Low temperature gas discharge system, structure, and low temperature gas discharge method

Abstract

This low temperature gas discharge system discharges boil-off gas of liquefied gas from a liquefied gas tank storing the liquefied gas to the atmosphere, said system comprising a discharge tower, and a discharge line that guides the boil-off gas from the liquefied gas tank to the discharge tower. The discharge line includes: a discharge main line that connects the liquefied gas tank and the discharge tower; a heat exchanger that is disposed in the discharge main line to heat the boil-off gas flowing through the discharge main line; a discharge valve that is disposed in the discharge main line to open and close the flow path; a bypass line that guides the boil-off gas to the discharge tower without passing the boil-off gas through the discharge valve and the heat exchanger; and an emergency discharge valve that is disposed in the bypass line to open and close the flow path in the bypass line.

Description

Low temperature gas release system, structure, and low temperature gas release method

[0001] The present disclosure relates to a cryogenic gas release system that releases boil-off gas generated in a liquefied gas tank from a release tower, and to a structure such as a ship that is equipped with this cryogenic gas release system.

[0002] Liquefied gas carriers are equipped with liquefied gas tanks containing low-temperature liquefied gas, such as liquefied hydrogen. In the liquefied gas tank, the liquefied gas vaporizes due to external heat input, generating boil-off gas. To prevent the increase in tank pressure caused by the boil-off gas, the boil-off gas is conventionally sent to the ship's gas combustion unit (GCU) or propulsion engine and consumed there. Furthermore, to prevent the tank pressure from exceeding the tank's design pressure, the boil-off gas is released into the atmosphere through a release tower. When the cryogenic boil-off gas is released from the release tower, liquid air and other substances may be generated, and the generated liquid may fall onto the hull around the release tower. Furthermore, when the generated liquid vaporizes again, a high-oxygen atmosphere may be created. Therefore, a technology has been proposed to heat the boil-off gas generated in the tank before releasing it into the atmosphere.

[0003] The liquefied gas tank of Patent Document 1 is connected to a discharge line that connects the top of the tank to a discharge tower. A heating line is connected to the middle of the discharge line. The liquefied gas is extracted from the tank, heated by a heater to a temperature higher than that of the boil-off gas in the tank, and then sent to the discharge line. The boil-off gas that flows out from the top of the tank to the discharge line is heated by the heated gas sent through the heating line and released into the atmosphere from the discharge tower.

[0004] Japanese Patent Application Laid-Open No. 2021-160619

[0005] In a system equipped with a heat exchanger that heats boil-off gas before it is released into the atmosphere, it is preferable that the heat exchanger can be used for purposes other than heating the boil-off gas to be released into the atmosphere. However, with the configuration of Patent Document 1, if boil-off gas is released into the atmosphere from the liquefied gas tank while the heat exchanger is being used for another purpose, there is a risk that the heating capacity of the heat exchanger for the other purpose will be reduced due to the heating of the boil-off gas.

[0006] The present disclosure has been made in consideration of the above circumstances, and its purpose is to propose a technology that, when boil-off gas generated in a liquefied gas tank is released from a release tower, prevents the generation of liquid oxygen around the release tower by releasing boil-off gas heated by a heat exchanger during steady state operation, and quickly reduces the pressure inside the tank without affecting the use of the heat exchanger for other purposes when gas needs to be released from the liquefied gas tank when the pressure inside the tank suddenly rises, when the heat exchanger is being used for other purposes, or when the heat exchanger is stopped.

[0007] In order to solve the above problems, a low-temperature gas release system according to one embodiment of the present disclosure is a system that releases boil-off gas of a liquefied gas from a liquefied gas tank that stores the liquefied gas into the atmosphere, and includes: a release tower; and a release line that guides the boil-off gas from the liquefied gas tank to the release tower. The release line includes a main release line that connects the liquefied gas tank and the release tower, a heat exchanger that is disposed in the main release line and heats the boil-off gas flowing through the main release line, a release valve that is disposed in the main release line and opens and closes the flow path, a bypass line that guides the boil-off gas to the release tower without passing through the release valve and the heat exchanger, and an emergency release valve that is disposed in the bypass line and opens and closes the flow path of the bypass line.

[0008] A structure according to one embodiment of the present disclosure comprises a liquefied gas tank for storing liquefied gas; the above-mentioned low-temperature gas release system; a cargo handling gas line connected to the liquefied gas tank and through which boil-off gas of the liquefied gas flowing out of the liquefied gas tank passes; and a cargo handling heat exchanger disposed on the cargo handling gas line and regulating the temperature of the boil-off gas passing through the cargo handling gas line, wherein the cargo handling heat exchanger also serves as the heat exchanger of the low-temperature gas release system.

[0009] A structure according to one embodiment of the present disclosure comprises: a liquefied gas tank for storing liquefied gas; the above-mentioned low-temperature gas release system; a fuel gas line connected to the liquefied gas tank and through which boil-off gas of the liquefied gas flowing out of the liquefied gas tank passes; and a fuel heat exchanger disposed on the fuel gas line and regulating the temperature of the boil-off gas passing through the fuel gas line, wherein the fuel heat exchanger also serves as the heat exchanger of the low-temperature gas release system.

[0010] A low-temperature gas release method according to one aspect of the present disclosure is a method for releasing boil-off gas of a liquefied gas from a liquefied gas tank storing the liquefied gas into the atmosphere, comprising: when the pressure in the liquefied gas tank exceeds a predetermined release start pressure, heating the boil-off gas in a heat exchanger and then releasing it into the atmosphere through a release tower; and when the pressure in the liquefied gas tank exceeds a predetermined emergency release start pressure that is higher than the release start pressure, releasing the boil-off gas into the atmosphere through the release tower without passing through the heat exchanger.

[0011] According to the present disclosure, when boil-off gas generated in a liquefied gas tank is released from a release tower, the generation of liquid oxygen around the release tower can be prevented, and if the pressure inside the tank suddenly rises, the pressure inside the tank can be quickly reduced.

[0012] FIG. 1 is a diagram showing a schematic configuration of a low-temperature gas release system according to an embodiment of the present disclosure. FIG. 2 is a diagram showing an example of the hardware configuration of a controller. FIG. 3 is a block diagram showing the configuration of a control system of the low-temperature gas release system. FIG. 4 is a diagram explaining an example of changes in pressure within a tank over time. FIG. 5 is a flowchart showing the processing of a controller that realizes an operating method according to a second embodiment. FIG. 6 is a diagram showing a schematic configuration of a low-temperature gas release system according to a first modified example. FIG. 7 is a diagram showing a schematic configuration of a low-temperature gas release system according to a second modified example. FIG. 8 is a diagram showing a schematic configuration of a low-temperature gas release system according to a third modified example.

[0013] Next, an embodiment of the present disclosure will be described with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a cryogenic gas release system 1 according to an embodiment of the present disclosure. The cryogenic gas release system 1 shown in FIG. 1 releases boil-off gas generated in a liquefied gas tank 10 into the atmosphere from a release tower 6. The liquefied gas tank 10 contains a low-temperature liquefied gas such as liquefied hydrogen or liquefied natural gas. The cryogenic gas release system 1 is provided in a land-based structure having the liquefied gas tank 10, or in an offshore structure such as a floating structure or a ship. The liquefied gas tank 10 is provided with an emergency vent line 55 that is independent from the cryogenic gas release system 1. The emergency vent line 55 is provided with an automatic safety valve 56. However, the emergency vent line 55 may be combined with the cryogenic gas release system 1.

[0014] The cryogenic gas release system 1 includes a release line 3, a release tower 6, and a controller 4. The release line 3 guides boil-off gas generated in the liquefied gas tank 10 from the liquefied gas tank 10 to the release tower 6. The controller 4 controls the cryogenic gas release system 1.

[0015] The discharge line 3 has a main discharge line 30 that connects the gas phase in the liquefied gas tank 10 with the discharge tower 6. The main discharge line 30 is composed of low-temperature resistant piping such as double piping. The main discharge line 30 is provided with a first release valve 32 and a second release valve 33. The first release valve 32 and the second release valve 33 are both normally closed on-off valves that open and close the flow path of the main discharge line 30.

[0016] In the main discharge line 30, a heat exchanger 34 and a compressor 35 are provided downstream of the first discharge valve 32 and upstream of the second discharge valve 33. However, the compressor 35 may be omitted. The boil-off gas flowing through the main discharge line 30 is heated to a predetermined discharge temperature while passing through the heat exchanger 34. This discharge temperature may be any temperature. The discharge temperature is not particularly limited, but is preferably higher than the boiling point of liquid oxygen from the viewpoint of suppressing the generation of liquid air near the outlet of the discharge tower 6. The heat exchange method of the heat exchanger 34 is not particularly limited, and known heat exchange methods such as a shell-and-tube method or a fin-plate method may be adopted. The compressor 35 is arranged upstream or downstream of the heat exchanger 34 in the main discharge line 30. By operating the compressor 35, the boil-off gas flowing through the main discharge line 30 is pressure-fed to the discharge tower 6.

[0017] In the main discharge line 30, an upstream end of a bypass line 7 is connected to a first connection 36 upstream of the first release valve 32. In addition, in the main discharge line 30, a downstream end of the bypass line 7 is connected to a second connection 37 downstream of the second release valve 33. The bypass line 7 guides the boil-off gas that has flowed into the main discharge line 30 to the release tower 6 without passing through the first release valve 32, the heat exchanger 34, the compressor 35, and the second release valve 33. In this embodiment, the bypass line 7 is connected to the discharge line 3, but the bypass line 7 may directly connect the liquefied gas tank 10 and the release tower 6 without passing through the discharge line 3. An emergency release valve 71 is provided in the bypass line 7. The emergency release valve 71 is a normally closed on-off valve that opens and closes the flow path of the bypass line 7. In this embodiment, one emergency release valve 71 is provided in the bypass line 7, but two or more emergency release valves 71 may be provided in the bypass line 7 in preparation for valve leakage or damage.

[0018] The low-temperature gas release system 1 having the above configuration may be configured so that the release tower 6 from which the boil-off gas is released can be selected from among a plurality of release towers 6. In the example shown in FIG. 6 , the structure 100 includes a plurality of liquefied gas tanks 10, and the low-temperature gas release system 1 includes a plurality of release lines 3 and a plurality of release towers 6. Here, the number of liquefied gas tanks 10 and the number of release towers 6 may be different. Each of the plurality of liquefied gas tanks 10 is connected to an upstream portion 30a of the main release line 30. Each of the upstream portions 30a of the plurality of main release lines 30 is connected to a single hub 64 via an on-off valve 62. Furthermore, each of the plurality of release towers 6 is connected to a single hub 63 via a downstream portion 30c of the main release line 30 and an on-off valve 61. The hubs 63 and 64 are connected by one or more shared lines 65. Each shared line 65 includes a midstream portion 30b of the main discharge line 30, a heat exchanger 34 through which the midstream portion 30b of the main discharge line 30 passes, a bypass line 7 connected to the midstream portion 30b of the main discharge line 30 so as to bypass the heat exchanger 34, and an on-off valve 66. The discharge line 3 connected to each liquefied gas tank 10 includes an upstream portion 30a of the main discharge line 30, a hub 64, at least one shared line 65, a hub 63, and a downstream portion 30c of the main discharge line 30. By opening and closing the on-off valve 61, it is possible to select the discharge tower 6 connected to the discharge line 3, i.e., one or more discharge towers 6 to which boil-off gas is discharged. By opening and closing the on-off valve 66, it is possible to arbitrarily select the shared line 65 to be used, i.e., the heat exchanger 34 to be used. In other words, one or more heat exchangers 34 are shared by multiple discharge lines 3.

[0019] Furthermore, in the low-temperature gas release system 1 having the above configuration, the heat exchanger 34 and the compressor 35 may be shared with those provided in the liquefied gas tank 10 for loading and unloading. For example, in a modified example shown in FIG. 7 , a structure 100 including the liquefied gas tank 10 includes a discharge line 3 and a loading gas line 5 for transporting boil-off gas of the liquefied gas from the liquefied gas tank 10 to the outside. The loading gas line 5 is composed of piping or the like. The loading gas line 5 is provided with a first release valve 32, a heat exchanger 34, a compressor 35, and a loading valve 52. The loading gas line 5 shares with the discharge line 3 a part of the main discharge line 30, and the first release valve 32, the heat exchanger 34, and the compressor 35 arranged in the main discharge line 30. A portion 51 of the cargo gas line 5 for sending out boil-off gas to the outside is connected to a connection portion 57 between the compressor 35 and the second discharge valve 33 of the main discharge line 30, and a cargo valve 52 is disposed in this portion 51 of the cargo gas line 5. The cargo valve 52 is a normally closed on-off valve that opens and closes the flow path of the cargo gas line 5. With this configuration, when the second discharge valve 33 is closed and the first discharge valve 32 and the cargo valve 52 are open, the heat exchanger 34 functions as a cargo heat exchanger for the cargo gas line 5, and the compressor 35 functions as a cargo compressor for the cargo gas line 5. When the first discharge valve 32 and the second discharge valve 33 are open and the cargo valve 52 is closed, the heat exchanger 34 functions as a heat exchanger for the discharge line 3, and the compressor 35 functions as a compressor for the discharge line 3.

[0020] In the above, an example has been described in which piping and equipment are shared between the discharge line 3 and the cargo gas line 5. However, piping and equipment may also be shared between the fuel gas line that sends boil-off gas from the liquefied gas tank 10 as fuel to the combustion appliance and the discharge line 3. In this case, as the combustion appliance, a part 51 of the fuel gas line that sends boil-off gas to the combustion appliance is connected to a connection part 57 between the compressor 35 and the second release valve 33 of the main discharge line 30 shown in Fig. 7 instead of a part of the cargo gas line 5. Then, when the second release valve 33 is closed and the first release valve 32 and the on-off valve 52 are open, the heat exchanger 34 functions as a fuel heat exchanger for the fuel gas line, and the compressor 35 functions as a fuel compressor for the fuel gas line.

[0021] FIG. 2 is a diagram illustrating an example of the hardware configuration of the controller 4. As illustrated in FIG. 2, the controller 4 is a so-called computer and includes a CPU (Central Processing Unit) 41, a memory 42, an I / F 44 for wirelessly or wirelessly connecting to the discharge valves 32, 33, and 71, the heat exchanger 34, the compressor 35, various meters, and a communication network, an input device 45 such as a mouse, keyboard, or touch panel, and an output device 46 such as a liquid crystal display or speaker. Each function of the controller 4, which will be described later, can be realized by loading a predetermined program stored in an auxiliary storage device or the like into the memory 42 and executing it by the CPU 41. Note that the predetermined program may be downloaded to the controller 4 from a network via the I / F 44 or may be loaded from a storage medium, for example.

[0022] The functions of the controller 4 disclosed herein can be performed using circuits or processing circuits, including general-purpose processors, special-purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, and / or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuitry. In this disclosure, a circuit, unit, or means is hardware that performs the recited functions. The hardware may be hardware disclosed herein or other known hardware that is programmed or configured to perform the recited functions. Where the hardware is a processor, which is considered a type of circuit, the circuit, means, or unit is a combination of hardware and software, and the software is used to configure the hardware and / or processor.

[0023] Fig. 3 is a block diagram showing the configuration of a control system of the low-temperature gas release system 1. As shown in Fig. 3, the first release valve 32, the second release valve 33, and the emergency release valve 71 are connected to the controller 4, and the controller 4 controls the first release valve 32, the second release valve 33, and the emergency release valve 71 to open and close.

[0024] A pressure sensor 11 and a temperature sensor 12 are connected to the controller 4, and detection data is output from these instruments to the controller 4. The pressure sensor 11 detects the pressure of the space containing the liquefied gas in the liquefied gas tank 10 (hereinafter referred to as the "tank pressure"). The temperature sensor 12 detects the temperature of the boil-off gas flowing into the release tower 6. In this embodiment, the temperature sensor 12 is arranged downstream of the second connection part 37 of the main release line 30, but the temperature sensor 12 may also be arranged in the release tower 6.

[0025] The controller 4 monitors the internal tank pressure detected by the pressure sensor 11 while the liquefied gas is stored in the liquefied gas tank 10. As shown in FIG. 4 , the controller 4 stores the operating pressure range of the liquefied gas tank 10, the alarm pressure P1, the discharge start pressure P2, the emergency discharge pressure P3, the design pressure P4 of the liquefied gas tank 10, and the discharge end pressure P5. The emergency discharge pressure P3 is equal to or lower than the design pressure P4, the discharge start pressure P2 is lower than the emergency discharge pressure P3, the alarm pressure P1 is equal to or lower than the discharge start pressure P2, and the discharge end pressure P5 is lower than the alarm pressure P1 (P4 ≥ P3 > P2 ≥ P1 > P5). The discharge end pressure P5 is a value within the operating pressure range of the liquefied gas tank 10. The alarm pressure P1 may be a pressure higher than or equal to the upper limit of the operating pressure range of the liquefied gas tank 10. The alarm pressure P1 is a pressure threshold that triggers the operation of the cryogenic gas discharge system 1. It is preferable that the pressure at which the safety valve 56 of the emergency vent line 55 operates be lower than the design pressure P4 and higher than the emergency release pressure P3.

[0026] The controller 4 monitors the temperature of the boil-off gas detected by the temperature sensor 12 while the boil-off gas is being released from the release tower 6. When the temperature of the boil-off gas detected by the temperature sensor 12 is lower than the release temperature, measures are taken, such as increasing the heating temperature of the boil-off gas by the heat exchanger 34 and reducing the amount of boil-off gas sent out by the compressor 35. When the temperature of the boil-off gas detected by the temperature sensor 12 is higher than the release temperature, measures are taken, such as decreasing the heating temperature of the boil-off gas by the heat exchanger 34 and increasing the amount of boil-off gas sent out by the compressor 35. In this way, the temperature of the boil-off gas detected by the temperature sensor 12 is maintained at the release temperature.

[0027] The discharge line 3 of the cryogenic gas discharge system 1 configured as described above has two flow paths: a "normal flow path" in which the boil-off gas discharged from the liquefied gas tank 10 is heated to a temperature higher than the boiling point of liquid oxygen in the heat exchanger 34 and then sent to the discharge tower 6; and an "emergency flow path" in which the boil-off gas is sent to the discharge tower 6 through a bypass line 7 that avoids the heat exchanger 34 and the compressor 35. The normal flow path is a flow path that passes through the main discharge line 30 and is activated by opening the first release valve 32 and the second release valve 33 and closing the emergency release valve 71. The emergency flow path is a flow path that passes through a part of the main discharge line 30 and the bypass line 7 and is activated by closing the first release valve 32 and the second release valve 33 and opening the emergency release valve 71. As shown in FIG. 8 , a third release valve 39 may be disposed in the discharge line 3 downstream of the second connection point 37, which is the junction of the main discharge line 30 and the bypass line 7. The third release valve 39 is an on-off valve that opens and closes in the same manner as the first release valve 32 and the second release valve 33 when the normal flow path is in use, and that opens and closes in the same manner as the emergency release valve 71 when the emergency flow path is in use. In the event of a failure of either the second release valve 33 or the emergency release valve 71, the third release valve 39 takes over the function of the failed valve.

[0028] The controller 4 controls the operation of the cryogenic gas discharge system 1 by operating the opening and closing of the valves 32, 33, and 71 to switch between one of the following states: a state in which the normal flow path functions and the emergency flow path does not function, a state in which the normal flow path does not function and the emergency flow path functions, and a state in which both the normal flow path and the emergency flow path do not function. However, in addition to the above-mentioned modes, there may also be a state in which both the normal flow path and the emergency flow path function. Below, first to third embodiments of the operating method of the cryogenic gas discharge system 1 will be described.

[0029] [First embodiment] As shown in Figure 4, the internal tank pressure P increases due to boil-off gas of the liquefied gas stored in the liquefied gas tank 10. Note that part of the boil-off gas may be sent to a fuel-consuming device as fuel. When the internal tank pressure P reaches an alarm pressure P1 (time t1), the controller 4 outputs an alarm from the output device 46. The alarm notifies the user of the upcoming release of boil-off gas.

[0030] When the tank internal pressure P reaches the discharge start pressure P2 (time t2), the controller 4 opens the first release valve 32 and the second release valve 33 and operates the compressor 35. Alternatively, when the pressure increase rate of the tank internal pressure P (i.e., the amount of pressure increase per unit time) reaches a predetermined discharge start pressure increase rate, the controller 4 opens the first release valve 32 and the second release valve 33 and operates the compressor 35. The controller 4 starts the heat exchanger 34 when or before the tank internal pressure P reaches the discharge start pressure P2. A stopped heat exchanger 34 requires a predetermined start-up time until it can heat the boil-off gas to the discharge temperature. Therefore, it is desirable to start the heat exchanger 34 before the tank internal pressure P reaches the discharge start pressure P2.

[0031] As described above, when the inside of the liquefied gas tank 10 and the release tower 6 are connected through the steady-state flow path of the release line 3 and the heat exchanger 34 and compressor 35 are operating, the boil-off gas that flows from the liquefied gas tank 10 into the release line 3 is heated to the release temperature and sent to the release tower 6, from which it is released into the atmosphere. Since the boil-off gas released into the atmosphere from the release tower 6 has a boiling point higher than that of liquid oxygen, it is possible to prevent the generation of liquid oxygen, liquid nitrogen, and liquid air at the outlet of the release tower 6 and its surroundings.

[0032] The release of the boil-off gas causes the tank internal pressure P to gradually decrease. When the tank internal pressure P reaches a discharge end pressure P5 (time t3), the controller 4 stops the heat exchanger 34 and the compressor 35 and closes the first release valve 32 and the second release valve 33. Alternatively, when the pressure increase rate of the tank internal pressure P (i.e., the amount of pressure increase per unit time) reaches a predetermined discharge end pressure increase rate, the controller 4 stops the heat exchanger 34 and the compressor 35 and closes the first release valve 32 and the second release valve 33. By such operation of the low-temperature gas release system 1, the tank internal pressure P is kept within the operating range.

[0033] If the tank internal pressure P becomes equal to or higher than the emergency release pressure P3 during the release of the boil-off gas, the controller 4 opens the emergency release valve 71. Alternatively, if the rate of pressure increase (i.e., the amount of pressure increase per unit time) of the tank internal pressure P becomes equal to or higher than a predetermined emergency release rate threshold, the controller 4 opens the emergency release valve 71. Note that, since the bypass line 7 upstream of the emergency release valve 71 is connected to the liquefied gas tank 10 via a part of the main release line 30, the pressure in the bypass line 7 is substantially the same as the tank internal pressure P.

[0034] By opening the emergency release valve 71, the emergency flow path of the release line 3 is opened, and part of the boil-off gas that has flowed into the release line 3 flows to the release tower 6 through the bypass line 7, i.e., without passing through the heat exchanger 34 and the compressor 35. In this way, when it becomes necessary to release boil-off gas urgently, part of the boil-off gas can be quickly released from the release tower 6 without passing through the heat exchanger 34 and the compressor 35, thereby preventing an excessive increase in the tank internal pressure P. Note that, since unheated boil-off gas is released from the release tower 6 when the tank internal pressure P is equal to or higher than the emergency release pressure P3, it is desirable that the controller 4 issue an alert from the output device 46 when the tank internal pressure P is equal to or higher than the emergency release pressure P3.

[0035] In the above, the opening and closing operations of the first release valve 32 and the second release valve 33 are controlled by the controller 4, but the opening and closing operations of at least one of the first release valve 32 and the second release valve 33 may be performed manually. Also, in the above, the emergency release valve 71 is a control valve controlled by the controller 4, but it may be a manual valve or a safety valve that is automatically opened by emergency release pressure P3. In this case, when the emergency release valve 71 is open, the first release valve 32 and the second release valve 33 may be closed or open.

[0036] [Second Example] Figure 5 is a flowchart of the processing of the controller 4 that realizes an operating method according to a second example. As shown in Figure 5, the tank internal pressure P rises due to boil-off gas of the liquefied gas contained in the liquefied gas tank 10 and reaches an alarm pressure P1. The controller 4 monitors the tank internal pressure P, and when the tank internal pressure P reaches the alarm pressure P1 (step S01), it calculates the rate of increase of the tank internal pressure P (step S02). The rate of increase of the tank internal pressure P can be obtained from the history of the tank internal pressure P.

[0037] A speed threshold value for the pressure rise rate is provided in advance and stored in the controller 4. The controller 4 compares the calculated pressure rise rate with the speed threshold value (step S03), and if the pressure rise rate is less than the speed threshold value (NO in step S03), outputs an alarm from the output device 46 (step S04). The alarm warns of the release of boil-off gas.

[0038] When the tank internal pressure P reaches the discharge start pressure P2 (YES in step S05), the controller 4 opens the first release valve 32 and the second release valve 33 and operates the compressor 35 (step S06). The controller 4 starts the heat exchanger 34 when or before the tank internal pressure P reaches the discharge start pressure P2. When the liquefied gas tank 10 and the release tower 6 are connected through the steady-state flow path of the discharge line 3 and the heat exchanger 34 and the compressor 35 are operated, the boil-off gas flowing from the liquefied gas tank 10 into the discharge line 3 is heated to a discharge temperature, sent to the release tower 6, and released from the release tower 6 to the atmosphere. When the tank internal pressure P decreases due to the release of the boil-off gas and reaches the discharge end pressure P5 (YES in step S07), the controller 4 closes the first release valve 32 and the second release valve 33 and stops the heat exchanger 34 and the compressor 35 (step S08).

[0039] On the other hand, in step S03, if the pressure increase rate is equal to or greater than the rate threshold (YES in step S03), the controller 4 generates an alert from the output device 46 (step S21) and opens the emergency release valve 71 (step S22). The emergency release valve 71 may be opened after the tank internal pressure P exceeds the emergency release pressure P3. Opening the emergency release valve 71 opens the emergency flow path of the release line 3, and the boil-off gas that has flowed into the release line 3 flows through the bypass line 7, i.e., without passing through the heat exchanger 34 and the compressor 35, to the release tower 6 and is released from the release tower 6 to the atmosphere. When the tank internal pressure P decreases due to the release of the boil-off gas and reaches the release end pressure P5 (YES in step S23), the controller 4 closes the emergency release valve 71 (step S24).

[0040] In the above, the opening and closing operations of the first release valve 32, the second release valve 33, and the emergency release valve 71 are controlled by the controller 4, but the opening and closing operations of at least one of the first release valve 32, the second release valve 33, and the emergency release valve 71 may be performed manually. In this case, the controller 4 generates an alarm or alert at the appropriate time, and the operator can operate the manual valve in response to the alarm or alert.

[0041] [Third Example] In the third example, the heat exchanger 34 of the low-temperature gas release system 1 has uses other than heating the boil-off gas to be released, such as a heat exchanger for cargo handling or a heat exchanger for fuel. Therefore, it is conceivable that the boil-off gas of liquefied gas may be released into the atmosphere from the liquefied gas tank 10 while the heat exchanger 34 is being used for purposes other than heating the boil-off gas to be released, such as cargo handling or fuel supply, or while the heat exchanger 34 is stopped. In such a case, the boil-off gas of liquefied gas is guided from the liquefied gas tank 10 to the release tower 6 without passing through the heat exchanger 34 using the emergency flow path, and the boil-off gas is released into the atmosphere. Specifically, the controller 4 opens the emergency release valve 71 while keeping the first release valve 32 and the second release valve 33 closed. When the release of the boil-off gas is completed, the controller 4 closes the emergency release valve 71. This allows the boil-off gas to be released into the atmosphere from the liquefied gas tank 10 without affecting the heating or loading of the fuel. Similarly, the emergency flow path can also be used as a flow path for releasing replacement gas during maintenance of the liquefied gas tank 10.

[0042] [Summary] The low-temperature gas release system 1 according to the first item of the present disclosure is a system for releasing boil-off gas of liquefied gas from a liquefied gas tank 10 that stores the liquefied gas into the atmosphere, and comprises a release tower 6, and a release line 3 that guides the boil-off gas from the liquefied gas tank 10 to the release tower 6. The release line 3 has a main release line 30 that connects the liquefied gas tank 10 and the release tower 6, a heat exchanger 34 that is disposed on the main release line 30 and heats the boil-off gas flowing through the main release line 30, release valves 32 and 33 that are disposed on the main release line 30 and open and close the flow path, a bypass line 7 that guides the boil-off gas to the release tower 6 without passing through the release valves 32 and 33 and the heat exchanger 34, and an emergency release valve 71 that is disposed on the bypass line 7 and opens and closes the flow path of the bypass line 7.

[0043] In the low-temperature gas release system 1 configured as described above, the release line 3 has a flow path (i.e., a steady-state flow path) through which the boil-off gas is heated by the heat exchanger 34 to a temperature higher than the boiling point of liquid oxygen and then sent to the release tower 6, and a flow path (i.e., an emergency flow path) through which the boil-off gas is sent to the release tower 6 without passing through the heat exchanger 34. When the steady-state flow path is used to release the boil-off gas, the boil-off gas is released from the release tower 6 at a temperature higher than the boiling point of liquid oxygen, thereby preventing the released boil-off gas from liquefying the surface of the release tower 6 and the air (air containing oxygen and nitrogen) around the release tower 6. When the emergency flow path is used to release the boil-off gas, the boil-off gas can be released from the liquefied gas tank 10 without waiting for the heat exchanger 34 to start up, so the internal tank pressure P of the liquefied gas tank 10 can be quickly reduced. For example, in the event of a sudden increase in the tank internal pressure P that could damage the liquefied gas tank 10, boil-off gas can be urgently released from the liquefied gas tank 10 using the emergency flow path of the release line 3 without waiting for the release valves 32, 33 to be operated or the heat exchanger 34 to be started up. Also, when it becomes necessary to release boil-off gas or other gases from the liquefied gas tank 10 while the heat exchanger 34 is being used for a purpose other than heating the boil-off gas to be released into the atmosphere in the liquefied gas tank 10, the boil-off gas can be urgently released from the liquefied gas tank 10 using the emergency flow path of the release line 3 without affecting the use of the heat exchanger 34 for that other purpose. Furthermore, the emergency flow path of the release line 3 can be used not only for the emergency release described above, but also for gas replacement in the liquefied gas tank 10. In this way, in the low-temperature gas release system 1, the normal flow path and the emergency flow path of the release line 3 can be used selectively depending on the application and situation.

[0044] The low-temperature gas release system 1 according to the second item of the present disclosure is the low-temperature gas release system 1 according to the first item, which is provided with a plurality of release towers 6 connectable to the release line 3, and the plurality of release lines 3 have a switch (in the above embodiment, an on-off valve 61) that switches the flow path of the boil-off gas so that any one or more of the plurality of release towers 6 are connected to the liquefied gas tank 10.

[0045] In this way, the release tower 6 from which the boil-off gas is released can be selected, so that the boil-off gas can be released from the release tower 6 under the most suitable conditions depending on the wind direction, maintenance conditions, and the like.

[0046] The low-temperature gas release system 1 relating to the third item of the present disclosure is a low-temperature gas release system 1 relating to the first or second item, which is provided with multiple release lines 3 connected to different liquefied gas tanks 10, and the multiple release lines 3 have a heat exchanger 34 shared by two or more release lines 3.

[0047] This allows a reduction in the number of heat exchangers 34. Furthermore, the heat exchangers 34 included in the discharge line 3 can be selected arbitrarily.

[0048] The low-temperature gas discharge system 1 according to the fourth item of the present disclosure is a low-temperature gas discharge system 1 according to any of the first to third items, in which the discharge valves 32, 33 include a first discharge valve 32 arranged upstream of the heat exchanger 34 in the main discharge line 30, a second discharge valve 33 arranged downstream of the heat exchanger 34 in the main discharge line 30, and a third discharge valve 39 arranged downstream of the junction 37 of the main discharge line 30 and the bypass line 7.

[0049] By arranging the double discharge valves 32, 33 in the main discharge line 30 in this way, it is possible to prevent the boil-off gas from being discharged into the atmosphere or the outside air from flowing into the liquefied gas tank 10 due to leakage or damage of one of the valves. In addition, the third discharge valve 39 can perform the function of the valve when any of the other valves is opened due to a failure.

[0050] The low-temperature gas discharge system 1 according to the fifth item of the present disclosure is the low-temperature gas discharge system 1 according to any one of the first to fourth items, and is provided with a temperature sensor 12 that detects the temperature of the boil-off gas flowing into the discharge tower 6, and the heat exchanger 34 heats the boil-off gas so that the temperature of the boil-off gas detected by the temperature sensor 12 becomes a predetermined discharge temperature.

[0051] This allows the boil-off gas flowing into the release tower 6 to be controlled to the release temperature.

[0052] The low-temperature gas release system 1 relating to the sixth item of the present disclosure is a low-temperature gas release system 1 relating to any one of the first to fifth items, in which the release valves 32, 33 are opened when the internal tank pressure P of the liquefied gas tank 10 exceeds a predetermined release start pressure P2 and closed when the internal tank pressure P reaches a predetermined release end pressure P5 due to the release of boil-off gas, or are opened when the pressure rise gradient of the internal tank pressure P of the liquefied gas tank 10 exceeds a predetermined release pressure rise gradient and closed when the pressure rise gradient of the internal tank pressure P reaches a predetermined release end pressure rise gradient due to the release of boil-off gas.

[0053] As a result, when the internal tank pressure P of the liquefied gas tank 10 exceeds the release start pressure P2, the boil-off gas is released, and the internal tank pressure P is maintained within a predetermined pressure range (i.e., within the operating pressure range).

[0054] The low-temperature gas release system 1 according to the seventh item of the present disclosure is the low-temperature gas release system 1 according to the sixth item, in which the emergency release valve 71 is opened when the pressure P inside the tank reaches a predetermined emergency release pressure P3 that is higher than the release start pressure P2.

[0055] As a result, when the tank internal pressure P exceeds the emergency release pressure P3, the boil-off gas is released through the emergency flow path, allowing the tank internal pressure P to be reduced quickly.

[0056] The low-temperature gas release system 1 relating to the eighth item of the present disclosure is a low-temperature gas release system 1 relating to any of the first to fifth items, in which when the internal tank pressure P of the liquefied gas tank 10 is equal to or higher than a predetermined pressure threshold, the release valve is opened if the rate of increase of the internal tank pressure P is less than the predetermined rate threshold, and the emergency release valve 71 is opened if the rate of increase of the internal tank pressure P is equal to or higher than the rate threshold.

[0057] This allows the normal flow path and the emergency flow path of the discharge line 3 to be used depending on the rate of increase in the tank internal pressure P of the liquefied gas tank 10.

[0058] The structure 100 relating to the ninth item of the present disclosure comprises a liquefied gas tank 10 for storing liquefied gas, a low-temperature gas release system 1 relating to any of the first to eighth items, a loading gas line 5 connected to the liquefied gas tank 10 and through which boil-off gas of the liquefied gas flowing out of the liquefied gas tank 10 passes, and a loading heat exchanger 34 arranged on the loading gas line 5 and regulating the temperature of the boil-off gas passing through the loading gas line 5, wherein the loading heat exchanger 34 also serves as the heat exchanger 34 of the low-temperature gas release system 1.

[0059] In the structure 100 having the above-described configuration, the heat exchanger 34 is shared by the cargo gas line 5 and the discharge line 3, so that a dedicated heat exchanger 34 for the discharge line 3 can be omitted.

[0060] The structure 100 according to the tenth item of the present disclosure comprises a liquefied gas tank 10 for storing liquefied gas, a low-temperature gas release system 1 according to any one of the first to eighth items, a fuel gas line connected to the liquefied gas tank 10 and through which boil-off gas of the liquefied gas flowing out of the liquefied gas tank 10 passes, and a fuel heat exchanger 34 arranged in the fuel gas line and regulating the temperature of the boil-off gas passing through the fuel gas line, wherein the fuel heat exchanger 34 also serves as the heat exchanger 34 of the low-temperature gas release system 1.

[0061] In the structure 100 having the above-described configuration, the heat exchanger 34 is shared by the fuel gas line and the discharge line 3, so that a dedicated heat exchanger 34 for the discharge line 3 can be omitted.

[0062] The low-temperature gas release method according to the eleventh item of the present disclosure is a method for releasing boil-off gas of liquefied gas from a liquefied gas tank (10) storing the liquefied gas into the atmosphere, in which, when the pressure inside the liquefied gas tank (10) exceeds a predetermined release start pressure (P2), the boil-off gas is heated in a heat exchanger (34) and then released into the atmosphere through a release tower (6), and, when the pressure inside the liquefied gas tank (10) exceeds a predetermined emergency release pressure (P3) higher than the release start pressure (P2), the boil-off gas is released into the atmosphere through the release tower (6) without passing through the heat exchanger (34).

[0063] According to the above-described low-temperature gas release method, when the tank internal pressure P exceeds the release start pressure P2, the steady-state flow path of the release line 3 is used, and boil-off gas having a temperature higher than the boiling point of liquid oxygen is released from the release tower 6. Furthermore, when the tank internal pressure P does not decrease when released through the release line 3, and the tank internal pressure P exceeds the emergency release pressure P3, the emergency flow path of the release line 3 is used, and the boil-off gas is quickly released from the liquefied gas tank 10, thereby reducing the tank internal pressure P.

[0064] The foregoing discussion of the present disclosure has been presented for purposes of illustration and description and is not intended to limit the present disclosure to the form disclosed herein. For example, in the foregoing detailed description, various features of the present disclosure are grouped together in several embodiments for the purpose of streamlining the disclosure, but some of the features may also be combined. Furthermore, the features included in the present disclosure may also be combined into alternative embodiments, configurations, or aspects other than those discussed above.

Claims

1. A system for discharging boil-off gas of a liquefied gas from a liquefied gas tank storing the liquefied gas into the atmosphere, comprising: A discharge tower; A discharge line for conducting the boil-off gas from the liquefied gas tank to the discharge tower, The discharge line includes a main discharge line connecting the liquefied gas tank and the discharge tower, a heat exchanger arranged in the main discharge line and heating the boil-off gas flowing through the main discharge line, a discharge valve arranged in the main discharge line and opening and closing a flow path, a bypass line that guides the boil-off gas to the discharge tower without passing through the discharge valve and the heat exchanger, and an emergency discharge valve arranged in the bypass line and opening and closing the flow path of the bypass line. Low temperature gas release system.

2. A plurality of the discharge towers connectable to the discharge line, the discharge line having a switch for switching a flow path of the boil-off gas so as to connect any one or more of the plurality of the discharge towers to the liquefied gas tank.

10. The cryogenic gas release system of claim 1.

3. A plurality of discharge lines are connected to different liquefied gas tanks, and the plurality of discharge lines have the heat exchanger shared by two or more of the discharge lines.

3. The low temperature gas release system according to claim 1 or 2.

4. the discharge valves include a first discharge valve disposed in the main discharge line upstream of the heat exchanger, a second discharge valve disposed in the main discharge line downstream of the heat exchanger, and a third discharge valve disposed downstream of a junction of the main discharge line and the bypass line.

3. The low temperature gas release system according to claim 1 or 2.

5. A temperature sensor is provided to detect the temperature of the boil-off gas flowing into the release tower, The heat exchanger heats the boil-off gas so that the temperature of the boil-off gas detected by the temperature sensor becomes a predetermined discharge temperature.

3. The low temperature gas release system according to claim 1 or 2.

6. the release valve is opened when the internal tank pressure of the liquefied gas tank exceeds a predetermined release start pressure, and is closed when the internal tank pressure reaches a predetermined release end pressure due to the release of the boil-off gas; or the release valve is opened when the internal tank pressure of the liquefied gas tank increases at a rate exceeding a predetermined release pressure increase rate, and is closed when the internal tank pressure increases at a rate reaching a predetermined release end pressure increase rate due to the release of the boil-off gas.

3. The low temperature gas release system according to claim 1 or 2.

7. The emergency release valve is opened when the tank internal pressure reaches a predetermined emergency release start pressure that is higher than the release start pressure.

7. The cryogenic gas release system of claim 6.

8. When the internal tank pressure of the liquefied gas tank is equal to or higher than a predetermined pressure threshold, if the rate of increase of the internal tank pressure is less than a predetermined rate threshold, the release valve is opened, and if the rate of increase of the internal tank pressure is equal to or higher than the rate threshold, the emergency release valve is opened.

3. The low temperature gas release system according to claim 1 or 2.

9. A liquefied gas tank for storing liquefied gas; A low temperature gas discharge system according to claim 1 or 2; A gas handling line connected to the liquefied gas tank and through which the boil-off gas of the liquefied gas flowing out of the liquefied gas tank passes; A cargo handling heat exchanger is provided in the cargo handling gas line and controls the temperature of the boil-off gas passing through the cargo handling gas line. The heat exchanger for loading and unloading also serves as the heat exchanger for the low-temperature gas release system; structure.

10. A liquefied gas tank for storing liquefied gas; A low temperature gas discharge system according to claim 1 or 2; a fuel gas line connected to the liquefied gas tank and through which the boil-off gas of the liquefied gas flowing out of the liquefied gas tank passes; a fuel heat exchanger disposed in the fuel gas line and configured to adjust the temperature of the boil-off gas passing through the fuel gas line; The fuel heat exchanger also serves as the heat exchanger of the low temperature gas discharge system; structure.

11. A method for releasing boil-off gas of a liquefied gas from a liquefied gas tank storing the liquefied gas into the atmosphere, comprising the steps of: When the pressure in the liquefied gas tank exceeds a predetermined release start pressure, the boil-off gas is heated by a heat exchanger and then released into the atmosphere through a release tower; When the pressure in the liquefied gas tank exceeds a predetermined emergency release start pressure higher than the release start pressure, the boil-off gas is released to the atmosphere through the release tower without passing through the heat exchanger. Low temperature gas release method.