Mixed medium precooling dynamic regulation and control system and method for oversize LNG (liquefied natural gas) storage tank

Abstract

The invention discloses an oversize LNG storage tank mixed medium precooling dynamic regulation and control system and method, and relates to the technical field of LNG storage tank precooling, and the system comprises a precooling medium supply module for providing two or more precooling media; the pre-cooling spraying distribution pipe network is arranged at the top of the inner tank of the LNG storage tank and is used for uniformly spraying a pre-cooling medium into the storage tank; the pre-cooling process control module is used for sequentially controlling the switching, proportioning and conveying rate of liquid nitrogen and LNG according to a preset stage; the thermal stress control and temperature difference balance subsystem is used for monitoring and adjusting the temperature difference of each area in the storage tank; the boil-off gas recovery module is used for collecting and treating natural gas boil-off gas caused by temperature drop in the pre-cooling process; the pre-cooling pre-safety control unit is used for executing air tightness detection, replacement, drying and cold brittleness pre-judgment operation before pre-cooling; a liquid nitrogen and LNG mixed medium pre-cooling scheme is adopted, and more efficient temperature reduction speed and uniformity can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field related to LNG storage tank pre-cooling, and in particular to a mixed medium pre-cooling dynamic control system and method for an extra-large LNG storage tank. Background Art

[0002] LNG storage tanks are divided into single-containment tanks, double-containment tanks, full-containment tanks and membrane tanks according to their structural form. They are divided into above-ground storage tanks and underground storage tanks according to their installation methods. They can be divided into small storage tanks (5-50m 3 ), medium-sized storage tanks (50-100m 3 ), large storage tanks (100~40000m 3 ) and extra large storage tanks (40000~200000m 3 ), extra-large storage tanks are mainly used for LNG receiving station configuration. LNG storage tanks must undergo a drying and replacement process before the pre-cooling procedure. The dew point of the inner tank is required to be less than -20°C, and the oxygen content is less than 4% Vol. The LNG storage tank pre-cooling procedure can only be entered after the supporting process pipelines are replaced, DCS configuration is completed, and important equipment and facilities such as SIS, FGS and LNG submersible pumps are debugged and qualified.

[0003] However, traditional pre-cooling of extra-large LNG storage tanks generally uses a single medium for pre-cooling. The single pre-cooling medium solution has high cost and low cooling efficiency, which in turn affects the pre-cooling effect. Summary of the Invention

[0004] In order to solve the defects of the existing technology, the present invention provides a mixed medium pre-cooling dynamic control system and method for an extra-large LNG storage tank.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] The present invention provides a mixed medium pre-cooling dynamic control system for an extra-large LNG storage tank, comprising a pre-cooling medium supply module, a pre-cooling spray distribution pipe network, a pre-cooling process control module, a thermal stress control and temperature difference balance subsystem, an evaporation gas recovery module, and a pre-cooling front safety control unit;

[0007] A pre-cooling medium supply module provides two or more pre-cooling media, wherein the pre-cooling media includes at least liquid nitrogen and LNG;

[0008] A pre-cooling spray distribution network is arranged on the top of the inner tank of the LNG storage tank and is used to evenly spray the pre-cooling medium into the storage tank;

[0009] The pre-cooling process control module is used to control the switching, ratio and delivery rate of liquid nitrogen and LNG in sequence according to the preset stages;

[0010] Thermal stress control and temperature difference balance subsystem, used to monitor and adjust the temperature difference between various areas inside the tank;

[0011] Boil-off gas recovery module, used to collect and process natural gas boil-off gas caused by temperature drop during the pre-cooling process;

[0012] The pre-cooling safety control unit is used to perform air tightness detection, replacement, drying and cold brittleness prediction operations before pre-cooling.

[0013] The present invention also provides a method for dynamically controlling pre-cooling of a mixed medium in an LNG storage tank, comprising the following steps:

[0014] Pre-cooling preparation stage: Execute pre-safety operations, including using low-leakage blind plates to isolate pre-cooling connecting pipes, testing the cold brittleness of valves and flange seals, using gaseous nitrogen to pre-cool temporary pipelines to below the boiling point of liquid nitrogen, and monitoring the safety status through temperature and pressure sensor interlocks;

[0015] Pre-cooling medium access: Liquid nitrogen and LNG are introduced through independent pipelines via liquid nitrogen tank truck access units and LNG low-pressure pump groups, and seamless transition of medium switching is achieved with the help of regulating valves and interlocking control;

[0016] Pre-cooling distribution and control: The medium is evenly sprayed using the top annular spray distribution network, and the nozzles are equipped with guide vanes and regulating valves. The DCS main control platform dynamically adjusts the medium flow according to temperature, flow rate, and pressure, achieving three-stage temperature curve control: liquid nitrogen rapid pre-cooling stage, LNG intermediate gradient temperature control stage, and liquid level stabilization stage.

[0017] Thermal stress and temperature difference balance adjustment: Through a number of temperature sensors arranged on the upper and lower layers and at the four corners, the temperature difference between any two points is monitored. If the temperature exceeds the threshold, the spray rhythm is automatically adjusted or the buffer valve group is activated;

[0018] Evaporative gas recovery: The evaporated gas is introduced into the evaporated gas recovery system through the top tank gas collection branch.

[0019] The beneficial effects of the present invention are:

[0020] 1. By adopting a "liquid nitrogen + LNG" mixed medium precooling solution in the present invention, extra-large LNG storage tanks can achieve a more efficient temperature drop rate and uniformity during the precooling process, while reducing the use of cooling materials. Mixed medium precooling not only optimizes the use ratio of liquid nitrogen and LNG, but also avoids the problem of excessive material consumption in a single medium system, thereby significantly reducing the overall material cost during the precooling process of LNG storage tanks.

[0021] 2. The thermal stress control and temperature difference balance subsystem in the system of the present invention can effectively control the temperature difference of the inner wall of the tank by accurately monitoring and adjusting the temperature difference between various areas inside the tank, preventing material stress or local freezing caused by excessive temperature difference. The system collects temperature data in real time and performs dynamic adjustments to ensure that the thermal stress during the cooling process of the tank is fully balanced, thereby avoiding equipment damage or safety hazards caused by excessive cooling. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0023] In the attached figure:

[0024] Figure 1 This is a schematic diagram of the overall structure of the mixed medium pre-cooling dynamic control system for super-large LNG storage tanks of the present invention.

[0025] Figure 2 This is a structural schematic diagram of the storage tank connected to the liquid nitrogen truck of the present invention.

[0026] Figure 3 This is a structural schematic diagram of the nitrogen replacement pipeline connected to the storage tank of the present invention.

[0027] Figure 4 This is a schematic diagram of the structure of the first sensor inside the storage tank of the present invention.

[0028] Figure 5 This is a schematic diagram of the structure of the second sensor and the third sensor inside the storage tank of the present invention. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0030] Example 1

[0031] like Figure 1 As shown, the mixed medium pre-cooling dynamic control system for super-large LNG storage tanks includes a pre-cooling medium supply module, a pre-cooling spray distribution pipe network, a pre-cooling process control module, a thermal stress control and temperature difference balance subsystem, an evaporation gas recovery module, and a pre-cooling safety control unit;

[0032] The pre-cooling medium supply module provides two or more pre-cooling media, which include at least liquid nitrogen and LNG; the pre-cooling spray distribution pipeline network is arranged on the top of the LNG storage tank inner tank, and is used to evenly spray the pre-cooling medium into the storage tank; the pre-cooling process control module is used to control the switching, ratio and delivery rate of liquid nitrogen and LNG in sequence according to preset stages; the thermal stress control and temperature difference balance subsystem is used to monitor and adjust the temperature difference of each area inside the storage tank to ensure that the temperature difference of the inner wall of the storage tank does not exceed the set safety threshold; the evaporation gas recovery module is used to collect and process the natural gas evaporation gas caused by temperature drop during the pre-cooling process; the pre-cooling front safety control unit is used to perform air tightness detection, replacement, drying and cold brittleness prediction operations before pre-cooling.

[0033] In LNG receiving stations or terminal storage and transportation facilities, the pre-cooling process of extra-large LNG storage tanks, due to their huge capacity and complex tank structure (usually double-containment or full-containment structures), not only directly affects the safety and efficiency of subsequent LNG liquid injection, but also affects the long-term stability of equipment operation. Traditional pre-cooling processes often use a single medium (such as liquid nitrogen or LNG), which has an insufficient trade-off between economy, safety and cooling efficiency:

[0034] Although liquid nitrogen has high cooling efficiency, its cooling capacity utilization rate is low and the unit cooling capacity cost is high;

[0035] LNG pre-cooling has a higher cooling efficiency, but the initial cooling rate is slow, making it difficult to quickly cross the high-temperature section, and there is a risk of local overcooling.

[0036] If the boil-off natural gas (BOG) in the pre-cooling stage is not handled properly, it will cause energy waste and safety risks;

[0037] Traditional control methods rely on manual adjustment, which has slow response speed and large errors.

[0038] Therefore, there is an urgent need for a pre-cooling system that integrates multi-media combined cooling, efficient thermal stress regulation, automatic process control and BOG recovery to meet the higher requirements of modern LNG receiving stations for safety, economy and automation level.

[0039] This invention aims to propose a "liquid nitrogen + LNG" mixed medium dynamic pre-cooling control system suitable for extra-large LNG storage tanks. The system has the following advantages:

[0040] Multi-medium coupling complements each other's advantages: Liquid nitrogen's extremely low temperature characteristics are used to quickly achieve initial cooling, and LNG is then used as a medium to achieve smooth gradient cooling and bottoming of the liquid level. By rationally utilizing the cold energy characteristics of liquid nitrogen and LNG, the "start with liquid nitrogen, then LNG" strategy can effectively reduce pre-cooling costs;

[0041] Intelligent control of temperature difference and thermal stress: Real-time temperature distribution perception and linkage adjustment system are used to avoid thermal shock and structural damage;

[0042] BOG (boil-off gas) recovery closed-loop management: During the pre-cooling stage, the vaporized natural gas is recovered for re-liquefaction or transportation, improving the system's cooling energy utilization efficiency;

[0043] Full-process safety closed-loop control: Pre-safety checks, automatic interlocking, and data diagnosis ensure the safety and controllability of the entire operation process, making the pre-cooling process visual, intelligent, and traceable. It is suitable for new storage tanks or the renovation of existing tanks without the need to completely replace the original system and supports modular integration.

[0044] Further, if Figure 2 As shown, the pre-cooling medium supply module includes:

[0045] The liquid nitrogen tank truck access unit includes a temporary unloading pipeline and a pre-cooling structure for the pre-cooling pipeline. The temporary unloading pipeline uses a DN100 (100mm diameter) reserved pipeline to access the low-pressure LNG external transmission main pipe and is connected to the tank top pre-cooling spray distribution pipeline network. A low-leakage blind plate is installed between the liquid nitrogen pipeline and the LNG conventional circuit to prevent liquid nitrogen from accidentally entering the LNG main line due to liquid backflow from the tank truck or internal leakage of the valve. Before unloading the liquid nitrogen, the temporary pre-cooling pipeline of the storage tank should be pre-cooled to -180℃, that is, the low-temperature gaseous nitrogen is used to pre-cool the pre-cooling pipeline of the storage tank when the liquid nitrogen truck is connected. The liquid nitrogen unloading operation can only be carried out after the upper and lower skin temperatures are lower than -180℃.

[0046] The LNG low-pressure pump unit is connected to the LNG liquid phase outlet of the operational storage tank. This part serves as the main cold source for the second stage of mixed pre-cooling. LNG is taken from the liquid phase outlet of the operational storage tank (such as tank 2# or 3#) and transported to the pre-cooling spray distribution network using a low-pressure pump. The pre-cooling rate can be precisely controlled through the dual adjustment mechanism of pump frequency conversion and valve positioning. In conjunction with the top pre-cooling spray distribution network and the DCS main control platform, the temperature drop is uniform. Compared with liquid nitrogen, LNG has higher adaptability in temperature control and lower evaporation loss, making it an ideal working fluid for establishing liquid level and completing low-temperature bottoming.

[0047] The liquid nitrogen and LNG correspond to independent pipeline inlets, regulating valves and interlocking control circuits respectively, which are used to achieve seamless transition during the medium switching process and prevent cross contamination;

[0048] To ensure the physical and logical isolation of liquid nitrogen and LNG, this module adopts the following safety designs:

[0049] Independent inlet interface and flow channel: between the external transmission main pipe and the liquid inlet point of the storage tank, liquid nitrogen and LNG are routed through separate pipelines to avoid intersection of the two;

[0050] Regulating valve and check valve design: Each inlet is equipped with a pressure regulating valve and a check valve group to prevent backflow;

[0051] Interlock control system: Based on the interlocking logic of PLC and DCS, once the liquid nitrogen is not unloaded or the temperature does not meet the standard, the system automatically prohibits the entry of LNG;

[0052] The medium-changing logic is embedded in the control platform: combining the triple signals of thermal data, pressure difference and flow rate to automatically execute the smooth switching instruction from liquid nitrogen to LNG.

[0053] The reason for setting up a liquid nitrogen tank truck access unit + LNG low-pressure pump + dual-path control structure is that liquid nitrogen is not a permanent medium in the factory and needs to be supplied by tank trucks. Temporary access to pipelines becomes a necessary solution. At the same time, it can also avoid solidification blockage caused by physical mixing. In particular, considering the difference in thermophysical properties between liquid nitrogen and LNG near the phase change point, while ensuring the controllability of working conditions and the adjustability of cooling rate, the dual-path adjustment system can flexibly respond to construction needs of different tank capacities and different climates.

[0054] Further, if Figure 3 As shown, the temporary unloading pipeline is set as follows:

[0055] Remove and short-circuit the original nitrogen replacement pipeline and install a high-throughput vent pipeline. The diameter of the vent pipeline can be set to 100-200mm. For example, during work, after completing the installation of the vent pipeline, you only need to ensure that valve 1 is closed and valve 2 is open to achieve normal operation of the vent pipeline;

[0056] In traditional LNG storage tank systems, nitrogen displacement pipelines are typically designed for drying and displacement processes before the tanks are put into operation. Their pipe diameter configuration and exhaust capacity are not designed for the high-density spray evaporation of liquid nitrogen, resulting in limited discharge. If the existing short-circuit connection method is used, large amounts of liquid nitrogen atomization spray will cause boil-off gas (BOG) to accumulate at the top of the tank, resulting in local overpressure, backflow, or frequent opening of the safety valve.

[0057] Therefore, the present invention temporarily removes the short circuit during the liquid nitrogen pre-cooling stage and adds a high-throughput venting pipeline to quickly evacuate the gas generated by the evaporation of liquid nitrogen, thereby improving the overall exhaust capacity, reducing the pressure fluctuation in the storage tank, avoiding sudden pressure rise, and ensuring the uniformity of spray atomization and heat exchange rate.

[0058] The original nitrogen replacement pipeline and the venting pipeline are distributed in a straight line along the center of the storage tank. When put into use at the same time, the overall temperature of the storage tank can be uniformly cooled, and the cooling rate of the liquid nitrogen pre-cooling tank can be increased from the original 0.5-1.5℃ / h to 2-3.5℃ / h.

[0059] Equipped with automatic vent valve and vacuum drain interface, used to quickly remove condensate in the pipeline when not in operation;

[0060] During the liquid nitrogen pre-cooling process, liquid residue or condensate accumulation is likely to form in the pipeline under low-temperature conditions. To address the above risks, the system adds a vacuum drain interface and an automatic vent valve in the temporary unloading pipeline, which can automatically remove or drain the residual liquid when not in operation, ensuring that there is no condensate residue in the pipeline, ensuring the safe reusability of the pipeline and the smoothness of medium switching.

[0061] All vent pipelines enter the gas collecting cap or safety burner and are equipped with bursting discs and back pressure check devices.

[0062] During the pre-cooling stage, gas emissions are concentrated and the airflow impact is strong. If directly released into the atmosphere, there will be a risk of flammable gas leakage and diffusion, causing low-temperature vaporization erosion to the environment and damaging adjacent facilities or personnel.

[0063] Therefore, all venting pipelines are ultimately connected to a safety burner or gas collecting cap system, supplemented by bursting discs and back-pressure check devices to control the direction of the exhaust path and prevent uncontrolled diffusion of BOG. ​​They can also ensure the stability of the system back pressure and prevent gas backflow. They also provide an overpressure relief function, which automatically releases pressure once it exceeds the limit to prevent overall system damage.

[0064] This design pre-installs a DN100 liquid nitrogen pre-cooling pipeline on the low-pressure external transmission main pipe of the 6# LNG storage tank (this embodiment mainly provides a pre-cooling process example for the 6# LNG storage tank). At the same time, the original nitrogen replacement pipeline is upgraded to a high-efficiency liquid nitrogen discharge path. Combined with the blasting device and the condensate discharge interface, the liquid nitrogen spray efficiency and the safety margin of the system operation are significantly improved.

[0065] From the application effect, in the first stage of liquid nitrogen pre-cooling, after the above optimization, the tank cooling rate was increased from 2℃ / h to 3.5℃ / h, far exceeding the traditional single-medium solution, effectively shortening the pre-cooling time and reducing the amount of liquid nitrogen used, with significant economic benefits. At the same time, the condensate treatment device avoided the subsequent LNG freezing problem, laying a safe foundation for the smooth transition of the medium.

[0066] Furthermore, the pre-cooling spray distribution network is arranged in a ring shape, with multiple nozzles evenly arranged along the top of the storage tank, and is provided with a regulating valve and a gas guide vane structure to optimize the spray coverage range and ensure uniform temperature distribution in the inner tank. The ring-shaped pre-cooling spray distribution network, regulating valve, and gas guide vane can be set up using conventional means and are no longer shown in the figure.

[0067] A circular spray network is installed on the top of the inner tank of the storage tank. This structure ensures that the pre-cooling medium (liquid nitrogen or LNG) evenly covers the inner tank surface. Through the pipe network layout, the liquid can be sprayed simultaneously from multiple nozzles within the pipe network, making the cooling process more uniform, avoiding localized overcooling or overheating, and reducing the risk of cold brittleness.

[0068] Each nozzle is equipped with a regulating valve that controls the flow of the spraying medium. In practice, the regulating valve adjusts the spraying flow and pressure in real time based on the temperature changes on the inner wall of the tank, according to the instructions of the DCS main control platform, thereby maintaining the uniformity of the temperature inside the tank.

[0069] The design of the gas guide vane helps optimize the spray coverage and prevents the nozzle's spray surface from being too concentrated or uneven, further improving cooling efficiency during the pre-cooling process. The guide vane more effectively guides the flow direction of the sprayed medium, ensuring efficient distribution of liquid nitrogen or LNG without causing excessive airflow interference.

[0070] Furthermore, the pre-cooling process control module includes:

[0071] The DCS main control platform is used to receive feedback data from temperature, flow rate and pressure sensors, and generate corresponding control logic instructions based on the data to achieve real-time monitoring and response adjustment of the system pre-cooling status;

[0072] The DCS main control platform is the core control unit of this system. Its main functions include real-time collection of sensor data such as temperature, pressure, and flow rate, executing preset control logic, quantitatively adjusting the output of the cooling medium, monitoring the system status, ensuring that the pre-cooling process can continue only when the safety threshold is not triggered, and realizing automatic connection between the liquid nitrogen stage and the LNG stage, without the need for human intervention, thereby reducing operational risks.

[0073] The switching program between the liquid nitrogen pre-cooling stage and the LNG pre-cooling stage is equipped with a multi-level threshold judgment mechanism based on the sensor data collected, which is used to automatically switch the pre-cooling medium when specific conditions are met. Specifically, the multi-level thresholds include the internal temperature of the storage tank reaching the first stage target temperature (-100℃), the system pressure fluctuation range ≤±0.01MPa (indicating that the system thermal stability meets the requirements), and the BOG generation rate ≤XNm 3 / h (indicating that precooling is close to steady state and the evaporation rate decreases. Optionally, the value range of X is 0.05 to 0.5);

[0074] During the liquid nitrogen pre-cooling stage, the DCS determines whether the first-stage target temperature (-100°C) has been reached based on temperature sensor data. Once the target temperature is reached, the system executes a medium switching command, simultaneously controlling the liquid nitrogen valve to close and the LNG regulating valve to open. This switching control prevents human error from causing residual liquid nitrogen to mix with the LNG flow, and improves heat exchange efficiency.

[0075] The cooling rate control unit, whose internal control logic supports dynamic adjustment of the cooling medium flow rate according to the set target cooling rate, ensures that the overall cooling process does not exceed the predetermined rate range, and avoids problems such as material stress or uneven phase change caused by excessive cooling;

[0076] Preferably, the cooling rate of the inner tank shall not exceed 3°C / h, and the maximum shall not exceed 5°C / h.

[0077] Further, if Figure 4-Figure 5 The thermal stress control and temperature difference balance subsystem shown includes:

[0078] Temperature monitoring points are located at the top, middle, and bottom layers, and several groups of sensors are deployed at the four corners and center of the tank. These groups of sensors are mainly divided into multiple first sensors for detecting the temperature of the inner tank wall, multiple second sensors for detecting the temperature of the bottom plate, and multiple third sensors for detecting the temperature of the annular gap. This ensures that the temperature gradient of different spaces in the entire tank can be monitored in real time. Specifically, there are 14 first sensors from top to bottom, 12 second sensors, and 8 third sensors.

[0079] The system calculates the temperature difference between adjacent points and any two points in real time. When the temperature difference exceeds the preset threshold, it adjusts the medium flow or activates the buffer valve group for dynamic balance adjustment. Generally speaking, the inner tank pre-cooling cooling rate is ≤3℃ / h, the maximum cooling rate cannot exceed 5℃ / h, and the temperature difference between two adjacent points (the temperature difference between two adjacent sensors) is ≤10℃, and the temperature difference between any two points (the temperature difference between any two sensors) is ≤30℃;

[0080] The control logic of the pre-cooling process is to set a three-stage temperature target curve, including: the first stage of liquid nitrogen rapid cooling, the target temperature range is -80°C to -120°C. In this embodiment, preferably, the target temperature range is 100°C, and the cooling rate is controlled within 3.5°C / h. This stage is dominated by liquid nitrogen to ensure that the overall temperature is quickly lowered in the initial stage;

[0081] In the middle of the second stage of LNG pre-cooling, LNG spraying is switched to target temperature drop to -158°C. This stage emphasizes temperature difference balance and gradient control, with a cooling rate of ≤3°C / h.

[0082] The third stage is the temperature stabilization stage before the LNG liquid level is laid at the bottom. The target temperature range is maintained, temperature fluctuations are reduced, and a stable temperature field is created for the tank bottom liquid level. That is, the temperature difference between any two points (the temperature difference between any two sensors) is ≤30℃.

[0083] Furthermore, the evaporation gas recovery module includes:

[0084] The top tank gas collection branch is used to guide the boil-off gas on the top of the storage tank through the venting pipeline to the boil-off gas compression recovery device for treatment;

[0085] During the pre-cooling process, liquid nitrogen and LNG are both cryogenic liquids. After being sprayed in the storage tank and contacting the inner tank wall, they quickly absorb heat and evaporate, producing a large amount of cryogenic gas (i.e. BOG). Since the gas density is lower than that of the liquid, it naturally gathers at the top of the tank. Therefore, a gas collection branch is set to quickly capture the top evaporation gas to prevent it from flowing back and accumulating in other areas of the tank, or suddenly releasing through the safety valve when the system pressure is too high, causing energy waste. The gas collection branch is connected to the evaporation gas compression recovery device, which has the ability to pump air and pressure regulation functions, and can realize the recovery and reuse of steam energy, on the one hand reducing environmental emissions, and on the other hand reducing operating energy consumption costs.

[0086] The pressure regulating valve and the anti-surge check assembly are used to adjust the pressure balance in the system exhaust path and prevent backflow and gas surge, ensuring a safe and stable recovery path;

[0087] During the operation of the precooling system, the generation rate of boil-off gas is affected by many factors (such as precooling flow rate, initial tank temperature, spray density, etc.), resulting in severe pressure fluctuations in the gas recovery path. Without an effective regulation mechanism, this may cause gas backflow (when the pressure in the recovery branch drops suddenly, external gas may flow back into the tank) and gas surge (when the pressure suddenly rises, high-speed gas impacts downstream pipelines and equipment, creating safety risks). To prevent the above problems, this module is designed with an integrated adaptive pressure regulating valve, combined with a check valve and surge prevention duct structure to achieve dynamic response to changes in the pressure difference between the inside and outside of the tank, quickly shut down the return path, and delay the release of surge pressure to ensure stable operation of downstream equipment.

[0088] The exhaust control unit is used to dynamically adjust the exhaust rate according to the real-time changes in the pressure at the top of the tank, so as to achieve the linkage management of system internal pressure control and stable recovery of evaporation gas;

[0089] The exhaust control unit is integrated into the DCS main control platform. By continuously collecting the pressure, temperature and evaporation gas composition at the top of the tank, it realizes an intelligent feedback adjustment mechanism. Its core function is to accurately match the evaporation rate and recovery rate, adjust the exhaust valve opening in real time according to the actual evaporation load, and achieve the stability of the internal pressure of the tank gas phase space within the set value range, so as to avoid the influence of pressure fluctuations on the continuity of the pre-cooling condition and the medium atomization effect.

[0090] Furthermore, the pre-cooling safety control unit performs the following steps before liquid nitrogen pre-cooling:

[0091] Low-leakage blind plates are used to isolate the connections. These blind plates are generally located at the front end of the common section between liquid nitrogen and LNG, fitting tightly against the pipeline flange surface. Metal sealing rings or PTFE-coated structures provide high sealing performance, allowing them to withstand the local thermal stress and pressure differential generated when liquid nitrogen is loaded. This helps prevent trace amounts of liquid nitrogen from leaking through the valve body into the LNG circuit, preventing freezing or thermal expansion rupture of the liquid phase pipeline.

[0092] Conduct cold brittleness tests on valves and flange seals. Under low-temperature conditions, materials such as ordinary carbon steel and alloy steel may enter the brittle transition zone, posing a risk of failure. This system performs cold brittleness tests on all key nodes before liquid nitrogen is unloaded and put into use. If there is a risk of material brittle cracking or poor sealing, the pre-cooling operation is terminated and the parts are replaced.

[0093] Use gaseous nitrogen to precool the temporary precooling pipeline to temperature T 管 , T 管 The temperature is preferably below -180°C;

[0094] The temporary unloading pipeline connecting the liquid nitrogen tank truck to the storage tank must be pre-cooled to below -180°C before the liquid nitrogen enters to prevent "hot and cold shock";

[0095] This system uses gaseous nitrogen to continuously fill the temporary pipeline. Leveraging its low heat capacity and uniform heat exchange, the system evenly reduces the temperature of the upper and lower pipe walls. A program is set to ensure that the temperature difference between the upper and lower pipe walls is ≤10°C, the minimum wall temperature is ≤-180°C, and the cooling time is ≥30 minutes.

[0096] The entire process of continuous nitrogen filling is monitored by the DCS main control platform, and the wall temperature change curve over time is recorded.

[0097] Furthermore, the control system also includes a multi-level automatic interlocking module for controlling the start and stop logic, operating parameters (temperature, flow, pressure, etc.) and abnormal response mechanism of each pre-cooling subsystem (such as DCS main control platform, regulating valve, evaporation gas recovery module, etc.);

[0098] The abnormal response mechanism includes:

[0099] When the temperature rise rate of any temperature monitoring point exceeds the set value, the input of the corresponding medium will be stopped immediately. If the temperature rise rate of any temperature monitoring point exceeds the preset upper limit (such as 3℃ / h), the system will automatically identify it as a cooling abnormality or temperature inversion problem and immediately issue a control instruction to stop the current cooling medium supply to prevent further expansion of the temperature difference and ensure the safety of the tank materials;

[0100] If local condensate accumulation or excessive pressure fluctuations in the pipeline are detected, emergency discharge will be initiated. During the pre-cooling process of liquid nitrogen or LNG, condensate accumulation or short-term drastic fluctuations in pipeline pressure are likely to form locally in the pipeline or storage tank due to factors such as medium evaporation or low-temperature condensation of the pipe. The system identifies such risks through pressure sensors and liquid level measurement modules. If it is determined that condensate accumulation or transient pressure exceeds the limit, it will automatically open a temporary vent branch and start the vacuum extraction unit to quickly remove the condensate. If the pressure fluctuation approaches the system safety boundary, the buffer circuit will be immediately linked to open to adjust the pressure, and the DC main control platform will be notified to mark the event.

[0101] If the gas mixture ratio approaches the lower explosion limit, nitrogen rapid replacement is initiated and the FGS alarm interlock is triggered. The multi-level automated interlock module collects real-time combustible gas concentration data on the top of the tank and the external pipeline area. If the methane concentration in the mixed gas approaches the lower explosion limit, the system will initiate a nitrogen rapid replacement program, pressurizing nitrogen to dilute the gas concentration in the pipeline and inside the tank, and simultaneously trigger the FGS alarm system and emergency shut-off valve, marking the system as "dangerous lock state" and prohibiting further pre-cooling operations until the concentration drops below a safe value.

[0102] The present invention also provides a method for dynamically controlling pre-cooling of a mixed medium in an LNG storage tank, comprising the following steps:

[0103] Pre-cooling preparation stage: Execute pre-safety operations, including using low-leakage blind plates to isolate pre-cooling connecting pipes, testing the cold brittleness of valves and flange seals, using gaseous nitrogen to pre-cool temporary pipelines to below the boiling point of liquid nitrogen, and monitoring the safety status through temperature and pressure sensor interlocks;

[0104] Pre-cooling medium access: Liquid nitrogen and LNG are introduced through independent pipelines via liquid nitrogen tank truck access units and LNG low-pressure pump groups, and seamless transition of medium switching is achieved with the help of regulating valves and interlocking control;

[0105] Pre-cooling distribution and control: The medium is evenly sprayed using the top annular spray distribution network, and the nozzles are equipped with guide vanes and regulating valves. The DCS main control platform dynamically adjusts the medium flow according to temperature, flow rate, and pressure, achieving three-stage temperature curve control: liquid nitrogen rapid pre-cooling stage, LNG intermediate gradient temperature control stage, and liquid level stabilization stage.

[0106] Thermal stress and temperature difference balance adjustment: Several temperature sensors are placed on the upper and lower layers and at the four corners to monitor the temperature difference between any two points. If the temperature exceeds the threshold, the spray rhythm is automatically adjusted or the buffer valve group is activated to ensure the stress balance of the tank wall.

[0107] Evaporative gas recovery: The evaporated gas is introduced into the evaporated gas recovery system through the top tank gas collection branch, and the discharge rhythm is automatically adjusted according to the real-time pressure fluctuation to prevent negative pressure or backflow.

[0108] The specific pre-cooling steps are as follows:

[0109] During the pre-cooling preparation phase, a series of pre-operations are first performed through the pre-cooling pre-safety control unit, including low-leakage blind plate isolation treatment on all pre-cooling connecting pipes, cold brittleness performance testing of valves and flange seals, pre-cooling the temporary pre-cooling pipelines to below -180°C using gaseous nitrogen, and installing temperature and pressure sensors on all temporary pipelines. Interlocking control is implemented through the safety interlock system to ensure that all safety conditions are met before the system can be put into use.

[0110] Then comes the pre-cooling medium access process. First, liquid nitrogen is introduced into the system through the liquid nitrogen tank truck access unit and then into the pre-cooling spray distribution network on the top of the storage tank via a temporary unloading pipeline. During the first stage of liquid nitrogen pre-cooling, the system is automatically controlled by the pre-cooling process control module. The DCS main control platform receives temperature, flow rate and pressure data in real time, and automatically adjusts the liquid nitrogen flow rate based on the feedback value collected by the temperature sensor. The system target temperature is set to -100°C. At this stage, the nitrogen replacement pipeline can be modified to remove the short circuit and install a high-throughput vent branch pipe to increase the discharge flow and enter the gas collecting cap or safety burner through the vent pipeline, maximizing the cooling rate to 3.5°C / h.

[0111] When the temperature inside the tank reaches -100°C, the system automatically switches between liquid nitrogen and LNG. This process achieves a seamless transition and avoids cross contamination through independent pipeline inlets, regulating valves, and interlocking control loops. LNG is then introduced via the LNG low-pressure pump unit, entering the second stage of LNG pre-cooling. The target temperature is preset at -158°C. During this stage, the temperature gradient is controlled to no more than 3°C per hour. The cooling rate control unit dynamically adjusts the flow rate to avoid rapid temperature differences that could lead to concentrated thermal stress in the material.

[0112] Throughout the pre-cooling process, the thermal stress control and temperature difference balance subsystem performs real-time analysis of sensor data located on the upper, middle, and lower layers, as well as at the four corners and center of the tank. The system automatically determines that the temperature difference between two adjacent points must not exceed 10°C, and that the temperature difference between any two points must not exceed 30°C. If this limit is exceeded, dynamic balancing adjustments are made through the buffer valve group. This process proceeds according to a preset three-stage temperature target curve, ensuring balanced tank wall stress and a smoothly controlled temperature drop.

[0113] At the same time, the BOG recovery module collects BOG evaporation gas through a gas collection branch installed on the top of the storage tank and guides it to the BOG compression recovery device. The system automatically adjusts the discharge rate according to the pressure changes at the top of the storage tank. It cooperates with the pressure regulating valve, non-return and surge prevention components and exhaust control unit to achieve full-process pressure balance and safety linkage management.

[0114] Finally, when the inner tank wall temperature and the bottom plate temperature reach -158°C, it means that the tank is ready for a large amount of liquid to enter. At this time, the minimum operating liquid level should be established for the LNG tank to create the necessary conditions for the subsequent trial operation of LNG submersible pumps and other equipment.

[0115] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. Extra-large LNG storage tank mixed medium pre-cooling dynamic control system, characterized by: It includes pre-cooling medium supply module, pre-cooling spray distribution network, pre-cooling process control module, thermal stress control and temperature difference balance subsystem, evaporation gas recovery module and pre-cooling front safety control unit; A pre-cooling medium supply module provides two or more pre-cooling media, wherein the pre-cooling media includes at least liquid nitrogen and LNG; A pre-cooling spray distribution network is arranged on the top of the inner tank of the LNG storage tank and is used to evenly spray the pre-cooling medium into the storage tank; The pre-cooling process control module is used to control the switching, ratio and delivery rate of liquid nitrogen and LNG in sequence according to the preset stages; Thermal stress control and temperature difference balance subsystem, used to monitor and adjust the temperature difference between various areas inside the tank; Boil-off gas recovery module, used to collect and process natural gas boil-off gas caused by temperature drop during the pre-cooling process; The pre-cooling safety control unit is used to perform air tightness detection, replacement, drying and cold brittleness prediction operations before pre-cooling.

2. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 1 is characterized in that: The pre-cooling medium supply module includes: Liquid nitrogen tank truck access unit, including temporary unloading pipeline and pre-cooling pipeline pre-cooling structure; LNG low-pressure pump unit, connected to the LNG liquid phase outlet of the commissioned storage tank; The liquid nitrogen and LNG correspond to independent pipeline inlets, regulating valves and interlocking control circuits respectively, so as to realize seamless transition of the medium switching process.

3. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 2 is characterized in that: The temporary unloading pipeline is set up as follows: Remove and short-circuit the original nitrogen replacement pipeline and install a vent pipeline; Equipped with automatic vent valve and vacuum drain interface, used to quickly remove condensate in the pipeline when not in operation; All vent pipelines enter the gas collecting cap or safety burner and are equipped with bursting discs and back pressure check devices.

4. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 1 is characterized in that: The pre-cooling spray distribution pipe network is arranged in a ring shape, with multiple nozzles evenly arranged along the top of the storage tank, and is equipped with a regulating valve and a gas guide plate structure to optimize the spray coverage range.

5. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 4 is characterized in that: The pre-cooling process control module includes: The DCS main control platform is used to receive feedback data from temperature, flow rate and pressure sensors and generate corresponding control logic instructions accordingly; The switching process between the liquid nitrogen pre-cooling stage and the LNG pre-cooling stage is equipped with a multi-level threshold judgment mechanism based on sensor data, which is used to automatically switch the pre-cooling medium when specific conditions are met; The cooling rate control unit has internal control logic that supports dynamic adjustment of the cooling medium flow rate according to the set target cooling rate.

6. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 1 is characterized in that: The thermal stress control and temperature difference balance subsystem and pre-cooling process control logic include: There are three temperature monitoring points on the upper, middle and lower layers, and several groups of sensors are arranged at the four corners and center of the tank; The system calculates the temperature difference between adjacent points and any two points in real time. When the temperature difference exceeds the preset threshold, it adjusts the medium flow or activates the buffer valve group to make dynamic balance adjustments. The pre-cooling process sets a three-stage temperature target curve, including: the first stage of rapid liquid nitrogen cooling, the second stage of LNG pre-cooling intermediate interval, and the third stage of temperature stabilization before the LNG liquid level reaches the bottom.

7. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 1 is characterized in that: The evaporation gas recovery module includes: The top tank gas collection branch is used to guide the boil-off gas on the top of the storage tank through the venting pipeline to the boil-off gas compression recovery device for treatment; Pressure regulating valve and check valve assembly are used to adjust the pressure balance in the exhaust passage of the system and prevent backflow and gas surge; The exhaust control unit is used to dynamically adjust the exhaust rate according to the real-time changes in the pressure at the top of the tank.

8. The super-large LNG storage tank mixed medium pre-cooling dynamic control system according to claim 1 is characterized in that: The pre-cooling safety control unit performs the following steps before liquid nitrogen pre-cooling: Use blind plates to isolate the joints; Conduct cold brittleness test on valves and flange seals; Use gaseous nitrogen to precool the temporary precooling line to temperature T 管 .

9. A method for dynamically controlling precooling of mixed media in an LNG storage tank, used in a dynamic control system for precooling mixed media in an extra-large LNG storage tank according to any one of claims 1 to 8, characterized in that: The following steps are involved: Pre-cooling preparation stage: Execute pre-safety operations, including using low-leakage blind plates to isolate pre-cooling connecting pipes, testing the cold brittleness of valves and flange seals, using gaseous nitrogen to pre-cool temporary pipelines to below the boiling point of liquid nitrogen, and monitoring the safety status through temperature and pressure sensor interlocks; Pre-cooling medium access: Liquid nitrogen and LNG are introduced through independent pipelines via liquid nitrogen tank truck access units and LNG low-pressure pump groups, and seamless transition of medium switching is achieved with the help of regulating valves and interlocking control; Pre-cooling distribution and control: The medium is evenly sprayed using the top annular spray distribution network, and the nozzles are equipped with guide vanes and regulating valves. The DCS main control platform dynamically adjusts the medium flow according to temperature, flow rate, and pressure, achieving three-stage temperature curve control: liquid nitrogen rapid pre-cooling stage, LNG intermediate gradient temperature control stage, and liquid level stabilization stage. Thermal stress and temperature difference balance adjustment: Through a number of temperature sensors arranged on the upper and lower layers and at the four corners, the temperature difference between any two points is monitored. If the temperature exceeds the threshold, the spray rhythm is automatically adjusted or the buffer valve group is activated; Evaporative gas recovery: The evaporated gas is introduced into the evaporated gas recovery system through the top tank gas collection branch.

Images