Nondestructive testing method for large-circulation liquid stealing of LNG (Liquefied Natural Gas) liquid adding machine

Abstract

The invention relates to the technical field of liquid filling machines, and aims to solve the problems that in the prior art, a filling muzzle is directly inserted into a filling vehicle for pre-cooling, unmetered liquid LNG enters the vehicle to cause site loss, and the cost is high when a sensing element is additionally arranged on a gas return pipe of a running LNG liquid filling machine in a damaged mode. The invention provides a nondestructive testing method for large-circulation liquid stealing of an LNG (Liquefied Natural Gas) liquid adding machine, which comprises the following steps of: 1, mounting temperature sensors on the outer surfaces of a liquid inlet pipe and a gas return pipe of the LNG liquid adding machine, and connecting the temperature sensors to an LNG filling control panel; a controller is arranged in the LNG filling control panel, and an algorithm of software in the controller can judge whether a filling gun of the LNG liquid filling machine is directly connected with a filled port of a vehicle or not; and 2, according to the result obtained in the step 1, whether a filling valve of the LNG liquid adding machine is closed or not is selected, if it is judged that a filling gun of the LNG liquid adding machine is directly inserted into a vehicle filled opening, the filling valve of the LNG liquid adding machine is closed, and otherwise, the filling valve of the LNG liquid adding machine is not closed.

Description

Technical Field

[0001] The invention relates to the technical field of liquid charging machines, and in particular to a nondestructive detection method for large-cycle liquid theft in an LNG liquid charging machine. Background Art

[0002] At present, the LNG filling machines on the market are all developed in accordance with the standards of JJF1524-2015 "Liquefied Natural Gas Filling Machine Type Evaluation Outline", JJG1114-2015 "Liquefied Natural Gas Filling Machine Verification Procedure", GB / T36126-2018 "Liquefied Natural Gas Filling Machine for Automobiles", and GB / T 41319-2022 "Liquefied Natural Gas (LNG) Filling Device".

[0003] The standard provides a block diagram of the principle of the LNG filling machine, but does not specify or display all the details, such as whether pressure transmitters and temperature transmitters should be installed, their installation locations, and the number of them. There are no clear requirements.

[0004] The process flow diagram of LNG filling machine in the market is as follows Figure 1 As shown in the flowchart, the filling function can be completed safely and accurately at a strictly managed site. However, if the operator is negligent, the filling site will suffer losses: the economic losses of the filling site are mainly due to the large cycle process in the LNG filling process.

[0005] The limit temperature of liquid LNG is -162℃. When the LNG filling machine is not filled for a long time, the residual liquid LNG in the pipeline will quickly vaporize into gas. In order to ensure accurate metering during filling, the station needs to fully precool the liquid flow meter before filling the vehicle, that is, replace the fuel in the liquid flow meter, SV01 valve and filling gun nozzle with liquid LNG. The conventional practice of the station is to first insert the filling gun nozzle into the gun socket of the LNG filling machine, and then open the SV01 valve to allow liquid LNG to flow through the liquid flow meter, SV01, filling gun nozzle, gun socket and gas phase pipeline back to the LNG storage tank of the filling station. When this circulation loop is fully precooled, the LNG filling machine stops precooling, and the operator inserts the filling gun nozzle into the filling vehicle for filling.

[0006] If the site staff is careless and inserts the filling gun into the filling vehicle before precooling, unmetered liquid LNG will enter the filling vehicle, causing site losses. Because there are no temperature, pressure or flow sensing components on the pipeline from the gun holder to the LNG storage tank, such losses are difficult to automatically correct and prevent.

[0007] There are a large number of LNG stations (more than a thousand) in the market. It is very costly to install pressure, temperature or flow sensing components on the return air pipeline of the running LNG filling machine, so a non-destructive liquid theft detection method is needed. Summary of the invention

[0008] The present invention aims to provide a nondestructive detection method for large-circulation liquid theft of an LNG filling machine, so as to solve the problem in the prior art that when the staff directly inserts the filling gun nozzle into the filling vehicle and then pre-cools it, unmetered liquid LNG will enter the filling vehicle, thereby causing site losses, and the cost of lossily installing pressure, temperature or flow sensing elements on the return air pipe of the running LNG filling machine is high.

[0009] The present invention is achieved by adopting the following technical solutions:

[0010] The present invention provides a nondestructive detection method for large-circulation liquid theft of an LNG liquid filling machine, comprising the following steps:

[0011] Step 1: Install temperature sensors on the outer surfaces of the liquid inlet pipe and the return air pipe of the LNG filling machine, and connect the temperature sensors to the LNG filling control board; the LNG filling control board has a controller, and the algorithm of the software in the controller can determine whether the filling gun of the LNG filling machine is directly connected to the vehicle filling port;

[0012] Step 2: Based on the result obtained in step 1, choose whether to close the filling valve of the LNG filling machine. If it is determined that the filling gun of the LNG filling machine is directly inserted into the vehicle filling port, then close the filling valve of the LNG filling machine; otherwise, do not close it.

[0013] As the preferred technical solution:

[0014] The software's algorithms include:

[0015] S1: The temperature distribution law of the air side of the liquid inlet pipe and the return pipe of the LNG charging machine is obtained by calculation, and the temperature change rate of the air side of the liquid inlet pipe and the return pipe is obtained according to the temperature distribution law of the air side of the liquid inlet pipe and the return pipe;

[0016] S2: Compare the temperature change rates of the air side of the liquid inlet pipe and the liquid return pipe. If the temperature change rate of the air side of the liquid return pipe is inconsistent with the temperature change rate of the air side of the liquid inlet pipe, it means that the filling gun of the LNG filling machine is directly connected to the vehicle filling port. At this time, close the filling valve of the LNG filling machine and stop filling LNG.

[0017] The inconsistency between the temperature change rate of the air side of the liquid return pipe and the temperature change rate of the air side of the liquid inlet pipe means that the trends of the two data curves of the temperature change rate of the air side of the liquid return pipe and the temperature change rate of the air side of the liquid inlet pipe are inconsistent.

[0018] As the preferred technical solution:

[0019] In order to further clarify the comparison of the temperature change rates of the air side of the liquid inlet pipe and the liquid return pipe, the temperature change rate of the air side of the liquid return pipe is judged according to the change rate comparison value ε to determine whether it is consistent with the temperature change rate of the air side of the liquid inlet pipe. When ε> the set value, the temperature change rate of the air side of the liquid return pipe is inconsistent with the temperature change rate of the air side of the liquid inlet pipe, otherwise, they are consistent;

[0020] The calculation formula of the change rate comparison value ε is:

[0021]

[0022] in, is the rate of change of the temperature on the air side of the inlet pipe, is the rate of change of temperature on the air side of the return pipe.

[0023] As the preferred technical solution:

[0024] Step S1 specifically includes:

[0025] S101: Establish a pipeline heat transfer model and simplify the model to obtain a one-dimensional unsteady-state heat conduction differential equation without internal heat source:

[0026]

[0027] in, represents the rate of change of temperature with time, represents the rate of change of temperature with position, λ is the thermal conductivity of the pipe, ρ is the density of the pipe material, c p is the specific heat capacity of the pipeline material;

[0028] S102: Meshing the pipe walls of the liquid inlet pipe and the air return pipe;

[0029] S103: According to Fourier's law and Newton's cooling formula, the following equation is obtained:

[0030] LNG side equation:

[0031]

[0032] Inferred:

[0033]

[0034] Air side equation:

[0035]

[0036] Inferred:

[0037]

[0038] Among them, h lis the surface convection coefficient of LNG, h a is the natural convection coefficient of air; is the rate of change of LNG side temperature with position, T l is the liquid temperature, T w is the tube wall temperature, is the rate of change of air side temperature with position, T a is the air temperature;

[0039] S104: Discretize and solve equations 1, 3, and 5 to obtain the pipe wall temperature T w , tube wall temperature T w is the temperature distribution along the wall thickness direction of the tube wall. Therefore, according to the tube wall temperature T w The temperature distribution law on the air side of the pipeline is obtained, and the temperature change rate on the air side of the liquid inlet pipe is calculated according to the temperature distribution law on the air side of the liquid inlet pipe;

[0040] S105: Calculate the mean square error between the temperature on the air side of the liquid inlet pipe and the temperature collected by the temperature sensor on the liquid inlet pipe, perform multiple iterative calculations to make the mean square error meet the set value, and then fit the voltage value of the temperature sensor on the liquid inlet pipe and the LNG liquid temperature in the liquid inlet pipe to obtain a fitting function of the voltage value of the temperature sensor and the LNG liquid temperature, that is, U k =f(T LNG ); According to U k =f(T LNG ) and the voltage value of the temperature sensor on the return air duct, the temperature distribution law in the return air duct is obtained, so as to obtain the temperature distribution law on the return air duct wall and the temperature distribution law on the air side of the return air duct according to steps S103 and S104; the temperature change rate on the air side of the return air duct is calculated according to the temperature distribution law on the air side of the return air duct.

[0041] In step S104, the temperature distribution law on the air side of the pipeline refers to the temperature distribution of the outer layer of the pipeline wall.

[0042] As the preferred technical solution:

[0043] In step S105, the mean square error is calculated according to the following formula:

[0044]

[0045] Among them, T c is the air side temperature of the liquid inlet pipe, T t It is the temperature collected by the temperature sensor on the liquid inlet pipe.

[0046] As the preferred technical solution:

[0047] A liquid flow meter is installed on the liquid inlet pipeline of the LNG charging machine, and a gas flow meter is installed on the gas return pipeline;

[0048] The algorithm in the software of the controller can also determine whether precooling is complete;

[0049] The software's algorithms also include:

[0050] S3: According to the temperature and pressure inside the return air pipe, the density of LNG is estimated to obtain the calculated density of LNG, and then the calculated density of LNG is compared with the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG under the current state. When the difference between the calculated density of LNG and the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG is within the preset range, it means that precooling is completed, the large circulation is stopped, and the vehicle is started to be filled with LNG. Otherwise, it is necessary to continue the large circulation until the precooling is completed.

[0051] As the preferred technical solution:

[0052] Step S3 specifically includes:

[0053] S301: Measure the LNG liquid density by the liquid phase flowmeter on the liquid inlet pipe;

[0054] S302: according to the temperature and pressure inside the gas return pipe, the density of the LNG is estimated by using the real gas equation to obtain the calculated density of the LNG;

[0055] S303: Compare the calculated density of LNG with the LNG liquid density and the LNG standard density measured by the liquid phase flow meter. When the difference between the calculated density of LNG and the LNG liquid density and the LNG standard density measured by the liquid phase flow meter is within a preset range, stop the large circulation and start filling the vehicle with LNG.

[0056] As the preferred technical solution:

[0057] The real gas equation in step S302 is:

[0058]

[0059] Among them, P is the pressure of the gas, R is the ideal gas constant, T is the temperature, V is the volume of the gas, and b is a constant related to the molecular volume. k is a constant related to the properties of the gas, T c is the critical temperature.

[0060] As the preferred technical solution:

[0061] The above real gas equation is only an example and is not limited to the above real gas equation. Other real gas equations may also be used.

[0062] As the preferred technical solution:

[0063] The temperature sensor adopts a K-type thermocouple. K-type thermocouple 01 is bonded on the outer surface of the liquid inlet pipe, and K-type thermocouple 02 is bonded on the outer surface of the air return pipe.

[0064] As the preferred technical solution:

[0065] The temperature sensor is not limited to K-type thermocouples. Other thermocouples or thermal resistors that can measure temperature can also be used. The selection can be made based on the applicable temperature range and price of the product.

[0066] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0067] 1. The present invention has a non-destructive LNG filling machine circulation detection function, which prevents operators from inserting the filling gun into the vehicle's filling port for circulation pre-cooling, resulting in unmetered LNG directly entering the vehicle, thereby causing losses to the filling station; at the same time, the non-destructive LNG filling machine circulation detection function can greatly reduce the transformation cost of equipment that has been put into use on the market. Lossy transformation not only requires the station to stop operating, but also requires relevant departments to conduct non-destructive testing on the transformation point after the transformation. The cost of stopping the station and testing will be much higher than the use of this technology.

[0068] 2. The present invention adopts Fourier's law and Newton's cooling formula in software to obtain the temperature change rate of the air side of the liquid inlet pipe and the liquid return pipe. The present invention determines whether there is a liquid flow channel by comparing the temperature change rate of the air side of the liquid inlet pipe and the liquid return pipe. If the temperature change rate of the air side of the liquid return pipe is inconsistent with the temperature change rate of the air side of the liquid inlet pipe, it means that the filling gun of the LNG filling machine is directly connected to the vehicle receiving port. At this time, the filling valve of the filling machine is closed to stop filling LNG.

[0069] 3. The present invention estimates the density of LNG based on the calculated temperature of the return air pipe and the collected return air pipe pressure, and uses the real gas equation to obtain the calculated density of LNG, and then compares the calculated density of LNG with the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG under the current state. When the difference between the calculated density of LNG and the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG is within a preset range, it proves that the quality of the liquid in the pipeline has met the requirements, and the large circulation can be stopped and filling can be started, thereby dynamically adjusting the pre-cooling time, shortening the large circulation time, and improving the filling efficiency.

[0070] 4. The present invention estimates the density of LNG and compares the calculated density of LNG with the liquid density of LNG measured by the liquid flow meter and the standard density of LNG under the current state to ensure that the LNG in the return pipe can meet the requirements, thereby improving the filling quality of the LNG filling machine.

[0071] 5. The present invention bonds K-type thermocouples to the outside of the liquid inlet pipe and the gas return pipe of the LNG liquid adding machine, which has a fast response speed and a wide temperature measurement range and has better applicability at -162°C to 55°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 It is a principle block diagram of the LNG liquid filling machine in the prior art.

[0073] Figure 2 It is a process flow chart of the LNG liquid filling machine in the prior art.

[0074] Figure 3 The present invention provides a process flow chart of the nondestructive detection method for large-cycle liquid theft in an LNG liquid filling machine.

[0075] Figure 4 It is a schematic diagram of the pipeline heat transfer model described in the present invention.

[0076] Figure 5 for Figure 4 A magnified schematic diagram of center A.

[0077] Figure 6 It is a schematic diagram of the model after mesh division described in the present invention.

[0078] Figure 7 This is a comparison chart of liquid density. DETAILED DESCRIPTION

[0079] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0080] Example 1

[0081] like Figure 3 As shown, this embodiment proposes a nondestructive detection method for large-cycle liquid theft of an LNG liquid filling machine, comprising the following steps:

[0082] Step 1: Bond a K-type thermocouple on the outer surface of the liquid inlet pipe and the return air pipe of the LNG filling machine, and connect the K-type thermocouple to the LNG filling control board; a liquid phase flow meter is provided on the liquid inlet pipe of the LNG filling machine, and a gas phase flow meter is provided on the return air pipe of the LNG filling machine; the LNG filling control board has a controller, which is used to analyze, calculate and process the received signals, and the algorithm of the software in the controller can determine whether the filling gun of the LNG filling machine is directly connected to the vehicle filling port and whether the pre-cooling is completed;

[0083] Step 2: According to the result obtained in step 1, choose whether to close the filling valve of the LNG filling machine. If it is determined that the filling gun of the LNG filling machine is directly inserted into the vehicle filling port, the filling valve of the LNG filling machine is closed; otherwise, it is not closed;

[0084] According to the result obtained in step 1, if precooling is completed, the large circulation is stopped and the vehicle is started to be filled with LNG. Otherwise, the large circulation needs to be continued until precooling is completed.

[0085] The K-type thermocouple bonded to the outer surface of the pipeline will receive the heat conducted along the pipeline wall by the LNG in the pipeline. At the same time, the K-type thermocouple will also receive the heat conducted from the environment. Since the K-type thermocouple is connected to the LNG filling control board, the temperature data collected by the K-type thermocouple is transmitted to the LNG filling control board.

[0086] The present invention adopts K-type thermocouple, but is not limited to K-type thermocouple. Other thermocouples or thermal resistors that can measure temperature can also be used, and can be selected according to the applicable temperature range and price of the product. The present embodiment preferably adopts K-type thermocouple, which has a faster response speed than PT100 / 1000 thermal resistor, has a wider temperature measurement range, and has better applicability at -162℃~55℃.

[0087] In this embodiment, a K-type thermocouple 01 is bonded to the outside of the liquid inlet pipe, and a K-type thermocouple 02 is bonded to the outside of the gas return pipe.

[0088] The algorithms in the controller software include:

[0089] S1: The temperature distribution law of the air side of the liquid inlet pipe and the return pipe of the LNG charging machine is obtained by calculation, and the temperature change rate of the air side of the liquid inlet pipe and the return pipe is obtained according to the temperature distribution law of the air side of the liquid inlet pipe and the return pipe.

[0090] Step S1 includes:

[0091] S101: Figure 4 As shown in Figure 2, a pipeline heat transfer model is established, as shown in Figure 2 Figure 5 As shown, a small part of the pipeline is taken for local magnification; by simplifying the model, the one-dimensional unsteady-state heat conduction differential equation without internal heat source is obtained:

[0092]

[0093] in, represents the rate of change of temperature with time, represents the rate of change of temperature with position, λ is the thermal conductivity of the pipe, ρ is the density of the pipe material, c p is the specific heat capacity of the pipe material.

[0094] Taking 06Cr19Ni10 stainless steel as an example, its thermal conductivity is 15W·m -1 ·K -1 , specific heat capacity is 500 J·kg -1 ·K -1 , density is about 7900kg·m -3 .

[0095] S102: Mesh the walls of the liquid inlet pipe and the gas return pipe. The meshed model is as follows: Figure 6 As shown, the left side of the tube wall is LNG liquid, denoted as l, and the right side is air, denoted as a.

[0096] S103: According to Fourier's law and Newton's cooling formula, the following equation is obtained:

[0097] Left LNG side equation:

[0098]

[0099] Inferred:

[0100]

[0101] Right air side equation:

[0102]

[0103] Inferred:

[0104]

[0105] Among them, h l is the surface convection coefficient of LNG, about 80 W·m -2 ·K -1 ,h a is the natural convection coefficient of air, about 5 W·m -2 ·K -1 ; is the rate of change of LNG side temperature with position, T l is the liquid temperature, T w is the tube wall temperature, is the rate of change of air side temperature with position, T a is the air temperature.

[0106] Air temperature T a and liquid temperature T l It can be collected by the temperature sensor, the pipe wall temperature T wIt is the temperature distribution along the wall thickness direction of the pipe wall. It varies along the wall thickness direction of the pipe. After the pipe wall is meshed, each mesh has its corresponding temperature.

[0107] S104: Discretize and solve equations 1, 3, and 5 to obtain the pipe wall temperature T at different times. w , that is, the temperature distribution law on the walls of the liquid inlet pipe and the return air pipe is obtained, and the temperature distribution law on the air side of the pipeline can also be obtained. The temperature distribution law on the air side of the pipeline refers to the temperature distribution of the outer layer of the pipe wall. According to the temperature distribution law on the air side of the liquid inlet pipe, the temperature change rate on the air side of the liquid inlet pipe can be calculated.

[0108] S105: Obtaining the temperature distribution law on the return air duct wall, the temperature distribution law on the air side of the return air duct, and the temperature change rate on the air side of the return air duct.

[0109] Step S105 specifically includes:

[0110] The air side temperature T of the liquid inlet pipe obtained in step S104 is calculated according to the following formula c The temperature T collected by the K-type thermocouple on the liquid inlet pipe t The mean square error between

[0111] σ=0.5(T c -T t ) 2

[0112] Perform multiple iterations to make the mean square error meet the set value. Then fit the voltage value of the K-type thermocouple on the inlet pipe and the LNG liquid temperature in the inlet pipe to obtain the fitting function of the voltage value of the K-type thermocouple and the LNG liquid temperature, that is, U k =f(T LNG ).

[0113] According to U k =f(T LNG ) and the voltage value of the K-type thermocouple on the return air duct, the temperature distribution law in the return air duct is obtained, and then the temperature distribution law on the return air duct wall and the temperature distribution law on the air side of the return air duct are obtained according to steps S103 and S104.

[0114] In the return air pipe of the LNG filling machine, the temperature rises due to the heat absorption of liquid LNG and the surrounding environment, resulting in unreliable temperature collected by the K-type thermocouple attached to the return air pipe. However, the voltage value of the K-type thermocouple here can be obtained. Therefore, according to U k =f(T LNG )The same change law is used to solve the temperature distribution law in the return air duct, the temperature distribution law on the return air duct wall and the temperature distribution law on the air side of the return air duct.

[0115] The temperature change rate on the air side of the return air duct can be calculated based on the temperature distribution law on the air side of the return air duct.

[0116] Further explanation of this step: According to the temperature of the liquid LNG liquid, the ambient temperature, the thermal conductivity, etc., the temperature change of the pipe wall when different LNG liquids flow in the pipeline can be obtained. The temperature change of the outer layer of the pipe wall can be detected by a thermocouple. In the gas phase pipeline, the temperature change of the liquid in the pipeline is unknown, but the change of the gas phase thermocouple voltage can be detected. The temperature change law of the pipeline with the internal liquid is known, and the temperature change law inside the pipeline can be deduced. Therefore, according to the above method, the temperature distribution law in the return gas pipe, the temperature distribution law on the return gas pipe wall and the temperature distribution law on the air side of the return gas pipe can be obtained.

[0117] S2: Compare the temperature change rate of the air side of the liquid inlet pipe and the liquid return pipe Determine whether there is a liquid flow channel. If the temperature change rate of the air side of the return pipe is inconsistent with the temperature change rate of the air side of the inlet pipe, it means that the filling gun of the LNG filling machine is directly connected to the vehicle's filling port. At this time, close the filling valve of the LNG filling machine and stop filling LNG.

[0118] The flow rate v, mass m and density ρ of the LNG liquid are measured by the liquid flow meter on the liquid inlet pipe. Given the length L of the liquid inlet pipe, the time t for the LNG liquid to fill the liquid inlet pipe can be calculated. Therefore, the temperature of the return air duct will change at most before time t. Since the temperature fluctuates, the rate of change of temperature over time is used for comparison here.

[0119] The inconsistency between the temperature change rate of the air side of the liquid return pipe and the temperature change rate of the air side of the liquid inlet pipe means that the trends of the two data curves of the temperature change rate of the air side of the liquid return pipe and the temperature change rate of the air side of the liquid inlet pipe are inconsistent.

[0120] In order to further clarify the comparison of the temperature change rate of the air side of the liquid inlet pipe and the liquid return pipe, the temperature change rate of the air side of the return pipe can be judged whether it is consistent with the temperature change rate of the air side of the liquid inlet pipe according to the change rate comparison value ε. When ε>set value, the temperature change rate of the air side of the return pipe is inconsistent with the temperature change rate of the air side of the liquid inlet pipe, otherwise, they are consistent.

[0121] The calculation formula of the change rate comparison value ε is:

[0122]

[0123] in, is the rate of change of the temperature on the air side of the inlet pipe, is the rate of change of temperature on the air side of the return pipe.

[0124] S3: According to the temperature and pressure inside the return air pipe, the density of LNG is estimated to obtain the calculated density of LNG, and then the calculated density of LNG is compared with the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG under the current state. When the difference between the calculated density of LNG and the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG is within the preset range, it proves that the quality of the liquid in the pipeline has met the requirements, and the large circulation can be stopped and filling can be started, thereby dynamically adjusting the precooling time, shortening the large circulation time, and improving the filling efficiency.

[0125] Step S3 includes:

[0126] S301: The LNG liquid density is measured by the liquid phase flowmeter on the liquid inlet pipe.

[0127] S302: According to the temperature and pressure inside the return gas pipe, the density of the LNG is estimated by using the real gas equation to obtain the calculated density of the LNG.

[0128] The real gas equation is exemplified by, but not limited to, the following formula:

[0129]

[0130] Where P is the pressure of the gas, R is the ideal gas constant, 8.314 J / (mol·K), T is the temperature, V is the volume of the gas, and b is a constant related to the molecular volume. k is a constant related to the properties of the gas, T c is the critical temperature.

[0131] S303: Compare the calculated density of LNG with the liquid density of LNG and the standard density of LNG measured by the liquid phase flowmeter. Generally speaking, the liquid density of LNG measured by the liquid phase flowmeter is greater than the calculated density of LNG. When the difference between the calculated density of LNG and the liquid density of LNG and the standard density of LNG measured by the liquid phase flowmeter is within the preset range, it proves that the quality of the liquid in the pipeline has met the requirements, and the large circulation can be stopped and filling can be started, thereby dynamically adjusting the pre-cooling time, shortening the large circulation time, improving the filling efficiency, and providing more filling services for vehicles.

[0132] like Figure 7 As shown in the figure, the blue line is the LNG liquid density measured by the liquid flow meter, the orange line is the LNG calculated density, the horizontal axis is time, and the vertical axis is density. This method can greatly shorten the cycle precooling time, especially when continuously refueling the vehicle.

[0133] The gaseous and liquid densities of LNG at different temperatures and pressures are shown in the following table:

[0134] Liquidus temperature K Gas phase temperature K Liquid pressure Bar Gas phase pressure Bar Liquid density Gas phase density 112.25 112.25 1.1013 1.1013 430.48 2.0558 116.31 116.31 1.5 1.5 424.53 2.7105 120.37 120.37 2 2 418.46 3.5121 …… …… …… …… …… …… 153.61 153.61 12 12 361.16 18.955 154.57 154.57 12.5 12.5 359.21 19.753 155.51 155.51 13 13 357.28 20.557

[0135] Furthermore, the premise for accurate metering of the LNG filling machine is that the internal filling flowmeter reaches the preset value, that is, the temperature reaches -110℃~-162℃, the density reaches 0.335~0.4kg / L, and the gain needs to be 0~10. The liquid can detect the above three data through the liquid phase flowmeter on the liquid inlet pipeline, but because the LNG liquid absorbs heat when it returns to the return air pipeline through the filling hose, its temperature, density and gain will change. If the precooling is not sufficient during the cycle precooling process, any of the three values ​​cannot reach the preset value, and the quality of the LNG entering the vehicle is low. Therefore, by estimating the density of the LNG in the return air pipe, the calculated density of LNG is obtained, and then the calculated density of LNG is compared with the LNG liquid density measured by the liquid phase flowmeter and the standard density of LNG in the current state to ensure that the LNG in the return air pipe can meet the requirements, thereby improving the filling quality of the LNG filling machine.

[0136] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A non-destructive detection method for large-cycle liquid theft in an LNG liquid filling machine, characterized in that: The following steps are involved: Step 1: Install temperature sensors on the outer surfaces of the liquid inlet pipe and the return air pipe of the LNG filling machine, and connect the temperature sensors to the LNG filling control board; the LNG filling control board has a controller, and the algorithm of the software in the controller can determine whether the filling gun of the LNG filling machine is directly connected to the vehicle filling port; Step 2: Based on the result obtained in step 1, choose whether to close the filling valve of the LNG filling machine. If it is determined that the filling gun of the LNG filling machine is directly inserted into the vehicle filling port, then close the filling valve of the LNG filling machine; otherwise, do not close it.

2. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 1 is characterized by: The software's algorithms include: S1: The temperature distribution law of the air side of the liquid inlet pipe and the return pipe of the LNG charging machine is obtained by calculation, and the temperature change rate of the air side of the liquid inlet pipe and the return pipe is obtained according to the temperature distribution law of the air side of the liquid inlet pipe and the return pipe; S2: Compare the temperature change rates of the air side of the liquid inlet pipe and the liquid return pipe. If the temperature change rate of the air side of the liquid return pipe is inconsistent with the temperature change rate of the air side of the liquid inlet pipe, it means that the filling gun of the LNG filling machine is directly connected to the vehicle filling port. At this time, close the filling valve of the LNG filling machine and stop filling LNG.

3. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 2 is characterized by: The change rate comparison value ε is used to determine whether the temperature change rate of the air side of the return pipe is consistent with the temperature change rate of the air side of the inlet pipe. When ε> the set value, the temperature change rate of the air side of the return pipe is inconsistent with the temperature change rate of the air side of the inlet pipe. Otherwise, they are consistent. The calculation formula of the change rate comparison value ε is: in, is the rate of change of the temperature on the air side of the inlet pipe, is the rate of change of temperature on the air side of the return pipe.

4. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 2 is characterized by: Step S1 specifically includes: S101: Establish a pipeline heat transfer model and simplify the model to obtain a one-dimensional unsteady-state heat conduction differential equation without internal heat source: in, represents the rate of change of temperature with time, represents the rate of change of temperature with position, λ is the thermal conductivity of the pipe, ρ is the density of the pipe material, c p is the specific heat capacity of the pipeline material; S102: Meshing the pipe walls of the liquid inlet pipe and the air return pipe; S103: According to Fourier's law and Newton's cooling formula, the following equation is obtained: LNG side equation: Inferred: Air side equation: Inferred: Among them, h l is the surface convection coefficient of LNG, h a is the natural convection coefficient of air; is the rate of change of LNG side temperature with position, T l is the liquid temperature, T w is the tube wall temperature, is the rate of change of air side temperature with position, T a is the air temperature; S104: Discretize and solve equations 1, 3, and 5 to obtain the pipe wall temperature T w , tube wall temperature T w is the temperature distribution along the wall thickness direction of the tube wall. Therefore, according to the tube wall temperature T w The temperature distribution law on the air side of the pipeline is obtained, and the temperature change rate on the air side of the liquid inlet pipe is calculated according to the temperature distribution law on the air side of the liquid inlet pipe; S105: Calculate the mean square error between the temperature on the air side of the liquid inlet pipe and the temperature collected by the temperature sensor on the liquid inlet pipe, perform multiple iterative calculations to make the mean square error meet the set value, and then fit the voltage value of the temperature sensor on the liquid inlet pipe and the LNG liquid temperature in the liquid inlet pipe to obtain a fitting function of the voltage value of the temperature sensor and the LNG liquid temperature, that is, U k =f(T LNG ); According to U k =f(T LNG ) and the voltage value of the temperature sensor on the return air duct, the temperature distribution law in the return air duct is obtained, so as to obtain the temperature distribution law on the return air duct wall and the temperature distribution law on the air side of the return air duct according to steps S103 and S104; the temperature change rate on the air side of the return air duct is calculated according to the temperature distribution law on the air side of the return air duct.

5. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 4 is characterized by: In step S105, the mean square error is calculated according to the following formula: Among them, T c is the air side temperature of the liquid inlet pipe, T t It is the temperature collected by the temperature sensor on the liquid inlet pipe.

6. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 1 is characterized by: A liquid flow meter is installed on the liquid inlet pipeline of the LNG charging machine, and a gas flow meter is installed on the gas return pipeline; The algorithm in the software of the controller can also determine whether precooling is complete; The software's algorithms also include: S3: According to the temperature and pressure inside the return air pipe, the density of LNG is estimated to obtain the calculated density of LNG, and then the calculated density of LNG is compared with the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG under the current state. When the difference between the calculated density of LNG and the liquid density of LNG measured by the liquid phase flowmeter and the standard density of LNG is within the preset range, it means that precooling is completed, the large circulation is stopped, and the vehicle is started to be filled with LNG. Otherwise, it is necessary to continue the large circulation until the precooling is completed.

7. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 6 is characterized by: Step S3 specifically includes: S301: Measure the LNG liquid density by the liquid phase flowmeter on the liquid inlet pipe; S302: according to the temperature and pressure inside the gas return pipe, the density of the LNG is estimated by using the real gas equation to obtain the calculated density of the LNG; S303: Compare the calculated density of LNG with the LNG liquid density and the LNG standard density measured by the liquid phase flow meter. When the difference between the calculated density of LNG and the LNG liquid density and the LNG standard density measured by the liquid phase flow meter is within a preset range, stop the large circulation and start filling the vehicle with LNG.

8. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 7 is characterized by: The real gas equation in step S302 is: Among them, P is the pressure of the gas, R is the ideal gas constant, T is the temperature, V is the volume of the gas, and b is a constant related to the molecular volume. k is a constant related to the properties of the gas, T c is the critical temperature.

9. The nondestructive detection method for large-cycle liquid theft of LNG liquid filling machine according to claim 1 is characterized by: The temperature sensor adopts a K-type thermocouple. K-type thermocouple 01 is bonded on the outer surface of the liquid inlet pipe, and K-type thermocouple 02 is bonded on the outer surface of the air return pipe.

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