Cryogenic gas discharge system, structure, and cryogenic gas discharge method

Abstract

A cryogenic gas discharge system for discharging boil-off gas of liquefied gas to the atmosphere from a liquefied gas tank for storing liquefied gas, the cryogenic gas discharge 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 having: 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 and heats the boil-off gas flowing through the discharge main line; a discharge valve that is disposed in the discharge main line and that opens and closes the flow path; a bypass line that guides the boil-off gas to the exhaust tower without passing through the exhaust valve and the heat exchanger; and an emergency discharge valve that is disposed in the bypass line and that opens and closes the flow path of the bypass line.

Description

Technical Field

[0001] The present disclosure relates to a low-temperature gas discharge system for discharging boil-off gas generated in a liquefied gas tank from a discharge tower, and a structure such as a ship having the low-temperature gas discharge system. Background Art

[0002] Liquefied gas transport ships are equipped with liquefied gas tanks for storing low-temperature liquefied gases such as liquefied hydrogen. In the liquefied gas tank, the liquefied gas is vaporized by heat input from the outside, generating boil-off gas. In order to suppress the increase in pressure in the tank caused by boil-off gas, boil-off gas has been transported to the GCU (gas combustion unit) and propulsion mechanism carried on the ship and consumed there. In addition, when the pressure in the tank exceeds the design pressure, the boil-off gas is discharged to the atmosphere through a discharge tower to prevent the pressure in the tank from becoming excessive and exceeding the design pressure of the tank. Here, when the extremely low-temperature boil-off gas is discharged from the discharge tower, liquid air and the like are generated, and there is a possibility that the generated liquid will drip onto the hull around the discharge tower or that a high oxygen concentration atmosphere will be generated when the generated liquid is vaporized again. Therefore, a technology has been proposed to heat the boil-off gas generated in the tank before discharging it to 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 midway along the discharge line. This line extracts liquefied gas from the tank and, after heating it to a temperature higher than the boil-off gas within the tank using a heater, delivers it to the discharge line. The boil-off gas flowing from the top of the tank into the discharge line is heated by the heated gas delivered through the heating line and then discharged from the discharge tower into the atmosphere.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-160619 Summary of the Invention

[0007] Problems to be solved by the invention

[0008] In a system that includes a heat exchanger for heating boil-off gas before discharge to the atmosphere, the heat exchanger should preferably be capable of being used for purposes other than heating the boil-off gas for discharge to the atmosphere. However, in the configuration of Patent Document 1, if boil-off gas is discharged from the liquefied gas tank to the atmosphere while the heat exchanger is being used for other purposes, the heat exchanger's heating capacity for other purposes may be reduced due to the need to heat the boil-off gas.

[0009] The present disclosure has been made in view of the above circumstances, and its purpose is to provide a technology that, when evacuating boil-off gas generated in a liquefied gas tank from a discharge tower, discharges the boil-off gas heated by a heat exchanger under normal circumstances to prevent the generation of liquid oxygen around the discharge tower. However, in situations where the pressure in the tank rises sharply, when the heat exchanger is being used for other purposes, or when the heat exchanger is stopped, which results in a situation requiring gas to be discharged from the liquefied gas tank, the pressure in the tank can be rapidly reduced without affecting the use of the heat exchanger for other purposes.

[0010] Means for solving problems

[0011] In order to solve the above-mentioned problems, a low-temperature gas discharge system of one embodiment of the present invention is a system for discharging boil-off gas of the liquefied gas from a liquefied gas tank storing liquefied gas to the atmosphere, wherein the low-temperature gas discharge system comprises: a discharge tower; and a discharge line, which guides the boil-off gas from the liquefied gas tank to the discharge tower, and the discharge line comprises: a discharge main line, which connects the liquefied gas tank and the discharge tower; a heat exchanger, which is arranged on the discharge main line, and heats the boil-off gas flowing in the discharge main line; a discharge valve, which is arranged on the discharge main line, and opens and closes the flow path; a bypass line, which guides the boil-off gas to the discharge tower without passing through the discharge valve and the heat exchanger; and an emergency discharge valve, which is arranged on the bypass line, and opens and closes the flow path of the bypass line.

[0012] A structure according to one embodiment of the present invention comprises: a liquefied gas tank for storing liquefied gas; the above-mentioned low-temperature gas discharge system; a loading and unloading gas line connected to the liquefied gas tank for allowing the evaporated gas of the liquefied gas flowing out of the liquefied gas tank to pass through; and a loading and unloading heat exchanger arranged on the loading and unloading gas line for regulating the temperature of the evaporated gas passing through the loading and unloading gas line, the loading and unloading heat exchanger also serving as the heat exchanger of the low-temperature gas discharge system.

[0013] A structure according to one embodiment of the present invention comprises: a liquefied gas tank for storing liquefied gas; the above-mentioned low-temperature gas discharge system; a fuel gas line connected to the liquefied gas tank for allowing the evaporated gas of the liquefied gas flowing out of the liquefied gas tank to pass through; and a fuel heat exchanger arranged on the fuel gas line for regulating the temperature of the evaporated gas passing through the fuel gas line, the fuel heat exchanger also serving as the heat exchanger of the low-temperature gas discharge system.

[0014] One embodiment of the present disclosure is a method for discharging boil-off gas of liquefied gas from a liquefied gas tank storing liquefied gas to the atmosphere, wherein when the pressure in the liquefied gas tank exceeds a prescribed discharge start pressure, the boil-off gas is heated by a heat exchanger and then discharged to the atmosphere through a discharge tower, and when the pressure in the liquefied gas tank exceeds a prescribed emergency discharge start pressure that is higher than the discharge start pressure, the boil-off gas is discharged to the atmosphere through the discharge tower without passing through the heat exchanger.

[0015] Effects of the Invention

[0016] According to the present disclosure, when the boil-off gas generated in the liquefied gas tank is discharged from the discharge tower, the generation of liquid oxygen around the discharge tower can be prevented, and the tank pressure can be quickly reduced when the tank pressure rises suddenly. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a diagram showing a schematic configuration of a low-temperature gas discharge system according to an embodiment of the present disclosure.

[0018] Figure 2 This is a diagram showing an example of the hardware configuration of the controller.

[0019] Figure 3 This is a block diagram showing the configuration of a control system of the low-temperature gas exhaust system.

[0020] Figure 4 This is a diagram illustrating an example of temporal changes in the pressure within the tank.

[0021] Figure 5 This is a flowchart of the processing of the controller for realizing the operating method of the second embodiment.

[0022] Figure 6 This is a diagram showing a schematic configuration of a low-temperature gas exhaust system according to Modification 1.

[0023] Figure 7 This is a diagram showing a schematic configuration of a low-temperature gas exhaust system according to Modification 2.

[0024] Figure 8 This is a diagram showing a schematic configuration of a low-temperature gas discharge system according to Modification 3. DETAILED DESCRIPTION

[0025] Next, embodiments of the present disclosure will be described with reference to the drawings. Figure 1 1 is a diagram showing a schematic configuration of a low-temperature gas discharge system 1 according to an embodiment of the present disclosure. Figure 1The illustrated cryogenic gas discharge system 1 discharges boil-off gas generated in a liquefied gas tank 10 from a discharge tower 6 to the atmosphere. The liquefied gas tank 10 contains cryogenic liquefied gas, such as liquid hydrogen or liquefied natural gas. Land structures, floating structures, and marine structures such as ships equipped with the liquefied gas tank 10 are equipped with the cryogenic gas discharge system 1. An emergency exhaust line 55, independent of the cryogenic gas discharge system 1, is provided in the liquefied gas tank 10. An automatic safety valve 56 is provided in the emergency exhaust line 55. However, the emergency exhaust line 55 may also be combined with the cryogenic gas discharge system 1.

[0026] The low-temperature gas discharge system 1 includes a discharge line 3, a discharge tower 6, and a controller 4. The discharge line 3 guides boil-off gas generated in the liquefied gas tank 10 from the liquefied gas tank 10 to the discharge tower 6. The controller 4 controls the low-temperature gas discharge system 1.

[0027] The discharge line 3 includes a main discharge line 30 that connects the gas phase within the liquefied gas tank 10 to the discharge tower 6. The main discharge line 30 is constructed of piping with low-temperature resistance, such as double piping. A first discharge valve 32 and a second discharge valve 33 are provided in the main discharge line 30. Both the first discharge valve 32 and the second discharge valve 33 are normally closed on-off valves that open and close the flow path of the main discharge line 30.

[0028] In the discharge main line 30, a heat exchanger 34 and a compressor 35 are provided on the downstream side of the first discharge valve 32 and the upstream side of the second discharge valve 33. However, the compressor 35 may be omitted. The evaporated gas flowing in the discharge main line 30 is heated to a predetermined discharge temperature while passing through the heat exchanger 34. The discharge temperature is an arbitrary temperature. The discharge temperature is not particularly limited, but from the viewpoint of suppressing the generation of liquid air near the outlet of the discharge tower 6, a temperature higher than the boiling point of liquid oxygen is preferably used. The heat exchange method of the heat exchanger 34 is not particularly limited, and a well-known heat exchange method such as a shell and tube method or a fin plate method may be adopted. The compressor 35 is arranged on the upstream side or downstream side of the heat exchanger 34 in the discharge main line 30. By the operation of the compressor 35, the evaporated gas flowing in the discharge main line 30 is pressurized and transported to the discharge tower 6.

[0029] The upstream end of the bypass line 7 is connected to the first connection 36 on the upstream side of the first discharge valve 32 in the main discharge line 30. The downstream end of the bypass line 7 is connected to the second connection 37 on the downstream side of the second discharge valve 33 in the main discharge line 30. The bypass line 7 directs the boil-off gas flowing into the main discharge line 30 to the discharge tower 6 without passing through the first discharge valve 32, the heat exchanger 34, the compressor 35, and the second discharge valve 33. In this embodiment, the bypass line 7 is connected to the discharge line 3, but the bypass line 7 can also directly connect the liquefied gas tank 10 to the discharge tower 6 without passing through the discharge line 3. An emergency discharge valve 71 is provided in the bypass line 7. The emergency discharge valve 71 is a normally closed on-off valve that opens and closes the flow path of the bypass line 7. In this embodiment, a single emergency discharge valve 71 is provided in the bypass line 7, but two or more emergency discharge valves 71 may be provided in the bypass line 7 to prevent valve leakage or damage.

[0030] The low-temperature gas discharge system 1 of the above structure may also be configured so that the discharge tower 6 for discharging boil-off gas can be selected from among a plurality of discharge towers 6. Figure 6 In the illustrated example, a structure 100 includes multiple liquefied gas tanks 10, and a low-temperature gas discharge system 1 includes multiple discharge lines 3 and multiple discharge towers 6. The number of liquefied gas tanks 10 and discharge towers 6 may vary. Each of the multiple liquefied gas tanks 10 is connected to the upstream portion 30a of the discharge main line 30. The upstream portions 30a of the multiple discharge main lines 30 are each connected to a hub 64 via an on-off valve 62. Furthermore, the multiple discharge towers 6 are each connected to a hub 63 via the downstream portion 30c of the discharge main line 30 and an on-off valve 61. The hubs 63 and 64 are connected by one or more common lines 65. The common lines 65 each include a midstream portion 30b of the discharge main line 30, a heat exchanger 34 through which the midstream portion 30b of the discharge main line 30 passes, a bypass line 7 connected to the midstream portion 30b of the discharge main line 30 to bypass the heat exchanger 34, and an on-off valve 66. The discharge line 3 connected to each liquefied gas tank 10 consists of the upstream portion 30a of the main discharge line 30, a hub 64, at least one common line 65, a hub 63, and the downstream portion 30c of the main discharge line 30. By opening and closing the on-off valve 61, the discharge tower 6 connected to the discharge line 3, i.e., the one or more discharge towers 6 that discharge boil-off gas, can be selected. By opening and closing the on-off valve 66, the common line 65 used, i.e., the heat exchanger 34 used, can be arbitrarily selected. In other words, one or more heat exchangers 34 are shared among multiple discharge lines 3.

[0031] In addition, in the low-temperature gas discharge system 1 having the above structure, the heat exchanger 34 and the compressor 35 may also be used as common equipment for loading and unloading the liquefied gas tank 10. Figure 7In the illustrated variation, a structure 100 having a liquefied gas tank 10 includes a discharge line 3 and a loading / unloading gas line 5 for discharging boil-off gas from the liquefied gas tank 10 to the outside. The loading / unloading gas line 5 is composed of piping and other components. The loading / unloading gas line 5 is equipped with a first discharge valve 32, a heat exchanger 34, a compressor 35, and a loading / unloading valve 52. The loading / unloading gas line 5 shares a portion of the main discharge line 30, the first discharge valve 32, the heat exchanger 34, and the compressor 35 disposed therein. A portion 51 of the loading / unloading gas line 5 for discharging boil-off gas to the outside is connected to a connection 57 between the compressor 35 and the second discharge valve 33 of the main discharge line 30. A loading / unloading valve 52 is disposed on this portion 51 of the loading / unloading gas line 5. The loading / unloading valve 52 is a normally closed on-off valve that opens and closes the flow path of the loading / unloading gas line 5. With this configuration, when the second discharge valve 33 is closed and the first discharge valve 32 and the loading / unloading valve 52 are open, the heat exchanger 34 functions as a loading / unloading heat exchanger for the loading / unloading gas line 5, and the compressor 35 functions as a loading / unloading compressor for the loading / unloading gas line 5. Furthermore, when the first discharge valve 32 and the second discharge valve 33 are open and the loading / unloading 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.

[0032] In the above description, an example in which pipes and equipment are shared between the discharge line 3 and the loading and unloading gas line 5 is described. However, pipes and equipment may also be shared between the fuel gas line that delivers boil-off gas from the liquefied gas tank 10 as fuel to the combustion equipment and the discharge line 3. In this case, as the combustion equipment, Figure 7 In the illustrated example, a portion of the fuel gas line 51 for transporting boil-off gas to the combustion equipment is connected to the connection 57 between the compressor 35 and the second discharge valve 33 of the main discharge line 30, replacing a portion of the loading and unloading gas line 5. Furthermore, when the second discharge valve 33 is closed and the first discharge 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.

[0033] Figure 2 4 is a diagram showing an example of the hardware structure of the controller 4. Figure 2As shown, the controller 4 is a so-called computer and includes a CPU (Central Processing Unit) 41, memory 42, an interface 44 for connecting to the discharge valves 32, 33, and 71, the heat exchanger 34, the compressor 35, various instruments, and a communication network via wired or wireless connections, an input device 45 such as a mouse, keyboard, and touch panel, and an output device 46 such as a liquid crystal display and a speaker. The various functions of the controller 4, described below, can be implemented 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. Furthermore, the predetermined program can be downloaded to the controller 4 from a network via the interface 44 or loaded from a storage medium.

[0034] The functions of the controller 4 disclosed in this specification can be performed using circuits or processing circuits including general-purpose processors, special-purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuit), conventional circuits, and / or combinations thereof that are constructed or programmed to perform the disclosed functions. The processor includes transistors or other circuits and is therefore considered a processing circuit or circuit. In this disclosure, a circuit, unit, or component is hardware that performs the listed functions. The hardware may be the hardware disclosed in this specification, or it may be other known hardware that is programmed or constructed to perform the listed functions. In the case where the hardware is a processor that is considered to be a type of circuit, the circuit, component, or unit is a combination of hardware and software, and the software is used for the structure of the hardware and / or processor.

[0035] Figure 3 1 is a block diagram showing the structure of the control system of the low-temperature gas exhaust system 1. Figure 3 As shown, the first discharge valve 32 , the second discharge valve 33 , and the emergency discharge valve 71 are connected to the controller 4 , and the controller 4 opens and closes the first discharge valve 32 , the second discharge valve 33 , and the emergency discharge valve 71 .

[0036] A pressure sensor 11 and a temperature sensor 12 are connected to the controller 4, and these sensors output detection data 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 "tank internal pressure"). The temperature sensor 12 detects the temperature of the boil-off gas flowing into the discharge tower 6. In this embodiment, the temperature sensor 12 is located downstream of the second connection portion 37 in the discharge main line 30. However, the temperature sensor 12 may also be located in the discharge tower 6.

[0037] The controller 4 monitors the internal pressure of the tank detected by the pressure sensor 11 while the liquefied gas is stored in the liquefied gas tank 10. Figure 4As shown, the controller 4 stores the operating pressure range of the liquefied gas tank 10, the warning 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 lower than the design pressure P4, the discharge start pressure P2 is lower than the emergency discharge pressure P3, the warning pressure P1 is lower than the discharge start pressure P2, and the discharge end pressure P5 is lower than the warning 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 warning pressure P1 can be the upper limit of the operating pressure range of the liquefied gas tank 10 or a higher pressure. The warning pressure P1 is the pressure threshold that triggers the operation of the low-temperature gas discharge system 1. In addition, the differential pressure of the safety valve 56 of the emergency exhaust line 55 is preferably lower than the design pressure P4 and higher than the emergency discharge pressure P3.

[0038] While the boil-off gas is being discharged from the discharge tower 6, the controller 4 monitors the boil-off gas temperature detected by the temperature sensor 12. If the boil-off gas temperature detected by the temperature sensor 12 is lower than the discharge temperature, the controller 4 takes measures such as increasing the boil-off gas heating temperature by the heat exchanger 34 or reducing the boil-off gas delivery rate by the compressor 35. If the boil-off gas temperature detected by the temperature sensor 12 is higher than the discharge temperature, the controller 4 takes measures such as lowering the boil-off gas heating temperature by the heat exchanger 34 or increasing the boil-off gas delivery rate by the compressor 35. This maintains the boil-off gas temperature detected by the temperature sensor 12 at the discharge temperature.

[0039] The discharge line 3 of the cryogenic gas discharge system 1 of the above-mentioned structure has two flow paths, namely a "normal flow path" and an "emergency flow path". The "normal flow path" uses the heat exchanger 34 to heat the evaporated gas coming out of the liquefied gas tank 10 to a temperature higher than the boiling point of liquid oxygen and then transports it to the discharge tower 6. The "emergency flow path" transports the evaporated gas to the discharge tower 6 through the 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 discharge main line 30. The first discharge valve 32 and the second discharge valve 33 are opened, and the emergency discharge valve 71 is closed, and the normal flow path functions. In addition, the emergency flow path is a flow path that passes through a part of the discharge main line 30 and the bypass line 7. The first discharge valve 32 and the second discharge valve 33 are closed, and the emergency discharge valve 71 is opened, and the emergency flow path functions. In addition, as Figure 8As shown, a third discharge valve 39 may be disposed in the discharge line 3 downstream of the second connection portion 37, which is the junction of the main discharge line 30 and the bypass line 7. The third discharge valve 39 is an on / off valve that opens and closes similarly to the first discharge valve 32 and the second discharge valve 33 during normal flow path operation, and opens and closes similarly to the emergency discharge valve 71 during emergency flow path operation. If either the second discharge valve 33 or the emergency discharge valve 71 fails, the third discharge valve 39 assumes the function of the failed valve.

[0040] The controller 4 operates the valves 32, 33, and 71 to open and close, controlling the operation of the low-temperature gas discharge system 1 by switching between one of the following states: a state in which the normal flow path is functional and the emergency flow path is inoperative; a state in which the normal flow path is inoperative and the emergency flow path is inoperative; or a state in which both the normal flow path and the emergency flow path are inoperative. However, in addition to the above state, a state in which both the normal flow path and the emergency flow path are functional is also possible. The following describes first to third embodiments of the method for operating the low-temperature gas discharge system 1.

[0041] [First embodiment]

[0042] like Figure 4 As shown, the boil-off gas from the liquefied gas stored in the liquefied gas tank 10 causes the internal pressure P to rise. Furthermore, some of the boil-off gas may be delivered as fuel to fuel-consuming equipment. When the internal pressure P reaches the warning pressure P1 (time t1), the controller 4 outputs a warning from the output device 46. This warning serves as a warning to inform the user of the discharge of the boil-off gas.

[0043] When the tank pressure P reaches the discharge start pressure P2 (time t2), the controller 4 opens the first discharge valve 32 and the second discharge valve 33 and simultaneously operates the compressor 35. Alternatively, when the pressure rise rate of the tank pressure P (i.e., the amount of pressure rise per unit time) reaches a predetermined discharge start pressure rise rate, the controller 4 opens the first discharge valve 32 and the second discharge valve 33 and simultaneously operates the compressor 35. In addition, the controller 4 starts the heat exchanger 34 when or before the tank pressure P reaches the discharge start pressure P2. The heat exchanger 34, which is currently stopped, requires a predetermined startup time until it reaches a state capable of heating the evaporated gas to the discharge temperature. Therefore, it is preferable that the heat exchanger 34 be started in advance before the tank pressure P reaches the discharge start pressure P2.

[0044] As described above, when the interior of the liquefied gas tank 10 is connected to the discharge tower 6 via the normal flow path of the discharge line 3 and the heat exchanger 34 and compressor 35 are operating, the boil-off gas flowing from the liquefied gas tank 10 into the discharge line 3 is heated to the discharge temperature and transported to the discharge tower 6, where it is discharged into the atmosphere. The boil-off gas discharged from the discharge tower 6 into the atmosphere has a temperature higher than the boiling point of liquid oxygen, thereby preventing the generation of liquid oxygen, liquid nitrogen, and liquid air at the outlet of the discharge tower 6 and its surroundings.

[0045] As the boil-off gas is discharged, the tank pressure P gradually decreases. When the tank pressure P reaches the discharge end pressure P5 (time t3), the controller 4 stops the heat exchanger 34 and compressor 35, and closes the first and second discharge valves 32 and 33. Alternatively, when the rate of increase in the tank 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 compressor 35, and closes the first and second discharge valves 32 and 33. Through the operation of the low-temperature gas discharge system 1 in this manner, the tank pressure P is controlled within the operating range.

[0046] During the boil-off gas discharge process described above, when the tank internal pressure P reaches or exceeds the emergency discharge pressure P3, the controller 4 opens the emergency discharge valve 71. Alternatively, when the rate of increase in the tank internal pressure P (i.e., the amount of pressure increase per unit time) reaches or exceeds a predetermined emergency discharge rate threshold, the controller 4 opens the emergency discharge valve 71. Furthermore, since the bypass line 7 is connected to the liquefied gas tank 10 upstream of the emergency discharge valve 71 via a portion of the main discharge line 30, the pressure in the bypass line 7 is substantially the same as the tank internal pressure P.

[0047] By opening the emergency discharge valve 71, the emergency flow path of the discharge line 3 is opened, allowing a portion of the boil-off gas flowing into the discharge line 3 to flow through the bypass line 7 (i.e., without passing through the heat exchanger 34 and compressor 35) and into the discharge tower 6. This allows for rapid discharge of the boil-off gas from the discharge tower 6 without passing through the heat exchanger 34 and compressor 35, thus preventing an excessive increase in the tank pressure P. Furthermore, since unheated boil-off gas is discharged from the discharge tower 6 when the tank pressure P exceeds the emergency discharge pressure P3, the controller 4 preferably issues an alarm from the output device 46 when the tank pressure P exceeds the emergency discharge pressure P3.

[0048] In the above description, the opening and closing of the first and second discharge valves 32 and 33 are controlled by the controller 4. However, at least one of the first and second discharge valves 32 and 33 can also be opened and closed manually. Furthermore, the emergency discharge valve 71 is a control valve controlled by the controller 4. However, it can also be a manual valve or a safety valve that automatically opens at the emergency discharge pressure P3. In this case, when the emergency discharge valve 71 is open, the first and second discharge valves 32 and 33 can be closed or opened.

[0049] [Second embodiment]

[0050] Figure 5 FIG. 4 is a flowchart of the processing of the controller 4 for realizing the operation method of the second embodiment. Figure 5 As shown, the tank pressure P rises due to the boil-off gas of the liquefied gas stored in the liquefied gas tank 10, reaching the warning pressure P1. The controller 4 monitors the tank pressure P, and when the tank pressure P reaches the warning pressure P1 (step S01), it calculates the pressure increase rate of the tank pressure P (step S02). The pressure increase rate of the tank pressure P can be determined based on the history of the tank pressure P.

[0051] A speed threshold value related to the pressure increase rate is pre-assigned and stored in the controller 4. The controller 4 compares the calculated pressure increase rate with the speed threshold value (step S03). If the pressure increase rate is less than the speed threshold value ("No" in step S03), the output device 46 outputs a warning (step S04). This warning serves as a warning to prevent the discharge of boil-off gas.

[0052] When the tank pressure P reaches the discharge start pressure P2 ("YES" in step S05), the controller 4 opens the first and second discharge valves 32 and 33 and simultaneously operates the compressor 35 (step S06). Furthermore, the controller 4 activates the heat exchanger 34 at or before the time when the tank pressure P reaches the discharge start pressure P2. When the normal flow path of the discharge line 3 connects the interior of the liquefied gas tank 10 to the discharge tower 6 and the heat exchanger 34 and compressor 35 are in operation, the boil-off gas flowing from the liquefied gas tank 10 into the discharge line 3 is heated to the discharge temperature and transported to the discharge tower 6, where it is discharged into the atmosphere. When the discharge of boil-off gas lowers the tank pressure P to the discharge end pressure P5 ("YES" in step S07), the controller 4 closes the first and second discharge valves 32 and 33 and simultaneously stops the heat exchanger 34 and compressor 35 (step S08).

[0053] On the other hand, in step S03, if the pressure rise rate exceeds the rate threshold ("Yes" in step S03), the controller 4 issues an alarm from the output device 46 (step S21) and opens the emergency discharge valve 71 (step S22). The emergency discharge valve 71 may also be opened after the tank pressure P exceeds the emergency discharge pressure P3. By opening the emergency discharge valve 71, the emergency flow path of the discharge line 3 is opened, and the boil-off gas flowing into the discharge line 3 flows through the bypass line 7 (i.e., without passing through the heat exchanger 34 and compressor 35) to the discharge tower 6, and is discharged from the discharge tower 6 into the atmosphere. When the tank pressure P decreases to the discharge end pressure P5 due to the discharge of boil-off gas ("Yes" in step S23), the controller 4 closes the emergency discharge valve 71 (step S24).

[0054] In the above description, the opening and closing operations of the first discharge valve 32, the second discharge valve 33, and the emergency discharge valve 71 are controlled by the controller 4. However, the opening and closing operations of at least one of the first discharge valve 32, the second discharge valve 33, and the emergency discharge valve 71 can also be performed manually. In this case, the controller 4 will issue a warning or alarm at an appropriate time, and the operator can operate the manual valve in response to the warning or alarm.

[0055] [Third embodiment]

[0056] In the third embodiment, the heat exchanger 34 of the low-temperature gas discharge system 1 has uses other than heating the boil-off gas for discharge, such as a loading and unloading heat exchanger or a fuel heat exchanger. Therefore, it is conceivable that while the heat exchanger 34 is being used for purposes other than heating the boil-off gas for discharge, such as loading and unloading or fuel supply, or while the heat exchanger 34 is stopped, the boil-off gas of the liquefied gas may be discharged from the liquefied gas tank 10 to the atmosphere. In this case, the emergency flow path is used to guide the boil-off gas from the liquefied gas tank 10 to the discharge tower 6 without passing through the heat exchanger 34, and the boil-off gas is discharged to the atmosphere. Specifically, the controller 4 opens the emergency discharge valve 71 while the first discharge valve 32 and the second discharge valve 33 are closed. When the boil-off gas discharge is completed, the controller 4 closes the emergency discharge valve 71. This allows the boil-off gas to be discharged from the liquefied gas tank 10 to the atmosphere without affecting fuel heating or loading and unloading. Similarly, the emergency flow path can also be used to discharge the replacement gas during maintenance of the liquefied gas tank 10.

[0057] 〔Summarize〕

[0058] The low-temperature gas discharge system 1 of the first item of the present disclosure is a system for discharging boil-off gas of liquefied gas from a liquefied gas tank 10 storing the liquefied gas to the atmosphere, wherein the low-temperature gas discharge system 1 includes: a discharge tower 6; and a discharge line 3, which guides the boil-off gas from the liquefied gas tank 10 to the discharge tower 6, the discharge line 3 including: a discharge main line 30, which connects the liquefied gas tank 10 and the discharge tower 6; a heat exchanger 34, which is arranged on the discharge main line 30 and heats the boil-off gas flowing in the discharge main line 30; discharge valves 32 and 33, which are arranged on the discharge main line 30 and open and close the flow path; a bypass line 7, which guides the boil-off gas to the discharge tower 6 without passing through the discharge valves 32 and 33 and the heat exchanger 34; and an emergency discharge valve 71, which is arranged on the bypass line 7 and opens and closes the flow path of the bypass line 7.

[0059] In the cryogenic gas discharge system 1 configured as described above, the discharge line 3 includes a flow path (i.e., a normal flow path) that heats the boil-off gas to a temperature higher than the boiling point of liquid oxygen using the heat exchanger 34 and then delivers it to the discharge tower 6, and a flow path (i.e., an emergency flow path) that delivers the boil-off gas to the discharge tower 6 without passing through the heat exchanger 34. When the normal flow path is used to discharge the boil-off gas, the boil-off gas, which has a temperature higher than the boiling point of liquid oxygen, is discharged from the discharge tower 6. This discharge prevents the liquefaction of the air (air containing oxygen and nitrogen) on the surface of the discharge tower 6 and around the discharge tower 6. When the emergency flow path is used to discharge the boil-off gas, the boil-off gas can be discharged from the liquefied gas tank 10 without waiting for the activation of the heat exchanger 34, thereby rapidly reducing the internal pressure P of the liquefied gas tank 10. For example, if the internal pressure P of the liquefied gas tank 10 rises suddenly due to damage, the emergency flow path of the discharge line 3 can be used to urgently discharge boil-off gas from the liquefied gas tank 10, without having to wait for the discharge valves 32 and 33 to be operated or the heat exchanger 34 to be activated. Furthermore, if the heat exchanger 34 is being used for purposes other than heating boil-off gas for atmospheric discharge from the liquefied gas tank 10, and if boil-off gas or other gases need to be discharged from the liquefied gas tank 10, the emergency flow path of the discharge line 3 can be used to urgently discharge boil-off gas from the liquefied gas tank 10 without affecting the use of the heat exchanger 34 for other purposes. Furthermore, the emergency flow path of the discharge line 3 is not limited to emergency discharge as described above; it can also be used for gas replacement in the liquefied gas tank 10. Thus, in the low-temperature gas discharge system 1, the normal flow path and the emergency flow path of the discharge line 3 can be used separately according to the application and situation.

[0060] The low-temperature gas discharge system 1 of the second item of the present disclosure is the low-temperature gas discharge system 1 of the first item, wherein the low-temperature gas discharge system 1 has a plurality of discharge towers 6 that can be connected to the discharge line 3, and the plurality of discharge lines 3 have a switcher (an on-off valve 61 in the above embodiment), which switches the flow path of the boil-off gas so that any one or more of the plurality of discharge towers 6 are connected to the liquefied gas tank 10.

[0061] In this manner, the discharge tower 6 from which the boil-off gas is discharged can be selected, and thus the boil-off gas can be discharged from the discharge tower 6 suitable for the conditions, such as the wind direction and maintenance.

[0062] The low-temperature gas discharge system 1 of the third item of the present disclosure is the low-temperature gas discharge system 1 of the first item or the second item, wherein the low-temperature gas discharge system 1 has a plurality of discharge lines 3 connected to different liquefied gas tanks 10, and the plurality of discharge lines 3 have a heat exchanger 34 shared by two or more discharge lines 3.

[0063] This can reduce the number of heat exchangers 34. In addition, the heat exchangers 34 included in the discharge line 3 can be arbitrarily selected.

[0064] The low-temperature gas discharge system 1 of the fourth item of the present disclosure is the low-temperature gas discharge system 1 of any one of the first to third items, wherein the discharge valves 32 and 33 include: a first discharge valve 32, which is arranged on the upstream side of the heat exchanger 34 in the discharge main line 30; a second discharge valve 33, which is arranged on the downstream side of the heat exchanger 34 in the discharge main line 30; and a third discharge valve 39, which is arranged downstream of the confluence 37 of the discharge main line 30 and the bypass line 7.

[0065] Thus, by arranging dual discharge valves 32 and 33 on the discharge main line 30, it is possible to prevent the discharge of boil-off gas to the atmosphere or the inflow of external air into the liquefied gas tank 10 due to leakage or damage of one valve. In addition, the third discharge valve 39 can function even if either valve is opened due to a malfunction.

[0066] The low-temperature gas discharge system 1 of item 5 of the present disclosure is the low-temperature gas discharge system 1 of any one of items 1 to 4, wherein the low-temperature gas discharge system 1 has a temperature sensor 12 for detecting 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.

[0067] Thereby, the boil-off gas flowing into the discharge tower 6 can be controlled to the discharge temperature.

[0068] The low-temperature gas discharge system 1 of item 6 of the present disclosure is the low-temperature gas discharge system 1 of any one of items 1 to 5, wherein the discharge valves 32 and 33 are opened when the tank pressure P of the liquefied gas tank 10 exceeds the prescribed discharge start pressure P2, and are closed when the tank pressure P becomes the prescribed discharge end pressure P5 due to the discharge of boil-off gas, or the discharge valves 32 and 33 are opened when the pressure rise gradient of the tank pressure P of the liquefied gas tank 10 exceeds the prescribed discharge pressure rise gradient, and are closed when the pressure rise gradient of the tank pressure P becomes the prescribed discharge end pressure rise gradient due to the discharge of boil-off gas.

[0069] Thus, when the internal pressure P of the liquefied gas tank 10 exceeds the discharge start pressure P2, boil-off gas is discharged, and the internal pressure P of the tank can be maintained within a predetermined pressure range (ie, within a use pressure range).

[0070] A low-temperature gas discharge system 1 according to a seventh aspect of the present disclosure is the low-temperature gas discharge system 1 according to the sixth aspect, wherein the emergency discharge valve 71 opens when the tank internal pressure P reaches a predetermined emergency discharge pressure P3 higher than the discharge start pressure P2.

[0071] Thus, when the tank internal pressure P exceeds the emergency discharge pressure P3, the boil-off gas is discharged through the emergency flow path, and the tank internal pressure P can be quickly reduced.

[0072] The low-temperature gas discharge system 1 of item 8 of the present disclosure is a low-temperature gas discharge system 1 of any one of items 1 to 5, wherein when the tank pressure P of the liquefied gas tank 10 is above a prescribed pressure threshold, the discharge valve opens when the rising speed of the tank pressure P is less than a prescribed speed threshold, and the emergency discharge valve 71 opens when the rising speed of the tank pressure P is above the speed threshold.

[0073] This allows the normal flow path and the emergency flow path of the discharge line 3 to be used separately according to the rising speed of the internal pressure P of the liquefied gas tank 10 .

[0074] The structure 100 of item 9 of the present disclosure comprises: a liquefied gas tank 10 for storing liquefied gas; a low-temperature gas discharge system 1 of any one of items 1 to 8; a loading and unloading gas line 5 connected to the liquefied gas tank 10 for allowing the evaporated gas of the liquefied gas flowing out of the liquefied gas tank 10 to pass through; and a loading and unloading heat exchanger 34, which is arranged on the loading and unloading gas line 5 and regulates the temperature of the evaporated gas passing through the loading and unloading gas line 5, and the loading and unloading heat exchanger 34 also serves as the heat exchanger 34 of the low-temperature gas discharge system 1.

[0075] In the structure 100 having the above-described structure, the heat exchanger 34 is commonly used in the loading and unloading gas line 5 and the discharge line 3 , and therefore a heat exchanger 34 dedicated to the discharge line 3 can be omitted.

[0076] The structure 100 of item 10 of the present disclosure comprises: a liquefied gas tank 10 for storing liquefied gas; a low-temperature gas discharge system 1 of any one of items 1 to 8; a fuel gas line connected to the liquefied gas tank 10 for allowing the evaporated gas of the liquefied gas flowing out of the liquefied gas tank 10 to pass through; and a fuel heat exchanger 34, which is arranged on the fuel gas line and regulates the temperature of the evaporated gas passing through the fuel gas line, and the fuel heat exchanger 34 also serves as the heat exchanger 34 of the low-temperature gas discharge system 1.

[0077] In the structure 100 having the above configuration, the heat exchanger 34 is commonly used in the fuel gas line and the discharge line 3 , and therefore a heat exchanger 34 dedicated to the discharge line 3 can be omitted.

[0078] The low-temperature gas discharge method of the eleventh aspect of the present disclosure is a method for discharging boil-off gas of liquefied gas from a liquefied gas tank 10 storing the liquefied gas to the atmosphere, wherein:

[0079] When the pressure in the liquefied gas tank 10 exceeds the specified discharge start pressure P2, the boil-off gas is heated by the heat exchanger 34 and then discharged to the atmosphere through the discharge tower 6. When the pressure in the liquefied gas tank 10 exceeds a specified emergency discharge pressure P3 which is higher than the discharge start pressure P2, the boil-off gas is discharged to the atmosphere through the discharge tower 6 without passing through the heat exchanger 34.

[0080] According to the above-described cryogenic gas discharge method, if the tank internal pressure P exceeds the discharge start pressure P2, boil-off gas, whose temperature is higher than the boiling point of liquid oxygen, is discharged from the discharge tower 6 using the normal flow path of the discharge line 3. Furthermore, if the tank internal pressure P does not decrease during discharge through the discharge line 3 and the tank internal pressure P exceeds the emergency discharge pressure P3, the boil-off gas is quickly discharged from the liquefied gas tank 10 using the emergency flow path of the discharge line 3, thereby reducing the tank internal pressure P.

[0081] The above discussion of the present disclosure is presented for purposes of illustration and description and is not intended to limit the present disclosure to the methods disclosed herein. For example, in the foregoing detailed description, various features of the present disclosure are summarized into several embodiments for the purpose of rationalizing the present disclosure, but several of the multiple features may be combined. Furthermore, the various features included in the present disclosure may also be combined with alternative embodiments, structures, or methods not discussed above.

Claims

1. A low-temperature gas discharge system for discharging boil-off gas of liquefied gas from a liquefied gas tank storing liquefied gas to the atmosphere, wherein: The low temperature gas exhaust system has: discharge tower; as well as a discharge line that guides the boil-off gas from the liquefied gas tank to the discharge tower, The discharge line has: a discharge main line connecting the liquefied gas tank and the discharge tower; a heat exchanger disposed in the exhaust main line and heating the boil-off gas flowing in the exhaust main line; a discharge valve, which is arranged on the discharge main line and opens and closes the flow path; a bypass line that guides the boil-off gas toward the discharge tower without passing through the discharge valve and the heat exchanger; and The emergency discharge valve is disposed in the bypass line and opens and closes the flow path of the bypass line.

2. The low-temperature gas exhaust system according to claim 1, wherein: The low-temperature gas discharge system includes a plurality of discharge towers that can be connected to the discharge line. The discharge line includes a switch that switches a flow path of the boil-off gas so that any one or more of the plurality of discharge towers communicate with the liquefied gas tank.

3. The low-temperature gas exhaust system according to claim 1 or 2, wherein: The low-temperature gas discharge system includes a plurality of discharge lines connected to different liquefied gas tanks. The plurality of discharge lines include the heat exchanger shared by two or more of the discharge lines.

4. The low-temperature gas exhaust system according to any one of claims 1 to 3, wherein: The discharge valve comprises: a first discharge valve disposed upstream of the heat exchanger in the discharge main line; a second discharge valve disposed downstream of the heat exchanger in the discharge main line; as well as The third discharge valve is arranged downstream of a junction of the discharge main line and the bypass line.

5. The low-temperature gas exhaust system according to any one of claims 1 to 4, wherein: The low-temperature gas discharge system includes a temperature sensor for detecting the temperature of the boil-off gas flowing into the discharge 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.

6. The low-temperature gas discharge system according to any one of claims 1 to 5, wherein: The discharge valve opens when the internal pressure of the liquefied gas tank exceeds a predetermined discharge start pressure, and closes when the internal pressure reaches a predetermined discharge end pressure due to the discharge of the boil-off gas, or The discharge valve opens when the pressure increase rate of the tank pressure of the liquefied gas tank exceeds a predetermined discharge pressure increase rate, and closes when the pressure increase rate of the tank pressure reaches a predetermined discharge end pressure increase rate due to the discharge of the boil-off gas.

7. The low-temperature gas exhaust system according to claim 6, wherein: The emergency discharge valve opens when the pressure in the tank reaches a predetermined emergency discharge start pressure that is higher than the discharge start pressure.

8. The low-temperature gas discharge system according to any one of claims 1 to 5, wherein: When the pressure inside the liquefied gas tank is above a specified pressure threshold, and the rising speed of the pressure inside the tank is less than a specified speed threshold, the discharge valve opens; and when the rising speed of the pressure inside the tank is above the speed threshold, the emergency discharge valve opens.

9. A structure comprising: a liquefied gas tank, which stores liquefied gas; The low-temperature gas discharge system according to any one of claims 1 to 8; a loading and unloading 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 loading and unloading heat exchanger, which is arranged in the loading and unloading gas line and controls the temperature of the boil-off gas passing through the loading and unloading gas line; The loading and unloading heat exchanger also serves as the heat exchanger of the low-temperature gas exhaust system.

10. A structure comprising: a liquefied gas tank, which stores liquefied gas; The low-temperature gas discharge system according to any one of claims 1 to 8; 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 in the fuel gas line and regulating the temperature of the evaporated gas passing through the fuel gas line; The fuel heat exchanger also serves as the heat exchanger of the low-temperature gas exhaust system.

11. A method for discharging low-temperature gas, which is a method for discharging boil-off gas of liquefied gas from a liquefied gas tank storing liquefied gas to the atmosphere, wherein: When the pressure in the liquefied gas tank exceeds the prescribed discharge start pressure, the boil-off gas is heated by a heat exchanger and then discharged to the atmosphere through a discharge tower. When the pressure in the liquefied gas tank exceeds a predetermined emergency discharge start pressure that is higher than the discharge start pressure, the boil-off gas is discharged to the atmosphere through the discharge tower without passing through the heat exchanger.

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