Natural gas supply system and operation method when restarting operation of the natural gas supply system

Abstract

To provide a natural gas supply system capable of preventing thermal stress from being applied abruptly to an LNG vaporizer, and capable of suppressing heat quantity fluctuation of natural gas to be supplied.SOLUTION: A natural gas supply system 101 comprises a vaporized gas return pipe 51 that returns natural gas in a buffer tank 41 to an LNG storage tank 1, and a control device 60 that controls at least the opening and closing of an LNG discharge valve 33 and the opening and closing of a shutoff valve 53. The control device 60 is configured to control the return of natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shutoff valve 53, and is also configured to control the sending of LNG from an LNG pressure transfer device 2 to an LNG vaporizer 31 by intermittently opening and closing the LNG discharge valve 33.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a natural gas supply system and an operation method for restarting the operation of a natural gas supply system. [Background technology]

[0002] Prior art related to natural gas supply systems is described, for example, in Patent Documents 1 and 2. Patent Document 1 describes a natural gas supply system including an LNG (Liquefied Natural Gas) receiving tank, an LNG delivery tank, and an LNG vaporizer. Although not described in Patent Document 1, a buffer tank may be disposed downstream of the LNG vaporizer, as described in Patent Document 2, for example.

[0003] Patent Document 2 describes an LNG storage facility that includes an LNG tank, a pump, a vaporizer, a BOG (Boil Off Gas) compressor, and a buffer tank. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-128061 [Patent Document 2] Japanese Patent Application Publication No. 2018-112218 Summary of the Invention [Problem to be solved by the invention]

[0005] The above-described conventional technology has the following problems. For example, during DSS (Daily Start and Stop) operation or after a long-term inspection of the system (equipment), the operation of a natural gas supply system (LNG storage facility) that has been shut down may be restarted. In this case, before the natural gas supply system (LNG storage facility) starts supplying natural gas, the LNG vaporizer is in a heated state with hot water flowing as a heat source. Therefore, immediately after the natural gas supply system (LNG storage facility) starts supplying natural gas, the LNG vaporizer is rapidly cooled by the inflow of LNG, which causes a sudden thermal stress to be applied to the LNG vaporizer. This may shorten the life of the LNG vaporizer.

[0006] Furthermore, when a natural gas supply system (LNG storage facility) supplies natural gas to gas-consuming equipment such as gas engine-powered power generation facilities, users require that fluctuations in the calorific value (heat value) of the natural gas be suppressed to ensure stable operation of the gas-consuming equipment. Here, the composition of the natural gas in the buffer tank remains constant during the shutdown of the natural gas supply system (LNG storage facility). However, in the LNG receiving tank (LNG tank), the light components in the LNG tend to decrease over time due to the release of boil-off gas (BOG). Therefore, when the operation of the natural gas supply system (LNG storage facility) is resumed after shutdown, there is a possibility that the composition of the LNG in the LNG receiving tank (LNG tank) will differ from that of the natural gas in the buffer tank. The LNG in the LNG receiving tank (LNG tank) may contain more heavy components than the natural gas in the buffer tank. This may result in the following problems: When the natural gas supply system resumes operation, the natural gas in the buffer tank is consumed, and the natural gas that has evaporated from the LNG in the LNG receiving tank (LNG tank) is mixed with the natural gas supplied to the gas consuming equipment. At this point, fluctuations in calorific value occur due to differences in composition. This can result in problems such as knocking in the gas engine, preventing the smooth operation of the user's gas consuming equipment.

[0007] An object of the present invention is to provide a natural gas supply system and an operating method for restarting operation of a natural gas supply system that can suppress sudden thermal stress on an LNG vaporizer and suppress fluctuations in the calorific value of the natural gas being supplied. [Means for solving the problem]

[0008] The natural gas supply system disclosed herein includes an LNG storage tank, an LNG vaporizer, an LNG pumping device, a buffer tank, a first LNG transfer pipe, a second LNG transfer pipe, an NG transfer pipe, an LNG discharge valve, an NG flow rate control valve, a vaporized gas return pipe, a shutoff valve, and a control device. The LNG storage tank stores LNG. The LNG vaporizer vaporizes LNG. The LNG pumping device pumps LNG from the LNG storage tank to the LNG vaporizer. The buffer tank stores natural gas vaporized by the LNG vaporizer. One end of the first LNG transfer pipe is connected to the LNG storage tank and the other end is connected to the LNG pumping device, and the first LNG transfer pipe sends LNG from the LNG storage tank to the LNG pumping device. The second LNG transfer pipe has one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, and sends LNG from the LNG pumping device to the LNG vaporizer. The NG transfer pipe has one end connected to the LNG vaporizer and the other end connected to the buffer tank, and sends natural gas from the LNG vaporizer to the buffer tank. The LNG discharge valve is provided on the second LNG transfer pipe and is opened when sending LNG to the LNG vaporizer. The NG flow rate control valve is provided on the NG transfer pipe and adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when sending LNG from the LNG pumping device to the LNG vaporizer. The vaporized gas return pipe returns natural gas in the buffer tank to the LNG storage tank. The shut-off valve is provided on the vaporized gas return pipe and is opened when returning natural gas to the LNG storage tank. The control device controls the opening and closing of the LNG discharge valve and the shutoff valve. The control device is configured to control the return of natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve, and is configured to control the supply of LNG from the LNG pumping device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve.

[0009] The above-described natural gas supply system has the following advantages. By intermittently opening and closing the LNG discharge valve, LNG can be introduced into the LNG vaporizer little by little. As a result, sudden thermal stress on the LNG vaporizer can be suppressed. Furthermore, by adjusting the flow rate of natural gas sent from the LNG vaporizer to the buffer tank using the NG flow control valve, the flow rate of LNG flowing into the LNG vaporizer can be adjusted. As a result, sudden thermal stress on the LNG vaporizer can be suppressed. Furthermore, by returning natural gas from the buffer tank to the LNG storage tank, the returned natural gas is re-liquefied in the LNG storage tank, thereby reducing the difference in composition between the LNG in the LNG storage tank and the natural gas in the buffer tank. As a result, fluctuations in the calorific value of the supplied natural gas can be suppressed. Although fluctuations in the calorific value of natural gas can be suppressed by temporarily releasing all of the natural gas in the buffer tank into the atmosphere (atmospheric release), this natural gas supply system does not require such an operation.

[0010] The natural gas supply system may further include a pressure gauge that detects the pressure of the buffer tank, and the control device may be configured as follows: when the control device determines that the pressure of the buffer tank is equivalent to the pressure of the LNG pumping device, the control device may close the LNG discharge valve and open the shutoff valve, and thereafter, when the control device determines that the pressure of the buffer tank has become lower than the pressure of the LNG pumping device by a predetermined value or more, the control device may open the LNG discharge valve and close the shutoff valve.

[0011] According to this configuration, the pressure in the buffer tank can be reduced so that it is lower than the pressure in the LNG pumping device by at least a predetermined value. This allows LNG to flow appropriately from the LNG pumping device to the buffer tank. In other words, it is possible to prevent an event in which the pressure in the buffer tank becomes equal to the pressure in the LNG pumping device, preventing LNG from flowing from the LNG pumping device to the buffer tank.

[0012] The natural gas supply system may further include a thermometer that detects the temperature of an inlet of the LNG vaporizer, and the control device may be configured as follows: When the control device determines that the temperature of the inlet of the LNG vaporizer is higher than a target temperature, the control device may be configured to open the LNG discharge valve and hold it for a first predetermined time, and thereafter close the LNG discharge valve and hold it for a second predetermined time, and open the NG flow rate control valve to a predetermined opening degree that is less than fully open.

[0013] This configuration allows LNG to flow into the LNG vaporizer little by little, thereby preventing the LNG vaporizer from being subjected to sudden thermal stress.

[0014] The vaporized gas return pipe may be connected to the second LNG transfer pipe or the NG transfer pipe.

[0015] This configuration makes it easy to install the vaporized gas return pipe in an existing system.

[0016] The vaporized gas return pipe may be connected to the first LNG transfer pipe.

[0017] This configuration makes it easy to install the vaporized gas return pipe in an existing system.

[0018] The LNG pressure transfer device may also include an LNG delivery tank that receives and stores LNG from the LNG storage tank, a pressurized evaporator that increases the pressure of the gas phase in the LNG delivery tank, and a depressurization pipe that releases pressure in the LNG delivery tank.

[0019] According to this configuration, compared to when a pump is used as the LNG pressure-feeding device, the generation of BOG can be suppressed, and therefore the loss of LNG can be reduced.

[0020] The natural gas supply system may further include an LNG tank pressure gauge that detects the pressure of the LNG tank, and an LNG tank pressure reducing valve that reduces the pressure of the LNG tank, and the control device may be configured as follows: When the pressure of the LNG tank detected by the LNG tank pressure gauge becomes equal to or greater than a first pressure threshold when the natural gas in the buffer tank is returned to the LNG tank, the control device closes the shutoff valve to stop the return and opens the LNG tank pressure reducing valve to reduce the pressure of the LNG tank. Thereafter, when the pressure of the LNG tank detected by the LNG tank pressure gauge becomes equal to or less than a second pressure threshold, the control device opens the shutoff valve to resume the return.

[0021] According to this configuration, even if the pressure in the LNG tank increases during the return (for example, when the natural gas cannot be reliquefied), it is possible to prevent the pressure in the LNG tank from increasing too much (for example, exceeding the allowable pressure). Furthermore, according to this configuration, after control to stop the return is performed, if the pressure in the LNG tank detected by the LNG tank pressure gauge becomes equal to or lower than a second pressure threshold, the return can be resumed.

[0022] The present application also discloses an operating method for restarting a natural gas supply system. The natural gas supply system includes an LNG storage tank, an LNG vaporizer, an LNG pumping device, a buffer tank, a first LNG transfer pipe, a second LNG transfer pipe, an NG transfer pipe, an LNG discharge valve, an NG flow rate control valve, a vaporized gas return pipe, and a shutoff valve. The LNG storage tank stores LNG. The LNG vaporizer vaporizes LNG. The LNG pumping device pumps LNG from the LNG storage tank to the LNG vaporizer. The buffer tank stores natural gas vaporized by the LNG vaporizer. The first LNG transfer pipe has one end connected to the LNG storage tank and the other end connected to the LNG pumping device, and sends LNG from the LNG storage tank to the LNG pumping device. The second LNG transfer pipe has one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, and sends LNG from the LNG pumping device to the LNG vaporizer. The NG transfer pipe has one end connected to the LNG vaporizer and the other end connected to the buffer tank, and sends natural gas from the LNG vaporizer to the buffer tank. The LNG discharge valve is provided on the second LNG transfer pipe and is opened when sending LNG to the LNG vaporizer. The NG flow rate control valve is provided on the NG transfer pipe and adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when sending LNG from the LNG pumping device to the LNG vaporizer. The vaporized gas return pipe returns natural gas in the buffer tank to the LNG storage tank. The shut-off valve is provided on the vaporized gas return pipe and is opened when returning natural gas to the LNG storage tank. The method for restarting the natural gas supply system includes a vaporized gas return process and an LNG supply process. The vaporized gas return process returns natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve. The LNG supply process, after the vaporized gas return process, sends LNG from the LNG compression device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve.

[0023] The above-described operating method has the following advantages. By intermittently opening and closing the LNG discharge valve, LNG can be introduced into the LNG vaporizer little by little. As a result, sudden thermal stress on the LNG vaporizer can be suppressed. Furthermore, by adjusting the flow rate of natural gas sent from the LNG vaporizer to the buffer tank using the NG flow control valve, the flow rate of LNG flowing into the LNG vaporizer can be adjusted. As a result, sudden thermal stress on the LNG vaporizer can be suppressed. Furthermore, by returning natural gas from the buffer tank to the LNG storage tank, the returned natural gas is re-liquefied in the LNG storage tank, thereby reducing the difference in composition between the LNG in the LNG storage tank and the natural gas in the buffer tank. As a result, fluctuations in the calorific value of the supplied natural gas can be suppressed. Although fluctuations in the calorific value of natural gas can be suppressed by temporarily releasing all of the natural gas in the buffer tank into the atmosphere (atmospheric release), this operating method does not require such an operation. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a natural gas supply system and an operating method for restarting operation of a natural gas supply system that can suppress sudden thermal stress on an LNG vaporizer and suppress fluctuations in the calorific value of the natural gas being supplied. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a block diagram of a natural gas supply system according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a block diagram of a natural gas supply system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0027] (First embodiment) FIG. 1 is a diagram for explaining the natural gas supply system 101 of the first embodiment. As shown in FIG. 1, the natural gas supply system 101 includes an LNG storage tank 1, an LNG pumping device 2, an LNG vaporizer 31, and a buffer tank 41.

[0028] <LNG storage tank> The LNG storage tank 1 is a tank for storing LNG. The LNG storage tank 1 is supplied with LNG transported by, for example, a tank truck (not shown) from the tank truck. The LNG storage tank 1 has a heat insulation structure to suppress the evaporation of the stored LNG. The natural gas supply system 101 includes an LNG storage tank pressure reducing valve 1a and an LNG storage tank pressure gauge 1p.

[0029] The LNG storage tank pressure reducing valve 1a is a valve for reducing the pressure inside the LNG storage tank 1. When the LNG storage tank pressure reducing valve 1a is opened, the natural gas inside the LNG storage tank 1 is released to the outside of the LNG storage tank 1 (for example, the atmosphere), thereby reducing the pressure inside the LNG storage tank 1. The LNG storage tank pressure gauge 1p detects the pressure of the LNG storage tank 1. The "pressure of the LNG storage tank 1" detected by the LNG storage tank pressure gauge 1p may be the pressure of the natural gas or LNG inside the LNG storage tank 1, or the pressure of the natural gas or LNG in the pipe connected to the LNG storage tank 1. The LNG storage tank pressure gauge 1p may be provided on the LNG storage tank 1 or on the pipe connected to the LNG storage tank 1.

[0030] <LNG pumping device> The LNG pumping device 2 is a device for pumping the LNG in the LNG storage tank 1 to the LNG vaporizer 31. Specifically, the LNG pumping device 2 is a device for pumping the LNG in the LNG discharge tank 5 that receives the LNG in the LNG storage tank 1 to the LNG vaporizer 31 by the pressurized evaporator 6. The LNG pumping device 2 includes an LNG pumping device pressure gauge 2p, an LNG discharge tank 5, a pressurized evaporator 6, and a pressure reducing pipe 7.

[0031] The LNG pumping device pressure gauge 2p detects the pressure of the LNG pumping device 2. The "pressure of the LNG pumping device 2" detected by the LNG pumping device pressure gauge 2p may be, for example, the pressure of natural gas or LNG inside the LNG delivery tank 5, or the pressure of natural gas or LNG in a pipe connected to the LNG delivery tank 5. The LNG pumping device pressure gauge 2p may be provided in the LNG delivery tank 5, or may be provided in a pipe connected to the LNG delivery tank 5 (see FIG. 2).

[0032] The LNG delivery tank 5 is a tank that receives and stores LNG from the LNG storage tank 1. A plurality of LNG delivery tanks 5 are provided downstream of the LNG storage tank 1. Like the LNG storage tank 1, the LNG delivery tank 5 has an insulated structure to prevent evaporation of the stored LNG. The LNG delivery tank 5 is installed, for example, in a horizontal position. The horizontal position refers to a state in which the LNG delivery tank 5 is placed with its longitudinal direction (the axial direction of the LNG delivery tank 5) horizontal (or approximately horizontal). The LNG delivery tank 5 may be installed in a vertical position. The vertical position refers to a state in which the LNG delivery tank 5 is placed with its longitudinal direction vertical (or approximately vertical). The LNG delivery tank 5 may be installed in a position other than a horizontal position or a vertical position. The shape of the LNG delivery tank 5 may not have a longitudinal direction. The height of the LNG liquid level in the LNG delivery tank 5 is made lower than the height of the LNG liquid level in the LNG storage tank 1. Although two LNG delivery tanks 5 are illustrated in FIG. 1, the number of LNG delivery tanks 5 is not limited to two. The two LNG delivery tanks 5 are referred to as LNG delivery tank 5A and LNG delivery tank 5B, respectively. The tank capacity of each of the LNG delivery tanks 5A and 5B is smaller than the tank capacity of the LNG storage tank 1. The tank capacity of the LNG delivery tank 5A is equal to the tank capacity of the LNG delivery tank 5B. Note that the tank capacity of the LNG delivery tank 5A may be different from the tank capacity of the LNG delivery tank 5B.

[0033] The pressurized evaporator 6 is a heat exchanger that increases the pressure of the gas phase portion in the LNG delivery tank 5. The LNG in the LNG delivery tank 5 is evaporated by the pressurized evaporator 6, and the volume-expanded (vaporized) natural gas is returned to the LNG delivery tank 5, thereby increasing the pressure of the gas phase portion in the LNG delivery tank 5. The pressure of the gas phase portion in the LNG delivery tank 5 is controlled to, for example, 0.65 to 0.7 MPa. Due to the pressure of the gas phase portion in the LNG delivery tank 5, the LNG in the LNG delivery tank 5 is pressure-fed from the LNG delivery tank 5 to the LNG vaporizer 31.

[0034] The depressurization pipe 7 is a pipe that releases pressure from the LNG delivery tank 5. The depressurization pipe 7 is connected to the LNG delivery tank 5 and the LNG storage tank 1. For example, the depressurization pipe 7 has depressurization gas vent pipes 7a, 7b and a depressurization junction pipe 7c. One end of the depressurization gas vent pipes 7a, 7b is connected to the upper part (e.g., the top) of each of the LNG delivery tanks 5A, 5B. The other end of the depressurization gas vent pipes 7a, 7b is connected to the depressurization junction pipe 7c. The downstream end of the depressurization junction pipe 7c is connected to the LNG storage tank 1, for example, to the upper part of the LNG storage tank 1. The downstream end of the depressurization junction pipe 7c may be connected to the bottom of the LNG storage tank 1. Furthermore, the depressurization junction pipe 7c may be branched, and the downstream ends of the depressurization junction pipe 7c may be connected to the upper part of the LNG storage tank 1 and the bottom of the LNG storage tank 1, respectively. Furthermore, the depressurization pipe 7 may be open to the atmosphere. Furthermore, the depressurization pipe 7 does not need to include the depressurization junction pipe 7c. In this case, the depressurization gas vent pipes 7a, 7b may not join together, and each of the depressurization gas vent pipes 7a, 7b may be connected to the LNG tank 1 independently.

[0035] The depressurization gas vent pipes 7a, 7b are each provided with a depressurization valve 8. The depressurization valve 8 may be an automatic valve whose opening and closing can be controlled by a command from a control device 60 (described later), or a manual valve. The same applies to other valves (such as the supply valve 10 and first receiving valve 11 described later). The depressurization valve 8 may be a valve that can be switched between fully open and fully closed, or a valve whose opening can be adjusted in stages or continuously. The same applies to other valves. The depressurization valve 8 may be a valve that can be switched between fully open and fully closed, or a valve whose opening can be adjusted. When it is detected that the LNG in the LNG delivery tank 5 has run out or that the amount of LNG has fallen below a predetermined LNG remaining amount threshold (for example, the volume of LNG remaining relative to the tank capacity of the LNG delivery tank 5 is less than a few percent), the depressurization valve 8 is opened. This allows the natural gas in the LNG delivery tank 5 to be returned to the LNG storage tank 1. As a result, the pressure of the gas phase in the LNG delivery tank 5 and the pressure of the gas phase in the LNG storage tank 1 become equal (pressure equalization). Here, the height of the liquid level of LNG in the LNG delivery tank 5 is made lower than the height of the liquid level of LNG in the LNG storage tank 1 (there is a difference in height). Note that the above-mentioned "height of the liquid level of LNG in the LNG delivery tank 5" is the height of the bottom of the LNG delivery tank 5 when the LNG in the LNG delivery tank 5 has run out. Once the pressure equalization is achieved, it becomes possible to supply (dispense) the LNG stored in the LNG storage tank 1 under gravity flow by utilizing the difference in height of the liquid level.

[0036] The LNG storage tank 1 and the LNG delivery tank 5 that constitutes the LNG pressure transfer device 2 are connected by a first LNG transfer pipe 9. The first LNG transfer pipe 9 is a pipe that transfers LNG from the LNG storage tank 1 to the LNG delivery tank 5. The first LNG transfer pipe 9 has a main transfer pipe 9a whose upstream end is connected to the bottom of the LNG storage tank 1, and branch transfer pipes 9b and 9c whose downstream ends are connected to the bottoms of the LNG delivery tanks 5A and 5B, respectively.

[0037] A supply valve 10 is provided at a portion on the side of the LNG storage tank 1 of the transfer main pipe 9a. First receiving valves 11 (receiving valves for the discharging tank) are provided in the transfer branch pipes 9b and 9c, respectively. After the pressure in the gas phase part in the LNG discharging tank 5 and the pressure in the gas phase part in the LNG storage tank 1 are equalized, the supply valve 10 and the first receiving valve 11 are opened. Thereby, the LNG stored in the LNG storage tank 1 is supplied (discharged) by gravity flow into the LNG discharging tank 5 that is empty or substantially empty.

[0038] <LNG vaporizer> The LNG vaporizer 31 is a heat exchanger that vaporizes LNG. The LNG vaporizer 31 vaporizes LNG by bringing LNG into indirect contact with a heat source (for example, warm water). The warm water as the heat source may be heated water or normal temperature water.

[0039] The LNG discharging tank 5 and the LNG vaporizer 31 that constitute the LNG pumping device 2 are connected by a second LNG transfer pipe 32. The second LNG transfer pipe 32 is a pipe that sends LNG from the LNG discharging tank 5 to the LNG vaporizer 31. The second LNG transfer pipe 32 has LNG discharge pipes 32a and 32b whose upstream ends are connected to the bottoms of the LNG discharging tanks 5A and 5B, respectively, and an LNG confluence pipe 32c whose downstream end is connected to the LNG vaporizer 31.

[0040] An LNG discharge valve 33 is provided on each of the LNG discharge pipes 32a, 32b. A thermometer 34 and a second receiving valve 35 (vaporizer receiving valve) are provided in a portion of the LNG junction pipe 32c near the LNG vaporizer 31. The LNG discharge valve 33 is a valve that is opened when LNG is sent to the LNG vaporizer 31. The thermometer 34 is an instrument for detecting the temperature at the inlet of the LNG vaporizer 31. The thermometer 34 is provided upstream of the second receiving valve 35, but the thermometer 34 may also be provided downstream of the second receiving valve 35. As described above, the LNG in the LNG delivery tank 5 is pressure-fed from the LNG delivery tank 5 to the LNG vaporizer 31 by utilizing the pressure of the gas phase in the LNG delivery tank 5. When the LNG discharge valve 33 and the second receiving valve 35 are opened, the LNG in the LNG delivery tank 5 flows from the LNG delivery tank 5 into the LNG vaporizer 31. The LNG that has flowed in is heated by hot water in the LNG vaporizer 31 and vaporized into natural gas (NG).

[0041] <Buffer tank> The buffer tank 41 is a tank that stores the natural gas vaporized by the LNG vaporizer 31. The natural gas stored in the buffer tank 41 is supplied to user-side facilities (gas consuming facilities) such as power generation facilities using gas engines. Note that the gas consuming facilities to which the natural gas is supplied are not limited to power generation facilities using gas engines.

[0042] A pressure gauge 42 (buffer tank pressure gauge) for detecting the pressure of the buffer tank 41 is provided in the buffer tank 41. The pressure gauge 42 detects the pressure of the natural gas inside the buffer tank 41.

[0043] The LNG vaporizer 31 and the buffer tank 41 are connected by an NG transfer pipe 43. The NG transfer pipe 43 is a pipe that transfers natural gas from the LNG vaporizer 31 to the buffer tank 41. One end of the NG transfer pipe 43 is connected to the LNG vaporizer 31. The other end of the NG transfer pipe 43 is connected to the buffer tank 41.

[0044] An NG transfer pipe 43 is provided with an NG flow rate control valve 44. The NG flow rate control valve 44 is a valve that adjusts the flow rate of natural gas sent from the LNG vaporizer 31 to the buffer tank 41. The NG flow rate control valve 44 also adjusts the flow rate of LNG flowing into the LNG vaporizer 31. The flow rate is adjusted by the NG flow rate control valve 44 when LNG is sent from the LNG compression device 2 to the LNG vaporizer 31, and when natural gas is sent from the LNG vaporizer 31 to the buffer tank 41.

[0045] (Regarding fluctuations in the calorific value of natural gas) The natural gas supply system 101 is operated, for example, in a DSS (daily shutdown) mode. The natural gas supply system 101 may also undergo long-term inspections. During shutdown of the natural gas supply system 101, the composition of the natural gas in the buffer tank 41 does not (or almost does not) change. However, in the LNG storage tank 1, the light components in the LNG tend to decrease over time due to the release of boil-off gas (BOG). Therefore, when restarting the operation of the natural gas supply system 101 after shutdown, there is a possibility that the composition of the LNG in the LNG storage tank 1 may differ from the composition of the natural gas in the buffer tank 41. There is a possibility that the LNG in the LNG storage tank 1 may contain more heavier components than the natural gas in the buffer tank 41. Therefore, the following problem may occur: When the natural gas supply system 101 resumes operation, the natural gas in the buffer tank 41 is consumed, and the natural gas vaporized from the LNG in the LNG storage tank 1 joins the natural gas supplied to the gas consumption facility. At this time, the calorific value (heat value) of natural gas fluctuates due to differences in its components, which may result in the user's gas consumption equipment not operating smoothly.

[0046] Therefore, in order to suppress fluctuations in the calorific value of the natural gas that the natural gas supply system 101 supplies to the gas consumption equipment, the natural gas supply system 101 is provided with a vaporized gas return pipe 51 that returns the natural gas in the buffer tank 41 to the LNG storage tank 1. When the natural gas is returned from the buffer tank 41 to the LNG storage tank 1, the returned natural gas is re-liquefied in the LNG storage tank 1. Since the natural gas returned from the buffer tank 41 has a lower heavier content than the LNG in the LNG storage tank 1, when the natural gas returned from the buffer tank 41 is re-liquefied in the LNG storage tank 1, the proportion of heavy components in the LNG in the LNG storage tank 1 decreases. Therefore, the difference between the components of the LNG in the LNG storage tank 1 and the natural gas in the buffer tank 41 becomes smaller. Then, when the operation of the natural gas supply system 101 is resumed, the natural gas in the buffer tank 41 is consumed, and the natural gas obtained by vaporizing the LNG in the LNG storage tank 1 is combined with the natural gas to be supplied to the gas consumption equipment. At this time, the difference between the components of the LNG in the LNG storage tank 1 and the components of the natural gas in the buffer tank 41 is reduced. As a result, it is possible to suppress fluctuations in the calorific value of the natural gas to be supplied to the gas consumption equipment on the user side.

[0047] The vaporized gas return pipe 51 is provided with a shutoff valve 53 that is opened when returning natural gas to the LNG storage tank 1 .

[0048] One end and the other end of the vaporized gas return pipe 51 are connected so as to return natural gas from the buffer tank 41 to the LNG storage tank 1. For example, one end of the vaporized gas return pipe 51 may be connected to the second LNG transfer pipe 32. One end of the vaporized gas return pipe 51 may be connected to a midpoint of the second LNG transfer pipe 32. The "midpoint of the second LNG transfer pipe 32" is a portion of the second LNG transfer pipe 32 that is downstream of the LNG discharge valve 33 and upstream of the second receiving valve 35. In the example shown in FIG. 1 , one end of the vaporized gas return pipe 51 is connected to a midpoint of the LNG junction pipe 32c that constitutes the second LNG transfer pipe 32. The "midpoint of the LNG junction pipe 32c" is a portion of the LNG junction pipe 32c that is downstream of the LNG discharge pipes 32a and 32b and upstream of the second receiving valve 35. One end of the vaporized gas return pipe 51 may be connected to the LNG junction pipe 32c downstream of the second receiving valve 35, or may be connected to the LNG junction pipe 32c upstream of the second receiving valve 35. One end of the vaporized gas return pipe 51 may be connected to the LNG junction pipe 32c upstream of the thermometer 34, or may be connected to the LNG junction pipe 32c downstream of the thermometer 34. If one end of the vaporized gas return pipe 51 is connected to a portion of the second LNG transfer pipe 32 downstream of the LNG discharge valve 33, natural gas can be returned from the buffer tank 41 to the LNG storage tank 1. Therefore, one end of the vaporized gas return pipe 51 may be connected to the portion of the LNG discharge pipes 32a, 32b downstream of the LNG discharge valve 33. One end of the vaporized gas return pipe 51 may be connected to the LNG vaporizer 31. One end of the vaporized gas return pipe 51 may be connected to the NG transfer pipe 43 (see the vaporized gas return pipe 51 shown by the two-dot chain line in FIG. 1). One end of the vaporized gas return pipe 51 may be connected to the NG transfer pipe 43 upstream of the NG flow rate adjustment valve 44, or may be connected to the NG transfer pipe 43 downstream of the NG flow rate adjustment valve 44. One end of the vaporized gas return pipe 51 may be directly connected to the buffer tank 41. One end of the vaporized gas return pipe 51 may be connected to a pipe downstream of the buffer tank 41 (a pipe for transferring natural gas).

[0049] Here, when adding the vaporized gas return pipe 51 to an existing natural gas supply system, connecting one end of the vaporized gas return pipe 51 to the buffer tank 41 requires modifying or replacing the buffer tank 41, which is a pressure vessel. Also, connecting one end of the vaporized gas return pipe 51 to the LNG vaporizer 31 requires modifying or replacing the LNG vaporizer 31. On the other hand, if one end of the vaporized gas return pipe 51 is connected to a pipe for transferring LNG or natural gas, such as the second LNG transfer pipe 32 or the NG transfer pipe 43, there is no need to modify the buffer tank 41. Therefore, the vaporized gas return pipe 51 can be easily installed in an existing system.

[0050] For example, the other end of the vaporized gas return pipe 51 may be connected to the first LNG transfer pipe 9. The other end of the vaporized gas return pipe 51 may be connected to a midpoint of the first LNG transfer pipe 9. The "midpoint of the first LNG transfer pipe 9" is a portion of the first LNG transfer pipe 9 that is downstream of the supply valve 10 and upstream of the first receiving valve 11. In the example shown in FIG. 1 , the other end of the vaporized gas return pipe 51 is connected to a midpoint of the main transfer pipe 9a that constitutes the first LNG transfer pipe 9. The "midpoint of the main transfer pipe 9a" is a portion of the first LNG transfer pipe 9 that is downstream of the supply valve 10 and upstream of the branch transfer pipes 9b and 9c. If the other end of the vaporized gas return pipe 51 is connected to a portion of the first LNG transfer pipe 9 that is upstream of the first receiving valve 11, natural gas can be returned from the buffer tank 41 to the LNG storage tank 1. Therefore, the other end of the vaporized gas return pipe 51 may be connected to the portion of the transfer branch pipe 9b, 9c upstream of the first receiving valve 11. The other end of the vaporized gas return pipe 51 may be connected to the first LNG transfer pipe 9 upstream of the supply valve 10. The other end of the vaporized gas return pipe 51 may be directly connected to the LNG storage tank 1. In the example shown in FIG. 2, the other end of the vaporized gas return pipe 51 is connected to the bottom of the LNG storage tank 1. The other end of the vaporized gas return pipe 51 shown in FIG. 1 may be connected to a depressurization pipe 7 (more specifically, a depressurization pipe 7 that is not open to the atmosphere).

[0051] When adding a vaporized gas return pipe 51 to an existing natural gas supply system, connecting the other end of the vaporized gas return pipe 51 to the LNG storage tank 1 requires modifying the LNG storage tank 1, which is a pressure vessel, or replacing the LNG storage tank 1. If the other end of the vaporized gas return pipe 51 is connected to a pipe for transferring LNG or natural gas, for example, the first LNG transfer pipe 9 or the depressurization pipe 7, as in this embodiment, there is no need to modify the LNG storage tank 1. Therefore, the vaporized gas return pipe 51 can be easily installed in an existing system.

[0052] (Thermal stress in LNG vaporizers) On the other hand, when restarting the operation of the natural gas supply system 101 that has been stopped, the LNG vaporizer 31 is heated before starting the supply of natural gas to the gas consuming equipment (before opening the LNG discharge valve 33). At this time, the LNG vaporizer 31 is in a heated state in which hot water as a heat source is flowing through the LNG vaporizer 31. In other words, before starting the supply of natural gas (before opening the LNG discharge valve 33), hot water as a heat source is flowed through the LNG vaporizer 31, thereby preheating the LNG vaporizer 31. Therefore, immediately after the natural gas supply system 101 starts supplying natural gas to the gas consuming equipment, the LNG vaporizer 31 is rapidly cooled by the inflow of LNG into the LNG vaporizer 31. This rapid cooling applies a sudden thermal stress to the LNG vaporizer 31. Therefore, in the conventional technology, the life of the LNG vaporizer 31 may be shortened.

[0053] <Control device> Therefore, the natural gas supply system 101 is equipped with a control device 60 having a control configuration that can prevent sudden thermal stress from being applied to the LNG vaporizer 31 and can suppress fluctuations in the calorific value of the natural gas being supplied.

[0054] The control device 60 controls the valves. For example, the control device 60 controls the opening and closing of the LNG discharge valve 33 and the shutoff valve 53. The control device 60 is built into, for example, a control panel. For example, a signal (electrical signal of a detected value) from the LNG pumping device pressure gauge 2p is input to the control device 60. For example, a signal (electrical signal of a detected value) from the pressure gauge 42 is input to the control device 60. For example, a signal (electrical signal of a detected value) from the thermometer 34 is input to the control device 60. The control device 60 is configured to control the return of natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shutoff valve 53. The control device 60 is also configured to control the transfer of LNG from the LNG delivery tank 5 constituting the LNG pumping device 2 to the LNG vaporizer 31 by intermittently opening and closing the LNG discharge valve 33.

[0055] When restarting the operation of the natural gas supply system 101 that has been stopped, the control device 60 performs the following control as operation restart control. At this time, hot water as a heat source has been flowing in advance into the LNG vaporizer 31. As a result, the LNG vaporizer 31 is in a heated state.

[0056] The control device 60 controls each valve so as to return the natural gas in the buffer tank 41 from the buffer tank 41 to the LNG storage tank 1. A specific example of this control is as follows: The control device 60 opens the shutoff valve 53. At this time, the first receiving valve 11 and the LNG discharge valve 33 are closed. The control device 60 opens each of these valves, for example, in the order of the supply valve 10, the shutoff valve 53, the second receiving valve 35, and the NG flow control valve 44. By this valve control, the natural gas in the buffer tank 41 flows through the NG transfer pipe 43, the LNG vaporizer 31, a portion of the LNG junction pipe 32c, the vaporized gas return pipe 51, and a portion of the main transfer pipe 9a due to the pressure within the buffer tank 41, and then flows into the LNG storage tank 1. In other words, the natural gas in the buffer tank 41 is returned from the buffer tank 41 to the LNG storage tank 1. The pressure within the buffer tank 41 is, for example, 0.4 MPa to 0.65 MPa. The returned natural gas is re-liquefied in the LNG storage tank 1. The natural gas returned from the buffer tank 41 has a lower heavier content than the LNG in the LNG storage tank 1, so when the natural gas returned from the buffer tank 41 is re-liquefied in the LNG storage tank 1, the proportion of heavy components in the LNG in the LNG storage tank 1 decreases. Therefore, the difference between the components of the LNG in the LNG storage tank 1 and the natural gas in the buffer tank 41 becomes smaller.

[0057] The control device 60 is preferably configured to control the LNG tank pressure reducing valve 1a and the shutoff valve 53 while monitoring the pressure of the LNG tank 1 with the LNG tank pressure gauge 1p when the natural gas in the buffer tank 41 is returned to the LNG tank 1. Depending on the conditions (e.g., temperature) inside the LNG tank 1, the natural gas returned from the buffer tank 41 may not be re-liquefied in the LNG tank 1. In this case, the pressure inside the LNG tank 1 increases, and the pressure in the LNG tank 1 may exceed the allowable pressure (the allowable pressure of the LNG tank 1). Therefore, when the pressure in the LNG tank 1 detected by the LNG tank pressure gauge 1p exceeds a predetermined threshold (first pressure threshold), the control device 60 closes the shutoff valve 53. This stops the return of natural gas from the buffer tank 41 to the LNG tank 1. Furthermore, when the pressure of the LNG tank 1 detected by the LNG tank pressure gauge 1p becomes equal to or greater than a predetermined threshold (first pressure threshold), the control device 60 opens the LNG tank pressure reducing valve 1a. This reduces the pressure of the LNG tank 1. Specifically, when the LNG tank pressure reducing valve 1a opens, the natural gas inside the LNG tank 1 is discharged (e.g., released into the atmosphere) from the LNG tank 1, and the pressure of the LNG tank 1 reduces. The above-mentioned return is stopped, and the pressure of the LNG tank 1 reduces, thereby preventing the pressure of the LNG tank 1 from exceeding the allowable pressure. Thereafter (after the pressure of the LNG tank 1 is reduced), when the pressure of the LNG tank 1 detected by the LNG tank pressure gauge 1p becomes equal to or less than a predetermined threshold (second pressure threshold), the control device 60 opens the shutoff valve 53. This resumes the return of natural gas from the buffer tank 41 to the LNG tank 1. At this time, the control device 60 may open the shutoff valve 53 and close the LNG tank pressure reducing valve 1a. The first and second pressure thresholds are set in advance in the control device 60 (before determining the pressure of the LNG tank 1). The second pressure threshold is smaller than the first pressure threshold.

[0058] The LNG tank pressure reducing valve 1a, the supply valve 10, the shutoff valve 53, and the second receiving valve 35 are all automatic valves that can be controlled to open and close. The LNG tank pressure reducing valve 1a, the supply valve 10, and the second receiving valve 35 may be manual valves instead of automatic valves. In this case, the LNG tank pressure reducing valve 1a, the supply valve 10, and the second receiving valve 35 are opened and closed manually, and the shutoff valve 53 is opened and closed by commands from the control device 60.

[0059] Here, the control device 60 may be configured to open the shutoff valve 53 while monitoring the pressure of the buffer tank 41 as follows. When the control device 60 determines that the pressure of the buffer tank 41 is equivalent to the pressure of the LNG pumping device 2, it closes the LNG discharge valve 33 and opens the shutoff valve 53. Thereafter, when the control device 60 determines that the pressure of the buffer tank 41 has become lower than the pressure of the LNG pumping device 2 by a predetermined value or more, it opens the LNG discharge valve 33 and closes the shutoff valve 53. This control allows the pressure of the buffer tank 41 to be reduced so that the pressure of the buffer tank 41 is lower than the pressure of the LNG pumping device 2 by a predetermined value or more. Therefore, LNG can be appropriately flowed from the LNG pumping device 2 to the buffer tank 41. In other words, it is possible to prevent an event in which the pressure of the buffer tank 41 becomes equivalent to the pressure of the LNG pumping device 2, preventing LNG from flowing from the LNG pumping device 2 to the buffer tank 41.

[0060] A specific example of this control is as follows. The control device 60 determines (monitors) whether the pressure in the buffer tank 41 is equivalent to the pressure in the LNG pumping device 2. For example, the control device 60 determines whether the differential pressure between the pressure in the buffer tank 41 and the pressure in the LNG pumping device 2 is equal to or less than a first differential pressure threshold. This first differential pressure threshold is set in the control device 60 in advance (before the determination). The value of the first differential pressure threshold is, for example, 0.05 MPa. If the differential pressure is equal to or less than the first differential pressure threshold, the control device 60 determines that the pressure in the buffer tank 41 is "equivalent to" the pressure in the LNG pumping device 2. In this case, the control device 60 closes the LNG discharge valve 33 and opens the shutoff valve 53. Thereafter, the control device 60 determines (monitors) whether the pressure in the buffer tank 41 has become lower than the pressure in the LNG pumping device 2 by equal to or more than a second differential pressure threshold (predetermined value). This second differential pressure threshold is set in the control device 60 in advance (before the determination). The value of the second differential pressure threshold is, for example, 0.25 MPa. When the pressure in the buffer tank 41 is lower than the pressure in the LNG pumping device 2 and the differential pressure is equal to or greater than the second differential pressure threshold (predetermined value), the control device 60 determines that "the pressure in the buffer tank 41 has become lower than the pressure in the LNG pumping device 2 by equal to or greater than the predetermined value." In this case, the control device 60 opens the LNG discharge valve 33 and closes the shutoff valve 53.

[0061] When a predetermined time has elapsed since the shutoff valve 53 was opened (when a predetermined time has elapsed since the shutoff valve was opened), the control device 60 returns the shutoff valve 53 to a closed state. The control device 60 may adjust the time interval from when the shutoff valve 53 is opened to when it is closed (the above-mentioned "predetermined time") in accordance with the above-mentioned differential pressure. For example, the control device 60 may increase the time interval from when the shutoff valve 53 is opened to when it is closed as the above-mentioned differential pressure increases. When the control device 60 controls the time interval from when the shutoff valve 53 is opened to when it is closed, a valve whose opening degree cannot be adjusted (such as an automatic shutoff valve) may be used as the shutoff valve 53. Note that the control device 60 may adjust the opening degree of the shutoff valve 53 in accordance with the above-mentioned differential pressure. For example, the control device 60 may increase the opening degree of the shutoff valve 53 as the above-mentioned differential pressure increases.

[0062] Next, the control device 60 controls each valve to send the LNG in the LNG delivery tank 5 to the LNG vaporizer 31. A specific example of this control is as follows: The control device 60 intermittently opens and closes the LNG discharge valve 33. At this time, the second receiving valve 35 and the NG flow rate control valve 44 are open. This valve control causes the LNG in the LNG delivery tank 5 to flow through the second LNG transfer pipe 32 and into the LNG vaporizer 31 due to the pressure of the gas phase in the LNG delivery tank 5. When the LNG discharge valve 33 is intermittently opened and closed, LNG can be gradually introduced into the LNG vaporizer 31 compared to when the LNG discharge valve 33 is simply opened to its full open position (immediately, immediately) and then maintained in the open position. As a result, sudden thermal stress on the LNG vaporizer 31 can be suppressed.

[0063] The LNG discharge valve 33 and the NG flow rate adjusting valve 44 are automatic valves that can be controlled to open and close.

[0064] Here, the control device 60 may be configured to control the LNG discharge valve 33 intermittently while monitoring the temperature at the inlet of the LNG vaporizer 31 as follows: When the control device 60 determines that the temperature at the inlet of the LNG vaporizer 31 is higher than the target temperature, the control device 60 opens the LNG discharge valve 33 and holds it open for a first predetermined time. Thereafter, the control device 60 closes the LNG discharge valve 33 and holds it open for a second predetermined time, and opens the NG flow rate adjustment valve 44 to a predetermined opening degree that is less than fully open. This control allows LNG to flow into the LNG vaporizer 31 little by little, making it possible to prevent the LNG vaporizer 31 from being subjected to sudden thermal stress.

[0065] A specific example of this control is as follows: The control device 60 determines (monitors) whether the temperature at the inlet of the LNG vaporizer 31 is higher than a target temperature. This target temperature is set in advance (before making the determination) in the control device 60. The value of the target temperature is, for example, a specific value below −100°C. If the control device 60 determines that the temperature at the inlet of the LNG vaporizer 31 is higher than the target temperature, the control device 60 opens the LNG discharge valve 33 and maintains this state for a first predetermined time. This first predetermined time is set in advance in the control device 60. The value of the first predetermined time is, for example, 8 seconds. Thereafter (after the first predetermined time has elapsed since the control device 60 opened the LNG discharge valve 33), the control device 60 closes the LNG discharge valve 33 and maintains this state for a second predetermined time. This second predetermined time is set in advance in the control device 60. The value of the second predetermined time is, for example, 8 seconds. The first predetermined time and the second predetermined time may be equal to or different from each other. Furthermore, after a first predetermined time has elapsed since the LNG discharge valve 33 was opened, the control device 60 opens the NG flow rate adjustment valve 44 to a predetermined opening degree that is less than fully open. This predetermined opening degree is set in advance in the control device 60. The value of the predetermined opening degree is less than fully open (100%), such as 30%. The control device 60 maintains the NG flow rate adjustment valve 44 at the predetermined opening degree for a third predetermined time. This third predetermined time is set in advance in the control device 60. The third predetermined time may be equal to or approximately equal to the second predetermined time. After maintaining the NG flow rate adjustment valve 44 at the predetermined opening degree for the third predetermined time, the control device 60 may fully open the NG flow rate adjustment valve 44, or may maintain the NG flow rate adjustment valve 44 at an opening degree that is less than fully open and different from the above-mentioned "predetermined opening degree."

[0066] The control device 60 performs control to intermittently open and close the LNG discharge valve 33, and after a predetermined time has elapsed (a predetermined time has elapsed since this control was started), the control device 60 leaves the LNG discharge valve 33 in an open state.

[0067] <Manual operation> An operator may manually perform an operation equivalent to the restart control performed by the control device 60. In this case, each of the supply valve 10, the first receiving valve 11, the LNG discharge valve 33, the second receiving valve 35, the NG flow rate adjustment valve 44, and the shutoff valve 53 may be an automatic valve that can be controlled to open and close (however, an automatic valve with manual operation), or may be a manual valve.

[0068] A specific example of an operation by an operator when manually performing an operation equivalent to operation restart control is as follows: The operator returns natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shutoff valve 53 (vaporized gas return process). At this time, the first receiving valve 11 and the LNG discharge valve 33 are closed. The operator opens each of these valves, for example, in the order of supply valve 10, shutoff valve 53, second receiving valve 35, and NG flow rate control valve 44. By operating these valves, the natural gas in the buffer tank 41 flows, due to the pressure within the buffer tank 41, through the NG transfer pipe 43, the LNG vaporizer 31, a portion of the LNG junction pipe 32c, the vaporized gas return pipe 51, and a portion of the main transfer pipe 9a, and then into the LNG storage tank 1.

[0069] The worker opens the shutoff valve 53 and when a predetermined time has elapsed (when a predetermined time has elapsed since the shutoff valve 53 was opened), closes the shutoff valve 53 again.

[0070] Next, an operator intermittently opens and closes the LNG discharge valve 33 to send LNG from the LNG delivery tank 5 to the LNG vaporizer 31 (LNG supply process). The LNG in the LNG delivery tank 5 flows through the second LNG transfer pipe 32 and into the LNG vaporizer 31 due to the pressure of the gas phase inside the LNG delivery tank 5.

[0071] The worker performs the operation of intermittently opening and closing the LNG discharge valve 33, and after a predetermined time has elapsed (a predetermined time has elapsed since the start of this operation), the worker leaves the LNG discharge valve 33 in an open state.

[0072] Even if an operator manually performs an operation equivalent to the restart control performed by the control device 60, as in the above example, it is possible to prevent sudden thermal stress from being applied to the LNG vaporizer 31 and to suppress fluctuations in the calorific value of the natural gas being supplied.

[0073] (effect) The effects of the natural gas supply system 101 shown in FIG. 1 are as follows.

[0074] (Effects of the first invention) The natural gas supply system 101 includes an LNG storage tank 1, an LNG vaporizer 31, an LNG pumping device 2, a buffer tank 41, a first LNG transfer pipe 9, a second LNG transfer pipe 32, an NG transfer pipe 43, an LNG discharge valve 33, an NG flow rate control valve 44, a vaporized gas return pipe 51, a shutoff valve 53, and a control device 60. The LNG storage tank 1 stores LNG. The LNG vaporizer 31 vaporizes the LNG. The LNG pumping device 2 pumps the LNG in the LNG storage tank 1 to the LNG vaporizer 31. The buffer tank 41 stores the natural gas vaporized by the LNG vaporizer 31. The first LNG transfer pipe 9 has one end connected to the LNG storage tank 1 and the other end connected to the LNG pumping device 2, and sends LNG from the LNG storage tank 1 to the LNG pumping device 2. The second LNG transfer pipe 32 has one end connected to the LNG pumping device 2 and the other end connected to the LNG vaporizer 31, and transfers LNG from the LNG pumping device 2 to the LNG vaporizer 31. The NG transfer pipe 43 has one end connected to the LNG vaporizer 31 and the other end connected to the buffer tank 41, and transfers natural gas from the LNG vaporizer 31 to the buffer tank 41. The LNG discharge valve 33 is provided on the second LNG transfer pipe 32, and is opened when LNG is transferred to the LNG vaporizer 31. The NG flow rate adjustment valve 44 is provided on the NG transfer pipe 43, and adjusts the flow rate of natural gas transferred from the LNG vaporizer 31 to the buffer tank 41 when LNG is transferred from the LNG pumping device 2 to the LNG vaporizer 31. The vaporized gas return pipe 51 returns the natural gas in the buffer tank 41 to the LNG storage tank 1. The shutoff valve 53 is provided in the vaporized gas return pipe 51, and is opened when returning natural gas to the LNG storage tank 1. The control device 60 controls the opening and closing of the LNG discharge valve 33 and the opening and closing of the shutoff valve 53. The control device 60 is configured to control the return of natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shutoff valve 53. The control device 60 is also configured to control the sending of LNG from the LNG compression device 2 to the LNG vaporizer 31 by intermittently opening and closing the LNG discharge valve 33.

[0075] The above-described natural gas supply system 101 provides the following advantageous effects. By intermittently opening and closing the LNG discharge valve 33, LNG can be caused to flow little by little into the LNG vaporizer 31. As a result, sudden thermal stress on the LNG vaporizer 31 can be suppressed. Furthermore, by adjusting the flow rate of natural gas sent from the LNG vaporizer 31 to the buffer tank 41 using the NG flow control valve 44, the flow rate of LNG flowing into the LNG vaporizer 31 can be adjusted. As a result, sudden thermal stress on the LNG vaporizer 31 can be suppressed. Furthermore, by returning natural gas from the buffer tank 41 to the LNG storage tank 1, the returned natural gas is re-liquefied in the LNG storage tank 1, thereby reducing the difference in composition between the LNG in the LNG storage tank 1 and the natural gas in the buffer tank 41. As a result, fluctuations in the calorific value of the natural gas supplied by the natural gas supply system 101 can be suppressed. Fluctuations in the calorific value of the natural gas can be suppressed by temporarily releasing all of the natural gas in the buffer tank 41 into the atmosphere (atmospheric release), but with the natural gas supply system 101, such an operation is not necessary.

[0076] (Effects of the second invention) The natural gas supply system 101 may further include a pressure gauge 42 that detects the pressure of the buffer tank 41. The control device 60 may be configured as follows: When the control device 60 determines that the pressure of the buffer tank 41 is equivalent to the pressure of the LNG pumping device 2, it closes the LNG discharge valve 33 and opens the shutoff valve 53. When the control device 60 subsequently determines that the pressure of the buffer tank 41 has become lower than the pressure of the LNG pumping device 2 by a predetermined value or more, it opens the LNG discharge valve 33 and closes the shutoff valve 53.

[0077] According to this configuration, the pressure in the buffer tank 41 can be reduced so that the pressure in the buffer tank 41 is lower than the pressure in the LNG pumping device 2 by a predetermined value or more. Therefore, LNG can be appropriately flowed from the LNG pumping device 2 to the buffer tank 41. In other words, it is possible to prevent an event such as the pressure in the buffer tank 41 becoming equal to the pressure in the LNG pumping device 2, preventing LNG from flowing from the LNG pumping device 2 to the buffer tank 41.

[0078] (Effect of the third invention) The natural gas supply system 101 may further include a thermometer 34 that detects the temperature at the inlet of the LNG vaporizer 31. The control device 60 may be configured as follows: When the control device 60 determines that the temperature at the inlet of the LNG vaporizer 31 is higher than the target temperature, the control device 60 opens the LNG discharge valve 33 and holds it open for a first predetermined time. The control device 60 then closes the LNG discharge valve 33 and holds it open for a second predetermined time, and opens the NG flow rate adjustment valve 44 to a predetermined opening degree that is less than fully open.

[0079] According to this configuration, LNG can be caused to flow little by little into the LNG vaporizer 31. As a result, it is possible to prevent the LNG vaporizer 31 from being subjected to sudden thermal stress.

[0080] (Effect of the fourth invention) The vaporized gas return pipe 51 may be connected to the second LNG transfer pipe 32 or the NG transfer pipe 43 .

[0081] According to this configuration, the vaporized gas return pipe 51 can be easily installed in an existing system.

[0082] (Effect of the fifth invention) The vaporized gas return pipe 51 may be connected to the first LNG transfer pipe 9 .

[0083] According to this configuration, the vaporized gas return pipe 51 can be easily installed in an existing system.

[0084] (Effect of the sixth aspect of the invention) The LNG pressure transfer device 2 may include an LNG delivery tank 5 that receives and stores LNG from the LNG storage tank 1, a pressurized evaporator 6 that increases the pressure of the gas phase within the LNG delivery tank 5, and a depressurization pipe 7 that releases pressure within the LNG delivery tank 5.

[0085] According to this configuration, compared to when a pump is used as the LNG pressure-feeding device 2, the generation of BOG can be suppressed, and therefore the loss of LNG can be reduced.

[0086] (Effect of the seventh invention) The natural gas supply system 101 may further include an LNG tank pressure gauge 1p that detects the pressure of the LNG tank 1, and an LNG tank pressure reducing valve 1a that reduces the pressure of the LNG tank 1. The control device 60 may be configured to perform the following control. When the pressure of the LNG tank 1 detected by the LNG tank pressure gauge 1p becomes equal to or higher than a first pressure threshold when the natural gas in the buffer tank 41 is returned to the LNG tank 1, the control device 60 performs the following control. In this case, the control device 60 closes the shutoff valve 53 to stop the return, and opens the LNG tank pressure reducing valve 1a to reduce the pressure of the LNG tank 1. Thereafter, when the pressure of the LNG tank 1 detected by the LNG tank pressure gauge 1p becomes equal to or lower than a second pressure threshold, the control device 60 opens the shutoff valve 53 to resume the return.

[0087] According to this configuration, even if the pressure in the LNG tank 1 increases during the return (for example, when the natural gas cannot be reliquefied), it is possible to prevent the pressure in the LNG tank 1 from increasing too much (for example, exceeding the allowable pressure). Furthermore, according to this configuration, after control to stop the return is performed, if the pressure in the LNG tank 1 detected by the LNG tank pressure gauge 1p becomes equal to or lower than the second pressure threshold, the return can be resumed.

[0088] (Effect of the eighth aspect of the invention) The effects of the operation method when the operation of the natural gas supply system 101 is restarted are as follows.

[0089] The natural gas supply system includes an LNG storage tank 1, an LNG vaporizer 31, an LNG pumping device 2, a buffer tank 41, a first LNG transfer pipe 9, a second LNG transfer pipe 32, an NG transfer pipe 43, an LNG discharge valve 33, an NG flow rate control valve 44, a vaporized gas return pipe 51, and a shutoff valve 53. The LNG storage tank 1 stores LNG. The LNG vaporizer 31 vaporizes the LNG. The LNG pumping device 2 pumps the LNG in the LNG storage tank 1 to the LNG vaporizer 31. The buffer tank 41 stores the natural gas vaporized by the LNG vaporizer 31. The first LNG transfer pipe 9 has one end connected to the LNG storage tank 1 and the other end connected to the LNG pumping device 2, and sends LNG from the LNG storage tank 1 to the LNG pumping device 2. The second LNG transfer pipe 32 has one end connected to the LNG pumping device 2 and the other end connected to the LNG vaporizer 31, and transfers LNG from the LNG pumping device 2 to the LNG vaporizer 31. The NG transfer pipe 43 has one end connected to the LNG vaporizer 31 and the other end connected to the buffer tank 41, and transfers natural gas from the LNG vaporizer 31 to the buffer tank 41. The LNG discharge valve 33 is provided on the second LNG transfer pipe 32, and is opened when LNG is transferred to the LNG vaporizer 31. The NG flow rate adjustment valve 44 is provided on the NG transfer pipe 43, and adjusts the flow rate of natural gas transferred from the LNG vaporizer 31 to the buffer tank 41 when LNG is transferred from the LNG pumping device 2 to the LNG vaporizer 31. The vaporized gas return pipe 51 returns the natural gas in the buffer tank 41 to the LNG storage tank 1. The shutoff valve 53 is provided in the vaporized gas return pipe 51 and is opened when returning natural gas to the LNG storage tank 1. The operating method for restarting the natural gas supply system 101 includes a vaporized gas return process and an LNG supply process. In the vaporized gas return process, the shutoff valve 53 is opened to return natural gas from the buffer tank 41 to the LNG storage tank 1. In the LNG supply process, after the vaporized gas return process, the LNG discharge valve 33 is intermittently opened and closed to send LNG from the LNG compression device 2 to the LNG vaporizer 31.

[0090] The above-described method of operating the natural gas supply system 101 has the following advantages. By intermittently opening and closing the LNG discharge valve 33, LNG can be caused to flow little by little into the LNG vaporizer 31. As a result, sudden thermal stress on the LNG vaporizer 31 can be suppressed. Furthermore, by adjusting the flow rate of natural gas sent from the LNG vaporizer 31 to the buffer tank 41 using the NG flow control valve 44, the flow rate of LNG flowing into the LNG vaporizer 31 can be adjusted. As a result, sudden thermal stress on the LNG vaporizer 31 can be suppressed. Furthermore, by returning natural gas from the buffer tank 41 to the LNG storage tank 1, the returned natural gas is re-liquefied in the LNG storage tank 1, thereby reducing the difference in composition between the LNG in the LNG storage tank 1 and the natural gas in the buffer tank 41. As a result, fluctuations in the calorific value of the natural gas supplied by the natural gas supply system 101 can be suppressed. Fluctuations in the calorific value of the natural gas can be suppressed by temporarily releasing all of the natural gas in the buffer tank 41 into the atmosphere (atmospheric release), but this operating method does not require such an operation.

[0091] (Second embodiment) 2 is a diagram illustrating a natural gas supply system 201 according to the second embodiment. In the components of the natural gas supply system 201 according to the second embodiment, the same reference numerals are used for components that correspond to the components of the natural gas supply system 101 according to the first embodiment. Regarding the natural gas supply system 201, differences from the natural gas supply system 101 according to the first embodiment will be mainly described, and descriptions of commonalities may be omitted.

[0092] A natural gas supply system 101 of the first embodiment shown in Fig. 1 uses an LNG pressure-feeding device 2 of a pressure-feeding type using an LNG discharge tank 5. In contrast, a natural gas supply system 201 of the second embodiment shown in Fig. 2 uses an LNG pump 202 as the LNG pressure-feeding device. That is, the natural gas supply system 201 of the second embodiment uses an LNG pressure-feeding device of a pump pressure-feeding type. Note that, although one LNG pump 202 is shown in Fig. 2, the number of LNG pumps 202 is not limited to one.

[0093] The LNG storage tank 1 and the LNG pump 202 are connected by a first LNG transfer pipe 9. The first LNG transfer pipe 9 is a pipe that transfers LNG from the LNG storage tank 1 to the LNG pump 202.

[0094] The first LNG transfer pipe 9 is provided with a supply valve 10 .

[0095] The LNG pump 202 and the LNG vaporizer 31 are connected by a second LNG transfer pipe 32. The second LNG transfer pipe 32 is a pipe that transfers LNG from the LNG pump 202 to the LNG vaporizer 31.

[0096] The second LNG transfer pipe 32 is provided with, in this order from the LNG pump 202 side, an LNG discharge valve 33, a thermometer 34, and a second receiving valve 35 (a receiving valve for a vaporizer).

[0097] The LNG in the LNG storage tank 1 is pumped from the LNG storage tank 1 to the LNG vaporizer 31 by the LNG pump 202. Here, the LNG pump 202 has a problem in that it will break down due to heat generation if a predetermined amount of LNG is not constantly flowing through the LNG pump 202 during operation. For this reason, an LNG return pipe 255 is provided to allow the minimum amount of LNG necessary for cooling to flow through the LNG pump 202 when the amount of natural gas required by the user (the amount supplied to the gas consumption facility) is small. The LNG return pipe 255 is a pipe for returning LNG from the LNG pump 202 to the LNG storage tank 1. One end of the LNG return pipe 255 is connected to the second LNG transfer pipe 32 upstream of the LNG discharge valve 33. One end of the LNG return pipe 255 may be connected to the LNG pump 202. The other end of the LNG return pipe 255 is connected to the LNG storage tank 1 (e.g., an upper part of the LNG storage tank 1).

[0098] The natural gas supply system 201 includes a vaporized gas return pipe 51 that returns the natural gas in the buffer tank 41 to the LNG storage tank 1, similar to the natural gas supply system 101 of the first embodiment (see FIG. 1).

[0099] <Control device> The control device 60 is configured to be able to control the driving (starting, stopping, etc.) of the LNG pump 202. When restarting the operation of the natural gas supply system 201 that has been stopped, the control device 60 performs the following control as operation restart control. Below, a specific example of the operation restart control will be described, focusing mainly on the differences from the specific example of the operation restart control in the first embodiment.

[0100] The control device 60 controls each valve so as to return the natural gas in the buffer tank 41 from the buffer tank 41 to the LNG storage tank 1. Specifically, for example, the control device 60 opens the shutoff valve 53. At this time, the LNG discharge valve 33 is closed. The control device 60 opens each of these valves, for example, in the order of the shutoff valve 53, the second receiving valve 35, and the NG flow rate control valve 44. By this valve control, the natural gas in the buffer tank 41 flows through the NG transfer pipe 43, the LNG vaporizer 31, a portion of the second LNG transfer pipe 32, and the vaporized gas return pipe 51 due to the pressure within the buffer tank 41, and then into the LNG storage tank 1. As a result, the difference between the components of the LNG in the LNG storage tank 1 and the natural gas in the buffer tank 41 becomes smaller (similar to the first embodiment).

[0101] The control device 60 may be configured to control the shutoff valve 53 to open while monitoring the pressure in the buffer tank 41 (similar to the first embodiment).

[0102] When a predetermined time has elapsed since the shutoff valve 53 was opened (when a predetermined time has elapsed since the shutoff valve 53 was opened), the control device 60 returns the shutoff valve 53 to a closed state.

[0103] Next, the control device 60 controls the LNG pump 202 and each valve so as to send the LNG in the LNG storage tank 1 to the LNG vaporizer 31. A specific example of this control is as follows: The control device 60 opens the supply valve 10, starts the LNG pump 202, and intermittently opens and closes the LNG discharge valve 33. At this time, the second receiving valve 35 and the NG flow rate control valve 44 are open. With this control, the LNG in the LNG storage tank 1 flows through the second LNG transfer pipe 32 and into the LNG vaporizer 31 as a result of the operation of the LNG pump 202. As in the first embodiment, the intermittent opening and closing of the LNG discharge valve 33 can prevent the LNG vaporizer 31 from being subjected to sudden thermal stress.

[0104] Alternatively, the supply valve 10 may be opened in advance (before natural gas is supplied from the natural gas supply system 201 to the gas consumption facility) and the LNG pump 202 may be started to circulate the LNG in the LNG storage tank 1 through the LNG return pipe 255. In this case, the control device 60 simply intermittently opens and closes the LNG discharge valve 33, causing the LNG in the LNG storage tank 1 to flow through the second LNG transfer pipe 32 and into the LNG vaporizer 31.

[0105] The control device 60 may be configured to control the LNG discharge valve 33 to open and close intermittently while monitoring the temperature at the inlet of the LNG vaporizer 31 (similar to the first embodiment).

[0106] The control device 60 performs control to intermittently open and close the LNG discharge valve 33, and after a predetermined time has elapsed (a predetermined time has elapsed since this control was started), the control device 60 leaves the LNG discharge valve 33 in an open state.

[0107] <Manual operation> An operator may manually perform an operation equivalent to the restart control performed by the control device 60. In this case, each of the supply valve 10, the LNG discharge valve 33, the second receiving valve 35, the NG flow rate adjustment valve 44, and the shutoff valve 53 may be an automatic valve that can be controlled to open and close (however, an automatic valve with manual operation), or may be a manual valve.

[0108] A specific example of an operation by an operator when manually performing an operation equivalent to operation restart control is as follows: The operator returns natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shutoff valve 53 (vaporized gas return process). At this time, the LNG discharge valve 33 is closed. The operator opens each of these valves, for example, in the order of the shutoff valve 53, the second receiving valve 35, and the NG flow rate adjustment valve 44. By operating these valves, the natural gas in the buffer tank 41 flows through the NG transfer pipe 43, the LNG vaporizer 31, a portion of the second LNG transfer pipe 32, and the vaporized gas return pipe 51 due to the pressure within the buffer tank 41, and then into the LNG storage tank 1.

[0109] The worker opens the shutoff valve 53 and when a predetermined time has elapsed (when a predetermined time has elapsed since the shutoff valve 53 was opened), closes the shutoff valve 53 again.

[0110] Next, the worker opens the supply valve 10 and starts the LNG pump 202. Then, the worker intermittently opens and closes the LNG discharge valve 33 to send the LNG in the LNG storage tank 1 from the LNG pump 202 to the LNG vaporizer 31 (LNG supply process).

[0111] Note that the supply valve 10 may be opened in advance (before natural gas is supplied from the natural gas supply system 201 to the gas consumption facility) and the LNG pump 202 may be started to circulate the LNG in the LNG storage tank 1 through the LNG return pipe 255. In this case, the LNG in the LNG storage tank 1 flows through the second LNG transfer pipe 32 and into the LNG vaporizer 31 simply by the LNG discharge valve 33 being intermittently opened and closed by an operator.

[0112] The worker performs the operation of intermittently opening and closing the LNG discharge valve 33, and after a predetermined time has elapsed (a predetermined time has elapsed since the start of this operation), the worker leaves the LNG discharge valve 33 in an open state.

[0113] Even if an operator manually performs an operation equivalent to the restart control performed by the control device 60, as in the above example, it is possible to prevent sudden thermal stress from being applied to the LNG vaporizer 31 and to suppress fluctuations in the calorific value of the natural gas being supplied.

[0114] The present invention is not limited to the above-described embodiments. The configurations of the above-described embodiments may be combined as appropriate, or various modifications may be made to the above-described embodiments. For example, the components (including modified examples) of the first and second embodiments may be combined. For example, the number of components of the above-described embodiments may be changed, or some of the components may not be provided. For example, modified examples of the above-described embodiments may be combined in various ways. For example, the connections between the devices may be modified in various ways. For example, the number of pipes, whether or not the pipes branch, and whether or not the pipes merge may be modified in various ways. For example, what has been described as multiple different components or parts may be combined into a single component or part. Specifically, for example, pipes, valves, sensors, etc. may be built into devices such as a tank and a vaporizer. For example, what has been described as a single component or part (such as the control device 60) may be provided as multiple different components or parts. For example, various parameters (such as threshold values) may be preset in the control device 60, or may be manually set directly by an operator. The various parameters may be calculated by the control device 60 based on information manually set by an operator, or may be calculated by the control device 60 based on detected information, etc. For example, the various parameters may be fixed values, may be changed manually, or may be automatically changed by the control device 60 in response to certain conditions. The order of control by the control device 60 may be changed in various ways within the scope of the invention. For example, each component may have only a part of its features (functions, operations, etc.). [Explanation of symbols]

[0115] 1:LNG storage tank 1a: LNG storage tank pressure reducing valve 1p: LNG storage tank pressure gauge 2: LNG pumping equipment 5: LNG discharge tank 6: Pressurized evaporator 7: Depressurization pipe 9: 1st LNG transfer pipe 31: LNG vaporizer 32:Second LNG transfer pipe 33: LNG discharge valve 34: Thermometer 41: Buffer tank 42: Pressure gauge 43:NG transfer pipe 44:NG flow control valve 51: Vaporized gas return pipe 53:Shut-off valve 60: Control device 101, 201: Natural gas supply system 202: LNG pump (LNG pressure transfer device)

Claims

1. an LNG storage tank for storing LNG; an LNG vaporizer that vaporizes LNG; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a pressure gauge for detecting the pressure of the buffer tank; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe that returns the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; a control device that controls the opening and closing of the LNG discharge valve and the opening and closing of the shutoff valve; Equipped with the control device is configured to control the return of natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve, and to control the delivery of LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve, The control device is configured to close the LNG discharge valve and open the shutoff valve when it determines that the pressure in the buffer tank is equivalent to the pressure of the LNG pumping device, and thereafter open the LNG discharge valve and close the shutoff valve when it determines that the pressure in the buffer tank has become lower than the pressure of the LNG pumping device by a predetermined value or more. Natural gas supply system.

2. an LNG storage tank for storing LNG; an LNG vaporizer that vaporizes LNG; a thermometer for detecting a temperature at an inlet of the LNG vaporizer; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe that returns the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; a control device that controls the opening and closing of the LNG discharge valve and the opening and closing of the shutoff valve; Equipped with the control device is configured to control the return of natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve, and to control the delivery of LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve, The control device is configured to control, when it is determined that the temperature at the inlet portion of the LNG vaporizer is higher than a target temperature, to open the LNG discharge valve and maintain the open state for a first predetermined time, and thereafter close the LNG discharge valve and maintain the open state for a second predetermined time. Natural gas supply system.

3. an LNG storage tank for storing LNG; an LNG vaporizer that vaporizes LNG; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe connected to the first LNG transfer pipe and returning the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; a control device that controls the opening and closing of the LNG discharge valve and the opening and closing of the shutoff valve; Equipped with The control device is configured to control the return of natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve, and is configured to control the delivery of LNG from the LNG pressure delivery device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve. Natural gas supply system.

4. an LNG storage tank for storing LNG; an LNG storage tank pressure gauge that detects the pressure of the LNG storage tank; an LNG storage tank pressure reducing valve that reduces the pressure of the LNG storage tank; an LNG vaporizer that vaporizes LNG; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe that returns the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; a control device that controls the opening and closing of the LNG discharge valve and the opening and closing of the shutoff valve; Equipped with the control device is configured to control the return of natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve, and to control the delivery of LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve, The control device When the natural gas in the buffer tank is returned to the LNG storage tank, if the pressure of the LNG storage tank detected by the LNG storage tank pressure gauge becomes equal to or greater than a first pressure threshold, the shut-off valve is closed to stop the return, and the LNG storage tank pressure reducing valve is opened to reduce the pressure of the LNG storage tank; Thereafter, when the pressure of the LNG storage tank detected by the LNG storage tank pressure gauge becomes equal to or lower than a second pressure threshold, the shutoff valve is opened to resume the return. The control is configured as follows: Natural gas supply system.

5. The natural gas supply system according to any one of claims 1 to 4, The vaporized gas return pipe is connected to the second LNG transfer pipe or the NG transfer pipe. Natural gas supply system.

6. The natural gas supply system according to any one of claims 1 to 4, The LNG pumping device is an LNG delivery tank that receives and stores LNG from the LNG storage tank; a pressure evaporator that increases the pressure of a gas phase portion in the LNG discharge tank; a depressurization pipe for releasing pressure in the LNG discharge tank; A natural gas supply system comprising:

7. an LNG storage tank for storing LNG; an LNG vaporizer that vaporizes LNG; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a pressure gauge for detecting the pressure of the buffer tank; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe that returns the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; An operation method for restarting an operation of a natural gas supply system, comprising: a vaporized gas returning step of returning natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve; an LNG supply step of supplying LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve after the vaporized gas returning step; a shutoff valve opening / closing step of closing the LNG discharge valve and opening the shutoff valve when the pressure of the buffer tank is equal to the pressure of the LNG pumping device, and then opening the LNG discharge valve and closing the shutoff valve when the pressure of the buffer tank becomes lower than the pressure of the LNG pumping device by a predetermined value or more; An operation method for restarting operation of a natural gas supply system, comprising:

8. an LNG storage tank for storing LNG; an LNG vaporizer that vaporizes LNG; a thermometer for detecting a temperature at an inlet of the LNG vaporizer; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe that returns the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; An operation method for restarting an operation of a natural gas supply system, comprising: a vaporized gas returning step of returning natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve; an LNG supply step of supplying LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve after the vaporized gas returning step; an LNG discharge valve opening / closing step of opening the LNG discharge valve and maintaining it for a first predetermined time when the temperature at the inlet of the LNG vaporizer is higher than a target temperature, and then closing the LNG discharge valve and maintaining it for a second predetermined time; An operation method for restarting operation of a natural gas supply system, comprising:

9. an LNG storage tank for storing LNG; an LNG vaporizer that vaporizes LNG; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe connected to the first LNG transfer pipe and returning the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; An operation method for restarting an operation of a natural gas supply system, comprising: a vaporized gas returning step of returning natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve; an LNG supply step of supplying LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve after the vaporized gas returning step; An operation method for restarting operation of a natural gas supply system, comprising:

10. an LNG storage tank for storing LNG; an LNG storage tank pressure gauge that detects the pressure of the LNG storage tank; an LNG storage tank pressure reducing valve that reduces the pressure of the LNG storage tank; an LNG vaporizer that vaporizes LNG; an LNG pressure transfer device that pressure transfers the LNG in the LNG storage tank to the LNG vaporizer; a buffer tank for storing the natural gas vaporized by the LNG vaporizer; a first LNG transfer pipe having one end connected to the LNG storage tank and the other end connected to the LNG pressure-feeding device, the first LNG transfer pipe transporting LNG from the LNG storage tank to the LNG pressure-feeding device; a second LNG transfer pipe having one end connected to the LNG pressure-feeding device and the other end connected to the LNG vaporizer, the second LNG transfer pipe transmitting LNG from the LNG pressure-feeding device to the LNG vaporizer; an NG transfer pipe having one end connected to the LNG vaporizer and the other end connected to the buffer tank, the NG transfer pipe transmitting natural gas from the LNG vaporizer to the buffer tank; an LNG discharge valve that is provided in the second LNG transfer pipe and that is opened when sending LNG to the LNG vaporizer; an NG flow rate adjustment valve provided in the NG transfer pipe, which adjusts the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when LNG is sent from the LNG pressure-feeding device to the LNG vaporizer; a vaporized gas return pipe that returns the natural gas in the buffer tank to the LNG storage tank; a shutoff valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; An operation method for restarting an operation of a natural gas supply system, comprising: a vaporized gas returning step of returning natural gas from the buffer tank to the LNG storage tank by opening the shutoff valve; an LNG supply step of supplying LNG from the LNG pressure-feeding device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve after the vaporized gas returning step; a return stop step of closing the shutoff valve to stop the return and opening the LNG storage tank pressure reduction valve to reduce the pressure in the LNG storage tank when the pressure in the LNG storage tank detected by the LNG storage tank pressure gauge becomes equal to or higher than a first pressure threshold when the natural gas in the buffer tank is returned to the LNG storage tank; a return restart step of opening the shutoff valve and restarting the return when the pressure of the LNG storage tank detected by the LNG storage tank pressure gauge becomes equal to or lower than a second pressure threshold after the return stop step; An operation method for restarting operation of a natural gas supply system, comprising:

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