A system and method for determining the overall leak rate of a primary containment barrier of an onshore storage tank

Abstract

The application discloses a kind of land storage tank main shielding layer integral leak rate determination system and method, for determining the integral leak rate of land LNG film type storage tank main shielding layer corrugated board, the main shielding layer integral leak rate determination system includes vacuum system, gas injection system, gas state monitoring system and safety system.The determination method of the application includes the following steps: setting land storage tank main shielding layer integral leak rate determination system;In the tank inner space of the land storage tank, under the condition of positive pressure, test gas is injected into the tank inner space, and the concentration change value of test gas in the main shielding space is continuously recorded;The pressure of the tank inner space is reduced to atmospheric pressure, and the concentration change value of test gas in the main shielding space is recorded again;Calculate the integral leak rate value of main shielding layer.The determination system and method can effectively determine the integral leak rate of main shielding layer in a safe, economical and reliable manner within the design pressure range of land LNG film type storage tank.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nondestructive testing, and relates to a system and a method for measuring the overall leakage rate of a main shielding layer of a land storage tank. TECHNICAL BACKGROUND

[0002] A land LNG thin-film type storage tank guarantees the overall tightness of the storage tank through an enclosure system. The enclosure system is divided into a main shielding space, a secondary shielding space, and a tank inner space for storing LNG, as shown in Figure 1 The main shielding layer of the enclosure system is a 304L stainless steel corrugated plate with longitudinal and transverse grooves, and a part with a height of more than 5 m directly forms the main shielding space by the main shielding layer and the concrete outer tank. The part with a height of less than 5 m of the storage tank comprises a secondary shielding layer, the secondary shielding layer is a three-in-one sheet material composed of two layers of glass fiber cloth and one layer of aluminum foil, the space between the main shielding layer and the secondary shielding layer is the main shielding space, and the space between the secondary shielding layer and the concrete outer tank is the secondary shielding space. The main shielding space and the secondary shielding space are both filled with thermal insulation materials, and the thermal insulation materials are reinforced polyurethane foam sandwiched between upper and lower layers of plywood. The main shielding layer directly contacts liquefied natural gas, and is the primary guarantee for the overall tightness of the storage tank. For a 29,000 m 3 For example, the main shielding layer of a land LNG storage tank is completed by lap welding of corrugated plates, and the total length of the manual welding and automatic welding is about 10,000 m, so that welding defects are difficult to avoid. The measurement of the overall leakage rate of the main shielding layer is limited by the volume of the storage tank and the structure of the storage tank. Due to the large volume of the storage tank, the pumping capacity of the front pump and the auxiliary pump cannot meet the starting pressure requirement of the high vacuum pump when the mass spectrometry is used, so it is difficult to measure the overall leakage rate by using the commonly used mass spectrometry. Moreover, the design pressure of the enclosure system as a normal pressure cryogenic system cannot meet the detection pressure requirement of the overall leakage rate measurement by using the conventional mass spectrometry.

[0003] Therefore, how to break away from the above conditions and realize the measurement of the overall leakage rate of the main shielding layer of the land LNG thin-film type storage tank is very important for guaranteeing the safe operation of the storage tank. SUMMARY

[0004] The application provides a system and a method for measuring the overall leakage rate of a main shielding layer of a land storage tank, which can effectively measure the overall leakage rate of the main shielding layer of a large-volume thin-film type storage tank by using a safe and convenient-to-assemble measurement system within the design working pressure of the thin-film type storage tank.

[0005] To achieve the above object, the application adopts the following technical scheme:

[0006] A kind of land storage tank main shielding layer integral leak rate determination system, the land storage tank is land LNG thin film type storage tank, it includes main shielding layer, secondary screen wall layer, outer shell and dome;The main shielding layer is sealedly connected with the dome to form the space in tank, the main shielding space is formed between the main shielding layer and the secondary screen wall layer, the outer shell and the dome, the secondary screen wall layer and the outer shell form secondary shielding space;Wherein, the determination system includes: vacuum system, gas injection system, gas state monitoring system and safety system.

[0007] Vacuum system is connected to the space in tank, for extracting the gas in the space in tank, the main shielding space and the secondary shielding space;Gas injection system is connected to the space in tank, for injecting test gas into the space in tank;Gas state monitoring system is used to monitor the pressure in the space in tank, the main shielding space and the secondary shielding space in real time, to monitor the temperature of the space in tank in real time, to monitor the test gas concentration of the space in tank and the main shielding space in real time;Safety system is connected to the main shielding space, and pressure relief is carried out when the pressure in the main shielding space exceeds the preset pressure threshold.

[0008] Further, the vacuum system includes vacuum pump, pumping speed regulating valve, pump gas pipeline;Wherein the vacuum pump is used for air extraction during gas replacement process;The pumping speed regulating valve is electrically connected to the vacuum pump, for controlling the air extraction rate of vacuum pump;One end of the pump gas pipeline is connected to the vacuum pump, and the other end of the pump gas pipeline is connected to the space in tank, the main shielding space and the secondary shielding space.

[0009] Further, the gas injection system includes air compressor, cold dryer, buffer tank and high-conductivity injection pipeline connected in sequence;Wherein the air compressor is used for air charging and pressurizing, the cold dryer is used to control the dew point value of air pressurized into the land storage tank by the air compressor, the buffer tank is used to buffer and store pressurized air, and the high-conductivity injection pipeline is used to connect the buffer tank and the space in tank.

[0010] Further, the gas state monitoring system includes pressure sensor, temperature sensor and test gas concentration sensor;Wherein the pressure sensor is arranged in the space in tank, the main shielding space and the secondary shielding space, for monitoring the pressure in the space in tank, the main shielding space and the secondary shielding space in real time;The temperature sensor is arranged in the space in tank, for monitoring the temperature of the space in tank in real time;The test gas concentration sensor is arranged in the space in tank and the main shielding space, for monitoring the test gas concentration in the space in tank and the main shielding space in real time.

[0011] Further, the safety system comprises a pressure relief valve; the pressure relief valve is communicated to the tank inner space, when the pressure in the main shielding space exceeds a preset pressure threshold, the pressure relief valve relieves pressure.

[0012] Further, the safety system further comprises an alarm, the alarm is electrically connected to the pressure relief valve, when the pressure relief valve relieves pressure, the alarm simultaneously alarms.

[0013] The application also provides a kind of land storage tank main shielding layer overall leak rate determination method, comprising the following steps:

[0014] Step 1, set the land storage tank main shielding layer overall leak rate determination system described above, start the vacuum system to pump to the tank inner space in positive pressure condition stop machine;

[0015] Step 2, in the tank inner space of the land storage tank in positive pressure condition, start the gas injection system to inject test gas into the tank inner space;

[0016] Step 3, in the tank inner space of the land storage tank in positive pressure condition, continuously record the test gas concentration change value in the main shielding space;

[0017] Step 4, start the safety system to reduce the pressure of the tank inner space to atmospheric pressure, again continuously record the test gas concentration change value in the main shielding space;

[0018] Step 5, make the test gas concentration change curve in the main shielding space during the test, calculate the quality leak rate and convert the PV leak rate, only change the test pressure of the tank inner space to make two tests, the difference between the two test results is used to eliminate the influence of the remaining variables except the main screen layer, and the difference value is used as the basis to finally obtain the main shielding layer overall leak rate value.

[0019] Further, in step 2, the tank inner space is pumped by the vacuum system, then the tank inner space is swept by dry air, the tank inner space maintains a constant temperature environment under positive pressure condition;Then inject a mixture of helium and dry air into the tank inner space, wherein helium is used as the test gas, and the concentration of helium is denoted as C 罐内 , the concentration should not be less than 1%.

[0020] Further, in step 3 and step 4, the helium concentration change value and temperature value are recorded every 1 hour, the helium concentration change value is in ppm level, and t hours are continuously recorded.

[0021] Further, in step 5, first calculate the mass leak rate of the main shielding layer to helium under test conditions according to formula (1).

[0022]

[0023] Wherein p is the pressure of the ideal gas, V is the volume of the ideal gas, n represents the amount of ideal gas substance, T represents the thermodynamic temperature of the ideal gas, and R is the ideal gas constant. 主屏蔽 is the volume of the main shielding space, and AC is the difference of the helium concentration change of the main shielding space, and t is the time recorded continuously in step 3.

[0024] When the mass leakage rate is calculated, there is a conversion relationship between the mass leakage rate and the PV leakage rate, and formula (3) can be obtained by dividing the ideal gas state formula (2) by the time variable t on both ends.

[0025] PV = nRT Formula (2);

[0026]

[0027] Wherein p is the pressure of the ideal gas, V is the volume of the ideal gas, n represents the amount of ideal gas substance, T represents the thermodynamic temperature of the ideal gas, and R is the ideal gas constant.

[0028] The left end of formula (3) is the PV leakage rate definition formula, and n can be represented as m / M, m is the mass of the test gas, and M is the molar mass of the test gas, and after derivation, formula (4) can be obtained:

[0029]

[0030] Wherein m / t is the mass leakage rate obtained in formula (1), and therefore the PV leakage rate under the pressure and temperature of the test condition can be represented as formula (5):

[0031]

[0032] After considering the influence of the concentration factor, the overall leakage rate of the main shielding space in the tank can be represented as formula (6):

[0033]

[0034] The application is a kind of main shielding layer overall leakage rate measuring system and method for land LNG film type storage tank, which can effectively measure the overall leakage rate of the main shielding layer of large volume film type storage tank on land within the design working pressure of the film type storage tank, through a safe and convenient engineering site assembly measuring system. BRIEF DESCRIPTION OF DRAWINGS

[0035] The technical solutions and other beneficial effects of the application will be apparent from the following detailed description of specific embodiments of the application, combined with the accompanying drawings.

[0036] Figure 1 It is a structural schematic diagram of the land storage tank.

[0037] Figure 2A The main shielding layer overall leakage rate measuring system of the land storage tank of the application is arranged inFigure 1 The connection diagram shown on the land storage tank.

[0038] Figure 2B It is the structural schematic diagram of the whole leakage rate measuring system of the main shielding layer of the land storage tank in the application.

[0039] Figure 3 It is the schematic diagram of the helium flow direction in the test process of the application.

[0040] Figure 4 It is the schematic diagram of the change of the helium concentration in the main shielding space in the test process of the application. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0042] In the description of the application, it should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connection" and "connection" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection or can communicate with each other; it can be directly connected, or indirectly connected through an intermediate medium, or the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0043] An embodiment of the application provides a whole leakage rate measuring system and method of the main shielding layer of a land LNG thin film type storage tank, which can effectively measure the whole leakage rate of the main shielding layer of a land large-volume thin film type storage tank by a safe and convenient engineering site assembly measuring system within the design working pressure of the thin film type storage tank.

[0044] An embodiment of the application provides a whole leakage rate measuring system of the main shielding layer of a land storage tank, which is installed on the land storage tank to measure the whole leakage rate of the main shielding layer.

[0045] As Figure 1 shown, the land storage tank is a land LNG thin film type storage tank enclosure system, which comprises a main shielding layer, a secondary shielding wall layer, an outer shell and a dome; the main shielding layer is sealingly connected with the dome to form an inner tank space, the main shielding layer and the secondary shielding wall layer, the outer shell and the dome form a main shielding space, and the secondary shielding wall layer and the outer shell form a secondary shielding space.

[0046] AsFigure 2A , Figure 2B As shown, the measuring system includes: a vacuum system 1, a gas injection system 2, a gas state monitoring system 3, and a safety system 4.

[0047] Vacuum system 1, connected to the tank interior space, is used to extract gas from the tank interior space, the main shielded space, and the secondary shielded space. Vacuum system 1 is designed according to the pressure requirements of each space in the onshore LNG membrane storage tank. A vacuum pump set suitable for the operation of each space in the onshore LNG membrane storage tank is selected, and the gas pumping speed is effectively adjusted through regulating valves.

[0048] Gas injection system 2 is connected to the internal space of the tank and is used to inject test gas into the internal space of the tank. Gas injection system 2 uses an air compressor and a refrigerated dryer to perform gas purging and cleaning of each space in the onshore LNG membrane storage tank and to pressurize and inject the gas required for the test.

[0049] The gas state monitoring system 3 is used to monitor the pressure in the tank interior, the main shielded space, and the secondary shielded space in real time; to monitor the temperature in the tank interior; and to monitor the concentration of the test gas in the tank interior and the main shielded space in real time. The gas state monitoring system 3 uses pressure, temperature, and gas concentration sensors to achieve real-time monitoring of key parameters such as pressure, temperature, and concentration of the test gas inside the onshore LNG membrane storage tank.

[0050] Safety system 4 is connected to the internal space of the tank and releases pressure when the pressure in the internal space exceeds a preset pressure threshold. Through the pressure relief protection device, safety system 4 ensures that the pressure in each space of the onshore LNG membrane storage tank does not exceed the design pressure requirements, thus achieving the purpose of safety testing.

[0051] like Figure 2A , Figure 2B As shown, the vacuum system 1 includes a vacuum pump 11, a pumping speed regulating valve 12, and a pumping gas pipeline 13. The vacuum pump 11 is used to perform evacuation operations during gas replacement. The pumping speed regulating valve 12 is electrically connected to the vacuum pump 11 and is used to control the pumping speed of the vacuum pump 11 to prevent damage to the tank enclosure system caused by excessively fast pumping speed. After reaching the preset pressure, the pumping speed regulating valve 12 is closed. One end of the pumping gas pipeline 13 is connected to the vacuum pump 11, and the other end of the pumping gas pipeline 13 is connected to the tank interior space, the main shielding space, and the secondary shielding space.

[0052] like Figure 2A , Figure 2BAs shown, the gas injection system 2 comprises an air compressor 21, a cold dryer 22, a buffer tank 23, and a high-conductivity injection pipeline 24 connected in sequence; wherein the air compressor 21 is used for air charging and pressurizing, the cold dryer 22 is used for controlling the dew point value of the air pressurized into the onshore storage tank by the air compressor 21, the buffer tank 23 is used for buffering and storing the pressurized air, and the high-conductivity injection pipeline 24 is used for connecting the buffer tank 23 and the tank space.

[0053] As shown in Figure 2A , Figure 2B , the gas state monitoring system 3 comprises a pressure sensor 31, a temperature sensor 32, and a test gas concentration sensor 33; wherein the pressure sensor 31 is arranged in the tank space, the main shielding space, and the secondary shielding space, and is used for monitoring the pressure in the tank space, the main shielding space, and the secondary shielding space in real time; the temperature sensor 32 is arranged in the tank space, and is used for monitoring the temperature of the tank space in real time; and the test gas concentration sensor 33 is arranged in the tank space and the main shielding space, and is used for monitoring the test gas concentration in the tank space and the main shielding space in real time.

[0054] As shown in Figure 2A , Figure 2B , the safety system 4 comprises a pressure relief valve 41; the pressure relief valve 41 is communicated to the tank space, and when the pressure in the main shielding space exceeds a preset pressure threshold, the pressure relief valve 41 performs pressure relief. The pressure relief valve 41 is also called a pipe valve.

[0055] As shown in Figure 2A , Figure 2B , the safety system 4 further comprises an alarm 42, which is electrically connected to the pressure relief valve 41, and when the pressure relief valve 41 performs pressure relief, the alarm 42 simultaneously performs alarm. The on-site detection personnel starts to confirm the pressure of each space.

[0056] The present application also provides a method for measuring the overall leakage rate of the main shielding layer of an onshore storage tank, which comprises the following steps:

[0057] Step 1, referring to FIG. 2, in the test preparation stage, connect the system components in place. Set up the overall leakage rate measuring system of the main shielding layer of the onshore storage tank as described above, and arrange the sensors of the gas state monitoring system and the pressure relief valve and alarm of the safety system in place. Specifically, connect the vacuum pumping system to the tank space; connect the gas injection system to the tank space; connect the gas state monitoring system to the main shielding space and the tank space; and connect the safety system to the tank space and the main shielding space.

[0058] Step 2, under the positive pressure condition of the tank inner space of the land storage tank, the gas injection system 2 is started to inject test gas into the tank inner space. Specifically, after the system is prepared for connection, the tank inner space is evacuated by the vacuum pumping system, and then the tank inner space is purged with dry air to ensure the cleanliness of the gas environment in the tank. This operation can be repeated several times. The temperature in the tank is kept as constant as possible, and the temperature sensor records the temperature change every 1 hour during the test. Then, a mixture of helium and dry air is injected into the tank inner space, and the concentration of helium is denoted as C 罐内 , which should not be less than 1%. The tank inner space is pressurized by the mixture of helium and air to a pressure P1 within a specified time t. The pressure should not be too high to prevent damage to the tank and to facilitate the full diffusion of the gas. The gas inlet of the tank inner space, as well as the primary and secondary shielding spaces, is closed, and the primary and secondary shielding spaces are maintained at atmospheric pressure.

[0059] Step 3, under the positive pressure condition of the tank inner space of the land storage tank, the concentration change value of the test gas in the primary shielding space is continuously recorded. Specifically, the change of the helium concentration in the primary shielding space is observed (the background value of helium content in the atmosphere is about 5 ppm), and the helium concentration change value is recorded every 1 hour (taking the average value of the readings of the helium concentration sensors in the tank inner space). The concentration change is generally in the ppm level, and the recording is continuous for t hours. The flow direction of helium in the primary shielding space is shown in Figure 3 .

[0060] Step 4, the safety system 4 is started to reduce the pressure of the tank inner space to atmospheric pressure, and the concentration change value of the test gas in the primary shielding space is continuously recorded again. Specifically, the tank inner space pressure relief valve 41 is slowly opened to reduce the pressure of the tank inner space to atmospheric pressure P AP , and the helium concentration change value in the primary shielding space is recorded again every 1 hour, continuously for t hours (the same as the recording time in Step 3). The only change in this step compared to Step 3 is the pressure of the tank inner space. The difference between the helium concentration values in the primary shielding space in the two tests can eliminate the influence of the leakage of the pipe valve and the secondary shielding layer, thereby obtaining the change of the helium concentration in the primary shielding space caused by the leakage of the primary shielding layer under a certain pressure difference.

[0061] Step 5, the curve of the concentration change of the test gas (helium) in the primary shielding space during the test is drawn, the quality leak rate is calculated, and the PV leak rate is converted. Only the test pressure of the tank inner space is changed to make two tests, and the difference between the two test results is used to eliminate the influence of the remaining variables except the primary shielding layer. Based on this difference, the overall leak rate of the primary shielding layer is finally obtained.

[0062] In Step 5, the mass leak rate of the primary shielding layer to helium under the test conditions is first calculated according to formula (1);

[0063]

[0064] wherein p is the density of helium, V 主屏蔽 is the volume of the main shielding space, AC is the difference of the helium concentration change of the main shielding space, t is the time recorded continuously in step 3;

[0065] When the mass leak rate is calculated, there is a conversion relationship between the mass leak rate and the PV leak rate, and formula (3) can be obtained by dividing the ideal gas state formula (2) by the time variable t on both ends;

[0066] PV = nRT formula (2);

[0067]

[0068] wherein p is the density of helium, V

[0069] The left end of formula (3) is the PV leak rate definition formula, n can be expressed as m / M, m is the mass of the test gas, and M is the molar mass of the test gas, and after derivation, formula (4) can be obtained:

[0070]

[0071] wherein m / t is the mass leak rate obtained from formula (1), so the PV leak rate under the pressure and temperature of the test condition can be expressed as formula (5):

[0072]

[0073] After considering the influence of concentration, the overall leak rate of the main shielding space in the tank can be expressed as formula (6):

[0074]

[0075] A specific example is listed below for illustration.

[0076] In specific use, the above system is used to determine the main shielding overall leak rate of a simulated body of a land LNG film type storage tank, the inner space of the simulated body tank is 5m 2 , and the main shielding space is 0.1m 2 .

[0077] First, connect the system components and connect them with the simulation body.

[0078] Subsequently, the simulated body is purged with dry air to keep the tank clean, and the test is carried out at a constant temperature of 20℃. Dry air mixed with helium is injected into the tank, and the concentration of helium is 20%. The mixed gas is injected into the simulated body tank space in 10 minutes to reach +150mbar. The main and secondary shielding spaces are closed.

[0079] The change of the concentration of helium in the main shielding space is observed, and the change value of the concentration of helium is recorded every 1 hour. The average value of the concentration of helium in the main shielding space of the simulated body is taken, and it is recorded every 1 hour. The continuous recording is t=72 hours.

[0080] Slowly open the pressure relief valve of the tank space, and reduce the pressure in the tank space to atmospheric pressure P AP The change value of the concentration of helium in the main shielding space is recorded again, and it is recorded every 1 hour. The continuous recording is t=72 hours.

[0081] The difference is obtained as ΔC=300ppm. According to the above formula (5), formula (6), the overall leakage rate of the main shielding layer under the test conditions is calculated as Q PV =6.3×10 -5 pa.m 3 / s.

[0082] The present application is a kind of main shielding layer overall leakage rate determination system and method for land LNG film type storage tank, can be in the design working pressure of film type storage tank, by safe, and convenient engineering site assembly determination system, effectively determine the overall leakage rate of the main shielding layer of large volume film type storage tank on land.

Claims

1. A method for determining the overall leakage rate of the main shielding layer of an onshore storage tank, characterized in that, Including the following steps: Step 1: Set up an overall leakage rate measurement system for the main shielding layer of the onshore storage tank; the onshore storage tank is an onshore LNG membrane-type storage tank, which includes a main shielding layer, a secondary shielding layer, an outer shell, and a dome; the main shielding layer and the dome are sealed together to form an internal space, the main shielding space is formed between the main shielding layer and the secondary shielding layer, the outer shell, and the dome, and the secondary shielding space is formed between the secondary shielding layer and the outer shell; wherein, the measurement system includes: a vacuum system (1), connected to the internal space of the tank, used to extract the leakage rate of the internal space of the tank, the main shielding layer, the secondary shielding layer, the secondary shielding layer, and the dome. The gas in the shielded space and the secondary shielded space; the gas injection system (2), connected to the tank space, for injecting test gas into the tank space; the gas status monitoring system (3), for real-time monitoring of the pressure in the tank space, the main shielded space and the secondary shielded space, for real-time monitoring of the temperature in the tank space, and for real-time monitoring of the test gas concentration in the tank space and the main shielded space; the safety system (4), connected to the tank space, for depressurizing when the pressure in the tank space exceeds a preset pressure threshold; Step 2: Under positive pressure conditions, start the gas injection system (2) to inject test gas into the space inside the onshore storage tank; Step 3: Under positive pressure conditions, continuously record the change value of the test gas concentration in the main shielding space inside the onshore storage tank; Step 4: Activate the safety system (4) to reduce the pressure inside the tank to atmospheric pressure, and record the change value of the test gas concentration in the main shielding space again. Step 5: Plot the curve of the change in test gas concentration in the main shielding space during the test, calculate the mass leakage rate and convert the PV leakage rate. Only change the test pressure in the tank space to conduct two tests, and calculate the difference between the two test results to eliminate the influence of other variables except the main shielding layer. Based on this difference, finally obtain the overall leakage rate value of the main shielding layer.

2. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 1, characterized in that, In step 2, the vacuum system (1) is used to evacuate the space inside the tank, followed by purging the space with dry air. The space inside the tank is kept at a constant temperature under positive pressure. Then, a mixture of helium and dry air is injected into the space inside the tank, with helium used as the test gas and its concentration denoted as C. 罐内 The concentration must not be less than 1%.

3. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 2, characterized in that, In steps 3 and 4, the helium concentration change and temperature value are recorded every hour. The helium concentration change is in the ppm range, and the records are continuously kept for t hours.

4. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 3, characterized in that, In step 5, First, the mass leakage rate of the main shielding layer to helium under the test conditions is calculated according to equation (1); Where ρ is the density of helium, and V 主屏蔽 ΔC is the volume of the main shielding space, ΔC is the difference in helium concentration in the main shielding space, and t is the time continuously recorded in step 3. After calculating the mass leakage rate, there is a conversion relationship between the mass leakage rate and the PV leakage rate. Dividing both sides of the ideal gas state formula in equation (2) by the time variable t, we can obtain equation (3). PV = nRT (Equation 2); Where p is the pressure of the ideal gas, V is the volume of the ideal gas, n is the amount of substance of the ideal gas, T is the thermodynamic temperature of the ideal gas, and R is the ideal gas constant. The left side of equation (3) is the definition formula for PV leakage rate, where n can be expressed as m / M, where m is the mass of the test gas and M is the molar mass of the test gas. After its derivation, equation (4) can be obtained: Where m / t is the mass leakage rate obtained by equation (1), therefore the PV leakage rate under the pressure and temperature conditions of the test can be expressed as equation (5): After considering the influence of concentration, the overall leakage rate of the main shielding space inside the tank can be expressed as equation (6):

5. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 1, characterized in that, The vacuum system (1) includes a vacuum pump (11), a pumping speed regulating valve (12), and a pumping gas pipeline (13); The vacuum pump (11) is used to perform evacuation during the gas replacement process; the pumping speed regulating valve (12) is electrically connected to the vacuum pump (11) and is used to control the evacuation rate of the vacuum pump (11); one end of the pumping pipeline (13) is connected to the vacuum pump (11), and the other end of the pumping pipeline (13) is connected to the tank space, the main shielding space and the secondary shielding space.

6. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 1, characterized in that, The air injection system (2) includes an air compressor (21), a refrigerated dryer (22), a buffer tank (23), and a high-conductivity injection line (24) connected in sequence. The air compressor (21) is used to pressurize and charge air, the refrigerated dryer (22) is used to control the dew point value of the air pressurized by the air compressor (21) into the onshore storage tank, the buffer tank (23) is used to buffer and store the pressurized air, and the high conductivity injection line (24) is used to connect the buffer tank (23) to the space inside the tank.

7. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 1, characterized in that, The gas state monitoring system (3) includes a pressure sensor (31), a temperature sensor (32), and a test gas concentration sensor (33); The pressure sensor (31) is disposed in the tank space, the main shielding space and the secondary shielding space, and is used to monitor the pressure in the tank space, the main shielding space and the secondary shielding space in real time; the temperature sensor (32) is disposed in the tank space and is used to monitor the temperature in the tank space in real time; the test gas concentration sensor (33) is disposed in the tank space and the main shielding space and is used to monitor the test gas concentration in the tank space and the main shielding space in real time.

8. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 1, characterized in that, The safety system (4) includes a pressure relief valve (41); the pressure relief valve (41) is connected to the tank space, and when the pressure in the tank space exceeds a preset pressure threshold, the pressure relief valve (41) releases pressure.

9. The method for determining the overall leakage rate of the main shielding layer of an onshore storage tank according to claim 8, characterized in that, The safety system (4) also includes an alarm (42) which is electrically connected to the pressure relief valve (41). When the pressure relief valve (41) is depressurized, the alarm (42) will sound an alarm simultaneously.

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