A nondestructive testing method for large circulation liquid theft of LNG filling machine

Abstract

The present application relates to the technical field of liquid filling machine, aiming at solving the problems of the prior art that the filling gun is directly inserted into the filling vehicle for pre-cooling, unmeasured liquid LNG enters the vehicle to cause site loss, and the cost of installing sensing elements on the gas return pipe of the LNG liquid filling machine is high, and provides a nondestructive testing method for large cycle liquid theft of LNG liquid filling machine, comprising the following steps: step one: installing temperature sensors on the outer surfaces of the liquid inlet pipe and the gas return pipe of the LNG liquid filling machine, and connecting 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 liquid filling machine is directly connected to the vehicle receiving nozzle; step two: according to the result obtained in step one, it is determined whether to close the filling valve of the LNG liquid filling machine, if it is determined that the filling gun of the LNG liquid filling machine is directly inserted into the vehicle receiving nozzle, the filling valve of the LNG liquid filling machine is closed, otherwise, it is not closed.

Description

Technical Field

[0001] This invention relates to the field of LNG dispenser technology, and more specifically, to a non-destructive testing method for LNG dispenser large-circulation LNG theft. Background Technology

[0002] Currently, LNG dispensers on the market are all developed in accordance with the standards JJF1524-2015 "Type Evaluation Outline for Liquefied Natural Gas Dispensers", JJG1114-2015 "Verification Procedure for Liquefied Natural Gas Dispensers", GB / T36126-2018 "Liquefied Natural Gas Dispensers for Automobiles" and GB / T 41319-2022 "Liquefied Natural Gas (LNG) Dispensing Devices".

[0003] The standard provides a block diagram illustrating the principle of the LNG dispenser, but it does not specify or show all the details, such as whether pressure transmitters and temperature transmitters are installed, their installation locations, and the number of transmitters.

[0004] The process flow diagram of LNG dispensers in the market is as follows: Figure 1 As shown in the flowchart, the refueling function can be completed safely and accurately at strictly managed sites. However, this process can cause losses to the refueling site if operators are negligent: the economic losses of the refueling site are mainly due to the large-loop process in the LNG refueling process.

[0005] The extreme temperature of liquid LNG is -162℃. If the LNG dispenser is not used for an extended period, the residual liquid LNG in the pipeline will rapidly vaporize. To ensure accurate metering during dispensing, the station must fully pre-cool the liquid flow meter before dispensing vehicles. This involves replacing the fuel in the liquid flow meter, SV01 valve, and dispensing nozzle with liquid LNG. The standard procedure at the station is to first insert the dispensing nozzle into the LNG dispenser's nozzle holder, then open the SV01 valve, allowing liquid LNG to flow through the liquid flow meter, SV01, dispensing nozzle, nozzle holder, and gas pipeline back to the LNG storage tank at the refueling station. Once this circulation loop has been sufficiently pre-cooled, the LNG dispenser stops pre-cooling, and the operator then inserts the dispensing nozzle into the vehicle to begin dispensing.

[0006] If station staff are negligent and insert the refueling nozzle into the refueling vehicle before pre-cooling, unmeasured liquid LNG will enter the vehicle, causing losses to the station. Because the pipeline from the nozzle to the LNG storage tank lacks temperature, pressure, or flow sensing components, such losses are difficult to automatically correct and prevent.

[0007] Currently, there are a large number of LNG stations in the market (more than a thousand). It is extremely costly to damage the return gas pipelines of the LNG dispensers. Therefore, a non-destructive method for detecting LNG theft is needed. Summary of the Invention

[0008] The present invention aims to provide a non-destructive detection method for LNG theft in the large circulation of LNG dispensers, in order to solve the problems in the prior art where unmetered liquid LNG will enter the dispenser vehicle when the staff directly inserts the dispensing nozzle into the dispenser vehicle and then pre-cools it, resulting in losses to the site, and the high cost of damagingly installing pressure, temperature or flow sensing elements on the return gas pipe of the operating LNG dispenser.

[0009] This invention is achieved using the following technical solution:

[0010] This invention provides a non-destructive testing method for LNG theft in a large-circulation LNG dispenser, comprising the following steps:

[0011] Step 1: Install temperature sensors on the outer surfaces of the LNG inlet pipe and return pipe of the LNG dispenser, and connect the temperature sensors to the LNG dispensing control board; the LNG dispensing control board contains a controller, and the algorithm in the software of the controller can determine whether the LNG dispenser's dispensing gun is directly connected to the vehicle's dispensing port.

[0012] Step 2: Based on the results obtained in Step 1, choose whether to close the LNG dispenser's filling valve. If it is determined that the LNG dispenser's filling nozzle is directly inserted into the vehicle's filling port, then close the LNG dispenser's filling valve; otherwise, do not close it.

[0013] As a preferred technical solution:

[0014] The software's algorithms include:

[0015] S1: The temperature distribution law of the air side of the LNG dispenser's inlet pipe and return pipe is obtained by calculation, and the temperature change rate of the air side of the inlet pipe and return pipe is obtained based on the temperature distribution law of the air side of the inlet pipe and return pipe.

[0016] S2: Compare the temperature change rates on the air side of the inlet pipe and the return pipe. If the temperature change rate on the air side of the return pipe is inconsistent with that on the air side of the inlet pipe, it indicates that the LNG dispenser's filling gun is directly connected to the vehicle's filling port. In this case, close the LNG dispenser's filling valve and stop filling LNG.

[0017] The inconsistency between the air-side temperature change rate of the return pipe and the air-side temperature change rate of the inlet pipe refers to the inconsistent trends of the two data curves.

[0018] As a preferred technical solution:

[0019] To further clarify the comparison of the air-side temperature change rates of the inlet and outlet pipes, the temperature change rate of the outlet pipe is determined to be consistent with that of the inlet pipe based on the comparison value ε. When ε > the set value, the temperature change rate of the outlet pipe is inconsistent with that of the inlet pipe; otherwise, they are consistent.

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

[0021]

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

[0023] As a preferred technical solution:

[0024] Step S1 specifically includes:

[0025] S101: Establish a pipe heat transfer model, and obtain a one-dimensional unsteady heat conduction differential equation without an internal heat source by simplifying the model:

[0026]

[0027] in, This represents the rate of change of temperature over time. The coefficient of performance is λ, where λ represents the rate of temperature change with position, ρ is the thermal conductivity of the pipe, and c is the density of the pipe material. p The specific heat capacity of the pipe material;

[0028] S102: Grid division of the pipe walls of the liquid inlet pipe and the gas return pipe;

[0029] S103: Based on Fourier's law and Newton's law of cooling, the following equation is obtained:

[0030] LNG-side equations:

[0031]

[0032] Inferred:

[0033]

[0034] Air-side equations:

[0035]

[0036] Inferred:

[0037]

[0038] Among them, h lh is the surface convection coefficient of LNG. a The natural convection coefficient of air; T represents the rate of change of LNG-side temperature with location. l T represents the liquid temperature. w For pipe wall temperature, T represents the rate of change of air temperature with position. a Air temperature;

[0039] S104: Discretize and solve equations 1, 3, and 5 to obtain the pipe wall temperature T. w Pipe wall temperature T w This represents the temperature distribution along the pipe wall thickness; therefore, based on the pipe wall temperature T... w The temperature distribution pattern 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 based on the temperature distribution pattern on the air side of the liquid inlet pipe.

[0040] S105: Calculate the root mean square error between the air-side temperature of the inlet pipe and the temperature collected by the temperature sensor on the inlet pipe. Perform multiple iterative calculations to ensure the root mean square error meets the set value. Then, fit the voltage value of the temperature sensor on the inlet pipe and the LNG liquid temperature inside the inlet pipe to obtain the fitting function between the voltage value of the temperature sensor and the LNG liquid temperature, i.e., U. k =f(T) LNG According to U k =f(T) LNG Based on the same changing pattern and the voltage value of the temperature sensor on the return pipe, the temperature distribution pattern inside the return pipe is obtained. Thus, according to steps S103 and S104, the temperature distribution pattern on the wall of the return pipe and the temperature distribution pattern on the air side of the return pipe are obtained. The temperature change rate on the air side of the return pipe is calculated based on the temperature distribution pattern on the air side of the return pipe.

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

[0042] As a preferred technical solution:

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

[0044]

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

[0046] As a preferred technical solution:

[0047] A liquid flow meter is installed on the inlet pipe of the LNG dispenser, and a gas flow meter is installed on the return gas pipe.

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

[0049] The software's algorithms also include:

[0050] S3: Based on the temperature and pressure inside the return gas pipe, estimate the density of LNG to obtain the calculated LNG density. Then compare the calculated LNG density with the LNG liquid density measured by the liquid flow meter and the standard LNG density under the current conditions. When the difference between the calculated LNG density and the LNG liquid density measured by the liquid flow meter and the standard LNG density is within the preset range, it means that the pre-cooling is completed, the large circulation stops, and LNG is added to the vehicle. Otherwise, the large circulation needs to continue until the pre-cooling is completed.

[0051] As a preferred technical solution:

[0052] Step S3 specifically includes:

[0053] S301: The density of LNG liquid is measured by the liquid phase flow meter on the inlet pipe;

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

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

[0056] As a preferred technical solution:

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

[0058]

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

[0060] As a preferred technical solution:

[0061] The above real gas equation is just one example, and is not limited to the real gas equation mentioned above; other real gas equations are also possible.

[0062] As a preferred technical solution:

[0063] The temperature sensor uses a type K thermocouple. Type K thermocouple 01 is bonded to the outer surface of the liquid inlet pipe, and type K thermocouple 02 is bonded to the outer surface of the gas return pipe.

[0064] As a preferred technical solution:

[0065] Temperature sensors are not limited to type K thermocouples; other thermocouples or resistance temperature detectors (RTDs) that can measure temperature can also be used. The choice 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. This invention features a non-destructive LNG dispenser circulation testing function, eliminating the need for operators to insert the dispensing nozzle into the vehicle's receiving port for pre-cooling, which could lead to unmetered LNG directly entering the vehicle and causing losses to the dispensing station. Furthermore, this non-destructive LNG dispenser circulation testing function significantly reduces the retrofitting costs of existing equipment. Destructive retrofitting not only requires the station to cease operations, but also necessitates non-destructive testing of the retrofitted site, with the costs of shutdown and testing far exceeding the cost of this technology.

[0068] 2. In this invention, Fourier's law and Newton's cooling formula are used in the software to obtain the temperature change rate of the air side of the inlet pipe and the return pipe. This invention determines whether there is a liquid flow channel by comparing the temperature change rate of the air side of the inlet pipe and the return pipe. 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 there is a situation where the LNG dispenser's filling gun is directly connected to the vehicle's filling port. At this time, the dispensing valve of the dispenser is closed and the filling of LNG is stopped.

[0069] 3. This invention calculates the LNG density based on the temperature and pressure of the return gas pipe and uses the real gas equation to estimate the LNG density. Then, it compares the calculated LNG density with the LNG liquid density measured by the liquid flow meter and the standard LNG density under the current conditions. When the difference between the calculated LNG density and the LNG liquid density measured by the liquid flow meter and the standard LNG density 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 refueling can begin. This dynamically adjusts the precooling time, shortens the large circulation time, and improves refueling efficiency.

[0070] 4. This invention estimates the density of LNG and compares the calculated LNG density with the LNG liquid density measured by the liquid phase flow meter and the standard LNG density under the current conditions to ensure that the LNG in the return gas pipe meets the requirements, thereby improving the filling quality of the LNG dispenser.

[0071] 5. In this invention, a type K thermocouple is bonded to the outside of the inlet pipe and return pipe of the LNG dispenser. It has a fast response speed and a wide temperature measurement range, with better applicability in the range of -162℃ to 55℃. Attached Figure Description

[0072] Figure 1 This is a schematic diagram of the principle of an existing LNG dispenser.

[0073] Figure 2 This is a process flow diagram of an existing LNG dispenser.

[0074] Figure 3 This is a flowchart illustrating the process flow of the non-destructive testing method for LNG dispenser large-circulation liquid theft described in this invention.

[0075] Figure 4 This is a schematic diagram of the pipe heat transfer model described in this invention.

[0076] Figure 5 for Figure 4 Enlarged diagram of point A in the middle.

[0077] Figure 6 This is a schematic diagram of the model after mesh division according to the present invention.

[0078] Figure 7 This is a comparison chart of liquid densities. Detailed Implementation

[0079] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0080] Example 1

[0081] like Figure 3 As shown in the figure, this embodiment proposes a non-destructive testing method for LNG theft in the large circulation system of an LNG dispenser, including the following steps:

[0082] Step 1: Attach K-type thermocouples to the outer surfaces of the LNG dispenser's inlet and return pipes, and connect the K-type thermocouples to the LNG refueling control board; a liquid flow meter is installed on the LNG dispenser's inlet pipe, and a gas flow meter is installed on the LNG dispenser's return pipe; the LNG refueling control board contains a controller, which is used to analyze, calculate, and process the received signals. The algorithm in the controller's software can determine whether the LNG dispenser's refueling gun is directly connected to the vehicle's refueling port and whether pre-cooling is complete;

[0083] Step 2: Based on the results obtained in Step 1, choose whether to close the LNG dispenser's filling valve. If it is determined that the LNG dispenser's filling nozzle is directly inserted into the vehicle's filling port, then close the LNG dispenser's filling valve; otherwise, do not close it.

[0084] Based on the results obtained in step one, if precooling is complete, the large circulation should be stopped and LNG should be added to the vehicle. Otherwise, the large circulation should continue until precooling is complete.

[0085] The K-type thermocouple, which is attached to the outer surface of the pipeline, receives the heat conducted by the LNG inside the pipeline along the pipeline wall. At the same time, the K-type thermocouple also receives the heat conducted by the environment. Since the K-type thermocouple is connected to the LNG refueling control board, the temperature data collected by the K-type thermocouple is transmitted to the LNG refueling control board.

[0086] This invention uses a type K thermocouple, but is not limited to type K thermocouples. Other thermocouples or resistance temperature detectors (RTDs) capable of measuring temperature can also be used, and the choice can be made based on the applicable temperature range and price of the product. In this embodiment, a type K thermocouple is preferred because it has a faster response speed and a wider temperature measurement range than PT100 / 1000 RTDs, and is more suitable for use in the range of -162℃ to 55℃.

[0087] In this embodiment, a type K thermocouple 01 is bonded to the outside of the liquid inlet pipe, and a type K 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 LNG dispenser's inlet pipe and return pipe is obtained by calculation, and the temperature change rate of the air side of the inlet pipe and return pipe is obtained based on the temperature distribution law of the air side of the inlet pipe and return pipe.

[0090] Step S1 includes:

[0091] S101: As Figure 4 As shown, a heat transfer model for the pipeline is established, such as... Figure 5 As shown, a small portion of the pipe is magnified locally; by simplifying the model, a one-dimensional unsteady heat conduction differential equation without an internal heat source is obtained:

[0092]

[0093] in, This represents the rate of change of temperature over time. The coefficient of performance is λ, where λ represents the rate of temperature change with position, ρ is the thermal conductivity of the pipe, and c is the density of the pipe material. p This refers to the specific heat capacity of the pipe material.

[0094] Taking 06Cr19Ni10 stainless steel as an example, its thermal conductivity is 15 W·m. -1 ·K -1 Specific heat capacity is 500 J·kg -1 ·K -1 Its density is approximately 7900 kg·m³. -3 .

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

[0096] S103: Based on Fourier's law and Newton's law of cooling, the following equation is obtained:

[0097] Left-side LNG equation:

[0098]

[0099] Inferred:

[0100]

[0101] Right-side air equation:

[0102]

[0103] Inferred:

[0104]

[0105] Among them, h l The surface convection coefficient of LNG is approximately 80 W·m⁻¹. -2 ·K -1 h a The natural convection coefficient of air is approximately 5 W·m. -2 ·K -1 ; T represents the rate of change of LNG-side temperature with location. l T represents the liquid temperature. w For pipe wall temperature, T represents the rate of change of air temperature with position. a This refers to the air temperature.

[0106] air temperature T a and liquid temperature T l The pipe wall temperature T can be obtained through a temperature sensor. wIt is the temperature distribution along the pipe wall thickness direction. It varies along the pipe wall thickness direction. After dividing the pipe wall into grids, each grid has its corresponding temperature.

[0107] S104: Discretize and solve equations 1, 3, and 5 together to obtain the pipe wall temperature T at different times. w This means that by obtaining the temperature distribution patterns on the walls of the inlet and outlet pipes, we can obtain the temperature distribution patterns on the air side of the pipeline. The air side temperature distribution pattern refers to the temperature distribution on the outer layer of the pipe wall. Based on the air side temperature distribution pattern of the inlet pipe, the rate of change of the air side temperature of the inlet pipe can be calculated.

[0108] S105: Obtain the temperature distribution pattern on the wall of the return pipe, the temperature distribution pattern on the air side of the return pipe, and the temperature change rate on the air side of the return pipe.

[0109] Step S105 specifically includes:

[0110] The air-side temperature T of the 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 inlet pipe t The mean squared error between them;

[0111] σ=0.5(T c -T t ) 2

[0112] Multiple iterative calculations were performed to ensure the mean square error met the set value. Then, the voltage value of the K-type thermocouple on the inlet pipe and the LNG liquid temperature inside the inlet pipe were fitted to obtain the fitting function between the K-type thermocouple voltage value and the LNG liquid temperature, i.e., U. k =f(T) LNG ).

[0113] According to U k =f(T) LNG By using the same variation pattern and the voltage value of the K-type thermocouple on the return pipe, the temperature distribution pattern inside the return pipe is obtained, and thus the temperature distribution pattern on the pipe wall and the temperature distribution pattern on the air side of the return pipe are obtained according to steps S103 and S104.

[0114] In the return gas pipe of the LNG dispenser, the liquid LNG absorbs heat from the surrounding environment and heats up, causing the temperature collected by the K-type thermocouple attached to the return gas pipe to be unreliable. However, the voltage value of the K-type thermocouple at this location can be obtained. Therefore, according to U k =f(T) LNG The same variation pattern was used to solve for the temperature distribution patterns inside the return pipe, on the pipe wall, and on the air side of the return pipe.

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

[0116] This step can be further explained as follows: Based on the temperature of the liquid LNG, the ambient temperature, and the thermal conductivity, the temperature change of the pipe wall can be obtained when different LNG liquids flow through it. The temperature change of the outer layer of the pipe wall can be detected by thermocouples. In the gas phase pipe, the temperature change of the liquid inside the pipe is unknown, but the change in the gas phase thermocouple voltage can be detected. The temperature change law of the pipe with the internal liquid is known, so the temperature change law inside the pipe can be deduced. Therefore, following 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 rate of temperature change on the air side of the inlet and outlet pipes. To determine if there is a liquid flow path, if the temperature change rate on the air side of the return pipe is inconsistent with the temperature change rate on the air side of the inlet pipe, it indicates that the LNG dispenser's filling gun is directly connected to the vehicle's filling port. In this case, close the LNG dispenser's filling valve and stop filling LNG.

[0118] The flow velocity v, mass m, and density ρ of the LNG liquid are measured by a liquid flow meter on the inlet pipe. Given the length L of the inlet pipe, the time t required for the LNG liquid to fill that section of the inlet pipe can be calculated. Therefore, the temperature of the return pipe will change no later than time t. Since the temperature fluctuates, the rate of change of temperature over time is used for comparison.

[0119] The inconsistency between the air-side temperature change rate of the return pipe and the air-side temperature change rate of the inlet pipe refers to the inconsistent trends of the two data curves.

[0120] To further clarify the comparison of the air-side temperature change rates of the inlet and outlet pipes, the comparison value ε can be used to determine whether the air-side temperature change rate of the outlet pipe is consistent with that of the inlet pipe. When ε > the set value, the air-side temperature change rate of the outlet pipe is inconsistent with that of the inlet pipe; otherwise, they are consistent.

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

[0122]

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

[0124] S3: Based on the temperature and pressure inside the return gas pipe, the density of LNG is estimated to obtain the calculated LNG density. Then, the calculated LNG density is compared with the LNG liquid density measured by the liquid flow meter and the standard LNG density under the current conditions. When the difference between the calculated LNG density and the LNG liquid density measured by the liquid flow meter and the standard LNG density 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 the filling can be started. This dynamically adjusts the precooling time, shortens the large circulation time, and improves the filling efficiency.

[0125] Step S3 includes:

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

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

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

[0129]

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

[0131] S303: Compare the calculated LNG density with the LNG liquid density and LNG standard density measured by the liquid flow meter. Generally, the LNG liquid density measured by the liquid flow meter is greater than the calculated LNG liquid density. When the difference between the calculated LNG density and the LNG liquid density and LNG standard density measured by the liquid flow meter 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 refueling can begin. This allows for dynamic adjustment of the precooling time, shortening of the large circulation time, and improvement of refueling efficiency, enabling more vehicles to be refueled.

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

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

[0134] 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, accurate metering by the LNG dispenser relies on the internal flow meter reaching preset values: a temperature of -110℃ to -162℃, a density of 0.335 to 0.4 kg / L, and a gain of 0 to 10. While the liquid phase flow meter on the inlet pipe detects these three values, the LNG absorbs heat as it returns to the return pipe via the dispensing hose, causing changes in temperature, density, and gain. If insufficient pre-cooling is not achieved during the circulation process, none of these three values ​​will reach the preset threshold, resulting in low-quality LNG entering the vehicle. Therefore, by estimating the LNG density in the return pipe, a calculated LNG density is obtained. This calculated density is then compared with the LNG liquid density measured by the liquid phase flow meter and the standard LNG density under current conditions to ensure the LNG in the return pipe meets the requirements, thereby improving the dispensing quality of the LNG dispenser.

[0136] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A non-destructive testing method for LNG dispenser large-circulation liquid theft, characterized in that: Includes the following steps: Step 1: Install temperature sensors on the outer surfaces of the LNG inlet pipe and return pipe of the LNG dispenser, and connect the temperature sensors to the LNG dispensing control board; the LNG dispensing control board contains a controller, and the algorithm in the software of the controller can determine whether the LNG dispenser's dispensing gun is directly connected to the vehicle's dispensing port. Step 2: Based on the results obtained in Step 1, choose whether to close the LNG dispenser's filling valve. If it is determined that the LNG dispenser's filling nozzle is directly inserted into the vehicle's filling port, then close the LNG dispenser's filling valve; otherwise, do not close it. The software's algorithms include: S1: The temperature distribution law of the air side of the LNG dispenser's inlet pipe and return pipe is obtained by calculation, and the temperature change rate of the air side of the inlet pipe and return pipe is obtained based on the temperature distribution law of the air side of the inlet pipe and return pipe. S2: Compare the temperature change rates on the air side of the inlet pipe and the return pipe. If the temperature change rate on the air side of the return pipe is inconsistent with the temperature change rate on the air side of the inlet pipe, it means that the LNG dispenser's filling gun is directly connected to the vehicle's filling port. At this time, close the LNG dispenser's filling valve and stop filling LNG. Step S1 specifically includes: S101: Establish a pipe heat transfer model, and obtain a one-dimensional unsteady heat conduction differential equation without an internal heat source by simplifying the model: (Equation 1) in, This represents the rate of change of temperature over time. This represents the rate of change of temperature with respect to location. The thermal conductivity of the pipe, The density of the pipe material, The specific heat capacity of the pipe material; S102: Grid division of the pipe walls of the liquid inlet pipe and the gas return pipe; S103: Based on Fourier's law and Newton's law of cooling, the following equation is obtained: LNG-side equations: (Equation 2) Inferred: (Equation 3) Air-side equations: (Equation 4) Inferred: (Equation 5) in, The surface convection coefficient of LNG. The natural convection coefficient of air; This represents the rate of change of LNG-side temperature with location. For liquid temperature, For pipe wall temperature, The rate of change of air temperature with position. Air temperature; S104: Discretize and solve equations 1, 3, and 5 to obtain the pipe wall temperature. Pipe wall temperature This refers to the temperature distribution along the pipe wall thickness; therefore, based on the pipe wall temperature... The temperature distribution pattern 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 based on the temperature distribution pattern on the air side of the liquid inlet pipe. S105: Calculate the root mean square error between the air-side temperature of the inlet pipe and the temperature collected by the temperature sensor on the inlet pipe. Perform multiple iterative calculations to ensure the root mean square error meets the set value. Then, fit the voltage value of the temperature sensor on the inlet pipe and the LNG liquid temperature inside the inlet pipe to obtain the fitting function between the voltage value of the temperature sensor and the LNG liquid temperature. ;according to Based on the same variation pattern and the voltage value of the temperature sensor on the return pipe, the temperature distribution pattern inside the return pipe is obtained. Thus, according to steps S103 and S104, the temperature distribution pattern on the wall of the return pipe and the temperature distribution pattern on the air side of the return pipe are obtained. The temperature change rate on the air side of the return pipe is calculated based on the temperature distribution pattern on the air side of the return pipe.

2. The non-destructive testing method for LNG dispenser large-circulation liquid theft according to claim 1, characterized in that: Based on the comparison value of the rate of change To determine whether the rate of temperature change on the air side of the return pipe is consistent with the rate of temperature change on the air side of the inlet pipe, when When the set value is greater than the set value, the temperature change rate on the air side of the return pipe is inconsistent with the temperature change rate on the air side of the inlet pipe; otherwise, they are consistent. Comparison of rates of change The calculation formula is: in, It is the rate of change of temperature on the air side of the inlet pipe. It is the rate of change of air temperature on the return air pipe.

3. The non-destructive testing method for LNG dispenser large-circulation liquid theft according to claim 1, characterized in that: In step S105, the root mean square error is calculated according to the following formula: in, The air-side temperature of the inlet pipe. The temperature is the temperature collected by the temperature sensor on the inlet pipe.

4. The non-destructive testing method for LNG dispenser large-circulation liquid theft according to claim 1, characterized in that: A liquid flow meter is installed on the inlet pipe of the LNG dispenser, and a gas flow meter is installed on the return gas pipe. The algorithm in the controller's software can also determine whether precooling is complete; The software's algorithms also include: S3: Based on the temperature and pressure inside the return gas pipe, estimate the density of LNG to obtain the calculated LNG density. Then compare the calculated LNG density with the LNG liquid density measured by the liquid flow meter and the standard LNG density under the current conditions. When the difference between the calculated LNG density and the LNG liquid density measured by the liquid flow meter and the standard LNG density is within the preset range, it means that the pre-cooling is completed, the large circulation stops, and LNG is added to the vehicle. Otherwise, the large circulation needs to continue until the pre-cooling is completed.

5. The non-destructive testing method for LNG dispenser large-circulation liquid theft according to claim 4, characterized in that: Step S3 specifically includes: S301: The density of LNG liquid is measured by the liquid phase flow meter on the inlet pipe; S302: Based on the temperature and pressure inside the return gas pipe, the density of LNG is estimated using the real gas equation to obtain the calculated density of LNG; S303: Compare the calculated LNG density with the LNG liquid density and LNG standard density measured by the liquid flow meter. If the difference between the calculated LNG density and the LNG liquid density and LNG standard density measured by the liquid flow meter is within the preset range, stop the large circulation and start refueling the vehicle with LNG.

6. The non-destructive testing method for LNG dispenser large-circulation liquid theft according to claim 1, characterized in that: The temperature sensor uses a type K thermocouple. A type K thermocouple (01) is bonded to the outer surface of the liquid inlet pipe, and a type K thermocouple (02) is bonded to the outer surface of the gas return pipe.

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