Supercooling degree maintaining system for LNG (Liquefied Natural Gas) storage tank and control method thereof

Abstract

The invention discloses an LNG storage tank supercooling degree maintaining system and a control method thereof. The LNG storage tank supercooling degree maintaining system comprises an LNG storage tank, an LNG pump pool, an LNG pump and a refrigerating machine, and the LNG pump is installed in the LNG pump pool; a liquid phase outlet and a bottom liquid inlet are formed in the bottom of the LNG storage tank, a top liquid inlet is formed in the top of the LNG storage tank, a liquid phase inlet is formed in the bottom of the LNG pump pool, when the system works, LNG in the LNG storage tank is conveyed to the refrigerating machine through the LNG pump to be cooled, the cooled LNG returns to the bottom liquid inlet of the LNG storage tank to reduce the temperature of the LNG in the LNG storage tank, and therefore the supercooling degree of the LNG storage tank is maintained; or returning to a top liquid inlet of the LNG storage tank to reduce the temperature of part of BOG in the LNG storage tank and liquefy the part of BOG to ensure that the LNG storage tank is not overpressure. The LNG storage tank can be cooled and depressurized, so that the supercooling degree of the LNG storage tank is maintained within the LNG inlet liquid supercooling degree range required by the operation of the immersed pump of the LNG station, and cavitation caused by insufficient supercooling degree of sucked LNG of the immersed pump is effectively avoided.

Description

Technical Field

[0001] The present application relates to cooling of a medium in an LNG storage tank, and in particular to a subcooling maintenance system for an LNG storage tank and a control method thereof. Background Art

[0002] At present, LNG stations (including LNG filling stations, L-CNG filling stations, LNG gasification stations, and LNG reserve stations) are all equipped with LNG storage tanks as LNG storage devices. The temperature of LNG is extremely low, reaching as low as -162°C. During the storage process, the LNG storage tanks will continuously absorb heat from the environment, and LNG will partially vaporize to form BOG (Boil Off Gas) stored in the LNG storage tanks, causing the pressure of the LNG storage tanks 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 temperature-pressure equilibrium state. The pressure in the equilibrium state is called the saturation pressure corresponding to the temperature. The relationship curve between the temperature and saturation pressure of the LNG storage tank is as follows: Figure 1 shown.

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

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

[0006] At present, the BOG recovery process of LNG stations adopts the method of extracting BOG from LNG storage tanks for recycling to reduce the pressure of LNG storage tanks. However, LNG in LNG storage tanks is mostly in a saturated state. Extracting BOG from LNG storage tanks will cause the pressure of LNG storage tanks to be lower than the saturation pressure at the corresponding temperature, resulting in insufficient supercooling of LNG sucked by the submersible pump, resulting in cavitation. Summary of the invention

[0007] In order to solve the above-mentioned technical problems, the present application provides an LNG storage tank subcooling maintenance system and a control method thereof, which can cool and reduce the pressure of the LNG storage tank, so that the subcooling of the LNG storage tank is maintained within the LNG inlet subcooling range required for the operation of the submersible pump of the LNG station, effectively avoiding cavitation of the submersible pump due to insufficient subcooling of the inhaled LNG, thereby ensuring the normal operation of the LNG station.

[0008] The first object of the present application is to provide a system for maintaining subcooling of an LNG storage tank.

[0009] The above-mentioned application objective 1 of the present application is achieved through the following technical solutions:

[0010] An LNG tank supercooling maintenance system comprises an LNG tank, an LNG pump pool, an LNG pump and a refrigerator, wherein the LNG pump is installed in the LNG pump pool;

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

[0012] A liquid phase inlet is provided at the bottom of the LNG pump pool;

[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 is connected between the liquid outlet of the LNG pump and the liquid inlet of the refrigerator, a second valve is installed on the liquid phase pipeline, the liquid outlet of the refrigerator is connected to the liquid inlet end of the liquid return main pipe, a third valve is installed on the liquid return main pipe,

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

[0016] in,

[0017] When the system is working, the LNG in the LNG storage tank is transported to the refrigeration machine through the LNG pump for cooling. The cooled LNG returns to the bottom liquid inlet of the LNG storage tank to reduce the LNG temperature in the LNG storage tank to maintain the supercooling degree of the LNG storage tank, or returns to the top liquid inlet of the LNG storage tank to reduce the temperature of part of the BOG in the LNG storage tank and liquefy it to ensure that the LNG storage tank is not over-pressurized.

[0018] Preferably, a gas phase inlet is provided at the top of the LNG storage tank, a gas phase outlet is provided at the top of the LNG pump pool, 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 tank subcooling maintenance system further includes a pressure transmitter, a thermometer and a controller.

[0021] The signal output end of the pressure transmitter and the signal output end of the thermometer are respectively connected to the input end of the controller, and the first control end, the second control end, the third control end, the fourth control end, the fifth control end, the sixth control end, the seventh control end, and the eighth control end of the controller are respectively connected to the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump, and the refrigerator.

[0022] in,

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

[0024] The thermometer is arranged in 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 pressure of the LNG tank according to the 实 and LNG temperature T 实 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 are controlled to maintain the supercooling degree of the LNG storage tank.

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

[0027] The second application objective of the present application is achieved through the following technical solutions:

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

[0029] S1, controlling the LNG pump and the refrigerator to start, and controlling 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 refrigerator through the second valve under the suction action of the LNG pump, and the LNG is cooled by the refrigerator;

[0030] S2, during the operation of the refrigerator, the pressure transmitter is used to detect the pressure P of the LNG storage tank in real time. 实 , and the temperature T of the LNG entering the LNG pump pool is detected in real time by the thermometer 实 ;

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

[0032] Preferably, 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 three valves, the fourth valve and the fifth valve according to a preset control strategy includes:

[0033] According to the LNG tank pressure P 实 and LNG temperature T 实 Select the corresponding liquid return control program to control the opening and closing states of the three valves, the fourth valve and the fifth valve, wherein the liquid return control program includes a top liquid return control program and a bottom liquid return control program, wherein:

[0034] When the selected liquid return control program is the top liquid return control program, the third valve and the fifth valve are controlled to open, so that the LNG cooled by the refrigerator returns to the top liquid inlet of the LNG storage tank through the third valve and the fifth valve, thereby reducing the temperature of part of the BOG in the LNG storage tank to reduce the pressure of the LNG storage tank and avoid overpressure of the storage tank;

[0035] When the selected liquid return control program is the bottom liquid return control program, the third valve and the fourth valve are controlled to open so that the LNG cooled by the refrigeration machine returns to the bottom liquid inlet of the LNG storage tank through the third valve and the fourth valve, thereby maintaining the supercooling degree of the LNG storage tank by mixing with the LNG in the LNG storage tank and lowering the LNG temperature in the LNG storage tank.

[0036] Preferably, the pressure P of the LNG storage tank is 实 and LNG temperature T 实 Selecting the corresponding liquid return control program to control the opening and closing states of the three valves, the fourth valve and the fifth valve 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 Treal;

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

[0039] Preferably, in step S31, the preset saturation pressure calculation model is constructed based on a temperature and saturation pressure relationship curve of LNG, wherein the preset saturation pressure calculation model is as follows:

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

[0041] Preferably, in step S32, the saturated pressure P1 and the LNG tank pressure P 实 The liquid return control program corresponding to the preset LNG tank pressure threshold value P2 includes:

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

[0043] When P 实 <P2,

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

[0045] If P 实 >P1+(1+x%)△P, then select the top liquid return control program;

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

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

[0048] S0, before starting the LNG pump and the refrigerator, precooling the LNG pump pool, the precooling process is as follows:

[0049] The first valve and the sixth valve are controlled to open, and the LNG in the LNG storage tank flows out from the liquid phase outlet under the action of gravity and enters the LNG pump pool through the first valve. 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, the present application discloses a LNG tank subcooling maintenance system and a control method thereof, wherein an LNG tank, an LNG pump pool, an LNG pump and a refrigeration machine are provided, wherein the LNG pump is installed in the LNG pump pool, a liquid phase outlet and a bottom liquid inlet are provided at the bottom of the LNG tank, a top liquid inlet is provided at the top of the LNG tank, and a liquid phase inlet is provided at the bottom of the LNG pump pool. When the system is working, the LNG in the LNG tank is transported to the refrigeration machine through the LNG pump for cooling, and the cooled LNG returns to the bottom liquid inlet of the LNG tank to reduce the LNG temperature in the LNG tank, or returns to the top liquid inlet of the LNG tank to reduce the temperature of part of the BOG in the LNG tank and liquefy it to reduce the pressure of the LNG tank, thereby cooling and reducing the pressure of the LNG tank, so that the subcooling of the LNG tank is maintained within the LNG inlet subcooling range required for the operation of the submersible pump of the LNG station without overpressure, thereby effectively avoiding cavitation of the submersible pump due to insufficient subcooling of the inhaled LNG, and ensuring the normal operation of the LNG station. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0052] Figure 1 It is the relationship curve between the temperature and saturation pressure of the LNG storage tank;

[0053] Figure 2 This is a structural schematic diagram of a subcooling maintenance system for an LNG storage tank in one embodiment of the present application;

[0054] Figure 3This is a control principle block diagram of a system for maintaining subcooling of an LNG storage tank in one embodiment of the present application;

[0055] Figure 4 This is a flow chart of a control method for a LNG tank subcooling maintenance system in one embodiment of the present application. DETAILED DESCRIPTION

[0056] In order to enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0057] In the embodiments provided in the present application, it should be understood that the disclosed methods and systems can be implemented in other ways. The system embodiments described below are merely schematic. For example, the division of units and modules is only a logical function division. There may be other division methods in actual implementation, such as: multiple units or modules can be combined, or can be integrated into another system, or some features can be ignored, or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, which can be electrical, mechanical or other forms.

[0058] In addition, all functional units in the embodiments of the present application may be integrated into one processor, or each unit may be a separate device, or two or more units may be integrated into one device; each functional unit in the embodiments of the present application may be implemented in the form of hardware or in the form of hardware plus software functional units.

[0059] A person skilled in the art can understand that all or part of the steps of the following method embodiments can be completed by program instructions and related hardware. The aforementioned program instructions can be stored in a computer-readable storage medium. When the program instructions are executed, the steps of the following method embodiments are executed; and the aforementioned storage medium includes: a mobile storage device, a read-only memory (ROM), a magnetic disk or an optical disk, and other media that can store program codes.

[0060] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, "multiple" and "several" mean two or more, unless otherwise clearly and specifically defined.

[0061] The present application embodiment provides a system for maintaining subcooling of an LNG storage tank, 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 refrigerator 4, wherein the LNG pump 3 is installed in the LNG pump pool 2;

[0062] 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.

[0063] A liquid phase inlet is provided at the bottom of the LNG pump pool 2;

[0064] A liquid 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, and a first valve 6 is installed on the liquid outlet pipe 5.

[0065] A liquid phase pipeline 7 is connected between the liquid outlet of the LNG pump 3 and the liquid inlet of the refrigerator 4, and a second valve 8 is installed on the liquid phase pipeline 7. The liquid outlet of the refrigerator 4 is connected to the liquid inlet end of the liquid return main pipe 9, and a third valve 10 is installed on the liquid return main pipe 9.

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

[0067] in,

[0068] When the system is working, the LNG in the LNG storage tank 1 is transported to the refrigeration machine 4 through the LNG pump 3 for cooling. The cooled LNG returns to the bottom liquid inlet of the LNG storage tank 1 to reduce the LNG temperature in the LNG storage tank 1, thereby maintaining the supercooling degree of the LNG storage tank 1, or returns to the top liquid inlet of the LNG storage tank 1 to reduce the temperature of part of the BOG in the LNG storage tank 1 and liquefy it to reduce the pressure of the LNG storage tank, thereby ensuring that the LNG storage tank 1 is not over-pressurized.

[0069] The LNG storage tank subcooling maintenance system of the embodiment of the present application is provided with an LNG storage tank 1, an LNG pump pool 2, an LNG pump 3 and a refrigerator 4, the LNG pump 3 is installed in the LNG pump pool 2, a liquid phase outlet and a bottom liquid inlet are provided at the bottom of the LNG storage tank 1, a top liquid inlet is provided at the top of the LNG storage tank 1, a liquid phase inlet is provided at the bottom of the LNG pump pool 2, and a liquid 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 liquid outlet pipe 5, a liquid phase pipe 7 is connected between the liquid outlet of the LNG pump 3 and the liquid inlet of the refrigerator 4, a second valve 8 is installed on the liquid phase pipe 7, the liquid outlet of the refrigerator 4 is connected to the liquid inlet end of the liquid return main pipe 9, a third valve 10 is installed on the liquid return main pipe 9, the liquid outlet end of the liquid return main pipe 9 is connected to the liquid inlet end of the first liquid return branch pipe 11 and the second liquid return branch pipe 12, and the liquid outlet end of the first liquid return branch pipe 11 is connected to the LN The bottom liquid inlet of the LNG storage tank 1 is connected to the fourth valve 13 on the first liquid return branch 11, the liquid outlet end of the second liquid return branch 12 is connected to the top liquid inlet of the LNG storage tank 1, and the fifth valve 14 is installed on the second liquid return branch 12. When the system is working, the LNG in the LNG storage tank 1 is transported to the refrigeration machine 4 through the LNG pump 3 for cooling. The cooled LNG returns to the bottom liquid inlet of the LNG storage tank 1 to reduce the LNG temperature in the LNG storage tank 1, or returns to the top liquid inlet of the LNG storage tank 1 to reduce the temperature of part of the BOG in the LNG storage tank 1 and liquefy it to reduce the pressure of the LNG storage tank. Therefore, by cooling and reducing the pressure of the LNG storage tank 1, the supercooling of the LNG storage tank 1 is maintained within the LNG inlet supercooling range required for the operation of the submersible pump of the LNG station without overpressure, effectively avoiding cavitation of the submersible pump due to insufficient supercooling of the inhaled LNG, thereby ensuring the normal operation of the LNG station.

[0070] In one embodiment, a gas phase inlet is provided at the top of the LNG storage tank 1, a gas phase outlet is provided at the top of the LNG pump pool 2, a return gas pipeline 15 is connected between 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 when the LNG pump pool 2 is preheated to enter the gas phase inlet of the LNG storage tank 1 through the return gas pipeline 15, and thus be 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 automation level of the system is effectively improved, and remote automatic control of the subcooling degree of the LNG storage tank 1 is achieved. Specifically, in this embodiment, each electrically controlled valve adopts an electrically controlled pneumatic valve.

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

[0073] The signal output end of the pressure transmitter 17 and the signal output end of the thermometer 18 are respectively connected to the input end of the controller 19. 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 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 refrigerator 4.

[0074] in,

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

[0076] The thermometer 18 is disposed in the LNG pump pool 2 and is used to detect the temperature T of the LNG entering the LNG pump pool 2. 实 ;

[0077] The controller 19 is used to adjust the pressure of the LNG tank according to the 实 and LNG temperature T 实 The working states 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 refrigerator 4 are controlled to maintain the supercooling degree 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 pressure P of the LNG storage tank. 实 The thermometer 18 detects the temperature T of the LNG entering the LNG pump pool 2. 实 , and the controller 19 can be based on the LNG tank pressure P 实 and LNG temperature T 实 To control the working states 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 refrigerator 4, so that the subcooling degree of the LNG storage tank 1 can be automatically adjusted and maintained according to the real-time detection signals of the pressure transmitter 17 and the thermometer 18.

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

[0080] Before the operation of the refrigerator 4, the LNG pump pool 2 is first 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 precooling of the LNG pump pool 2 is completed, the LNG pump 3 is started, and the LNG enters the refrigerator 4 through the second valve 8 for cooling. The cooled LNG returns to the LNG storage tank 1 from the liquid outlet of the refrigerator 4 through the third valve 10 and the fifth valve 14 / fourth valve 13.

[0082] Specifically, during the operation of the refrigerator 4, the pressure transmitter 17 on the LNG storage tank 1 detects the LNG storage tank pressure P in real time. 实 The thermometer 18 in the LNG pump pool 2 detects the temperature T of the LNG entering the LNG pump pool 2 in real time. 实 , the controller 19 is based on the LNG tank pressure P 实 and LNG temperature T 实 Through program control, the fifth valve 14 is selected to enter the top liquid inlet of the LNG storage tank 1, or the fourth valve 13 is selected to enter the bottom liquid inlet of the LNG storage tank 1.

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

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

[0085] S1, control the LNG pump and the refrigerator 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 refrigerator through the second valve under the suction action of the LNG pump, and the LNG is cooled by the refrigerator;

[0086] S2, during the operation of the refrigerator, the LNG tank pressure P is detected in real time through the pressure transmitter 实 , and use a thermometer to detect the LNG temperature T entering the LNG pump pool in real time 实 ;

[0087] S3, according to the LNG tank pressure P 实 and LNG temperature T 实 The opening and closing states of the third valve, the fourth valve and the fifth valve are controlled according to the preset control strategy to control the LNG cooled by the refrigeration machine to return to the bottom liquid inlet of the LNG storage tank through the third valve and the fourth valve, or to return to the top liquid inlet of the LNG storage tank through the third valve and the fifth valve, so as to maintain the supercooling degree of the LNG storage tank without overpressure.

[0088] In the control method of the present embodiment, through the above steps, it is possible to return the LNG cooled by the refrigeration machine to the bottom liquid inlet of the LNG storage tank to cool the LNG in the LNG storage tank, or return to the top liquid inlet to cool part of the BOG in the LNG storage tank to liquefy it and reduce the pressure of the LNG storage tank, thereby maintaining the supercooling of the LNG storage tank without overpressure, and keeping the supercooling of the LNG storage tank within the supercooling range required for the operation of the LNG station, thereby avoiding cavitation of the submersible pump of the LNG station.

[0089] In one embodiment, in step S3, according to the LNG tank pressure P 实 and LNG temperature T 实 Controlling the opening and closing states of the third valve, the fourth valve and the fifth valve according to the preset control strategy includes:

[0090] According to the LNG tank pressure P 实 and LNG temperature T 实 Select the corresponding liquid return control program to control the opening and closing states of the third valve, the fourth valve and the fifth valve. The liquid return control program includes a top liquid return control program and a bottom liquid return control program, wherein:

[0091] When the selected liquid return control program is the top liquid return control program, the third valve and the fifth valve are controlled to open, so that the LNG cooled by the refrigerator returns to the top liquid inlet of the LNG storage tank through the third valve and the fifth valve, thereby maintaining the supercooling degree of the LNG storage tank by reducing the temperature of part of the BOG in the LNG storage tank to reduce the pressure of the LNG storage tank;

[0092] When the selected liquid return control program is the bottom liquid return control program, the third valve and the fourth valve are controlled to open so that the LNG cooled by the refrigeration machine returns to the bottom liquid inlet of the LNG storage tank through the third valve and the fourth valve, thereby maintaining the supercooling degree of the LNG storage tank by mixing with the LNG in the LNG storage tank and lowering the LNG temperature in the LNG storage tank.

[0093] In one embodiment, according to the LNG tank pressure P 实 and LNG temperature T 实 Select the corresponding liquid return control program to control the opening and closing status of the third valve, the fourth valve and the fifth valve, including:

[0094] S31, based on the preset 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, LNG tank pressure P 实The corresponding liquid return control program is selected according to the preset LNG tank pressure threshold value P2 to control the opening and closing states of the third valve, the fourth valve and the fifth valve.

[0096] In one embodiment, in step S31, a preset saturation pressure calculation model is constructed based on a temperature and saturation pressure relationship curve of LNG, wherein the preset saturation pressure calculation model is as follows:

[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 a temperature and saturation pressure relationship curve diagram of LNG, and then the saturation pressure corresponding to the current temperature can be accurately calculated through the model.

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

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

[0101] When P 实 <P2,

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

[0103] If P 实 >P1+(1+x%)△P, then select the top liquid return control program;

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

[0105] Specifically, the preset LNG tank pressure threshold value 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 of the LNG tank subcooling maintenance system further includes the following steps:

[0107] S0, before the LNG pump and refrigerator are started, the LNG pump pool is precooled. The precooling process is as follows:

[0108] Control the first valve and the sixth valve to open, 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] The LNG pump pool is pre-cooled before the refrigerator is started. On the one hand, it can ensure that the liquid in the LNG pump pool can submerge the LNG pump when the refrigerator is started, meeting the liquid level required for the LNG pump to work. On the other hand, it can avoid the LNG pump from being vaporized in large quantities due to the high temperature of the LNG pump pool in the initial stage of the refrigerator startup, causing cavitation to the LNG pump.

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

[0111] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0112] Professionals may further appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the composition and steps of each example have been generally described in the above description according to function. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0113] The steps of the methods or algorithms described in conjunction with the embodiments disclosed herein may be implemented directly using hardware, software modules executed by a processor, or a combination of the two. The software modules may be placed in a random access memory (RAM), a memory, a read-only memory (ROM), an electrically programmable ROM, an electrically erasable programmable ROM, a register, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.

[0114] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A LNG tank subcooling maintenance system, characterized in that: It includes an LNG storage tank, an LNG pump pool, an LNG pump and a refrigerator, wherein the LNG pump is installed in the LNG pump pool; The bottom of the LNG storage tank is provided with a liquid phase outlet and a bottom liquid inlet, and the top of the LNG storage tank is provided with a top liquid inlet. A liquid phase inlet is provided at the bottom of the LNG pump pool; 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 is connected between the liquid outlet of the LNG pump and the liquid inlet of the refrigerator, a second valve is installed on the liquid phase pipeline, the liquid outlet of the refrigerator is connected to the liquid inlet end of the liquid return main pipe, a third valve is installed on the liquid return main pipe, The liquid outlet of the liquid return main pipe is connected to the liquid inlet of the first liquid return branch pipe and the second liquid return branch pipe, the liquid outlet of the first liquid return branch pipe is connected to the bottom liquid inlet of the LNG storage tank, a fourth valve is installed on the first liquid return branch pipe, the liquid outlet of the second liquid return branch pipe is connected to the top liquid inlet of the LNG storage tank, and a fifth valve is installed on the second liquid return branch pipe; in, When the system is working, the LNG in the LNG storage tank is transported to the refrigeration machine through the LNG pump for cooling, and the cooled LNG returns to the bottom liquid inlet of the LNG storage tank to reduce the LNG temperature in the LNG storage tank, thereby maintaining the supercooling degree of the LNG storage tank, or returns to the top liquid inlet of the LNG storage tank to reduce the temperature of part of the BOG in the LNG storage tank and liquefy it to ensure that the LNG storage tank is not over-pressurized.

2. The LNG storage tank subcooling maintenance system according to claim 1, characterized in that: A gas phase inlet is provided at the top of the LNG storage tank, a gas phase outlet is provided at the top of the LNG pump pool, 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 tank subcooling maintenance system according to claim 2, 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.

4. The LNG tank subcooling maintenance system according to claim 3, characterized in that: Also includes pressure transmitters, thermometers and controllers, The signal output end of the pressure transmitter and the signal output end of the thermometer are respectively connected to the input end of the controller, and the first control end, the second control end, the third control end, the fourth control end, the fifth control end, the sixth control end, the seventh control end, and the eighth control end of the controller are respectively connected to the first valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the LNG pump, and the refrigerator. in, The pressure transmitter is arranged on the LNG storage tank and is used to detect the pressure P of the LNG storage tank. 实 ; The thermometer is arranged in the LNG pump pool, and is used to detect the temperature T of the LNG entering the LNG pump pool. 实 ; The controller is used to adjust the pressure of the LNG tank according to the 实 and LNG temperature T 实 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 are controlled to maintain the supercooling degree of the LNG storage tank.

5. A control method for a subcooling maintenance system for an LNG storage tank according to any one of claims 2 to 4, characterized in that: The steps include: S1, controlling the LNG pump and the refrigerator to start, and controlling 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 refrigerator through the second valve under the suction action of the LNG pump, and the LNG is cooled by the refrigerator; S2, during the operation of the refrigerator, the pressure transmitter is used to detect the pressure P of the LNG storage tank in real time. 实 , and the temperature T of the LNG entering the LNG pump pool is detected in real time by the thermometer 实 ; S3, according to the LNG storage tank pressure P 实 and LNG temperature T 实 The opening and closing states of the three valves, the fourth valve and the fifth valve are controlled according to a preset control strategy to control the LNG cooled by the refrigeration machine to return to the bottom liquid inlet of the LNG storage tank through the third valve and the fourth valve, or to return to the top liquid inlet of the LNG storage tank through the third valve and the fifth valve, so as to maintain the supercooling degree of the LNG storage tank.

6. The control method of the LNG tank subcooling maintenance system according to claim 5, characterized in that: 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 three valves, the fourth valve and the fifth valve according to a preset control strategy includes: According to the LNG tank pressure P 实 and LNG temperature T 实 Select the corresponding liquid return control program to control the opening and closing states of the three valves, the fourth valve and the fifth valve, wherein the liquid return control program includes a top liquid return control program and a bottom liquid return control program, wherein: When the selected liquid return control program is the top liquid return control program, the third valve and the fifth valve are controlled to open, so that the LNG cooled by the refrigerator returns to the top liquid inlet of the LNG storage tank through the third valve and the fifth valve, thereby reducing the temperature of part of the BOG in the LNG storage tank to ensure that the LNG storage tank is not over-pressurized; When the selected liquid return control program is the bottom liquid return control program, the third valve and the fourth valve are controlled to open so that the LNG cooled by the refrigeration machine returns to the bottom liquid inlet of the LNG storage tank through the third valve and the fourth valve, thereby maintaining the supercooling degree of the LNG storage tank by mixing with the LNG in the LNG storage tank and lowering the LNG temperature in the LNG storage tank.

7. The control method of the LNG tank subcooling maintenance system according to claim 6, characterized in that: According to the LNG tank pressure P 实 and LNG temperature T 实 Selecting the corresponding liquid return control program to control the opening and closing states of the three valves, the fourth valve and the fifth valve 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 saturated pressure P1, the LNG tank pressure P 实 The liquid return control program corresponding to the preset LNG tank pressure threshold value P2 is selected to control the opening and closing states of the three valves, the fourth valve and the fifth valve.

8. The control method of the LNG tank subcooling maintenance system according to claim 7, characterized in that: In step S31, the preset saturation pressure calculation model is constructed based on the temperature and saturation pressure relationship curve of LNG, wherein the preset saturation pressure calculation model is as follows: P1=exp(39.205-1324.4 / T 实 -3.4366*LnT 实 +0.000031019*T 实 2 )。 9. The control method of the LNG tank subcooling maintenance system according to claim 7, characterized in that: In step S32, the saturated pressure P1 and the LNG tank pressure P 实 The liquid return control program corresponding to the preset LNG tank pressure threshold value P2 includes: When P 实 When ≥P2, select the top liquid return control program; When P 实 <P2, If P 实 ≤P1+(1+x%)△P, then select the bottom liquid return control program, If P 实 >P1+(1+x%)△P, then select the top liquid return control program; Wherein, △P represents the required subcooling degree of the LNG station, and x% represents the preset subcooling margin.

10. The control method of the LNG storage tank subcooling maintenance system according to any one of claims 5 to 9, characterized in that: The following steps are also included: S0, before starting the LNG pump and the refrigerator, precooling the LNG pump pool, the precooling process is as follows: The first valve and the sixth valve are controlled to open, and the LNG in the LNG storage tank flows out from the liquid phase outlet under the action of gravity and enters the LNG pump pool through the first valve. The gas in the LNG pump pool returns to the gas phase inlet of the LNG storage tank through the sixth valve.

Images