BOG liquefaction recovery system of LNG storage tank and control method of BOG liquefaction recovery system

Abstract

The invention discloses an LNG storage tank BOG liquefaction recovery system and a control method thereof.The system comprises an LNG storage tank, an LNG buffer tank, an LNG pump and a refrigerating machine, the LNG pump is installed in the LNG buffer tank, when BOG in the LNG storage tank needs to be recovered, BOG conveyed by the LNG storage tank is cooled and liquefied through the refrigerating machine and then returns to the LNG storage tank, recovery of BOG gas in the LNG storage tank is achieved, and in the BOG recovery process, the BOG is recovered through the refrigerating machine. And when the supercooling degree of the LNG storage tank needs to be maintained, the LNG conveyed by the LNG storage tank is cooled through the refrigerating machine, the cooled LNG returns into the LNG storage tank, and after the cooled LNG is mixed with the LNG in the LNG storage tank, the temperature of the LNG in the LNG storage tank is reduced, so that the supercooling degree of the LNG storage tank is maintained. The whole system is simple in structure, BOG in the LNG storage tank can be liquefied and recycled, and the supercooling degree of the LNG storage tank can be maintained.

Description

Technical Field

[0001] The present application relates to the technical field of BOG recovery from LNG storage tanks, and in particular to a BOG liquefaction recovery system for LNG storage tanks and a control method thereof. Background Art

[0002] At present, LNG (Liquefied Natural Gas) stations (including LNG filling stations, L-CNG filling stations, LNG gasification stations, and LNG reserve stations) are all equipped with LNG storage tanks as storage devices. The temperature of LNG is extremely low, reaching as low as -162°C. During the storage process, LNG will continuously absorb heat from the environment and evaporate BOG (Boil Off Gas) which is stored in the LNG storage tank. At the same time, the BOG generated by the LNG absorbing heat in other equipment and pipelines of the LNG station will eventually return to the LNG storage tank. The pressure in the LNG storage tank will gradually increase with the increase of BOG. ​​Once the maximum working pressure of the LNG storage tank is reached, emissions will occur if there is no BOG recovery equipment.

[0003] When LNG is stored in a closed insulated container, it often exists as a boiling liquid and is in a temperature-pressure equilibrium state. The pressure in the equilibrium state is called the saturation pressure corresponding to the temperature. The relationship curve between the temperature and saturation pressure of the LNG storage tank is as follows: Figure 1 shown.

[0004] pass Figure 1 It can be seen that when the pressure of the LNG storage tank is lower than the saturation pressure, LNG will accelerate gasification until a new equilibrium is formed. When the pressure of the LNG storage tank is higher than the saturation pressure, the evaporation of LNG is suppressed and the liquefaction of BOG is intensified until a new equilibrium is formed. We call the part of the LNG storage tank pressure higher than its saturation pressure the supercooling (or net positive pressure head), that is, even if the LNG with supercooling absorbs heat or produces a partial pressure drop, as long as the pressure is higher than its saturation pressure, LNG will not produce BOG.

[0005] Submersible pumps are often used as LNG station conveying and pressurizing equipment. Submersible pumps are centrifugal pumps. When centrifugal pumps are working, once there is gas in the liquid, cavitation will occur. In order to avoid the generation of BOG gas, the LNG at the inlet of the submersible pump must have sufficient subcooling to offset the pressure drop at the pump suction port.

[0006] At present, most LNG stations are not equipped with BOG recovery equipment. The emissions generated by some stations with small gas filling volumes even reach 50%. LNG stations equipped with BOG recovery equipment mostly extract BOG from LNG storage tanks and heat and pressure-regulate the BOG for use in boilers and kitchens on the station. However, due to the small amount of gas used in boilers and kitchens on the station, it is still difficult to fully utilize the BOG in the LNG storage tanks, and most of the BOG will still be discharged.

[0007] Moreover, recycling BOG by extracting it from the LNG tank will reduce the pressure of the LNG tank. The LNG in the LNG tank is mostly in a saturated state when stored. Extracting BOG from the LNG tank will cause the pressure of the LNG tank to be lower than the saturation pressure at the corresponding temperature, resulting in insufficient supercooling of the LNG sucked in by the submersible pump, causing cavitation.

[0008] Therefore, how to realize BOG recovery in LNG storage tanks and maintain the supercooling degree of LNG storage tanks is a problem to be solved urgently in the art. Summary of the invention

[0009] In order to solve the above technical problems, the present application provides an LNG storage tank BOG liquefaction and recovery system and a control method thereof, which can liquefy and recover the BOG in the LNG storage tank and maintain the supercooling degree of the LNG storage tank.

[0010] The first object of the present application is to provide an LNG storage tank BOG liquefaction recovery system capable of recovering BOG in an LNG storage tank.

[0011] The above-mentioned application objective 1 of the present application is achieved through the following technical solutions:

[0012] An LNG storage tank BOG liquefaction recovery system comprises an LNG storage tank, an LNG buffer tank, an LNG pump and a refrigerator, wherein the LNG pump is installed in the LNG buffer tank;

[0013] The bottom of the LNG storage tank is provided with a liquid inlet and a liquid outlet, the top of the LNG storage tank is provided with a gas phase port, the bottom of the LNG buffer tank is provided with a liquid phase inlet, and the refrigerator is provided with a medium inlet and a medium outlet;

[0014] The gas phase port of the LNG storage tank is connected to the medium inlet of the refrigerator through a first pipeline, and a first valve is installed on the first pipeline; the medium outlet of the refrigerator is connected to the liquid phase inlet of the LNG buffer tank through a second pipeline, and a second valve is installed on the second pipeline; the liquid outlet of the LNG pump is connected to the liquid inlet of the LNG storage tank through a third pipeline, and a third valve is installed on the third pipeline;

[0015] The liquid outlet of the LNG storage tank is connected to the liquid phase inlet of the LNG buffer tank through a fourth pipeline, and a fourth valve is installed on the fourth pipeline; the liquid outlet of the LNG pump is connected to the medium inlet of the refrigerator through a fifth pipeline, and a fifth valve is installed on the fifth pipeline, and the liquid inlet end of the fifth valve is connected to the liquid inlet pipe of the third valve, and the liquid outlet end of the fifth valve is connected to an end of the first valve close to the medium inlet of the refrigerator; a sixth pipeline is connected between the medium outlet of the refrigerator and the liquid inlet of the LNG storage tank, and a sixth valve is installed on the sixth pipeline, and an end of the sixth valve close to the medium outlet of the refrigerator is connected to an end of the second valve close to the medium outlet of the refrigerator, and an end of the sixth valve close to the liquid inlet of the LNG storage tank is connected to an end of the third valve away from the LNG pump.

[0016] Preferably, the first valve, the second valve, the third valve, the fourth valve, the fifth valve and the sixth valve are all electrically controlled valves.

[0017] Preferably, the storage tank BOG liquefaction recovery system further includes a first pressure detection device, a temperature detection device and a controller, wherein the first pressure detection device and the temperature detection device are both installed on the LNG storage tank, the signal output ends of the first pressure detection device and the temperature detection device are respectively connected to the first signal input end and the second signal input end of the controller, and the first control end, the second control end, the third control end, the fourth control end, the fifth control end, the sixth control end, the seventh control end and the eighth control end of the controller are respectively connected to the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator, wherein:

[0018] The first pressure detection device is used to detect the real-time pressure P in the LNG storage tank;

[0019] The temperature detection device is used to detect the real-time temperature T in the LNG storage tank;

[0020] The controller is used to output corresponding control instructions according to the real-time pressure P and the real-time temperature T in the LNG storage tank, so as to control the working states of the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator through the control instructions, so that the LNG storage tank BOG liquefaction recovery system can achieve BOG recovery in the LNG storage tank while maintaining the supercooling degree of the LNG storage tank.

[0021] Preferably, the controller executes a control instruction corresponding to the real-time pressure P and the real-time temperature T in the LNG storage tank, so as to control the working states of the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator according to a preset control process through the control instruction, so that the LNG storage tank BOG liquefaction recovery system can realize BOG recovery in the LNG storage tank while maintaining the supercooling degree of the LNG storage tank. Specifically, it is used to:

[0022] When the real-time pressure P is greater than or equal to a preset LNG tank pressure threshold value P2, the LNG tank BOG liquefaction recovery system is controlled to execute a preset BOG liquefaction recovery workflow, wherein the preset LNG tank pressure threshold value P2 is less than or equal to the maximum working pressure of the LNG tank;

[0023] When the real-time pressure P is less than the preset LNG tank pressure threshold value P2, and P≤P1+(1+x%)ΔP, the LNG tank BOG liquefaction recovery system is controlled to execute the preset LNG tank subcooling maintenance work flow, wherein:

[0024] P1 represents the saturation pressure corresponding to the real-time temperature T in the LNG storage tank at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling margin.

[0025] Preferably, the preset BOG liquefaction recovery workflow is as follows:

[0026] Control the refrigeration machine and the LNG pump to start, and control the first valve, the second valve, and the third valve to open, so that the BOG in the LNG storage tank passes through the gas phase port of the LNG storage tank and then enters the refrigeration machine from the medium inlet of the refrigeration machine for cooling, and after the BOG is cooled and liquefied into LNG, it passes through the medium outlet of the refrigeration machine through the second valve and enters the LNG buffer tank through the liquid phase inlet of the LNG buffer tank, and the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank and recovers it into the LNG storage tank from the liquid inlet of the LNG storage tank through the third valve;

[0027] The preset LNG tank subcooling maintenance workflow is as follows:

[0028] The refrigeration machine and the LNG pump are controlled to start, and the fourth valve, the fifth valve and the sixth valve are controlled to open, so that the LNG in the LNG storage tank enters the LNG buffer tank through the liquid outlet of the LNG storage tank via the fourth valve, the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank, and enters the refrigeration machine through the medium inlet of the refrigeration machine through the fifth valve for cooling, the cooled LNG is discharged through the medium outlet of the refrigeration machine, enters the LNG storage tank through the liquid inlet of the LNG storage tank through the sixth valve, mixes with the LNG in the LNG storage tank, and then reduces the LNG temperature in the LNG storage tank to maintain the supercooling degree of the LNG storage tank.

[0029] Preferably, the saturated pressure P1 corresponding to the real-time temperature T in the LNG storage tank at the current moment is calculated based on a preset saturated pressure calculation model and the real-time temperature T, and the preset saturated pressure calculation model is constructed based on a temperature and saturated pressure relationship curve of LNG, wherein the preset saturated pressure calculation model is as follows:

[0030] P1=exp(39.205-1324.4 / T-3.4366*LnT+0.000031019*T 2 ).

[0031] Preferably, the storage tank BOG liquefaction recovery system further comprises a liquid level detection device, which is installed on the LNG buffer tank and is used to detect a liquid level signal of the LNG in the buffer tank.

[0032] Preferably, the storage tank BOG liquefaction recovery system further comprises a second pressure detection device, which is installed on the LNG buffer tank and is used to detect the air pressure signal in the LNG storage tank.

[0033] A gas phase outlet is provided at the top of the LNG buffer tank, and the gas phase outlet of the LNG buffer tank is connected to the gas phase port of the LNG storage tank through a seventh pipeline, and a seventh valve is installed on the seventh pipeline.

[0034] A second object of the present application is to provide an LNG storage tank BOG liquefaction recovery system capable of recovering BOG in an LNG storage tank.

[0035] The second application objective of the present application is achieved through the following technical solutions:

[0036] A control method for a storage tank BOG liquefaction recovery system according to any one of the above application purposes, the method comprising the following steps:

[0037] S1, monitoring the real-time pressure P in the LNG storage tank, and determining whether the real-time pressure P meets a first preset condition, and if so, executing S2;

[0038] S2, controlling the LNG storage tank BOG liquefaction recovery system to execute a preset BOG liquefaction recovery workflow, wherein the preset BOG liquefaction recovery workflow is as follows:

[0039] Control the refrigeration machine and the LNG pump to start, and control the first valve, the second valve, and the third valve to open, so that the BOG in the LNG storage tank passes through the gas phase port of the LNG storage tank and then enters the refrigeration machine from the medium inlet of the refrigeration machine for cooling, and after the BOG is cooled and liquefied into LNG, it passes through the medium outlet of the refrigeration machine through the second valve and enters the LNG buffer tank through the liquid phase inlet of the LNG buffer tank, and the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank and recovers it into the LNG storage tank from the liquid inlet of the LNG storage tank through the third valve;

[0040] S3, during the process of the LNG storage tank BOG liquefaction recovery system executing the preset BOG liquefaction recovery workflow, continuously monitoring the real-time pressure P in the LNG storage tank, and determining whether the real-time pressure P meets a first preset condition, if not, executing S4;

[0041] S4, determining whether the real-time pressure P meets a second preset condition, if so, executing S5;

[0042] S5, controlling the LNG storage tank BOG liquefaction recovery system to execute a preset LNG storage tank subcooling maintenance workflow, wherein the preset LNG storage tank subcooling maintenance workflow is as follows:

[0043] The refrigeration machine and the LNG pump are controlled to start, and the fourth valve, the fifth valve and the sixth valve are controlled to open, so that the LNG in the LNG storage tank enters the LNG buffer tank through the liquid outlet of the LNG storage tank via the fourth valve, the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank, and enters the refrigeration machine through the medium inlet of the refrigeration machine through the fifth valve for cooling, the cooled LNG is discharged through the medium outlet of the refrigeration machine, enters the LNG storage tank through the liquid inlet of the LNG storage tank through the sixth valve, mixes with the LNG in the LNG storage tank, and then reduces the LNG temperature in the LNG storage tank to maintain the supercooling degree of the LNG storage tank.

[0044] Preferably, the first preset condition is as follows:

[0045] P≥P2, where P2 represents a preset LNG storage tank pressure threshold, and the preset LNG storage tank pressure threshold P2 is less than or equal to the maximum working pressure of the LNG storage tank;

[0046] The second preset condition is as follows:

[0047] P≤P1+(1+x%)△P, where P1 represents the saturation pressure corresponding to the real-time temperature T in the LNG storage tank at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling margin.

[0048] The beneficial effects of this application are as follows:

[0049] The LNG tank BOG liquefaction and recovery system of the present application, when it is necessary to recover the BOG in the LNG tank, cools down and liquefies the BOG transported from the LNG tank through a refrigerator and then returns it to the LNG tank, so as to realize the recovery of the BOG gas in the LNG tank, reduce the pressure of the LNG tank, and avoid the discharge of the BOG inside the LNG tank due to the excessive pressure inside the LNG tank. During the BOG recovery process, when it is necessary to maintain the supercooling degree of the LNG tank, the LNG transported from the LNG tank is cooled down through a refrigerator, and the cooled LNG is returned to the LNG tank, mixed with the LNG in the LNG tank, and the LNG temperature in the LNG tank is reduced to maintain the supercooling degree of the LNG tank. The entire system of the present application has a simple structure, and can liquefy and recover the BOG in the LNG tank and maintain the supercooling degree of the LNG tank. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] Figure 1 It is the relationship curve between the temperature and saturation pressure of the LNG storage tank;

[0052] Figure 2 This is a structural schematic diagram of a LNG storage tank BOG liquefaction recovery system in one embodiment of the present application;

[0053] Figure 3 for Figure 2 The control principle block diagram of the LNG storage tank BOG liquefaction recovery system shown;

[0054] Figure 4 This is a flow chart of a control method for a BOG liquefaction recovery system of an LNG storage tank in one embodiment of the present application. DETAILED DESCRIPTION

[0055] In order to enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0056] In the embodiments provided in the present application, it should be understood that the disclosed methods and systems can be implemented in other ways. The system embodiments described below are merely schematic. For example, the division of units and modules is only a logical function division. There may be other division methods in actual implementation, such as: multiple units or modules can be combined, or can be integrated into another system, or some features can be ignored, or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, which can be electrical, mechanical or other forms.

[0057] In addition, all functional units in the embodiments of the present application may be integrated into one processor, or each unit may be a separate device, or two or more units may be integrated into one device; each functional unit in the embodiments of the present application may be implemented in the form of hardware or in the form of hardware plus software functional units.

[0058] A person skilled in the art can understand that all or part of the steps of the following method embodiments can be completed by program instructions and related hardware. The aforementioned program instructions can be stored in a computer-readable storage medium. When the program instructions are executed, the steps of the following method embodiments are executed; and the aforementioned storage medium includes: a mobile storage device, a read-only memory (ROM), a magnetic disk or an optical disk, and other media that can store program codes.

[0059] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, "multiple" and "several" mean two or more, unless otherwise clearly and specifically defined.

[0060] The present application embodiment provides a LNG storage tank BOG liquefaction recovery system, such as Figure 2 , Figure 3As shown, the system may include an LNG storage tank 1, an LNG buffer tank 2, an LNG pump 3 and a refrigerator 4, wherein the LNG pump 3 is installed in the LNG buffer tank 2;

[0061] The bottom of the LNG storage tank 1 is provided with a liquid inlet and a liquid outlet, the top of the LNG storage tank 1 is provided with a gas phase port, the bottom of the LNG buffer tank 2 is provided with a liquid phase inlet, and the refrigerator 4 is provided with a medium inlet and a medium outlet;

[0062] The gas phase port of the LNG storage tank 1 is connected to the medium inlet of the refrigerator 4 through a first pipeline 5, and a first valve 6 is installed on the first pipeline 5; the medium outlet of the refrigerator 4 is connected to the liquid phase inlet of the LNG buffer tank 2 through a second pipeline 7, and a second valve 8 is installed on the second pipeline 7; the liquid outlet of the LNG pump 3 is connected to the liquid inlet of the LNG storage tank 1 through a third pipeline 9, and a third valve 10 is installed on the third pipeline 9;

[0063] The liquid outlet of the LNG storage tank 1 is connected to the liquid phase inlet of the LNG buffer tank 2 through a fourth pipeline 11, and a fourth valve 12 is installed on the fourth pipeline 11; the liquid outlet of the LNG pump 3 is connected to the medium inlet of the refrigerator 4 through a fifth pipeline 13, and a fifth valve 14 is installed on the fifth pipeline 13, and the liquid inlet end of the fifth valve 14 is connected to the liquid inlet pipe of the third valve 10, and the liquid outlet end of the fifth valve 14 is connected to one end of the first valve 6 close to the medium inlet of the refrigerator 4; a sixth pipeline 15 is connected between the medium outlet of the refrigerator 4 and the liquid inlet of the LNG storage tank 1, and a sixth valve 16 is installed on the sixth pipeline 15, and one end of the sixth valve 16 close to the medium outlet of the refrigerator 4 is connected to one end of the second valve 8 close to the medium outlet of the refrigerator 4, and one end of the sixth valve 16 close to the liquid inlet of the LNG storage tank 1 is connected to one end of the third valve 10 away from the LNG pump 3.

[0064] The working principle of the LNG storage tank BOG liquefaction recovery system in the embodiment of the present application is as follows:

[0065] When it is necessary to recover the BOG in the LNG storage tank 1, the refrigerator 4 and the LNG pump 3 are started, and the first valve 6, the second valve 8 and the third valve 10 are opened (the other valves are closed), so that the BOG in the LNG storage tank 1 enters the refrigerator 4 through the gas phase port of the LNG storage tank 1 through the first valve 6 and then from the medium inlet of the refrigerator 4 for cooling. After the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank 2 through the medium outlet of the refrigerator 4 through the second valve 8 and from the liquid phase inlet of the LNG buffer tank 2. The LNG pump 3 in the LNG buffer tank 2 extracts the LNG in the LNG buffer tank 2 and enters the LNG storage tank 1 through the liquid inlet of the LNG storage tank 1 through the third valve 10, so that the BOG transported from the LNG storage tank 1 is cooled and liquefied by the refrigerator 4 and then returned to the LNG storage tank 1, so as to realize the recovery of the BOG gas in the LNG storage tank 1, reduce the pressure of the LNG storage tank 1, and avoid the discharge of the BOG inside the LNG storage tank 1 due to the excessive pressure inside the LNG storage tank 1;

[0066] During the BOG recovery process, as the BOG in the LNG storage tank 1 is continuously liquefied and recovered, the pressure in the LNG storage tank 1 will decrease. At this time, the supercooling degree of the LNG storage tank 1 will decrease and the supercooling degree may be insufficient. Therefore, when it is necessary to maintain the supercooling degree of the LNG storage tank 1, the refrigerator 4 and the LNG pump 3 are started, and the fourth valve 12, the fifth valve 14 and the sixth valve 16 are opened (the remaining valves are closed), so that the LNG in the LNG storage tank 1 passes through the liquid outlet of the LNG storage tank 1 through the fourth valve 12 and enters the LNG buffer tank 2. The LNG pump 3 in the LNG buffer tank 2 transfers the LNG buffer tank 2 to the LNG buffer tank 2. The LNG in the LNG storage tank 1 is extracted, enters the refrigeration machine 4 from the medium inlet of the refrigeration machine 4 through the fifth valve 14 for cooling, and the cooled LNG is discharged through the medium outlet of the refrigeration machine 4, enters the LNG storage tank 1 from the liquid inlet through the sixth valve 16, and is mixed with the LNG in the LNG storage tank 1 to reduce the LNG temperature in the LNG storage tank 1. As the LNG temperature drops, the corresponding saturation pressure of the LNG storage tank 1 will also drop. Therefore, at this time, the supercooling degree of the LNG storage tank 1 will increase rapidly, thereby maintaining the supercooling degree of the LNG storage tank 1 within the range required for the operation of the LNG station.

[0067] The entire system of the embodiment of the present application has a simple structure and is capable of liquefying and recovering BOG in the LNG storage tank 1 and maintaining the supercooling degree of the LNG storage tank 1 .

[0068] In one embodiment, the first valve 6, the second valve 8, the third valve 10, the fourth valve 12, the fifth valve 14 and the sixth valve 16 are all electrically controlled valves. By setting each valve as an electrically controlled valve, the automation level of the system is effectively improved, and remote automatic control of liquefaction recovery of BOG and maintenance of subcooling degree of the LNG storage tank is realized. Specifically, in this embodiment, each electrically controlled valve adopts an electrically controlled pneumatic valve.

[0069] In one embodiment, the storage tank BOG liquefaction recovery system further includes a first pressure detection device 19, a temperature detection device 20 and a controller 21. The first pressure detection device 19 and the temperature detection device 20 are both installed on the LNG storage tank. The signal output ends of the first pressure detection device 19 and the temperature detection device 20 are respectively connected to the first signal input end and the second signal input end of the controller 21. The first control end, the second control end, the third control end, the fourth control end, the fifth control end, the sixth control end, the seventh control end and the eighth control end of the controller 21 are respectively connected to the first valve 6, the second valve 8, the third valve 10, the fourth valve 12, the fifth valve 14, the sixth valve 16, the LNG pump 3 and the refrigerator 4.

[0070] A first pressure detection device 19, which is used to detect the real-time pressure P in the LNG storage tank 1;

[0071] A temperature detection device 20, which is used to detect the real-time temperature T in the LNG storage tank 1;

[0072] The controller 21 is used to output corresponding control instructions according to the real-time pressure P and real-time temperature T in the LNG storage tank 1, so as to control the working states of the first valve 6, the second valve 8, the third valve 10, the fourth valve 12, the fifth valve 14, the sixth valve 16, the LNG pump 3 and the refrigerator 4 through the control instructions, so that the LNG storage tank BOG liquefaction recovery system can achieve BOG recovery in the LNG storage tank 1 while maintaining the supercooling degree of the LNG storage tank 1.

[0073] In this embodiment, a first pressure detection device 19, a temperature detection device 20 and a controller 21 are provided, and the real-time pressure P in the LNG storage tank 1 is detected by the first pressure detection device 19, and the real-time temperature T in the LNG storage tank 1 is detected by the temperature detection device 20, and the controller 21 can output corresponding control instructions according to the real-time pressure P and the real-time temperature T in the LNG storage tank 1 to control the working states of the first valve 6, the second valve 8, the third valve 10, the fourth valve 12, the fifth valve 14, the sixth valve 16, the LNG pump 3 and the refrigerator 4, so that the system can automatically switch the two control processes of BOG liquefaction recovery and maintenance of the supercooling degree of the LNG storage tank 1 according to the real-time detection signals of the first pressure detection device 19 and the temperature detection device 20.

[0074] Specifically, in this embodiment, the controller 21 adopts a PLC, the first pressure detection device 19 adopts a pressure transmitter, and the temperature detection device 20 adopts a temperature sensor.

[0075] In one embodiment, the controller 21 executes the control instructions corresponding to the real-time pressure P and the real-time temperature T in the LNG storage tank 1, so as to control the working states of the first valve 6, the second valve 8, the third valve 10, the fourth valve 12, the fifth valve 14, the sixth valve 16, the LNG pump 3 and the refrigerator 4 according to the preset control process through the control instructions, so that the LNG storage tank BOG liquefaction recovery system can realize the BOG recovery in the LNG storage tank 1 while maintaining the supercooling degree of the LNG storage tank 1. Specifically, it is used to:

[0076] When the real-time pressure P is greater than or equal to the preset LNG storage tank 1 pressure threshold value P2, the LNG storage tank BOG liquefaction recovery system is controlled to execute the preset BOG liquefaction recovery work process, wherein the preset LNG storage tank 1 pressure threshold value P2 is less than or equal to the maximum working pressure of the LNG storage tank 1;

[0077] When the real-time pressure P is less than the preset LNG tank 1 pressure threshold value P2, and P≤P1+(1+x%)△P, the LNG tank BOG liquefaction recovery system is controlled to execute the preset LNG tank 1 supercooling maintenance work flow, wherein,

[0078] P1 represents the saturation pressure corresponding to the real-time temperature T in the LNG storage tank 1 at the current moment, ΔP represents the required subcooling degree of the LNG station, and x% represents the preset subcooling degree margin.

[0079] In this embodiment, by setting the pressure threshold value P2 of the LNG storage tank 1, when the real-time pressure P is greater than or equal to the preset pressure threshold value P2 of the LNG storage tank 1, it indicates that at this time, there is a lot of BOG in the LNG storage tank 1, resulting in excessive pressure in the LNG storage tank 1. If BOG recovery is not performed, the pressure of the LNG storage tank 1 will be higher than the maximum working pressure of the LNG storage tank 1, resulting in BOG discharge. Therefore, when the real-time pressure P is greater than or equal to the preset pressure threshold value P2 of the LNG storage tank 1, the LNG storage tank BOG liquefaction recovery system is controlled to execute the preset BOG liquefaction recovery workflow to achieve BOG recovery in the LNG storage tank 1;

[0080] During the BOG recovery process, the real-time pressure P in the LNG storage tank 1 will drop, which may cause insufficient supercooling of the LNG storage tank 1. When the real-time pressure P is less than the preset LNG storage tank 1 pressure threshold value P2, it means that the pressure of the LNG storage tank 1 will no longer cause BOG discharge. At this time, if the real-time pressure P≤P1+(1+x%)△P, it means that the corresponding supercooling of the LNG storage tank 1 under the real-time pressure P cannot meet the working requirements of the LNG station, which will cause cavitation in the submersible pump. At this time, the LNG storage tank BOG liquefaction recovery system is controlled to execute the preset LNG storage tank 1 supercooling maintenance work flow to increase the supercooling of the LNG storage tank 1, so that the supercooling of the LNG tank 1 is maintained within the range required for normal operation.

[0081] Specifically, the preset LNG storage tank 1 pressure threshold value P2 is set according to actual needs. In this embodiment, the preset LNG storage tank 1 pressure threshold value P2 is between the maximum working pressure of the LNG storage tank 1 minus 0.2MPa and the maximum working pressure of the LNG storage tank 1, that is, if the maximum working pressure of the LNG storage tank 1 is P0, then P0-0.2MPa≤P2≤P0.

[0082] In one embodiment, the preset BOG liquefaction recovery workflow is as follows:

[0083] Control the refrigerator 4 and the LNG pump 3 to start, and control the first valve 6, the second valve 8 and the third valve 10 to open, so that the BOG in the LNG storage tank 1 enters the refrigerator 4 through the gas phase port of the LNG storage tank 1 through the first valve 6 and then from the medium inlet of the refrigerator 4 to cool down, and after the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank 2 through the medium outlet of the refrigerator 4 through the second valve 8 and through the liquid phase inlet of the LNG buffer tank 2, and the LNG pump 3 in the LNG buffer tank 2 extracts the LNG in the LNG buffer tank 2 and recovers it into the LNG storage tank 1 from the liquid inlet of the LNG storage tank 1 through the third valve 10;

[0084] The preset subcooling maintenance workflow of LNG tank 1 is as follows:

[0085] The refrigerator 4 and the LNG pump 3 are controlled to start, and the fourth valve 12, the fifth valve 14 and the sixth valve 16 are controlled to open, so that the LNG in the LNG storage tank 1 enters the LNG buffer tank 2 through the liquid outlet of the LNG storage tank 1 via the fourth valve 12, the LNG pump 3 in the LNG buffer tank 2 extracts the LNG in the LNG buffer tank 2, and enters the refrigerator 4 from the medium inlet of the refrigerator 4 through the fifth valve 14 for cooling, the cooled LNG is discharged through the medium outlet of the refrigerator 4, enters the LNG storage tank 1 from the liquid inlet of the LNG storage tank 1 through the sixth valve 16, mixes with the LNG in the LNG storage tank 1, and then reduces the LNG temperature in the LNG storage tank 1 to maintain the supercooling degree of the LNG storage tank 1.

[0086] In one embodiment, the saturated pressure P1 corresponding to the real-time temperature T in the LNG storage tank 1 at the current moment is calculated based on a preset saturated pressure calculation model and the real-time temperature T. The preset saturated pressure calculation model is constructed based on a temperature and saturated pressure relationship curve of the LNG, wherein the preset saturated pressure calculation model is as follows:

[0087] P1=exp(39.205-1324.4 / T-3.4366*LnT+0.000031019*T 2 ).

[0088] In this embodiment, a saturation pressure calculation model is constructed based on a temperature and saturation pressure relationship curve of LNG, and then the saturation pressure P1 corresponding to the current temperature T can be accurately calculated through the model.

[0089] In one embodiment, the storage tank BOG liquefaction recovery system further includes a liquid level detection device 23 , which is installed on the LNG buffer tank 2 , and is used to detect a liquid level signal of the LNG in the buffer tank.

[0090] In this embodiment, a liquid level detection device 23 is provided on the LNG buffer tank 2 to detect the liquid level signal of the LNG in the buffer tank. When the system executes the preset BOG liquefaction recovery work process and the preset LNG storage tank 1 supercooling maintenance work process, in order to avoid the LNG pump 3 from being damaged due to insufficient liquid level in the LNG buffer tank 2, when it is detected that the liquid level of the LNG buffer tank 2 reaches a preset high liquid level threshold, the controller 21 starts the LNG pump 3 in an interlocking manner, and the pumped LNG enters the LNG storage tank 1 through the liquid inlet at the bottom of the LNG storage tank 1. When it is detected that the liquid level of the LNG buffer tank 2 reaches a preset low liquid level threshold, the controller 21 shuts down the LNG pump 3 in an interlocking manner, thereby realizing automatic control of the start and stop of the LNG pump 3, thereby improving the automation level of the BOG liquefaction recovery device.

[0091] Specifically, in this embodiment, the liquid level detection device 23 adopts a liquid level sensor.

[0092] In one embodiment, the storage tank BOG liquefaction recovery system further includes a second pressure detection device 22, which is installed on the LNG buffer tank 2 and is used to detect the air pressure signal in the LNG storage tank 1.

[0093] A gas phase outlet is provided at the top of the LNG buffer tank 2 , and the gas phase outlet of the LNG buffer tank 2 is connected to the gas phase port of the LNG storage tank 1 through a seventh pipeline 17 , and a seventh valve 18 is installed on the seventh pipeline 17 .

[0094] In this embodiment, a second pressure detection device 22 is provided on the LNG buffer tank 2 to detect the air pressure signal of the LNG buffer tank 2. When it is detected that the pressure of the LNG buffer tank 2 reaches a preset high pressure threshold, the controller 21 opens the seventh valve 18 in an interlocked manner, and the BOG in the LNG buffer tank 2 enters the LNG storage tank 1 from the gas phase port of the LNG storage tank 1 through the gas phase outlet of the LNG buffer tank 2 through the seventh valve 18, thereby avoiding excessive pressure in the LNG buffer tank 2 and realizing automatic control of the pressure reduction of the LNG buffer tank 2.

[0095] It should be noted that before starting the refrigerator 4 to cool and liquefy the BOG in the LNG storage tank 1 or maintaining the supercooling of the LNG storage tank 1, the LNG buffer tank 2 and the pipeline can be pre-cooled. This can ensure that the liquid in the LNG buffer tank 2 can submerge the LNG pump 3 when the refrigerator 4 is started and meet the liquid level required for the LNG pump 3 to work, and can also avoid the initial stage of the refrigerator 4 starting The temperature of the LNG buffer tank 2 is too high, causing a large amount of LNG to be vaporized, causing cavitation to the LNG pump 3.

[0096] The specific process of precooling is as follows:

[0097] Before the operation of the refrigeration machine 4, the fourth valve 12 and the seventh valve 18 are opened, and the LNG in the LNG storage tank 1 flows out from the liquid outlet at the bottom under the action of gravity, and enters the LNG buffer tank 2 through the fourth valve 12. The BOG generated in the LNG tank during the precooling process flows from the gas phase outlet at the top of the LNG buffer tank 2 through the seventh valve 18 to the gas phase port at the top of the LNG storage tank 1 and enters the LNG storage tank 1, thereby achieving precooling of the LNG storage tank 1 and part of the pipeline.

[0098] like Figure 4 As shown, the embodiment of the present application also provides a control method for the BOG liquefaction recovery system of the storage tank in any of the above embodiments, and the method may include the following steps:

[0099] S1, monitor the real-time pressure P in the LNG storage tank, and determine whether the real-time pressure P meets the first preset condition, if so, execute S2;

[0100] S2, controlling the LNG storage tank BOG liquefaction recovery system to execute a preset BOG liquefaction recovery workflow, wherein the preset BOG liquefaction recovery workflow is as follows:

[0101] Control the refrigeration machine and the LNG pump to start, and control the first valve, the second valve, and the third valve to open, so that the BOG in the LNG storage tank passes through the gas phase port of the LNG storage tank and then enters the refrigeration machine from the medium inlet of the refrigeration machine for cooling, and after the BOG is cooled and liquefied into LNG, it passes through the medium outlet of the refrigeration machine through the second valve and enters the LNG buffer tank through the liquid phase inlet of the LNG buffer tank, and the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank and recovers it into the LNG storage tank from the liquid inlet of the LNG storage tank through the third valve;

[0102] S3, during the process of the LNG tank BOG liquefaction recovery system executing the preset BOG liquefaction recovery workflow, continuously monitor the real-time pressure P in the LNG tank, and determine whether the real-time pressure P meets the first preset condition, if not, execute S4;

[0103] S4, determining whether the real-time pressure P meets the second preset condition, if so, executing S5;

[0104] S5, controlling the LNG storage tank BOG liquefaction recovery system to execute a preset LNG storage tank subcooling maintenance workflow, wherein the preset LNG storage tank subcooling maintenance workflow is as follows:

[0105] The refrigeration machine and the LNG pump are controlled to start, and the fourth valve, the fifth valve and the sixth valve are controlled to open, so that the LNG in the LNG storage tank enters the LNG buffer tank through the liquid outlet of the LNG storage tank via the fourth valve, the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank, and enters the refrigeration machine through the medium inlet of the refrigeration machine through the fifth valve for cooling, the cooled LNG is discharged through the medium outlet of the refrigeration machine, enters the LNG storage tank through the liquid inlet of the LNG storage tank through the sixth valve, mixes with the LNG in the LNG storage tank, and then reduces the LNG temperature in the LNG storage tank to maintain the supercooling degree of the LNG storage tank.

[0106] The above steps S1 to S5 are executed cyclically to realize automatic switching between LNG tank BOG recovery and LNG tank subcooling maintenance, so that the subcooling of the LNG tank is maintained within a normal range while realizing LNG tank BOG recovery and reducing the LNG tank pressure.

[0107] In one embodiment, the first preset condition is as follows:

[0108] P≥P2, where P2 represents a preset LNG tank pressure threshold, and the preset LNG tank pressure threshold P2 is less than or equal to the maximum working pressure of the LNG tank;

[0109] The second precondition is as follows:

[0110] P≤P1+(1+x%)△P, where P1 represents the saturation pressure corresponding to the real-time temperature T in the LNG storage tank at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling margin.

[0111] The control method of the storage tank BOG liquefaction recovery system in this embodiment is implemented based on the storage tank BOG liquefaction recovery system in any of the above embodiments. Therefore, the control method of the storage tank BOG liquefaction recovery system has the same working principle and technical effect as the storage tank BOG liquefaction recovery system in the above embodiments, which will not be repeated here.

[0112] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0113] Professionals may further appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the composition and steps of each example have been generally described in the above description according to function. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0114] The steps of the methods or algorithms described in conjunction with the embodiments disclosed herein may be implemented directly using hardware, software modules executed by a processor, or a combination of the two. The software modules may be placed in a random access memory (RAM), a memory, a read-only memory (ROM), an electrically programmable ROM, an electrically erasable programmable ROM, a register, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.

[0115] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A LNG storage tank BOG liquefaction recovery system, characterized in that: It includes an LNG storage tank, an LNG buffer tank, an LNG pump and a refrigerator, wherein the LNG pump is installed in the LNG buffer tank; The bottom of the LNG storage tank is provided with a liquid inlet and a liquid outlet, the top of the LNG storage tank is provided with a gas phase port, the bottom of the LNG buffer tank is provided with a liquid phase inlet, and the refrigerator is provided with a medium inlet and a medium outlet; The gas phase port of the LNG storage tank is connected to the medium inlet of the refrigerator through a first pipeline, and a first valve is installed on the first pipeline; the medium outlet of the refrigerator is connected to the liquid phase inlet of the LNG buffer tank through a second pipeline, and a second valve is installed on the second pipeline; the liquid outlet of the LNG pump is connected to the liquid inlet of the LNG storage tank through a third pipeline, and a third valve is installed on the third pipeline; The liquid outlet of the LNG storage tank is connected to the liquid phase inlet of the LNG buffer tank through a fourth pipeline, and a fourth valve is installed on the fourth pipeline; the liquid outlet of the LNG pump is connected to the medium inlet of the refrigerator through a fifth pipeline, and a fifth valve is installed on the fifth pipeline, and the liquid inlet end of the fifth valve is connected to the liquid inlet pipe of the third valve, and the liquid outlet end of the fifth valve is connected to an end of the first valve close to the medium inlet of the refrigerator; a sixth pipeline is connected between the medium outlet of the refrigerator and the liquid inlet of the LNG storage tank, and a sixth valve is installed on the sixth pipeline, and an end of the sixth valve close to the medium outlet of the refrigerator is connected to an end of the second valve close to the medium outlet of the refrigerator, and an end of the sixth valve close to the liquid inlet of the LNG storage tank is connected to an end of the third valve away from the LNG pump.

2. The storage tank BOG liquefaction recovery system according to claim 1, characterized in that: The first valve, the second valve, the third valve, the fourth valve, the fifth valve and the sixth valve are all electrically controlled valves.

3. The storage tank BOG liquefaction recovery system according to claim 2, characterized in that: It also includes a first pressure detection device, a temperature detection device and a controller, wherein the first pressure detection device and the temperature detection device are both installed on the LNG storage tank, the signal output ends of the first pressure detection device and the temperature detection device are respectively connected to the first signal input end and the second signal input end of the controller, and the first control end, the second control end, the third control end, the fourth control end, the fifth control end, the sixth control end, the seventh control end and the eighth control end of the controller are respectively connected to the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator, wherein, The first pressure detection device is used to detect the real-time pressure P in the LNG storage tank; The temperature detection device is used to detect the real-time temperature T in the LNG storage tank; The controller is used to output corresponding control instructions according to the real-time pressure P and the real-time temperature T in the LNG storage tank, so as to control the working states of the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator through the control instructions, so that the LNG storage tank BOG liquefaction recovery system can achieve BOG recovery in the LNG storage tank while maintaining the supercooling degree of the LNG storage tank.

4. The storage tank BOG liquefaction recovery system according to claim 3, characterized in that: The controller executes a control instruction corresponding to the real-time pressure P and the real-time temperature T in the LNG storage tank, so as to control the working states of the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator according to a preset control process through the control instruction, so that the LNG storage tank BOG liquefaction recovery system can realize the BOG recovery in the LNG storage tank while maintaining the supercooling degree of the LNG storage tank. Specifically used for: When the real-time pressure P is greater than or equal to a preset LNG tank pressure threshold value P2, the LNG tank BOG liquefaction recovery system is controlled to execute a preset BOG liquefaction recovery workflow, wherein the preset LNG tank pressure threshold value P2 is less than or equal to the maximum working pressure of the LNG tank; When the real-time pressure P is less than the preset LNG tank pressure threshold value P2, and P≤P1+(1+x%)ΔP, the LNG tank BOG liquefaction recovery system is controlled to execute the preset LNG tank subcooling maintenance work flow, wherein: P1 represents the saturation pressure corresponding to the real-time temperature T in the LNG storage tank at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling margin.

5. The storage tank BOG liquefaction recovery system according to claim 4, characterized in that: The preset BOG liquefaction recovery workflow is as follows: Control the refrigeration machine and the LNG pump to start, and control the first valve, the second valve, and the third valve to open, so that the BOG in the LNG storage tank passes through the gas phase port of the LNG storage tank and then enters the refrigeration machine from the medium inlet of the refrigeration machine for cooling, and after the BOG is cooled and liquefied into LNG, it passes through the medium outlet of the refrigeration machine through the second valve and enters the LNG buffer tank through the liquid phase inlet of the LNG buffer tank, and the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank and recovers it into the LNG storage tank from the liquid inlet of the LNG storage tank through the third valve; The preset LNG tank subcooling maintenance workflow is as follows: The refrigeration machine and the LNG pump are controlled to start, and the fourth valve, the fifth valve and the sixth valve are controlled to open, so that the LNG in the LNG storage tank enters the LNG buffer tank through the liquid outlet of the LNG storage tank via the fourth valve, the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank, and enters the refrigeration machine through the medium inlet of the refrigeration machine through the fifth valve for cooling, the cooled LNG is discharged through the medium outlet of the refrigeration machine, enters the LNG storage tank through the liquid inlet of the LNG storage tank through the sixth valve, mixes with the LNG in the LNG storage tank, and then reduces the LNG temperature in the LNG storage tank to maintain the supercooling degree of the LNG storage tank.

6. The storage tank BOG liquefaction recovery system according to claim 4, characterized in that: The saturation pressure P1 corresponding to the real-time temperature T in the LNG storage tank at the current moment is calculated based on a preset saturation pressure calculation model and the real-time temperature T. The preset saturation pressure calculation model is constructed based on a temperature and saturation pressure relationship curve of LNG, wherein the preset saturation pressure calculation model is as follows: P1=exp(39.205-1324.4 / T-3.4366*LnT+0.000031019*T 2 )。 7. The storage tank BOG liquefaction recovery system according to claim 1, characterized in that: It also includes a liquid level detection device, which is installed on the LNG buffer tank and is used to detect a liquid level signal of the LNG in the buffer tank.

8. The storage tank BOG liquefaction recovery system according to any one of claims 1 to 7, characterized in that: The second pressure detection device is installed on the LNG buffer tank and is used to detect the air pressure signal in the LNG storage tank. A gas phase outlet is provided at the top of the LNG buffer tank, and the gas phase outlet of the LNG buffer tank is connected to the gas phase port of the LNG storage tank through a seventh pipeline, and a seventh valve is installed on the seventh pipeline.

9. A control method for a storage tank BOG liquefaction recovery system according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: S1, monitoring the real-time pressure P in the LNG storage tank, and determining whether the real-time pressure P meets a first preset condition, and if so, executing S2; S2, controlling the LNG storage tank BOG liquefaction recovery system to execute a preset BOG liquefaction recovery workflow, wherein the preset BOG liquefaction recovery workflow is as follows: Control the refrigeration machine and the LNG pump to start, and control the first valve, the second valve, and the third valve to open, so that the BOG in the LNG storage tank passes through the gas phase port of the LNG storage tank and then enters the refrigeration machine from the medium inlet of the refrigeration machine for cooling, and after the BOG is cooled and liquefied into LNG, it passes through the medium outlet of the refrigeration machine through the second valve and enters the LNG buffer tank through the liquid phase inlet of the LNG buffer tank, and the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank and recovers it into the LNG storage tank from the liquid inlet of the LNG storage tank through the third valve; S3, during the process of the LNG storage tank BOG liquefaction recovery system executing the preset BOG liquefaction recovery workflow, continuously monitoring the real-time pressure P in the LNG storage tank, and determining whether the real-time pressure P meets a first preset condition, if not, executing S4; S4, determining whether the real-time pressure P meets a second preset condition, if so, executing S5; S5, controlling the LNG storage tank BOG liquefaction recovery system to execute a preset LNG storage tank subcooling maintenance workflow, wherein the preset LNG storage tank subcooling maintenance workflow is as follows: The refrigeration machine and the LNG pump are controlled to start, and the fourth valve, the fifth valve and the sixth valve are controlled to open, so that the LNG in the LNG storage tank enters the LNG buffer tank through the liquid outlet of the LNG storage tank via the fourth valve, the LNG pump in the LNG buffer tank extracts the LNG in the LNG buffer tank, and enters the refrigeration machine through the medium inlet of the refrigeration machine through the fifth valve for cooling, the cooled LNG is discharged through the medium outlet of the refrigeration machine, enters the LNG storage tank through the liquid inlet of the LNG storage tank through the sixth valve, mixes with the LNG in the LNG storage tank, and then reduces the LNG temperature in the LNG storage tank to maintain the supercooling degree of the LNG storage tank.

10. The control method according to claim 9, characterized in that: The first preset condition is as follows: P≥P2, where P2 represents a preset LNG storage tank pressure threshold, and the preset LNG storage tank pressure threshold P2 is less than or equal to the maximum working pressure of the LNG storage tank; The second preset condition is as follows: P≤P1+(1+x%)△P, where P1 represents the saturation pressure corresponding to the real-time temperature T in the LNG storage tank at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling margin.

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