Large-scale lng storage tank mixed medium precooling dynamic regulation system and method

Abstract

The application discloses a super-large LNG storage tank mixed medium precooling dynamic regulation and control system and method, relates to the technical field of LNG storage tank precooling, and comprises a precooling medium supply module, provides two or more than two precooling media; a precooling spray distribution pipe network is arranged on the top of the inner tank of the LNG storage tank and is used for uniformly spraying the precooling medium into the storage tank; a precooling process control module is used for sequentially controlling the switching, ratio and conveying rate of liquid nitrogen and LNG according to preset stages; a thermal stress control and temperature difference balance subsystem is used for monitoring and adjusting the temperature difference of each region in the storage tank; an evaporation gas recovery module is used for collecting and processing natural gas evaporation gas caused by temperature drop in the precooling process; a precooling preposition safety control unit is used for performing air tightness detection, replacement, drying and cold brittle pre-judgment operation before precooling; the application adopts the "liquid nitrogen+LNG" mixed medium precooling scheme and can realize more efficient temperature drop speed and uniformity.

Description

Technical Field

[0001] This invention relates to the technical field of LNG storage tank precooling, specifically to a dynamic control system and method for precooling mixed media in extra-large LNG storage tanks. Background Technology

[0002] LNG storage tanks are classified by structure into single-containment tanks, double-containment tanks, full-containment tanks, and membrane tanks; by installation method into above-ground tanks and underground tanks; and by capacity into small tanks (5–50 m³). 3 Medium-sized storage tanks (50-100m³) 3 Large storage tanks (100-40000m³) 3 ) and extra-large storage tanks (40,000 to 200,000 m³) 3 Extra-large storage tanks are mainly used in LNG receiving terminals. Before the pre-cooling process, the LNG storage tank must undergo a drying and replacement process. The inner tank dew point is required to be <-20℃ and the oxygen content is required to be <4% Vol. The supporting process pipeline replacement, DCS configuration completion, and important equipment and facilities such as SIS, FGS and LNG submerged pumps can only enter the LNG storage tank pre-cooling process after the system is successfully commissioned.

[0003] However, traditional precooling of extra-large LNG storage tanks generally uses a single medium for precooling. The single precooling medium scheme is costly and has low cooling capacity utilization efficiency, which in turn affects the precooling effect. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a dynamic control system and method for precooling mixed media in extra-large LNG storage tanks.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] This invention provides a dynamic control system for precooling mixed media in an extra-large LNG storage tank, including a precooling medium supply module, a precooling spray distribution network, a precooling process control module, a thermal stress control and temperature difference balance subsystem, an evaporation gas recovery module, and a precooling pre-safety control unit.

[0007] A precooling medium supply module provides two or more precooling media, wherein the precooling media include at least liquid nitrogen and LNG;

[0008] A pre-cooling spray distribution network is installed on the top of the inner tank of the LNG storage tank to uniformly spray the pre-cooling medium into the storage tank.

[0009] The precooling process control module is used to control the switching, proportioning, and delivery rate of liquid nitrogen and LNG in sequence according to preset stages.

[0010] The thermal stress control and temperature difference balance subsystem is used to monitor and regulate the temperature difference between different areas inside the storage tank.

[0011] The evaporation gas recovery module is used to collect and process the natural gas evaporation gas caused by temperature drop during the precooling process;

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

[0013] This invention also provides a method for dynamic control of precooling of mixed media in LNG storage tanks, comprising the following steps:

[0014] Pre-cooling preparation stage: Perform pre-safe operations, including using low-leakage blind flanges to isolate pre-cooled connecting pipelines, testing the cold brittleness of valves and flange seals, pre-cooling temporary pipelines with gaseous nitrogen 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 tanker access unit and LNG low-pressure pump group respectively, and seamless transition of medium switching is achieved by means of regulating valves and interlock control;

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

[0017] Thermal stress and temperature difference balance regulation: By using several 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 difference exceeds the threshold, the spray rhythm is automatically adjusted or the buffer valve group is activated.

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

[0019] The beneficial effects of this invention are:

[0020] 1. In this invention, by adopting a "liquid nitrogen + LNG" mixed medium precooling scheme, extra-large LNG storage tanks can achieve a more efficient temperature drop rate and uniformity during the precooling process, while reducing the amount of cooling materials used. Mixed medium precooling not only optimizes the usage ratio of liquid nitrogen and LNG, but also avoids the problem of excessive material consumption in a single medium system. Therefore, it can significantly reduce the overall material cost in the LNG storage tank precooling process.

[0021] 2. The thermal stress control and temperature difference balance subsystem 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 of each area inside the tank, and prevent material stress or local freezing caused by excessive temperature difference. The system ensures that the thermal stress is fully balanced during the cooling process of the tank by collecting temperature data in real time and making dynamic adjustments, thereby avoiding equipment damage or safety hazards caused by excessive cooling. Attached Figure Description

[0022] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0023] In the attached diagram:

[0024] Figure 1 This is a schematic diagram of the overall structure of the dynamic control system for precooling mixed media in the extra-large LNG storage tank of the present invention.

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

[0026] Figure 3 This is a schematic diagram of the structure 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 and third sensors inside the storage tank of the present invention. Detailed Implementation

[0029] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0030] Example 1

[0031] like Figure 1 As shown, the dynamic control system for precooling mixed media in an extra-large LNG storage tank includes a precooling medium supply module, a precooling spray distribution network, a precooling process control module, a thermal stress control and temperature difference balance subsystem, an evaporation gas recovery module, and a precooling pre-safety control unit.

[0032] The system includes a precooling medium supply module, providing two or more precooling media, including at least liquid nitrogen and LNG; a precooling spray distribution network, installed on the top of the LNG storage tank, for uniformly spraying the precooling media into the tank; a precooling process control module, for sequentially controlling the switching, proportioning, and delivery rate of liquid nitrogen and LNG according to preset stages; a thermal stress control and temperature difference balance subsystem, for monitoring and adjusting the temperature difference between different areas inside the tank to ensure that the temperature difference of the tank wall does not exceed a set safety threshold; an evaporation gas recovery module, for collecting and processing natural gas evaporation gas caused by temperature drop during the precooling process; and a precooling pre-safety control unit, for performing airtightness testing, replacement, drying, and cold embrittlement prediction operations before precooling.

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

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

[0035] LNG precooling has a higher efficiency in utilizing cold energy, but the initial cooling rate is slow, making it difficult to quickly cross the high temperature range, and there is a risk of local overcooling.

[0036] Improper handling of boil-off gas (BOG) during the pre-cooling stage can lead to energy waste and safety risks.

[0037] Traditional control methods rely on manual adjustment, resulting in slow response and large errors.

[0038] Therefore, there is an urgent need for a precooling 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 terminals for safety, economy and automation.

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

[0040] Multi-medium coupling offers complementary advantages: The extremely low temperature characteristics of liquid nitrogen are used to quickly achieve initial cooling, and then LNG is used as a medium to complete stable gradient cooling and liquid leveling. By making reasonable use of the cold energy characteristics of liquid nitrogen and LNG, the strategy of "starting with liquid nitrogen and then replacing with LNG" can effectively reduce pre-cooling costs.

[0041] Intelligent control of temperature difference and thermal stress: Through a real-time temperature distribution sensing and linkage adjustment system, thermal shock and structural damage are avoided;

[0042] BOG (evaporated gas) recovery closed-loop management: During the pre-cooling stage, gasified natural gas is recovered for reliquefaction or transportation and utilization, improving the system's cold energy utilization efficiency;

[0043] Full-process safety closed-loop control: Through pre-operation safety checks, automatic interlocks, and data diagnostics, the entire operation process is ensured to be safe and controllable. The pre-cooling process is made visible, 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] Furthermore, such as Figure 2 As shown, the precooling medium supply module includes:

[0045] The liquid nitrogen tanker 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 connect to the low-pressure LNG export main pipeline and is connected to the pre-cooling spray distribution network on the top of the tank. A low-leakage blind flange 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 tanker backflow or valve internal leakage. Before unloading liquid nitrogen, the temporary pre-cooling pipeline of the storage tank should be pre-cooled to -180℃. That is, the liquid nitrogen vehicle is connected to the storage tank pre-cooling pipeline by using low-temperature gaseous nitrogen to pre-cool it so that the temperature of the upper and lower surfaces is below -180℃ before the liquid nitrogen unloading operation can be carried out.

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

[0047] The liquid nitrogen and LNG each have independent pipeline inlets, regulating valves, and interlocking control loops, which are used to achieve a seamless transition during the medium switching process and prevent cross-contamination.

[0048] To ensure physical and logical isolation between liquid nitrogen and LNG, this module employs the following safety design:

[0049] Independent inlet interface and flow channel: Between the main export pipeline and the liquid inlet point of the storage tank, liquid nitrogen and LNG run through their own pipelines to avoid them from converging;

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

[0051] Interlocking control system: Based on the PLC and DCS interlocking logic, the system automatically prevents LNG from entering if liquid nitrogen is not completely unloaded or the temperature does not meet the standard.

[0052] Medium switching logic is embedded in the control platform: by combining thermal data, pressure difference and flow rate signals, it automatically executes the smooth switching command from liquid nitrogen to LNG.

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

[0054] Furthermore, such as Figure 3 As shown, the temporary unloading pipeline is configured as follows:

[0055] The original nitrogen replacement pipeline is dismantled and short-connected, and a high-throughput venting pipeline is installed. The diameter of the venting pipeline can be set to 100-200mm. For example, during operation, after the installation of the venting pipeline is completed, it is only necessary to ensure that valve 1 is closed and valve 2 is open to achieve normal operation of the venting pipeline.

[0056] In traditional LNG storage tank systems, nitrogen replacement pipelines are typically designed for drying and replacement processes before the tank is put into operation. Their pipe diameter and exhaust capacity are not designed for the high-density spray evaporation of liquid nitrogen, resulting in emission limitations. If the original short-connection method is used, the large amount of liquid nitrogen atomized and sprayed will cause the evaporating gas (BOG) at the top of the tank to stagnate, resulting in problems such as local overpressure, backflow, or frequent opening of safety valves.

[0057] Therefore, the present invention temporarily removes the short connection during the liquid nitrogen precooling stage and adds a high-throughput venting pipeline to quickly vent the gas formed by liquid nitrogen evaporation, improve the overall venting capacity, reduce pressure fluctuations in the storage tank, avoid sudden pressure rises, and ensure the uniformity of spray atomization and heat exchange rate.

[0058] The original nitrogen replacement pipeline and venting pipeline are distributed in a straight line along the center of the storage tank. When put into use at the same time, they can achieve uniform cooling of the entire storage tank, which can increase the cooling rate of the liquid nitrogen precooling storage tank from the original 0.5-1.5℃ / h to 2-3.5℃ / h.

[0059] It is equipped with an automatic vent valve and a vacuum drain interface for quickly removing condensate from the pipeline when not in operation;

[0060] During the liquid nitrogen precooling process, liquid residue or condensate accumulation is likely to form in the pipeline under low temperature conditions. To address the above risks, the system is equipped with a vacuum drain interface and an automatic vent valve in the temporary unloading pipeline. This allows for the automatic removal or drainage of residual liquid when not in operation, ensuring that there is no condensate residue inside the pipeline and guaranteeing the pipeline's safety reusability and smooth media switching.

[0061] All vent lines should lead to a gas collection cap or safety burner and be equipped with rupture discs and backpressure check devices.

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

[0063] Therefore, all vent lines are ultimately connected to a safety burner or gas collection cap system, and are equipped with rupture discs and backpressure check devices to control the direction of the exhaust path, prevent uncontrolled diffusion of BOG, ensure stable system back pressure, prevent gas backflow, and provide overpressure relief function. Once the pressure exceeds the limit, it will automatically release to prevent overall system damage.

[0064] The design pre-installs a DN100 liquid nitrogen pre-cooling pipeline on the low-pressure external transmission main 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 explosion device and condensate discharge interface, the liquid nitrogen spraying efficiency and the safety margin of system operation are significantly improved.

[0065] In terms of application results, after the above optimization, the cooling rate of the storage tank in the first stage of liquid nitrogen precooling increased from 2℃ / h to 3.5℃ / h, far exceeding the traditional single-medium scheme. This effectively shortened the precooling time and reduced the amount of liquid nitrogen used, resulting in significant economic benefits. At the same time, the condensate treatment device avoided the subsequent LNG freezing problem, laying a safe foundation for a smooth transition of the medium.

[0066] Furthermore, the precooling spray distribution network is arranged in a ring, with multiple nozzles evenly arranged along the top of the storage tank, and equipped with regulating valves and gas guide vanes to optimize the spray coverage and ensure uniform temperature distribution inside the tank. The ring-shaped precooling spray distribution network, regulating valves, and gas guide vanes can all be set using conventional methods, and are not shown in the figure.

[0067] The inner tank of the storage tank is equipped with a ring-shaped spray pipe network structure at its top. This structure ensures that the pre-cooling medium (liquid nitrogen or LNG) evenly covers the surface of the inner tank. Through the pipe network arrangement, the liquid can be sprayed simultaneously from multiple nozzles within the network, resulting in a more uniform cooling process, avoiding localized overcooling or overheating, and reducing the risk of cold embrittlement.

[0068] Each nozzle is equipped with a regulating valve to control the flow rate of the sprayed medium. In practical applications, based on the temperature changes of the inner wall of the storage tank, the regulating valve can adjust the spray flow rate and pressure in real time according to the instructions of the DCS main control platform, thereby maintaining the temperature uniformity inside the storage tank.

[0069] The design of the gas guide vanes helps optimize the spray coverage, preventing the nozzle spray surface from being too concentrated or uneven, thereby further improving the cooling efficiency during the precooling process. The guide vanes also more effectively guide the flow direction of the ejected medium, ensuring efficient distribution of liquid nitrogen or LNG without generating excessive airflow interference.

[0070] Furthermore, the precooling 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 commands to realize real-time monitoring and response adjustment of the system precooling state.

[0072] The DCS main control platform is the core control unit of this system. Its main functions include real-time acquisition of sensor data such as temperature, pressure, and flow rate; execution of preset control logic; quantitative output adjustment of cooling medium; monitoring of system status to ensure that the precooling process can continue only when the safety threshold is not triggered; and automatic connection between the liquid nitrogen stage and the LNG stage without manual intervention, thus reducing operational risks.

[0073] The switching procedure between the liquid nitrogen precooling stage and the LNG precooling stage is equipped with a multi-level threshold judgment mechanism based on sensor data. This mechanism automatically switches the precooling 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℃); system pressure fluctuation range ≤ ±0.01MPa (indicating that the system thermal stability meets the requirements); and BOG generation rate ≤ XNm. 3 / h (characterizing that precooling approaches steady state and the evaporation rate decreases; optionally, the value of X ranges from 0.05 to 0.5);

[0074] During the liquid nitrogen precooling stage, the DCS determines whether the first-stage target temperature (-100℃) has been reached based on temperature sensor data. Once the target is met, the system executes a medium switching command and simultaneously controls the liquid nitrogen valve to close and the LNG regulating valve to open. This switching control can prevent human error from causing liquid nitrogen residue to mix with LNG and improve heat exchange efficiency.

[0075] 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, so as to ensure that the overall cooling process does not exceed the predetermined rate range and avoid problems such as material stress or uneven phase transformation caused by excessive cooling.

[0076] Preferably, the cooling rate of the inner tank shall not exceed 3℃ / h, and shall not exceed 5℃ / h at most.

[0077] Furthermore, such as Figures 4-5 The thermal stress control and temperature difference balance subsystem shown includes:

[0078] Temperature monitoring points are set up in three layers (top, middle, and bottom), and several sets of sensors are deployed at the four corners and center of the tank. These sets 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, 12 second sensors, and 8 third sensors from top to bottom.

[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 dynamically balances the temperature by adjusting the medium flow rate or activating the buffer valve group. Generally, the pre-cooling rate of the inner tank 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 precooling process control logic is to set a three-stage temperature target curve, including: the first stage is the rapid cooling stage of liquid nitrogen, with a target temperature range of -80℃ to -120℃. In this embodiment, preferably, the target temperature range is 100℃, and the cooling rate is controlled within 3.5℃ / h. This stage is dominated by liquid nitrogen to ensure that the overall temperature is quickly lowered in the initial stage.

[0081] The second stage is the LNG pre-cooling intermediate zone, which is switched to LNG spraying, with the target temperature dropping to -158℃. This stage emphasizes temperature difference balance and gradient control, with a cooling rate of ≤3℃ / h.

[0082] The third stage is the temperature stabilization stage before the LNG level is laid at the bottom. This stage maintains the target temperature range, reduces temperature fluctuations, and creates a stable temperature field for the LNG level at the bottom of the tank, i.e., 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 evaporated gas from the top of the storage tank to the evaporated gas compression and recovery device for treatment via the venting pipeline.

[0085] During the precooling process, both liquid nitrogen and LNG are cryogenic liquids. After being sprayed into the tank and coming into contact with the inner tank wall, they rapidly absorb heat and evaporate, generating a large amount of cryogenic gas (i.e., BOG). Since the gas density is less than that of the liquid, it naturally accumulates at the top of the tank. Therefore, a gas collection branch is set up to quickly capture the top evaporating gas, preventing it from flowing back and accumulating in other areas of the tank, or being released through a safety valve when the system pressure is too high, thus avoiding energy waste. This gas collection branch is connected to an evaporating gas compression and recovery device, which has the ability to pump gas and regulate pressure, enabling the recovery and reuse of steam energy. This reduces environmental emissions on the one hand, and lowers operating energy costs on the other.

[0086] Pressure regulating valves and anti-backflow and anti-surge components are used to regulate the pressure balance in the system exhaust passage and prevent backflow and gas surge, ensuring the safety and stability of the recovery path;

[0087] During operation, the precooling system is affected by multiple factors (such as precooling flow rate, initial tank temperature, spray density, etc.), resulting in drastic 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 rises suddenly, high-speed gas impacts downstream pipelines and equipment, creating safety risks). To prevent these problems, this module is designed to integrate an adaptive pressure regulating valve, combined with a check valve and anti-surge conduit structure, to dynamically respond to changes in the pressure difference inside and outside the tank, quickly shut off the backflow path, and delay and release surge pressure, ensuring 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 storage tank, so as to realize the linkage management of internal pressure control and stable recovery of evaporated gas.

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

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

[0091] Low-leakage blind flanges are used to isolate the connection. The blind flanges are generally installed at the front end of the common section between liquid nitrogen and LNG, and are tightly fitted to the pipeline flange face. High sealing performance is achieved through metal sealing rings or PTFE-coated structures. They can withstand the local thermal stress and pressure difference generated when liquid nitrogen is loaded, which helps to prevent trace amounts of liquid nitrogen from leaking into the LNG circuit through the valve body and prevent the liquid phase pipeline from freezing or thermal expansion and rupture.

[0092] The cold brittleness performance of valves and flange seals is tested. Under low temperature conditions, materials such as ordinary carbon steel and alloy steel may enter the brittle transition zone and are at risk of failure. Before unloading and putting into use, this system performs cold brittleness performance testing on all key nodes. If there is a risk of material cracking or poor sealing, the pre-cooling operation is stopped and the parts are replaced.

[0093] The temporary precooling pipeline was precooled to temperature T using gaseous nitrogen. 管 T 管 The preferred temperature is below -180℃;

[0094] The temporary unloading pipeline connecting the liquid nitrogen tanker to the storage tank must be pre-cooled to below -180°C before the liquid nitrogen enters to prevent thermal shock.

[0095] This system uses gaseous nitrogen to continuously fill the temporary pipeline. Taking advantage of its low heat capacity and uniform heat exchange, the temperature of the upper and lower pipe walls is reduced uniformly. The system also ensures that the temperature difference between the upper and lower pipe walls is ≤10℃, the minimum wall temperature is ≤-180℃, and the cooling time is ≥30min through a set program.

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

[0097] Furthermore, the control system also includes a multi-level automated interlocking module, which is used to control the start-stop logic, operating parameters (temperature, flow rate, pressure, etc.) and abnormal response mechanisms of various precooling subsystems (such as DCS main control platform, regulating valves, evaporator recovery module, etc.).

[0098] The anomaly response mechanism includes:

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

[0100] If localized condensate accumulation or excessive pressure fluctuations in the pipeline are detected, emergency venting will be initiated. During the precooling process of liquid nitrogen or LNG, condensate accumulation or short-term drastic pressure fluctuations can easily occur in localized areas of pipelines or storage tanks due to factors such as medium evaporation or low-temperature condensation in the conduit. The system identifies such risks through pressure sensors and a liquid level measurement module. If it determines that there is condensate accumulation or instantaneous pressure exceeding the limit, it will automatically open the temporary vent branch and start the vacuum pumping unit to quickly remove the condensate. If the pressure fluctuation approaches the system's safety boundary, it will immediately activate the buffer circuit to regulate the pressure and simultaneously notify the DC main control platform to mark the event.

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

[0102] This invention also provides a method for dynamic control of precooling of mixed media in LNG storage tanks, comprising the following steps:

[0103] Pre-cooling preparation stage: Perform pre-safe operations, including using low-leakage blind flanges to isolate pre-cooled connecting pipelines, testing the cold brittleness of valves and flange seals, pre-cooling temporary pipelines with gaseous nitrogen 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 tanker access unit and LNG low-pressure pump group respectively, and seamless transition of medium switching is achieved by means of regulating valves and interlock control;

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

[0106] Thermal stress and temperature difference balance regulation: By using several temperature sensors arranged in the upper and lower layers and at the four corners, the temperature difference between any two points is monitored. If the temperature difference exceeds the threshold, the spraying rhythm is automatically adjusted or the buffer valve group is activated to ensure the stress balance of the tank wall.

[0107] Evaporation gas recovery: Evaporation gas is introduced into the evaporation gas recovery system through the gas collection branch of the top tank. The discharge rhythm is automatically adjusted according to real-time pressure fluctuations 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 flange isolation treatment of all pre-cooling connection pipelines, cold brittleness performance testing of valves and flange seals, pre-cooling temporary pre-cooling pipelines to below -180°C using gaseous nitrogen, and installing temperature and pressure sensors on all temporary pipelines. The safety interlock system is used to achieve linkage control, and the system can only be put into operation after all safety conditions are met.

[0110] The pre-cooling medium inlet process then begins. Liquid nitrogen is first introduced into the system via a liquid nitrogen tanker inlet unit and then fed into the pre-cooling spray distribution network at the top of the storage tank via a temporary unloading pipeline. In 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℃. In this stage, the nitrogen replacement pipeline can be modified by removing the short circuit and adding a high-throughput vent branch pipe to increase the discharge flow rate and allow it to enter the gas collection cap or safety burner through the vent pipeline, maximizing the cooling rate to 3.5℃ / h.

[0111] When the temperature inside the tank reaches -100℃, the system automatically executes the liquid nitrogen and LNG switching procedure. This process achieves a seamless transition and avoids cross-contamination through independent pipeline inlets, regulating valves and interlocking control loops. Subsequently, LNG medium is introduced through the LNG low-pressure pump group to enter the second stage LNG pre-cooling intermediate zone, with a preset target temperature of -158℃. During this stage, the temperature gradient is controlled to not exceed 3℃ per hour. At the same time, the flow rate is dynamically adjusted through the cooling rate control unit to avoid the material thermal stress concentration caused by excessive temperature difference.

[0112] Throughout the precooling process, the thermal stress control and temperature difference balance subsystem analyzes the sensor data deployed in the upper, middle and lower layers of the storage tank as well as the four corners and the center in real time. The system automatically determines that the temperature difference between two adjacent points should not exceed 10℃ and the temperature difference between any two points should not exceed 30℃. Once the limits are exceeded, dynamic balance adjustment is performed through the buffer valve group. This process proceeds according to the preset three-stage temperature target curve to ensure the stress balance of the tank wall and the smooth and controlled temperature drop process.

[0113] Meanwhile, the evaporation gas recovery module collects BOG evaporation gas through a gas collection branch set at the top of the storage tank and guides it to the evaporation gas compression and recovery device. The system automatically adjusts the discharge rate according to the pressure change at the top of the storage tank, and works with the pressure regulating valve, check valve and anti-surge component and exhaust control unit to achieve full-process pressure balance and safety linkage management.

[0114] Finally, when the temperature of the inner tank wall and the bottom plate reaches -158°C, it indicates that the storage tank is ready for a large amount of liquid to be introduced. At this time, the minimum operating liquid level of the LNG storage tank should be established to create the necessary conditions for the trial operation of equipment such as the LNG submersible pump in the later stage.

[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 foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A large LNG storage tank mixed medium pre-cooling dynamic regulation system, characterized in that, The precooling medium supply module, the precooling spray distribution pipe network, the precooling process control module, the thermal stress control and temperature difference balancing subsystem, the evaporation gas recovery module and the precooling front safety control unit are provided. The precooling medium supply module provides two or more precooling media, and the precooling media at least include liquid nitrogen and LNG. The precooling medium supply module comprises: The liquid nitrogen tank truck access unit comprises a temporary unloading pipeline and a precooling front pipeline precooling structure. The temporary unloading pipeline is arranged as follows: The original nitrogen replacement pipeline is removed and connected with a venting pipeline. An automatic venting valve and a vacuum liquid discharge interface are arranged. The precooling spray distribution pipe network is arranged on the top of the LNG storage tank and is used for uniformly spraying the precooling medium into the storage tank. The precooling process control module is used for sequentially controlling the switching, proportioning and conveying rate of liquid nitrogen and LNG according to preset stages. The thermal stress control and temperature difference balancing subsystem is used for monitoring and adjusting the temperature difference of each region inside the storage tank. The precooling process is provided with a three-stage temperature target curve, including a first-stage liquid nitrogen rapid cooling stage, a second-stage LNG precooling intermediate interval and a third-stage LNG liquid level bottoming temperature stabilization stage. The evaporation gas recovery module is used for collecting and processing natural gas evaporation gas caused by temperature drop during the precooling process. The precooling front safety control unit is used for performing air tightness detection, replacement, drying and cold brittle pre-judgment operations before precooling. The precooling front safety control unit performs the following steps before liquid nitrogen precooling: Blind plates are used to separate the connections. Valves and flange sealing rings are subjected to cold brittle performance detection. Pre-cooling the temporary pre-cooling line to a temperature T using gaseous nitrogen 管 .

2. The extra large LNG storage tank hybrid medium precooling dynamic regulation system according to claim 1, characterized in that, The precooling medium supply module further comprises: The LNG low-pressure pump group is 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 extra large LNG storage tank hybrid medium precooling dynamic regulation system according to claim 2, characterized in that, The automatic venting valve and the vacuum liquid discharge interface are used for quickly pumping out the condensate in the pipeline in a non-operating state. All venting pipelines enter a gas collection cap or a safety burner, and a bursting disc and a back pressure check valve device are arranged. The precooling spray distribution pipe network is arranged in a ring shape, a plurality of nozzles are uniformly arranged along the top of the storage tank, and an adjusting valve and a gas flow guide plate structure are arranged, so as to optimize the spraying coverage.

4. The extra large LNG storage tank hybrid medium precooling dynamic regulation system according to claim 1, characterized in that, The precooling process control module comprises:

5. The extra large LNG storage tank hybrid medium precooling dynamic regulation system according to claim 4, characterized in that, The DCS main control platform is used for receiving feedback data from temperature, flow rate and pressure sensors and generating corresponding control logic instructions accordingly. The switching program of the liquid nitrogen precooling stage and the LNG precooling stage is provided with a multi-level threshold value judgment mechanism based on sensor collected data, so as to automatically switch the precooling medium when specific conditions are met. The cooling rate control unit supports dynamic adjustment of the cooling medium flow rate according to the set target cooling rate. The thermal stress control and temperature difference balancing subsystem and the precooling process control logic comprise:

6. The extra large LNG storage tank hybrid medium precooling dynamic regulation system according to claim 1, characterized in that, Upper, middle and lower three layers of temperature monitoring points are arranged at the four corners and the center of the tank body. The system calculates the temperature difference between adjacent points and any two points in real time, and when the temperature difference exceeds the preset threshold value, the medium flow rate is adjusted or the buffer valve group is enabled for dynamic balancing adjustment. ​ 7. The extra large LNG storage tank hybrid medium precooling dynamic regulation system according to claim 1, characterized in that, The evaporation gas recovery module comprises: A top tank gas collection branch for guiding the evaporation gas at the top of the storage tank to the evaporation gas compression recovery device through a vent line for treatment; A pressure regulating valve and a check anti-surge assembly for regulating the pressure balance in the system exhaust passage and preventing backflow and gas surge phenomena; An exhaust control unit for dynamically adjusting the exhaust rate according to the real-time changes of the pressure at the top of the storage tank.

8. The LNG storage tank hybrid medium precooling dynamic regulation method, for the super large LNG storage tank hybrid medium precooling dynamic regulation system of any one of claims 1-7, characterized in that, The method comprises the following steps: A precooling preparation stage: performing pre-safety operations, including using a low-leakage blind plate to separate the precooling connecting pipeline, detecting the cold brittleness of the valve and the flange sealing ring, using gas-phase nitrogen to precool the temporary pipeline to below the boiling point of liquid nitrogen, and monitoring the safety state through temperature and pressure sensor interlocking; Precooling medium access: liquid nitrogen and LNG are introduced in independent pipelines through the liquid nitrogen tank truck access unit and the LNG low-pressure pump group, respectively, and the seamless transition of medium switching is realized by adjusting the valves and interlocking control; Precooling distribution and control: the medium is uniformly sprayed by using the top annular spray distribution pipe network, the nozzles are provided with guide vanes and regulating valves; the DCS main control platform dynamically adjusts the medium flow according to the temperature, flow rate and pressure, realizes three-stage temperature target curve control of the liquid nitrogen rapid cooling stage, the LNG precooling intermediate interval and the temperature stable stage before the LNG liquid level bottoming, and realizes three-stage temperature target curve control of the liquid nitrogen rapid cooling stage, the LNG precooling intermediate interval and the temperature stable stage before the LNG liquid level bottoming; Thermal stress and temperature difference balance adjustment: the temperature difference between any two points is monitored through a plurality of temperature sensors arranged on the upper and lower layers and the four corners, and if the threshold is exceeded, the spraying rhythm is automatically adjusted or the buffer valve group is enabled; Evaporation gas recovery: the evaporation gas is guided into the evaporation gas recovery system through the top tank gas collection branch.

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