are erected from the periphery of the inner tank bottom plate 41 and have a cylindrical shape. The inner tank roof 43 is attached to the upper end of the inner tank side plates 42 and has a dome shape. Liquefied hydrogen LH is stored in a storage space 4A inside the inner tank 4. The upper layer of the inner tank 4 forms a gas phase portion LA where hydrogen gas vaporized from the liquefied hydrogen LH accumulates.

[0016] Gaps of a predetermined width are formed between the inner tank 4 and the intermediate tank 3, and between the intermediate tank 3 and the outer tank 2, respectively, to serve as thermal insulation spaces. The gap between the inner tank 4 and the intermediate tank 3 serves as an inner cold insulation layer 11, and the gap between the intermediate tank 3 and the outer tank 2 serves as an outer cold insulation layer 12. The inner cold insulation layer 11 and the outer cold insulation layer 12 are filled with powder insulation material to enhance cold insulation. Granular perlite, for example, can be used as the powder insulation material. An insulation material 15 such as glass wool is disposed in the area between the intermediate tank side plate 32 and the inner tank side plate 42 that surrounds the sides of the stored liquefied hydrogen LH. The 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 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 underside 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 underside 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 material. Examples of the solid insulation material include foam glass blocks. The inner bottom cold insulation layer 13 can be composed of blocks of lightweight aerated concrete (ALC), for example, laid on top of 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. Hydrogen gas or helium gas is preferably used as the seal gas for the inner cold insulation layer 11. In this embodiment, a communication pipe 44 is disposed through the inner tank roof 43, connecting the storage space 4A of the inner tank 4 with the space of the inner cold insulation layer 11. Therefore, hydrogen gas in the gas phase LA of the liquefied hydrogen LH stored in the inner tank 4 can be used as the seal gas. Furthermore, by providing 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 prevents the seal gas in the inner cold insulation layer 11 from being liquefied or solidified due to the cold energy of the liquefied hydrogen LH stored in the inner tank 4. The outer cold insulation layer 12 is filled with an inert gas with 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 a seal gas improves insulation performance.

[0019] The tank body 1T includes a plurality of pipes extending from the inside to the outside of the tank body 1T. Shown in Fig. 1 as the plurality of pipes are 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. Attached to each of the pipes 51 to 56 are 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 the respective pipes.

[0020] The BOG piping 51 is a piping for removing BOG of 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. The upper end of the BOG piping 51 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 of the tank body 1T. The BOG removed from the BOG piping 51 is used for re-liquefaction into liquefied hydrogen in a liquefaction facility, for delivery as hydrogen gas to a consumer, for delivery 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 installed so as to penetrate vertically through the tank body 1T and equipped with a discharge port. In the purging method of this embodiment, the discharge pipe 52 is used as a pipe for discharging 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 a gas exhaust pipe for 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 passes vertically through the intermediate tank roof 33 and the outer tank roof 23 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. The first purge ring 71 is disposed 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. The other end of the inner cold insulation layer piping 54 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 simplified diagram illustrating the structure of the first purge ring 71. The first purge ring 71 is a structure in which multiple short pipes 73 are connected in a ring or arc shape by breathable connecting portions 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 or diffusion device for purged gas and post-purge gas containing the purge gas. When the first purge ring 71 is used in a supply system, purge gas is supplied to the base pipe 75 as indicated by arrow F11, and the purge gas is released from the connecting portion 74 as indicated by arrow F12. On the other hand, when the first purge ring 71 is used in an exhaust system, negative pressure acts on the base pipe 75 as indicated by arrow F21, and the post-purge gas is recovered from the connecting portion 74.

[0025] 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. The second purge ring 72 is 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. The other end of the outer cold insulation layer piping 56 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] [Overview of Purging Process] In the purging method for 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] In this embodiment, the first purge path is a path through which purge gas is supplied from the BOG piping 51 and the 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 the purged gas is exhausted from the manhole 24 in the outer tank roof 23.

[0029] Figure 3 is a process chart showing an example of the purging method of this embodiment. Figure 3 assumes that, at time T0, the inner tank 4, inner cold insulation layer 11, and outer cold insulation layer 12 are in an air environment exposed to air. The outer cold insulation layer 12 is purged by replacing the air with an inert gas, in this case, nitrogen gas. During this purging of the outer cold insulation layer 12, the inner tank 4 and the inner cold insulation layer 11 are subjected to a first-stage purge in which the air present in both spaces is replaced with nitrogen gas, and a second-stage purge in which the nitrogen gas filling both spaces during the first-stage purge is replaced with hydrogen gas.

[0030] In Figure 3, a purge that replaces air with nitrogen gas is referred to as an "air purge," and a purge that replaces nitrogen gas with hydrogen gas is referred to as an "N2 purge." An "air purge" of the inner tank 4 is performed from time T0, and an "air purge" of the inner cold insulation layer 11 is performed 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. An "air purge" of the outer cold insulation layer 12 is also performed continuously from time T0. After the "air purge" of the inner tank 4 and the inner cold insulation layer 11 is completed, an "N2 purge" of the inner cold insulation layer 11 is performed from time T3, and then an "N2 purge" of the inner cold insulation layer 11 is performed from time T4. An "air purge" of the outer cold insulation layer 12 is also performed during this "N2 purge."

[0031] According to the above-described 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, thereby shortening the time required for purging the triple-shell tank 1. That is, the "air purge" and "N2 purge" are performed on the inner tank 4 and the inner cold insulation layer 11 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, the two purging targets are purged simultaneously, thereby shortening the purging time compared to purging each target in stages. Furthermore, during the "air purge" of the outer cold insulation layer 12, two-stage purging of the inner tank 4 and the inner cold insulation layer 11, namely, the "air purge" and the "N2 purge," are performed sequentially. While two-stage purging inevitably requires a longer purging time, this embodiment shortens the purging time for the triple-shell tank 1 as a whole.

[0032] [Specific Example of Purging Process] The purging method of this embodiment is divided into the following steps 1 to 4, which will be sequentially described with reference to Figures 4 to 7. Note that Figures 4 to 7 illustrate an example of the purging process from time T0 to time T2 in Figure 3, i.e., an "air purge" of the inner tank 4 and inner cold insulation layer 11 and an "air purge" of the outer cold insulation layer 12, performed in parallel. In Figures 4 to 7, "air" and "N2" represent air and nitrogen gas, respectively. Furthermore, among the first to sixth valves 61 to 66 shown in Figures 4 to 7, valves that are "closed" are indicated in black, while valves that are "open" are indicated in white. Step 1: Purging the storage space 4A of the inner tank 4 and purging the outer cold insulation layer 12 in parallel (Figure 4). Step 2: Purging the roof portion 11A of the inner cold insulation layer 11 and purging the outer cold insulation layer 12 in parallel (Figure 5). Step 3: Purging the side portion 11B of the inner cold insulation layer 11 and the outer cold insulation layer 12 in parallel (FIG. 6). Step 4: Purging the bottom portion 11C of the inner cold insulation layer 11 and the outer cold insulation layer 12 in parallel (FIG. 7).

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

[0034] As indicated by arrow A1, nitrogen gas is supplied as a purge gas to the BOG piping 51. As indicated by arrow A2, the nitrogen gas is introduced into the storage space 4A of the inner tank 4. Instead of the BOG piping 51, other piping that connects the upper portion of the storage space 4A to the outside air may be used as the introduction path for introducing nitrogen gas into the inner tank 4. In this embodiment, purging is performed using a piston flow method in which an interface (IF) is created between two gases based on the density difference between the two gases, and one gas is gradually forced out and discharged. Specifically, nitrogen gas heated to approximately 60°C is introduced into the storage space 4A to make the density difference more apparent. Because room-temperature air is present 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] Because the storage space 4A of the inner tank 4 and the inner cold insulation layer 11 are connected by the connecting pipe 44, when implementing the piston flow method, it is necessary to narrow the outlet of the purged air to the discharge pipe 52. The third valve 63, fourth valve 64, and fifth valve 65, which connect the inner cold insulation layer 11 to the outside air, are closed for the reasons described above, and this valve operation maintains the interface IF. 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. As shown by arrow A3, the air is dissipated from the discharge pipe 52 to the outside air while being pressed against the nitrogen gas layer.

[0036] As shown by arrow A4, nitrogen gas is also supplied as a purge gas to the outer cold insulation layer piping 56. 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 supplied nitrogen gas and dissipates into the outside air through the opened manhole 24 as shown by arrow A5. The purging operation for this outer cold insulation layer 12 is the same as in the subsequent steps 2 to 4, so a description thereof will be omitted below.

[0037] <Step 2> Figure 5 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging status in step 2. The valves that are "open" 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 with nitrogen gas, and the second valve 62 is closed while the third valve 63 is opened. By operating the valves in this manner, the process transitions from step 1 to step 2.

[0038] The purge path in step 2 is a path that sequentially passes through the BOG piping 51 (inlet path), the inner tank 4, the connecting pipe 44, the roof portion 11A of the inner cold insulation layer 11, and the roof piping 53 (outlet path). At the time of transition to step 2, the storage space 4A of the inner tank 4 has been replaced with nitrogen gas. Nitrogen gas is introduced from the inner tank 4 through the connecting pipe 44 into the roof portion 11A of the inner cold insulation layer 11, as shown by arrow A6. The other piping that communicates with the inner cold insulation layer 11, the inner cold insulation layer piping 54 and the bottom cold insulation layer piping 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 into the outside air through the roof piping 53, as shown by arrow A7.

[0039] In step 2, after purging of the inner tank 4 is completed, nitrogen gas is sent as a purge gas from the inner tank 4 side through the connecting pipe 44 to the roof portion 11A of the inner cold insulation layer 11, and air is then discharged from the roof portion 11A, thereby performing purging. The nitrogen gas introduction path may be set at an appropriate location in the inner cold insulation layer 11. In this case, however, an on-off valve must be provided in the connecting pipe 44. Furthermore, when the introduction pressure of the nitrogen gas is applied to the inner cold insulation layer 11, pressure (negative pressure) that tends to contract the inner tank 4 is generated, which may cause the inner tank 4 to buckle. In contrast, if a path is used to send purge gas from the inner tank 4 side to the inner cold insulation layer 11, at least positive pressure can be applied to the inner tank 4, thereby eliminating the above-mentioned concern.

[0040] <Step 3> Figure 6 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging state in 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 for nitrogen gas as purge gas is the path from the inner tank 4 through the connecting pipe 44 to the roof portion 11A of the inner cold insulation layer 11, as shown by arrows A2 and A6.

[0041] Because 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 open inner cold insulation layer piping 54. This flow of nitrogen gas causes the air present in the side portion 11B to be dispersed to the outside air through the inner cold insulation layer piping 54, as shown by arrow A8. The first purge ring 71 is disposed so as to surround the bottom of the side portion 11B, so that the air remaining in the annular side portion 11B can be evenly collected and the collected air can be dispersed from the inner cold insulation layer piping 54.

[0042] <Step 4> Figure 7 is a diagram showing the open / closed states of the first valve 61 to the sixth valve 66 and the purging status in 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 for nitrogen gas as 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, and the negative pressure burden on the inner tank 4 is negligible. Therefore, 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 around the bottom 11C of the inner cold insulation layer 11 through the first purge ring 71. That is, the first purge ring 71, which served to collect the replaced air in step 3, now serves to supply 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 collected through the exhaust opening 551 of the bottom cold insulation layer piping 55 and, as shown by arrow A10, is released to the outside air through the bottom cold insulation layer piping 55.

[0044] 8 is a top view of the tank bottom, illustrating the purging process for the inner bottom cold insulation layer 13 in step 4. Nitrogen gas is introduced from a first purge ring 71 disposed radially outside the inner tank bottom plate 41 so as to surround the inner bottom cold insulation layer 13. As indicated by arrow A9 in Fig. 8 , when nitrogen gas is supplied to the inner cold insulation layer piping 54, the nitrogen gas is sent from the connecting portion 74 (Fig. 2) of the first purge ring 71 toward the vicinity of the peripheral edge of the inner tank bottom plate 41.

[0045] The nitrogen gas then enters the radial center of the inner tank bottom plate 41 while pushing out air present in 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 through the exhaust opening 551 and exhausted as shown by 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 extracted from the exhaust opening 551 near the radial center, thereby enabling more efficient purging.

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

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

[0048] (1) In the above step 1, an example is shown in which purge gas is introduced into the inner tank 4 from the BOG piping 51 and purged by the piston flow method. Instead of adopting the piston flow method, a method of purging by gas flow of purge gas may be adopted. In this case, a purge path may be adopted in which 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 purge gas is introduced into the inner cold insulation layer 11 from the inner cold insulation layer piping 54 and the first purge ring 71. Alternatively, in step 4, as in steps 2 and 3, purge gas may be introduced into 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 purge gas is introduced from the inner cold insulation layer piping 54 and exhausted from the roof piping 53, or alternatively, exhausted from both the roof piping 53 and the bottom cold insulation layer piping 55. In this case, it is desirable to adopt a configuration in which a portion of the communicating pipe 44 is exposed to the outside of the tank body 1T and an on-off valve is provided at the exposed portion. After purging of the inner tank 4 is completed, when purging the inner cold insulation layer 11, the communicating pipe 44 is closed with the on-off valve to isolate the storage space 4A from the inner cold insulation layer 11, and then purge gas is introduced from the inner cold insulation layer piping 54.

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

[0052] A method for purging a triple-shelled tank according to a first aspect of the present disclosure is a method for purging a triple-shelled 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-described 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, thereby shortening the time required for purging the triple-shell tank. That is, the purging of the inner tank and the inner cold insulation layer with the required replacement gas can be performed using the first purge path, while the purging of the outer cold insulation layer with the required replacement gas can be performed using the second purge path. In this way, the two purging targets are purged simultaneously, thereby shortening the purging process time compared to purging each purging target in stages.

[0054] The second aspect of the purging method for a triple-shell tank is the same as the first aspect of the purging method, except that, while the gas in the outer cold insulation layer is being replaced with the replacement gas, a first stage purge is performed to replace the gas in the inner tank and the inner cold insulation layer with an inert gas, and a second stage purge 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, while the outer cold insulation layer is being purged, the inner tank and the inner cold insulation layer are subjected to a first-stage purge and a second-stage purge, respectively. When two purging stages are required, the purge time is inevitably long. However, according to the second aspect, the purge time for the entire triple-shell tank 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 purging 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 connecting pipe.

[0057] According to a third aspect, after purging of the inner tank is completed, purging is performed using the first purge path by sending replacement gas from the inner tank side through a connecting pipe to the inner cold insulation layer and then discharging the replacement gas from the inner cold insulation layer. If a path is used to send replacement gas directly to the inner cold insulation layer, pressure (negative pressure) will be generated in a direction that causes the inner tank to contract, which could cause the inner tank to buckle. In contrast, if a path is used to send replacement gas from the inner tank side to the inner cold insulation layer, at least positive pressure can be applied to the inner tank, thereby eliminating the above-mentioned concern.

[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 by 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 portion of the inner cold insulation layer that is close to the position of the communicating pipe is purged first, followed by the side portion. Therefore, the inner cold insulation layer can be subjected to a purging process that is appropriate for the tank structure that includes 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 further includes, following purging of the sides, 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, after the side portions of the inner cold insulation layer are purged, the bottom portion of the inner cold insulation layer can also be purged, thereby ensuring that the inner cold insulation layer is purged without leaking.

[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 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 near the periphery of the inner tank bottom plate and discharged from near the center of the inner tank bottom plate. In this case, the gas in the bottom to be purged is driven from the radially outer side to the radially inner side of the inner tank bottom plate and discharged from near the radial center, thereby enabling more efficient purging.

[0064] REFERENCE SIGNS LIST 1 Triple-shell tank 11 Inner cold insulation layer 11A, 11B, 11C Roof, side, bottom 12 Outer cold insulation layer 2 Outer tank 21, 22, 23 Outer tank bottom plate, outer tank side plate, outer tank roof 24 Manhole (second purge path) 3 Intermediate tank 31, 32, 33 Intermediate tank bottom plate, intermediate tank side plate, intermediate tank roof 4 Inner tank 4A Storage space (inside of inner tank) 41, 42, 43 Inner tank bottom plate, inner tank side plate, inner tank roof 44 Connecting pipe 51 BOG piping (introduction path / first purge path) 52 Discharge piping (exhaust path / first purge path) 53 Roof piping (exhaust path / first purge path) 54 Inner cold insulation layer piping (exhaust path / introduction path / first purge path) 55 Bottom cold insulation layer piping (outlet path / first purge path) 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-insulating layer between the inner tank and the intermediate tank, and an outer cold-insulating layer between the intermediate tank and the outer tank, the interior of the inner tank being in communication with the inner cold-insulating layer via a connecting pipe, the method comprising: replacing gas present in the inner tank and the inner cold-insulating layer with a replacement gas using a predetermined first purge path; and replacing gas present in the outer cold-insulating layer with a replacement gas using a second purge path different from the first purge path.

2. A method for purging a triple-shell tank as described in claim 1, comprising carrying out 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 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. A method for purging a triple-shelled tank as described in claim 1 or 2, 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.

4. A method for purging a triple-shell tank as described in claim 3, 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 by 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. A method for purging a triple-shell tank as described in claim 4, 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 the 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.

6. A method for purging a triple-shell tank as described in claim 5, 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.

Images