LNG storage tank safety detection and control system

Abstract

The invention discloses an LNG storage tank safety detection and control system, and relates to the technical field of liquefied natural gas storage safety, the system comprises a DCS control system, a safety instrument system, a medium input system, a BOG processing system, a safety discharge system, a detection system and an interlocking execution system; the medium input system comprises an LNG upper liquid inlet input pipeline, an LNG lower liquid inlet input pipeline and a natural gas supplementing and conveying pipeline; the detection system comprises an LTD density meter, a multi-point average thermometer, a plurality of servo liquid level meters and a pressure transmitter; the interlocking execution system comprises a first pneumatic emergency cut-off valve, a second pneumatic emergency cut-off valve, a third pneumatic emergency cut-off valve, a BOG recycling adjusting valve, a torch removing adjusting valve, a first flow adjusting valve, a second flow adjusting valve, a third flow adjusting valve, a fourth flow adjusting valve and an immersed pump pumping mechanism. An anti-tumbling algorithm is embedded in the DCS control system. According to the system and the method, real-time early warning-active suppression-emergency blocking full-process closed-loop control on the core risk of the LNG storage tank can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of liquefied natural gas storage safety, in particular to a LNG storage tank safety detection and control system. BACKGROUND

[0002] As a core carrier of clean energy, liquefied natural gas (LNG) is stored in large quantities under normal pressure and at a low temperature of-162℃, which faces multiple complex safety risks. The traditional LNG storage tank system has the following significant defects in key risk monitoring and active control: Rollover risk: after LNG forms stable stratification due to density difference, the heat absorption density of the lower layer decreases and the evaporation density of the upper layer increases, and the density tends to be the same, which causes violent rollover, a large amount of boil-off gas (BOG) is generated instantaneously, and the storage tank is over-pressured or even explodes; leakage risk: low-temperature environment is easy to cause sealing failure and material brittle fracture, the traditional LNG storage tank system has insufficient coverage of leakage monitoring points, and emergency shutdown is lagging, and the combustible gas cloud formed by the gasification of leaked LNG is easy to cause explosion; Geysering phenomenon: BOG accumulates in the liquid phase pipeline connected to the storage tank and suddenly erupts, causing pressure impact and frequent opening of the safety valve; pressure control defects: BOG continuously generates and needs to be dynamically balanced and recovered and discharged, the safety valve in the traditional LNG storage tank system responds passively, and the discharge capacity is insufficient when the pressure rises sharply due to rollover or geysering, and there is a risk of storage tank collapse in negative pressure working condition; insufficient system coordination: monitoring parameters (liquid level, temperature, pressure) are analyzed in isolation, and key risk factors such as density gradient and temperature stratification are not fused; that is, the existing LNG storage tank system cannot meet the high safety requirements of large-scale LNG storage tanks for real-time risk early warning, active inhibition and multi-system coordinated control. SUMMARY

[0003] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a LNG storage tank safety detection and control system, which realizes real-time early warning-active inhibition-emergency blocking full-process closed-loop control of core risks (rollover, leakage, geysering, overpressure) of the LNG storage tank through multi-dimensional sensing fusion (density / temperature / liquid level), intelligent algorithm decision (rollover prevention algorithm), multi-path execution mechanism (liquid inlet switching / submerged liquid pump / gas supplement) and safety level coordination (DCS-SIS linkage).

[0004] The present application provides a LNG storage tank safety detection and control system, comprising: a DCS control system, a safety instrument system, a medium input system, a BOG processing system, a safety discharge system, a detection system and an interlocking execution system. The medium input system includes an LNG upper liquid inlet input pipeline, an LNG lower liquid inlet input pipeline, and a natural gas replenishment pipeline; the output end of the LNG upper liquid inlet input pipeline is connected to the liquid inlet at the top of the LNG storage tank, the output end of the LNG lower liquid inlet input pipeline is connected to the liquid inlet at the bottom of the LNG storage tank, and the output end of the natural gas replenishment pipeline is connected to the interior of the LNG storage tank through a gas replenishment port opened on the side wall near the top of the LNG storage tank; The BOG processing system includes a BOG recovery pipeline, a flare-to-flare pipeline, and a vacuum relief valve. The input end of the BOG recovery pipeline is connected to the interior of the LNG storage tank through a first BOG outlet opened on the top of the LNG storage tank. The input end of the flare-to-flare pipeline is connected to the interior of the LNG storage tank through a second BOG outlet opened on the top of the LNG storage tank. The vacuum relief valve is installed at the first vent on the top of the LNG storage tank. The safety relief system includes a pilot-operated exhalation valve, which is installed at the second vent on the top of the LNG storage tank; The detection system includes an LTD density meter, a multi-point average temperature meter, multiple servo level gauges, and a pressure transmitter. The LTD density meter is vertically layered and installed inside the LNG storage tank. The multi-point average temperature meter is vertically distributed in the LNG liquid phase inside the LNG storage tank. The multiple servo level gauges are vertically installed on the inner wall of the LNG storage tank and are evenly distributed vertically. The pressure transmitter is installed on the top of the LNG storage tank. The interlocking execution system includes a first pneumatic emergency shut-off valve, a second pneumatic emergency shut-off valve, a third pneumatic emergency shut-off valve, a BOG recovery regulating valve, a flare removal regulating valve, a first flow regulating valve, a second flow regulating valve, a third flow regulating valve, a fourth flow regulating valve, and a submerged pump pumping mechanism. The first pneumatic emergency shut-off valve and the second flow regulating valve are both located on the upper LNG inlet pipeline, the second pneumatic emergency shut-off valve and the third flow regulating valve are both located on the lower LNG inlet pipeline, the third pneumatic emergency shut-off valve and the fourth flow regulating valve are both located on the natural gas replenishment pipeline, the BOG recovery regulating valve is located on the BOG recovery pipeline, the flare removal regulating valve is located on the flare removal pipeline, the first flow regulating valve is located on the natural gas replenishment pipeline, and the submerged pump pumping mechanism is submerged in the LNG liquid phase near the bottom of the LNG storage tank. The DCS control system is embedded with a roll-over prevention algorithm, and is connected with the LTD density meter, the multi-point average temperature meter, the BOG recovery regulating valve, the flare knockout regulating valve, the first flow regulating valve, the second flow regulating valve, the third flow regulating valve, the fourth flow regulating valve, the submersible pump pumping mechanism, the pressure transmitter and each of the servo liquid level meters respectively, the safety instrument system is connected with the vacuum relief valve, the pilot type blowdown valve, the flare knockout regulating valve, the first pneumatic emergency shutdown valve, the second pneumatic emergency shutdown valve and the third pneumatic emergency shutdown valve respectively, and the DCS control system is also connected with the safety instrument system.

[0005] According to some embodiments of the present application, the LNG storage tank safety detection and control system further comprises a first combustible gas detector, a plurality of second combustible gas detectors and a plurality of third combustible gas detectors, the first combustible gas detector is arranged at a connecting flange between the LNG lower liquid inlet pipeline and the bottom of the LNG storage tank, the plurality of second combustible gas detectors are uniformly distributed in the circumferential direction of the annular space formed between the inner tank and the outer tank of the LNG storage tank, and the second combustible gas detectors are arranged close to the bottom weld of the LNG storage tank, the plurality of third combustible gas detectors are installed on the inner wall close to the top of the LNG storage tank, and the first combustible gas detector, each of the second combustible gas detectors and each of the third combustible gas detectors are connected with the DCS control system and the safety instrument system respectively.

[0006] According to some embodiments of the present application, the LNG storage tank safety detection and control system further comprises a purging and inerting system, the purging and inerting system comprises a nitrogen purging pipeline and a liquid nitrogen storage tank, the interlocking execution system comprises a fifth flow regulating valve, the input end of the nitrogen purging pipeline is connected with the first output end of the liquid nitrogen storage tank, the output end of the nitrogen purging pipeline is connected to the annular space formed between the inner tank and the outer tank of the LNG storage tank, the sealed interlayer between the ceiling of the LNG storage tank and the top plate of the inner tank, and the submersible pump pumping mechanism respectively, the fifth flow regulating valve is arranged close to the liquid nitrogen storage tank in the nitrogen purging pipeline, and the fifth flow regulating valve is connected with the DCS control system.

[0007] According to some embodiments of the present application, the purging and inerting system further comprises an instrument air pipeline and an instrument air station, the instrument air pipeline is laid along the periphery of the LNG storage tank, the input end of the instrument air pipeline is connected to the output end of the instrument air station, and the output end of the instrument air pipeline is connected to the pilot outlet valve, the first pneumatic emergency shut-off valve, the second pneumatic emergency shut-off valve, the third pneumatic emergency shut-off valve, the BOG recovery regulating valve, the flare knockout regulating valve, the first flow regulating valve, the second flow regulating valve, the third flow regulating valve, the fourth flow regulating valve, and the fifth flow regulating valve, respectively.

[0008] According to some embodiments of the present application, the medium input system further comprises a liquid nitrogen pre-cooling pipeline, and the interlocking execution system further comprises a sixth flow regulating valve, the input end of the liquid nitrogen pre-cooling pipeline is connected to the liquid nitrogen storage tank, the output end of the liquid nitrogen pre-cooling pipeline is connected to the liquid inlet at the bottom of the LNG storage tank, the sixth flow regulating valve is arranged on the liquid nitrogen pre-cooling pipeline, and the sixth flow regulating valve is connected to the DCS control system.

[0009] According to some embodiments of the present application, the safety relief system further comprises a first pipeline safety valve, a second pipeline safety valve, and a third pipeline safety valve, the first pipeline safety valve is installed on the LNG upper liquid inlet input pipeline, the second pipeline safety valve is installed on the LNG lower liquid inlet input pipeline, and the third pipeline safety valve is installed on the flare knockout pipeline.

[0010] According to some embodiments of the present application, the safety relief system further comprises five flame arresters, the safety relief system further comprises a first relief pipeline, the BOG treatment system further comprises a second relief pipeline, the pilot outlet valve is arranged on the first relief pipeline, the vacuum relief valve is installed on the second relief pipeline, the first relief pipeline is communicated with the inside of the LNG storage tank through the second relief port at the top of the LNG storage tank, the second relief pipeline is communicated with the inside of the LNG storage tank through the first relief port at the top of the LNG storage tank, and the five flame arresters are installed on the LNG upper liquid inlet input pipeline, the LNG lower liquid inlet input pipeline, the flare knockout pipeline, the first relief pipeline, and the second relief pipeline, respectively.

[0011] According to some embodiments of the present application, the LNG storage tank safety detection and control system further comprises a storage tank liquid phase pipeline, the input end of the storage tank liquid phase pipeline is connected to the liquid outlet at the bottom of the LNG storage tank, the output end of the submerged pump pumping mechanism is connected to the liquid outlet at the bottom of the LNG storage tank, and the output end of the storage tank liquid phase pipeline is connected to the input end of the submerged pump pumping mechanism and the input end of a downstream system through a three-way valve, respectively.

[0012] According to some embodiments of the present application, the safety relief system further comprises a low-temperature safety valve installed at a bend of the liquid-phase pipeline of the storage tank.

[0013] According to some embodiments of the present application, the detection system further comprises a radar liquid level meter arranged at the top of the LNG storage tank.

[0014] The beneficial effects of the present application are embodied in: 1. LTD density meter: vertically layered installation, direct measurement of LNG density gradient change in the storage tank; multi-point average thermometer: vertical distribution monitoring of liquid-phase temperature stratification, identification of density changes caused by heat absorption; anti-rolling algorithm (embedded in DCS): based on density and temperature data to predict stratification stability and actively intervene; 2. Interlock control: dynamically adjust the liquid inlet position and flow through the second / third flow regulating valve (upper liquid inlet / lower liquid inlet pipeline) to break the density stratification; release BOG in advance through the BOG recovery / fire flare regulating valve to avoid overpressure; interlock control effect: actively predict and suppress stratification formation to avoid instantaneous overpressure explosion caused by rolling; 3. Multiple leakage monitoring: servo liquid level meter (uniform distribution): high-precision monitoring of liquid level abnormal drop (leakage feature); pressure transmitter: monitoring of abnormal pressure fluctuation.

[0015] 4. Rapid emergency shutdown: safety instrument system (SIS) independent control: pneumatic emergency shutdown valve (first, second, and third valves): all input pipelines (LNG upper / lower liquid inlet, gas makeup pipeline) are shut off within seconds when leakage occurs; fire flare regulating valve: emergency BOG release to reduce pressure; rapid emergency shutdown effect: early and accurate identification of leakage and millisecond pipeline shutdown to prevent the formation of flammable vapor clouds.

[0016] 5. Geysering phenomenon suppression: submersible pump pumping mechanism: immersed in the LNG liquid phase at the bottom of the tank for pumping to avoid LNG stagnation and heat absorption in the pipeline; temperature monitoring: multi-point average thermometer for real-time monitoring of liquid-phase supercooling degree to prevent the formation of gasification core; pressure control: BOG recovery / fire flare regulating valve (DCS control) to stabilize tank pressure and reduce liquid-phase pipeline pressure fluctuation; effect: eliminate the conditions for BOG accumulation in the pipeline and suppress geysering from the source.

[0017] 6. Optimize pressure control defects: Active pressure balance system: BOG recovery pipeline + flare pipeline: dual-channel dynamic processing of BOG (recycling or safe combustion); BOG recovery / flare regulation valve (DCS control): real-time adjustment of the discharge amount according to the pressure transmitter data. Negative pressure protection: vacuum relief valve (SIS control): emergency opening to suck in air to prevent the tank from collapsing under negative pressure; Overpressure protection: pilot-operated outlet valve (SIS control): main discharge channel, with a response speed much higher than that of the traditional safety valve; Flare regulation valve (SIS / DCS dual control): auxiliary pressure relief; Effect: Achieve dynamic balance of BOG and active protection against positive / negative pressure, avoiding the risk of insufficient passive discharge capacity.

[0018] 7. Improve system synergy; DCS system integrates all key data: Synchronous analysis of multi-dimensional parameters such as density (LTD), temperature stratification (multi-point thermometer), liquid level (servo liquid level gauge), and pressure (transmitter); Anti-rolling algorithm: Predict risks based on fused data and output control instructions (such as adjusting liquid inflow / BOG processing); DCS and SIS layered linkage: DCS executes active control (such as regulating valve, anti-rolling); SIS executes safety interlocking (such as emergency shutdown, relief valve), independent of DCS and with a communication interface, ensuring that emergency states are responded to first; Natural gas supplement pipeline: Adjust the gas supplement through the fourth flow regulating valve (DCS) and the third pneumatic emergency shut-off valve (SIS) to assist in pressure balance; Effect: Break data silos, achieve risk factor fusion analysis, and coordinate the execution of control instructions.

[0019] The present application predicts and eliminates stratification through LTD density gauge + temperature stratification monitoring + anti-rolling algorithm + liquid inflow / position adjustment, avoiding rolling overpressure; Servo liquid level gauge group + pressure transmitter + SIS independently controlled emergency shut-off valve for early leak identification and millisecond-level shut-off; Submerged pump + liquid phase temperature monitoring + BOG dynamic pressure control to eliminate pipeline BOG accumulation conditions; Dual BOG processing channels + vacuum relief valve + pilot-operated outlet valve + DCS / SIS hierarchical control to dynamically balance BOG and provide positive / negative pressure double active protection; DCS fusion multi-parameter analysis + anti-rolling algorithm + DCS and SIS layered linkage (control + safety) for multi-system data-driven collaborative decision-making, safety and efficiency integration; Through the combination of high-precision multi-dimensional monitoring (density, temperature, liquid level, pressure) + active control algorithm (anti-rolling) + hierarchical safety architecture (DCS + SIS) + dedicated actuator (regulating valve, shut-off valve, submerged pump), the passive safety of traditional LNG storage tanks is upgraded to active prediction and collaborative control, systematically solving the high-risk problem of large LNG storage tanks.

[0020] Additional aspects and advantages of the application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0021] Additional aspects and advantages of the present application will become apparent and appreciated from the following description of embodiments, taken in conjunction with the accompanying drawings, wherein: Figure 1 Connection diagram between DCS control system, safety instrument system and other components provided for embodiments of the present application; Figure 2 Connection diagram between instrument air pipeline and other components provided for embodiments of the present application. DETAILED DESCRIPTION

[0022] Embodiments of the present application are described below in detail, examples of which are shown in the accompanying drawings, wherein the same or similar notations represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by reference to the accompanying drawings are exemplary only, only for explaining the present application, and cannot be understood as limiting the present application.

[0023] In the description of the present application, it is to be understood that, if there is a description of orientation, for example, the orientation or position relationship indicated by up, down, front, back, left, right, etc. is based on the orientation or position relationship shown in the drawings, and is only for the purpose of facilitating the description of the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0024] In the description of the present application, if there is a description of first, second, only for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features or implicitly indicating the order of indicated technical features.

[0025] In the description of the present application, unless otherwise explicitly limited, the words such as setting, installing, connecting, etc. should be broadly understood, and the person skilled in the art can reasonably determine the specific meaning of the above words in the present application in combination with the specific content of the technical solution.

[0026] Liquefied natural gas (LNG) as the core carrier of clean energy, its large-scale storage in normal pressure, -162 ℃ low temperature environment faces multiple complex security risks. The traditional LNG storage tank system has the following significant defects in key risk monitoring and active control: Rollover risk: after LNG forms stable stratification due to density difference, the heat absorption density of the lower layer decreases, the evaporation density of the upper layer increases, the density tends to be the same, causing violent rollover, a large amount of boil-off gas (BOG) is generated instantaneously, leading to overpressure of the storage tank and even explosion; Leakage risk: low temperature environment is easy to cause sealing failure and material brittle fracture, the traditional LNG storage tank system has insufficient coverage of leakage monitoring points, and the emergency shutdown is lagging, the combustible vapor cloud formed by the gasification of leaked LNG is easy to cause explosion; Geysering phenomenon: BOG accumulates in the liquid phase pipeline connected to the storage tank and suddenly erupts, causing pressure shock and frequent opening of safety valve; Pressure control defects: BOG continues to generate and needs to be dynamically balanced with recovery and discharge, the safety valve in the traditional LNG storage tank system responds passively, and the discharge capacity is insufficient when the pressure rises sharply due to rollover or geysering, and there is a risk of storage tank collapse under negative pressure conditions; Insufficient system synergy: isolated analysis of monitoring parameters (liquid level, temperature, pressure), without fusion of key risk factors such as density gradient and temperature stratification; That is, the existing LNG storage tank system is difficult to meet the high safety requirements of large-scale LNG storage tanks for real-time risk early warning, active inhibition and multi-system collaborative control.

[0027] In order to solve the above problems, the application provides a LNG storage tank safety detection and control system, and the embodiments of the application are further described below with reference to the drawings.

[0028] Reference Figure 1The application provides an LNG storage tank safety detection and control system, which comprises a DCS control system, a safety instrument system, a medium input system, a BOG processing system, a safety relief system, a detection system and an interlock execution system; the medium input system comprises an LNG upper liquid inlet input pipeline, an LNG lower liquid inlet input pipeline and a natural gas supplementary conveying pipeline; the output end of the LNG upper liquid inlet input pipeline is connected to a liquid inlet at the top of the LNG storage tank, the output end of the LNG lower liquid inlet input pipeline is connected to a liquid inlet at the bottom of the LNG storage tank, and the output end of the natural gas supplementary conveying pipeline is in communication with the inside of the LNG storage tank through a gas supplementing opening formed in the side wall of the LNG storage tank near the top; the BOG processing system comprises a BOG recovery pipeline, a flare pipeline and a vacuum relief valve, the input end of the BOG recovery pipeline is in communication with the inside of the LNG storage tank through a first BOG outlet formed in the top of the LNG storage tank, the input end of the flare pipeline is in communication with the inside of the LNG storage tank through a second BOG outlet formed in the top of the LNG storage tank, and the vacuum relief valve is installed at a first relief opening in the top of the LNG storage tank; the safety relief system comprises a pilot operated vent valve, which is installed at a second relief opening in the top of the LNG storage tank; the detection system comprises an LTD density meter, a multi-point average temperature meter, a plurality of servo liquid level meters and a pressure transmitter, the LTD density meter is installed vertically in layers in the LNG storage tank, the multi-point average temperature meter is vertically distributed in the LNG liquid phase in the LNG storage tank, the plurality of servo liquid level meters are vertically installed on the inner wall of the LNG storage tank and are vertically and uniformly distributed, and the pressure transmitter is installed at the top of the LNG storage tank; the interlock execution system comprises a first pneumatic emergency shut-off valve, a second pneumatic emergency shut-off valve, a third pneumatic emergency shut-off valve, a BOG recovery regulating valve, a flare regulating valve, a first flow regulating valve, a second flow regulating valve, a third flow regulating valve, a fourth flow regulating valve and a submerged pump pumping mechanism, the first pneumatic emergency shut-off valve and the second flow regulating valve are arranged on the LNG upper liquid inlet input pipeline, the second pneumatic emergency shut-off valve and the third flow regulating valve are arranged on the LNG lower liquid inlet input pipeline, the third pneumatic emergency shut-off valve and the fourth flow regulating valve are arranged on the natural gas supplementary conveying pipeline, the BOG recovery regulating valve is arranged on the BOG recovery pipeline, the flare regulating valve is arranged on the flare pipeline, the first flow regulating valve is arranged on the natural gas supplementary conveying pipeline, and the submerged pump pumping mechanism is immersed in the LNG liquid phase near the bottom in the LNG storage tank; an anti-rolling algorithm is embedded in the DCS control system, and the DCS control system is connected with the LTD density meter, the multi-point average temperature meter, the BOG recovery regulating valve, the flare regulating valve, the first flow regulating valve, the second flow regulating valve, the third flow regulating valve, the fourth flow regulating valve, the submerged pump pumping mechanism, the pressure transmitter and each servo liquid level meter, the safety instrument system is connected with the vacuum relief valve, the pilot operated vent valve, the flare regulating valve, the first pneumatic emergency shut-off valve, the second pneumatic emergency shut-off valve and the third pneumatic emergency shut-off valve, and the DCS control system is also connected with the safety instrument system.

[0029] In some embodiments, a pressure reducing valve is further arranged on the natural gas supplement pipeline, the input end of the natural gas supplement pipeline is connected to the input end of the gas supplement system, a pressure reducing valve is further arranged on the natural gas supplement pipeline, the pressure reducing valve is arranged close to the input end of the natural gas supplement pipeline, the fourth flow regulating valve is arranged between the pressure reducing valve and the third pneumatic emergency shut-off valve; the gas supplement system is connected to the gas phase space at the top of the LNG storage tank through the pressure reducing valve, the fourth flow regulating valve and the third pneumatic emergency shut-off valve, so as to maintain stable tank pressure (5.0 MPa natural gas supplement).

[0030] In some embodiments, the detection system further comprises six surface thermometers, the plurality of surface thermometers are arranged at the tank bottom, wall plate, ceiling, annular space and the like of the LNG storage tank respectively, each surface thermometer is connected to the DCS control system; the six surface thermometers are used for detecting temperature gradient and assisting in layered early warning.

[0031] It should be noted that the annular space of the LNG storage tank is located in the interlayer region between the inner tank (main container) and the outer tank (secondary container) in the LNG storage tank; functions: ① thermal insulation barrier: reduce external heat transfer, maintain LNG low temperature state; ② safety protection: when the inner tank leaks, contain the leaked LNG to prevent direct external leakage; ③ inert environment: nitrogen filling inhibits the accumulation of flammable gas (methane explosion lower limit 5%).

[0032] It should be noted that the pump area: the area composed of the submerged pump in the LNG at the bottom of the storage tank and its auxiliary pipelines; the storage tank gas phase, LNG liquid phase ← pump area (submerged pump × 2 + import and export pipeline + backflow pipe), the low temperature failure of the pump shaft seal leads to LNG leakage into the liquid phase and gasification, and the vibration of the backflow pipe causes the flange to loosen.

[0033] It should be noted that the submerged pump pumping mechanism includes two submerged pumps, inlet and outlet pipelines and a return pipe. Specifically, the two submerged pumps: immersed in the liquid phase at the bottom of the LNG storage tank (≥1.5 m from the tank bottom), using vertical multi-stage centrifugal pump structure, material for low temperature austenitic stainless steel (such as S31603); inlet and outlet pipelines: inlet pipeline: connecting the pump suction end to the tank bottom outlet (inner diameter DN200); outlet pipeline: extending from the pump outlet to the tank bottom flange (inner diameter DN150); return pipe: connecting the pump outlet pipeline with the tank bottom return interface (inner diameter DN80), forming a closed loop circulation. The submerged pump pumping mechanism is installed in the tank bottom pump well (special caisson structure) as a whole, welded and fixed with the tank bottom, the pump body is fixed through spring shock absorber support, and the inlet and outlet pipelines use corrugated pipe compensator to absorb thermal expansion and contraction stress. The pump body flange of the submerged pump body is connected with the inlet and outlet pipeline flanges (bolt + metal winding gasket sealing) to provide LNG conveying power, maintain continuous flow of fluid in the pipeline, and avoid stagnation vaporization; the inlet end of the inlet pipeline is connected with the tank bottom outlet (DN200 flange), and the outlet end is connected with the pump suction chamber, which is used for pumping LNG from the liquid phase layer of the tank to ensure that the pump suction inlet is always immersed below the liquid level; the outlet pipeline connection relationship: pump outlet → check valve → pneumatic shut-off valve → tank bottom flange (connected with downstream system), used for conveying LNG to downstream users (such as gasifier / car loading system), and an exhaust valve is arranged at the highest point of the pipeline to discharge BOG accumulation; the return pipe connection relationship: three-way valve branch connects the outlet pipeline → tank bottom return interface (DN80), used for forcing part of LNG to return to the tank to maintain continuous flow in the pipeline and eliminate BOG accumulation conditions; the sealing liquid system connection relationship: isolation liquid tank → double mechanical seal cavity (through pressurizing pipeline), which blocks the infiltration of LNG into the motor cavity, and the pressure of the isolation liquid is higher than the pressure of the pump cavity by 0.2 MPa, and the leakage triggers DCS alarm. The working principle and control logic of the submerged pump pumping mechanism: during normal operation (anti-intermittent spring): ① DCS control system controls the flow by adjusting the pneumatic regulating valve (not marked in the figure, actually needs to be added) on the outlet pipeline ② 10-15% flow returns to the tank through the return pipe to ensure that the LNG flow rate in the pipeline is ≥1 m / s to avoid stagnation and heat absorption vaporization; the multi-point average thermometer detects the liquid phase temperature in real time, and if the temperature approaches the saturation temperature (indicating the risk of vaporization), the DCS increases the submerged pump speed to increase the flow. Anti-rolling mode: ① forced circulation start: when the anti-rolling algorithm detects the density stratification risk (density difference >5%), the DCS starts the double pump parallel operation; ② the return proportion is increased to 30%, and the high-speed LNG is injected upward from the tank bottom to destroy the stratification structure. Leakage emergency response: interlocking shutdown: if the pump area combustible gas detector (first type) alarm ≥20% LEL, the SIS system within 0.5 seconds: ① close the outlet pipeline pneumatic emergency shut-off valve; ② cut off the power supply of the submerged pump; ③ start nitrogen purging (50 Nm 3h) Cover the pump seal area. Double pump configuration (one active and one standby), standby pump automatic switching logic: when the active pump fails or the flow is insufficient, the DCS automatically starts the standby pump. Seal redundancy: double mechanical seal: when the main seal fails, the standby seal + barrier fluid system prevents leakage; leakage monitoring: real-time data from the seal cavity pressure sensor to the DCS, pressure drop > 10% triggers an early warning; Anti-vibration measures: hydraulic pulse damper is installed on the outlet pipeline to absorb the pressure impact caused by the start / stop of the submersible pump and intermittent spring. The DCS control system adjusts the pump speed (flow 0-100 m 3 / h) in response to the anti-rollover algorithm instructions; the safety instrument system makes the SIS independent hard-wire control through the outlet pneumatic emergency shut-off valve, which is cut off in milliseconds when there is a leak; the detection system monitors the bearing temperature through the pump area temperature sensor, and alarms when the temperature is >-150°C, and stops the pump when the temperature is >-145°C (to prevent overheating vaporization); The purge system injects nitrogen through a branch pipe in the pump well, and after the SIS is triggered, the nitrogen blows through the annular distribution pipe to purge the pump seal area and dilute the leaked methane.

[0034] It should be noted that the pilot-operated outlet valve is directly installed in the gas phase space at the highest point of the storage tank dome, without a transition pipe, ensuring zero resistance gas release when the pressure is too high; the flare regulation valve (DN200) is located on the gas phase outlet pipeline at the top of the storage tank, and is connected to the BOG recovery pipeline (the flare pipeline is connected in parallel with the BOG recovery pipeline). When the pressure is too high, the recovery valve is opened first, and when the pressure continues to rise, it is switched to the flare; the vacuum relief valve (DN250x3) is symmetrically installed with the outlet valve at the dome, and is provided with a one-way check structure inside.

[0035] It should be noted that LNG stands for Liquefied natural gas, which means liquid natural gas; Carbon dioxide; DCS stands for Distributed Control System, which means distributed control system; SIS stands for Safety Instrumentation System, which means safety instrument system; GDS stands for Gas Detection System, which means combustible gas / toxic gas detection and alarm system; BOG stands for Boil-Off Gas, which means the gas generated by the evaporation of LNG (liquefied natural gas) due to insufficient insulation performance of the storage tank or increase of ambient temperature during storage and transportation of LNG.

[0036] The beneficial effects of the present application are: 1. LTD density meter: vertically stratified installation, direct measurement of LNG density gradient change in the tank; multi-point average thermometer: vertical distribution monitoring of liquid phase temperature stratification, identifying density changes caused by heat absorption; anti-rolling algorithm (embedded in DCS): predicting stratification stability based on density and temperature data, active intervention; 2. Interlocking control: dynamically adjust the liquid inlet position and flow through the second / third flow regulating valve (upper inlet / lower inlet pipeline) to break the density stratification; release BOG in advance through the BOG recovery / flare regulating valve to avoid overpressure; interlocking control effect: actively predict and suppress stratification formation to avoid instantaneous overpressure explosion caused by rolling 3. Multiple leak monitoring: servo liquid level meter (uniform distribution): high-precision monitoring of sudden drop in liquid level (leakage feature); pressure transmitter: monitoring abnormal pressure fluctuations.

[0037] 4. Rapid emergency shutdown: safety instrument system (SIS) independent control: pneumatic emergency shutdown valve (first, second, and third valves): seconds-level shutdown of all input pipelines (LNG upper / lower inlet, makeup gas pipeline) in the event of a leak; flare regulating valve: emergency BOG release to reduce pressure; rapid emergency shutdown effect: early and accurate identification of leaks and millisecond pipeline shutdown to prevent the formation of flammable vapor clouds.

[0038] 5. Geysering phenomenon suppression: submersible pump pumping mechanism: immersed in the tank bottom LNG liquid phase for pumping, avoiding LNG stagnation and heat absorption in the pipeline; temperature monitoring: multi-point average thermometer for real-time monitoring of liquid phase supercooling degree to prevent the formation of gasification core; pressure control: BOG recovery / flare regulating valve (DCS control) to stabilize tank pressure and reduce liquid phase pipeline pressure fluctuations; effect: eliminating the conditions for BOG accumulation in the pipeline and suppressing geysering from the source.

[0039] 6. Optimization of pressure control defects: active pressure balancing system: BOG recovery pipeline + flare pipeline: dual-channel dynamic handling of BOG (recycling or safe combustion); BOG recovery / flare regulating valve (DCS control): real-time adjustment of discharge based on pressure transmitter data. Negative pressure protection: vacuum relief valve (SIS control): emergency opening to suck in air to prevent tank collapse due to negative pressure; overpressure protection: pilot-operated outlet valve (SIS control): main discharge channel with much faster response speed than traditional safety valves; flare regulating valve (SIS / DCS dual control): auxiliary pressure relief; effect: achieving dynamic balance of BOG and active protection against positive / negative pressure, avoiding the risk of insufficient passive discharge capacity.

[0040] 7. The system has the following features: 1. The system has the function of system cooperation. The DCS system integrates all the key data: synchronous analysis of density (LTD), temperature stratification (multi-point temperature gauge), liquid level (servo liquid level gauge), pressure (transmitter) and other multi-dimensional parameters. 2. The system has the function of anti-rollover algorithm. The system predicts the risk based on the fusion data and outputs the control instruction (such as adjusting the liquid inlet flow / BOG treatment). 3. The system has the function of DCS and SIS hierarchical linkage. The DCS executes the active control (such as adjusting valve, anti-rollover). The SIS executes the safety interlock (such as emergency shutdown, relief valve). The SIS is independent of the DCS and has a communication interface, which ensures that the emergency state is responded preferentially. 4. The system has the function of natural gas supplement pipeline. The pipeline is adjusted by the fourth flow regulating valve (DCS) and the third pneumatic emergency shutdown valve (SIS) to supplement the gas and assist the pressure balance. The function of the system is to break the data island, realize the fusion analysis of risk factors and the cooperative execution of control instruction.

[0041] The application can avoid the rollover overpressure by predicting and eliminating the stratification through the LTD density gauge + temperature stratification monitoring + anti-rollover algorithm + liquid inlet flow / position adjustment. The system can identify the early leakage and shut down in milliseconds through the servo liquid level gauge group + pressure transmitter + SIS independently controlled emergency shutdown valve. The system can eliminate the pipeline BOG accumulation condition through the submersible pump + liquid phase temperature monitoring + BOG dynamic pressure control. The system can dynamically balance the BOG through the double BOG treatment channel + vacuum relief valve + pilot type exhalation valve + DCS / SIS hierarchical control. The system can have the double active protection of positive / negative pressure through the DCS fusion multi-parameter analysis + anti-rollover algorithm + DCS and SIS hierarchical linkage (control + safety). The system can have the cooperative decision of multi-system data driving. The system can unify the safety and efficiency. The system can upgrade the passive safety of the traditional LNG storage tank to the active prediction and cooperative control through the combination of high-precision multi-dimensional monitoring (density, temperature, liquid level, pressure) + active control algorithm (anti-rollover) + hierarchical safety architecture (DCS + SIS) + special execution mechanism (regulating valve, shutdown valve, submersible pump). The system can systematically solve the high-risk problem of large LNG storage tank.

[0042] Reference Figure 1 It can be understood that the LNG storage tank safety detection and control system further comprises a first combustible gas detector, a plurality of second combustible gas detectors and a plurality of third combustible gas detectors. The first combustible gas detector is arranged at a connection flange between the LNG lower liquid inlet pipeline and the bottom of the LNG storage tank. The plurality of second combustible gas detectors are uniformly distributed in the circumferential direction of the annular space formed between the inner tank and the outer tank of the LNG storage tank, and are arranged close to the bottom weld of the LNG storage tank. The plurality of third combustible gas detectors are installed on the inner wall close to the top of the LNG storage tank. The first combustible gas detector, each second combustible gas detector and each third combustible gas detector are respectively connected with the DCS control system and the safety instrument system.

[0043] In some embodiments, the LNG storage tank safety detection and control system further comprises a GDS system, an input end of the GDS system being connected with the first combustible gas detector, each second combustible gas detector and each third combustible gas detector, and an output end of the GDS system being connected with the DCS control system and the instrument safety system. The GDS system functions to cover the annular space / pump area of the storage tank in real time by the multi-point combustible gas detector. The triggering condition is that when two or more detectors alarm, the GDS system sends a hard-wired signal to the instrument safety system to make the instrument safety system take the dominant action: ① close all liquid inlet / gas supplement cut-off valves within 2s, start the nitrogen purging system and switch the BOG to full open discharge to the flare. At this time, the auxiliary function of the DCS control system is to record the accident process data to provide the basis for post-analysis.

[0044] In some embodiments, three first combustible gas detectors are provided, and the three first combustible gas detectors are respectively installed at: the LNG lower liquid inlet pipeline root flange (≤1m from the tank body of the LNG storage tank), the outlet flange of the output pipeline of the submerged pump pumping mechanism (high-pressure leakage point), and the nitrogen purging pipeline flange (welded joint dense area).

[0045] It should be noted that the tank bottom flange sealing place refers to the flange connection point of the pipeline interface at the bottom of the storage tank; the LNG liquid inlet pipe → [flange A] → the lower liquid inlet of the storage tank; the submerged pump outlet pipe → [flange B] → the bottom of the storage tank; the return pipe → [flange C] → the pump well interface; low-temperature shrinkage can cause flange sealing failure (-162℃ metal embrittlement), causing LNG to spray and leak, and real-time monitoring is required for the “small leakage high-risk point”, and the interlocking logic uses 1oo2 (two out of one), and single-point alarm triggers the DCS control system to adjust the purging flow.

[0046] It can be understood that the LNG storage tank safety detection and control system further comprises a purging and inerting system, and an interlocking execution system, the purging and inerting system comprising a nitrogen purging pipeline and a liquid nitrogen storage tank, the interlocking execution system comprising a fifth flow regulating valve, an input end of the nitrogen purging pipeline being connected with a first output end of the liquid nitrogen storage tank, an output end of the nitrogen purging pipeline being connected to an annular space formed between an inner tank and an outer tank in the LNG storage tank, a sealed interlayer between a ceiling of the LNG storage tank and a top plate of the inner tank, and a submerged pump pumping mechanism, the fifth flow regulating valve being arranged near the liquid nitrogen storage tank, and the fifth flow regulating valve being connected with the DCS control system.

[0047] In some embodiments, the arrangement is that: 4 second combustible gas detectors are evenly distributed in the circumferential direction of the annular space (one every 90°), and the installation height is close to the inner tank bottom weld (the highest risk point of leakage); the function is to monitor the methane concentration in real time, if the inner tank wall is broken and LNG seeps into the annular space, the detector triggers DCS alarm when the concentration is ≥10% LEL (low alarm), and starts SIS interlock (nitrogen purging + cutting liquid) when the concentration is ≥20% LEL (high alarm).

[0048] It should be noted that the output end of the nitrogen purging pipeline is connected to the annular space: the annular area between the inner and outer tank walls of the storage tank (4-6 purging inlets are evenly distributed), the inner interlayer: the sealed interlayer between the storage tank ceiling and the inner tank top plate (a purging distribution pipe is arranged at the top), the pump area: the sealing cavity of the submersible pump and the pump well (the flange / axle seal area is covered), and the secondary bottom plate: the interlayer between the concrete foundation and the storage tank bottom plate (a ring-shaped purging pipe is arranged along the edge). The action scene and mechanism of the nitrogen purging pipeline are as follows: ① inerting of combustible gas, replacing the methane gas in the annular space / pump area; ② emergency dilution in case of leakage, 50 Nm 3 / h purging to suppress explosion when the safety instrument system is triggered; ③ protection against condensation, dry nitrogen to prevent instrument air pipeline from icing; ④ purging after pre-cooling, discharging residual liquid nitrogen to prevent valve freezing.

[0049] In some embodiments, the valves provided on the nitrogen purging pipeline include: ① a fifth flow regulating valve provided at the front end of the purging main pipe, which controls the basic flow through the DCS control system; ② an electromagnetic valve provided at the inlet of each branch pipe, which is fully opened when the safety instrument system is interlocked (emergency purging mode); ③ a manual root valve provided at the outlet of the nitrogen source, which is used for physical isolation and maintenance (DN80 ball valve); and ④ a check valve provided at the end of the branch pipe, which prevents LNG from flowing back.

[0050] Referring to Figure 2 It can be understood that the purging and inerting system also includes an instrument air pipeline and an instrument air station. The input end of the instrument air pipeline is connected to the output end of the instrument air station, and the output end of the instrument air pipeline is connected to the pilot-operated outlet valve, the first pneumatic emergency shut-off valve, the second pneumatic emergency shut-off valve, the third pneumatic emergency shut-off valve, the BOG recovery regulating valve, the flare removal regulating valve, the first flow regulating valve, the second flow regulating valve, the third flow regulating valve, the fourth flow regulating valve, and the fifth flow regulating valve.

[0051] Specifically, the position of the instrument air pipeline is set as follows: ① main pipeline: DN50 stainless steel ring pipeline laid along the periphery of the storage tank; ② branch pipeline: radiatingly connected to each actuator: pneumatic regulating valve (BOG / flare / gas supplement pipeline), emergency shut-off valve (liquid inlet / gas supplement pipeline), pilot-operated outlet valve control chamber. The connection relationship of the instrument air pipeline is as follows: instrument air compressor→gas storage tank (1.0 MPa)→filtering pressure reducing valve→main pipe (DN50)→branch hose, with the regulating valve positioner (controlled by the DCS control system) and the shut-off valve cylinder (controlled by the safety instrument system); the input end of the instrument air pipeline is directly connected to 0.6 MPa dry air output by the instrument air station; the instrument air pipeline is redundantly designed as double compressors + gas storage tank (with a capacity to meet full valve action 3 times); the instrument air pipeline serves to provide driving force for the valve stem of the pneumatic regulating valve, drive the spring return cylinder of the emergency shut-off valve to quickly close the valve, control the opening pressure of the pilot-operated outlet valve, drive the limit switch signal and return the position signal to the DCS control system.

[0052] In some embodiments, the instrument air pipeline is connected to the DCS through an electric positioner for accurate control of the valve opening degree; and the instrument air pipeline is connected to the safety instrument system through an electromagnetic valve for supplying air to the cylinder to shut off the valve when triggered.

[0053] In some embodiments, the valves provided on the instrument air pipeline include: ① filtering pressure reducing valve, provided at the inlet of each branch pipe; ② quick exhaust valve, provided at the cylinder exhaust port, for accelerating the closing of the shut-off valve; ③ pneumatic three-way fitting, provided at the branch of the main pipe, for air filtering / lubricating / pressure stabilizing; and ④ manual isolation valve, provided at the front end of the air supply of each device, for isolating the air source during maintenance.

[0054] Reference Figure 1 It can be understood that the medium input system further includes a liquid nitrogen pre-cooling pipeline, and the interlocking execution system further includes a sixth flow regulating valve, the input end of the liquid nitrogen pre-cooling pipeline is connected to a liquid nitrogen storage tank, the output end of the liquid nitrogen pre-cooling pipeline is connected to a liquid inlet at the bottom of the LNG storage tank, the sixth flow regulating valve is provided on the liquid nitrogen pre-cooling pipeline, and the sixth flow regulating valve is connected to the DCS control system.

[0055] It can be understood that the safety relief system further includes a first pipeline safety valve, a second pipeline safety valve and a third pipeline safety valve, the first pipeline safety valve is installed on the LNG upper liquid inlet input pipeline, the second pipeline safety valve is installed on the LNG lower liquid inlet input pipeline, and the third pipeline safety valve is installed on the flare pipeline.

[0056] It should be noted that the first pipeline safety valve, the second pipeline safety valve and the third pipeline safety valve are respectively installed at the turning portions of the LNG upper liquid inlet pipeline, the LNG lower liquid inlet pipeline and the flare pipeline, and are directly screwed or flanged connected on the pipelines for preventing overpressure caused by fluid impact or blockage in the local pipeline.

[0057] It can be understood that the safety relief system further comprises five flame arresters, the safety relief system further comprises a first relief pipeline, the BOG treatment system further comprises a second relief pipeline, the pilot operated vent valve is arranged on the first relief pipeline, the vacuum relief valve is installed on the second relief pipeline, the first relief pipeline is communicated with the inside of the LNG storage tank through a second relief opening at the top of the LNG storage tank, the second relief pipeline is communicated with the inside of the LNG storage tank through a first relief opening at the top of the LNG storage tank, and the five flame arresters are respectively installed on the LNG upper liquid inlet pipeline, the LNG lower liquid inlet pipeline, the flare pipeline, the first relief pipeline and the second relief pipeline.

[0058] It should be noted that the flame arrester is arranged at a distance of ≤1.5m from the outlet of the safety valve, the pilot operated vent valve and the vacuum relief valve to block the external flame from being transmitted back to the LNG storage tank or the pipeline through the relief pipeline, and each flame arrester has a DN300 aperture to meet the maximum relief flow requirement; the outlet pipeline height: the outlet of the first relief pipeline and the second relief pipeline is higher than the highest liquid level of the LNG storage tank by 2m to ensure the high-altitude diffusion of the gas; the flame arrester has a fireproof mechanism, an internal stainless steel corrugated plate structure and a gap between the slits to extinguish the flame propagation.

[0059] It can be understood that the LNG storage tank safety detection and control system further comprises a storage tank liquid phase pipeline, an input end of the storage tank liquid phase pipeline is connected to a liquid outlet at the bottom of the LNG storage tank, an output end of the submersible pump pumping mechanism is connected to the liquid outlet at the bottom of the LNG storage tank, and an output end of the storage tank liquid phase pipeline is connected to an input end of the submersible pump pumping mechanism and an input end of a downstream system through a three-way valve.

[0060] It can be understood that the safety relief system further comprises a low-temperature safety valve, and the low-temperature safety valve is installed at the turning portion of the storage tank liquid phase pipeline.

[0061] It should be noted that the low temperature safety valve is welded or flanged to the turn of the liquid phase pipeline of the storage tank, which is used to prevent overpressure of the liquid phase pipeline caused by low temperature gasification or "geyser" phenomenon. The DCS control system indirectly monitors the inlet pressure of each safety valve through the pressure transmitter (PT series), displays the pressure state in real time, and warns of potential overpressure risk, but does not directly control the action of the safety valve (the safety valve is a pure mechanical structure). The safety valve, as a mechanical redundant protection layer independent of the safety instrument system, can still physically release pressure when the safety instrument system interlock or the DCS control system notifies the system failure. Working principle: mechanical take-off: when the local pressure of the pipeline exceeds the set value of the safety valve (such as 1.5 MPa for the pipeline safety valve, and the low temperature safety valve is customized according to the working condition), the spring in the valve is compressed, and the valve is automatically opened to release pressure; automatic reset: when the pressure drops to below 95% of the set value, the spring force pushes the valve to close, stopping the discharge; both the pipeline safety valve and the low temperature safety valve are independent of external dependence: no power or gas source is needed, and they meet the independent protection of safety integrity level (SIL).

[0062] In some embodiments, a surface thermometer is welded to the outer wall of the liquid phase pipeline of the storage tank for detecting the low temperature stress risk of the pipeline.

[0063] Referring to Figure 1 It can be understood that the detection system further includes a radar liquid level meter, which is arranged at the top of the LNG storage tank.

[0064] It should be noted that the radar liquid level meter and the sixth flow regulating valve are respectively connected with the DCS control system.

[0065] It should be noted that LNG is a mixture of hydrocarbons dominated by methane. When stored for a long time, mixed from different sources, or improperly fed (e.g., high-density LNG injected from the top or low-density LNG injected from the bottom), the LNG in the storage tank will form a stable upper and lower layer due to the density difference. The lower layer LNG gradually decreases in density due to heat absorption (mainly through the tank wall and tank bottom), and the upper layer LNG gradually increases in density due to the preferential evaporation of light components. When the densities of the upper and lower layers tend to be the same, the stratification stability is destroyed, causing violent mixing (tumbling), and a large amount of boil-off gas (BOG) is generated instantaneously, causing the pressure in the storage tank to rise rapidly, which may cause an overpressure explosion, equipment damage, or a large amount of LNG leakage. The traditional system lacks real-time accurate monitoring of the LNG density gradient and active intervention means (such as forced circulation, intelligent switching of feeding method) based on this, which cannot effectively prevent and suppress tumbling; The extremely low temperature characteristics of LNG require extremely high material and sealing performance of the equipment such as storage tank, pipeline, valve, flange, and pump. Sealing failure, low-temperature brittle fracture of materials, and improper pre-cooling operation (such as excessive speed leading to excessive cold shrinkage stress) can easily cause LNG leakage. The leaked LNG quickly vaporizes, forming a low-temperature vapor cloud and a flammable gas cloud, which is the source of fire and explosion. The traditional LNG storage tank system lacks sufficient coverage of leakage monitoring points, has slow response speed, and emergency shutdown is not quick and thorough enough; Explosion and fire risk: Leaked natural gas (methane) mixes with air to form an explosive mixture, which can easily cause fire and explosion accidents when exposed to ignition sources (such as static electricity spark, electrical spark, open flame, mechanical impact spark, etc.). Misconnection / disconnection of loading and unloading vehicles (such as crane pipe), vehicle movement pulling off the pipe, poor static grounding, incomplete equipment maintenance and replacement, electrical explosion failure, insufficient safety distance, improper or failed safety relief system design, etc. are all high-risk links. The traditional LNG storage tank system has poor linkage in ignition source control and rapid disposal of leaked gas; LNG in the liquid phase pipeline (especially vertical or U-shaped pipe sections) connected to the storage tank will gasify if the insulation fails or the environment heats up, causing the BOG to accumulate in the pipeline. When the gas pressure accumulates enough to overcome the liquid column static pressure and the gas phase pressure in the storage tank, it will suddenly erupt, violently pushing the liquid in the pipeline back into the storage tank, causing the liquid in the storage tank to flash and the pressure to fluctuate violently. This phenomenon, known as "geyser", not only causes the safety valve to frequently trip (which may lead to sealing failure or ice blockage), but also may damage the pipeline and equipment. The traditional system lacks effective suppression design for this phenomenon; The production of BOG is continuous, and an effective recovery (such as liquefaction or as fuel gas) or discharge (such as flare combustion) system is needed to maintain the stability of the storage tank pressure. The pressure relief device (such as safety valve) of the traditional LNG storage tank system is mainly passive response, with a single response pressure setting. In the case of pressure rising rapidly due to tumbling or geyser, the relief capacity may be insufficient or the response may not be timely enough, leading to overpressure risk.At the same time, the storage tank has a risk of collapse under negative pressure (e.g. due to excessive liquid discharge), and needs reliable vacuum protection. Traditional LNG storage tank monitoring systems usually independently measure parameters such as liquid level, temperature, pressure, etc., but lack comprehensive analysis capabilities for key risk factors (such as density gradient, temperature stratification, early signs of leakage), the control logic is relatively simple, and the information sharing and collaborative action among the various safety subsystems (such as DCS process control, SIS safety instrument system, leakage monitoring) is insufficient, making it difficult to achieve early warning and proactive intervention of risks. That is, the existing LNG storage tank safety technology has problems such as single monitoring dimension, delayed risk warning, weak proactive control capability, poor subsystem collaboration, insufficient targeted solutions for core risks (stratification and rolling, intermittent spring), and is difficult to meet the increasingly high safety and reliability requirements of large LNG storage tanks. Therefore, there is an urgent need for a comprehensive safety detection and control system that integrates advanced sensing technology, intelligent analysis algorithms, and rapid linkage execution mechanisms to prevent accidents from the source, provide early warning of risks, and respond quickly and effectively in emergency situations.

[0066] Based on this, the present application provides a LNG storage tank safety detection and control system, which integrates LTD density meters and multi-point average temperature meters, vertically stratified / distributed in the LNG liquid phase in the storage tank, the LTD density meters provide real-time and accurate measurement of the density of different liquid layers, and the multi-point temperature meters monitor the vertical temperature gradient. These data are input into the anti-rolling algorithm embedded in the DCS control system. When the algorithm determines that the density difference exceeds the safety threshold (e.g. > 5%) or the temperature gradient is abnormal, indicating that there is a risk of stratification, the DCS system immediately triggers: starts the submersible pump pumping mechanism to forcibly circulate the mixed liquid layer, destroys the stratification structure, according to the comparison of the density of the incoming LNG and the density in the tank, through the DCS control of the second flow regulating valve (upper liquid inlet) or the third flow regulating valve (lower liquid inlet), intelligently switches the liquid inlet mode (when the density is high, changes to upper liquid inlet to inject into the upper layer, when the density is low, changes to lower liquid inlet to inject into the lower layer), uses the kinetic energy of the incoming liquid to promote mixing, prevents stratification from forming from the source, adjusts the opening of the BOG recovery regulating valve and the flare regulating valve to optimize the BOG discharge rate, avoids the sudden increase in pressure caused by the generation of a large amount of BOG during rolling, to achieve early identification and proactive intervention of stratification risks, significantly reducing the likelihood of rolling accidents and the severity of consequences.

[0067] The application arranges first, second and third combustible gas detectors at key leakage risk points (such as the connection flange between the LNG lower liquid inlet pipeline and the bottom of the storage tank, the bottom weld of the storage tank annular space, and the gas phase space of the storage tank), which monitor the concentration of combustible gas in real time; fast interlocking: once the detector alarm signal is confirmed to have a leakage by the safety instrument system (SIS), the SIS system (independent of the DCS and more focused on safety) immediately triggers the highest priority interlocking: within 2 seconds, the first, second and third pneumatic emergency shut-off valves are closed in an emergency, cutting off the LNG liquid inlet (upper and lower liquid inlet) and natural gas supplement path, and isolating the leakage source. Open the flare regulating valve (if not automatically opened) to guide the leakage gas and the overpressure BOG in the tank to the flare for emergency discharge, start the purging and inerting system (control the nitrogen flow through the fifth flow regulating valve), inject nitrogen into the annular space, interlayer, pump area, etc., inert and dilute the leaked combustible gas, reduce the risk of combustion and explosion, realize early detection and extremely fast (second-level) source shutdown, supplemented by inerting protection, maximize the containment of leakage development, prevent fire and explosion, and install a flame arrestor on the key pipeline (liquid inlet, discharge, and flare pipeline) to prevent external flames from returning to the inside of the storage tank through the pipeline; the submersible pump pumping mechanism and its return pipeline (connected to the tank bottom outlet through a three-way valve), when the submersible pump is running, part of the LNG can return to the bottom of the storage tank through the return pipeline, keeping the LNG in the liquid phase pipeline (especially between the pump outlet and the downstream valve) continuously flowing, reducing the opportunity for LNG to stay in the pipeline and be heated and gasified, pre-cooling the storage tank through the liquid nitrogen pre-cooling pipeline and the sixth flow regulating valve, avoiding low-temperature brittle fracture, and also reducing abnormal gasification in the pipeline due to insufficient pre-cooling, effectively reducing LNG gasification accumulation in the pipeline, and reducing the frequency and intensity of geyser phenomenon by maintaining fluid flow in the pipeline and ensuring good pre-cooling.

[0068] Need to explain, by building three level pressure control and relief system to optimize pressure control and multi-stage safety relief: daily pressure regulation: DCS control system control BOG recovery regulating valve (preferential recycling BOG) and torch adjusting valve (auxiliary regulation, when the recovery capacity is insufficient, discharge), maintain the tank pressure in the normal working range, the first flow regulating valve control the additional natural gas, for maintaining pressure. First-stage rapid pressure relief: pilot callout valve (installed in the second relief port at the top of the tank), when the pressure exceeds the set value (such as 1.2MPa), quickly and massively release BOG to the atmosphere or torch (through the first relief pipeline), fast response speed; second-stage controllable relief: SIS system controlled torch adjusting valve, when the pressure rises (such as >1.0MPa) but has not yet reached the callout valve action point, or as a supplement to the callout valve, SIS can instruct it to open large for controllable discharge; redundant safety backup: first, second and third pipeline safety valves (installed in the liquid inlet, torch pipeline) and low-temperature safety valve (installed in the liquid phase pipeline bend), provide local overpressure protection, safety valve as the final physical relief guarantee; negative pressure protection: vacuum relief valve (installed on the second relief pipeline at the first relief port at the top of the tank). When negative pressure (such as <-0.5kPa) is formed in the tank, automatically open to suck in air (usually need to pass through the flame arrester), prevent the tank from being deflated; further realize the grading, priority and redundancy of pressure control and relief mechanism, significantly improve the system's ability to respond to normal fluctuations, BOG increment (including the risk of rolling) and overpressure accidents, ensure pressure safety.

[0069] Need to explain, by building comprehensive detection system (liquid level: servo liquid level meter x3 + radar liquid level meter to improve monitoring dimension and system coordination: density: LTD density meter; temperature: multi-point average thermometer (liquid phase) + platinum resistance (gas phase) + surface thermometer (tank structure); pressure: pressure transmitter x4; flammable gas: multi-position detector) and control layer (DCS control system + safety instrument system), multi-parameter, multi-point real-time monitoring provides a comprehensive data basis for anti-rolling algorithm, leakage judgment, pressure control, etc. DCS control system is responsible for continuous process monitoring, optimization control and active prevention based on algorithm (such as anti-rolling); safety instrument system is independently set, focusing on safety interlocking (SIL level), executing emergency shutdown (ESD) and key safety actions (such as emergency shutdown, forced relief) when reaching safety limit; DCS control system and safety instrument system are connected to realize necessary information interaction (such as safety instrument system can inform DCS control system of trigger signals for recording and subsequent processing), pneumatic actuators (valves, pumps) are reliably driven by instrument air system. Effect: realized the leap from single parameter monitoring to multi-dimensional risk perception, the division of labor between DCS control system and safety instrument system ensures the unity of process optimization and functional safety, greatly improves the overall intelligent level and reliability of the system.

[0070] That is, the LNG storage tank safety detection and control system provided by the application forms a complete LNG storage tank safety guarantee system combining active prevention and rapid response through sensor network layout (LTD density meter, multi-point temperature, multi-position combustible gas detection), DCS control embedded with intelligent algorithm (anti-rolling, pressure optimization), high-reliability and rapid-response SIS safety interlocking (emergency shutdown, relief), multi-stage pressure control and relief architecture (regulating valve, outlet valve, safety valve, vacuum valve), and design optimization for specific risks (double liquid inlet, pump backflow, pre-cooling, nitrogen inerting), effectively solves the core pain points of traditional systems in layered rolling prevention, leakage rapid disposal, geyser suppression, pressure safety control and multi-system cooperation, and significantly improves the intrinsic safety level and operation reliability of the LNG storage tank.

[0071] For example, the workflow of the application is as follows: I. System initialization and pre-cooling stage. Pre-cooling of the storage tank (first use / after overhaul), start the liquid nitrogen pre-cooling pipeline: the DCS control system opens the sixth flow regulating valve to control the injection of liquid nitrogen from the liquid nitrogen storage tank through the LNG storage tank bottom inlet; temperature monitoring: the multi-point average thermometer monitors the temperature gradient in the tank in real time to ensure that the pre-cooling rate is ≤5℃ / h (to avoid low-temperature brittle fracture of materials); purging protection: the DCS control system opens the fifth flow regulating valve to inject nitrogen into the annular space and pump area (flow rate 20 Nm 3Prevent air condensation icing. II. Normal storage and transportation operation process. 1. LNG feeding control: ① High-density LNG, DCS control system controls the second flow regulating valve (upper liquid inlet pipeline), and is injected from the top of the storage tank to avoid increasing the density of the lower layer; ② Low-density LNG, DCS control system controls the third flow regulating valve (lower liquid inlet pipeline), and is injected from the bottom of the storage tank to destroy the upper layer stability; ③ Supplementary gas pressure regulation, when the pressure is <0.8 MPa, DCS opens the fourth flow regulating valve to supplement 5.0 MPa natural gas to the gas phase space. 2. BOG dynamic balance control: ① BOG recovery mode (pressure <0.95 MPa): DCS opens the BOG recovery regulating valve to transport the evaporated gas to the re-liquefaction system; ② Flare discharge mode (pressure ≥0.95 MPa): DCS switches to the flare regulating valve, and the overpressure gas is safely burned; ③ Vacuum protection: when the pressure is <-0.5 kPa, the vacuum relief valve automatically opens to suck in air (through the flame arrester) to prevent the storage tank from collapsing. III. Core risk active prevention and control process. 1. Rollover prevention: the LTD density meter monitors the density gradient in real time, and confirms whether the density difference is >5% or the temperature stratification is abnormal; if not, the current feeding strategy is maintained; if yes, the anti-rollover algorithm of the DCS control system is started, the submersible pump is forced to circulate the mixed liquid layer, the liquid inlet position is switched: high-density → top liquid inlet, low-density → bottom liquid inlet, the BOG recovery regulating valve opening is increased, and the BOG is discharged in advance. 2. Geysering suppression: ① Pipeline flow guarantee: the submersible pump is continuously operated to maintain the LNG flow in the liquid phase pipeline through the backflow pipeline. ② Subcooling degree monitoring: the multi-point average thermometer monitors the liquid phase temperature in real time to ensure that it is lower than the saturation temperature by more than 3℃. ③ Pressure stabilization: DCS dynamically adjusts the BOG recovery / flare valve to control the tank pressure fluctuation within ±0.05 MPa. IV. Leakage monitoring and second-level emergency response. 1. Leakage detection matrix Probe location Probe type Interlock logic Response action Lower liquid inlet flange / pump seal First combustible gas probe 1oo2 (one out of two) Concentration > 10% LEL: DCS alarm; > 20% LEL: SIS triggers interlock Annulus space bottom weld Second combustible gas probe 2oo4 (two out of four) Concentration > 20% LEL: SIS initiates emergency shutdown + nitrogen purge Top gas phase space Third combustible gas probe Single point trigger Concentration > 5% vol: SIS forces opening of flare relief valve 2. SIS interlocking execution: ① Cut off the feeding source: close the first, second and third pneumatic emergency shut-off valves (LNG upper / lower liquid inlet + supplementary gas pipeline); ② Emergency pressure relief: fully open the flare regulating valve, and BOG is directly discharged to the flare system; ③ Inertization explosion suppression: SIS opens the electromagnetic valve, and the nitrogen gas blowing flow is increased to 50 Nm 3 / h (annular space / pump area). V. Graded pressure safety protection, as shown in the following table: Pressure regime Safety protection measures Normal range 0.8-0.95 MPa DCS control system regulates BOG recovery regulating valve Pressure > 0.95 MPa DCS control system switches flare valve Safety instrumented system assists opening of flare valve Pressure > 1.2 MPa Pilot operated vent valve mechanically opened Local piping overpressure Piping relief valve / low temperature relief valve tripped Negative pressure < 0.5 kPa Vacuum relief valve automatically opened Six, system coordination and data fusion. 1, DCS central decision layer: ①synchronous analysis of density (LTD), temperature (≥8 points), liquid level (servo liquid level meter ×3+radar), pressure, combustible gas data; ②execute anti-rollover algorithm, BOG optimization control, air supplement and pressure regulation, etc. Active strategy. 2, SIS safety protection layer: ①independent hard-wired connection of emergency shut-off valve, relief valve, combustible gas detector; ②communication with DCS: DCS provides risk warning, SIS executes SIL2 level interlocking (such as rollover prediction confirmation to prepare for cutting off); 3, mechanical redundancy layer: pure mechanical action of pilot-operated outlet valve / vacuum valve / safety valve, no dependence on control system. Seven, shutdown and maintenance mode. ①Drainage protection, open submersible pump to pump residual LNG, DCS monitors liquid level to <5% when pump stops; ②SIS interlocking ensures that the air supplement pipeline is opened when draining, preventing negative pressure; ③Nitrogen replacement: nitrogen is injected into the purge system at a flow rate of 100 Nm 3 / h, until the oxygen content is <1%; ④Instrument calibration: servo liquid level meter / density meter is calibrated online every month, and combustible gas detector is calibrated every quarter.

[0072] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the exemplary description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0073] The above is the preferred embodiment of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which are also considered to be within the scope of protection of the present application.

[0074] Although the embodiments of the present application have been shown and described, those skilled in the art can understand that various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and purposes of the present application, and the scope of the present application is defined by the claims and their equivalents.

Claims

1. A safety detection and control system for LNG storage tanks, characterized in that, include: DCS control system, safety instrumented system, media input system, BOG processing system, safety relief system, detection system and interlocking execution system; The medium input system includes an LNG upper liquid inlet input pipeline, an LNG lower liquid inlet input pipeline, and a natural gas replenishment pipeline; the output end of the LNG upper liquid inlet input pipeline is connected to the liquid inlet at the top of the LNG storage tank, the output end of the LNG lower liquid inlet input pipeline is connected to the liquid inlet at the bottom of the LNG storage tank, and the output end of the natural gas replenishment pipeline is connected to the interior of the LNG storage tank through a gas replenishment port opened on the side wall near the top of the LNG storage tank; The BOG processing system includes a BOG recovery pipeline, a flare-to-flare pipeline, and a vacuum relief valve. The input end of the BOG recovery pipeline is connected to the interior of the LNG storage tank through a first BOG outlet opened on the top of the LNG storage tank. The input end of the flare-to-flare pipeline is connected to the interior of the LNG storage tank through a second BOG outlet opened on the top of the LNG storage tank. The vacuum relief valve is installed at the first vent on the top of the LNG storage tank. The safety relief system includes a pilot-operated exhalation valve, which is installed at the second vent on the top of the LNG storage tank; The detection system includes an LTD density meter, a multi-point average temperature meter, multiple servo level gauges, and a pressure transmitter. The LTD density meter is vertically layered and installed inside the LNG storage tank. The multi-point average temperature meter is vertically distributed in the LNG liquid phase inside the LNG storage tank. The multiple servo level gauges are vertically installed on the inner wall of the LNG storage tank and are evenly distributed vertically. The pressure transmitter is installed on the top of the LNG storage tank. The interlocking execution system includes a first pneumatic emergency shut-off valve, a second pneumatic emergency shut-off valve, a third pneumatic emergency shut-off valve, a BOG recovery regulating valve, a flare removal regulating valve, a first flow regulating valve, a second flow regulating valve, a third flow regulating valve, a fourth flow regulating valve, and a submerged pump pumping mechanism. The first pneumatic emergency shut-off valve and the second flow regulating valve are both located on the upper LNG inlet pipeline, the second pneumatic emergency shut-off valve and the third flow regulating valve are both located on the lower LNG inlet pipeline, the third pneumatic emergency shut-off valve and the fourth flow regulating valve are both located on the natural gas replenishment pipeline, the BOG recovery regulating valve is located on the BOG recovery pipeline, the flare removal regulating valve is located on the flare removal pipeline, the first flow regulating valve is located on the natural gas replenishment pipeline, and the submerged pump pumping mechanism is submerged in the LNG liquid phase near the bottom of the LNG storage tank. The DCS control system is embedded with an anti-rollover algorithm. The DCS control system is connected to the LTD density meter, the multi-point average thermometer, the BOG recovery regulating valve, the flare removal regulating valve, the first flow regulating valve, the second flow regulating valve, the third flow regulating valve, the fourth flow regulating valve, the submersible pumping mechanism, the pressure transmitter, and each of the servo level gauges. The safety instrument system is connected to the vacuum relief valve, the pilot-operated exhalation valve, the flare removal regulating valve, the first pneumatic emergency shut-off valve, the second pneumatic emergency shut-off valve, and the third pneumatic emergency shut-off valve. The DCS control system is also connected to the safety instrument system.

2. The LNG storage tank safety detection and control system according to claim 1, characterized in that, The LNG storage tank safety detection and control system further includes a first combustible gas detector, multiple second combustible gas detectors, and multiple third combustible gas detectors. The first combustible gas detector is located at the connection flange between the LNG lower liquid inlet pipeline and the bottom of the LNG storage tank. The multiple second combustible gas detectors are evenly distributed in the circumferential direction of the annular space formed between the inner tank and the outer tank of the LNG storage tank, and all the second combustible gas detectors are located near the bottom weld of the LNG storage tank. The multiple third combustible gas detectors are installed on the inner wall of the LNG storage tank near the top. The first combustible gas detector, each of the second combustible gas detectors, and each of the third combustible gas detectors are respectively connected to the DCS control system and the safety instrument system.

3. The LNG storage tank safety detection and control system according to claim 1, characterized in that, The LNG storage tank safety detection and control system also includes a purging and inerting system, which includes a nitrogen purging pipeline and a liquid nitrogen storage tank. The interlocking execution system includes a fifth flow regulating valve. The input end of the nitrogen purging pipeline is connected to the first output end of the liquid nitrogen storage tank. The output end of the nitrogen purging pipeline is connected to the annular space formed between the inner and outer tanks of the LNG storage tank, the sealed interlayer between the ceiling of the LNG storage tank and the top plate of the inner tank, and the submersible pump pumping mechanism. The fifth flow regulating valve is located on the nitrogen purging pipeline near the liquid nitrogen storage tank and is connected to the DCS control system.

4. The LNG storage tank safety detection and control system according to claim 3, characterized in that, The purging and inerting system also includes an instrument air line and an instrument air station. The instrument air line is laid along the periphery of the LNG storage tank. The input end of the instrument air line is connected to the output end of the instrument air station. The output end of the instrument air line is respectively connected to the pilot-operated exhalation valve, the first pneumatic emergency shut-off valve, the second pneumatic emergency shut-off valve, the third pneumatic emergency shut-off valve, the BOG recovery regulating valve, the flare removal regulating valve, the first flow regulating valve, the second flow regulating valve, the third flow regulating valve, the fourth flow regulating valve, and the fifth flow regulating valve.

5. The LNG storage tank safety detection and control system according to claim 3, characterized in that, The medium input system also includes a liquid nitrogen precooling pipeline, and the interlocking execution system also includes a sixth flow regulating valve. The input end of the liquid nitrogen precooling pipeline is connected to the liquid nitrogen storage tank, and the output end of the liquid nitrogen precooling pipeline is connected to the liquid inlet at the bottom of the LNG storage tank. The sixth flow regulating valve is installed on the liquid nitrogen precooling pipeline and is connected to the DCS control system.

6. The LNG storage tank safety detection and control system according to claim 1, characterized in that, The safety relief system also includes a first pipeline safety valve, a second pipeline safety valve, and a third pipeline safety valve. The first pipeline safety valve is installed on the LNG upper inlet pipeline, the second pipeline safety valve is installed on the LNG lower inlet pipeline, and the third pipeline safety valve is installed on the flare-to-flare pipeline.

7. The LNG storage tank safety detection and control system according to claim 1, characterized in that, The safety release system also includes five flame arresters, a first release pipeline, and a second release pipeline. The pilot-operated exhalation valve is installed on the first release pipeline, and the vacuum release valve is installed on the second release pipeline. The first release pipeline is connected to the interior of the LNG storage tank through a second release port on the top of the LNG storage tank, and the second release pipeline is connected to the interior of the LNG storage tank through a first release port on the top of the LNG storage tank. The five flame arresters are respectively installed on the LNG upper liquid inlet pipeline, the LNG lower liquid inlet pipeline, the flare removal pipeline, the first release pipeline, and the second release pipeline.

8. The LNG storage tank safety detection and control system according to claim 1, characterized in that, The LNG storage tank safety detection and control system also includes a storage tank liquid phase pipeline. The input end of the storage tank liquid phase pipeline is connected to the liquid outlet at the bottom of the LNG storage tank, and the output end of the submersible pump pumping mechanism is connected to the liquid outlet at the bottom of the LNG storage tank. The output end of the storage tank liquid phase pipeline is connected to the input end of the submersible pump pumping mechanism and the input end of the downstream system respectively through a three-way valve.

9. The LNG storage tank safety detection and control system according to claim 8, characterized in that, The safety relief system also includes a cryogenic safety valve, which is installed at the bend in the liquid phase pipeline of the storage tank.

10. The LNG storage tank safety detection and control system according to claim 1, characterized in that, The detection system also includes a radar level gauge, which is installed on top of the LNG storage tank.

Images