A pressure control system for a marine liquid ammonia storage tank

Abstract

The utility model provides a kind of ship liquid ammonia storage tank pressure control system, the system includes ammonia gas mixing liquefier, for the BOG compressed after pressurization BOG and the deep cooling liquid ammonia obtained after liquid ammonia subcooling system are mixed, realize the rapid liquefaction of BOG as liquid ammonia;Liquid ammonia throttle valve's outlet is connected with the liquid ammonia inlet of the ammonia gas mixing liquefier, ammonia gas throttle valve's outlet is connected with the ammonia gas inlet of the ammonia gas mixing liquefier, the liquid ammonia outlet of the ammonia gas mixing liquefier is connected with the inlet of spraying device one.The utility model introduces secondary spraying BOG liquefaction mechanism by introduction, uses deep cooling liquid ammonia as cold source, successively in ammonia gas mixing liquefier and liquid ammonia storage tank and BOG mixed liquefaction, to realize the continuous, accurate control of pressure in storage tank, improve system overall security and operating stability.

Description

Technical Field

[0001] This utility model relates to the technical field of ammonia-fueled ships and liquid ammonia transport ships, and more specifically, to a pressure control system for a ship's liquid ammonia storage tank. Background Technology

[0002] With the increasing global shipping industry's demand for low-carbon and zero-carbon energy, ammonia, as a carbon-free fuel and a highly efficient hydrogen carrier, is gradually being applied to ship propulsion systems and bulk liquid transportation. Currently, research and demonstration projects on ammonia-fueled ships and liquid ammonia transport ships are progressing steadily, and liquid ammonia storage tanks, as key core equipment, are widely used to store liquid ammonia in fuel or liquid cargo form.

[0003] Liquid ammonia is typically stored in liquid form at atmospheric pressure (approximately 1 bar) and -33°C. In practical engineering, the design pressure of storage tanks is generally 3-5 barg to cope with pressure fluctuations during operation. However, during ship navigation, due to heat exchange between the hull structure and the external environment, heat continuously seeps into the storage tank, causing the liquid ammonia to absorb heat, its temperature rises, and it partially vaporizes, producing blow-off gas (BOG). The accumulation of BOG will cause the pressure inside the storage tank to rise continuously. If not handled in time, it may trigger the release of safety valves, creating a risk of ammonia leakage and affecting the safety and environmental protection of ship operation.

[0004] In existing technologies, common BOG (Boiled Air Gaseous) treatment methods include combustion, compression recovery, or direct emission. However, these methods suffer from high energy consumption, poor control precision, and environmental pollution. Especially for toxic, flammable, and highly irritating media like ammonia, traditional pressure control methods struggle to meet the comprehensive requirements of safety, stability, and energy efficiency during long-term navigation.

[0005] Chinese patent CN222527422U discloses a shipboard ammonia vapor treatment system, including a liquid ammonia storage tank, a liquid ammonia pump, a liquid ammonia subcooling system, and a first throttle valve. A spray device is installed at the top of the liquid ammonia storage tank. The liquid ammonia pump is located inside the liquid ammonia storage tank, and its outlet is connected to the liquid ammonia inlet of the liquid ammonia subcooling system. The liquid ammonia outlet of the liquid ammonia subcooling system is connected to the inlet of the first throttle valve, and the outlet of the first throttle valve is connected to the spray device. This shipboard ammonia vapor treatment system can reliquefy ammonia vapor in the liquid ammonia storage tank, allowing the ammonia vapor to be reused, thereby reducing fuel waste. However, the system's pressure regulation performance is not ideal.

[0006] Therefore, there is an urgent need for an efficient, energy-saving, and reliable liquid ammonia storage tank pressure control technology that can maintain the tank pressure at near-normal pressure levels during the full operating conditions of a ship, ensuring the stability and safety of the system. Utility Model Content

[0007] To address the aforementioned problems, this utility model provides a pressure control system for ship liquid ammonia storage tanks. By introducing a secondary spray BOG liquefaction mechanism, it utilizes cryogenic liquid ammonia as a cold source, mixing and liquefying the ammonia with BOG in the ammonia mixing liquefaction unit and the liquid ammonia storage tank successively. This achieves continuous and precise control of the pressure inside the storage tank, improving the overall safety and operational stability of the system.

[0008] To achieve the above objectives, this utility model provides a pressure control system for a ship's liquid ammonia storage tank. The technical solution of this utility model is implemented as follows:

[0009] A pressure control system for a marine liquid ammonia storage tank includes a liquid ammonia storage tank, a liquid ammonia supply pump, a liquid ammonia subcooling system, a liquid ammonia throttling valve, a BOG compressor, an ammonia throttling valve, and a spray device. The spray device is located in the upper space of the liquid ammonia storage tank and is used to condense and liquefy the BOG in the liquid ammonia storage tank into liquid ammonia. The system also includes an ammonia mixing liquefaction unit for mixing the BOG pressurized by the BOG compressor with the cryogenic liquid ammonia obtained by the liquid ammonia subcooling system to achieve rapid liquefaction of the BOG. ​​The outlet of the liquid ammonia throttling valve is connected to the liquid ammonia inlet of the ammonia mixing liquefaction unit, the outlet of the ammonia throttling valve is connected to the ammonia inlet of the ammonia mixing liquefaction unit, and the liquid ammonia outlet of the ammonia mixing liquefaction unit is connected to the inlet of the spray device.

[0010] Furthermore, a second spray device is installed in the upper space of the ammonia mixing liquefaction unit, and the liquid ammonia inlet of the ammonia mixing liquefaction unit is connected to the second spray device to condense and liquefy the BOG in the ammonia mixing liquefaction unit into liquid ammonia.

[0011] Furthermore, the liquid ammonia subcooling system includes a liquid ammonia subcooler, a refrigeration compressor, a refrigeration condenser, and a refrigerant throttling valve; the refrigerant inlet of the liquid ammonia subcooler is connected to the outlet of the refrigerant throttling valve, and the refrigerant outlet of the liquid ammonia subcooler is connected to the inlet of the refrigeration compressor; the refrigerant inlet of the refrigeration condenser is connected to the outlet of the refrigeration compressor, and the refrigerant outlet of the refrigeration condenser is connected to the inlet of the refrigerant throttling valve.

[0012] Furthermore, a heat exchange tube is provided inside the liquid ammonia subcooler. The refrigerant inlet of the heat exchange tube is connected to the outlet of the refrigerant throttling valve, and the refrigerant outlet of the heat exchange tube is connected to the inlet of the refrigeration compressor.

[0013] Furthermore, a second heat exchange tube is provided inside the refrigeration condenser. The refrigerant inlet of the second heat exchange tube is connected to the outlet of the refrigeration compressor, and the refrigerant outlet of the second heat exchange tube is connected to the inlet of the refrigerant throttling valve.

[0014] Furthermore, the liquid ammonia subcooling system also includes a cooling water pipeline, which works in conjunction with the refrigeration condenser to condense and cool the refrigerant.

[0015] Furthermore, the inlet of the BOG compressor is connected to the ammonia outlet of the liquid ammonia storage tank, and the outlet of the BOG compressor is connected to the ammonia inlet of the ammonia mixing liquefaction unit, for extracting BOG from the liquid ammonia storage tank and pressurizing it before sending it to the ammonia mixing liquefaction unit.

[0016] Furthermore, one end of the liquid ammonia throttling valve is connected to the liquid ammonia outlet of the liquid ammonia subcooler, and the other end is connected to the cryogenic liquid ammonia inlet of the ammonia mixing liquefaction unit, which is used to throttle and depressurize the cryogenic liquid ammonia from the liquid ammonia subcooler and then send it into the ammonia mixing liquefaction unit.

[0017] Furthermore, one end of the ammonia throttling valve is connected to the outlet of the BOG compressor, and the other end is connected to the BOG inlet of the ammonia mixing liquefaction unit, for throttling control of the BOG after it has been pressurized by the BOG compressor.

[0018] Furthermore, one end of the liquid ammonia supply pump is located below the liquid ammonia level in the liquid ammonia storage tank, and the other end is located outside the liquid ammonia storage tank and connected to the liquid ammonia inlet of the liquid ammonia subcooling system.

[0019] Compared with existing technologies, the pressure control system for marine liquid ammonia storage tanks described in this utility model has the following advantages:

[0020] 1. This utility model realizes the efficient liquefaction and reuse of BOG in liquid ammonia storage tanks and pressure self-balancing control, thereby improving system safety, energy efficiency and environmental friendliness.

[0021] 2. This invention significantly improves the pressure control efficiency of liquid ammonia storage tanks by introducing a secondary spray BOG liquefaction mechanism. It accelerates pressure response speed and significantly enhances system stability and safety, ensuring that the tank pressure remains at atmospheric pressure during ship operation. Attached Figure Description

[0022] Figure 1 This is a flow chart of the pressure control process for the ammonia fuel storage tank as described in Embodiment 1 of this utility model.

[0023] Explanation of reference numerals in the attached figures:

[0024] 1. Liquid ammonia storage tank; 2. Liquid ammonia supply pump; 3. Liquid ammonia subcooler; 4. Liquid ammonia throttling valve; 5. Ammonia gas mixing liquefaction unit; 6. BOG compressor; 7. Ammonia gas throttling valve; 8. Spray device one; 9. Refrigeration compressor; 10. Refrigeration condenser; 11. Refrigerant throttling valve; 12. Spray device two; 13. Heat exchange tube one; 14. Heat exchange tube two; 15. Cooling water pipeline; 16. Liquid ammonia control valve one; 17. Ammonia gas control valve two. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the described embodiments are only some, not all, of the embodiments of this utility model. The specific embodiments described herein are merely illustrative of the present utility model and are not intended to limit the present utility model. It should be noted that, unless otherwise specified, the embodiments and features described in the present utility model can be combined with each other.

[0026] Example 1

[0027] This invention provides a pressure control system for ammonia storage tanks on ships, which is suitable for the design requirements of ammonia-fueled ships or liquid ammonia transport vessels. It can effectively control the temperature and pressure of the ammonia storage tanks throughout the entire operation of the ship, which is of great significance for the stable and safe operation of ammonia-fueled ships or liquid ammonia transport vessels.

[0028] A pressure control system for a marine liquid ammonia storage tank, such as Figure 1 As shown, it includes a liquid ammonia storage tank 1, a liquid ammonia supply pump 2, a liquid ammonia subcooling system, a liquid ammonia throttling valve 4, a BOG compressor 6, an ammonia throttling valve 7, and a spray device 8.

[0029] One end of the liquid ammonia supply pump 2 is located below the liquid ammonia level in the liquid ammonia storage tank 1, and the other end is located outside the liquid ammonia storage tank 1 and connected to the liquid ammonia inlet of the liquid ammonia subcooling system. The liquid ammonia supply pump 2 delivers cryogenic liquid ammonia (-10~-30℃) from the liquid ammonia storage tank 1 to the liquid ammonia subcooling system, which is used to cool the liquid ammonia to cryogenic liquid ammonia of approximately -45℃.

[0030] The liquid ammonia subcooling system includes a liquid ammonia subcooler 3, a refrigeration compressor 9, a refrigeration condenser 10, and a refrigerant throttling valve 11. The refrigerant inlet of the liquid ammonia subcooler 3 is connected to the outlet of the refrigerant throttling valve 11, and the refrigerant outlet of the liquid ammonia subcooler 3 is connected to the inlet of the refrigeration compressor 9. The refrigerant inlet of the refrigeration condenser 10 is connected to the outlet of the refrigeration compressor 9, and the refrigerant outlet of the refrigeration condenser 10 is connected to the inlet of the refrigerant throttling valve 11. The refrigeration compressor 9 is used to pressurize the low-pressure refrigerant. The refrigeration condenser 10 is used to cool the refrigerant. The refrigerant throttling valve 11 is used for refrigerant flow control. The liquid ammonia subcooler 3 exchanges heat with the cryogenic liquid ammonia, absorbing heat from the cryogenic liquid ammonia and lowering its temperature to a cryogenic state.

[0031] The liquid ammonia subcooler 3 is equipped with a heat exchange tube 13. The refrigerant inlet of the heat exchange tube 13 is connected to the outlet of the refrigerant throttle valve 11, and the refrigerant outlet of the heat exchange tube 13 is connected to the inlet of the refrigeration compressor 9. The refrigerant can circulate among the refrigeration compressor 9, the refrigeration condenser 10, the refrigerant throttle valve 11 and the heat exchange tube 13.

[0032] The refrigeration condenser 10 is equipped with a second heat exchange tube 14. The refrigerant inlet of the second heat exchange tube 14 is connected to the outlet of the refrigeration compressor 9, and the refrigerant outlet of the second heat exchange tube 14 is connected to the inlet of the refrigerant throttling valve 11.

[0033] The liquid ammonia subcooling system also includes a cooling water pipeline 15, which works in conjunction with the refrigeration condenser 10 to condense and cool the refrigerant.

[0034] The refrigeration compressor 9 compresses the low-pressure gaseous refrigerant into a high-temperature, high-pressure gas, which is then sent to the refrigeration condenser 10. There, it is condensed and cooled by cooling fresh water, transforming into a high-pressure liquid refrigerant. Subsequently, the refrigerant is depressurized by the refrigerant throttling valve 11, becoming a low-temperature, low-pressure refrigerant that enters the liquid ammonia subcooler 3. In the liquid ammonia subcooler 3, the refrigerant exchanges heat counter-currently with the low-temperature liquid ammonia, absorbing heat from the liquid ammonia and evaporating, further cooling the liquid ammonia to a cryogenic state. Simultaneously, the refrigerant itself returns to a low-temperature, low-pressure gaseous state and returns to the inlet of the refrigeration compressor 9, completing the cycle. The liquid ammonia subcooling system achieves deep cooling of liquid ammonia through continuous refrigeration cycles.

[0035] The pressure control system for the ship's liquid ammonia storage tank also includes an ammonia mixing and liquefying device 5, used to mix BOG pressurized by the BOG compressor 6 with cryogenic liquid ammonia obtained from the liquid ammonia subcooling system, thereby achieving rapid liquefaction of the BOG. ​​The outlet of the liquid ammonia throttle valve 4 is connected to the liquid ammonia inlet of the ammonia mixing and liquefying device 5, the outlet of the ammonia throttle valve 7 is connected to the ammonia inlet of the ammonia mixing and liquefying device 5, and the liquid ammonia outlet of the ammonia mixing and liquefying device 5 is connected to the inlet of spray device 1 8. A second spray device 12 is installed inside the ammonia mixing and liquefying device 5, and the liquid ammonia inlet of the ammonia mixing and liquefying device 5 is connected to the second spray device 12. The second spray device 12 is located inside the ammonia mixing and liquefying device 5 and is used to spray the cryogenic liquid ammonia (approximately -45°C) processed by the liquid ammonia subcooling system into the upper gas phase space inside the mixing and liquefying device 5, causing the BOG from the liquid ammonia storage tank 1 to condense and liquefy into liquid ammonia. The second spraying device 12 includes a spray pipe and a plurality of nozzles disposed on the spray pipe.

[0036] Cryogenic liquid ammonia (approximately -45°C) comes into contact with and exchanges heat with the throttled BOG in ammonia mixing liquefaction unit 5, cooling the BOG to a liquefied state and forming cryogenic liquid ammonia at approximately -40°C. This process achieves simultaneous liquefaction of BOG inside and outside the storage tank, improving liquefaction efficiency and effectively reducing the temperature and pressure of the evaporated gas inside the storage tank. This keeps the tank pressure consistently around atmospheric pressure, ensuring the safe and stable operation of the system.

[0037] One end of the liquid ammonia throttling valve 4 is connected to the outlet of the liquid ammonia subcooler 3, and the other end is connected to the cryogenic liquid ammonia inlet of the ammonia mixing liquefaction unit 5. It is used to throttle and reduce the pressure of the cryogenic liquid ammonia (about -45℃, 16 barg) from the liquid ammonia subcooler 3 and send it into the ammonia mixing liquefaction unit 5 to mix with BOG and achieve further liquefaction.

[0038] The inlet of the BOG compressor 6 is connected to the ammonia outlet of the liquid ammonia storage tank (1), and the outlet of the BOG compressor 6 is connected to the ammonia inlet of the ammonia mixing liquefaction unit (5). This is used to extract BOG (-10~-30℃) from the liquid ammonia storage tank 1 and pressurize it to approximately 16 barg, providing sufficient pressure for subsequent liquefaction processes.

[0039] One end of the ammonia throttling valve 7 is connected to the outlet of the BOG compressor 6, and the other end is connected to the BOG inlet of the ammonia mixing liquefaction unit 5. It is used to throttle the BOG after it has been pressurized by the BOG compressor 6, so that it has suitable pressure and temperature conditions when it enters the ammonia mixing liquefaction unit 5, so as to fully mix with the cryogenic liquid ammonia and achieve rapid liquefaction.

[0040] A spray device 8 is installed inside the liquid ammonia storage tank 1 and is used to spray the cryogenic liquid ammonia (approximately -40°C) processed by the ammonia mixing liquefaction unit 5 into the upper gas phase space inside the liquid ammonia storage tank 1, so that the BOG in the liquid ammonia storage tank 1 is condensed and liquefied into liquid ammonia. The spray device 8 includes a spray pipe and multiple nozzles installed on the spray pipe.

[0041] The pressure control system for the ship's liquid ammonia storage tank also includes a liquid ammonia control valve 16 and an ammonia control valve 2 17. The inlet of the liquid ammonia control valve 16 is connected to the outlet of the liquid ammonia supply pump 2, and the outlet of the liquid ammonia control valve 16 is connected to the liquid ammonia inlet of the liquid ammonia subcooling system, for controlling the flow rate of liquid ammonia; the inlet of the ammonia control valve 2 17 is connected to the ammonia outlet of the liquid ammonia storage tank (1), and the outlet of the ammonia control valve 2 17 is connected to the inlet of the BOG compressor 6, for controlling the flow rate of BOG.

[0042] The pressure control system for marine liquid ammonia storage tanks of this utility model mainly includes the following four coordinated operation processes:

[0043] Refrigerant cycle process: The refrigeration compressor 9 compresses and increases the pressure of the low-pressure gaseous refrigerant, which is then sent to the refrigeration condenser 10. Cooling water is used to condense and cool the high-temperature, high-pressure refrigerant. The refrigerant then passes through the refrigerant throttling valve 11 to reduce its pressure before entering the liquid ammonia subcooler 3. In the liquid ammonia subcooler 3, the low-temperature, low-pressure liquid refrigerant exchanges heat with the liquid ammonia in a counter-current manner, absorbing heat and evaporating before returning to the compressor, thus completing the refrigeration cycle; simultaneously, the liquid ammonia is further cooled.

[0044] Cryogenic Liquid Ammonia Process: Liquid ammonia supply pump 2 draws cryogenic liquid ammonia at a temperature of -10~-30℃ from liquid ammonia storage tank 1, pressurizes it to approximately 16 barg, and then sends it to liquid ammonia subcooler 3. Here, it exchanges heat with refrigerant and is cooled to approximately -45℃ to form cryogenic liquid ammonia. Subsequently, it is throttled and depressurized by liquid ammonia throttling valve 4 and enters ammonia mixing and liquefying tank 5 to provide a cold source for BOG liquefaction.

[0045] The ammonia mixing and liquefaction process is as follows: BOG (-10~-30℃) generated in the storage tank is pressurized to approximately 16 barg by the BOG compressor 6, and then throttled and cooled by the ammonia throttling valve 7 before being sent to the ammonia mixing and liquefaction unit 5. In this unit, the BOG is sprayed by the spray device 12 installed inside the ammonia mixing and liquefaction unit 5, causing direct contact and heat exchange between the BOG and the cryogenic liquid ammonia (approximately -45℃) from the cryogenic liquid ammonia process. The BOG is condensed and liquefied into cryogenic liquid ammonia at approximately -40℃, achieving closed-loop recovery.

[0046] Tank-based spray liquefaction process: Cryogenic liquid ammonia (approximately -40°C) generated from the ammonia mixing liquefaction unit 5 returns to the top of the liquid ammonia storage tank 1 and is evenly sprayed into the upper gas phase space of the liquid ammonia storage tank 1 through uniformly distributed spray devices 8, causing the BOG inside the liquid ammonia storage tank 1 to condense and liquefy into liquid ammonia. During the spraying process, the cryogenic liquid ammonia absorbs heat from the BOG, lowering the gas phase temperature, thereby effectively suppressing the rise in tank pressure and maintaining the pressure stably at an atmospheric pressure level during ship operation.

[0047] The synergistic effect of the above four processes enables efficient liquefaction and reuse of BOG in liquid ammonia storage tank 1, as well as pressure self-balancing control, thereby improving system safety, energy efficiency, and environmental friendliness.

[0048] This invention significantly improves the pressure control efficiency of liquid ammonia storage tank 1 by introducing a secondary spray BOG liquefaction mechanism. Specifically, after primary liquefaction via spray device 12 within the ammonia mixing liquefaction unit 5, the cryogenic liquid ammonia (approximately -40°C) generated in the ammonia mixing liquefaction unit 5 is returned to the top of the liquid ammonia storage tank 1, where it undergoes secondary spray cooling of the BOG inside the tank via a uniformly distributed spray device 8. This process directly reduces the temperature of the gas phase space, promotes further condensation of residual BOG, and achieves continuous pressure reduction inside the storage tank.

[0049] Combining an external BOG compressor-driven mixed liquefaction process, the system simultaneously liquefies BOG both inside and outside the storage tank. Externally, primary liquefaction is achieved through pressurization and spraying, mixing with cryogenic liquid ammonia; internally, auxiliary deep liquefaction is achieved through spraying, forming a closed-loop, highly efficient pressure control system. This setup not only accelerates pressure response but also significantly enhances system stability and safety, ensuring the tank pressure remains at atmospheric pressure during ship operation.

[0050] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A pressure control system for a marine liquid ammonia storage tank, comprising a liquid ammonia storage tank (1), a liquid ammonia supply pump (2), a liquid ammonia subcooling system, a liquid ammonia throttling valve (4), a BOG compressor (6), an ammonia throttling valve (7), and a spray device (8), wherein the spray device (8) is disposed in the upper space within the liquid ammonia storage tank (1) for condensing and liquefying the BOG within the liquid ammonia storage tank (1) into liquid ammonia, characterized in that, The pressure control system also includes an ammonia mixing liquefaction unit (5), which is used to mix the BOG pressurized by the BOG compressor (6) with the cryogenic liquid ammonia obtained by the liquid ammonia subcooling system to achieve rapid liquefaction of the BOG; the outlet of the liquid ammonia throttle valve (4) is connected to the liquid ammonia inlet of the ammonia mixing liquefaction unit (5), the outlet of the ammonia throttle valve (7) is connected to the ammonia inlet of the ammonia mixing liquefaction unit (5), and the liquid ammonia outlet of the ammonia mixing liquefaction unit (5) is connected to the inlet of the spray device (8).

2. The pressure control system according to claim 1, characterized in that, A second spray device (12) is installed in the upper space of the ammonia mixing liquefaction unit (5). The liquid ammonia inlet of the ammonia mixing liquefaction unit (5) is connected to the second spray device (12) to condense and liquefy the BOG in the ammonia mixing liquefaction unit (5) into liquid ammonia.

3. The pressure control system according to claim 2, characterized in that, The liquid ammonia subcooling system includes a liquid ammonia subcooler (3), a refrigeration compressor (9), a refrigeration condenser (10), and a refrigerant throttling valve (11). The refrigerant inlet of the liquid ammonia subcooler (3) is connected to the outlet of the refrigerant throttling valve (11), and the refrigerant outlet of the liquid ammonia subcooler (3) is connected to the inlet of the refrigeration compressor (9). The refrigerant inlet of the refrigeration condenser (10) is connected to the outlet of the refrigeration compressor (9), and the refrigerant outlet of the refrigeration condenser (10) is connected to the inlet of the refrigerant throttling valve (11).

4. The pressure control system according to claim 3, characterized in that, The liquid ammonia subcooler (3) is equipped with a heat exchange tube (13). The refrigerant inlet of the heat exchange tube (13) is connected to the outlet of the refrigerant throttle valve (11), and the refrigerant outlet of the heat exchange tube (13) is connected to the inlet of the refrigeration compressor (9).

5. The pressure control system according to claim 3, characterized in that, The refrigeration condenser (10) is provided with a second heat exchange tube (14). The refrigerant inlet of the second heat exchange tube (14) is connected to the outlet of the refrigeration compressor (9), and the refrigerant outlet of the second heat exchange tube (14) is connected to the inlet of the refrigerant throttle valve (11).

6. The pressure control system according to claim 3, characterized in that, The liquid ammonia subcooling system also includes a cooling water pipeline (15), which works in conjunction with the refrigeration condenser (10) to condense and cool the refrigerant.

7. The pressure control system according to claim 1, characterized in that, The inlet of the BOG compressor (6) is connected to the ammonia outlet of the liquid ammonia storage tank (1), and the outlet of the BOG compressor (6) is connected to the ammonia inlet of the ammonia mixing liquefaction unit (5). It is used to extract BOG from the liquid ammonia storage tank (1), pressurize it, and then transport it to the ammonia mixing liquefaction unit (5).

8. The pressure control system according to claim 3, characterized in that, One end of the liquid ammonia throttling valve (4) is connected to the liquid ammonia outlet of the liquid ammonia subcooler (3), and the other end is connected to the cryogenic liquid ammonia inlet of the ammonia mixing liquefaction unit (5). It is used to throttle and depressurize the cryogenic liquid ammonia from the liquid ammonia subcooler and send it into the ammonia mixing liquefaction unit (5).

9. The pressure control system according to claim 1, characterized in that, One end of the ammonia throttling valve (7) is connected to the outlet of the BOG compressor (6), and the other end is connected to the BOG inlet of the ammonia mixing liquefaction unit (5), which is used to throttle the BOG after it has been pressurized by the BOG compressor (6).

10. The pressure control system according to claim 1, characterized in that, One end of the liquid ammonia supply pump (2) is located below the liquid ammonia level in the liquid ammonia storage tank (1), and the other end is located outside the liquid ammonia storage tank (1) and connected to the liquid ammonia inlet of the liquid ammonia subcooling system.

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