Natural gas supply system, and operation method during operation resumption of natural gas supply system

Abstract

To provide a natural gas supply system capable of preventing abrupt heat stress from being applied 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: an LNG vaporizer 31 that vaporizes LNG; a buffer tank 41 that stores natural gas vaporized by the LNG vaporizer; an LNG discharge valve 33 that is opened when sending LNG to the LNG vaporizer 31; a shutoff valve 53 that is opened when returning the natural gas to an LNG storage tank 1; and a control device 60. The control device 60 is configured to change a pressure in the buffer tank 41 by opening and closing the LNG discharge valve 33 and the shutoff valve 53 according to the pressure in the buffer tank 41.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] As a prior art related to a natural gas supply system, there are, for example, those described 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 discharging tank, and an LNG vaporizer. Although not described in Patent Document 1, as described in, for example, Patent Document 2, a buffer tank may be arranged on the downstream side of the LNG vaporizer.

[0003] Patent Document 2 describes an LNG storage facility including 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

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] The above prior art has the following problems. For example, in DSS (Daily Start and Stop) operation, or after a long-term inspection of the system (facility), the operation of the natural gas supply system (LNG storage facility) that has been stopped 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 where warm water as the heat source is flowing through it. 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, and as a result, a rapid thermal stress is applied to the LNG vaporizer. Therefore, the life of the LNG vaporizer may be impaired.

[0006] Also, when the natural gas supply system (LNG storage facility) supplies natural gas to gas-consuming facilities such as power generation facilities using gas engines, in order to achieve stable operation of the gas-consuming facilities, it is required by users to suppress fluctuations in the calorific value (heating value) of natural gas. Here, during the stop period of the natural gas supply system (LNG storage facility), the components of the natural gas in the buffer tank do not change. However, in the LNG receiving tank (LNG tank), due to the release of BOG (Boil Off Gas), the light components in the LNG tend to decrease over time. Therefore, when restarting the operation of the natural gas supply system (LNG storage facility) that has been stopped, there may be a difference between the components of the LNG in the LNG receiving tank (LNG tank) and the components of the natural gas in the buffer tank. The LNG in the LNG receiving tank (LNG tank) may have more heavy components than the natural gas in the buffer tank. Therefore, the following problems may occur. When the natural gas supply system resumes operation, the natural gas in the buffer tank is consumed, and the natural gas vaporized from the LNG in the LNG receiving tank (LNG tank) merges into the natural gas supplied to the gas-consuming facilities. At this timing, heat quantity fluctuations due to the component difference occur. As a result, problems such as knocking in the gas engine may occur, and it may become impossible to smoothly operate the user's gas-consuming facilities.

[0007] An object of the present invention is to provide a natural gas supply system capable of suppressing the application of abrupt thermal stress to an LNG vaporizer and suppressing fluctuations in the calorific value of the supplied natural gas, and an operation method at the time of restarting the operation of the natural gas supply system.

Means for Solving the Problems

[0008] The natural gas supply system disclosed in the present application 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 control valve, a vaporized gas return pipe, a shut-off valve, and a control device. The LNG storage tank stores LNG. The LNG vaporizer vaporizes LNG. The LNG pumping device pumps the LNG in the LNG storage tank to the LNG vaporizer. The buffer tank stores the 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. One end of the second LNG transfer pipe is connected to the LNG pumping device and the other end is connected to the LNG vaporizer, and the second LNG transfer pipe sends LNG from the LNG pumping device to the LNG vaporizer. One end of the NG transfer pipe is connected to the LNG vaporizer and the other end is connected to the buffer tank, and the NG transfer pipe sends natural gas from the LNG vaporizer to the buffer tank. The LNG discharge valve is provided in the second LNG transfer pipe and is opened when sending LNG to the LNG vaporizer. The NG flow control valve is provided in the NG transfer pipe and adjusts the flow rate of the 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 the natural gas in the buffer tank to the LNG storage tank. The shut-off valve is provided in 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 opening and closing of the shut-off valve. The control device is configured to control to return natural gas from the buffer tank to the LNG storage tank by opening the shut-off valve, and is configured to control to send LNG from the LNG pumping device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve.

[0009] The above natural gas supply system has the following effects. By intermittently opening and closing the LNG discharge valve, LNG can be made to flow into the LNG vaporizer little by little. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer. Also, by adjusting the flow rate of the natural gas sent from the LNG vaporizer to the buffer tank with the NG flow rate adjustment valve, the flow rate of the LNG flowing into the LNG vaporizer can be adjusted. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer. Further, by returning the natural gas from the buffer tank to the LNG storage tank, the returned natural gas is re-liquefied in the LNG storage tank, and the difference between the components of the LNG in the LNG storage tank and the components of the natural gas in the buffer tank can be reduced. As a result, fluctuations in the calorific value of the supplied natural gas can be suppressed. By once discharging all of the natural gas in the buffer tank to the atmosphere (venting to the atmosphere), fluctuations in the calorific value of the natural gas can be suppressed, but according to this natural gas supply system, such an operation is not necessary.

[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 to control as follows. When the control device determines that the pressure of the buffer tank is equal to the pressure of the LNG pumping device, the control device closes the LNG discharge valve and opens the shut-off valve, and then, when it 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 be configured to control to open the LNG discharge valve and close the shut-off valve.

[0011] According to this configuration, the pressure of the buffer tank can be reduced so that the pressure of the buffer tank becomes lower than the pressure of the LNG pumping device by a predetermined value or more. Therefore, LNG can be appropriately flowed from the LNG pumping device to the buffer tank. That is, it is possible to suppress an event in which the pressure of the buffer tank becomes equal to the pressure of the LNG pumping device and LNG cannot flow from the LNG pumping device to the buffer tank.

[0012] In addition, the natural gas supply system may further include a thermometer for detecting the temperature at the inlet of the LNG vaporizer, and the control device may be configured as follows. When the control device determines that the temperature at the inlet of the LNG vaporizer is higher than the target temperature, the control device may open the LNG discharge valve and hold it for a first predetermined time, and then close the LNG discharge valve and hold it for a second predetermined time, and control the NG flow rate adjustment valve to open to a predetermined opening less than full open.

[0013] According to this configuration, LNG can be made to flow into the LNG vaporizer little by little. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer.

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

[0015] According to this configuration, it is possible to easily install the vaporized gas return pipe to the existing system.

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

[0017] According to this configuration, it is possible to easily install the vaporized gas return pipe to the existing system.

[0018] In addition, the LNG pumping device may include an LNG discharge tank that receives and stores LNG from the LNG storage tank, a pressurizing evaporator that raises the pressure in the gas phase portion in the LNG discharge tank, and a pressure relief pipe that relieves the pressure in the LNG discharge tank.

[0019] According to this configuration, compared with the case of using a pump as the LNG pumping device, the generation of BOG can be suppressed, so the loss of LNG can be reduced.

[0020] In addition, the natural gas supply system further includes an LNG storage tank pressure gauge for detecting the pressure of the LNG storage tank and an LNG storage tank pressure reducing valve for reducing the pressure of the LNG storage tank, and the control device may be configured to control as follows. 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 is equal to or higher than a first pressure threshold value, the control device closes the shut-off valve to stop the return and opens the LNG storage tank pressure reducing valve to reduce the pressure of the LNG storage tank. After that, when the pressure of the LNG storage tank detected by the LNG storage tank pressure gauge is equal to or lower than a second pressure threshold value, the control device opens the shut-off valve to resume the return.

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

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

[0023] The above operation method has the following effects. By intermittently opening and closing the LNG discharge valve, LNG can be made to flow into the LNG vaporizer little by little. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer. Also, by adjusting the flow rate of the natural gas sent from the LNG vaporizer to the buffer tank with the NG flow rate adjustment valve, the flow rate of the LNG flowing into the LNG vaporizer can be adjusted. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer. Further, by returning the natural gas from the buffer tank to the LNG storage tank, the returned natural gas is re-liquefied in the LNG storage tank, and the difference between the components of the LNG in the LNG storage tank and the components of the natural gas in the buffer tank can be reduced. As a result, fluctuations in the calorific value of the supplied natural gas can be suppressed. Although it is possible to suppress fluctuations in the calorific value of natural gas by once discharging (venting to the atmosphere) all of the natural gas in the buffer tank, such an operation is not necessary according to the present operation method.

Effect of the Invention

[0024] According to the present invention, it is possible to provide a natural gas supply system and an operation method at the time of restarting the operation of the natural gas supply system, which can suppress the application of sudden thermal stress to the LNG vaporizer and can suppress fluctuations in the calorific value of the supplied natural gas.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0026] Hereinafter, modes for carrying out 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-insulating 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 has received the LNG in the LNG storage tank 1 to the LNG vaporizer 31 by the pressurizing evaporator 6. The LNG pumping device 2 includes an LNG pumping device pressure gauge 2p, an LNG discharge tank 5, a pressurizing 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 dispensing tank 5, or the pressure of natural gas or LNG in the pipeline connected to the LNG dispensing tank 5. The LNG pumping device pressure gauge 2p may be provided in the LNG dispensing tank 5 or may be provided in the pipeline connected to the LNG dispensing tank 5 (see Fig. 2).

[0032] The LNG dispensing tank 5 is a tank that receives and stores LNG from the LNG storage tank 1. A plurality of LNG dispensing tanks 5 are provided on the downstream side of the LNG storage tank 1. Similar to the LNG storage tank 1, the LNG dispensing tank 5 has a heat-insulating structure to suppress the evaporation of the stored LNG. The LNG dispensing tank 5 is installed, for example, in a horizontal position. The horizontal position means a state in which the longitudinal direction (axial direction of the LNG dispensing tank 5) of the LNG dispensing tank 5 is horizontal (or substantially horizontal) and the LNG dispensing tank 5 is placed. The LNG dispensing tank 5 may be installed in a vertical position. The vertical position means a state in which the longitudinal direction of the LNG dispensing tank 5 is vertical (or substantially vertical) and the LNG dispensing tank 5 is placed. Note that the LNG dispensing tank 5 may be installed in a state that is neither horizontal nor vertical. The shape of the LNG dispensing tank 5 may be a shape without a longitudinal direction. The height of the LNG liquid level in the LNG dispensing tank 5 is made lower than the height of the LNG liquid level in the LNG storage tank 1. Two LNG dispensing tanks 5 are shown in Fig. 1, but the number of LNG dispensing tanks 5 is not limited to two. Let the two LNG dispensing tanks 5 be the LNG dispensing tank 5A and the LNG dispensing tank 5B, respectively. The tank capacity of each of the LNG dispensing tanks 5A and 5B is smaller than the tank capacity of the LNG storage tank 1. The tank capacity of the LNG dispensing tank 5A is equal to the tank capacity of the LNG dispensing tank 5B. Note that the tank capacity of the LNG dispensing tank 5A and the tank capacity of the LNG dispensing tank 5B may be different.

[0033] The pressurizing evaporator 6 is a heat exchanger that increases the pressure in the gas phase portion of the LNG discharge tank 5. By evaporating the LNG in the LNG discharge tank 5 with the pressurizing evaporator 6 and returning the natural gas with an expanded (vaporized) volume to the LNG discharge tank 5, the pressure in the gas phase portion of the LNG discharge tank 5 is increased. The pressure in the gas phase portion of the LNG discharge tank 5 is controlled, for example, to 0.65 to 0.7 MPa. Due to the pressure in the gas phase portion of the LNG discharge tank 5, the LNG in the LNG discharge tank 5 is pumped to the LNG vaporizer 31 from the LNG discharge tank 5.

[0034] The pressure relief pipe 7 is a pipe that relieves the pressure in the LNG discharge tank 5. The pressure relief pipe 7 is connected to the LNG discharge tank 5 and the LNG storage tank 1. For example, the pressure relief pipe 7 has pressure relief gas extraction pipes 7a, 7b and a pressure relief confluence pipe 7c. One ends of the pressure relief gas extraction pipes 7a, 7b are connected to the upper parts (for example, the tops) of the LNG discharge tanks 5A, 5B respectively. The other ends of the pressure relief gas extraction pipes 7a, 7b are connected to the pressure relief confluence pipe 7c. The downstream end of the pressure relief confluence pipe 7c is connected to the LNG storage tank 1, for example, to the upper part of the LNG storage tank 1. Note that the downstream end of the pressure relief confluence pipe 7c may be connected to the bottom of the LNG storage tank 1. Also, the pressure relief confluence pipe 7c may be branched and the downstream end of the pressure relief confluence pipe 7c may be connected to each of the upper part and the bottom of the LNG storage tank 1. Also, the pressure relief pipe 7 may be open to the atmosphere. Also, the pressure relief pipe 7 may not include the pressure relief confluence pipe 7c. In this case, without the pressure relief gas extraction pipes 7a, 7b merging, each of the pressure relief gas extraction pipes 7a, 7b may be individually connected to the LNG storage tank 1.

[0035] The pressure relief gas extraction pipes 7a and 7b are each provided with a pressure relief valve 8. The pressure relief valve 8 may be an automatic valve capable of opening and closing control according to a command from a control device 60 (described later), or a manual valve. Whether it is an automatic valve or a manual valve, the same applies to other valves (such as the supply valve 10 and the first receiving valve 11 described later). The pressure relief valve 8 may be a valve that can be switched between fully open and fully closed, or a valve whose opening degree can be adjusted stepwise or continuously. Whether it is a valve that can be switched between fully open and fully closed or a valve whose opening degree can be adjusted, the same applies to other valves. When it is detected that the LNG in the LNG dispensing tank 5 has run out or the amount of LNG is less than a predetermined LNG remaining amount threshold (for example, the volume of LNG with respect to the tank capacity of the LNG dispensing tank 5 is less than a few percent remaining), the pressure relief valve 8 is opened. As a result, the natural gas in the LNG dispensing tank 5 is returned to the LNG storage tank 1. As a result, the pressure of the gas phase part in the LNG dispensing tank 5 becomes equal to the pressure of the gas phase part in the LNG storage tank 1 (equalized). Here, the height of the liquid level of the LNG in the LNG dispensing tank 5 is made lower than the height of the liquid level of the LNG in the LNG storage tank 1 (there is a height difference). Note that the "height of the liquid level of the LNG in the LNG dispensing tank 5" is the height of the bottom of the LNG dispensing tank 5 when the LNG in the LNG dispensing tank 5 has run out. When the above equalization of pressure is performed, by utilizing the above height difference of the liquid level, the LNG stored in the LNG storage tank 1 can be supplied (dispensed) by natural flow into the LNG dispensing tank 5 that is empty or almost empty.

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

[0037] A supply valve 10 is provided at a portion on one side of the LNG storage tank 1 in the transfer main pipe 9a. First receiving valves 11 (receiving valves for the dispensing tank) are provided in the transfer branch pipes 9b and 9c, respectively. After the pressure in the gas phase portion in the LNG dispensing tank 5 and the pressure in the gas phase portion 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 (dispensed) by gravity flow into the LNG dispensing tank 5 that is empty or almost 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 dispensing 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 dispensing 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 dispensing tanks 5A and 5B, respectively, and an LNG confluence pipe 32c whose downstream end is connected to the LNG vaporizer 31.

[0040] LNG discharge pipes 32a and 32b are each provided with an LNG discharge valve 33. A thermometer 34 and a second receiving valve 35 (vaporizer receiving valve) are provided in a portion of the LNG confluence pipe 32c near the LNG vaporizer 31. The LNG discharge valve 33 is a valve that is opened when sending LNG 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 be provided downstream of the second receiving valve 35. As described above, using the pressure in the gas phase portion in the LNG dispensing tank 5, the LNG in the LNG dispensing tank 5 is pressure-fed from the LNG dispensing tank 5 to the LNG vaporizer 31. When the LNG discharge valve 33 and the second receiving valve 35 are opened, the LNG in the LNG dispensing tank 5 flows from the LNG dispensing tank 5 into the LNG vaporizer 31. The flowing-in LNG is heated by warm water in the LNG vaporizer 31 and vaporizes into natural gas (NG; Natural Gas).

[0041] <Buffer Tank> The buffer tank 41 is a tank for storing the natural gas vaporized in the LNG vaporizer 31. The natural gas stored in the buffer tank 41 is supplied to equipment on the user side (gas consumption equipment) such as power generation equipment using a gas engine. Note that the gas consumption equipment, which is the destination of the natural gas supply, is not limited to power generation equipment using a gas engine.

[0042] A pressure gauge 42 (buffer tank pressure gauge) for detecting the pressure of the buffer tank 41 is provided on 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 for sending 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 adjustment valve 44. The NG flow rate adjustment 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 adjustment valve 44 is also a valve that adjusts the flow rate of LNG flowing into the LNG vaporizer 31. The adjustment of the flow rate by the NG flow rate adjustment valve 44 is performed when sending LNG from the LNG pumping device 2 to the LNG vaporizer 31, and is also performed when sending natural gas from the LNG vaporizer 31 to the buffer tank 41.

[0045] (Regarding the heat quantity fluctuation of natural gas) The natural gas supply system 101 is, for example, operated in DSS (daytime start-stop) operation. Also, the natural gas supply system 101 may be subject to long-term inspection. During the stop period of the natural gas supply system 101, the components of the natural gas in the buffer tank 41 do not change (or hardly change). However, in the LNG storage tank 1, due to the release of BOG (Boil Off Gas), the light components in the LNG tend to decrease over time. Therefore, when restarting the operation of the natural gas supply system 101 that has been stopped, there may be a 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. The LNG in the LNG storage tank 1 may have more heavy components than the natural gas in the buffer tank 41. Therefore, the following problems may occur. When the natural gas supply system 101 restarts 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 merges into the natural gas supplied to the gas consumption equipment. At this timing, a heat quantity (calorific value) fluctuation due to a component difference occurs in the natural gas. As a result, it may become impossible to smoothly operate the gas consumption equipment on the user side.

[0046] Therefore, in order to suppress the calorific value fluctuation of the natural gas supplied by the natural gas supply system 101 to the gas consumption equipment, the natural gas supply system 101 is provided with a vapor return pipe 51 for returning 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 reliquefied in the LNG storage tank 1. Since the natural gas returned from the buffer tank 41 has less heavy components than the LNG in the LNG storage tank 1, when the natural gas returned from the buffer tank 41 is reliquefied in the LNG storage tank 1, the proportion of the heavy components of 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 components of the natural gas in the buffer tank 41 becomes smaller. When the operation of the natural gas supply system 101 is restarted, the natural gas in the buffer tank 41 is consumed, and the natural gas vaporized from the LNG in the LNG storage tank 1 merges into the natural gas 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 has become smaller. As a result, the calorific value fluctuation of the natural gas supplied to the gas consumption equipment on the user side can be suppressed.

[0047] The vapor return pipe 51 is provided with a shut-off valve 53 that is opened when returning the 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 that natural gas can be returned 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 middle part of the second LNG transfer pipe 32. The above-mentioned "middle part of the second LNG transfer pipe 32" is a part 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 middle part of the LNG confluence pipe 32c that constitutes the second LNG transfer pipe 32. The above-mentioned "middle part of the LNG confluence pipe 32c" is a part of the LNG confluence 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 confluence pipe 32c downstream of the second receiving valve 35, or may be connected to the LNG confluence pipe 32c upstream of the second receiving valve 35. One end of the vaporized gas return pipe 51 may be connected to the LNG confluence pipe 32c upstream of the thermometer 34, or may be connected to the LNG confluence pipe 32c downstream of the thermometer 34. If one end of the vaporized gas return pipe 51 is connected to the part 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 parts of the LNG discharge pipes 32a and 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 (refer to the vaporized gas return pipe 51 shown by the dashed-dotted 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 (a pipe for transferring natural gas) downstream of the buffer tank 41.

[0049] Here, when adding the vaporized gas return pipe 51 to an existing natural gas supply system, in order to connect one end of the vaporized gas return pipe 51 to the buffer tank 41, it is necessary to modify or replace the buffer tank 41 which is a pressure vessel. Also, in order to connect one end of the vaporized gas return pipe 51 to the LNG vaporizer 31, modification or replacement of the LNG vaporizer 31 etc. is necessary. 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, for example, the second LNG transfer pipe 32 or the NG transfer pipe 43 etc., it is not necessary to perform modification of the buffer tank 41 etc. Therefore, the installation of the vaporized gas return pipe 51 to the existing system can be easily carried out.

[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 middle part of the first LNG transfer pipe 9. The above-mentioned "middle part of the first LNG transfer pipe 9" is a part 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 middle part of the main transfer pipe 9a that constitutes the first LNG transfer pipe 9. The above-mentioned "middle part of the main transfer pipe 9a" is a part of the first LNG transfer pipe 9 that is downstream of the supply valve 10 and upstream of the transfer branch pipes 9b, 9c. If the other end of the vaporized gas return pipe 51 is connected to a part 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 transfer branch pipes 9b, 9c part that is 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 that is 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 the pressure relief pipe 7 (more specifically, the pressure relief pipe 7 that is not open to the atmosphere).

[0051] When adding a vapor return pipe 51 to an existing natural gas supply system, in order to connect the other end of the vapor return pipe 51 to the LNG storage tank 1, it is necessary to modify the LNG storage tank 1 which is a pressure vessel, or replace the LNG storage tank 1. As in this embodiment, if the other end of the vapor return pipe 51 is connected to a pipe for transferring LNG or natural gas, for example, the first LNG transfer pipe 9 or the pressure relief pipe 7, it is not necessary to perform modifications such as modifying the LNG storage tank 1. Therefore, the installation of the vapor return pipe 51 to the existing system can be easily carried out.

[0052] (Regarding the thermal stress of the LNG vaporizer) On the other hand, when restarting the operation of the natural gas supply system 101 that has been stopped, before starting the supply of natural gas to the gas consumption facility (before opening the LNG discharge valve 33, in advance), the LNG vaporizer 31 is put into a heating state. At this time, the LNG vaporizer 31 is in a heating state where hot water as a heat source is flowing into 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 flowing into the LNG vaporizer 31, whereby the LNG vaporizer 31 is pre-heated. Therefore, immediately after the natural gas supply system 101 starts supplying natural gas to the gas consumption facility, the LNG vaporizer 31 is rapidly cooled by the inflow of LNG into the LNG vaporizer 31. This rapid cooling causes a sudden thermal stress on the LNG vaporizer 31. Therefore, in the prior art, there is a possibility of reducing the life of the LNG vaporizer 31.

[0053] <Control device> Therefore, the natural gas supply system 101 is provided with a control device 60 having a control configuration capable of suppressing the application of sudden thermal stress to the LNG vaporizer 31 and suppressing fluctuations in the calorific value of the supplied natural gas.

[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 opening and closing of the shut-off valve 53. The control device 60 is built into, for example, the control panel. Signals (electrical signals of the detected values) from, for example, the LNG pumping device pressure gauge 2p are input to the control device 60. Signals (electrical signals of the detected values) from, for example, the pressure gauge 42 are input to the control device 60. Signals (electrical signals of the detected values) from, for example, the thermometer 34 are 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 shut-off valve 53. Further, the control device 60 is configured to control the supply of LNG from the LNG discharge 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 restart control. At this time, hot water as a heat source is pre-flowed through the LNG vaporizer 31. Thereby, 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 shut-off valve 53. At this time, the first intake valve 11 and the LNG discharge valve 33 are in the closed state. The control device 60 opens these valves in the order of, for example, the supply valve 10, the shut-off valve 53, the second intake 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 part of the LNG confluence pipe 32c, the vaporized gas return pipe 51, and a part of the main transfer pipe 9a due to the pressure in the buffer tank 41 and flows into the LNG storage tank 1. That is, the natural gas in the buffer tank 41 is returned from the buffer tank 41 to the LNG storage tank 1. The pressure in the buffer tank 41 is, for example, 0.4 MPa to 0.65 MPa. The returned natural gas is reliquefied in the LNG storage tank 1. Since the natural gas returned from the buffer tank 41 has less heavy components than the LNG in the LNG storage tank 1, when the natural gas returned from the buffer tank 41 is reliquefied in the LNG storage tank 1, the proportion of heavy components of 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 components of the natural gas in the buffer tank 41 becomes smaller.

[0057] When the natural gas in the buffer tank 41 is returned to the LNG storage tank 1, the control device 60 is preferably configured to control the LNG storage tank pressure reducing valve 1a and the shut-off valve 53 while monitoring the pressure of the LNG storage tank 1 with the LNG storage tank pressure gauge 1p. Depending on the conditions (such as temperature) inside the LNG storage tank 1, the natural gas returned from the buffer tank 41 may not be re-liquefied inside the LNG storage tank 1. In this case, the pressure inside the LNG storage tank 1 rises, and there is a risk that the pressure of the LNG storage tank 1 exceeds the allowable pressure (the allowable pressure of the LNG storage tank 1). Therefore, when the pressure of the LNG storage tank 1 detected by the LNG storage tank pressure gauge 1p becomes equal to or higher than a predetermined threshold value (first pressure threshold value), the control device 60 closes the shut-off valve 53. As a result, the return of the natural gas from the buffer tank 41 to the LNG storage tank 1 stops. Also, when the pressure of the LNG storage tank 1 detected by the LNG storage tank pressure gauge 1p becomes equal to or higher than a predetermined threshold value (first pressure threshold value), the control device 60 opens the LNG storage tank pressure reducing valve 1a. As a result, the pressure of the LNG storage tank 1 drops. Specifically, when the LNG storage tank pressure reducing valve 1a opens, the natural gas inside the LNG storage tank 1 is discharged (for example, released to the atmosphere) from the LNG storage tank 1, and the pressure of the LNG storage tank 1 drops. By stopping the above return and lowering the pressure of the LNG storage tank 1, it is possible to prevent the pressure of the LNG storage tank 1 from exceeding the allowable pressure. After that (after the pressure of the LNG storage tank 1 has been reduced), when the pressure of the LNG storage tank 1 detected by the LNG storage tank pressure gauge 1p becomes equal to or lower than a predetermined threshold value (second pressure threshold value), the control device 60 opens the shut-off valve 53. As a result, the return of the natural gas from the buffer tank 41 to the LNG storage tank 1 is restarted. At this time, the control device 60 may close the LNG storage tank pressure reducing valve 1a while opening the shut-off valve 53. The above first pressure threshold value and second pressure threshold value are set in advance in the control device 60 (before the determination of the pressure of the LNG storage tank 1). The second pressure threshold value is smaller than the first pressure threshold value.

[0058] Note that the LNG storage tank pressure reducing valve 1a, supply valve 10, shut-off valve 53, and second receiving valve 35 are all automatic valves capable of opening and closing control. The LNG storage tank pressure reducing valve 1a, supply valve 10, and second receiving valve 35 may be replaced with manual valves. In this case, the LNG storage tank pressure reducing valve 1a, supply valve 10, and second receiving valve 35 are opened and closed manually, and the shut-off valve 53 is opened and closed by a command from the control device 60.

[0059] Here, the control device 60 may be configured to control as follows to open the shut-off valve 53 while monitoring the pressure of the buffer tank 41. When the control device 60 determines that the pressure of the buffer tank 41 is equal to the pressure of the LNG pumping device 2, it closes the LNG discharge valve 33 and opens the shut-off valve 53. After that, 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 shut-off valve 53. By this control, the pressure of the buffer tank 41 can be lowered so that the pressure of the buffer tank 41 becomes 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. That is, it is possible to suppress an event such that the pressure of the buffer tank 41 becomes equal to the pressure of the LNG pumping device 2 and LNG cannot flow 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 of the buffer tank 41 is equal to the pressure of the LNG pumping device 2. For example, the control device 60 determines whether the differential pressure between the pressure of the buffer tank 41 and the pressure of the LNG pumping device 2 is equal to or less than a first differential pressure threshold value. This first differential pressure threshold value is set in advance (before determination) in the control device 60. The value of the first differential pressure threshold value is, for example, 0.05 MPa or the like. When the above differential pressure is equal to or less than the first differential pressure threshold value, the control device 60 determines that the pressure of the buffer tank 41 is "equal" to the pressure of the LNG pumping device 2. In this case, the control device 60 closes the LNG discharge valve 33 and opens the shut-off valve 53. Thereafter, the control device 60 determines (monitors) whether the pressure of the buffer tank 41 has become lower than the pressure of the LNG pumping device 2 by a second differential pressure threshold value (predetermined value) or more. This second differential pressure threshold value is set in advance (before determination) in the control device 60. The value of the second differential pressure threshold value is, for example, 0.25 MPa or the like. When the pressure of the buffer tank 41 is lower than the pressure of the LNG pumping device 2 and the above differential pressure is equal to or more than the second differential pressure threshold value (predetermined value), 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". In this case, the control device 60 opens the LNG discharge valve 33 and closes the shut-off valve 53.

[0061] When a predetermined time has elapsed after the control device 60 opens the shut-off valve 53 (when a predetermined time has elapsed since it was opened), the control device 60 closes the shut-off valve 53 again. The control device 60 may adjust the time interval (the above "predetermined time") from the opening to the closing of the shut-off valve 53 according to the above differential pressure. For example, the control device 60 may increase the time interval from the opening to the closing of the shut-off valve 53 as the above differential pressure increases. When the control device 60 controls the time interval from the opening to the closing of the shut-off valve 53, a valve whose opening cannot be adjusted (such as an automatic shut-off valve) can be used as the shut-off valve 53. Note that the control device 60 may adjust the opening degree of the shut-off valve 53 according to the above differential pressure. For example, the control device 60 may increase the opening degree of the shut-off valve 53 as the above differential pressure increases.

[0062] Next, the control device 60 controls each valve so as to send the LNG in the LNG dispensing 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 adjustment valve 44 are in the open state. By this valve control, the LNG in the LNG dispensing tank 5 flows through the second LNG transfer pipe 32 due to the pressure in the gas phase part in the LNG dispensing tank 5 and flows into the LNG vaporizer 31. When the LNG discharge valve 33 intermittently opens and closes, compared with the case where the LNG discharge valve 33 is simply opened all at once (immediately, instantly) to the fully open state and maintained in the open state, the LNG can flow into the LNG vaporizer 31 little by little. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31.

[0063] Note that the LNG discharge valve 33 and the NG flow rate adjustment valve 44 are automatic valves capable of opening and closing control.

[0064] Here, the control device 60 may be controlled as follows so as to intermittently open and close the LNG discharge valve 33 while monitoring the temperature at the inlet of the LNG vaporizer 31. 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 for a first predetermined time. After that, the control device 60 closes the LNG discharge valve 33 and holds it for a second predetermined time, and at the same time, opens the NG flow rate adjustment valve 44 to a predetermined opening degree less than the fully open state. By this control, the LNG can flow into the LNG vaporizer 31 little by little, and it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31.

[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 the target temperature. This target temperature is set in advance (before the determination) in the control device 60. The value of the target temperature is, for example, a specific value such as -100 °C or lower. When the control device 60 determines that the temperature at the inlet of the LNG vaporizer 31 is higher than the target temperature, it opens the LNG discharge valve 33 and holds 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. After that (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 holds 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. Note that the first predetermined time and the second predetermined time may be equal to each other or different from each other. Also, after the first predetermined time has elapsed since the control device 60 opened the LNG discharge valve 33, the control device 60 opens the NG flow rate adjustment valve 44 and sets the opening degree of the NG flow rate adjustment valve 44 to a predetermined opening degree 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%), for example, 30%. The control device 60 holds the state where the opening degree of the NG flow rate adjustment valve 44 is 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 the second predetermined time or approximately equal to the second predetermined time. After the control device 60 holds the state where the opening degree of the NG flow rate adjustment valve 44 is the predetermined opening degree for the third predetermined time, the NG flow rate adjustment valve 44 may be fully opened or set to an opening degree less than fully open and different from the above-mentioned "predetermined opening degree".

[0066] When a predetermined time has elapsed after the control device 60 performs control to intermittently open and close the LNG discharge valve 33 (when a predetermined time has elapsed since the start of this control), the LNG discharge valve 33 is kept open.

[0067] <Manual operation> An operator may manually perform an operation corresponding to the restart operation control implemented 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 shut-off valve 53 may be an automatic valve capable of opening and closing control (however, an automatic valve with manual operation), or may be a manual valve.

[0068] Specific examples of the operator's operations when the operator manually performs an operation corresponding to the restart operation control are as follows. The operator returns natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shut-off valve 53 (vapor return process). At this time, the first receiving valve 11 and the LNG discharge valve 33 are in the closed state. The operator opens these valves in the order of the supply valve 10, the shut-off valve 53, the second receiving valve 35, and the NG flow rate adjustment valve 44, for example. By this valve operation, the natural gas in the buffer tank 41 flows through the NG transfer pipe 43, the LNG vaporizer 31, a part of the LNG confluence pipe 32c, the vapor return pipe 51, and a part of the transfer main pipe 9a due to the pressure in the buffer tank 41 and flows into the LNG storage tank 1.

[0069] When a predetermined time has elapsed after the operator opens the shut-off valve 53 (when a predetermined time has elapsed since it was opened), the operator closes the shut-off valve 53 again.

[0070] Next, the operator sends LNG from the LNG discharge tank 5 to the LNG vaporizer 31 by intermittently opening and closing the LNG discharge valve 33 (LNG supply process). The LNG in the LNG discharge tank 5 flows through the second LNG transfer pipe 32 due to the pressure in the gas phase part of the LNG discharge tank 5 and flows into the LNG vaporizer 31.

[0071] When a predetermined time has elapsed after the operator performs the operation of intermittently opening and closing the LNG discharge valve 33 (when a predetermined time has elapsed since this operation was started), the operator leaves the LNG discharge valve 33 in the open state.

[0072] Even if an operator manually performs an operation corresponding to the restart control implemented by the control device 60, for example, as in the above example, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31, and it is also possible to suppress fluctuations in the calorific value of the supplied natural gas.

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

[0074] (Effect 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 control valve 44, a vaporized gas return pipe 51, a shut-off valve 53, and a control device 60. The LNG storage tank 1 stores LNG. The LNG vaporizer 31 vaporizes 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. One end of the first LNG transfer pipe 9 is connected to the LNG storage tank 1 and the other end is connected to the LNG pumping device 2, and the first LNG transfer pipe 9 sends LNG from the LNG storage tank 1 to the LNG pumping device 2. One end of the second LNG transfer pipe 32 is connected to the LNG pumping device 2 and the other end is connected to the LNG vaporizer 31, and the second LNG transfer pipe 32 sends LNG from the LNG pumping device 2 to the LNG vaporizer 31. One end of the NG transfer pipe 43 is connected to the LNG vaporizer 31 and the other end is connected to the buffer tank 41, and the NG transfer pipe 43 sends 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 sending LNG to the LNG vaporizer 31. The NG flow control valve 44 is provided on the NG transfer pipe 43 and adjusts the flow rate of the natural gas sent from the LNG vaporizer 31 to the buffer tank 41 when sending LNG 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 shut-off valve 53 is provided on 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 shut-off valve 53. The control device 60 is configured to control to return natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shut-off valve 53. And the control device 60 is configured to control to send LNG from the LNG pumping 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 has the following operational effects. By intermittently opening and closing the LNG discharge valve 33, LNG can be made to flow into the LNG vaporizer 31 little by little. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31. Also, by adjusting the flow rate of the natural gas sent from the LNG vaporizer 31 to the buffer tank 41 with the NG flow rate adjustment valve 44, the flow rate of the LNG flowing into the LNG vaporizer 31 can be adjusted. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31. Further, by returning the 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, and 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 can be reduced. As a result, fluctuations in the calorific value of the natural gas supplied by the natural gas supply system 101 can be suppressed. Although fluctuations in the calorific value of the natural gas can be suppressed by once discharging (venting to the atmosphere) all of the natural gas in the buffer tank 41, according to this natural gas supply system 101, such an operation is not necessary.

[0076] (Effect 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 to control as follows. When the control device 60 determines that the pressure of the buffer tank 41 is equal to the pressure of the LNG pumping device 2, the control device 60 closes the LNG discharge valve 33 and opens the shut-off valve 53. After that, 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, the control device 60 opens the LNG discharge valve 33 and closes the shut-off valve 53.

[0077] According to this configuration, the pressure of the buffer tank 41 can be reduced so that the pressure of the buffer tank 41 is lower than a predetermined value or more than the pressure of the LNG pumping device 2. Therefore, LNG can be appropriately flowed from the LNG pumping device 2 to the buffer tank 41. That is, it is possible to suppress an event in which the pressure of the buffer tank 41 becomes equal to the pressure of the LNG pumping device 2 and LNG cannot flow 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 to control 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 LNG discharge valve 33 is opened and held for a first predetermined time. Thereafter, the control device 60 closes the LNG discharge valve 33 and holds it for a second predetermined time, and at the same time opens the NG flow rate adjustment valve 44 to a predetermined opening less than fully open.

[0079] According to this configuration, LNG can be made to flow into the LNG vaporizer 31 little by little. As a result, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31.

[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 installation of the vaporized gas return pipe 51 to the existing system can be easily performed.

[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 installation of the vaporized gas return pipe 51 to the existing system can be easily performed.

[0084] (Effect of the sixth invention) The LNG pumping device 2 may include an LNG discharge tank 5 that receives and stores LNG from the LNG storage tank 1, a pressurizing evaporator 6 that increases the pressure of the gas phase portion in the LNG discharge tank 5, and a pressure relief pipe 7 that relieves the pressure in the LNG discharge tank 5.

[0085] According to this configuration, compared with the case where a pump is used as the LNG pumping device 2, the generation of BOG can be suppressed, so the loss of LNG can be reduced.

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

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

[0088] (Effect of the eighth invention) The effect of the operation method at the restart of the operation of the natural gas supply system 101 is 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 control valve 44, a vaporized gas return pipe 51, and a shut-off valve 53. The LNG storage tank 1 stores LNG. The LNG vaporizer 31 vaporizes 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. One end of the first LNG transfer pipe 9 is connected to the LNG storage tank 1 and the other end is connected to the LNG pumping device 2, and the first LNG transfer pipe 9 sends LNG from the LNG storage tank 1 to the LNG pumping device 2. One end of the second LNG transfer pipe 32 is connected to the LNG pumping device 2 and the other end is connected to the LNG vaporizer 31, and the second LNG transfer pipe 32 sends LNG from the LNG pumping device 2 to the LNG vaporizer 31. One end of the NG transfer pipe 43 is connected to the LNG vaporizer 31 and the other end is connected to the buffer tank 41, and the NG transfer pipe 43 sends natural gas from the LNG vaporizer 31 to the buffer tank 41. The LNG discharge valve 33 is provided in the second LNG transfer pipe 32 and is opened when sending LNG to the LNG vaporizer 31. The NG flow control valve 44 is provided in the NG transfer pipe 43 and adjusts the flow rate of the natural gas sent from the LNG vaporizer 31 to the buffer tank 41 when sending LNG 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 shut-off 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 operation method during restart of the operation of the natural gas supply system 101 includes a vaporized gas return step and an LNG supply step. The vaporized gas return step returns natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shut-off valve 53. The LNG supply step, after performing the vaporized gas return step, sends LNG from the LNG pumping device 2 to the LNG vaporizer 31 by intermittently opening and closing the LNG discharge valve 33.

[0090] The operation method of the above natural gas supply system 101 has the following effects. By intermittently opening and closing the LNG discharge valve 33, LNG can be made to flow into the LNG vaporizer 31 little by little. As a result, it is possible to suppress the application of a sudden thermal stress to the LNG vaporizer 31. Also, by adjusting the flow rate of the natural gas sent from the LNG vaporizer 31 to the buffer tank 41 with the NG flow rate adjustment valve 44, the flow rate of the LNG flowing into the LNG vaporizer 31 can be adjusted. As a result, it is possible to suppress the application of a sudden thermal stress to the LNG vaporizer 31. Further, by returning the 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, and 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 can be reduced. As a result, fluctuations in the calorific value of the natural gas supplied by the natural gas supply system 101 can be suppressed. By once discharging (venting to the atmosphere) all of the natural gas in the buffer tank 41, fluctuations in the calorific value of the natural gas can be suppressed, but according to this operation method, such an operation is not necessary.

[0091] (Second Embodiment) FIG. 2 is a diagram for explaining the natural gas supply system 201 of the second embodiment. In each configuration of the natural gas supply system 201 of the second embodiment, those corresponding to the respective configurations of the natural gas supply system 101 of the first embodiment are given the same reference numerals. Regarding the natural gas supply system 201, mainly, the differences from the natural gas supply system 101 of the first embodiment will be described, and the description of the common points may be omitted.

[0092] In the natural gas supply system 101 of the first embodiment shown in FIG. 1, the LNG pumping device 2 of the pumping type by the LNG dispensing tank 5 is used. On the other hand, in the natural gas supply system 201 of the second embodiment shown in FIG. 2, an LNG pump 202 is used as the LNG pumping device. That is, in the natural gas supply system 201 of the second embodiment, an LNG pumping device of the pumping type by a pump is used. 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 for sending LNG from the LNG storage tank 1 to the LNG pump 202.

[0094] A supply valve 10 is provided in the first LNG transfer pipe 9.

[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 for sending LNG from the LNG pump 202 to the LNG vaporizer 31.

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

[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 that it malfunctions due to heat generation if a predetermined amount of LNG does not always flow into the LNG pump 202 during operation. Therefore, when the amount of natural gas required by the user side (the supply amount to the gas consumption facility) is small, an LNG return pipe 255 is provided to flow the minimum amount of LNG required for cooling into the LNG pump 202. 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 on the upstream side 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 (for example, the upper part of the LNG storage tank 1).

[0098] Similar to the natural gas supply system 101 (see FIG. 1) of the first embodiment, the natural gas supply system 201 includes a vaporized gas return pipe 51 for returning the natural gas in the buffer tank 41 to the LNG storage tank 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 restart control. Hereinafter, the specific example of the restart control will be mainly described with differences from the specific example of the 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 shut-off valve 53. At this time, the LNG discharge valve 33 is in a closed state. The control device 60 opens these valves in the order of, for example, the shut-off valve 53, the second intake valve 35, and the NG flow rate adjustment 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 part of the second LNG transfer pipe 32, and the vapor gas return pipe 51 and flows into the LNG storage tank 1 due to the pressure in the buffer tank 41. As a result, 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 becomes smaller (the same as in the first embodiment).

[0101] The control device 60 may be configured to control to open the shut-off valve 53 while monitoring the pressure of the buffer tank 41 (the same as in the first embodiment).

[0102] When a predetermined time has elapsed after the control device 60 opens the shut-off valve 53 (when a predetermined time has elapsed since it was opened), the control device 60 returns the shut-off valve 53 to the 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 adjustment valve 44 are in the open state. By this control, the LNG in the LNG storage tank 1 flows through the second LNG transfer pipe 32 due to the operation of the LNG pump 202 and flows into the LNG vaporizer 31. Similar to the first embodiment, by intermittently opening and closing the LNG discharge valve 33, it is possible to suppress the application of sudden thermal stress to the LNG vaporizer 31.

[0104] Also, prior to (before natural gas is supplied from the natural gas supply system 201 to the gas consumption facility), the supply valve 10 may be opened, the LNG pump 202 may be started, and the LNG in the LNG storage tank 1 may be circulated through the LNG return pipe 255. In this case, by the control device 60, only by intermittently opening and closing the LNG discharge valve 33, the LNG in the LNG storage tank 1 flows through the second LNG transfer pipe 32 and flows into the LNG vaporizer 31.

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

[0106] When a predetermined time has elapsed after the control device 60 performs the control of intermittently opening and closing the LNG discharge valve 33 (when a predetermined time has elapsed since the start of this control), the LNG discharge valve 33 is kept open.

[0107] <Manual operation> An operator may manually perform an operation corresponding to the restart operation 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 shut-off valve 53 may be an automatic valve capable of opening and closing control (however, an automatic valve with manual operation) or a manual valve.

[0108] Specific examples of the operator's operations when the operator manually performs an operation corresponding to restart control of operation are as follows. The operator returns natural gas from the buffer tank 41 to the LNG storage tank 1 by opening the shut-off valve 53 (vaporized gas return process). At this time, the LNG discharge valve 33 is in a closed state. The operator opens each of these valves in the order of, for example, the shut-off valve 53, the second receiving valve 35, and the NG flow rate adjustment valve 44. By this valve operation, the natural gas in the buffer tank 41 flows through the NG transfer pipe 43, the LNG vaporizer 31, a part of the second LNG transfer pipe 32, and the vaporized gas return pipe 51 due to the pressure in the buffer tank 41 and flows into the LNG storage tank 1.

[0109] When a predetermined time has elapsed after the operator opens the shut-off valve 53 (when a predetermined time has elapsed since it was opened), the operator closes the shut-off valve 53.

[0110] Next, the operator opens the supply valve 10 and starts the LNG pump 202. Then, the operator sends the LNG in the LNG storage tank 1 from the LNG pump 202 to the LNG vaporizer 31 by intermittently opening and closing the LNG discharge valve 33 (LNG supply process).

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

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

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

[0114] The present invention is not limited to the above-described embodiments. It is possible to appropriately combine the respective configurations of the above-described embodiments or to make various modifications to the above-described embodiments. For example, components (including modified examples) of the first embodiment and the second embodiment may be combined with each other. 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 with each other in various ways. For example, the connection between each device may be changed in various ways. For example, the number of pipes, the presence or absence of pipe branches, the presence or absence of pipe junctions, etc. may be changed in various ways. For example, what has been described as a plurality of different members or parts may be made into one member or part. Specifically, for example, pipes, valves, sensors, etc. may be built into devices such as tanks and vaporizers. For example, what has been described as one member or part (for example, the control device 60, etc.) may be divided into a plurality of different members or parts. For example, various parameters (such as threshold values) may be preset in the control device 60, or may be directly set by a manual operation of an operator. The various parameters may be calculated in the control device 60 based on information set by a manual operation of an operator, or may be calculated in the control device 60 based on detected information, etc. For example, the various parameters may be fixed values, may be changed by a manual operation, or may be automatically changed by the control device 60 according to some conditions. The order of control by the control device 60 may be changed in various ways within the scope where the invention is valid. For example, each component may have only a part of each feature (action function, operation, etc.).

Explanation of Reference Numerals

[0115] 1: LNG storage tank 1a: LNG storage tank pressure reducing valve 1p: LNG storage tank pressure gauge 2: LNG pumping device 5: LNG discharge tank 6: Pressure evaporator 7: Pressure relief pipe 9: First 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: Vapor return pipe 53: Shut-off valve 60: Control device 101, 201: Natural gas supply system 202: LNG pump (LNG pumping device)

Claims

1. An LNG storage tank for storing LNG; An LNG vaporizer for vaporizing LNG; An LNG pumping device for pumping 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device; A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping 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 for sending natural gas from the LNG vaporizer to the buffer tank; An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer; An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting the flow rate of the natural gas sent from the LNG vaporizer to the buffer tank when sending LNG from the LNG pumping device to the LNG vaporizer; A vaporized gas return pipe for returning the natural gas in the buffer tank to the LNG storage tank; A shut-off valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; A control device for controlling the opening and closing of the LNG discharge valve and the shut-off valve; Comprising; The control device is configured to control to return natural gas from the buffer tank to the LNG storage tank by opening the shut-off valve, and is configured to control to send LNG from the LNG pumping device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve; The control device is configured to control the opening and closing of the LNG discharge valve and the shut-off valve according to the pressure of the buffer tank so that the pressure of the buffer tank is not less than a predetermined value lower than the pressure of the LNG pumping device; A natural gas supply system.

2. An LNG storage tank for storing LNG; An LNG vaporizer for vaporizing LNG; A thermometer for detecting the temperature at the inlet of the LNG vaporizer; An LNG pumping device for pumping 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device, A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping 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 for sending natural gas from the LNG vaporizer to the buffer tank, An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer, An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting 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, A control device for controlling the opening and closing of the LNG discharge valve, Comprising, The control device is configured to control to send LNG from the LNG pumping device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve, The control device is configured to control to change the flow rate of LNG flowing into the LNG vaporizer by opening and closing the LNG discharge valve according to the temperature at the inlet of the LNG vaporizer, Natural gas supply system.

3. An LNG storage tank for storing LNG, An LNG vaporizer for vaporizing LNG, An LNG pumping device for pumping LNG in the LNG storage tank to the LNG vaporizer, A buffer tank for storing 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device, A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping 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 for sending natural gas from the LNG vaporizer to the buffer tank, An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer, An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting 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; A vaporized gas return pipe connected to the first LNG transfer pipe for returning the natural gas in the buffer tank to the LNG storage tank; A shut-off valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; A control device for controlling the opening and closing of the LNG discharge valve and the opening and closing of the shut-off valve; Comprising; The control device is configured to control to return natural gas from the buffer tank to the LNG storage tank by opening the shut-off valve; Natural gas supply system.

4. An LNG storage tank for storing LNG; An LNG storage tank pressure gauge for detecting the pressure of the LNG storage tank; An LNG storage tank pressure reducing valve for reducing the pressure of the LNG storage tank; An LNG vaporizer for vaporizing LNG; An LNG pumping device for pumping 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 with one end connected to the LNG storage tank and the other end connected to the LNG pumping device, for sending LNG from the LNG storage tank to the LNG pumping device; A second LNG transfer pipe with one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, for sending LNG from the LNG pumping device to the LNG vaporizer; An NG transfer pipe with one end connected to the LNG vaporizer and the other end connected to the buffer tank, for sending natural gas from the LNG vaporizer to the buffer tank; An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer; An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting 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; A vaporized gas return pipe for returning the natural gas in the buffer tank to the LNG storage tank; A shut-off valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank; A control device for controlling the opening and closing of the LNG discharge valve and the opening and closing of the shut-off valve; Comprising; The control device is configured to control to return natural gas from the buffer tank to the LNG storage tank by opening the shut-off 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 higher than a first pressure threshold value, 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. After that, 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 value, the shut-off valve is opened to resume the return. It is configured to control in such a manner. Natural gas supply system. [

5. ] An LNG storage tank for storing LNG, An LNG vaporizer for vaporizing LNG, An LNG pumping device for pumping LNG in the LNG storage tank to the LNG vaporizer, A buffer tank for storing 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device, A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping 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 for sending natural gas from the LNG vaporizer to the buffer tank, An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer, An NG flow rate adjustment valve provided in the NG transfer pipe and adjusting 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, A vaporized gas return pipe for returning natural gas in the buffer tank to the LNG storage tank, A shut-off valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank, It is an operation method at the time of resuming operation of a natural gas supply system, comprising: A vaporized gas return step of returning natural gas from the buffer tank to the LNG storage tank by opening the shut-off valve. After performing the vapor gas return step, an LNG supply step of sending LNG from the LNG pumping device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve; A shut-off valve opening / closing step of opening and closing the LNG discharge valve and the shut-off valve according to the pressure of the buffer tank so that the pressure of the buffer tank is lower than the pressure of the LNG pumping device by a predetermined value or more; A method of operating when restarting the operation of a natural gas supply system, comprising:

6. An LNG storage tank for storing LNG; An LNG vaporizer for vaporizing LNG; A thermometer for detecting the temperature at the inlet of the LNG vaporizer; An LNG pumping device for pumping 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device; A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping 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 for sending natural gas from the LNG vaporizer to the buffer tank; An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer; An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting the flow rate of the natural gas sent from the LNG vaporizer to the buffer tank when sending LNG from the LNG pumping device to the LNG vaporizer; A method of operating when restarting the operation of a natural gas supply system, comprising: An LNG supply step of sending LNG from the LNG pumping device to the LNG vaporizer by intermittently opening and closing the LNG discharge valve; An LNG discharge valve opening / closing step of changing the flow rate of the LNG flowing into the LNG vaporizer by opening and closing the LNG discharge valve according to the temperature at the inlet of the LNG vaporizer; A method of operating when restarting the operation of a natural gas supply system, comprising:

7. An LNG storage tank for storing LNG; An LNG vaporizer for vaporizing LNG; An LNG pumping device for pumping 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device, A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping 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 for sending natural gas from the LNG vaporizer to the buffer tank, An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer, An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting 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, A vaporized gas return pipe connected to the first LNG transfer pipe for returning the natural gas in the buffer tank to the LNG storage tank, A shut-off valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank, A method of operating at the restart of operation of a natural gas supply system, comprising: A vaporized gas return step of returning natural gas from the buffer tank to the LNG storage tank by opening the shut-off valve, A method of operating at the restart of operation of a natural gas supply system, comprising:

8. An LNG storage tank for storing LNG, An LNG storage tank pressure gauge for detecting the pressure of the LNG storage tank, An LNG storage tank pressure reducing valve for reducing the pressure of the LNG storage tank, An LNG vaporizer for vaporizing LNG, An LNG pumping device for pumping 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 pumping device, the first LNG transfer pipe for sending LNG from the LNG storage tank to the LNG pumping device, A second LNG transfer pipe having one end connected to the LNG pumping device and the other end connected to the LNG vaporizer, the second LNG transfer pipe for sending LNG from the LNG pumping device to the LNG vaporizer, An NG transfer pipe with one end connected to the LNG vaporizer and the other end connected to the buffer tank, which is an NG transfer pipe for sending natural gas from the LNG vaporizer to the buffer tank, An LNG discharge valve provided in the second LNG transfer pipe and opened when sending LNG to the LNG vaporizer, An NG flow rate adjustment valve provided in the NG transfer pipe for adjusting the flow rate of natural gas sent from the LNG vaporizer to the buffer tank when sending LNG from the LNG pressure feeding device to the LNG vaporizer, A vaporized gas return pipe for returning the natural gas in the buffer tank to the LNG storage tank, A shut-off valve provided in the vaporized gas return pipe and opened when returning natural gas to the LNG storage tank, A method of operating when restarting the operation of a natural gas supply system, comprising: A vaporized gas return step of returning natural gas from the buffer tank to the LNG storage tank by opening the shut-off valve, When the pressure of the LNG storage tank detected by the LNG storage tank pressure gauge becomes equal to or higher than a first pressure threshold value when the natural gas in the buffer tank is returned to the LNG storage tank, a return stop step of closing the shut-off valve to stop the return and opening the LNG storage tank pressure reducing valve to reduce the pressure of the LNG storage tank, After the return stop step, 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 value, a return restart step of opening the shut-off valve to restart the return, A method of operating when restarting the operation of a natural gas supply system, comprising the above steps.

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