LNG storage tank pressure control system and method based on fuzzy control algorithm

Abstract

The invention relates to the technical field of LNG filling, and discloses an LNG storage tank pressure control system and method based on a fuzzy control algorithm, the system comprises an LNG storage tank, an LNG pump pool and an LNG immersed pump, the LNG immersed pump is arranged in the LNG pump pool, the LNG storage tank is connected with the LNG immersed pump, a liquid level sensor and pressure sensors are arranged on the LNG storage tank, and the pressure sensors are arranged at an inlet and an outlet of the LNG pump pool. The problem that in the prior art, the unloading efficiency is difficult to improve under the condition that liquid adding is not affected is solved.

Description

Technical Field

[0001] The invention relates to the technical field of LNG filling, and in particular to an LNG storage tank pressure control system and method based on a fuzzy control algorithm. Background Art

[0002] LNG filling stations mainly consist of LNG storage tanks, LNG submersible pumps, LNG pump pools, LNG filling machines and control systems, which are used to pressurize liquefied natural gas and provide safe and efficient filling services to natural gas vehicles.

[0003] Due to the large filling volume, the liquid level in the LNG tanks of the central and western regions of the LNG filling stations dropped rapidly, the gas phase space in the tanks increased, and the pressure dropped accordingly. When the LNG tank pressure was too low, the inlet pressure of the LNG submersible pump was also low, resulting in the filling pressure not being able to meet the filling requirements. At the same time, the LNG tanks, pump pools, and pipelines absorbed heat during operation, and the LNG temperature in the tanks gradually increased. The cavitation margin of the device was insufficient, and the system had a cavitation alarm interlock, affecting the normal operation of the station.

[0004] In the existing technology, there is a function to adjust the saturation of the storage tank, but the system pressure and temperature control are rough, and some equipment will affect the filling of vehicles when pressurizing, so it is rarely used during the operation of LNG filling stations.

[0005] Another example is a net positive pressure head detection and control method and system for an LNG submersible pump with patent number 202211421268.3 and application date November 14, 2022. A submersible pump is required for control, which takes more than 30 minutes. The control time is long and the LNG submersible pump will be occupied, resulting in the system being unable to operate normally during control. Summary of the invention

[0006] In order to overcome the deficiencies of the prior art, the present invention provides an LNG storage tank pressure control system and method based on a fuzzy control algorithm, which solves the problems existing in the prior art such as difficulty in improving the unloading efficiency without affecting the liquid filling.

[0007] The technical solution adopted by the present invention to solve the above problems is:

[0008] An LNG storage tank pressure control system based on fuzzy control algorithm includes an LNG storage tank, an LNG pump pool, and an LNG submersible pump. The LNG submersible pump is arranged in the LNG pump pool. The LNG storage tank and the LNG submersible pump are connected to each other. A liquid level sensor and a pressure sensor are arranged on the LNG storage tank. Pressure sensors are arranged at the inlet and outlet of the LNG pump pool.

[0009] A LNG tank pressure control method based on fuzzy control algorithm is adopted. The LNG tank pressure control system based on fuzzy control algorithm is adopted. The pressure control process is divided into three stages: filling stage, unloading preparation stage and unloading stage according to the operating conditions of the gas station. The fuzzy control algorithm is combined to realize the control of the LNG tank pressure.

[0010] As a preferred technical solution, during the filling stage, the optimal tank pressure is calculated by the control system to allow the LNG submersible pump to operate in the optimal working condition; during the unloading preparation stage, the pressure of the LNG tank is reduced to prepare for unloading; during the unloading stage, the optimal tank pressure is calculated by the control system.

[0011] As a preferred technical solution, when constructing a fuzzy control algorithm, the LNG tank pressure, liquid level and cavitation margin are used as input variables for LNG tank pressure control, and three fuzzy sets of low, medium and high are defined for each input variable.

[0012] As a preferred technical solution, when constructing a fuzzy control algorithm, the minimum working pressure of the LNG storage tank is calculated in the control system, and the pipeline friction resistance loss is calculated, and then the average value of the total resistance coefficient is calculated based on different existing sites to estimate the pipeline friction resistance loss applicable to various sites; the inlet pressure of the LNG submersible pump is calculated using the following formula: LNG submersible pump inlet pressure = filling pressure - LNG submersible pump rated boost pressure + pipeline friction resistance loss; the inlet pressure of the LNG submersible pump is the minimum working pressure of the LNG storage tank; wherein, the rated boost pressure of the LNG submersible pump is calculated by the liquid pressure formula, and the filling pressure is a modifiable pressure preset by the control system.

[0013] As a preferred technical solution, the liquid level of the LNG storage tank is divided into filling level, unloading preparation level and unloading level. The liquid level value ranges corresponding to the three stages of filling stage, unloading preparation stage and unloading stage are divided according to the different forms of LNG storage tanks and statistical empirical data.

[0014] As a preferred technical solution, the cavitation margin is calculated based on the LNG pump pool temperature. The calculation formula is:

[0015] Cavitation allowance = device cavitation allowance – required cavitation allowance;

[0016] The control system calculates the difference between consecutive data points of the cavitation margin through a differential algorithm to predict the rising, falling or flat trend of the cavitation margin data.

[0017] As a preferred technical solution, the calculation formula of the cavitation margin of the device is:

[0018] NPSHa=P+Ph-Pf–Ps;

[0019] Wherein, P is the gas phase pressure of the LNG storage tank, Ph is the static pressure generated by the liquid in the LNG storage tank, Pf is the friction resistance loss of the pipeline from the LNG storage tank to the pump pool inlet, Ps is the saturated vapor pressure of LNG in the LNG pump pool, and the required cavitation margin is calculated based on the real-time speed and flow rate of the LNG submersible pump combined with the characteristic curve provided by the pump manufacturer.

[0020] As a preferred technical solution, the fuzzy control rules of the fuzzy control algorithm are:

[0021] If the LNG tank level is at the filling level and the cavitation margin is high, and the cavitation margin trend is rising or flat, the control signal is to maintain the current pressure;

[0022] If the LNG tank level is at the filling level and the cavitation margin is high, and the cavitation margin trend is decreasing, the control signal is a slight pressurization;

[0023] If the LNG tank level is at the filling level and the cavitation margin is low, and the cavitation margin trend is rising, the control signal is a slight pressurization;

[0024] If the LNG tank level is at the filling level and the cavitation margin is low, and the cavitation margin trend is flat, the control signal is medium boost;

[0025] If the LNG tank level is at the filling level and the cavitation margin is low, and the cavitation margin trend is decreasing, the control signal is full pressure boost;

[0026] If the LNG tank level is at the filling level and the cavitation margin is negative, and the cavitation margin trend is rising, the control signal is full pressure boost;

[0027] If the LNG tank level is at the unloading preparation level and the cavitation margin is high, and the cavitation margin trend is rising or flat, the control signal is to reduce the pressure;

[0028] If the LNG tank level is at the unloading preparation level and the cavitation margin is high, and the cavitation margin trend is decreasing, the control signal is to maintain the current pressure;

[0029] If the LNG tank level is at the unloading preparation level and the cavitation margin is low, and the cavitation margin trend is rising, the control signal is to maintain the current pressure;

[0030] If the LNG tank liquid level is at the unloading preparation level and the cavitation margin is low, and the cavitation margin trend is gentle, the control signal is a slight pressurization;

[0031] If the LNG tank liquid level is at the unloading preparation level and the cavitation margin is low, and the cavitation margin trend is decreasing, the control signal is light to medium pressure increase;

[0032] If the LNG tank level is at the unloading preparation level and the cavitation margin is negative, and the cavitation margin trend is rising, the control signal is light to medium pressure increase;

[0033] If the LNG tank liquid level is at the unloading preparation level and the cavitation margin is negative, and the cavitation margin trend is gentle, the control signal is light to moderate pressurization;

[0034] If the LNG tank level is at the unloading preparation level and the cavitation margin is negative, and the cavitation margin trend is negative, the control signal is medium boost;

[0035] If the LNG tank level is at the unloading level and the cavitation margin is high, and the cavitation margin trend is rising or flat, the control signal is to maintain the current pressure;

[0036] If the LNG tank level is at the unloading level and the cavitation margin is high, and the cavitation margin trend is decreasing, the control signal is to maintain the current pressure;

[0037] If the LNG tank level is at the unloading level and the cavitation margin is low, and the cavitation margin trend is rising, the control signal is to maintain the current pressure;

[0038] If the LNG tank level is at the unloading level and the cavitation margin is low, and the cavitation margin trend is flat, the control signal is to maintain the current pressure;

[0039] If the LNG tank level is at the unloading level and the cavitation margin is low, and the cavitation margin trend is decreasing, the control signal is a slight pressurization;

[0040] If the LNG tank level is at the unloading level and the cavitation margin is negative, and the cavitation margin trend is rising, the control signal is a slight pressurization;

[0041] If the LNG tank level is at the unloading level and the cavitation margin is negative, and the cavitation margin trend is flat, the control signal is a slight pressurization;

[0042] If the LNG tank level is at the unloading level and the cavitation margin is negative, and the cavitation margin trend is decreasing, the control signal is light to moderate pressurization.

[0043] As a preferred technical solution, in the fuzzy control algorithm, the weighted average method is used to calculate the weighted average of the membership degree of each control signal output fuzzy set and the control value of each control signal output fuzzy set, convert the fuzzy control value into an accurate control value, and use the accurate control value as the target pressure of the LNG tank pressure.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention combines fuzzy control algorithm to realize efficient intelligent control of LNG storage tank pressure, reduces personnel operation, has no influence on vehicle filling during pressure control, and improves the operation efficiency of LNG filling station. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a structural diagram of a LNG tank pressure control system based on fuzzy control algorithm;

[0047] Figure 2 for Figure 1 One of the partial enlarged pictures;

[0048] Figure 3 for Figure 1 The second partial enlarged picture;

[0049] Figure 4 for Figure 1 The third partial enlarged image. DETAILED DESCRIPTION

[0050] The present invention will be further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0051] Example 1

[0052] like Figures 1 to 4 As shown, in order to solve the problems of the prior art, the present invention proposes a LNG storage tank pressure control method and system based on fuzzy control algorithm, liquid level and pressure sensors are set in the LNG storage tank, temperature and pressure sensors are set at the inlet of the LNG pump pool, and a pressure sensor is set at the rear end of the LNG submersible pump (the outlet of the LNG pump pool). According to the operating conditions of the gas station, the pressure control process in the system is divided into three stages, namely, the filling stage, the unloading preparation stage, and the unloading stage. The fuzzy control algorithm is combined to realize efficient and intelligent control of the LNG storage tank pressure, reduce personnel operation, and have no effect on vehicle filling during pressure control, thereby improving the operating efficiency of the LNG gas station.

[0053] Figure 1 Among them, No. E-101 is a vaporizer, which is an executive device for pressurizing the LNG storage tank. The LNG storage tank is pressurized by opening and closing the boosting valve GV002; No. M-101 is an LNG submersible pump.

[0054] During the filling stage, the pressure range of the LNG storage tank can be from the minimum working pressure to the upper limit of pressure control. The control system calculates the optimal tank pressure at that time according to the actual situation of the site, allowing the LNG submersible pump to operate in the optimal working condition, thereby achieving energy saving and extending the service life of the equipment.

[0055] During the unloading preparation stage, the pressure of the LNG storage tank will begin to decrease in preparation for unloading.

[0056] During the unloading stage, the optimal tank pressure is calculated through the control system to improve the unloading efficiency without affecting the liquid addition.

[0057] Implementation plan:

[0058] Build the fuzzy algorithm:

[0059] LNG tank pressure, liquid level, and cavitation margin are the key input variables for LNG tank pressure control, and three fuzzy sets of "low", "medium", and "high" are defined for each variable;

[0060] LNG tank pressure: "minimum working pressure", "target pressure", "upper limit of pressure control"

[0061] 1. Calculate the minimum working pressure of the LNG storage tank in the control system, calculate the pipeline friction resistance loss according to the mechanical energy conservation formula, and then calculate the average value of the total resistance coefficient according to the different built sites to obtain a reasonable friction resistance loss estimate suitable for various sites; the rated boost pressure of the LNG submersible pump is calculated by the liquid pressure formula, and the filling pressure is a preset modifiable pressure of the system, which is a constant. The calculation formula for the inlet pressure of the LNG submersible pump is: LNG submersible pump inlet pressure = filling pressure - LNG submersible pump rated boost pressure + pipeline friction resistance loss; since the inlet pressure of the LNG submersible pump is similar to the LNG storage tank pressure, this value is the minimum working pressure of the LNG storage tank.

[0062] 2. Determine the maximum working pressure of the LNG storage tank, which can be preset in the control system according to the nameplate marking of the LNG storage tank as the tank protection pressure value.

[0063] 3. The upper limit of pressure control is reset in the control system based on statistical experience data.

[0064] 4. The target pressure is the value that needs to be calculated by the control system in real time.

[0065] LNG tank liquid level: "Filling level", "Unloading preparation level", "Unloading level"

[0066] 1. According to the different forms of LNG storage tanks, horizontal storage tanks or vertical storage tanks and statistical experience data, the liquid level value range corresponding to the three stages is divided.

[0067] LNG pump pool temperature: "minimum temperature", "operating temperature", "maximum temperature"

[0068] 1. The minimum and maximum temperatures of the LNG pump pool can be set based on statistical empirical data. The operating temperature is the temperature measured in real time by the LNG pump pool. The LNG pump pool temperature is used to calculate the cavitation margin.

[0069] Cavitation allowance = device cavitation allowance – required cavitation allowance;

[0070] Cavitation allowance: "High Cavitation Allowance", "Low Cavitation Allowance", "Negative Cavitation Allowance"

[0071] Calculation of cavitation margin of device:

[0072] NPSHa=P+Ph-Pf–Ps

[0073] P is the gas phase pressure of the LNG storage tank, which is detected in real time by the tank pressure sensor.

[0074] Ph is the static pressure generated by the liquid in the LNG storage tank, which is calculated by the tank level sensor based on the real-time detection of the liquid level.

[0075] Pf is the friction resistance loss of the pipeline from the LNG storage tank to the pump pool inlet, and the average value is calculated from the existing sites.

[0076] Ps is the saturated vapor pressure of LNG in the LNG pump pool. The LNG temperature is detected in real time by the LNG pump pool temperature sensor, and then calculated by the control system based on the corresponding data of saturated vapor pressure and temperature.

[0077] The required cavitation margin is calculated based on the real-time speed and flow rate of the LNG submersible pump and the characteristic curve provided by the pump manufacturer.

[0078] The control system calculates the difference between consecutive data points of the cavitation margin through a differential algorithm to predict the rising, falling or flat trend of the cavitation margin data.

[0079] Fuzzy control rules:

[0080] If the LNG tank level is at the "filling level" and the "cavitation margin is high", and the cavitation margin trend is rising or flat, the control signal is "maintain current pressure".

[0081] If the LNG tank level is at the "filling level" and the "cavitation margin is high", and the cavitation margin trend is decreasing, the control signal is "slight pressurization".

[0082] If the LNG tank level is at "filling level" and "cavitation margin is low", and the cavitation margin trend is rising, the control signal is "slight boost".

[0083] If the LNG tank level is at "filling level" and "cavitation margin is low", and the cavitation margin trend is flat, the control signal is "medium boost".

[0084] If the LNG tank level is at "filling level" and "cavitation margin is low", and the cavitation margin trend is decreasing, the control signal is "full boost".

[0085] If the LNG tank level is at the "filling level" and the "cavitation margin is negative", and the cavitation margin trend is rising, the control signal is "full boost".

[0086] If the LNG tank level is at the "unloading preparation level" and the "cavitation margin is high", and the cavitation margin trend is rising or flat, the control signal is "reduce pressure".

[0087] If the LNG tank level is at the "unloading preparation level" and the "cavitation margin is high", and the cavitation margin trend is decreasing, the control signal is "maintain current pressure".

[0088] If the LNG tank level is at the "unloading preparation level" and the "cavitation margin is low", and the cavitation margin trend is rising, the control signal is "maintain current pressure".

[0089] If the LNG tank level is at the "unloading preparation level" and the "cavitation margin is low", and the cavitation margin trend is flat, the control signal is "slight pressurization".

[0090] If the LNG tank level is at the "unloading preparation level" and the "cavitation margin is low", and the cavitation margin trend is decreasing, the control signal is "light to medium pressurization".

[0091] If the liquid level in the LNG tank is at the "unloading preparation level" and the "cavitation margin is negative", and the cavitation margin trend is rising, the control signal is "light to medium pressurization".

[0092] If the liquid level in the LNG tank is at the "unloading preparation level" and the "cavitation margin is negative", and the cavitation margin trend is flat, the control signal is "light to moderate pressurization".

[0093] If the LNG tank level is at the "unloading preparation level" and the "cavitation margin is negative", and the cavitation margin trend is negative, the control signal is "medium boost".

[0094] If the LNG tank level is at the "unloading level" and the "cavitation margin is high", and the cavitation margin trend is rising or flat, the control signal is "maintain current pressure".

[0095] If the LNG tank level is at the "unloading level" and the "cavitation margin is high", and the cavitation margin trend is decreasing, the control signal is "maintain current pressure".

[0096] If the LNG tank level is at the "unloading level" and the "cavitation margin is low", and the cavitation margin trend is rising, the control signal is "maintain current pressure".

[0097] If the LNG tank level is at the "unloading level" and the "cavitation margin is low", and the cavitation margin trend is flat, the control signal is "maintain current pressure".

[0098] If the LNG tank level is at the "unloading level" and the "cavitation margin is low", and the cavitation margin trend is decreasing, the control signal is "slight pressurization".

[0099] If the LNG tank level is at the "unloading level" and the "cavitation margin is negative", and the cavitation margin trend is rising, the control signal is "slight pressurization".

[0100] If the LNG tank level is at the "unloading level" and the "cavitation margin is negative", and the cavitation margin trend is flat, the control signal is "slight pressurization".

[0101] If the LNG tank level is at the "unloading level" and the "cavitation margin is negative", and the cavitation margin trend is decreasing, the control signal is "light to medium boost".

[0102] Defuzzification and output:

[0103] Using the weighted average method, the membership degree of each control signal output fuzzy set and the weighted average of its control value are calculated and converted into an accurate control value. The control value is used as the "target pressure" of the LNG tank pressure to control the opening and closing of the boost valve GV002.

[0104] As described above, the present invention can be preferably implemented.

[0105] All features disclosed in all embodiments in this specification, or steps in all methods or processes implicitly disclosed, except for mutually exclusive features and / or steps, can be combined and / or expanded or replaced in any manner.

[0106] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. According to the technical essence of the present invention, within the spirit and principles of the present invention, any simple modification, equivalent replacement and improvement made to the above embodiment still falls within the protection scope of the technical solution of the present invention.

Claims

1. A LNG tank pressure control system based on fuzzy control algorithm, characterized in that: It includes an LNG storage tank, an LNG pump pool, and an LNG submersible pump. The LNG submersible pump is installed in the LNG pump pool. The LNG storage tank and the LNG submersible pump are connected to each other. The LNG storage tank is equipped with a liquid level sensor and a pressure sensor. The inlet and outlet of the LNG pump pool are equipped with pressure sensors.

2. A LNG tank pressure control method based on fuzzy control algorithm, characterized in that: An LNG storage tank pressure control system based on a fuzzy control algorithm as described in claim 1 is adopted, and the pressure control process is divided into three stages: filling stage, unloading preparation stage, and unloading stage according to the operating conditions of the gas station, and the fuzzy control algorithm is combined to realize the control of the LNG storage tank pressure.

3. The LNG tank pressure control method based on fuzzy control algorithm according to claim 2 is characterized in that: During the filling stage, the optimal tank pressure is calculated by the control system to allow the LNG submersible pump to operate in the optimal working condition; during the unloading preparation stage, the pressure of the LNG tank is reduced in preparation for unloading; during the unloading stage, the optimal tank pressure is calculated by the control system.

4. The LNG tank pressure control method based on fuzzy control algorithm according to claim 3 is characterized in that: When constructing the fuzzy control algorithm, the LNG tank pressure, liquid level and cavitation margin are used as the input variables of the LNG tank pressure control, and three fuzzy sets of low, medium and high are defined for each input variable.

5. The LNG storage tank pressure control method based on fuzzy control algorithm according to claim 4 is characterized in that: When constructing the fuzzy control algorithm, the minimum working pressure of the LNG tank is calculated in the control system, and the pipeline friction resistance loss is calculated. Then, the average value of the total resistance coefficient is calculated based on the different built sites to estimate the pipeline friction resistance loss applicable to various sites. Calculate the inlet pressure of the LNG submersible pump using the following formula: LNG submersible pump inlet pressure = filling pressure - rated boost pressure of the LNG submersible pump + pipeline friction loss; The inlet pressure of the LNG submersible pump is the minimum working pressure of the LNG storage tank; the rated boost pressure of the LNG submersible pump is calculated by the liquid pressure formula, and the filling pressure is a modifiable pressure preset by the control system.

6. The LNG tank pressure control method based on fuzzy control algorithm according to claim 5 is characterized in that: The liquid level of the LNG storage tank is divided into filling level, unloading preparation level and unloading level. The liquid level value ranges corresponding to the three stages of filling stage, unloading preparation stage and unloading stage are divided according to the different forms of LNG storage tanks and statistical experience data.

7. The LNG tank pressure control method based on fuzzy control algorithm according to claim 6 is characterized in that: The cavitation margin is calculated based on the LNG pump pool temperature. The calculation formula is: Cavitation allowance = device cavitation allowance – required cavitation allowance; The control system calculates the difference between consecutive data points of the cavitation margin through a differential algorithm to predict the rising, falling or flat trend of the cavitation margin data.

8. The LNG storage tank pressure control method based on fuzzy control algorithm according to claim 7 is characterized in that: The calculation formula of the cavitation margin of the device is: NPSHa=P+Ph-Pf–Ps; Wherein, P is the gas phase pressure of the LNG storage tank, Ph is the static pressure generated by the liquid in the LNG storage tank, Pf is the friction resistance loss of the pipeline from the LNG storage tank to the pump pool inlet, Ps is the saturated vapor pressure of LNG in the LNG pump pool, and the required cavitation margin is calculated based on the real-time speed and flow rate of the LNG submersible pump combined with the characteristic curve provided by the pump manufacturer.

9. The LNG storage tank pressure control method based on fuzzy control algorithm according to claim 8, characterized in that: The fuzzy control rules of the fuzzy control algorithm are: If the LNG tank level is at the filling level and the cavitation margin is high, and the cavitation margin trend is rising or flat, the control signal is to maintain the current pressure; If the LNG tank level is at the filling level and the cavitation margin is high, and the cavitation margin trend is decreasing, the control signal is a slight pressurization; If the LNG tank level is at the filling level and the cavitation margin is low, and the cavitation margin trend is rising, the control signal is a slight pressurization; If the LNG tank level is at the filling level and the cavitation margin is low, and the cavitation margin trend is flat, the control signal is medium boost; If the LNG tank level is at the filling level and the cavitation margin is low, and the cavitation margin trend is decreasing, the control signal is full pressure boost; If the LNG tank level is at the filling level and the cavitation margin is negative, and the cavitation margin trend is rising, the control signal is full pressure boost; If the LNG tank level is at the unloading preparation level and the cavitation margin is high, and the cavitation margin trend is rising or flat, the control signal is to reduce the pressure; If the LNG tank level is at the unloading preparation level and the cavitation margin is high, and the cavitation margin trend is decreasing, the control signal is to maintain the current pressure; If the LNG tank level is at the unloading preparation level and the cavitation margin is low, and the cavitation margin trend is rising, the control signal is to maintain the current pressure; If the LNG tank liquid level is at the unloading preparation level and the cavitation margin is low, and the cavitation margin trend is gentle, the control signal is a slight pressurization; If the LNG tank liquid level is at the unloading preparation level and the cavitation margin is low, and the cavitation margin trend is decreasing, the control signal is light to medium pressure increase; If the LNG tank level is at the unloading preparation level and the cavitation margin is negative, and the cavitation margin trend is rising, the control signal is light to medium pressure increase; If the LNG tank liquid level is at the unloading preparation level and the cavitation margin is negative, and the cavitation margin trend is gentle, the control signal is light to moderate pressurization; If the LNG tank level is at the unloading preparation level and the cavitation margin is negative, and the cavitation margin trend is negative, the control signal is medium boost; If the LNG tank level is at the unloading level and the cavitation margin is high, and the cavitation margin trend is rising or flat, the control signal is to maintain the current pressure; If the LNG tank level is at the unloading level and the cavitation margin is high, and the cavitation margin trend is decreasing, the control signal is to maintain the current pressure; If the LNG tank level is at the unloading level and the cavitation margin is low, and the cavitation margin trend is rising, the control signal is to maintain the current pressure; If the LNG tank level is at the unloading level and the cavitation margin is low, and the cavitation margin trend is flat, the control signal is to maintain the current pressure; If the LNG tank level is at the unloading level and the cavitation margin is low, and the cavitation margin trend is decreasing, the control signal is a slight pressurization; If the LNG tank level is at the unloading level and the cavitation margin is negative, and the cavitation margin trend is rising, the control signal is a slight pressurization; If the LNG tank level is at the unloading level and the cavitation margin is negative, and the cavitation margin trend is flat, the control signal is a slight pressurization; If the LNG tank level is at the unloading level and the cavitation margin is negative, and the cavitation margin trend is decreasing, the control signal is light to moderate pressurization.

10. The LNG storage tank pressure control method based on fuzzy control algorithm according to any one of claims 2 to 9, characterized in that: In the fuzzy control algorithm, the weighted average method is used to calculate the weighted average of the membership degree of each control signal output fuzzy set and the control value of each control signal output fuzzy set, convert the fuzzy control value into an accurate control value, and use the accurate control value as the target pressure of the LNG tank pressure.

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