A method for cooling and depressurizing a large LNG storage tank after long-term quiescence

Abstract

This invention discloses a method for cooling and depressurizing a large LNG storage tank after long-term quiescence, relating to the field of LNG storage tank technology. The method includes the following steps: filling detection, valve detection, spray control, pressure display, pipeline control, temperature and pressure detection, and valve closure. Before resuming LNG transport, this invention uses a pre-cooling pipeline and spray device to pre-cool the LNG storage tank, reducing the temperature and pressure of the top gas phase space. In the early stages of spraying, the pressure of the LNG storage tank tends to decrease. When the tank pressure drops to a certain value, the vaporization of the LNG entering the tank causes the pressure to rise. Before the LNG tank pressure begins to rise, a BOG compressor needs to be started to extract BOG gas from the LNG storage tank. At this time, the pressure of the LNG storage tank is low, and the temperature of the top gas is reduced, thereby achieving control of the tank pressure before resuming transport after a long period of quiescence, and reducing the time required to start the BOG compressor and the amount of BOG gas flare.

Description

Technical Field

[0001] This invention relates to the field of LNG storage tank technology, and in particular to a method for cooling and depressurizing a large LNG storage tank after it has been stationary for a long time. Background Technology

[0002] An LNG storage tank is a device or structure specifically designed for storing liquefied natural gas. The liquefied natural gas stored therein is a liquid state formed by cooling natural gas to extremely low temperatures, which greatly reduces the volume of the natural gas, making it easier to store and transport.

[0003] In existing technologies where there is external transmission, the vaporized gas in the LNG storage tank is mainly processed by the BOG compressor. When there is no external transmission, the pressure in the storage tank rises, which is controlled by measures such as flare venting, on-site venting of the storage tank, or on-site discharge through safety valves. However, when the LNG storage tank has no external transmission for a long time, the BOG generated inside the tank causes the pressure in the storage tank to rise continuously, and the temperature of the gas atmosphere inside the tank also rises accordingly. When the pressure reaches the flare venting pressure (26 kPa), the pressure in the storage tank is reduced by flare venting.

[0004] However, before resuming external transmission after a long period of LNG storage tank quiescence, the BOG compressor cannot be directly started to process the BOG in the tank due to the high temperature and pressure of the gas phase space at the top of the LNG storage tank. If started directly, the BOG compressor will be in a state of reflux cooling (cooling by spraying through the BOG inlet desuperheating valve) for a long time due to the temperature protection logic of the BOG compressor, and will not be able to compress the BOG and deliver it downstream. At this time, the pressure of the LNG storage tank will rise and can only be vented through a flare. However, flaring will produce a large amount of carbon dioxide and other greenhouse gases, which will have a negative impact on the environment. At the same time, flaring requires fuel and equipment for combustion, which will also indirectly increase the operating cost of the LNG storage tank. Summary of the Invention

[0005] The main objective of this invention is to provide a method for cooling and depressurizing large LNG storage tanks after long-term quiescence, which solves the problem mentioned in the background art that the BOG compressor cannot be directly started to process the BOG inside the tank due to the high temperature and pressure of the gas phase space at the top of the LNG storage tank.

[0006] To achieve the above objectives, this invention proposes a method for cooling and depressurizing a large LNG storage tank after long-term quiescence, comprising the following steps:

[0007] S1: Filling Detection

[0008] Confirm that the unloading pipeline of the LNG storage tank is filled with LNG and the pressure is controlled at around 5 bar;

[0009] S2: Valve Inspection

[0010] Confirm that the upper and lower inlet valves and bypass of the LNG storage tank are closed, and that the PV valve of the flare line is in normal operation under automatic control.

[0011] S3: Spray Control

[0012] By controlling the HV valve of the precooling pipeline of the LNG storage tank, the pressure at the spray head is controlled above 3.0 bar, ensuring that the LNG liquid enters the storage tank in a spraying state.

[0013] S4: Pressure Display

[0014] Observe the temperature and pressure curve changes of the LNG storage tank, adjust the opening and closing of the precooling pipeline HV in a timely manner, and start the BOG compressor to extract the gas in the LNG storage tank when the pressure of the storage tank drops and begins to rebound.

[0015] S5: Piping Control

[0016] When the BOG compressor starts, the gas temperature in the pipeline from the LNG storage tank to the BOG inlet is high, so the inlet TV valve needs to be opened to cool it down. The TV valve is closed after the BOG compressor inlet temperature meets the logic requirements.

[0017] S6: Temperature and Pressure Detection

[0018] The pressure and temperature inside the LNG storage tank are detected by temperature and pressure sensors, and the corresponding values ​​are received through a remote terminal.

[0019] S7: Valve closed

[0020] When the temperature of the LNG storage tank's vapor space drops to the normal temperature for external transport, and the pressure of the LNG storage tank is slowly decreasing, close the HV valve of the LNG storage tank precooling pipeline.

[0021] Preferably, the pressure detection in S1 is mainly used to monitor the filling status inside the LNG storage tank, including the following steps:

[0022] S101: Liquid level indicator

[0023] By observing the liquid level indicator, ensure that the liquid level is at the position where the unloading pipeline enters the storage tank;

[0024] S102: Check pipeline pressure

[0025] The presence of LNG in a pipeline is usually manifested as a certain pressure inside the pipeline, which can be determined by observing the data displayed on the pressure gauge.

[0026] S103: Ultrasonic Testing

[0027] The depth of the liquid level in the pipeline is detected by installing an ultrasonic testing device on the outer wall of the pipeline, and the displayed height of the liquid level indicator is verified.

[0028] Preferably, in the valve testing process of S2, the performance of each valve needs to be tested simultaneously, including the following steps:

[0029] S201: Visual Inspection

[0030] Perform a visual inspection to ensure that the valve has no obvious damage or signs of leakage, and determine the valve's open / closed status based on the standard on the outside of the valve.

[0031] S202: Reagent Testing

[0032] Apply a leak detection agent to the valve sealing surface or near the connection joint and observe whether bubbles are generated to test the valve's sealing performance.

[0033] S203: Ultrasonic Testing

[0034] Use an ultrasonic detector to scan around the valve and look for signs of ultrasonic signal leakage.

[0035] Preferably, the spray control in S3 is mainly used for cooling the LNG storage tank, including the following steps:

[0036] S301: Equipment Pre-inspection

[0037] Inspect the sprinkler system equipment, including sprinkler heads, pipes, water supply, and control systems, to ensure everything is functioning properly;

[0038] S302: Determine sprinkler requirements

[0039] Based on the temperature and environmental conditions of the LNG pipeline, determine the required precooling temperature, and estimate the spraying time based on the difference between the current temperature inside the LNG storage tank and the set temperature.

[0040] S303: Control parameter adjustment

[0041] Configure the control parameters of the sprinkler system, including sprinkler flow rate, sprinkler time, and sprinkler pressure, and then...

[0042] S304: Start the sprinkler system

[0043] Turn on the main switch or valve of the sprinkler system to start the sprinkler process and ensure that the water supply is sufficient to meet the required flow and pressure requirements;

[0044] S305: Temperature Monitoring

[0045] During the spraying process, infrared thermal imagers are used to continuously perform thermal imaging operations on the inside of the LNG storage tank and pipelines to monitor the temperature of the LNG pipelines.

[0046] S306: Adaptive Adjustment

[0047] Based on the temperature monitoring results in step S305, the system automatically controls and adjusts the parameters of the spray system to ensure that the pipeline surface temperature is within the required range.

[0048] S307: Pre-cooling inspection

[0049] After precooling is completed, inspect the LNG pipeline to ensure there are no freezing or icing issues and that the pipeline is in normal condition.

[0050] S308: Record Report

[0051] Record all relevant data for the precooling process, including temperature monitoring, spray parameters, and spray duration.

[0052] Preferably, the S4 pressure display is used to receive data signals from each group of sensors in the LNG storage tank via a data terminal and present them in graphical form on a large data screen, including the following steps:

[0053] S401: Terminal Reception

[0054] The various sensors in the LNG storage tank are interconnected through a data terminal, and the received data is filtered, rectified, and processed.

[0055] S402: Chart Feedback

[0056] The data received from the data terminal is displayed in charts, and alarm markers are set at its thresholds.

[0057] S403: Gas Extraction

[0058] When the internal pressure of the LNG storage tank reaches the normal range, the gas at the top of the LNG storage tank is extracted and condensed and recovered by the BOG compressor at the top of the LNG storage tank.

[0059] Preferably, the pipeline control in S5 is mainly used to detect the temperature of the gas or liquid passing through through the sensor inside the temperature control valve, and automatically adjust the opening and closing degree of the valve according to the real-time feedback data of the temperature sensor.

[0060] Preferably, the temperature and pressure detection process in S6 is mainly used for sensors with different functions at various locations inside the LNG storage tank, and can determine the various states during the cooling and depressurization process based on the different data fed back by different sensors.

[0061] Preferably, the valve closing process in S7 is used to determine whether the pressure and temperature of the LNG storage tank can be at normal values ​​after processing, and can close the HV valve of the LNG storage tank precooling pipeline through automatic control when the icon curve fed back by the corresponding sensor is approximately coincident with the calibration curve.

[0062] Preferably, in the S1 process, the filling status of the LNG storage tank is detected by three sets of detection methods, and the final determination of the filling status of the LNG storage tank is obtained by mutual verification and comparison between the three sets of data.

[0063] Preferably, the adaptive adjustment performed in S306 can compare the temperature curve generated in S305 with the predicted curve generated in the system, and can adjust the spray flow rate and spray pressure of the spray system when the deviation is greater than the preset value. In addition, when an abnormal temperature line image appears, the control data during manual intervention can be stored, and the spray system can be automatically adjusted adaptively when an approximate curve appears.

[0064] Compared with the prior art, the present invention has the following beneficial effects:

[0065] (1) The cooling and depressurization method for the large LNG storage tank after long-term static operation involves spraying the LNG storage tank with a pre-cooling pipeline and spray device before the LNG storage tank resumes external operation. This reduces the temperature and pressure of the gas phase space at the top of the LNG storage tank. In the early stage of spraying, the pressure of the LNG storage tank will tend to decrease. When the pressure of the storage tank drops to a certain value, the pressure of the storage tank will start to rise due to the vaporization of the LNG entering the LNG storage tank. Before the pressure of the LNG storage tank starts to rise, the BOG compressor is started to extract the BOG gas in the LNG storage tank. At this time, the pressure of the LNG storage tank is low and the temperature of the gas at the top also decreases. The compressor can start quickly to compress the BOG and transport it downstream for re-condensation or direct external operation. This achieves the control of the storage tank pressure before the LNG storage tank resumes external operation after long-term static operation, and reduces the time to start the BOG compressor and the amount of BOG gas flare.

[0066] (2) The cooling and depressurization method for the long-term static LNG storage tank requires a sealing test on the valve of the long-term static LNG storage tank during the process of checking whether the valve is closed. The method involves applying a leak detection agent to the valve sealing surface or near the connection joint and observing whether bubbles are generated to determine whether the valve of the long-term static LNG storage tank has aging leakage caused by long-term static operation. This avoids LNG gas leakage during the opening and closing of the LNG storage tank valve.

[0067] (3) When performing adaptive adjustment in step S306, the cooling and depressurization method for the large LNG storage tank after long-term static operation can read the temperature data fed back by the infrared thermal imager in step S305, and can automatically adjust the temperature and pressure of the storage tank according to the temperature data, and maintain a safe temperature and pressure range under different environmental conditions, thereby reducing the need for manual intervention and monitoring by operators. In addition, through automated control, human error in the process of cooling the LNG storage tank can be significantly reduced and the stability of operation can be improved. Attached Figure Description

[0068] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0069] Figure 1 This is a schematic diagram of the structure of the present invention;

[0070] Figure 2 This is a schematic diagram of the operation process of the present invention;

[0071] Figure 3 This is a schematic diagram of the S1 operation process of the present invention;

[0072] Figure 4 This is a schematic diagram of the S2 operation process of the present invention;

[0073] Figure 5 This is a schematic diagram of the S3 operation process of the present invention;

[0074] Figure 6 This is a schematic diagram of the S4 operation process of the present invention.

[0075] In the diagram: 1. BOG compressor; 2. Inlet TV valve; 3. Precooling line HV valve; 4. LNG storage tank; 5. Flare line PV valve; 6. Flare. Detailed Implementation

[0076] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0077] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0078] Furthermore, the use of terms such as "first" and "second" in this invention is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. When the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed by this invention.

[0079] Please see Figure 1-6 This invention provides a technical solution: a method for cooling and depressurizing a large LNG storage tank after long-term quiescence, comprising the following steps:

[0080] S1: Filling Detection

[0081] Confirm that the unloading pipeline of the LNG storage tank is filled with LNG and the pressure is controlled at around 5 bar;

[0082] S2: Valve Inspection

[0083] Confirm that the upper and lower inlet valves and bypass of the LNG storage tank are closed, and that the PV valve of the flare line is in normal operation under automatic control.

[0084] S3: Spray Control

[0085] By controlling the HV valve of the precooling pipeline of the LNG storage tank, the pressure at the spray head is controlled above 3.0 bar, ensuring that the LNG liquid enters the storage tank in a spraying state.

[0086] S4: Pressure Display

[0087] Observe the temperature and pressure curve changes of the LNG storage tank, adjust the opening and closing of the precooling pipeline HV in a timely manner, and start the BOG compressor to extract the gas in the LNG storage tank when the pressure of the storage tank drops and begins to rebound.

[0088] S5: Piping Control

[0089] When the BOG compressor starts, the gas temperature in the pipeline from the LNG storage tank to the BOG inlet is high, so the inlet TV valve needs to be opened to cool it down. The TV valve is closed after the BOG compressor inlet temperature meets the logic requirements.

[0090] S6: Temperature and Pressure Detection

[0091] The pressure and temperature inside the LNG storage tank are detected by temperature and pressure sensors, and the corresponding values ​​are received through a remote terminal.

[0092] S7: Valve closed

[0093] When the temperature of the LNG storage tank's vapor space drops to the normal temperature for external transport, and the pressure of the LNG storage tank is slowly decreasing, close the HV valve of the LNG storage tank precooling pipeline.

[0094] The pressure detection in S1 is mainly used to monitor the filling status inside the LNG storage tank, including the following steps:

[0095] S101: Liquid level indicator

[0096] By observing the liquid level indicator, ensure that the liquid level is at the position where the unloading pipeline enters the storage tank;

[0097] S102: Check pipeline pressure

[0098] The presence of LNG in a pipeline is usually manifested as a certain pressure inside the pipeline, which can be determined by observing the data displayed on the pressure gauge.

[0099] S103: Ultrasonic Testing

[0100] The depth of the liquid level in the pipeline is detected by installing an ultrasonic testing device on the outer wall of the pipeline, and the displayed height of the liquid level indicator is verified.

[0101] During the valve testing process in S2, the performance of each valve needs to be tested simultaneously, including the following steps:

[0102] S201: Visual Inspection

[0103] Perform a visual inspection to ensure that the valve has no obvious damage or signs of leakage, and determine the valve's open / closed status based on the standard on the outside of the valve.

[0104] S202: Reagent Testing

[0105] Apply a leak detection agent to the valve sealing surface or near the connection joint and observe whether bubbles are generated to test the valve's sealing performance.

[0106] S203: Ultrasonic Testing

[0107] Use an ultrasonic detector to scan around the valve and look for signs of ultrasonic signal leakage.

[0108] The spray control in S3 is mainly used to cool the LNG storage tank, including the following steps:

[0109] S301: Equipment Pre-inspection

[0110] Inspect the sprinkler system equipment, including sprinkler heads, pipes, water supply, and control systems, to ensure everything is functioning properly;

[0111] S302: Determine sprinkler requirements

[0112] Based on the temperature and environmental conditions of the LNG pipeline, determine the required precooling temperature, and estimate the spraying time based on the difference between the current temperature inside the LNG storage tank and the set temperature.

[0113] S303: Control parameter adjustment

[0114] Configure the control parameters of the sprinkler system, including sprinkler flow rate, sprinkler time, and sprinkler pressure, and then...

[0115] S304: Start the sprinkler system

[0116] Turn on the main switch or valve of the sprinkler system to start the sprinkler process and ensure that the water supply is sufficient to meet the required flow and pressure requirements;

[0117] S305: Temperature Monitoring

[0118] During the spraying process, infrared thermal imagers are used to continuously perform thermal imaging operations on the inside of the LNG storage tank and pipelines to monitor the temperature of the LNG pipelines.

[0119] S306: Adaptive Adjustment

[0120] Based on the temperature monitoring results in step S305, the system automatically controls and adjusts the parameters of the spray system to ensure that the pipeline surface temperature is within the required range.

[0121] S307: Pre-cooling inspection

[0122] After precooling is completed, inspect the LNG pipeline to ensure there are no freezing or icing issues and that the pipeline is in normal condition.

[0123] S308: Record Report

[0124] Record all relevant data for the precooling process, including temperature monitoring, spray parameters, and spray duration.

[0125] The S4 pressure display is used to receive data signals from various sensors in the LNG storage tank via a data terminal and present them in graphical form on a large data screen, including the following steps:

[0126] S401: Terminal Reception

[0127] The various sensors in the LNG storage tank are interconnected through a data terminal, and the received data is filtered, rectified, and processed.

[0128] S402: Chart Feedback

[0129] The data received from the data terminal is displayed in charts, and alarm markers are set at its thresholds.

[0130] S403: Gas Extraction

[0131] When the internal pressure of the LNG storage tank reaches the normal range, the gas at the top of the LNG storage tank is extracted and condensed and recovered by the BOG compressor at the top of the LNG storage tank.

[0132] Specifically, the temperature and pressure detection process in S6 primarily targets sensors of varying functions located within the LNG storage tank. It assesses the different states during the cooling and depressurization process based on the data returned by these sensors. This allows maintenance personnel to monitor various indicators within the LNG storage tank and the current status of the equipment in real time through sensor feedback. The valve closing process in S7 determines whether the pressure and temperature of the LNG storage tank are within normal range after processing. When the corresponding sensor's feedback curve approximately coincides with the calibration line, it automatically closes the HV valve of the LNG storage tank pre-cooling pipeline. This results in a high degree of automation in the operation and reduces operating costs. To mitigate operational errors caused by manual operation, the S1 process employs three sets of detection methods to assess the filling status of the LNG storage tank. The final determination of the LNG storage tank's internal filling status is obtained through cross-verification and comparison of the three sets of data. Therefore, compared to a single detection method, this method provides higher verification accuracy. The adaptive adjustment performed in S306 compares the temperature curve generated in S305 with the predicted curve generated within the system. Furthermore, it can adjust the spray flow rate and spray pressure of the spray system when the deviation exceeds the preset value. In addition, when abnormal temperature curve images appear, it can store the control data from manual intervention and automatically adjust the spray system adaptively when an approximate curve appears.

[0133] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for cooling and depressurizing a large LNG storage tank after long-term static operation, characterized in that: Includes the following steps: S1: Filling Detection Confirm that the unloading pipeline of the LNG storage tank is filled with LNG and the pressure is controlled at around 5 bar; S2: Valve Inspection Confirm that the upper and lower inlet valves and bypass of the LNG storage tank are closed, and that the PV valve of the flare line is in normal operation under automatic control. S3: Spray Control By controlling the HV valve of the precooling pipeline of the LNG storage tank, the pressure at the spray head is controlled above 3.0 bar, ensuring that the LNG liquid enters the storage tank in a spraying state. S4: Pressure Display Observe the temperature and pressure curve changes of the LNG storage tank, adjust the opening and closing of the precooling pipeline HV in a timely manner, and start the BOG compressor to extract the gas in the LNG storage tank when the pressure of the storage tank drops and begins to rebound. S5: Piping Control When the BOG compressor starts, the gas temperature in the pipeline from the LNG storage tank to the BOG inlet is high, so the inlet TV valve needs to be opened to cool it down. The TV valve is closed after the BOG compressor inlet temperature meets the logic requirements. S6: Temperature and Pressure Detection The pressure and temperature inside the LNG storage tank are detected by temperature and pressure sensors, and the corresponding values ​​are received through a remote terminal. S7: Valve closed When the temperature of the LNG storage tank's vapor space drops to the normal temperature for external transport, and the pressure of the LNG storage tank is slowly decreasing, close the HV valve of the LNG storage tank precooling pipeline. S1 uses pressure detection to monitor the filling status inside the LNG storage tank, including the following steps: S101: Liquid level indicator By observing the liquid level indicator, ensure that the liquid level is at the position where the unloading pipeline enters the storage tank; S102: Check pipeline pressure The presence of LNG in a pipeline is usually manifested as a certain pressure inside the pipeline, which can be determined by observing the data displayed on the pressure gauge. S103: Ultrasonic Testing The depth of the liquid level in the pipeline is detected by installing an ultrasonic testing device on the outer wall of the pipeline, and the display height of the liquid level indicator is verified. During the valve testing process in S2, the performance of each valve needs to be tested simultaneously, including the following steps: S201: Visual Inspection Perform a visual inspection to ensure that the valve has no obvious damage or signs of leakage, and determine the valve's open / closed status based on the standard on the outside of the valve. S202: Reagent Testing Apply a leak detection agent to the valve sealing surface or near the connection joint and observe whether bubbles are generated to test the valve's sealing performance. S203: Ultrasonic Testing Use an ultrasonic detector to scan around the valve and look for signs of ultrasonic signal leakage. The spray control in S3 is used to cool the LNG storage tank, and includes the following steps: S301: Equipment Pre-inspection Inspect the sprinkler system equipment, including sprinkler heads, pipes, water supply, and control systems, to ensure everything is functioning properly; S302: Determine sprinkler requirements Based on the temperature and environmental conditions of the LNG pipeline, determine the required precooling temperature, and estimate the spraying time based on the difference between the current temperature inside the LNG storage tank and the set temperature. S303: Control parameter adjustment Configure the control parameters of the sprinkler system, including sprinkler flow rate, sprinkler time, and sprinkler pressure, and then... S304: Start the sprinkler system Turn on the main switch or valve of the sprinkler system to start the sprinkler process and ensure that the water supply is sufficient to meet the required flow and pressure requirements; S305: Temperature Monitoring During the spraying process, infrared thermal imagers are used to continuously perform thermal imaging operations on the inside of the LNG storage tank and pipelines to monitor the temperature of the LNG pipelines. S306: Adaptive Adjustment Based on the temperature monitoring results in step S305, the system automatically controls and adjusts the parameters of the spray system to ensure that the pipeline surface temperature is within the required range. S307: Pre-cooling inspection After precooling is completed, inspect the LNG pipeline to ensure there are no freezing or icing issues and that the pipeline is in normal condition. S308: Record Report Record all relevant data for the precooling process, including temperature monitoring, spray parameters, and spray duration.

2. The method for cooling and depressurizing a large LNG storage tank after long-term quiescence, as described in claim 1, is characterized in that: The S4 pressure display is used to receive data signals from various sensors in the LNG storage tank via a data terminal and present them in graphical form on a large data screen, including the following steps: S401: Terminal Reception The various sensors in the LNG storage tank are interconnected through a data terminal, and the received data is filtered, rectified, and processed. S402: Chart Feedback The data received from the data terminal is displayed in charts, and alarm markers are set at its thresholds. S403: Gas Extraction When the internal pressure of the LNG storage tank reaches the normal range, the gas at the top of the LNG storage tank is extracted and condensed and recovered by the BOG compressor at the top of the LNG storage tank.

3. The method for cooling and depressurizing a large LNG storage tank after long-term quiescence, as described in claim 1, is characterized in that: The pipeline control in S5 is used to detect the temperature of the gas or liquid passing through the valve via a sensor inside the temperature control valve, and automatically adjust the valve opening degree based on the real-time feedback data from the temperature sensor.

4. The method for cooling and depressurizing a large LNG storage tank after long-term quiescence, as described in claim 1, is characterized in that: The temperature and pressure detection process in S6 is as follows: It can read the data from sensors of different functions located at various locations inside the LNG storage tank and determine the various states during the cooling and depressurization process based on the different data fed back by the different sensors.

5. The method for cooling and depressurizing a large LNG storage tank after long-term quiescence, as described in claim 1, is characterized in that: The valve closing process in S7 is used to determine whether the pressure and temperature of the LNG storage tank can be at normal values ​​after processing, and to automatically close the HV valve of the LNG storage tank precooling pipeline when the icon curve fed back by the corresponding sensor is approximately coincident with the calibration curve.

6. The method for cooling and depressurizing a large LNG storage tank after long-term quiescence, as described in claim 1, is characterized in that: In the S1 process, the filling status of the LNG storage tank is detected by three sets of detection methods, and the final determination of the filling status of the LNG storage tank is obtained by cross-verification and comparison of the three sets of data.

7. The method for cooling and depressurizing a large LNG storage tank after long-term quiescence, as described in claim 1, is characterized in that: The adaptive adjustment performed in S306 can compare the temperature curve generated in S305 with the predicted curve generated in the system, and can adjust the spray flow rate and spray pressure of the spray system when the deviation is greater than the preset value. In addition, when an abnormal temperature line image appears, the control data during manual intervention can be stored, and the spray system can be automatically adjusted adaptively when an approximate curve appears.

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