A supercooling degree maintaining system for an LNG storage tank and a control method thereof

Abstract

This application discloses an LNG storage tank subcooling maintenance system and its control method. The LNG storage tank subcooling maintenance system includes an LNG storage tank, an LNG pump pool, an LNG pump, and a chiller. The LNG pump is installed in the LNG pump pool. The LNG storage tank has a liquid phase outlet and a bottom inlet at the bottom, and a top inlet at the top. The LNG pump pool has a liquid phase inlet at the bottom. When the system is working, the LNG in the LNG storage tank is pumped to the chiller for cooling. The cooled LNG returns to the bottom inlet of the LNG storage tank to further reduce the LNG temperature inside the tank, thereby maintaining the subcooling of the LNG storage tank. Alternatively, it returns to the top inlet to lower the temperature of some of the BOG (Body-Off Gas) inside the LNG storage tank and liquefy it, ensuring that the LNG storage tank does not overpressure. This application can cool and depressurize the LNG storage tank, thereby maintaining the subcooling of the LNG storage tank within the LNG inlet subcooling range required for the operation of the submersible pump in the LNG station, effectively preventing cavitation caused by insufficient subcooling of the LNG drawn into the submersible pump.

Description

Technical Field

[0001] This application relates to the cooling of media inside LNG storage tanks, and in particular to an LNG storage tank subcooling maintenance system and its control method. Background Technology

[0002] Currently, LNG stations (including LNG refueling stations, L-CNG refueling stations, LNG vaporization stations, and LNG storage stations) are all equipped with LNG storage tanks as LNG storage devices. LNG has an extremely low temperature, which can be as low as -162℃. During the storage process, the LNG storage tank will continuously absorb heat from the environment, and LNG will partially vaporize to form BOG (Boil Off Gas) stored in the LNG storage tank. This causes the pressure of the LNG storage tank to gradually increase. Once the maximum working pressure of the LNG storage tank is reached, emissions will occur.

[0003] When LNG is stored in a closed, insulated container, it often exists as a boiling liquid and is in a state of temperature-pressure equilibrium. The pressure in this equilibrium state is called the saturation pressure at that temperature. The relationship between temperature and saturation pressure of an LNG storage tank is shown in the figure below. Figure 1 As shown.

[0004] pass Figure 1 It is known that when the pressure in an LNG storage tank is below its saturation pressure, LNG will accelerate vaporization until a new equilibrium is reached. When the pressure in an LNG storage tank is above its saturation pressure, LNG evaporation is suppressed, and BOG liquefaction is intensified until a new equilibrium is reached. We call the portion of the LNG storage tank pressure above its saturation pressure subcooling (or net positive head). That is, even if LNG with subcooling absorbs heat or experiences a partial pressure drop, as long as the pressure is above its saturation pressure, LNG will not produce BOG.

[0005] Submersible pumps are commonly used as pressurization equipment in LNG terminals. As centrifugal pumps, they are susceptible to cavitation if the liquid contains gas. To prevent the generation of bulky gas (BOG), the LNG at the submersible pump inlet must have sufficient subcooling to offset the pressure drop at the pump suction inlet.

[0006] Currently, LNG terminal BOG recovery processes all involve extracting BOG from LNG storage tanks for reuse, in order to reduce the pressure in the LNG storage tanks. However, LNG in LNG storage tanks is often in a saturated state during storage. Extracting BOG from the LNG storage tanks will cause the LNG storage tank pressure to fall below the saturation pressure at the corresponding temperature, resulting in insufficient subcooling of the LNG drawn into the submersible pump and causing cavitation. Summary of the Invention

[0007] To address the aforementioned technical problems, this application provides an LNG storage tank subcooling maintenance system and its control method, which can cool and depressurize the LNG storage tank, thereby maintaining the subcooling of the LNG storage tank within the LNG inlet subcooling range required for the operation of the LNG station's submersible pump. This effectively prevents cavitation caused by insufficient subcooling of the LNG drawn into the submersible pump, ensuring the normal operation of the LNG station.

[0008] The first objective of this application is to provide a subcooling maintenance system for LNG storage tanks.

[0009] The aforementioned objective of this application is achieved through the following technical solution:

[0010] An LNG storage tank subcooling maintenance system includes an LNG storage tank, an LNG pump pool, an LNG pump, and a chiller, wherein the LNG pump is installed in the LNG pump pool;

[0011] The LNG storage tank is equipped with a liquid phase outlet and a bottom inlet at the bottom, and a top inlet at the top.

[0012] The bottom of the LNG pump tank is equipped with a liquid phase inlet;

[0013] A liquid outlet pipe is connected between the liquid phase outlet of the LNG storage tank and the liquid phase inlet of the LNG pump pool, and a first valve is installed on the liquid outlet pipe.

[0014] A liquid phase pipeline connects the outlet of the LNG pump to the inlet of the refrigeration unit, and a second valve is installed on the liquid phase pipeline. The outlet of the refrigeration unit is connected to the inlet of the return liquid main, and a third valve is installed on the return liquid main.

[0015] The outlet end of the main return pipe is connected to the inlet end of the first return branch pipe and the second return branch pipe. The outlet end of the first return branch pipe is connected to the bottom inlet of the LNG storage tank. A fourth valve is installed on the first return branch pipe. The outlet end of the second return branch pipe is connected to the top inlet of the LNG storage tank. A fifth valve is installed on the second return branch pipe.

[0016] in,

[0017] When the system is working, the LNG in the LNG storage tank is pumped to the refrigeration unit for cooling. The cooled LNG is returned to the bottom inlet of the LNG storage tank to reduce the temperature of the LNG in the LNG storage tank and thus maintain the subcooling of the LNG storage tank, or it is returned to the top inlet of the LNG storage tank to reduce the temperature of some of the BOG in the LNG storage tank and liquefy it to ensure that the LNG storage tank does not overpressure.

[0018] Preferably, the LNG storage tank is provided with a gas phase inlet at the top, the LNG pump pool is provided with a gas phase outlet at the top, a return gas pipeline is connected between the gas phase outlet of the LNG pump pool and the gas phase inlet of the LNG storage tank, and a sixth valve is installed on the return gas pipeline.

[0019] 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.

[0020] Preferably, the LNG storage tank subcooling maintenance system further includes a pressure transmitter, a thermometer, and a controller.

[0021] The signal output terminals of the pressure transmitter and the thermometer are respectively connected to the input terminal of the controller. The first, second, third, fourth, fifth, sixth, seventh, and eighth control terminals of the controller are respectively connected to the first, second, third, fourth, fifth, and sixth valves, the LNG pump, and the refrigeration unit.

[0022] in,

[0023] The pressure transmitter is installed on the LNG storage tank and is used to detect the LNG storage tank pressure P. 实 ;

[0024] The thermometer is installed inside the LNG pump pool and is used to detect the temperature T of the LNG entering the LNG pump pool. 实 ;

[0025] The controller is used to adjust the LNG storage tank pressure P. 实 and LNG temperature T 实 The operating status of the first valve, second valve, third valve, fourth valve, fifth valve, sixth valve, LNG pump, and refrigeration unit is controlled to maintain the subcooling of the LNG storage tank.

[0026] The second objective of this application is to provide a control method for the LNG storage tank subcooling maintenance system described in the first objective above.

[0027] The second objective of this application is achieved through the following technical solution:

[0028] A control method for an LNG storage tank subcooling maintenance system includes the following steps:

[0029] S1, control the LNG pump and the refrigeration unit to start, and control the first valve and the second valve to open, so that the LNG in the LNG storage tank enters the LNG pump pool through the first control valve, and enters the refrigeration unit through the second valve under the suction action of the LNG pump, and the refrigeration unit cools down the LNG;

[0030] S2, during the operation of the refrigeration unit, the pressure P of the LNG storage tank is monitored in real time by the pressure transmitter. 实 And the temperature T of the LNG entering the LNG pump pool is detected in real time by the thermometer. 实 ;

[0031] S3, based on the LNG storage tank pressure P 实 and LNG temperature T 实 The opening and closing states of the third, fourth, and fifth valves are controlled according to a preset control strategy to control the LNG cooled by the refrigeration unit to return to the bottom inlet of the LNG storage tank through the third and fourth valves, thereby maintaining the subcooling of the LNG storage tank, or to return to the top inlet of the LNG storage tank through the third and fifth valves, thereby avoiding overpressure in the storage tank.

[0032] Preferably, in step S3, the step of adjusting the pressure P of the LNG storage tank... 实 and LNG temperature T 实 Controlling the opening and closing states of the third, fourth, and fifth valves according to a preset control strategy includes:

[0033] Based on the LNG storage tank pressure P 实 and LNG temperature T 实 The corresponding return liquid control program is selected to control the opening and closing states of the third, fourth, and fifth valves. The return liquid control program includes a top return liquid control program and a bottom return liquid control program.

[0034] When the selected return liquid control program is the top return liquid control program, the third valve and the fifth valve are opened so that the LNG cooled by the refrigeration unit returns to the top inlet of the LNG storage tank through the third valve and the fifth valve, thereby reducing the temperature of some BOG in the LNG storage tank and reducing the pressure of the LNG storage tank to avoid overpressure of the storage tank.

[0035] When the selected return liquid control program is the bottom return liquid control program, the third valve and the fourth valve are opened so that the LNG cooled by the refrigeration unit returns to the bottom inlet of the LNG storage tank through the third valve and the fourth valve. This reduces the temperature of the LNG in the LNG storage tank by mixing with the LNG in the LNG storage tank, thereby maintaining the subcooling of the LNG storage tank.

[0036] Preferably, the step of adjusting the pressure P of the LNG storage tank... 实 and LNG temperature T 实 Selecting the corresponding return liquid control program to control the opening and closing states of the third, fourth, and fifth valves includes:

[0037] S31, based on the preset saturation pressure calculation model and the LNG temperature T 实 Calculate the saturation pressure P1 corresponding to the LNG temperature T.

[0038] S32, based on the saturation pressure P1 and the LNG storage tank pressure P 实 The corresponding return control program is selected based on the preset LNG storage tank pressure threshold P2 to control the opening and closing states of the third, fourth, and fifth valves.

[0039] Preferably, in step S31, 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:

[0040] P1 = exp(39.205 - 1324.4 / T) 实 -3.4366*LnT 实 +0.000031019*T 实 2 ).

[0041] Preferably, in step S32, the pressure based on the saturation pressure P1 and the LNG storage tank pressure P... 实 The corresponding return control program selected based on the preset LNG tank pressure threshold P2 includes:

[0042] When P 实 When ≥P2, select the top return liquid control program;

[0043] When P 实 When <P2,

[0044] If P 实 If ≤P1+(1+x%)△P, then select the bottom return control program.

[0045] If P 实 If the value is greater than P1 + (1 + x%)ΔP, then the top return liquid control program is selected;

[0046] Where △P represents the required subcooling of the LNG station, and x% represents the preset subcooling margin.

[0047] Preferably, the control method for the LNG storage tank subcooling maintenance system further includes the following steps:

[0048] S0, before the LNG pump and the refrigeration unit are started, the LNG pump pool is pre-cooled. The pre-cooling process is as follows:

[0049] When the first and sixth valves are opened, the LNG in the LNG storage tank flows out from the liquid phase outlet under the action of gravity, enters the LNG pump pool through the first valve, and the gas in the LNG pump pool returns to the gas phase inlet of the LNG storage tank through the sixth valve.

[0050] In summary, this application discloses an LNG storage tank subcooling maintenance system and its control method. The system comprises an LNG storage tank, an LNG pump pool, an LNG pump, and a chiller. The LNG pump is installed in the LNG pump pool. The LNG storage tank has a liquid phase outlet and a bottom inlet at its bottom, and a top inlet at its top. The LNG pump pool has a liquid phase inlet at its bottom. During system operation, the LNG in the storage tank is pumped to the chiller for cooling. The cooled LNG returns to the bottom inlet of the storage tank to further reduce the LNG temperature, or returns to the top inlet to lower the temperature of some of the BOG (Boiled Air Gaseous) and liquefy it, thereby reducing the LNG storage tank pressure. This cooling and depressurization of the LNG storage tank maintains the subcooling within the range required for the submersible pumps of the LNG station to operate without overpressure. This effectively prevents cavitation caused by insufficient subcooling of the LNG inlet, ensuring the normal operation of the LNG station. Attached Figure Description

[0051] 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.

[0052] Figure 1 A graph showing the relationship between temperature and saturation pressure in an LNG storage tank.

[0053] Figure 2 This is a schematic diagram of the structure of an LNG storage tank subcooling maintenance system according to one embodiment of this application;

[0054] Figure 3This is a block diagram illustrating the control principle of an LNG storage tank subcooling maintenance system according to one embodiment of this application.

[0055] Figure 4 This is a flowchart of a control method for an LNG storage tank subcooling maintenance system according to an embodiment of this application. Detailed Implementation

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] This application provides an LNG storage tank subcooling maintenance system, such as... Figure 2-3 As shown, the system may include an LNG storage tank 1, an LNG pump pool 2, an LNG pump 3, and a refrigeration unit 4, with the LNG pump 3 installed in the LNG pump pool 2;

[0062] LNG storage tank 1 is equipped with a liquid phase outlet and a bottom inlet at the bottom, and a top inlet at the top.

[0063] The bottom of LNG pump tank 2 is equipped with a liquid phase inlet;

[0064] A liquid outlet pipe 5 is connected between the liquid phase outlet of LNG storage tank 1 and the liquid phase inlet of LNG pump pool 2. A first valve 6 is installed on the liquid outlet pipe 5.

[0065] A liquid phase pipe 7 connects the outlet of LNG pump 3 to the inlet of refrigeration unit 4. A second valve 8 is installed on the liquid phase pipe 7. The outlet of refrigeration unit 4 is connected to the inlet of return main pipe 9. A third valve 10 is installed on return main pipe 9.

[0066] The outlet of the return main pipe 9 is connected to the inlet of the first return branch pipe 11 and the second return branch pipe 12. The outlet of the first return branch pipe 11 is connected to the bottom inlet of the LNG storage tank 1. A fourth valve 13 is installed on the first return branch pipe 11. The outlet of the second return branch pipe 12 is connected to the top inlet of the LNG storage tank 1. A fifth valve 14 is installed on the second return branch pipe 12.

[0067] in,

[0068] When the system is working, the LNG in LNG storage tank 1 is transported to the refrigeration unit 4 by LNG pump 3 for cooling. The cooled LNG is returned to the bottom inlet of LNG storage tank 1 to reduce the temperature of LNG in LNG storage tank 1, thereby maintaining the subcooling of LNG storage tank 1, or returned to the top inlet of LNG storage tank 1 to reduce the temperature of some BOG in LNG storage tank 1 and liquefy it to reduce the pressure of LNG storage tank, thereby ensuring that LNG storage tank 1 does not overpressure.

[0069] The LNG storage tank subcooling maintenance system of this application embodiment includes an LNG storage tank 1, an LNG pump pool 2, an LNG pump 3, and a chiller 4. The LNG pump 3 is installed in the LNG pump pool 2. The bottom of the LNG storage tank 1 is provided with a liquid phase outlet and a bottom liquid inlet, and the top of the LNG storage tank 1 is provided with a top liquid inlet. The bottom of the LNG pump pool 2 is provided with a liquid phase inlet. An outlet pipe 5 is connected between the liquid phase outlet of the LNG storage tank 1 and the liquid phase inlet of the LNG pump pool 2. A first valve 6 is installed on the outlet pipe 5. A liquid phase pipe 7 is connected between the outlet of the LNG pump 3 and the inlet of the chiller 4. A second valve 8 is installed on the liquid phase pipe 7. The outlet of the chiller 4 is connected to the inlet of the return liquid main pipe 9. A third valve 10 is installed on the return liquid main pipe 9. The outlet of the return liquid main pipe 9 is connected to the inlet of the first return liquid branch pipe 11 and the second return liquid branch pipe 12. The outlet of the first return liquid branch pipe 11 is connected to the LNG pump pool 4. The bottom inlet of LNG storage tank 1 is connected to the first return branch pipe 11, where a fourth valve 13 is installed. The outlet of the second return branch pipe 12 is connected to the top inlet of LNG storage tank 1, where a fifth valve 14 is installed. During system operation, the LNG in LNG storage tank 1 is pumped by LNG pump 3 to the refrigeration unit 4 for cooling. The cooled LNG returns to the bottom inlet of LNG storage tank 1 to further reduce the LNG temperature, or returns to the top inlet of LNG storage tank 1 to reduce the temperature of some BOG in LNG storage tank 1 and liquefy it, thereby reducing the pressure of the LNG storage tank. By cooling and depressurizing LNG storage tank 1, the subcooling of LNG storage tank 1 is maintained within the LNG inlet subcooling range required for the operation of the LNG submersible pump in the LNG station without overpressure. This effectively prevents cavitation caused by insufficient subcooling of LNG in the submersible pump, ensuring the normal operation of the LNG station.

[0070] In one embodiment, the LNG storage tank 1 has a gas phase inlet at its top, and the LNG pump pool 2 has a gas phase outlet at its top. A return gas pipeline 15 connects the gas phase outlet of the LNG pump pool 2 and the gas phase inlet of the LNG storage tank 1, and a sixth valve 16 is installed on the return gas pipeline 15. This arrangement allows the gas generated during the preheating of the LNG pump pool 2 to enter the gas phase inlet of the LNG storage tank 1 through the return gas pipeline 15, thereby being recovered into the LNG storage tank 1.

[0071] In one embodiment, the first valve 6, the second valve 8, the third valve 10, the fourth valve 13, 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 the subcooling degree of LNG storage tank 1. Specifically, in this embodiment, each electrically controlled valve is an electro-pneumatic valve.

[0072] In one embodiment, the LNG storage tank subcooling maintenance system further includes a pressure transmitter 17, a thermometer 18, and a controller 19.

[0073] The signal output terminals of pressure transmitter 17 and thermometer 18 are respectively connected to the input terminals of controller 19. The first, second, third, fourth, fifth, sixth, seventh, and eighth control terminals of controller 19 are respectively connected to the first valve 6, the second valve 8, the third valve 10, the fourth valve 13, the fifth valve 14, the sixth valve 16, the LNG pump 3, and the refrigeration unit 4.

[0074] in,

[0075] Pressure transmitter 17 is installed on LNG storage tank 1 and is used to detect the LNG storage tank pressure P. 实 ;

[0076] Thermometer 18 is installed inside LNG pump pool 2 and is used to detect the temperature T of LNG entering LNG pump pool 2. 实 ;

[0077] Controller 19 is used to adjust the pressure P of the LNG storage tank. 实 and LNG temperature T 实 The operating status of the first valve 6, the second valve 8, the third valve 10, the fourth valve 13, the fifth valve 14, the sixth valve 16, the LNG pump 3, and the refrigeration unit 4 are controlled to maintain the subcooling of the LNG storage tank 1.

[0078] In this embodiment, a pressure transmitter 17, a thermometer 18, and a controller 19 are provided, and the pressure transmitter 17 is used to detect the LNG storage tank pressure P. 实 Thermometer 18 is used to detect the temperature T of the LNG entering LNG pump pool 2. 实 Furthermore, controller 19 can adjust the pressure P of the LNG storage tank. 实 and LNG temperature T 实 It controls the working status of the first valve 6, the second valve 8, the third valve 10, the fourth valve 13, the fifth valve 14, the sixth valve 16, the LNG pump 3, and the refrigeration unit 4, so as to automatically adjust and maintain the subcooling of the LNG storage tank 1 based on the real-time detection signals of the pressure transmitter 17 and the thermometer 18.

[0079] The working principle of the above-mentioned LNG storage tank subcooling maintenance system is as follows:

[0080] Before the refrigeration unit 4 is put into operation, the LNG pump pool 2 is pre-cooled. That is, the LNG liquid in the LNG storage tank 1 flows out from the liquid phase outlet under the action of gravity, enters the LNG pump pool 2 after passing through the first valve 6, and the gas in the LNG pump pool 2 returns to the gas phase inlet of the LNG storage tank 1 through the sixth valve 16.

[0081] After the LNG pump pool 2 is pre-cooled, the LNG pump 3 is started. The LNG enters the refrigeration unit 4 through the second valve 8 for cooling. The cooled LNG returns to the LNG storage tank 1 from the outlet of the refrigeration unit 4 through the third valve 10 and the fifth valve 14 / fourth valve 13.

[0082] Specifically, during the operation of the refrigeration unit 4, the pressure transmitter 17 on the LNG storage tank 1 monitors the LNG storage tank pressure P in real time. 实 The thermometer 18 inside LNG pump pool 2 monitors the temperature T of the LNG entering LNG pump pool 2 in real time. 实 Controller 19 adjusts the pressure P of the LNG storage tank. 实 and LNG temperature T 实 The program can be used to select either the top inlet of LNG storage tank 1 from the fifth valve 14 or the bottom inlet of LNG storage tank 1 from the fourth valve 13.

[0083] Specifically, in this embodiment, the controller 19 is a PLC.

[0084] like Figure 4 As shown in the embodiments, this application also provides a control method for the LNG storage tank subcooling maintenance system described in the above embodiments, which may include the following steps:

[0085] S1 controls the LNG pump and refrigeration unit to start, and controls the opening of the first and second valves, so that the LNG in the LNG storage tank enters the LNG pump pool through the first control valve, and enters the refrigeration unit through the second valve under the suction action of the LNG pump, and the refrigeration unit cools down the LNG.

[0086] S2, during the operation of the refrigeration unit, uses a pressure transmitter to monitor the LNG storage tank pressure P in real time. 实 And the LNG temperature T entering the LNG pump pool is monitored in real time by a thermometer. 实 ;

[0087] S3, based on the LNG storage tank pressure P 实 and LNG temperature T 实 The opening and closing states of the third, fourth, and fifth valves are controlled according to the preset control strategy to control the LNG cooled by the refrigeration unit to return to the bottom inlet of the LNG storage tank through the third and fourth valves, or to return to the top inlet of the LNG storage tank through the third and fifth valves, thereby maintaining the subcooling of the LNG storage tank and preventing overpressure.

[0088] In the control method of this embodiment, through the above steps, the LNG cooled by the refrigeration unit can be returned to the bottom inlet of the LNG storage tank to cool the LNG in the LNG storage tank, or returned to the top inlet to cool part of the BOG in the LNG storage tank to liquefy it, thereby reducing the pressure of the LNG storage tank. This achieves the maintenance of the subcooling of the LNG storage tank without overpressure, keeping the subcooling of the LNG storage tank within the subcooling range required for the operation of the LNG station, thereby avoiding cavitation of the LNG station's submersible pump.

[0089] In one embodiment, in step S3, based on the LNG tank pressure P 实 and LNG temperature T 实 The opening and closing states of valves three, four, and five are controlled according to a preset control strategy, including:

[0090] Based on the LNG storage tank pressure P 实 and LNG temperature T 实 Select the corresponding return liquid control program to control the opening and closing status of valves three, four, and five. The return liquid control program includes a top return liquid control program and a bottom return liquid control program.

[0091] When the selected return liquid control program is the top return liquid control program, the third and fifth valves are opened so that the LNG cooled by the refrigeration unit returns to the top inlet of the LNG storage tank through the third and fifth valves. This reduces the temperature of some of the BOG in the LNG storage tank, thereby reducing the pressure of the LNG storage tank and maintaining the subcooling of the LNG storage tank.

[0092] When the selected return liquid control program is the bottom return liquid control program, the third and fourth valves are opened so that the LNG cooled by the refrigeration unit returns to the bottom inlet of the LNG storage tank through the third and fourth valves. This reduces the temperature of the LNG in the LNG storage tank by mixing with the LNG in the tank, thereby maintaining the subcooling of the LNG storage tank.

[0093] In one embodiment, based on the LNG tank pressure P 实 and LNG temperature T 实 Selecting the corresponding return liquid control program to control the opening and closing states of valves three, four, and five includes:

[0094] S31, based on a pre-set saturation pressure calculation model and LNG temperature T 实 Calculate LNG temperature T 实 The corresponding saturation pressure P1;

[0095] S32, based on saturation pressure P1 and LNG tank pressure P 实The corresponding return liquid control program is selected based on the preset LNG storage tank pressure threshold P2 to control the opening and closing status of the third, fourth, and fifth valves.

[0096] In one embodiment, in step S31, 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:

[0097] P1 = exp(39.205 - 1324.4 / T) 实 -3.4366*LnT 实 +0.000031019*T 实 2 ).

[0098] 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 corresponding to the current temperature can be accurately calculated using this model.

[0099] In one embodiment, in step S32, based on the saturation pressure P1 and the LNG storage tank pressure P 实 The corresponding return control program selected based on the preset LNG tank pressure threshold P2 includes:

[0100] When P 实 When ≥P2, select the top return liquid control program;

[0101] When P 实 When <P2,

[0102] If P 实 If ≤P1+(1+x%)△P, then select the bottom return control program.

[0103] If P 实 If the value is greater than P1 + (1 + x%)ΔP, then the top return liquid control program is selected;

[0104] Where △P represents the required subcooling of the LNG station, and x% represents the preset subcooling margin.

[0105] Specifically, the preset LNG storage tank pressure threshold P2, the required subcooling degree ΔP of the LNG station, and the preset subcooling degree margin x% are all constants, and are set according to the actual situation of the LNG station.

[0106] In one embodiment, the control method for the LNG storage tank subcooling maintenance system further includes the following steps:

[0107] S0, before starting the LNG pump and refrigeration unit, the LNG pump pool is pre-cooled. The pre-cooling process is as follows:

[0108] When the first and sixth valves are opened, the LNG in the LNG storage tank flows out from the liquid phase outlet under the action of gravity, enters the LNG pump pool through the first valve, and the gas in the LNG pump pool returns to the gas phase inlet of the LNG storage tank through the sixth valve.

[0109] Pre-cooling the LNG pump pool before starting the refrigeration unit serves two purposes: firstly, it ensures that the liquid in the LNG pump pool can submerge the LNG pump when the refrigeration unit starts, meeting the liquid level required for the LNG pump to operate; secondly, it prevents the LNG pump from cavitating due to excessively high LNG pump pool temperature during the initial stage of refrigeration unit startup.

[0110] It should be noted that the control method of the LNG storage tank subcooling maintenance system in the above embodiments has the same working principle and technical effect as the LNG storage tank subcooling maintenance system in the above embodiments, and will not be repeated here.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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. A subcooling maintenance system for an LNG storage tank, characterized in that, It includes an LNG storage tank, an LNG pump pool, an LNG pump, and a refrigeration unit, wherein the LNG pump is installed in the LNG pump pool; The LNG storage tank is equipped with a liquid phase outlet and a bottom inlet at the bottom, and a top inlet at the top. The bottom of the LNG pump tank is equipped with a liquid phase inlet; A liquid outlet pipe is connected between the liquid phase outlet of the LNG storage tank and the liquid phase inlet of the LNG pump pool, and a first valve is installed on the liquid outlet pipe. A liquid phase pipeline connects the outlet of the LNG pump to the inlet of the refrigeration unit, and a second valve is installed on the liquid phase pipeline. The outlet of the refrigeration unit is connected to the inlet of the return liquid main, and a third valve is installed on the return liquid main. The outlet end of the main return pipe is connected to the inlet end of the first return branch pipe and the second return branch pipe. The outlet end of the first return branch pipe is connected to the bottom inlet of the LNG storage tank. A fourth valve is installed on the first return branch pipe. The outlet end of the second return branch pipe is connected to the top inlet of the LNG storage tank. A fifth valve is installed on the second return branch pipe. The first valve, the second valve, the third valve, the fourth valve, and the fifth valve are all electrically controlled valves; It also includes pressure transmitters, thermometers, and controllers. The signal output terminals of the pressure transmitter and the thermometer are respectively connected to the input terminal of the controller. The first, second, third, fourth, fifth, seventh, and eighth control terminals of the controller are respectively connected to the first valve, second valve, third valve, fourth valve, fifth valve, LNG pump, and refrigeration unit. in, The pressure transmitter is installed on the LNG storage tank and is used to detect the LNG storage tank pressure P. The thermometer is installed inside the LNG pump pool and is used to detect the actual temperature T of the LNG entering the LNG pump pool. The controller is used to adjust the LNG storage tank pressure P. 实 and LNG temperature T 实 The operating status of the first valve, second valve, third valve, fourth valve, fifth valve, LNG pump, and refrigeration unit is controlled to maintain the subcooling of the LNG storage tank. The control method for the LNG storage tank subcooling maintenance system includes the following steps: S1, control the LNG pump and the refrigeration unit to start, and control the first valve and the second valve to open, so that the LNG in the LNG storage tank enters the LNG pump pool through the first valve, and enters the refrigeration unit through the second valve under the suction action of the LNG pump, and the refrigeration unit cools down the LNG; S2, during the operation of the refrigeration unit, the pressure P of the LNG storage tank is detected in real time by the pressure transmitter, and the temperature T of the LNG entering the LNG pump pool is detected in real time by the thermometer; S3, based on the LNG storage tank pressure P 实 and LNG temperature T 实 The opening and closing states of the third, fourth, and fifth valves are controlled according to a preset control strategy to control the LNG cooled by the refrigeration unit to return to the bottom inlet of the LNG storage tank through the third and fourth valves, or to return to the top inlet of the LNG storage tank through the third and fifth valves, thereby maintaining the subcooling of the LNG storage tank. In step S3, the pressure P of the LNG storage tank is... 实 and LNG temperature T 实 Controlling the opening and closing states of the third, fourth, and fifth valves according to a preset control strategy includes: Based on the LNG storage tank pressure P 实 and LNG temperature T 实 The corresponding return liquid control program is selected to control the opening and closing states of the third, fourth, and fifth valves. The return liquid control program includes a top return liquid control program and a bottom return liquid control program. When the selected return liquid control program is the top return liquid control program, the third valve and the fifth valve are opened so that the LNG cooled by the refrigeration unit returns to the top inlet of the LNG storage tank through the third valve and the fifth valve, thereby reducing the temperature of some BOG in the LNG storage tank to ensure that the LNG storage tank does not overpressure. When the selected return liquid control program is the bottom return liquid control program, the third valve and the fourth valve are controlled to open so that the LNG cooled by the refrigeration unit returns to the bottom inlet of the LNG storage tank through the third valve and the fourth valve, thereby maintaining the subcooling of the LNG storage tank by mixing with the LNG in the LNG storage tank and reducing the temperature of the LNG in the LNG storage tank. The LNG storage tank pressure P is used as a reference. 实 and LNG temperature T 实 Selecting the corresponding return liquid control program to control the opening and closing states of the third, fourth, and fifth valves includes: S31, based on the preset saturation pressure calculation model and the LNG temperature T 实 Calculate the LNG temperature T 实 The corresponding saturation pressure P1; S32, based on the saturation pressure P1 and the LNG storage tank pressure P 实 The corresponding return control program is selected based on the preset LNG storage tank pressure threshold P2 to control the opening and closing states of the third, fourth, and fifth valves. In step S32, the pressure based on the saturation pressure P1 and the LNG storage tank pressure P... 实 The corresponding return control program selected based on the preset LNG tank pressure threshold P2 includes: When P 实 When ≥P2, select the top return liquid control program; When P 实 <P2, If P 实 If ≤P1+(1+x%)△P, then select the bottom return control program. If P 实 If P1 + (1 + x%) △P, then select the top return liquid control program; Where △P represents the required subcooling of the LNG station, and x% represents the preset subcooling margin.

2. The LNG storage tank subcooling maintenance system according to claim 1, characterized in that, The LNG storage tank is provided with a gas phase inlet at the top, and the LNG pump pool is provided with a gas phase outlet at the top. A return gas pipeline is connected between the gas phase outlet of the LNG pump pool and the gas phase inlet of the LNG storage tank, and a sixth valve is installed on the return gas pipeline.

3. The LNG storage tank subcooling maintenance system according to claim 2, characterized in that, The sixth valve is an electrically controlled valve.

4. The LNG storage tank subcooling maintenance system according to claim 3, characterized in that, The sixth control terminal of the controller is connected to the sixth valve. The controller is also used to control the operating state of the sixth valve in order to maintain the subcooling of the LNG storage tank.

5. The LNG storage tank subcooling maintenance system according to claim 1, characterized in that, In step S31, 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: 。 6. The LNG storage tank subcooling maintenance system according to claim 4, characterized in that, The control method for the LNG storage tank subcooling maintenance system also includes the following steps: S0, before the LNG pump and the refrigeration unit are started, the LNG pump pool is pre-cooled. The pre-cooling process is as follows: When the first and sixth valves are opened, the LNG in the LNG storage tank flows out from the liquid phase outlet under the action of gravity, enters the LNG pump pool through the first valve, and the gas in the LNG pump pool returns to the gas phase inlet of the LNG storage tank through the sixth valve.

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