Experimental system and method for diffusion of leaked natural gas along soil-adjacent underground limited space

Abstract

The invention discloses an experiment system and method for diffusion of leaked natural gas along a soil-adjacent underground limited space. According to the experiment, a full-scale experiment is adopted, and a natural gas inspection well and a valve inspection well are arranged around a buried pipeline to simulate a real urban underground environment. A certain number of methane concentration sensors are laid at specific positions around leakage holes of a buried pipeline and in an inspection well and used for monitoring diffusion concentration of natural gas in an underground soil area and in the inspection well at different time periods and different positions. According to data obtained through experiments, on one hand, the relational expression of the diffusion distance of natural gas in soil after leakage and the time required for reaching the saturation state can be obtained; on the other hand, an experimental model and verification data are provided for theoretical research and simulation research.

Description

Technical Field

[0001] The invention relates to an experimental system and method for gas leakage and diffusion, in particular to an experimental system and method for leakage of natural gas along soil-adjacent underground limited space. Background Art

[0002] Direct buried natural gas pipelines in some cities have been in service for a long time, and natural gas leakage accidents caused by corrosion, aging, third-party damage and other factors frequently occur. At the same time, direct buried natural gas pipelines in cities intersect or are close to underground spaces such as various pipeline inspection wells, rainwater and sewage pipes, and thermal pipe trenches. Once a direct buried natural gas pipeline leaks, natural gas can easily spread along soil cracks to these limited underground spaces and accumulate in concentration. Once the concentration reaches the explosion limit, it will cause explosions, fires and other accidents when it encounters a fire source.

[0003] CN114239193A proposes a pipeline leakage simulation method. By establishing a three-dimensional numerical model of a natural gas pipeline, it simulates the physical process of natural gas leakage caused by third-party damage to the urban natural gas pipeline diffusing into the atmosphere, and considers the influence of different leakage locations, wind directions, etc. on the distribution of natural gas concentration in the atmosphere. CN115791530A proposes a concentration prediction method for the diffusion of buried gas pipeline leakage to adjacent underground spaces. By analyzing the natural gas soil diffusion process of different ground covering media, the formula of natural gas concentration and distance in the soil is calculated using the steam intrusion theory, and the diffusion radius of natural gas in the soil is predicted.

[0004] Although the above two patents have calculated the diffusion concentration distribution along the atmosphere and soil after natural gas leakage from a theoretical level, which is a big step forward in predicting the consequences of natural gas pipeline leakage. However, there are certain limitations to the research on leakage and diffusion of natural gas pipelines caused by third-party damage. In actual projects, natural gas pipelines often leak due to small holes caused by corrosion, which are not easy to be found in the early stage and easily cause concentration accumulation in adjacent underground limited spaces. At the same time, the steam intrusion theory used in the second patent has yet to be experimentally verified, and when a real accident occurs, the diffusion time is unknown and the applicable conditions of the formula are limited. Both are theoretical calculations, lacking experimental verification, and cannot truly reflect the diffusion of natural gas in the soil and adjacent underground limited spaces after leakage. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and to provide an experimental system and method for the diffusion of leaked natural gas along the soil-adjacent underground limited space. Through this experiment, on the one hand, the relationship between the diffusion distance of natural gas in the soil after leakage and the time required to reach the saturation state can be obtained; on the other hand, an experimental model and verification data can be provided for theoretical research and simulation research.

[0006] The present invention discloses an experimental device for leaking natural gas and diffusing it along a soil-adjacent underground limited space, comprising a gas storage tank, wherein the gas storage tank is connected to a high-pressure gas cylinder via a gas transmission line equipped with a delivery valve, a first pressure gauge is installed on the gas storage tank, and an outlet of the gas storage tank is sequentially connected to a stop valve, a first-level electric regulating valve, a second-level electric regulating valve, a flow meter, and a second pressure gauge via a natural gas pipeline, and then connected to a gas collecting tank through an experimental soil area, a third pressure gauge is connected to the gas collecting tank, and a pipe section of the natural gas pipeline passing through the experimental soil area is an underground pipe.

[0007] A natural gas inspection well is excavated in the experimental soil area, a valve inspection well is excavated in the experimental area on the side of the natural gas inspection well, a leakage hole is opened on the buried pipe in the experimental soil area, and multiple nozzles are installed on the leakage hole, and the directions of the multiple nozzles are upward, sideways and downward; multiple methane concentration sensors are installed in the soil of the experimental soil area around the leakage hole, and a methane concentration sensor is installed in the natural gas inspection well and the valve inspection well respectively, and the signal output end of the control platform is connected with the signal input end of the delivery valve, the stop valve, the first electric regulating valve, the second electric regulating valve, the flow meter, the first pressure gauge, the second pressure gauge, the third pressure gauge and the nozzle installed on the leakage hole; the software monitoring platform is used to receive the monitoring data of the methane concentration output by each methane concentration sensor and convert the data into images, a natural gas inspection well crack is set on the natural gas inspection well, and a valve inspection well crack is set on the valve inspection well.

[0008] The present invention provides an experimental method for leaking natural gas to diffuse along soil-adjacent underground limited space, comprising the following steps:

[0009] Step 1: The control platform outputs a control signal to the delivery valve between the high-pressure gas cylinder and the gas storage tank to open the delivery valve, and injects the experimental gas from the high-pressure gas cylinder into the gas storage tank. The control platform reads the pressure signal output by the first pressure gauge to monitor the pressure of the gas storage tank. When the pressure reaches the rated pressure of the gas storage tank, the filling process of the gas storage tank is completed.

[0010] Step 2: The control platform outputs a control signal to open the stop valve, and the control platform receives the pressure data monitoring signal output by the second pressure gauge to observe whether the pressure of the natural gas pipeline changes. If it changes, recheck the connection of the pipeline; if there is no change, execute step 3;

[0011] Step 3. According to the designed experimental working conditions, the flow rate and pressure of the experimental gas flowing into the buried pipe are set through the control platform, and then the control platform outputs a control signal to open the nozzle directly below the leakage hole. The specific process is as follows: First, the control platform outputs a control signal to the first-level electric regulating valve and the second-level electric regulating valve to control the opening of the first-level electric regulating valve and the second-level electric regulating valve to adjust the experimental gas delivery flow rate and pressure. The experimental gas flows into the buried pipe along the natural gas pipeline and flows into the experimental soil area from the leakage hole. As the leakage time continues, the experimental gas enters the natural gas inspection well and the valve inspection well along the cracks in the natural gas inspection well and the cracks in the valve inspection well respectively. In this process, the control platform outputs a control signal to adjust the opening of the second-level electric regulating valve to keep the flow rate and pressure in the experiment unchanged;

[0012] Step 4: read the data of all methane concentration sensors through the software monitoring platform to observe the changes in the experimental gas concentration. If the data output by all methane concentration sensors show that the experimental gas has reached a saturated state or reached a preset leakage time, the control platform outputs a closing signal to the stop valve to close the stop valve, thus ending the experiment.

[0013] Step 5. Use the ring knife method to sample the soil in the experimental soil area. The specific process is as follows: select a 30cm×30cm square in the center of the ground surface of the experimental soil area, compact and level it, shovel away the floating and uneven parts of the ground surface, use a ring knife, and sample the soil at the plane where the buried pipe is located directly below the square area and test the soil parameters, including porosity, moisture content and saturation. When the readings of all methane concentration sensors drop to 0, the experimental system is left to stand for 24 hours, and then the value of the third pressure gauge is checked through the control platform. If the value of the third pressure gauge is close to the rated pressure of the gas collecting tank, replace the gas collecting tank, otherwise perform step 6; otherwise, replace the gas collecting tank;

[0014] Step 6: According to the designed experimental working conditions, the flow rate and pressure of the experimental gas flowing into the buried pipe are reset through the control platform, and steps 3 to 6 are repeated continuously until all designed experimental working condition tests are completed;

[0015] Step seven, the control platform outputs a control signal to the nozzle facing downward to close the nozzle, and outputs a control signal to the nozzle facing sideways to open the nozzle, and then repeats steps three to six. Finally, according to the method of step seven, the control platform outputs a control signal to the nozzle facing sideways to close the nozzle, and outputs a control signal to the nozzle facing upward to open the nozzle, and then repeats steps three to six until all designed experimental working condition tests are completed. The control platform outputs a closing signal to the nozzle facing upward to close the nozzle facing upward.

[0016] Advantages of the present invention:

[0017] The method and device of the present invention analyze the influence of factors such as leakage flow, pipeline pressure, leakage hole direction, leakage hole size, etc. on the diffusion characteristics of leaked natural gas under different leakage conditions. On the one hand, the relationship between the diffusion distance of natural gas in the soil after leakage and the time required to reach the saturation state can be obtained, and the authenticity of the relationship can be guaranteed by referring to real engineering cases such as natural gas leakage underground. On the other hand, experimental models and verification data are provided for theoretical research and simulation research. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0019] Figure 1 It is a structural schematic diagram of an experimental device for leaked natural gas diffusion along soil-adjacent underground limited space of the present invention;

[0020] Figure 2 It is the location map of the leakage point and sensor detection point;

[0021] Figure 3 A flow chart of an experimental method for the diffusion of leaked natural gas along soil-adjacent underground limited space of the present invention; DETAILED DESCRIPTION

[0022] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present application have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0023] like Figure 1 As shown, an experimental device for leaking natural gas diffusion along soil-adjacent underground limited space of the present invention comprises a gas storage tank 1, the gas storage tank is connected to a high-pressure gas cylinder through a gas transmission pipeline equipped with a delivery valve, a first pressure gauge 2-1 is installed on the gas storage tank, and the outlet of the gas storage tank 1 is connected to a stop valve 3, a first-level electric regulating valve 4, a second-level electric regulating valve 5, a flow meter 6, and a second pressure gauge 2-2 in sequence through a natural gas pipeline, and then connected to a gas collecting tank 15 through an experimental soil area 7. A third pressure gauge 2-3 is connected to the gas collecting tank 15. The pipe section of the natural gas pipeline passing through the experimental soil area is a buried pipe.

[0024] A natural gas inspection well 12 is excavated in the experimental soil area 7, and a valve inspection well 13 is excavated in the experimental area on the side of the natural gas inspection well 12 to simulate the real underground environment of the city. Preferably, the particles of the soil in the experimental soil area 7 outside the natural gas inspection well 12 and the valve inspection well 13 are 300-625 mesh, which can be screened by a sieve to ensure the uniformity of soil particle distribution. The soil in the experimental soil area 7 is layered and compacted and leveled. Further preferably, the soil in the experimental soil area 7 has a depth of 20 cm per layer, which can ensure the uniformity of the soil environment.

[0025] A leakage hole 8 is opened on the buried pipe in the experimental soil area 7, and a plurality of nozzles are installed on the leakage hole 8, and the directions of the plurality of nozzles are upward, sideways and downward; a plurality of methane concentration sensors are installed in the soil of the experimental soil area 7 around the leakage hole 8, and a methane concentration sensor is installed in the natural gas inspection well 12 and the valve inspection well 13 respectively. Preferably, a methane concentration sensor is respectively arranged 1m below, above, in front of and behind the leakage hole 8, and the first methane concentration sensor is S1, the fifteenth methane concentration sensor is S15, the eighth methane concentration sensor is S8, and the ninth methane concentration sensor is S9. With the leakage hole 8 as a reference point, the horizontal plane where the leakage hole 8 is located is at a height of Z=-2.0m from the ground, and from the leakage hole 8 upward from the height of Z=-1.0m below the ground to the ground Z=0 and from the location of the leakage hole to the height of Z=-3.0m below the ground every other methane concentration sensor. A group of methane concentration sensors are arranged in each height gradient horizontal plane at a height gradient of 0.5m, each group of methane concentration sensors is three and forms a right triangle, the vertical central axis of the leakage hole 8 intersects with the midpoint of the hypotenuse of the right triangle in each height gradient horizontal plane, and the methane concentration sensors located at the right angle position of each right triangle are arranged in a direction away from the valve inspection well 13, and the methane concentration sensors in the natural gas inspection well 12 and the valve inspection well 13 are located at a height Z = -1.5m below the ground, as shown in the attached figure. Figure 2As shown, in the horizontal plane where the leakage hole 8 is located, a fifth methane concentration sensor S5 is arranged 3m to the left and 2m in front of the leakage hole 8; a seventh methane concentration sensor S7 is arranged 3m to the left and 2m in the back of the leakage hole 8; a sixth methane concentration sensor S6 is arranged 3m to the right and 2m in front of the leakage hole 8; a second methane concentration sensor S2, a twelfth methane concentration sensor S12 and a sixteenth methane concentration sensor S16 are arranged 0.5m below, 1.0m and 1.5m above the fifth methane concentration sensor S5; a fourth methane concentration sensor S7 is arranged 0.5m below, 1.0m and 1.5m above the seventh methane concentration sensor S7 concentration sensor S4, the fourteenth methane concentration sensor S14 and the eighteenth methane concentration sensor S18; a third methane concentration sensor S3, a thirteenth methane concentration sensor S13 and a seventeenth methane concentration sensor S17 are respectively arranged at 0.5m below, 1.0m above and 1.5m above the sixth methane concentration sensor S6; a tenth methane concentration sensor S10 is arranged in the natural gas inspection well 12 in the horizontal plane 0.5m above the horizontal plane where the leakage hole 8 is located, and an eleventh methane concentration sensor S11 is arranged in the valve inspection well 13 in the horizontal plane 0.5m above the horizontal plane where the leakage hole 8 is located. This arrangement can grasp the distribution of methane concentration in each area of ​​the experimental soil area 7 diffused by the experimental gas.

[0026] The signal output end of the control platform 10 is connected to the signal input end of the delivery valve, the stop valve 3, the first electric regulating valve 4, the second electric regulating valve 5, the flow meter 6, the first pressure gauge 2-1, the second pressure gauge 2-2, the third pressure gauge 2-3 and the nozzle installed on the leakage hole 8; the software monitoring platform 11 is used to receive the monitoring data of the methane concentration output by each methane concentration sensor and convert the data into an image. The data processing can use existing software, such as Origin.

[0027] A natural gas inspection well crack 14-1 is provided on the natural gas inspection well 12, and a valve inspection well crack 14-2 is provided on the valve inspection well 13. In actual engineering, the position of the crack is random, and the shape of the crack is also arbitrary, which can be regular or irregular. In order to optimize the verification of the concentration numerical model of the experimental gas leakage and diffusion into the natural gas inspection well 12 and the valve inspection well 13, preferably, along the height direction of the side walls of the natural gas inspection well 12 and the valve inspection well 13, the natural gas inspection well crack 14-1 and the valve inspection well crack 14-2 are respectively set in the middle position of the side walls of the natural gas inspection well 12 and the valve inspection well 13, and the shape is set to be rectangular, which is convenient for the establishment of a three-dimensional model and meshing.

[0028] The present invention provides an experimental method for leaking natural gas to diffuse along soil-adjacent underground limited space, comprising the following steps:

[0029] Step 1, the control platform 10 outputs a control signal to the delivery valve between the high-pressure gas cylinder and the gas storage tank 1 to open the delivery valve, and injects the experimental gas from the high-pressure gas cylinder into the gas storage tank 1. The control platform 10 reads the pressure signal output by the first pressure gauge 2-1 to monitor the pressure of the gas storage tank 1. When the pressure reaches the rated pressure of the gas storage tank 1, the filling process of the gas storage tank 1 ends. Preferably, under the premise of ensuring safety, the experimental gas is set to natural gas; if the experimental site is limited, in order to ensure the safety of the experiment, the components of natural gas are a mixed gas composed of methane with a volume fraction of 4%vol and nitrogen with a volume fraction of 96%vol. This is because natural gas generally has flammable and explosive chemical properties. The main component of natural gas is methane, and the explosion limit of methane is 5%vol-15%vol.

[0030] Step 2: The control platform 10 outputs a control signal to open the stop valve 3. The control platform 10 receives the pressure data monitoring signal output by the second pressure gauge 2-2 to observe whether the natural gas pipeline pressure changes. If it changes, it means that there is a problem with the air tightness of the experimental system, and the connection of the pipeline should be rechecked; if there is no change, step 3 is executed, which means that the experimental air tightness is good.

[0031] Step 3: According to the designed experimental working conditions, the flow rate and pressure of the experimental gas flowing into the buried pipe are set through the control platform 10. For example, the flow rate of the experimental gas flowing into the buried pipe can be set to 0.19Nm 3 / h, the pressure can be set to 0.022MPa. Then the control platform 10 outputs a control signal to open the nozzle directly below the leak hole. The specific process is as follows: First, the control platform 10 outputs a control signal to the first-level electric regulating valve 4 and the second-level electric regulating valve 5 to control the opening of the first-level electric regulating valve 4 and the second-level electric regulating valve 5 to adjust the experimental gas delivery flow and pressure. The experimental gas flows into the buried pipe along the natural gas pipeline and flows into the experimental soil area 7 from the leak hole 8. As the leakage time continues, the experimental gas enters the natural gas inspection well 12 and the valve inspection well 13 along the cracks 14-1 of the natural gas inspection well and the cracks 14-2 of the valve inspection well. In this process, the control platform 10 outputs a control signal to adjust the opening of the second-level electric regulating valve 5 to keep the flow and pressure unchanged in the experiment. This is because the same working condition should be guaranteed to be stable during the experiment. However, the pressure of the gas storage tank 1 will inevitably decrease gradually during the experiment. If the flow of the leak hole does not change, the leakage experiment continues; if the flow changes, the control platform 10 outputs a control signal to slightly adjust the opening of the second-level electric valve 5, thereby ensuring the stability of the gas flow during the experiment.

[0032] Step 4: Read the data of all methane concentration sensors through the software monitoring platform 11 and observe the changes in the experimental gas concentration. If the data output by all methane concentration sensors show that the experimental gas has reached a saturated state (taking the experimental gas as 4% vol methane and 96% vol nitrogen as an example, the saturated state is when the methane volume fraction reaches 4% vol) or reaches the preset leakage time, the control platform 10 outputs a closing signal to the stop valve 3 to close the stop valve 3, thereby ending the group of experiments.

[0033] Step 5. Use the ring knife method (for the specific operation of the ring knife method, please refer to the second edition of "Soil Analysis Technical Specifications" published by China Agricultural Press in June 2006, page 17) to sample the soil in the experimental soil area 7. The specific process is as follows: select a 30cm×30cm square in the center of the surface of the experimental soil area 7, compact and level it, shovel away the floating and uneven parts of the surface, and use a ring knife to sample the soil and test the soil parameters, including porosity, moisture content and saturation, on the plane where the buried pipe is located directly below the square area. When the readings of all methane concentration sensors drop to 0, the experimental system is left to stand for 24 hours. Then check the value of the third pressure gauge 2-3 through the control platform 10. If the value of the third pressure gauge 2-3 is close to the rated pressure of the gas collecting tank 15, replace the gas collecting tank 15, otherwise perform step 6; otherwise, replace the gas collecting tank 15 to avoid the risk of explosion due to excessive pressure. A specific example is as follows: If the rated pressure of the selected gas collecting tank 15 is 0.84MPa, the pressure of the gas collecting tank should be less than 80% of the rated pressure at the end of the experiment, that is, the pressure is less than 0.672MPa. Check whether the data of the third pressure gauge 2-3 is less than 0.672MPa. If it is, execute step 6;

[0034] Step 6: According to the designed experimental working conditions, the flow rate and pressure of the experimental gas flowing into the buried pipe are reset through the control platform 10, and steps 3 to 6 are repeated continuously until all the designed experimental working condition tests are completed.

[0035] Preferably, the second set of experimental gas operating parameters can be set to a flow rate of 0.24 Nm2 of experimental gas flowing into the buried pipe. 3 / h, pressure 0.031MPa; the third group of experimental gas operating parameters were adjusted to the experimental gas flow rate of 0.29Nm 3 / h, pressure 0.039MPa; the fourth group of experimental gas operating parameters were adjusted to the experimental gas flow rate of 0.15Nm 3 / h, pressure 0.018MPa; the fifth group of experimental gas operating parameters were adjusted to the experimental gas flow rate of 0.20Nm 3 / h, pressure 0.024MPa; the sixth group of experimental gas operating parameters were adjusted to the experimental gas flow rate of 0.25Nm 3 / h, pressure 0.037MPa. Setting the flow rate and pressure of the experimental gas in the buried pipeline in this way can ensure both the completion and safety of the experiment.

[0036] Step seven, the control platform 10 outputs a control signal to the nozzle facing downward to close the nozzle, and outputs a control signal to the nozzle facing sideways to open the nozzle, and then repeats steps three to six. Finally, according to the method of step seven, the control platform 10 outputs a control signal to the nozzle facing sideways to close the nozzle, and outputs a control signal to the nozzle facing upward to open the nozzle, and then repeats steps three to six until all designed experimental working condition tests are completed. The control platform 10 outputs a closing signal to the nozzle facing upward to close the nozzle facing upward.

[0037] Through this experiment, on the one hand, we can simulate the real leakage scenario through the experimental result data, obtain the diffusion characteristics of methane in the soil and inspection wells, convert the experimental data output into images through Origin software, and fit the curve through the fitting function of the software to obtain the relationship between the diffusion distance of methane in the soil and the time required to reach the saturation concentration. This provides a scientific basis for the prediction of the farthest diffusion distance when the natural gas leakage diffusion reaches the saturation state, the time required for the natural gas leakage to reach the saturation state, etc.; on the other hand, the experimental result data obtained also provide verification data for the numerical model of studying the leakage diffusion law of direct buried gas pipelines.

[0038] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An experimental device for the diffusion of leaked natural gas along the soil-adjacent underground limited space, characterized in that: The invention comprises a gas storage tank (1), wherein the gas storage tank (1) is connected to a high-pressure gas cylinder via a gas delivery pipeline equipped with a delivery valve, a first pressure gauge (2-1) is installed on the gas storage tank (1), an outlet of the gas storage tank (1) is connected to a stop valve (3), a first-level electric regulating valve (4), a second-level electric regulating valve (5), a flow meter (6), and a second pressure gauge (2-2) in sequence via a natural gas pipeline, and then passes through an experimental soil area (7) to connect to a gas collecting tank (15), a third pressure gauge (2-3) is connected to the gas collecting tank (15), and the pipe section of the natural gas pipeline passing through the experimental soil area is an underground pipe. A natural gas inspection well (12) is excavated in the experimental soil area (7), a valve inspection well (13) is excavated in the experimental area on the side of the natural gas inspection well (12), a leakage hole (8) is opened on the buried pipe in the experimental soil area (7), and a plurality of nozzles are installed on the leakage hole (8), and the directions of the plurality of nozzles are upward, sideways and downward; a plurality of methane concentration sensors are installed in the soil of the experimental soil area (7) around the leakage hole, and a methane concentration sensor is installed in each of the natural gas inspection well (12) and the valve inspection well (13), and a control platform (10) is provided. The signal output end is connected to the signal input end of the delivery valve, the stop valve (3), the first-level electric regulating valve (4), the second-level electric regulating valve (5), the flow meter (6), the first pressure gauge (2-1), the second pressure gauge (2-2), the third pressure gauge (2-3) and the nozzle installed on the leakage hole (8); the software monitoring platform (11) is used to receive the monitoring data of the methane concentration output by each methane concentration sensor and process the data into an image; a natural gas inspection well crack (14-1) is provided on the natural gas inspection well (12), and a valve inspection well crack (14-2) is provided on the valve inspection well (13).

2. The experimental device for the diffusion of leaked natural gas along the soil-adjacent underground limited space according to claim 1 is characterized by: The particles of the soil in the experimental soil area (7) outside the natural gas inspection well (12) and the valve inspection well (13) are 300-625 meshes, and the soil in the experimental soil area (7) is filled in layers and compacted and leveled.

3. The experimental device for the diffusion of leaked natural gas along the soil-adjacent underground limited space according to claim 2 is characterized by: The soil layer depth of each layer in the experimental soil area is 20 cm.

4. The experimental device for the diffusion of leaked natural gas along the soil-adjacent underground limited space according to claim 2 is characterized by: A methane concentration sensor is arranged at 1 m below, above, in front of and behind the leakage hole (8), respectively. The leakage hole (8) is used as a reference point. The height of the horizontal plane where the leakage hole (8) is located is Z=-2.0 m from the ground. From the leakage hole (8) upward from the height Z=-1.0 m below the ground to the point Z=0 on the ground and from the position of the leakage hole (8) to the point Z=-3.0 m below the ground, a group of methane concentration sensors is arranged in each height gradient horizontal plane at every 0.5 m height gradient. Each group of methane concentration sensors has three methane concentration sensors and forms a right triangle. The vertical center axis of the leakage hole (8) intersects with the midpoint of the hypotenuse of the right triangle in each height gradient horizontal plane. The methane concentration sensors located at the right angle of each right triangle are arranged in a direction away from the valve inspection well (13). The methane concentration sensors in the natural gas inspection well (12) and the valve inspection well (13) are located at a height Z=-1.5 m below the ground.

5. The experimental device for the diffusion of leaked natural gas along the soil-adjacent underground limited space according to claim 4 is characterized by: In the horizontal plane where the leakage hole (8) is located, a fifth methane concentration sensor is arranged 3m to the left and 2m in front of the leakage hole; a seventh methane concentration sensor is arranged 3m to the left and 2m in the back of the leakage hole (8); a sixth methane concentration sensor is arranged 3m to the right and 2m in front of the leakage hole (8); a second methane concentration sensor, a twelfth methane concentration sensor and a sixteenth methane concentration sensor are arranged 0.5m below the fifth methane concentration sensor and 1.0m and 1.5m above the fifth methane concentration sensor; a fourth methane concentration sensor is arranged 0.5m below the seventh methane concentration sensor and 1.0m and 1.5m above the seventh methane concentration sensor. A methane concentration sensor, a fourteenth methane concentration sensor and an eighteenth methane concentration sensor; a third methane concentration sensor, a thirteenth methane concentration sensor and a seventeenth methane concentration sensor are respectively arranged at 0.5 m directly below, 1.0 m directly above and 1.5 m above the sixth methane concentration sensor; a tenth methane concentration sensor is arranged in a natural gas inspection well (12) in a horizontal plane 0.5 m above the horizontal plane where the leakage hole (8) is located, and an eleventh methane concentration sensor is arranged in a valve inspection well (13) in a horizontal plane above the horizontal plane where the leakage hole (8) is located.

6. The experimental device for leaked natural gas diffusion along soil-adjacent underground limited space according to claim 2 is characterized by: Along the height direction of the side walls of the natural gas inspection well (12) and the valve inspection well (13), the natural gas inspection well crack (14-1) and the valve inspection well crack (14-2) are respectively arranged at the middle position of the side walls of the natural gas inspection well (12) and the valve inspection well (13), and the shape is set to be rectangular.

7. A method for conducting an experiment using the experimental device for the diffusion of leaked natural gas along the soil-adjacent underground limited space as claimed in any one of claims 1 to 6, characterized in that The following steps are involved: Step 1: The control platform (10) outputs a control signal to the delivery valve between the high-pressure gas cylinder and the gas storage tank (1) to open the delivery valve, and injects the experimental gas from the high-pressure gas cylinder into the gas storage tank (1). The control platform (10) reads the pressure signal output by the first pressure gauge (2-1) to monitor the pressure of the gas storage tank (1). When the pressure reaches the rated pressure of the gas storage tank (1), the gas filling process of the gas storage tank (1) is completed. Step 2: The control platform (10) outputs a control signal to open the stop valve (3), and the control platform (10) receives the pressure data monitoring signal output by the second pressure gauge (2-2) to observe whether the pressure of the natural gas pipeline changes. If it changes, recheck the connection of the pipeline; if it does not change, execute step 3; Step 3: According to the designed experimental working conditions, the flow rate and pressure of the experimental gas flowing into the buried pipe are set through the control platform (10), and then the control platform (10) outputs a control signal to open the nozzle directly below the leakage hole (8). The specific process is as follows: First, the control platform (10) outputs a control signal to the first-level electric regulating valve (4) and the second-level electric regulating valve (5) to control the opening of the first-level electric regulating valve (4) and the second-level electric regulating valve (5) to adjust the experimental gas transmission flow rate and pressure. The experimental gas flows into the buried pipe along the natural gas pipeline and flows into the experimental soil area (7) from the leakage hole (8). As the leakage time continues, the experimental gas enters the natural gas inspection well (12) and the valve inspection well (13) along the cracks (14-1) of the natural gas inspection well and the cracks (14-2) of the valve inspection well. During this process, the control platform (10) outputs a control signal to adjust the opening of the second-level electric regulating valve (5) to keep the flow rate and pressure in the experiment unchanged; Step 4: read the data of all methane concentration sensors through the software monitoring platform (11) to observe the change of experimental gas concentration. If the data output by all methane concentration sensors show that the experimental gas has reached a saturated state or reached a preset leakage time, the control platform (10) outputs a closing signal to the stop valve (3) to close the stop valve (3), thereby ending the experiment. Step 5, using the ring knife method to sample the soil in the experimental soil area (7), the specific process is as follows: select a 30cm×30cm square in the center of the surface of the experimental soil area (7), compact and level it, shovel away the floating and uneven parts of the surface, use a ring knife to sample the soil and test the soil parameters, including porosity, moisture content and saturation, at the plane where the buried pipe is located directly below the square area. When the readings of all methane concentration sensors drop to 0, the experimental system is left to stand for 24 hours, and then the value of the third pressure gauge (2-3) is checked through the control platform. If the value of the third pressure gauge is close to the rated pressure of the gas collecting tank (15), replace the gas collecting tank (15), otherwise perform step 6; otherwise, replace the gas collecting tank (15); Step 6: according to the designed experimental working conditions, the flow rate and pressure of the experimental gas flowing into the buried pipe are reset through the control platform (10), and steps 3 to 6 are continuously repeated until all designed experimental working condition tests are completed; Step seven, the control platform (10) outputs a control signal to the nozzle facing downward to close the nozzle, and outputs a control signal to the nozzle facing sideways to open the nozzle, and then repeats steps three to six. Finally, according to the method of step seven, the control platform (10) outputs a control signal to the nozzle facing sideways to close the nozzle, and outputs a control signal to the nozzle facing upward to open the nozzle, and then repeats steps three to six until all designed experimental working condition tests are completed. The control platform (10) outputs a closing signal to the nozzle facing upward to close the nozzle facing upward.

8. The experimental method for diffusion of leaked natural gas along soil-adjacent underground limited space according to claim 7, characterized in that: The experimental gas is set as natural gas, and the components of the natural gas are a mixed gas consisting of methane with a volume fraction of 4% vol and nitrogen with a volume fraction of 96% vol.

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