Automatic control method and system for liquid level of after-condenser of liquefied natural gas receiving station

Abstract

The invention provides an automatic liquid level control method and system for an after-condenser of a liquefied natural gas receiving station. The automatic liquid level control method for the after-condenser of the liquefied natural gas receiving station comprises the steps that S1, the bottom pressure of the after-condenser is kept stable, and the bottom pressure of the after-condenser is obtained; s2, according to the bottom pressure of the after-condenser, the set operation temperature of the after-condenser is calculated; s3, obtaining a plurality of parameter values and BOG mass flow which influence the liquid-gas ratio R value for controlling the recondenser to operate at the set operation temperature; s4, according to the set operation temperature of the recondenser and the multiple parameter values, the liquid-gas ratio R value required by the set operation temperature of the recondenser is calculated; s5, according to the liquid-gas ratio R value required by the re-condenser at the set operating temperature and the BOG mass flow, the LNG mass flow required by feeding of the re-condenser is calculated; and S6, feeding according to the LNG mass flow rate required by feeding of the re-condenser.

Description

Technical Field

[0001] The present invention relates to a liquefied natural gas receiving, storage and external transmission process of a liquefied natural gas receiving station, and in particular to a recondenser liquid level automatic control method and system of a liquefied natural gas receiving station. Background Art

[0002] Liquefied natural gas is an important part of natural gas supply. LNG (Liquefied Natural Gas) receiving stations are important facilities for receiving, storing and gasifying LNG. Recondensers are equipment for processing boil-off gas (BOG) in LNG receiving stations. They are connected to low-pressure pumps, high-pressure pumps, compressors and other equipment in the process flow. They are key equipment and the most difficult to control. Fluctuations in the recondenser liquid level will cause fluctuations in upstream and downstream operating parameters, and even cause compressors to shut down or the entire plant to shut down. Automatic control of the recondenser liquid level has always been a difficult problem in the industry, and the recondenser liquid level in most receiving stations is under manual control.

[0003] The recondenser structure mainly includes a tank body, a gas-liquid distribution plate, a packing, etc., and has two main functions. The first function is to handle the evaporation gas generated by the evaporation of liquefied natural gas caused by the operation or heating of low-temperature equipment (such as high-pressure pumps, storage tanks, low-temperature pipelines, etc.). When the recondenser is in operation, the supercooled low-temperature LNG introduced from the low-pressure output main pipe is mixed with the BOG (Boil Off Gas, evaporation gas) compressor after pressurization, and the BOG temperature is reduced and condensed to a liquid state, and then mixed with the LNG bypassed by the recondenser and enters the high-pressure pump, and is transported to downstream users after being pressurized and gasified. The second function is to buffer the pressure fluctuations caused by the start and stop of the high-pressure pump and the adjustment of the upstream and downstream LNG flow, and enhance the stability of the system. At the same time, the gas in the high-pressure pump tank can be continuously vented to the recondenser to ensure that there is enough liquid level in the high-pressure pump tank to prevent the high-pressure pump from being cavitated.

[0004] When the recondenser is in operation, if the LNG entering the recondenser is too little and the BOG entering the recondenser cannot be completely condensed, the liquid level of the recondenser will be reduced. When it is reduced to a low liquid level, it will trigger a chain stop of the BOG compressor and the high-pressure pump, causing the entire plant to shut down; when there is too much LNG entering the recondenser, the BOG condensation amount in the gas phase space of the recondenser is too much, and the condensation amount in the packing is too little. Due to the unstable gas phase condensation, the liquid level and pressure of the recondenser will fluctuate up and down. In order to ensure the normal operation of the recondenser and the high-pressure pump, the liquid level of the recondenser needs to be controlled within the normal range and kept stable.

[0005] There are two main ways to automatically control the liquid level of the recondenser. The first is to control the LNG flow at the inlet of the recondenser by the pressure at the top of the recondenser to maintain the top pressure of the recondenser stable, and at the same time adjust the regulating valve at the bottom of the recondenser by the liquid level of the recondenser to control the liquid level of the recondenser within the normal range. The disadvantage of this automatic liquid level control method is that the liquid level of the recondenser is greatly affected by the pressure fluctuations of the upstream and downstream, and requires the operator to operate carefully. When the pressure fluctuations of the upstream and downstream are large, it is easy to cause the whole plant to shut down. The second method is to fit the relationship between the liquid-gas mass ratio (R value) of the recondenser LNG and BOG feed and the BOG temperature, BOG flow, recondenser outlet pressure, relative density of the external natural gas, and nitrogen content based on the historical manual operation data of the recondenser. The system calculates the R value in real time through the relationship and obtains the LNG flow. According to the calculated LNG flow, the recondenser inlet regulating valve is adjusted to control the LNG flow, thereby adjusting the recondenser liquid level. This liquid level automatic control method has many participating control instruments and some instruments have low safety levels, resulting in weak control stability and the inability to predict the relationship between the R value and the recondenser liquid level and temperature. The recondenser temperature fluctuates greatly, and the pressure at the bottom of the recondenser is controlled. The upstream and downstream pressure fluctuations have a great impact on the recondenser liquid level. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a method and system for automatically controlling the liquid level of a recondenser in a liquefied natural gas receiving station in view of the deficiencies in the prior art.

[0007] The technical solution of the present invention to solve the above technical problems is as follows: a method for automatically controlling the liquid level of a recondenser at a liquefied natural gas receiving station, comprising: S1, maintaining the bottom pressure of the recondenser stable, and obtaining the bottom pressure of the recondenser; S2, calculating the set operating temperature of the recondenser according to the bottom pressure of the recondenser; S3, obtaining multiple parameter values ​​and BOG mass flow that affect the liquid-to-gas ratio R value for controlling the recondenser to operate at the set operating temperature; S4, calculating the liquid-to-gas ratio R value required at the set operating temperature of the recondenser according to the set operating temperature of the recondenser and multiple parameter values; S5, calculating the LNG mass flow required for recondenser feed according to the liquid-to-gas ratio R value required at the set operating temperature of the recondenser and the BOG mass flow; S6, feeding according to the LNG mass flow required for the recondenser feed.

[0008] The beneficial effects of the technical solution of the present invention are: by controlling the operating temperature of the recondenser, the operating temperature of the recondenser is controlled below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the recondenser liquid level to be stable. The recondenser liquid level is controlled by the automatic control method of the recondenser liquid level, the recondenser liquid level is less affected by the upstream and downstream pressure fluctuations, the recondenser operating temperature is stable and controllable, there are fewer field instruments involved in the automatic adjustment, the calculation program is safe and stable, the parameters involved in the adjustment are few and stable, and the stable automatic control of the recondenser liquid level of the LNG receiving station is realized. The operator does not need to perform fine operations when performing recondenser-related operations, the operator's recondenser operation skills are reduced, and there is no need to monitor the operating status of the receiving station recondenser at all times. Compared with manual adjustment by the operator, the automatic adjustment of the liquid level is more stable, there will be no adjustment errors, and the liquid level is automatically adjusted accurately when the parameters related to the recondenser liquid level control change. The recondenser operation is safer and more stable, reducing the risk of parking due to recondenser liquid level fluctuations, and reducing the intensity of the operator's monitoring of the recondenser operation status. There is no need to wait for the upstream and downstream pressures to completely stabilize before proceeding to the next step, which improves the production adjustment speed and emergency response speed of the receiving station and enhances the safety and stability of the production operation of the receiving station.

[0009] Further, it includes: the multiple parameter values ​​include: recondenser feed BOG temperature, recondenser feed LNG temperature, recondenser set operating temperature, LNG vaporization latent heat related parameters, BOG specific heat capacity, LNG specific heat capacity, LNG feed mass flow rate and BOG feed mass flow rate error correction coefficient.

[0010] The beneficial effects of adopting the above further technical solution are: fewer on-site instruments involved in automatic adjustment, safer and more stable calculation procedures, fewer parameters involved in adjustment and stronger stability, and stable automatic control of the recondenser liquid level of the LNG receiving station. There is no need to wait for the upstream and downstream pressures to be completely stable before proceeding to the next step, which improves the production adjustment speed of the receiving station and the emergency response speed, and enhances the safety and stability of the production operation of the receiving station.

[0011] Further, in step S4, the liquid-to-gas ratio R value required at the set operating temperature of the recondenser is calculated by the following formula: Wherein, R is the ratio of the recondenser LNG feed mass flow rate to the BOG feed mass flow rate, in units of 1; t1 is the recondenser feed BOG temperature, in units of °C; t2 is the recondenser feed LNG temperature, in units of °C; t is the set operating temperature of the recondenser, in units of °C; r is a parameter related to the latent heat of LNG vaporization, in units of kJ / kg; c1 is the BOG specific heat capacity, in units of kJ / kg / °C; c2 is the LNG specific heat capacity, in units of kJ / kg / °C; α is the error correction coefficient between the LNG feed mass flow rate and the BOG feed mass flow rate, in units of 1.

[0012] The beneficial effects of adopting the above further technical solution are: the liquid-gas ratio R value required at the set operating temperature of the recondenser is calculated through data and formulas to improve accuracy and reliability. There are fewer on-site instruments involved in automatic adjustment, the calculation program is safe and stable, and the parameters involved in adjustment are few and stable, so as to achieve stable automatic control of the liquid level of the recondenser of the LNG receiving station. There is no need to wait for the upstream and downstream pressures to be completely stable before proceeding to the next step, which improves the production adjustment speed of the receiving station and the emergency response speed, and enhances the safety and stability of the production operation of the receiving station.

[0013] Furthermore, in step S5, the liquid-to-gas ratio R value required at the set operating temperature of the recondenser is multiplied by the BOG mass flow rate to calculate the LNG mass flow rate required for feeding the recondenser.

[0014] The beneficial effect of adopting the above further technical solution is: the liquid-gas ratio R value is multiplied by the BOG mass flow rate to obtain the LNG mass flow rate required for feeding the recondenser, and the feeding is carried out according to the LNG mass flow rate, so that the operating temperature of the recondenser can be controlled at the set temperature, ensuring that the BOG entering the recondenser is completely condensed, thereby maintaining the recondenser liquid level within a normal range.

[0015] Further, step S2 includes: S21, calculating the top pressure of the recondenser according to the bottom pressure of the recondenser; S22, determining the safety margin and the material bubble point temperature corresponding to the top pressure of the recondenser according to the top pressure of the recondenser; S23, calculating the set operating temperature of the recondenser according to the safety margin and the material bubble point temperature.

[0016] The beneficial effect of adopting the above further technical solution is: by controlling the operating temperature of the recondenser, the operating temperature of the recondenser is controlled below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the liquid level of the recondenser to be stable.

[0017] Furthermore, in step S23, the material bubble point temperature is added to the safety margin to calculate the set operating temperature of the recondenser.

[0018] The beneficial effect of adopting the above further technical solution is: by controlling the operating temperature of the recondenser, the operating temperature of the recondenser is controlled below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the liquid level of the recondenser to be stable.

[0019] In addition, the present invention also provides a liquefied natural gas receiving station recondenser liquid level automatic control system, which is used to implement any one of the above-mentioned liquefied natural gas receiving station recondenser liquid level automatic control methods, and the liquefied natural gas receiving station recondenser liquid level automatic control system includes: a recondenser, a low-pressure output main pipe, a recondenser bottom pressure gauge, a recondenser bottom pressure controller, a recondenser bottom pressure regulating valve, a low-pressure pump, a high-pressure pump, a recondenser BOG inlet pipeline, a BOG pressure gauge, a BOG volume flow meter, a BOG thermometer, a BOG mass flow calculation device, a recondenser temperature control device, a recondenser inlet LNG flow calculation device, a recondenser upstream LNG pipeline, LNG The recondenser is provided with a thermometer, an LNG mass flowmeter, an LNG regulating valve, and a flow controller. The bottom of the recondenser and one end of the LNG pipeline upstream of the recondenser are connected to the low-pressure output main pipe. The two ends of the low-pressure output main pipe are connected to the low-pressure pump and the high-pressure pump in a one-to-one correspondence. The recondenser bottom pressure gauge and the recondenser bottom pressure regulating valve are both installed on the low-pressure output main pipe. The recondenser bottom pressure controller is respectively connected to the recondenser bottom pressure gauge and the recondenser bottom pressure regulating valve; the recondenser BOG inlet pipeline is connected to the top of the recondenser. The BOG pressure gauge, the BOG volume flowmeter, and the BOG temperature The BOG mass flow calculation device is connected to the BOG pressure gauge, the BOG volume flow meter, the BOG thermometer, and the LNG flow calculation device at the inlet of the recondenser respectively. The LNG flow calculation device at the inlet of the recondenser is connected to the flow controller and the recondenser temperature control device respectively. The recondenser temperature control device is connected to the LNG thermometer and the BOG thermometer respectively. The other end of the LNG pipeline upstream of the recondenser is connected to the top of the recondenser. The LNG thermometer, the LNG mass flow meter, and the LNG regulating valve are all installed on the recondenser. On the upstream LNG pipeline, the flow controller is connected to the LNG mass flowmeter and the LNG regulating valve respectively; the recondenser bottom pressure controller and the recondenser bottom pressure regulating valve are used to maintain the recondenser bottom pressure stable; the recondenser bottom pressure gauge is used to obtain the recondenser bottom pressure; the BOG mass flow calculation device is used to calculate the BOG mass flow according to the values ​​obtained by the BOG pressure gauge, the BOG volume flowmeter and the BOG thermometer; the recondenser temperature control device is used to calculate the liquid-to-gas ratio R value required at the set operating temperature of the recondenser according to the values ​​obtained by the BOG thermometer and the LNG thermometer;The recondenser inlet LNG flow calculation device is used to calculate the LNG mass flow required for the recondenser feed by multiplying the BOG mass flow output by the BOG mass flow calculation device and the liquid-gas ratio R value required at the recondenser set operating temperature output by the recondenser temperature control device; the flow controller is used to control the opening of the LNG regulating valve according to the LNG mass flow meter. ;

[0020] The beneficial effects of the technical solution of the present invention are: first, the pressure at the bottom of the recondenser is maintained stable by the recondenser bottom pressure regulating valve, and then the recondenser temperature is controlled below the material bubble point temperature corresponding to the recondenser pressure by the recondenser temperature control device, ensuring that the BOG entering the recondenser is completely condensed to a liquid state in the packing, thereby controlling the stability of the recondenser liquid level. The pressure at the bottom of the recondenser is maintained stable by regulating the recondenser bottom regulating valve by the pressure controller. The liquid-gas ratio R value required for the recondenser to set the operating temperature is calculated based on the BOG temperature and the LNG temperature. In the recondenser inlet LNG flow calculation device, the output value of the BOG mass flow calculation device is multiplied by the output value of the control device for adjusting the recondenser operating temperature to obtain the recondenser inlet LNG flow required to control the recondenser temperature at the set operating temperature. The flow controller controls the opening of the regulating valve according to the LNG mass flowmeter to control the LNG flow to maintain the required LNG flow at the recondenser operating temperature under the set value.

[0021] Furthermore, the recondenser includes: a tank body, a material thermometer in the recondenser, a material level meter in the recondenser, a feed liquid distributor, a gas-liquid distribution plate, and a filler. The feed liquid distributor, the gas-liquid distribution plate, and the filler are all installed in the tank body, the feed liquid distributor is located above the gas-liquid distribution plate, the gas-liquid distribution plate is located above the filler, and the material thermometer in the recondenser and the material level meter in the recondenser are both connected to the tank body.

[0022] The beneficial effect of adopting the above further technical solution is that the LNG entering from the side of the recondenser enters the gas-liquid distribution plate through the liquid distributor, and then enters the packing evenly after passing through the gas-liquid distribution plate, and the BOG entering the recondenser is condensed into liquid in the packing and then output. When the BOG is completely condensed in the packing, the liquid level of the recondenser will be maintained within the packing height range.

[0023] Further, a BOG discharge pipeline is connected to the top of the recondenser, and the BOG discharge pipeline is connected to a pressure safety valve, a discharge pressure gauge, a discharge pressure regulating valve and a discharge pressure controller. The pressure safety valve is installed on the BOG discharge pipeline, the discharge pressure gauge and the discharge pressure regulating valve are both installed on the BOG discharge pipeline, and the discharge pressure controller is respectively connected to the discharge pressure gauge and the discharge pressure regulating valve.

[0024] The beneficial effect of adopting the above further technical solution is that when the top of the recondenser is over-pressured, the BOG discharge pipeline at the top of the recondenser is discharged to the BOG main pipe, thereby improving the stability and reliability of the system. The setting of the discharge pressure controller facilitates automatic analysis and control, thereby improving automation.

[0025] Furthermore, there are two recondenser bottom pressure regulating valves, the first recondenser bottom pressure regulating valve is installed on the low-pressure output main pipe, the second recondenser bottom pressure regulating valve is connected in parallel with the first recondenser bottom pressure regulating valve, the first recondenser bottom pressure regulating valve and the second recondenser bottom pressure regulating valve are respectively connected to the recondenser bottom pressure controller, and valves are respectively provided between one end of the LNG pipeline upstream of the recondenser and the low-pressure output main pipe, between the low-pressure output main pipe and the low-pressure pump, and between the recondenser and the low-pressure output main pipe.

[0026] The beneficial effect of adopting the above further technical solution is: the setting of multiple valves and regulating valves makes it convenient for users to make corresponding adjustments according to actual needs, facilitates the system to adapt to different application scenarios, improves control flexibility, and improves user experience.

[0027] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic flowchart of a method for automatically controlling the liquid level of a recondenser at a liquefied natural gas receiving station provided in an embodiment of the present invention.

[0029] Figure 2 A schematic diagram of the structure of an automatic control system for liquid level of a recondenser at a liquefied natural gas receiving station provided by an embodiment of the present invention.

[0030] Explanation of the accompanying drawings: 10. Low-pressure output main pipe; 101. Low-pressure pump; 102. High-pressure pump; 11. Pressure gauge at the bottom of recondenser; 12. Pressure controller at the bottom of recondenser; 13. Pressure regulating valve at the bottom of recondenser; 20. Tank; 21. Material thermometer in recondenser; 22. Material level gauge in recondenser; 23. Packing; 30. BOG inlet pipeline of recondenser; 31. BOG pressure gauge; 32. BOG volume flowmeter; 33. BOG thermometer; 34. BOG mass flow calculation device; 35. Recondenser temperature control device; 36. LNG flow calculation device at the inlet of recondenser; 40. BOG discharge pipeline; 50. LNG pipeline upstream of recondenser; 51. LNG thermometer; 52. LNG mass flowmeter; 53. LNG regulating valve; 531. Flow controller. DETAILED DESCRIPTION

[0031] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0032] like Figure 1 As shown, an embodiment of the present invention provides a method for automatically controlling the liquid level of a recondenser at a liquefied natural gas receiving station, comprising: S1, maintaining the bottom pressure of the recondenser stable, and obtaining the bottom pressure of the recondenser; S2, calculating the set operating temperature of the recondenser according to the bottom pressure of the recondenser; S3, obtaining multiple parameter values ​​and BOG mass flow rate that affect the liquid-to-gas ratio R value for controlling the recondenser to operate at the set operating temperature; S4, calculating the liquid-to-gas ratio R value required at the set operating temperature of the recondenser according to the set operating temperature of the recondenser and the multiple parameter values; S5, calculating the LNG mass flow rate required for recondenser feed according to the liquid-to-gas ratio R value required at the set operating temperature of the recondenser and the BOG mass flow rate; S6, feeding according to the LNG mass flow rate required for the recondenser feed.

[0033] The beneficial effects of the technical solution of the present invention are: by controlling the operating temperature of the recondenser, the operating temperature of the recondenser is controlled below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the recondenser liquid level to be stable. The recondenser liquid level is controlled by the automatic control method of the recondenser liquid level, the recondenser liquid level is less affected by the upstream and downstream pressure fluctuations, the recondenser operating temperature is stable and controllable, there are fewer field instruments involved in the automatic adjustment, the calculation program is safe and stable, the parameters involved in the adjustment are few and stable, and the stable automatic control of the recondenser liquid level of the LNG receiving station is realized. The operator does not need to perform fine operations when performing recondenser-related operations, the operator's recondenser operation skills are reduced, and there is no need to monitor the operating status of the receiving station recondenser at all times. Compared with manual adjustment by the operator, the automatic adjustment of the liquid level is more stable, there will be no adjustment errors, and the liquid level is automatically adjusted accurately when the parameters related to the recondenser liquid level control change. The recondenser operation is safer and more stable, reducing the risk of parking due to recondenser liquid level fluctuations, and reducing the intensity of the operator's monitoring of the recondenser operation status. There is no need to wait for the upstream and downstream pressures to completely stabilize before proceeding to the next step, which improves the production adjustment speed and emergency response speed of the receiving station and enhances the safety and stability of the production operation of the receiving station.

[0034] The embodiment of the present invention provides a method for automatic liquid level control of a recondenser at a liquefied natural gas receiving station, so as to solve the problems in the prior art that the liquid level of the recondenser is greatly affected by the upstream and downstream pressure fluctuations, the stability of the participating control parameters is weak, and the operating temperature of the recondenser fluctuates greatly.

[0035] The method controls the operating temperature of the recondenser to be below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the liquid level of the recondenser to be stable.

[0036] A method for automatically controlling the liquid level of a recondenser at a liquefied natural gas receiving station:

[0037] (1) Maintain the bottom pressure of the recondenser stable by regulating the valve, determine the material bubble point temperature corresponding to the top pressure of the recondenser at this time, and add an appropriate safety margin to obtain the set operating temperature of the recondenser;

[0038] Among them, the liquid level of the recondenser is stable at a certain level, and the difference between the top pressure and the bottom pressure is equal to the static pressure of the liquid, so what is controlled is the bottom pressure. After the bottom pressure is determined, the top pressure is also basically determined.

[0039] (2) Determine the parameters that affect the liquid-gas ratio R value of the adjustable recondenser operating temperature (the liquid-gas ratio R value required at the set operating temperature of the recondenser), including: BOG temperature, LNG temperature, etc.;

[0040] (3) Quantify the above parameters that affect the liquid-gas ratio R value of the adjustable recondenser operating temperature (the liquid-gas ratio R value required at the set operating temperature of the recondenser), and calculate the liquid-gas ratio R value required at the set operating temperature of the recondenser. The R value calculation formula is as follows:

[0041]

[0042] Where:

[0043] R—ratio of LNG feed mass flow rate to BOG feed mass flow rate of the recondenser, i.e., the liquid-gas ratio R value required at the set operating temperature of the recondenser, 1;

[0044] t1—recondenser feed BOG temperature, °C;

[0045] t2—LNG feed temperature to recondenser, °C;

[0046] t—recondenser set operating temperature, °C;

[0047] r—LNG vaporization latent heat related parameters, kJ / kg;

[0048] c1—BOG specific heat capacity, kJ / kg / ℃;

[0049] c2—LNG specific heat capacity, kJ / kg / ℃;

[0050] α—Error correction coefficient of LNG feed mass flow rate and BOG feed mass flow rate, 1.

[0051] The liquid-gas ratio R value (ratio of recondenser LNG feed mass flow rate to BOG feed mass flow rate) is multiplied by the BOG mass flow rate to obtain the LNG mass flow rate required for the recondenser feed. The recondenser operating temperature can be controlled at the set temperature according to the LNG mass flow rate, ensuring that the BOG entering the recondenser is completely condensed, thereby maintaining the recondenser liquid level within the normal range.

[0052] By using the automatic control method for recondenser liquid level (automatic control method for recondenser liquid level at liquefied natural gas receiving station) of the present invention to control the recondenser liquid level, the recondenser liquid level is less affected by upstream and downstream pressure fluctuations, the operating temperature of the recondenser is stable, the number of parameters involved in regulation is small and the stability is strong, thereby achieving stable automatic control of the recondenser liquid level at the LNG receiving station.

[0053] Under the automatic control method of the recondenser liquid level, the recondenser can realize automatic liquid level control under the working conditions of normal gasification and external transmission, external transmission volume adjustment, tank truck loading, compressor load adjustment, compressor start and shutdown, ship loading and unloading, storage tank switching, etc. The operator does not need to perform delicate operations when performing recondenser related operations, and the requirements for the operator's recondenser operating skills are reduced. There is no need to monitor the operating status of the recondenser at the receiving station all the time, which reduces the operator's labor intensity.

[0054] Further, it includes: the multiple parameter values ​​include: recondenser feed BOG temperature, recondenser feed LNG temperature, recondenser set operating temperature, LNG vaporization latent heat related parameters, BOG specific heat capacity, LNG specific heat capacity, LNG feed mass flow rate and BOG feed mass flow rate error correction coefficient.

[0055] The beneficial effects of adopting the above further technical solution are: fewer on-site instruments involved in automatic adjustment, safer and more stable calculation procedures, fewer parameters involved in adjustment and stronger stability, and stable automatic control of the recondenser liquid level of the LNG receiving station. There is no need to wait for the upstream and downstream pressures to be completely stable before proceeding to the next step, which improves the production adjustment speed of the receiving station and the emergency response speed, and enhances the safety and stability of the production operation of the receiving station.

[0056] Further, in step S4, the liquid-to-gas ratio R value required at the set operating temperature of the recondenser is calculated by the following formula: Wherein, R is the ratio of the recondenser LNG feed mass flow rate to the BOG feed mass flow rate, in units of 1; t1 is the recondenser feed BOG temperature, in units of °C; t2 is the recondenser feed LNG temperature, in units of °C; t is the set operating temperature of the recondenser, in units of °C; r is a parameter related to the latent heat of LNG vaporization, in units of kJ / kg; c1 is the BOG specific heat capacity, in units of kJ / kg / °C; c2 is the LNG specific heat capacity, in units of kJ / kg / °C; α is the error correction coefficient between the LNG feed mass flow rate and the BOG feed mass flow rate, in units of 1.

[0057] The beneficial effects of adopting the above further technical solution are: the liquid-gas ratio R value required at the set operating temperature of the recondenser is calculated through data and formulas to improve accuracy and reliability. There are fewer on-site instruments involved in automatic adjustment, the calculation program is safe and stable, and the parameters involved in adjustment are few and stable, so as to achieve stable automatic control of the liquid level of the recondenser of the LNG receiving station. There is no need to wait for the upstream and downstream pressures to be completely stable before proceeding to the next step, which improves the production adjustment speed of the receiving station and the emergency response speed, and enhances the safety and stability of the production operation of the receiving station.

[0058] Furthermore, in step S5, the liquid-to-gas ratio R value required at the set operating temperature of the recondenser is multiplied by the BOG mass flow rate to calculate the LNG mass flow rate required for feeding the recondenser.

[0059] The beneficial effect of adopting the above further technical solution is: the liquid-gas ratio R value is multiplied by the BOG mass flow rate to obtain the LNG mass flow rate required for feeding the recondenser, and the feeding is carried out according to the LNG mass flow rate, so that the operating temperature of the recondenser can be controlled at the set temperature, ensuring that the BOG entering the recondenser is completely condensed, thereby maintaining the recondenser liquid level within a normal range.

[0060] Further, step S2 includes: S21, calculating the top pressure of the recondenser according to the bottom pressure of the recondenser; S22, determining the safety margin and the material bubble point temperature corresponding to the top pressure of the recondenser according to the top pressure of the recondenser; S23, calculating the set operating temperature of the recondenser according to the safety margin and the material bubble point temperature.

[0061] The beneficial effect of adopting the above further technical solution is: by controlling the operating temperature of the recondenser, the operating temperature of the recondenser is controlled below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the liquid level of the recondenser to be stable.

[0062] Furthermore, in step S23, the material bubble point temperature is added to the safety margin to calculate the set operating temperature of the recondenser.

[0063] The beneficial effect of adopting the above further technical solution is: by controlling the operating temperature of the recondenser, the operating temperature of the recondenser is controlled below the material bubble point temperature corresponding to the pressure at the top of the recondenser, so that the BOG entering the recondenser is completely condensed, thereby controlling the liquid level of the recondenser to be stable.

[0064] An automatic control method for the liquid level of a recondenser at a liquefied natural gas receiving station provided in an embodiment of the present invention first maintains the bottom pressure of the recondenser stable, and determines the required set operating temperature of the recondenser according to the top pressure of the recondenser; confirms 7 factors that affect the liquid-gas ratio R value (the liquid-gas ratio R value required at the set operating temperature of the recondenser) for controlling the recondenser to operate at the set temperature, which are the following 7 parameters: recondenser feed BOG temperature, recondenser feed LNG temperature, recondenser set operating temperature, LNG vaporization latent heat related parameters, BOG specific heat capacity, LNG specific heat capacity, LNG feed mass flow rate and the BOG feed mass flow error correction coefficient, where the recondenser feed BOG temperature and the recondenser feed LNG temperature are derived from the on-site real-time instrument measurement values, the recondenser set operating temperature is determined according to the recondenser operating pressure, and the LNG vaporization latent heat related parameters, BOG specific heat capacity, LNG specific heat capacity, LNG feed mass flow rate and the BOG feed mass flow error correction coefficient are the recondenser system characteristic parameters; then, after calculation, their influence on the liquid-gas ratio R value required for the recondenser to operate at the set temperature (the liquid-gas ratio R value required at the recondenser set operating temperature) is quantified, and then calculated by the following formula:

[0065]

[0066] Where:

[0067] R—ratio of LNG feed mass flow rate to BOG feed mass flow rate of the recondenser, unit is 1;

[0068] t1—recondenser feed BOG temperature, in °C;

[0069] t2—LNG feed temperature of recondenser, in °C;

[0070] t—recondenser set operating temperature, in °C;

[0071] r—LNG vaporization latent heat related parameters, unit is kJ / kg;

[0072] c1—BOG specific heat capacity, in kJ / kg / ℃;

[0073] c2—LNG specific heat capacity, in kJ / kg / ℃;

[0074] α—Error correction coefficient of LNG feed mass flow rate and BOG feed mass flow rate, unit is 1.

[0075] Taking a certain LNG receiving station as an example, the bottom pressure of the recondenser is 0.74MPa, the temperature is set to -132℃, the relevant parameters of LNG vaporization latent heat are 371kJ / kg, the specific heat capacity of BOG is 2.217kJ / kg / ℃, the specific heat capacity of LNG is 3.28kJ / kg / ℃, the error correction coefficient of LNG feed mass flow rate and BOG feed mass flow rate is 0.9876, and the on-site instrument shows that the recondenser feed BOG temperature is 18.76℃, and the recondenser feed LNG temperature is -157.98℃. The recondenser liquid-gas ratio R value (the liquid-gas ratio R value required at the set operating temperature of the recondenser) is calculated by substituting it into the formula: R=(2.217*(18.76-(-132))+371) / (3.28*(-132-(-157.98)))*0.9876=8.17.

[0076] At this time, the BOG mass flow rate entering the recondenser is 13.35t / h, and the current LNG mass flow rate required at the recondenser inlet is 109t / h. The recondenser inlet flow regulating valve automatically adjusts the recondenser inlet LNG flow rate to the required LNG flow rate, and the recondenser operating temperature will be stabilized at -132℃. At this temperature and pressure, the BOG entering the recondenser will be completely condensed to liquid in the recondenser packing, and the recondenser liquid level will be stabilized within the packing height range.

[0077] like Figure 2As shown, in addition, the present invention also provides a liquefied natural gas receiving station recondenser liquid level automatic control system, which is used to implement any one of the above-mentioned liquefied natural gas receiving station recondenser liquid level automatic control methods, and the liquefied natural gas receiving station recondenser liquid level automatic control system includes: a recondenser, a low-pressure output main pipe 10, a recondenser bottom pressure gauge 11, a recondenser bottom pressure controller 12, a recondenser bottom pressure regulating valve 13, a low-pressure pump 101, a high-pressure pump 102, a recondenser BOG inlet pipeline 30, a BOG pressure gauge 31, a BOG volume flowmeter 32, a BOG thermometer 33, a BOG mass flow calculation device 34, a recondenser temperature control device 35, a recondenser The condenser inlet LNG flow calculation device 36, the recondenser upstream LNG pipeline 50, the LNG thermometer 51, the LNG mass flowmeter 52, the LNG regulating valve 53, and the flow controller 531, the bottom of the recondenser and one end of the recondenser upstream LNG pipeline 50 are connected to the low-pressure output main pipe 10, and the two ends of the low-pressure output main pipe 10 are connected to the low-pressure pump 101 and the high-pressure pump 102 in a one-to-one correspondence. The recondenser bottom pressure gauge 11 and the recondenser bottom pressure regulating valve 13 are both installed on the low-pressure output main pipe 10, and the recondenser bottom pressure controller 12 is respectively connected to the recondenser bottom pressure gauge 11 and the recondenser bottom pressure regulating valve 13. The bottom pressure regulating valve 13 of the condenser is connected; the recondenser BOG inlet pipeline 30 is connected to the top of the recondenser, the BOG pressure gauge 31, the BOG volume flow meter 32 and the BOG thermometer 33 are all installed on the recondenser BOG inlet pipeline 30, the BOG mass flow calculation device 34 is respectively connected to the BOG pressure gauge 31, the BOG volume flow meter 32, the BOG thermometer 33, and the recondenser inlet LNG flow calculation device 36, the recondenser inlet LNG flow calculation device 36 is respectively connected to the flow controller 531 and the recondenser temperature control device 35, the recondenser temperature control The device 35 is connected to the LNG thermometer 51 and the BOG thermometer 33 respectively, the other end of the recondenser upstream LNG pipeline 50 is connected to the top of the recondenser, the LNG thermometer 51, the LNG mass flowmeter 52 and the LNG regulating valve 53 are all installed on the recondenser upstream LNG pipeline 50, and the flow controller 531 is connected to the LNG mass flowmeter 52 and the LNG regulating valve 53 respectively; the recondenser bottom pressure controller 12 and the recondenser bottom pressure regulating valve 13 are used to maintain the recondenser bottom pressure stable; the recondenser bottom pressure gauge 11 is used to obtain the recondenser bottom pressure;The BOG mass flow calculation device 34 is used to calculate the BOG mass flow according to the values ​​obtained by the BOG pressure gauge 31, the BOG volume flow meter 32 and the BOG thermometer 33; the recondenser temperature control device 35 is used to calculate the liquid-gas ratio R value required at the set operating temperature of the recondenser according to the values ​​obtained by the BOG thermometer 33 and the LNG thermometer 51; the recondenser inlet LNG flow calculation device 36 is used to multiply the BOG mass flow output by the BOG mass flow calculation device 34 by the liquid-gas ratio R value required at the set operating temperature of the recondenser output by the recondenser temperature control device 35 to calculate the LNG mass flow required for the recondenser feed; the flow controller 531 is used to control the opening of the LNG regulating valve 53 according to the LNG mass flow meter 52.;

[0078] The beneficial effects of the technical solution of the present invention are: first, the pressure at the bottom of the recondenser is maintained stable by the recondenser bottom pressure regulating valve, and then the recondenser temperature is controlled below the material bubble point temperature corresponding to the recondenser pressure by the recondenser temperature control device, ensuring that the BOG entering the recondenser is completely condensed to a liquid state in the packing, thereby controlling the stability of the recondenser liquid level. The pressure at the bottom of the recondenser is maintained stable by regulating the recondenser bottom regulating valve by the pressure controller. The liquid-gas ratio R value required for the recondenser to set the operating temperature is calculated based on the BOG temperature and the LNG temperature. In the recondenser inlet LNG flow calculation device, the output value of the BOG mass flow calculation device is multiplied by the output value of the control device for adjusting the recondenser operating temperature to obtain the recondenser inlet LNG flow required to control the recondenser temperature at the set operating temperature. The flow controller controls the opening of the regulating valve according to the LNG mass flowmeter to control the LNG flow to maintain the required LNG flow at the recondenser operating temperature under the set value.

[0079] Through theoretical research on the packed recondenser of the receiving station (liquefied natural gas receiving station), it is proved that within the design operating range, when the BOG entering the recondenser is completely condensed, the recondenser liquid level will be stable within the packing height range.

[0080] According to this theory, the present invention firstly maintains the pressure at the bottom of the recondenser stable through the recondenser bottom pressure regulating valve, and then controls the recondenser temperature below the material bubble point temperature corresponding to the recondenser pressure through the recondenser temperature control device of the present invention, to ensure that the BOG entering the recondenser is completely condensed to a liquid state in the packing, thereby controlling the liquid level of the recondenser to be stable.

[0081] The liquefied natural gas receiving station recondenser liquid level automatic control system provided by the embodiment of the present invention has a stable recondenser liquid level, is less affected by upstream and downstream pressure fluctuations, can withstand recondenser bottom pressure fluctuations exceeding 140kPa, and the recondenser temperature is stable and controllable, the temperature control error is within 1°C, and the field instruments involved in the temperature control calculation are reduced to 2, and the automatic control is safe and stable. When the recondenser liquid level is automatically controlled, the recondenser liquid level and temperature are stable, the anti-interference ability is strong, the external transmission speed is improved, and the number of parking times is reduced. All indicators are better than similar technologies.

[0082] Furthermore, the recondenser includes: a tank body 20, a material thermometer 21 in the recondenser, a material level meter 22 in the recondenser, a feed liquid distributor, a gas-liquid distribution plate, and a filler 23. The feed liquid distributor, the gas-liquid distribution plate, and the filler 23 are all installed in the tank body 20, the feed liquid distributor is located above the gas-liquid distribution plate, the gas-liquid distribution plate is located above the filler 23, and the material thermometer 21 in the recondenser and the material level meter 22 in the recondenser are both connected to the tank body 20.

[0083] The beneficial effect of adopting the above further technical solution is that the LNG entering from the side of the recondenser enters the gas-liquid distribution plate through the liquid distributor, and then enters the packing evenly after passing through the gas-liquid distribution plate, and the BOG entering the recondenser is condensed into liquid in the packing and then output. When the BOG is completely condensed in the packing, the liquid level of the recondenser will be maintained within the packing height range.

[0084] Furthermore, a BOG discharge pipeline 40 is connected to the top of the recondenser, and a pressure safety valve, a discharge pressure gauge, a discharge pressure regulating valve and a discharge pressure controller are connected to the BOG discharge pipeline 40. The pressure safety valve is installed on the BOG discharge pipeline 40, and the discharge pressure gauge and the discharge pressure regulating valve are both installed on the BOG discharge pipeline 40. The discharge pressure controller is connected to the discharge pressure gauge and the discharge pressure regulating valve, respectively.

[0085] The beneficial effect of adopting the above further technical solution is that when the top of the recondenser is over-pressured, the BOG discharge pipeline at the top of the recondenser is discharged to the BOG main pipe, thereby improving the stability and reliability of the system. The setting of the discharge pressure controller facilitates automatic analysis and control, thereby improving automation.

[0086] Figure 2 In the embodiment, the PSV (Pressure Safety Valve) on the BOG discharge pipeline 40 is a pressure safety valve. Figure 2 The solid arrows in the figure represent the flow direction and trajectory of LNG and BOG, and the dotted arrows represent the signal transmission direction and trajectory.

[0087] Furthermore, there are two recondenser bottom pressure regulating valves 13, the first recondenser bottom pressure regulating valve is installed on the low-pressure output main pipe 10, the second recondenser bottom pressure regulating valve is connected in parallel with the first recondenser bottom pressure regulating valve, the first recondenser bottom pressure regulating valve and the second recondenser bottom pressure regulating valve are respectively connected to the recondenser bottom pressure controller 12, and valves are respectively provided between one end of the recondenser upstream LNG pipeline 50 and the low-pressure output main pipe 10, between the low-pressure output main pipe 10 and the low-pressure pump 101, and between the recondenser and the low-pressure output main pipe 10.

[0088] The beneficial effect of adopting the above further technical solution is: the setting of multiple valves and regulating valves makes it convenient for users to make corresponding adjustments according to actual needs, facilitates the system to adapt to different application scenarios, improves control flexibility, and improves user experience.

[0089] The recondenser liquid level automatic control system (liquefied natural gas receiving station recondenser liquid level automatic control system) includes a low-pressure output main pipe 10, a recondenser bottom pressure gauge 11, a recondenser bottom pressure controller 12, and a recondenser bottom pressure regulating valve 13. The LNG from the upstream low-pressure pump 101 (the low-pressure pump 101 is connected to the LNG) and the LNG from the recondenser bottom outlet pipeline merge into LNG and go to the high-pressure pump 102. The recondenser bottom pressure (measured by the recondenser bottom pressure gauge) is maintained stable by adjusting the recondenser bottom regulating valve 13 through the recondenser bottom pressure controller 12.

[0090] The recondenser includes a tank body 20, a material temperature meter 21 in the recondenser, a material level meter 22 in the recondenser, a feed liquid distributor, a gas-liquid distribution plate, and a packing 23. The LNG entering from the side of the recondenser passes through the liquid distributor and enters the gas-liquid distribution plate, and then evenly enters the packing 23 after passing through the gas-liquid distribution plate. The BOG entering the recondenser is condensed to liquid in the packing 23 and then output. When the BOG is completely condensed in the packing, the liquid level of the recondenser will be maintained within the packing height range.

[0091] The compressed BOG enters the recondenser from the top through the recondenser BOG inlet pipeline 30. The pipeline (recondenser BOG inlet pipeline 30) is provided with a BOG pressure gauge 31, a BOG volume flow meter 32, and a BOG thermometer 33. The BOG mass flow calculation device 34 calculates the BOG mass flow according to the BOG pressure gauge 31, the BOG volume flow meter 32, and the BOG thermometer 33. The present invention designs a new recondenser temperature control device 35. The liquid-gas ratio R value required for the recondenser set operating temperature (the liquid-gas ratio R value required at the recondenser set operating temperature) is calculated according to the BOG temperature (measured by the BOG thermometer) and the LNG temperature (measured by the LNG thermometer) provided by the present invention. In the recondenser inlet LNG flow calculation device 36, the output value of the BOG mass flow calculation device 34 is multiplied by the output value of the recondenser temperature control device 35 for adjusting the recondenser operating temperature to obtain the recondenser inlet LNG flow required to control the recondenser temperature at the set operating temperature.

[0092] When the top of the recondenser is over-pressured, the BOG is discharged to the BOG main pipe through the BOG discharge line 40 at the top of the recondenser.

[0093] An LNG thermometer 51, an LNG mass flowmeter 52, and an LNG regulating valve 53 are provided on the LNG pipeline 50 upstream of the recondenser. The flow controller 531 controls the opening of the LNG regulating valve 53 according to the LNG mass flowmeter 52, and controls the LNG flow to maintain the required LNG flow at the recondenser operating temperature (measured by the material thermometer 21 in the recondenser) at the set value.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for automatically controlling the liquid level of a recondenser at a liquefied natural gas receiving station, characterized in that: include: S1. Maintain the bottom pressure of the recondenser stable and obtain the bottom pressure of the recondenser; S2. Calculating a set operating temperature of the recondenser according to the bottom pressure of the recondenser; S3, obtaining multiple parameter values ​​and BOG mass flow rate that affect the liquid-gas ratio R value of controlling the recondenser to operate at a set operating temperature; S4, calculating the liquid-to-gas ratio R value required at the set operating temperature of the recondenser according to the set operating temperature of the recondenser and the plurality of parameter values; S5. Calculate the LNG mass flow rate required for the recondenser feed according to the liquid-to-gas ratio R value required at the set operating temperature of the recondenser and the BOG mass flow rate; S6. Feeding according to the LNG mass flow rate required for feeding the recondenser.

2. A method for automatic liquid level control of a recondenser at a liquefied natural gas receiving station according to claim 1, characterized in that: include: The plurality of parameter values ​​include: recondenser feed BOG temperature, recondenser feed LNG temperature, recondenser set operating temperature, LNG vaporization latent heat related parameters, BOG specific heat capacity, LNG specific heat capacity, LNG feed mass flow rate and BOG feed mass flow rate error correction coefficient.

3. The method for automatic liquid level control of a recondenser at a liquefied natural gas receiving station according to claim 1, characterized in that: In step S4, the liquid-to-gas ratio R value required at the set operating temperature of the recondenser is calculated by the following formula: Wherein, R is the ratio of the recondenser LNG feed mass flow rate to the BOG feed mass flow rate, in units of 1; t1 is the recondenser feed BOG temperature, in units of °C; t2 is the recondenser feed LNG temperature, in units of °C; t is the set operating temperature of the recondenser, in units of °C; r is a parameter related to the latent heat of LNG vaporization, in units of kJ / kg; c1 is the BOG specific heat capacity, in units of kJ / kg / °C; c2 is the LNG specific heat capacity, in units of kJ / kg / °C; α is the error correction coefficient between the LNG feed mass flow rate and the BOG feed mass flow rate, in units of 1.

4. The method for automatic liquid level control of a recondenser at a liquefied natural gas receiving station according to claim 1, characterized in that: In step S5, the liquid-to-gas ratio R value required at the set operating temperature of the recondenser is multiplied by the BOG mass flow rate to calculate the LNG mass flow rate required for feeding the recondenser.

5. The method for automatic liquid level control of a recondenser at a liquefied natural gas receiving station according to claim 1, characterized in that: Step S2 includes: S21, calculating the recondenser top pressure according to the recondenser bottom pressure; S22, determining a safety margin and a material bubble point temperature corresponding to the recondenser top pressure according to the recondenser top pressure; S23. Calculate a set operating temperature of the recondenser according to the safety margin and the material bubble point temperature.

6. A method for automatic liquid level control of a recondenser at a liquefied natural gas receiving station according to claim 5, characterized in that: In step S23, the material bubble point temperature is added to the safety margin to calculate the set operating temperature of the recondenser.

7. A liquefied natural gas receiving station recondenser liquid level automatic control system, characterized in that: A method for automatically controlling the liquid level of a recondenser at a liquefied natural gas receiving station according to any one of claims 1 to 6, the automatic liquid level control system for the recondenser at a liquefied natural gas receiving station comprises: a recondenser, a low-pressure output main pipe (10), a pressure gauge at the bottom of the recondenser (11), a pressure controller at the bottom of the recondenser (12), a pressure regulating valve at the bottom of the recondenser (13), a low-pressure pump (101), a high-pressure pump (102), a BOG inlet pipeline (30) of the recondenser, a BOG pressure gauge (31), a BOG volume flow meter (32), a BOG thermometer (33), a BOG mass flow calculation device (34), a recondenser temperature control device (35), a recondenser inlet An LNG flow calculation device (36), an LNG pipeline (50) upstream of a recondenser, an LNG thermometer (51), an LNG mass flowmeter (52), an LNG regulating valve (53), and a flow controller (531); the bottom of the recondenser and one end of the LNG pipeline (50) upstream of the recondenser are connected to the low-pressure output main pipe (10); the two ends of the low-pressure output main pipe (10) are connected to the low-pressure pump (101) and the high-pressure pump (102) in a one-to-one correspondence; the pressure gauge (11) at the bottom of the recondenser and the pressure regulating valve (13) at the bottom of the recondenser are both installed on the low-pressure output main pipe (10); the pressure controller (13) at the bottom of the recondenser 2) are respectively connected to the recondenser bottom pressure gauge (11) and the recondenser bottom pressure regulating valve (13); the recondenser BOG inlet pipeline (30) is connected to the top of the recondenser, the BOG pressure gauge (31), the BOG volume flow meter (32) and the BOG thermometer (33) are all installed on the recondenser BOG inlet pipeline (30), the BOG mass flow calculation device (34) is respectively connected to the BOG pressure gauge (31), the BOG volume flow meter (32), the BOG thermometer (33) and the recondenser inlet LNG flow calculation device (36), the recondenser inlet LNG flow meter The calculation device (36) is respectively connected to the flow controller (531) and the recondenser temperature control device (35), the recondenser temperature control device (35) is respectively connected to the LNG thermometer (51) and the BOG thermometer (33), the other end of the recondenser upstream LNG pipeline (50) is connected to the top of the recondenser, the LNG thermometer (51), the LNG mass flowmeter (52) and the LNG regulating valve (53) are all installed on the recondenser upstream LNG pipeline (50), and the flow controller (531) is respectively connected to the LNG mass flowmeter (52) and the LNG regulating valve (53); The recondenser bottom pressure controller (12) and the recondenser bottom pressure regulating valve (13) are used to maintain the recondenser bottom pressure stable; The recondenser bottom pressure gauge (11) is used to obtain the recondenser bottom pressure; The BOG mass flow calculation device (34) is used to calculate the BOG mass flow according to the values ​​obtained by the BOG pressure gauge (31), the BOG volume flow meter (32) and the BOG thermometer (33); The recondenser temperature control device (35) is used to adjust the temperature of the recondenser according to the BOG thermometer (33). and the value obtained by the LNG thermometer (51), calculating the liquid-to-gas ratio R value required at the set operating temperature of the recondenser; The recondenser inlet LNG flow calculation device (36) is used to compare the BOG mass flow output by the BOG mass flow calculation device (34) with the recondenser temperature control device (35). The output is multiplied by the liquid-gas ratio R value required at the set operating temperature of the recondenser to calculate the LNG mass flow rate required for the recondenser feed; The flow controller (531) is used to control the opening of the LNG regulating valve (53) according to the LNG mass flow meter (52).

8. The automatic control system for liquid level of a recondenser of a liquefied natural gas receiving station according to claim 7, characterized in that: The recondenser comprises: a tank body (20), a material temperature meter (21) in the recondenser, a material level meter (22) in the recondenser, a feed liquid distributor, a gas-liquid distribution plate, and a filler (23); the feed liquid distributor, the gas-liquid distribution plate, and the filler (23) are all installed in the tank body (20); the feed liquid distributor is located above the gas-liquid distribution plate; the gas-liquid distribution plate is located above the filler (23); the material temperature meter (21) in the recondenser and the material level meter (22) in the recondenser are both connected to the tank body (20).

9. The automatic control system for liquid level of a recondenser of a liquefied natural gas receiving station according to claim 7, characterized in that: A BOG discharge pipeline (40) is connected to the top of the recondenser, and the BOG discharge pipeline (40) is connected to a pressure safety valve, a discharge pressure gauge, a discharge pressure regulating valve and a discharge pressure controller. The pressure safety valve is installed on the BOG discharge pipeline (40), the discharge pressure gauge and the discharge pressure regulating valve are both installed on the BOG discharge pipeline (40), and the discharge pressure controller is connected to the discharge pressure gauge and the discharge pressure regulating valve, respectively.

10. The automatic control system for liquid level of a recondenser of a liquefied natural gas receiving station according to claim 7, characterized in that: There are two recondenser bottom pressure regulating valves (13), the first recondenser bottom pressure regulating valve is installed on the low-pressure output main pipe (10), the second recondenser bottom pressure regulating valve is connected in parallel with the first recondenser bottom pressure regulating valve, the first recondenser bottom pressure regulating valve and the second recondenser bottom pressure regulating valve are respectively connected to the recondenser bottom pressure controller (12), and valves are respectively provided between one end of the recondenser upstream LNG pipeline (50) and the low-pressure output main pipe (10), between the low-pressure output main pipe (10) and the low-pressure pump (101), and between the recondenser and the low-pressure output main pipe (10).

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