An LNG storage tank BOG liquefaction and recovery system and its control method

Abstract

This application discloses an LNG storage tank BOG liquefaction and recovery system and its control method. The system includes an LNG storage tank, an LNG buffer tank, an LNG pump, and a chiller. The LNG pump is installed inside the LNG buffer tank. When BOG needs to be recovered from the LNG storage tank, the chiller cools and liquefies the BOG transported from the LNG storage tank and returns it to the LNG storage tank, thus realizing the recovery of BOG gas in the LNG storage tank. During the BOG recovery process, when it is necessary to maintain the subcooling of the LNG storage tank, the chiller cools the LNG transported from the LNG storage tank. The cooled LNG is then returned to the LNG storage tank, mixed with the LNG in the LNG storage tank, and further reduces the temperature of the LNG in the LNG storage tank to maintain the subcooling of the LNG storage tank. The entire system of this application has a simple structure and can liquefy and recover BOG from the LNG storage tank while maintaining the subcooling of the LNG storage tank.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of LNG storage tank BOG recovery, in particular to a LNG storage tank BOG liquefaction recovery system and a control method thereof. BACKGROUND

[0002] At present, LNG (Liquefied Natural Gas) stations (including LNG filling stations, L-CNG filling stations, LNG gasification stations, LNG storage stations) are all provided with LNG storage tanks as storage devices. The temperature of LNG is extremely low, and the minimum temperature can reach -162℃. During the storage process, LNG will continuously absorb heat from the environment and evaporate BOG (Boil Off Gas) to be stored in the LNG storage tank. At the same time, BOG produced by the absorption of 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, if there is no BOG recovery equipment, emission will occur.

[0003] When LNG is stored in a closed and heat-insulated container, it usually 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. It can be known from the temperature-saturation pressure relationship curve of the LNG storage tank that when the pressure of the LNG storage tank is lower than the saturation pressure, the LNG will accelerate vaporization 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 inhibited and the liquefaction of BOG is intensified until a new equilibrium is formed. The part of the LNG storage tank with a pressure higher than the saturation pressure is called the subcooling degree (or net positive suction head), that is, even if the LNG absorbs heat or produces a partial pressure drop, as long as the pressure is higher than the saturation pressure, the LNG will not produce BOG.

[0004] Submerged pumps are often used as LNG station conveying and pressurizing equipment. The submerged pump is a centrifugal pump. When the centrifugal pump is working, if the liquid contains gas, cavitation will occur. In order to avoid the generation of BOG gas, it is required that the LNG at the inlet of the submerged pump has sufficient subcooling degree to offset the pressure drop at the suction inlet of the pump.

[0005] At present, most LNG stations do not have BOG recovery equipment. The emission amount of some stations with small gas filling amount even reaches 50%. However, the LNG stations provided with BOG recovery equipment mostly use the BOG in the LNG storage tank to heat and adjust the pressure and then use it as the boiler and kitchen on the station. However, due to the small amount of gas used by the boiler and kitchen on the station, the BOG in the LNG storage tank is still difficult to be completely utilized, and most of the BOG is still emitted.

[0006] Moreover, the BOG in the LNG storage tank is extracted for recycling, which reduces the pressure of the LNG storage tank, and the LNG in the LNG storage tank is usually in a saturated state, and the extraction of the BOG in the LNG storage tank causes the pressure of the LNG storage tank to be lower than the saturated pressure at the corresponding temperature, which causes the submergence pump to be insufficiently supercooled when sucking LNG, and cavitation occurs.

[0007] Therefore, how to realize BOG recycling in the LNG storage tank while maintaining the supercooling degree of the LNG storage tank is a problem to be solved in the field. SUMMARY

[0008] To solve the above technical problems, the present application provides an LNG storage tank BOG liquefaction recycling system and a control method thereof, which can liquefy and recycle the BOG in the LNG storage tank and maintain the supercooling degree of the LNG storage tank.

[0009] The first object of the present application is to provide an LNG storage tank BOG liquefaction recycling system capable of recycling the BOG in the LNG storage tank.

[0010] The above application object of the present application is achieved by the following technical scheme:

[0011] An LNG storage tank BOG liquefaction recycling system, comprising an LNG storage tank, an LNG buffer tank, an LNG pump and a refrigeration machine, wherein the LNG pump is installed in the LNG buffer tank;

[0012] The bottom of the LNG storage tank is provided with a liquid inlet and a liquid outlet, and the top of the LNG storage tank is provided with a gas phase port, and the bottom of the LNG buffer tank is provided with a liquid phase inlet, and the refrigeration machine is provided with a medium inlet and a medium outlet;

[0013] The gas phase port of the LNG storage tank is connected to the medium inlet of the refrigeration machine through a first pipeline, and a first valve is installed on the first pipeline; the medium outlet of the refrigeration machine 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;

[0014] The liquid outlet of the LNG storage tank is connected with 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 with 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 with the liquid inlet pipe of the third valve, and the liquid outlet end of the fifth valve is connected with one end of the first valve close to the medium inlet of the refrigerator; the medium outlet of the refrigerator is connected with the liquid inlet of the LNG storage tank through a sixth pipeline, and a sixth valve is installed on the sixth pipeline, and one end of the sixth valve close to the medium outlet of the refrigerator is connected with one end of the second valve close to the medium outlet of the refrigerator, and one end of the sixth valve close to the liquid inlet of the LNG storage tank is connected with one end of the third valve away from the LNG pump.

[0015] Preferably, the first valve, the second valve, the third valve, the fourth valve, the fifth valve and the sixth valve are electrically controlled valves.

[0016] Preferably, the BOG liquefaction recovery system of the storage tank further comprises a first pressure detection device, a temperature detection device and a controller, 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 connected with the first signal input end and the second signal input end of the controller respectively, 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 connected with the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump and the refrigerator respectively, wherein,

[0017] The first pressure detection device is used for detecting the real-time pressure P in the LNG storage tank;

[0018] The temperature detection device is used for detecting the real-time temperature T in the LNG storage tank;

[0019] The controller is used for outputting 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 realize BOG recovery in the LNG storage tank while maintaining the supercooling degree of the LNG storage tank.

[0020] Preferably, the controller outputs corresponding control instructions according to the real-time pressure P and the real-time temperature T in the LNG tank, and controls 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 the preset control procedure through the control instructions, so that the LNG tank BOG liquefaction recovery system can achieve BOG recovery in the LNG tank while maintaining the subcooling degree of the LNG tank.

[0021] When the real-time pressure P is greater than or equal to a preset LNG tank pressure threshold P2, the LNG tank BOG liquefaction recovery system is controlled to execute a preset BOG liquefaction recovery working procedure, wherein the preset LNG tank pressure threshold P2 is less than or equal to the maximum working pressure of the LNG tank.

[0022] When the real-time pressure P is less than the preset LNG tank pressure threshold P2, and P≤P1+ (1+x%)△P, the LNG tank BOG liquefaction recovery system is controlled to execute a preset LNG tank subcooling degree maintenance working procedure, wherein,

[0023] P1 represents the saturation pressure corresponding to the real-time temperature T at the current time in the LNG tank,△P represents the required subcooling degree of the LNG station, and x% represents a preset subcooling degree allowance.

[0024] Preferably, the preset BOG liquefaction recovery working procedure is as follows:

[0025] The refrigerator and the LNG pump are controlled to start, and the first valve, the second valve and the third valve are controlled to open, so that the BOG in the LNG tank enters the refrigerator through the medium inlet of the refrigerator after passing through the first valve from the gas phase port of the LNG tank, and the BOG is cooled and liquefied into LNG after passing through the medium outlet of the refrigerator, and then enters the LNG buffer tank through the liquid phase inlet of the LNG buffer tank from the second valve, and the LNG pump in the LNG buffer tank pumps the LNG in the LNG buffer tank out through the third valve and recycles it into the LNG tank through the liquid inlet of the LNG tank.

[0026] The preset LNG tank subcooling degree maintenance working procedure is as follows:

[0027] The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the fourth, fifth, and sixth valves. This allows LNG from the LNG storage tank to enter the LNG buffer tank through the outlet of the LNG storage tank and the fourth valve. The LNG pump in the LNG buffer tank extracts the LNG from the buffer tank, which then enters the refrigeration unit through the medium inlet of the refrigeration unit via the fifth valve for cooling. The cooled LNG is then discharged through the medium outlet of the refrigeration unit and enters the LNG storage tank through the inlet of the LNG storage tank via the sixth valve. After mixing with the LNG in the LNG storage tank, the temperature of the LNG in the LNG storage tank is reduced to maintain the subcooling of the LNG storage tank.

[0028] Preferably, 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 the temperature-saturation pressure relationship curve of LNG, wherein the preset saturation pressure calculation model is as follows:

[0029] .

[0030] Preferably, the BOG liquefaction and recovery system in the storage tank further includes a level detection device, which is installed on the LNG buffer tank and is used to detect the LNG level signal in the buffer tank.

[0031] Preferably, the BOG liquefaction and recovery system further includes a second pressure detection device, which is installed on the LNG buffer tank and is used to detect the gas pressure signal inside the LNG storage tank.

[0032] The top of the LNG buffer tank is provided with a gas phase outlet, which 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.

[0033] The second objective of this application is to provide an LNG storage tank BOG liquefaction and recovery system capable of recovering BOG from LNG storage tanks.

[0034] The second objective of this application is achieved through the following technical solution:

[0035] A control method for a tank BOG liquefaction and recovery system as described in any one of the above-mentioned claims, the method comprising the following steps:

[0036] 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.

[0037] S2, control the LNG storage tank BOG liquefaction and recovery system to execute a preset BOG liquefaction and recovery workflow, wherein the preset BOG liquefaction and recovery workflow is as follows:

[0038] The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the first, second, and third valves. This allows the BOG in the LNG storage tank to enter the refrigeration unit through the gas phase port of the LNG storage tank, through the first valve, and then through the medium inlet of the refrigeration unit for cooling. After the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank through the medium outlet of the refrigeration unit, through the second valve, and through the liquid phase inlet of the LNG buffer tank. The LNG pump in the LNG buffer tank extracts the LNG from the LNG buffer tank and returns it to the LNG storage tank through the liquid inlet of the LNG storage tank via the third valve.

[0039] S3, during the execution of the preset BOG liquefaction and recovery process of the LNG storage tank BOG liquefaction and recovery system, the real-time pressure P in the LNG storage tank is continuously monitored, and it is determined whether the real-time pressure P meets the first preset condition. If not, S4 is executed.

[0040] S4, determine whether the real-time pressure P meets the second preset condition; if so, execute S5.

[0041] S5, control the LNG storage tank BOG liquefaction and recovery system to execute a preset LNG storage tank subcooling maintenance workflow, wherein the preset LNG storage tank subcooling maintenance workflow is as follows:

[0042] The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the fourth, fifth, and sixth valves. This allows LNG from the LNG storage tank to enter the LNG buffer tank through the outlet of the LNG storage tank and the fourth valve. The LNG pump in the LNG buffer tank extracts the LNG from the buffer tank, which then enters the refrigeration unit through the medium inlet of the refrigeration unit via the fifth valve for cooling. The cooled LNG is then discharged through the medium outlet of the refrigeration unit and enters the LNG storage tank through the inlet of the LNG storage tank via the sixth valve. After mixing with the LNG in the LNG storage tank, the temperature of the LNG in the LNG storage tank is reduced to maintain the subcooling of the LNG storage tank.

[0043] Preferably, the first preset condition is as follows:

[0044] 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.

[0045] The second preset condition is as follows:

[0046] 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 degree margin.

[0047] The beneficial effects of this application are as follows:

[0048] The LNG storage tank BOG liquefaction and recovery system of this application, when it is necessary to recover BOG from the LNG storage tank, uses a chiller to cool and liquefy the BOG transported from the LNG storage tank before returning it to the LNG storage tank. This achieves the recovery of BOG gas in the LNG storage tank, reduces the pressure of the LNG storage tank, and prevents the release of BOG due to excessive pressure. During the BOG recovery process, when it is necessary to maintain the subcooling of the LNG storage tank, the chiller cools the LNG transported from the LNG storage tank. The cooled LNG is then returned to the LNG storage tank, mixes with the LNG in the tank, and further reduces the temperature of the LNG in the tank to maintain the subcooling. The entire system of this application has a simple structure and can liquefy and recover BOG from the LNG storage tank while maintaining the subcooling of the LNG storage tank. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a schematic diagram of the structure of an LNG storage tank BOG liquefaction and recovery system according to one embodiment of this application;

[0051] Figure 2 for Figure 1 The diagram shown is a block diagram illustrating the control principle of the LNG storage tank BOG liquefaction and recovery system.

[0052] Figure 3 This is a flowchart of a control method for an LNG storage tank BOG liquefaction and recovery system according to one embodiment of this application. Detailed Implementation

[0053] 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 a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0054] In the embodiments provided in this application, it should be understood that the disclosed methods and systems can be implemented in other ways. The system embodiments described below are merely illustrative. For example, the division of units and modules is only a logical functional division, and in actual implementation, there may be other division methods, such as: multiple units or modules can be combined, or integrated into another system, or some features can be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the various components shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, and can be electrical, mechanical, or other forms.

[0055] In addition, each functional unit in the various embodiments of this application can be integrated into a single processor, or each unit can be a separate device, or two or more units can be integrated into a single device; each functional unit in the various embodiments of this application can be implemented in hardware or in the form of hardware plus software functional units.

[0056] Those skilled in the art will understand that all or part of the steps of the following method embodiments can be implemented 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, they perform the steps of the following method embodiments. The aforementioned storage medium includes various media capable of storing program code, such as mobile storage devices, read-only memory (ROM), magnetic disks, or optical disks.

[0057] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "a plurality of" or "several" means two or more, unless otherwise explicitly specified.

[0058] This application provides an LNG storage tank BOG liquefaction and recovery system, such as... Figure 1 , Figure 2As shown, the system may include an LNG storage tank 1, an LNG buffer tank 2, an LNG pump 3, and a refrigeration unit 4, with the LNG pump 3 installed inside the LNG buffer tank 2;

[0059] The bottom of LNG storage tank 1 is provided with a liquid inlet and a liquid outlet, the top of LNG storage tank 1 is provided with a gas phase inlet, the bottom of LNG buffer tank 2 is provided with a liquid phase inlet, and the refrigeration unit 4 is provided with a medium inlet and a medium outlet.

[0060] The gas phase port of LNG storage tank 1 is connected to the medium inlet of refrigeration machine 4 through the first pipeline 5, and the first valve 6 is installed on the first pipeline 5; the medium outlet of refrigeration machine 4 is connected to the liquid phase inlet of LNG buffer tank 2 through the second pipeline 7, and the second valve 8 is installed on the second pipeline 7; the liquid outlet of LNG pump 3 is connected to the liquid inlet of LNG storage tank 1 through the third pipeline 9, and the third valve 10 is installed on the third pipeline 9.

[0061] The outlet of LNG storage tank 1 is connected to the liquid phase inlet of LNG buffer tank 2 via a fourth pipeline 11, and a fourth valve 12 is installed on the fourth pipeline 11. The outlet of LNG pump 3 is connected to the medium inlet of refrigeration unit 4 via a fifth pipeline 13, and a fifth valve 14 is installed on the fifth pipeline 13. The inlet of the fifth valve 14 is connected to the inlet pipe of the third valve 10, and the outlet of the fifth valve 14 is connected to the end of the first valve 6 near the medium inlet of refrigeration unit 4. A sixth pipeline 15 connects the medium outlet of refrigeration unit 4 to the inlet of LNG storage tank 1. A sixth valve 16 is installed on the sixth pipeline 15, and the end of the sixth valve 16 near the medium outlet of refrigeration unit 4 is connected to the end of the second valve 8 near the medium outlet of refrigeration unit 4. The end of the sixth valve 16 near the inlet of LNG storage tank 1 is connected to the end of the third valve 10 away from LNG pump 3.

[0062] The working principle of the LNG storage tank BOG liquefaction and recovery system in this embodiment is as follows:

[0063] When it is necessary to recover BOG in LNG storage tank 1, start the refrigeration unit 4 and LNG pump 3, and open the first valve 6, the second valve 8 and the third valve 10 (the other valves are closed). This allows the BOG in LNG storage tank 1 to enter the refrigeration unit 4 through the gas phase port of LNG storage tank 1, through the first valve 6, and then through the medium inlet of the refrigeration unit 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 refrigeration unit 4, through the second valve 8, and through the liquid phase inlet of the LNG buffer tank 2. The LNG pump 3 in the LNG buffer tank 2 extracts the LNG from the LNG buffer tank 2 and enters the LNG storage tank 1 through the liquid inlet of the LNG storage tank 1 via the third valve 10. Thus, the BOG transported from LNG storage tank 1 is cooled and liquefied by the refrigeration unit 4 and returned to the LNG storage tank 1, realizing the recovery of BOG gas in LNG storage tank 1, reducing the pressure of LNG storage tank 1, and preventing the emission of BOG inside LNG storage tank 1 due to excessive pressure.

[0064] During BOG recovery, as BOG is continuously liquefied and recovered in LNG storage tank 1, the pressure inside LNG storage tank 1 decreases. At this time, the subcooling of LNG storage tank 1 may decrease and become insufficient. Therefore, when it is necessary to maintain the subcooling of LNG storage tank 1, the refrigeration unit 4 and LNG pump 3 are started, and the fourth valve 12, fifth valve 14, and sixth valve 16 are opened (the remaining valves are closed). This allows LNG in LNG storage tank 1 to enter LNG buffer tank 2 through the outlet of LNG storage tank 1 and the fourth valve 12. LNG pump 3 in LNG buffer tank 2 then pumps LNG into LNG buffer tank 2. The LNG is extracted from the tank and enters the refrigeration unit 4 through the medium inlet of the refrigeration unit 4 via the fifth valve 14 for cooling. The cooled LNG is discharged through the medium outlet of the refrigeration unit 4 and enters the LNG storage tank 1 through the liquid inlet of the LNG storage tank 1 via the sixth valve 16. After mixing with the LNG in the LNG storage tank 1, the temperature of the LNG in the LNG storage tank 1 is reduced. Since the LNG temperature drops, the saturation pressure of the LNG storage tank 1 will also drop. Therefore, the subcooling of the LNG storage tank 1 will increase rapidly at this time, so that the subcooling of the LNG storage tank 1 is maintained within the range required for the operation of the LNG station.

[0065] The system structure of this application embodiment is simple, and it can liquefy and recover BOG in LNG storage tank 1 and maintain the subcooling of LNG storage tank 1.

[0066] 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 system's automation level is effectively improved, enabling remote automatic control of LNG storage tank BOG liquefaction recovery and subcooling maintenance. Specifically, in this embodiment, each electrically controlled valve is an electro-pneumatic valve.

[0067] In one embodiment, the BOG liquefaction and recovery system for the storage tank further includes a first pressure detection device 19, a temperature detection device 20, and a controller 21. Both the first pressure detection device 19 and the temperature detection device 20 are installed on the LNG storage tank. The signal output terminals of the first pressure detection device 19 and the temperature detection device 20 are respectively connected to the first signal input terminal and the second signal input terminal of the controller 21. The first control terminal, second control terminal, third control terminal, fourth control terminal, fifth control terminal, sixth control terminal, seventh control terminal, and eighth control terminal 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 refrigeration unit 4.

[0068] The first pressure detection device 19 is used to detect the real-time pressure P inside the LNG storage tank 1;

[0069] Temperature detection device 20, which is used to detect the real-time temperature T inside LNG storage tank 1;

[0070] The controller 21 is used to output corresponding control commands based on the real-time pressure P and real-time temperature T in the LNG storage tank 1, so as to control the working status 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 refrigeration unit 4 through the control commands, so that the LNG storage tank BOG liquefaction and recovery system can realize the recovery of BOG in the LNG storage tank 1 while maintaining the subcooling of the LNG storage tank 1.

[0071] In this embodiment, a first pressure detection device 19, a temperature detection device 20, and a controller 21 are provided. The first pressure detection device 19 detects the real-time pressure P inside the LNG storage tank 1, and the temperature detection device 20 detects the real-time temperature T inside the LNG storage tank 1. The controller 21 can output corresponding control commands based on the real-time pressure P and real-time temperature T inside the LNG storage tank 1 to control the working status 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 chiller 4. Thus, the system can automatically switch between the two control processes of BOG liquefaction and recovery and the maintenance of subcooling of LNG storage tank 1 based on the real-time detection signals of the first pressure detection device 19 and the temperature detection device 20.

[0072] Specifically, in this embodiment, the controller 21 is a PLC, the first pressure detection device 19 is a pressure transmitter, and the temperature detection device 20 is a temperature sensor.

[0073] In one embodiment, the controller 21 executes control commands based on the real-time pressure P and real-time temperature T within the LNG storage tank 1. These commands control the operating states of the first valve 6, second valve 8, third valve 10, fourth valve 12, fifth valve 14, sixth valve 16, LNG pump 3, and chiller 4 according to a preset control flow. Specifically, this allows the LNG storage tank BOG liquefaction and recovery system to achieve BOG recovery within the LNG storage tank 1 while simultaneously maintaining the subcooling of the LNG storage tank 1.

[0074] When the real-time pressure P is greater than or equal to the preset pressure threshold P2 of LNG storage tank 1, the LNG storage tank BOG liquefaction and recovery system is controlled to execute the preset BOG liquefaction and recovery workflow, wherein the preset pressure threshold P2 of LNG storage tank 1 is less than or equal to the maximum working pressure of LNG storage tank 1.

[0075] When the real-time pressure P is less than the preset pressure threshold P2 of LNG storage tank 1, and P≤P1+(1+x%)△P, the LNG storage tank BOG liquefaction and recovery system is controlled to execute the preset subcooling maintenance workflow of LNG storage tank 1.

[0076] P1 represents the saturation pressure corresponding to the real-time temperature T inside LNG storage tank 1 at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling degree margin.

[0077] In this embodiment, by setting a pressure threshold P2 for LNG storage tank 1, when the real-time pressure P is greater than or equal to the preset pressure threshold P2 for LNG storage tank 1, it indicates that there is a lot of BOG in LNG storage tank 1, resulting in excessive pressure in LNG storage tank 1. If BOG is not recovered, the pressure in LNG storage tank 1 will be higher than the maximum working pressure of LNG storage tank 1, resulting in BOG emission. Therefore, when the real-time pressure P is greater than or equal to the preset pressure threshold P2 for LNG storage tank 1, the LNG storage tank BOG liquefaction and recovery system is controlled to execute the preset BOG liquefaction and recovery workflow to realize the recovery of BOG in LNG storage tank 1.

[0078] During BOG recovery, the real-time pressure P in LNG storage tank 1 will decrease, which may lead to insufficient subcooling of LNG storage tank 1. When the real-time pressure P is less than the preset pressure threshold P2 of LNG storage tank 1, it means that the pressure of 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 subcooling of LNG storage tank 1 corresponding to the real-time pressure P cannot meet the working requirements of the LNG station, which will cause cavitation of the submersible pump. At this time, the LNG storage tank BOG liquefaction and recovery system is controlled to execute the preset subcooling maintenance process of LNG storage tank 1 to increase the subcooling of LNG storage tank 1 and keep the subcooling of LNG storage tank 1 within the range required for normal operation.

[0079] Specifically, the preset pressure threshold P2 of LNG storage tank 1 is set according to actual needs. In this embodiment, the preset pressure threshold P2 of LNG storage tank 1 is between the maximum working pressure of LNG storage tank 1 minus 0.2MPa and the maximum working pressure of LNG storage tank 1. That is, if the maximum working pressure of LNG storage tank 1 is P0, then P0-0.2MPa≤P2≤P0.

[0080] In one embodiment, the preset BOG liquefaction and recycling workflow is as follows:

[0081] The system controls the start of the refrigeration unit 4 and the LNG pump 3, and controls the opening of the first valve 6, the second valve 8 and the third valve 10, so that the BOG in the LNG storage tank 1 enters the refrigeration unit 4 through the gas phase port of the LNG storage tank 1, passes through the first valve 6 and enters the refrigeration unit 4 through the medium inlet to cool down. After the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank 2 through the medium outlet of the refrigeration unit 4, passes through the second valve 8 and enters the LNG buffer tank 2 through the liquid phase inlet. The LNG pump 3 in the LNG buffer tank 2 extracts the LNG in the LNG buffer tank 2 and returns it to the LNG storage tank 1 through the liquid inlet of the LNG storage tank 1 via the third valve 10.

[0082] The pre-defined subcooling maintenance process for LNG storage tank 1 is as follows:

[0083] The system controls the start of the refrigeration unit 4 and the LNG pump 3, and controls the opening of the fourth valve 12, the fifth valve 14 and the sixth valve 16, so that the LNG in the LNG storage tank 1 enters the LNG buffer tank 2 through the outlet of the LNG storage tank 1 and the fourth valve 12. The LNG pump 3 in the LNG buffer tank 2 draws out the LNG in the LNG buffer tank 2 and enters the refrigeration unit 4 through the medium inlet of the refrigeration unit 4 through the fifth valve 14 for cooling. The cooled LNG is discharged through the medium outlet of the refrigeration unit 4 and enters the LNG storage tank 1 through the inlet of the LNG storage tank 1 through the sixth valve 16. After mixing with the LNG in the LNG storage tank 1, the temperature of the LNG in the LNG storage tank 1 is reduced to maintain the subcooling of the LNG storage tank 1.

[0084] In one embodiment, the saturated pressure P1 corresponding to the real-time temperature T in 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 the temperature-saturated pressure relationship curve of LNG. The preset saturated pressure calculation model is as follows:

[0085] .

[0086] In this embodiment, a saturation pressure calculation model is constructed based on the temperature-saturation pressure relationship curve of LNG, and then the saturation pressure P1 corresponding to the current temperature T can be accurately calculated using this model.

[0087] In one embodiment, the BOG liquefaction and recovery system for the storage tank further includes a level detection device 23, which is installed on the LNG buffer tank 2 and is used to detect the LNG level signal in the buffer tank.

[0088] In this embodiment, a liquid level detection device 23 is installed 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 and recovery process and the preset LNG storage tank 1 subcooling maintenance process, in order to avoid the LNG pump 3 being damaged due to insufficient liquid level in the LNG buffer tank 2, when the liquid level of the LNG buffer tank 2 is detected to reach the preset high liquid level threshold, the controller 21 interlocks and starts the LNG pump 3. The pumped LNG enters the LNG storage tank 1 through the liquid inlet at the bottom of the LNG storage tank 1. When the liquid level of the LNG buffer tank 2 is detected to reach the preset low liquid level threshold, the controller 21 interlocks and shuts down the LNG pump 3, realizing the automatic control of the start and stop of the LNG pump 3, and improving the automation level of the BOG liquefaction and recovery device.

[0089] Specifically, in this embodiment, the liquid level detection device 23 uses a liquid level sensor.

[0090] In one embodiment, the BOG liquefaction and recovery system for the storage tank further includes a second pressure detection device 22, which is installed on the LNG buffer tank 2 and is used to detect the gas pressure signal inside the LNG storage tank 1.

[0091] The top of the LNG buffer tank 2 is provided with a gas phase outlet. The gas phase outlet of the LNG buffer tank 2 is connected to the gas phase port of the LNG storage tank 1 through the seventh pipeline 17. The seventh valve 18 is installed on the seventh pipeline 17.

[0092] In this embodiment, a second pressure detection device 22 is installed on the LNG buffer tank 2 to detect the gas pressure signal of the LNG buffer tank 2. When the pressure of the LNG buffer tank 2 reaches the preset high pressure threshold, the controller 21 interlocks and opens the seventh valve 18. BOG in the LNG buffer tank 2 enters the LNG storage tank 1 through the gas phase outlet of the LNG buffer tank 2 and the seventh valve 18, thereby preventing the pressure of the LNG buffer tank 2 from being too high and realizing automatic control of depressurization of the LNG buffer tank 2.

[0093] It should be noted that before starting the refrigeration unit 4 to cool and liquefy the BOG in the LNG storage tank 1 or to maintain the subcooling of the LNG storage tank 1, the LNG buffer tank 2 and pipelines can be pre-cooled. This ensures that the liquid in the LNG buffer tank 2 can submerge the LNG pump 3 when the refrigeration unit 4 is started, meeting the liquid level required for the operation of the LNG pump 3. It also avoids the large amount of LNG vaporization caused by the excessively high temperature of the LNG buffer tank 2 during the initial stage of the refrigeration unit 4 startup, which could cause cavitation on the LNG pump 3.

[0094] The pre-cooling process is as follows:

[0095] Before the refrigeration unit 4 is started, the fourth valve 12 and the seventh valve 18 are opened. Under the action of gravity, LNG in LNG storage tank 1 flows out from the bottom outlet and enters LNG buffer tank 2 through the fourth valve 12. During the precooling process, the BOG generated in the LNG tank flows from the gas phase outlet at the top of LNG buffer tank 2 through the seventh valve 18 to the gas phase port at the top of LNG storage tank 1 and enters LNG storage tank 1, thereby achieving the precooling of LNG storage tank 1 and part of the pipeline.

[0096] like Figure 3 As shown in the embodiments of this application, a control method for the storage tank BOG liquefaction and recovery system in any of the above embodiments is also provided. The method may include the following steps:

[0097] 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.

[0098] S2 controls the LNG storage tank BOG liquefaction and recovery system to execute a preset BOG liquefaction and recovery workflow, which is as follows:

[0099] The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the first, second, and third valves. This allows the BOG in the LNG storage tank to enter the refrigeration unit through the gas phase port of the LNG storage tank, through the first valve, and then through the medium inlet of the refrigeration unit for cooling. After the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank through the medium outlet of the refrigeration unit, through the second valve, and through the liquid phase inlet of the LNG buffer tank. The LNG pump in the LNG buffer tank extracts the LNG from the LNG buffer tank and returns it to the LNG storage tank through the liquid inlet of the LNG storage tank via the third valve.

[0100] S3, during the execution of the preset BOG liquefaction and recovery process of the LNG storage tank BOG liquefaction and recovery system, continuously 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 not, execute S4.

[0101] S4, determine whether the real-time pressure P meets the second preset condition; if so, execute S5.

[0102] S5 controls the LNG storage tank BOG liquefaction and recovery system to execute a preset LNG storage tank subcooling maintenance workflow, wherein the preset LNG storage tank subcooling maintenance workflow is as follows:

[0103] The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the fourth, fifth, and sixth valves. This allows LNG from the LNG storage tank to enter the LNG buffer tank through the outlet of the LNG storage tank and the fourth valve. The LNG pump in the LNG buffer tank extracts the LNG from the buffer tank, which then enters the refrigeration unit through the medium inlet of the refrigeration unit via the fifth valve for cooling. The cooled LNG is then discharged through the medium outlet of the refrigeration unit and enters the LNG storage tank through the inlet of the LNG storage tank via the sixth valve. After mixing with the LNG in the LNG storage tank, the temperature of the LNG in the LNG storage tank is reduced to maintain the subcooling of the LNG storage tank.

[0104] The above steps S1 to S5 are executed cyclically to achieve automatic switching between BOG recovery and LNG tank subcooling maintenance, so that while reducing LNG tank pressure through BOG recovery, the subcooling of the LNG tank is maintained within the normal range.

[0105] In one embodiment, the first preset condition is as follows:

[0106] P≥P2, where P2 represents the 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.

[0107] The second preset condition is as follows:

[0108] 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 degree margin.

[0109] The control method of the BOG liquefaction and recovery system in this embodiment is based on the BOG liquefaction and recovery system in any of the above embodiments. Therefore, the control method of the BOG liquefaction and recovery system has the same working principle and technical effect as the BOG liquefaction and recovery system in the above embodiments, and will not be described again here.

[0110] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0111] Those skilled in the art will further recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components and steps of the various examples have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0112] The steps of the methods or algorithms described in conjunction with the embodiments disclosed herein can be implemented directly using hardware, a software module executed by a processor, or a combination of both. The software module can be located in random access memory (RAM), main memory, read-only memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, hard disk, removable disk, CD-ROM, or any other form of storage medium known in the art.

[0113] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily 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 this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An LNG storage tank BOG liquefaction and recovery system, characterized in that, It includes an LNG storage tank, an LNG buffer tank, an LNG pump, and a refrigeration unit, wherein the LNG pump is installed inside the LNG buffer tank; The LNG storage tank is provided with an inlet and an outlet at the bottom, a gas phase inlet at the top, a liquid phase inlet at the bottom of the LNG buffer tank, and a medium inlet and a medium outlet at the refrigeration unit. The gas phase port of the LNG storage tank is connected to the medium inlet of the refrigeration unit through a first pipeline, and a first valve is installed on the first pipeline; the medium outlet of the refrigeration unit 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 outlet of the LNG storage tank is connected to the liquid phase inlet of the LNG buffer tank via a fourth pipeline, and a fourth valve is installed on the fourth pipeline. The outlet of the LNG pump is connected to the medium inlet of the refrigeration unit via a fifth pipeline, and a fifth valve is installed on the fifth pipeline. The inlet of the fifth valve is connected to the inlet pipe of the third valve, and the outlet of the fifth valve is connected to the end of the first valve near the medium inlet of the refrigeration unit. A sixth pipeline connects the medium outlet of the refrigeration unit to the inlet of the LNG storage tank. A sixth valve is installed on the sixth pipeline, and the end of the sixth valve near the medium outlet of the refrigeration unit is connected to the end of the second valve near the medium outlet of the refrigeration unit. The end of the sixth valve near the inlet of the LNG storage tank is connected to the end of the third valve away from the LNG pump. 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 first pressure detection device is used to detect the real-time pressure P inside the LNG storage tank. The temperature detection device is used to detect the real-time temperature T inside the LNG storage tank.

2. The BOG liquefaction and recovery system for storage tanks 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 BOG liquefaction and recovery system for storage tanks according to claim 2, characterized in that, The signal output terminals of the first pressure detection device and the temperature detection device are respectively connected to the first signal input terminal and the second signal input terminal of the controller. The first control terminal, the second control terminal, the third control terminal, the fourth control terminal, the fifth control terminal, the sixth control terminal, the seventh control terminal, and the eighth control terminal 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 refrigeration unit. The controller is used to output corresponding control commands based on the real-time pressure P and real-time temperature T in the LNG storage tank, so as to control the working status of the first valve, second valve, third valve, fourth valve, fifth valve, sixth valve, LNG pump and refrigeration unit through the control commands, so that the LNG storage tank BOG liquefaction and recovery system can realize the recovery of BOG in the LNG storage tank while maintaining the subcooling of the LNG storage tank.

4. The BOG liquefaction and recovery system for storage tanks according to claim 3, characterized in that, The controller executes corresponding control commands based on the real-time pressure P and real-time temperature T inside the LNG storage tank. These commands, in accordance with a preset control flow, control the operating status of the first valve, second valve, third valve, fourth valve, fifth valve, sixth valve, LNG pump, and chiller. Specifically, the LNG storage tank BOG liquefaction and recovery system, while recovering BOG from the LNG storage tank, also maintains the subcooling of the LNG storage tank. When the real-time pressure P is greater than or equal to the preset LNG tank pressure threshold P2, the LNG tank BOG liquefaction and recovery system is controlled to execute the preset BOG liquefaction and recovery workflow, wherein the preset LNG tank pressure threshold 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 P2, and P≤P1+(1+x%)△P, the LNG tank BOG liquefaction and recovery system is controlled to execute the preset LNG tank subcooling maintenance workflow, wherein... P1 represents the saturation pressure corresponding to the real-time temperature T inside the LNG storage tank at the current moment, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling degree margin.

5. The BOG liquefaction and recovery system for storage tanks according to claim 4, characterized in that, The preset BOG liquefaction and recycling process is as follows: The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the first, second, and third valves. This allows the BOG in the LNG storage tank to enter the refrigeration unit through the gas phase port of the LNG storage tank, through the first valve, and then through the medium inlet of the refrigeration unit for cooling. After the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank through the medium outlet of the refrigeration unit, through the second valve, and through the liquid phase inlet of the LNG buffer tank. The LNG pump in the LNG buffer tank extracts the LNG from the LNG buffer tank and returns it to the LNG storage tank through the liquid inlet of the LNG storage tank via the third valve. The pre-defined LNG storage tank subcooling maintenance process is as follows: The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the fourth, fifth, and sixth valves. This allows LNG from the LNG storage tank to enter the LNG buffer tank through the outlet of the LNG storage tank and the fourth valve. The LNG pump in the LNG buffer tank extracts the LNG from the buffer tank, which then enters the refrigeration unit through the medium inlet of the refrigeration unit via the fifth valve for cooling. The cooled LNG is then discharged through the medium outlet of the refrigeration unit and enters the LNG storage tank through the inlet of the LNG storage tank via the sixth valve. After mixing with the LNG in the LNG storage tank, the temperature of the LNG in the LNG storage tank is reduced to maintain the subcooling of the LNG storage tank.

6. The BOG liquefaction and recovery system for storage tanks according to claim 4, characterized in that, The saturation pressure P1 corresponding to the current real-time temperature T inside the LNG storage tank 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 the relationship curve between LNG temperature and saturation pressure. The preset saturation pressure calculation model is as follows: 。 7. The BOG liquefaction and recovery system for storage tanks 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 the liquid level signal of the LNG in the buffer tank.

8. The tank BOG liquefaction and recovery system according to any one of claims 1-7, characterized in that, It also includes a second pressure detection device, which is installed on the LNG buffer tank and is used to detect the gas pressure signal inside the LNG storage tank. The top of the LNG buffer tank is provided with a gas phase outlet, which 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 tank BOG liquefaction and recovery system according to any one of claims 1-8, characterized in that, The method includes the following steps: 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. S2, control the LNG storage tank BOG liquefaction and recovery system to execute a preset BOG liquefaction and recovery workflow, wherein the preset BOG liquefaction and recovery workflow is as follows: The system controls the start-up of the refrigeration unit and LNG pump, and controls the opening of the first, second, and third valves. This allows the BOG in the LNG storage tank to enter the refrigeration unit through the gas phase port of the LNG storage tank, through the first valve, and then through the medium inlet of the refrigeration unit for cooling. After the BOG is cooled and liquefied into LNG, it enters the LNG buffer tank through the medium outlet of the refrigeration unit, through the second valve, and through the liquid phase inlet of the LNG buffer tank. The LNG pump in the LNG buffer tank extracts the LNG from the LNG buffer tank and returns it to the LNG storage tank through the liquid inlet of the LNG storage tank via the third valve. S3, during the execution of the preset BOG liquefaction and recovery process of the LNG storage tank BOG liquefaction and recovery system, the real-time pressure P in the LNG storage tank is continuously monitored, and it is determined whether the real-time pressure P meets the first preset condition. If not, S4 is executed. S4, determine whether the real-time pressure P meets the second preset condition; if so, execute S5. S5, control the LNG storage tank BOG liquefaction and 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 system controls the start of the refrigeration unit and LNG pump, and controls the opening of the fourth, fifth and sixth valves. This allows LNG in the LNG storage tank to enter the LNG buffer tank through the outlet of the LNG storage tank and the fourth valve. The LNG pump in the LNG buffer tank extracts the LNG from the LNG buffer tank and enters the refrigeration unit through the medium inlet of the refrigeration unit via the fifth valve for cooling. The cooled LNG is discharged through the medium outlet of the refrigeration unit and enters the LNG storage tank through the inlet of the LNG storage tank via the sixth valve. After mixing with the LNG in the LNG storage tank, the temperature of the LNG in the LNG storage tank is reduced to maintain the subcooling of the LNG storage tank. 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 degree margin.

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