A step-by-step constant load full-scale test system and method for a defective steel pipe

Abstract

The application discloses a kind of step-by-step constant load full-size test system and method of steel pipe containing defect, by pressure controller opens first pump and first water valve, increases the test pressure to below the level of direct failure pressure by injecting liquid into sealed steel pipe, then keep under this pressure until failure occurs, then by pressure controller opens second pump and second water valve, extracts the liquid in sealed steel pipe.In the process of test, by measuring device and pressure sensor, the information such as pressure, crack propagation and strain of defect site is collected to predict the failure pressure of sealed steel pipe containing defect related to time, and then determine the empirical threshold of pipeline delayed fracture.The application can obtain the creep fracture response state under different defect sizes and different pressures, maximize the observed time response of each defect in the test container, reduce the number of tests, reduce the cost and defect processing time, etc., to provide a test basis for accurately predicting the delayed failure pressure and service life of steel pipe.

Description

Technical Field

[0001] This invention belongs to the technical field of natural gas pipeline structural integrity, and relates to a step-by-step constant load full-size test system and method for defective steel pipes. Background Technology

[0002] In terms of pipeline service safety, delayed fracture is increasingly becoming a focus of attention. There are two main considerations regarding the mechanism of delayed fracture. One is hydrogen intrusion into components, primarily occurring during electrochemical reactions accompanying corrosion. The other mechanism is room temperature creep. Creep is a deformation mode of metallic materials under high-temperature conditions and has long been a subject of widespread interest. Studies have found that even high-strength steel undergoes slow and permanent plastic deformation under constant loads. However, recent research has revealed that even at relatively low isotropic temperatures (such as room temperature), the deformation of certain metallic materials (such as stainless steel, pipeline steel, high-strength steel, and titanium alloys) increases over time when subjected to constant loads. Especially in situations with stress concentration, even under relatively low nominal stress conditions, significant room temperature creep can occur in certain areas of severe stress concentration, leading to pipeline leaks and explosions, causing enormous disasters and losses.

[0003] Currently, many scholars have conducted small-scale creep tests in the laboratory to investigate the effects of room temperature creep on fatigue crack propagation behavior in X70 pipeline steel and SUS304 stainless steel, and have analyzed and discussed the microscopic mechanisms. However, these studies are all based on the results of small-scale samples and lack the actual changes in defects and failure behavior during pipeline operation. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies where creep tests are based on research results from small-sized samples, lacking the ability to accurately reflect the actual changes in defects and failure behaviors during pipeline operation. This invention provides a step-by-step constant load full-size test system and method for steel pipes with defects.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] This invention proposes a step-by-step constant load full-size test system for defective steel pipes, comprising a pressure controller and a measuring device for measuring crack defects. The measuring device is installed at the crack defect on the surface of the sealed steel pipe. A first pipe and a second pipe are respectively connected to the two ends of the sealed steel pipe. A first pressure measuring device, a first water valve, and a first pump are sequentially installed on the first pipe. A second pressure measuring device, a second water valve, an air vent valve, and a second pump are sequentially installed on the second pipe. Both the first pressure measuring device and the second pressure measuring device are positioned close to the end face of the sealed steel pipe.

[0007] The first input port of the pressure controller is connected to the first pressure measuring device, the second input port of the pressure controller is connected to the second pressure measuring device, the first output port of the pressure controller is connected to the first water valve, the second output port of the pressure controller is connected to the first pump, the third output port of the pressure controller is connected to the second water valve, the fourth output port of the pressure controller is connected to the second pump, the fifth output port of the pressure controller is connected to the vent valve, and the third input port of the pressure controller is connected to the measuring device.

[0008] Preferably, an air inlet valve is provided on the first pipeline, the air inlet valve is located between the first water valve and the first pump, and the sixth output port of the pressure controller is connected to the air inlet valve.

[0009] Preferably, the measuring device includes a temperature measuring device, a strain measuring device, and a CMOD measuring device, all of which are connected to the pressure controller.

[0010] Preferably, the temperature measuring device is a patch-type temperature sensor with a range of -60℃ to 50℃, and the temperature measuring device is installed 500mm away from the tip of the crack defect in the sealed steel pipe along the axial direction of the sealed steel pipe; the CMOD measuring device is a clamp-type extensometer, and there are two clamp-type extensometers located at the center of the defect and at one-quarter of the length of the defect, respectively.

[0011] Preferably, the strain measuring device is a strain gauge, the range of the strain measuring device shall not be less than 20000με, and the strain measuring device is located at the tip and middle of the crack defect in the sealed steel pipe.

[0012] Four strain gauges are distributed at the tip of the crack defect in the sealed steel pipe and two strain gauges are distributed in the middle part of the crack defect in the sealed steel pipe; each strain gauge has three channels.

[0013] Preferably, both the first and second pipes are made of hard stainless steel or are high-pressure flexible hoses with a rated capacity of 100 MPa; both the first and second pressure measuring devices are pressure sensors with a range of 0 MPa to 40 MPa.

[0014] The present invention proposes a method for a step-by-step constant load full-scale test system for defective steel pipes, comprising the following steps:

[0015] The two ends of the test steel pipe are sealed to form a sealed steel pipe, and crack defects are machined on the surface of the sealed steel pipe;

[0016] The second pump and second water valve are shut off by the pressure controller, while the first pump and first water valve are opened to inject liquid into the sealed steel pipe. At the same time, the vent valve is opened to extract the gas inside the sealed steel pipe, and then the vent valve is closed. The first pressure measuring device collects the liquid pressure flowing through the first pipeline and transmits it to the pressure controller. When the test pressure of the sealed steel pipe increases to the desired pressure value, it is maintained at the desired pressure value for at least 2 days, wherein the desired pressure value is 80% of the direct failure pressure. The desired pressure value is then increased by an increment and maintained for at least 2 days until the sealed steel pipe leaks, wherein the increment is at least 5% of the direct failure pressure. During the test, the measuring device collects the crack defect temperature, crack size, and crack deformation of the sealed steel pipe in real time and transmits the collected data to the pressure controller.

[0017] The pressure controller shuts off the first pump and the first water valve, and opens the second pump and the second water valve to extract liquid from the sealed steel pipe. The second pressure measuring device collects the liquid pressure flowing through the second pipe and transmits it to the pressure controller.

[0018] Preferably, the direct failure pressure P is obtained by formula (1);

[0019]

[0020]

[0021]

[0022]

[0023] Where T is the pipe wall thickness, D is the pipe outer diameter, CVN is the Charpy impact energy, E is the elastic modulus, and σ F For the failure circumferential stress, σ flow For rheological stress, σ YS For yield strength, σ TS For tensile strength, A c Let M be the cross-sectional area of ​​the impact specimen, c be the crack length, and M be the cross-sectional area of ​​the specimen. s It is a swelling factor.

[0024] Preferably, defect processing is performed using wire cutting or tool cutting.

[0025] The wire cutting method includes the following steps:

[0026] A 6mm wide notch is machined on the surface of the sealing steel pipe, and the depth of the notch is approximately 30% of the total depth of the sealing steel pipe;

[0027] A 0.15mm wide slit is introduced using electrical discharge machining;

[0028] The cutting method includes the following steps:

[0029] The sealing steel pipe surface is directly cut using a 50.8mm diameter disc;

[0030] The last 0.75mm of the defect depth is machined using a 0.15mm thick square profile tool.

[0031] Preferably, a sealed steel pipe is formed by sealing and welding a pipe cap to the test steel pipe.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] This invention proposes a step-by-step constant load full-size test system for defective steel pipes. In failure analysis tests for creep rupture in high-grade steel pipes, existing technologies all employ small-scale tensile tests to obtain the material's sensitivity and stability response to load and time. However, for long-distance oil and gas pipelines, the pipe wall thickness is often greater than that of small-scale samples, and small-scale samples cannot fully represent the true performance of the steel pipe, especially for defective pipes. Therefore, a full-size constant load test with defects is the most effective method. Secondly, this invention installs a first water valve and a first pump on a first pipeline. The purpose is to simultaneously close the second pump and the second water valve by opening the first water valve and the first pump through a pressure controller, injecting liquid into the sealed steel pipe, and then testing the first pipeline... A first pressure measuring device is installed at the inlet of the sealed steel pipe to monitor the pressure value flowing through the first pipe. A second water valve and a second pump are installed on the second pipe. The purpose is to allow the liquid in the sealed steel pipe to be pumped out by simultaneously closing the first water valve and the first pump while the second water valve and the first pump are opened, controlled by a pressure controller, after the sealed steel pipe is filled with liquid. A second pressure measuring device is installed at the outlet of the sealed steel pipe on the second pipe to monitor the pressure value flowing through the second pipe. Then, an air vent valve is installed on the second pipe to completely vent the air in the sealed steel pipe when liquid is injected. Finally, by installing a measuring device at the crack defect in the sealed steel pipe and transmitting the measured data to the pressure controller, the crack defect can be measured and its changes can be monitored in real time. Therefore, the test system proposed in this invention monitors the time-related failure pressure of the defective sealed steel pipe in real time through the first pressure measuring device, the second pressure measuring device, and the measuring device, thereby determining the empirical threshold for delayed pipe fracture. This allows for accurate prediction of the actual changes and failure behavior of the steel pipe, providing technical support for accurately predicting the delayed failure pressure and service life of the steel pipe.

[0034] Furthermore, by installing an air inlet valve on the first pipeline, gas can be injected into the sealed steel pipe as needed for the test.

[0035] Furthermore, four strain gauges and two strain gauges are distributed at the tip and middle of the crack defect in the sealed steel pipe, respectively. These are mainly used to measure the strain changes at the tip and middle of the defect as it expands. The strain gauges on both sides can obtain the difference in strain changes on both sides as the defect expands. Each strain gauge has three channels, which can be used to measure the circumferential, 45-degree, and axial strain of the sealed steel pipe.

[0036] Furthermore, the use of a crack CMOD measurement device can effectively monitor the deformation of the cracked area over time and pressure, obtain the CMOD response over time, and thus determine whether the pipe has reached a stable response state or a failure state under the pressure during the test.

[0037] Furthermore, a first pressure measuring device and a second measuring device with a range of 0-40MPa are adopted, and the range of the strain gauge must not be less than 20000με, which can ensure the accuracy and effectiveness of pressure and strain measurement; the temperature measuring device has a range of -60℃ to 50℃, which can realize measurement in low temperature environment and room temperature environment.

[0038] This invention proposes a test method for a step-by-step constant load full-scale test system for defective steel pipes. This method, by gradually increasing the pressure by at least 5%, not only avoids direct pipe failure but also monitors crack deformation under different pressures, providing a large amount of experimental data for accurately predicting the delayed failure pressure and service life of the steel pipe. This method allows observation of crack propagation of different sizes in a single full-scale test, making it more economical and simple. This invention is not only economical, simple, and easy to operate, but it can also effectively obtain the creep rupture response state under different defect sizes and pressures, maximizing the observed correlation time response of each defect in the test vessel.

[0039] Furthermore, for high-grade steel pipes, crack-type defects are currently the most serious type of pipe failure. Because the actual crack tip is very narrow, fatigue testing is usually required to pre-induce the crack. However, for full-size steel pipes, due to their unique geometry, current fatigue pre-indulation equipment cannot meet the requirements. Using wire cutting or direct cutting methods to process steel pipe defects can effectively replace fatigue pre-indulation of cracks, ensuring the effectiveness of the test. By machining defects on the surface of the test steel pipe, the creep fracture behavior of the defects in response to time is monitored. In the wire cutting method, a 6mm wide notch is first machined using a larger tool, and then a 0.15mm wide narrow slit is introduced using electrical discharge machining (EDM). This not only simulates a real crack but also effectively improves processing efficiency. Using a disc tool directly, although the machined defect is relatively large, the test results are similar to those of EDM, while further improving processing efficiency and saving costs. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a structural diagram of the full-size test system for step-by-step constant load on steel pipes according to the present invention.

[0042] Figure 2 This is a machining diagram of the defects in the sealing steel pipe of the present invention.

[0043] Figure 3 This is a structural diagram showing the location of the strain gauge and extensometer in this invention.

[0044] Figure 4 This is a time-dependent steady-state response diagram of CMOD in this invention.

[0045] Figure 5 This is a time-dependent instability response diagram of CMOD in this invention. Detailed Implementation

[0046] 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, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0047] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0048] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0049] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper," "lower," "horizontal," or "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, terms such as "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0050] Furthermore, the use of the term "horizontal" does not imply that the component must be absolutely horizontal, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0051] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0052] The present invention will now be described in further detail with reference to the accompanying drawings:

[0053] This invention proposes a step-by-step constant load full-scale test system for defective steel pipes, such as... Figure 1 As shown, it includes a pressure controller 1, a first pressure measuring device 2, a first water valve 3, a first pump 4, an air inlet valve 12, a second pressure measuring device 5, a second water valve 6, an exhaust valve 11, a second pump 7, and a measuring device, which includes a temperature measuring device 8, a strain measuring device 9, and a CMOD measuring device 10.

[0054] A first pipe and a second pipe are connected to the two ends of a sealed steel pipe, respectively. A first pressure measuring device 2, a first water valve 3, an air inlet valve 12, and a first pump 4 are sequentially installed on the first pipe. A second pressure measuring device 5, a second water valve 6, an air vent valve 11, and a second pump 7 are sequentially installed on the second pipe. Both the first pressure measuring device 2 and the second pressure measuring device 5 are close to the end face of the sealed steel pipe, while the first pump 4 and the second pump 7 are far from the end face of the sealed steel pipe. The first input port of a pressure controller 1 is connected to the first pressure measuring device 2, the second input port of a pressure controller 1 is connected to the second pressure measuring device 5, the first output port of a pressure controller 1 is connected to the first water valve 3, the second output port of a pressure controller 1 is connected to the first pump 4, the third output port of a pressure controller 1 is connected to the second water valve 6, the fourth output port of a pressure controller 1 is connected to the second pump 7, the fifth output port of a pressure controller 1 is connected to the air vent valve 11 to completely expel air from inside the sealed steel pipe, and the sixth output port of a pressure controller 1 is connected to the air inlet valve 12. Both the first and second pipelines are made of hard stainless steel or high-pressure flexible hoses with a rated capacity of 100 MPa. Both the first pressure measuring device 2 and the second pressure measuring device 5 are pressure sensors with a range of 0 MPa-40 MPa, ensuring the accuracy and effectiveness of pressure measurement. An inlet valve 12 is installed on the first pipeline, allowing gas to be injected into the sealed steel pipe as needed for the test.

[0055] The measuring devices are installed at the crack defect on the surface of the sealed steel pipe. Temperature measuring device 8, strain measuring device 9, and CMOD measuring device 10 are all connected to the third input port of pressure controller 1. Specifically, temperature measuring device 8 is a patch-type temperature sensor with a range of -60℃ to 50℃, suitable for measurements in low-temperature and room-temperature environments. Temperature measuring device 8 is installed 500mm from the tip of the crack defect along the axial direction of the sealed steel pipe. Strain measuring device 9 is a strain gauge with a range of not less than 20000με to ensure the accuracy and effectiveness of strain measurement. Strain measuring device 9 is located at the tip and middle of the crack defect in the sealed steel pipe, with four strain gauges at the tip and two at the middle, respectively. These are mainly used to measure strain changes at the tip and middle of the defect during expansion. The strain gauges on both sides can obtain the difference in strain changes on both sides during expansion. Each strain gauge has three channels, which can be used to measure the circumferential, 45-degree, and axial strain of the steel pipe. The CMOD measuring device has a 10-position extensometer, with two clamp-type extensometers located at the defect center and one at a quarter-length position of the defect, respectively.

[0056] The present invention proposes a test method for a step-by-step constant load full-scale test system for defective steel pipes, comprising the following steps:

[0057] The two ends of the test steel pipe are sealed to form a sealed steel pipe, and crack defects are machined on the surface of the sealed steel pipe;

[0058] The second pump 7 and the second water valve 6 are shut off by the pressure controller 1, and the first pump 4 and the first water valve 3 are opened to inject liquid into the sealed steel pipe. At the same time, the exhaust valve 11 is opened to extract the gas in the sealed steel pipe, and then the exhaust valve 11 is closed. The first pressure measuring device 2 collects the liquid pressure flowing through the first pipeline and transmits it to the pressure controller 1. When the test pressure of the sealed steel pipe increases to the expected pressure value, it is maintained at the expected pressure value for at least 2 days, wherein the expected pressure value is 80% of the direct failure pressure. The expected pressure value is increased by an increment and maintained for at least 2 days until leakage failure occurs. During the test, the measuring device collects the crack defect temperature, crack size and crack deformation of the sealed steel pipe in real time and transmits the collected data to the pressure controller 1.

[0059] The first pump 4 and the first water valve 3 are shut off by the pressure controller 1, and the second pump 7 and the second water valve 6 are opened to extract the liquid in the sealed steel pipe. The second pressure measuring device 5 collects the liquid pressure flowing through the second pipeline and transmits it to the pressure controller 1; wherein the increment is at least 5% of the direct failure pressure.

[0060] The direct failure pressure P is obtained through formula (1):

[0061]

[0062]

[0063]

[0064]

[0065] Where T is the pipe wall thickness, D is the pipe outer diameter, CVN is the Charpy impact energy, E is the elastic modulus, and σ F For the failure circumferential stress, σ flow For rheological stress, σ YS For yield strength, σ TS For tensile strength, A c Let M be the cross-sectional area of ​​the impact specimen, c be the crack length, and M be the cross-sectional area of ​​the specimen. s It is a swelling factor.

[0066] The test method for a step-by-step, full-scale constant-load test system for defective steel pipes includes the following steps:

[0067] S1. Using machining methods, one or more surface defects are machined on the test steel pipe according to the test requirements; pre-cracks are pre-fabricated in the middle of the steel pipe, and one or more defects of different sizes can be machined on the test steel pipe, such as... Figure 2 As shown, the number and size of defects are determined according to the test plan;

[0068] Defects are removed by mechanical processing, which can be done using the following two methods.

[0069] 1) Wire cutting method: First, process a 6mm wide notch on the surface of the steel pipe, with a depth of about 30% of the total depth. Second, use electrical discharge machining to introduce a 0.15mm wide slit.

[0070] 2) Tool cutting method: Direct cutting is performed on the surface of the steel pipe using a disc with a diameter of 50.8mm (or other). For the last 0.75mm of the defect depth, a square profile tool with a thickness of 0.15mm (or less) is used for machining.

[0071] When processing multiple defects, the nominal distances between the defects around the surface of the steel pipe are equal and far enough that they do not interact with each other or at the ends of the steel pipe.

[0072] S2. Seal both ends of the test steel pipe: Use pipe caps or end caps to weld to the test steel pipe to completely seal both ends of the test steel pipe, and perform non-destructive testing on the welded circumferential weld of the pipe cap to ensure that there is no risk of leakage in the steel pipe;

[0073] A first pressure measuring device 2, a first water valve 3, an air inlet valve 12, and a first pump 4 are sequentially installed on the first pipeline. A second pressure measuring device 5, a second water valve 6, an air vent valve 11, and a second pump 7 are sequentially installed on the second pipeline. Both the first and second pipelines are made of hard stainless steel or high-pressure flexible hoses. The first pressure measuring device 2 and the second pressure measuring device 5 are respectively installed at the inlet and outlet of the sealed steel pipe to monitor the pressure changes in the sealed steel pipe during the test. The first input port of pressure controller 1 is connected to the first pressure measuring device 2, the second input port of pressure controller 1 is connected to the second pressure measuring device 5, the first output port of pressure controller 1 is connected to the first water valve 3, the second output port of pressure controller 1 is connected to the first pump 4, the third output port of pressure controller 1 is connected to the second water valve 6, the fourth output port of pressure controller 1 is connected to the second pump 7, the fifth output port of pressure controller 1 is connected to the exhaust valve 11, and the sixth output port of pressure controller 1 is connected to the air inlet valve 12; the measuring device includes a temperature measuring device 8, a strain measuring device 9, and a CMOD measuring device 10, used to measure the pressure, crack propagation, and strain at the crack defect.

[0074] The strain measuring device 9 is a strain gauge, which is used to measure the strain in three directions (0°, 45°, and 90°) at one or more defects on the test steel pipe. The range of the strain measuring device 9 must not be less than 20000 με to ensure the accuracy and effectiveness of the strain measurement. Figure 3As shown, strain measurement device 9 is located at the tip and middle of the crack defect in the sealed steel pipe. Four strain gauges are distributed at the tip and two at the middle of the crack defect, respectively, primarily used to measure strain changes at the tip and middle of the defect during its propagation. The strain gauges on both sides can obtain the difference in strain changes on both sides during propagation. Each strain gauge has three channels, which can be used to measure the circumferential, 45-degree, and axial strain of the steel pipe. Temperature measurement device 8 is a patch-type temperature sensor with a range of -60℃ to 50℃, suitable for measurements in low-temperature and room-temperature environments. Temperature measurement device 8 is installed 500mm from the tip of the crack defect along the axial direction of the sealed steel pipe.

[0075] At the pre-fabricated defect opening in the sealed steel pipe, two clamp-on extensometers are used to monitor and record the CMOD of each defect; the first is located at the center of the defect, and the second at a quarter-length position. The cutting edge is secured to the sealed pipe by attaching a stud to the pipe surface via a threaded connector. The stud base provides sufficient height to create a clearance between the pipe surface and the tip of the gauge arm. The cutting edge opening is set to 2.5 mm, and the cutting edge height above the pipe surface should be recorded.

[0076] S3. Pressurize the sealed steel pipe using a pressurization device, and monitor the deformation of the steel pipe defects and its response to time using a measuring device.

[0077] Using a pressurization device, liquid is injected into the sealed steel pipe through the inlet of the sealed pipe. The exhaust valve 11 of the second pipe is opened until the air inside the sealed steel pipe is completely removed, and then the exhaust valve 11 is closed.

[0078] The pressure is continuously increased by the pressurizing equipment until the sealed steel pipe reaches the expected pressure value, which is 80% of the direct failure pressure. Then the pressurization is stopped, and the test pressure is maintained for a certain period of time (at least 48 hours) according to the test requirements. At the same time, the CMOD response of the crack defect to time is monitored, and the CMOD response curve to time is plotted to determine the stable state of the defect.

[0079] like Figure 4 and Figure 5 As shown, when the CMOD reaches a stable response state, the sealed steel pipe is pressurized until the pressure increases by 5%, then the pressurization is stopped, and the test pressure is maintained for a certain period of time (at least 48 hours). The response curve of CMOD versus time is plotted to determine the stable state of the defect.

[0080] Based on the monitored test results, the pressure was repeatedly increased and maintained, and the CMOD of the crack was monitored in relation to time until the defect became unstable or failed.

[0081] S4. Turn off the pressurization equipment, control the exhaust valve 11 and the second pump 7 through the pressure controller 1 to completely empty the remaining gas and liquid in the sealed steel pipe, and hoist the tested steel pipe to other dry areas to carry out rust prevention treatment on the defective parts of the steel pipe.

[0082] S5. Cut off the defective part of the failed steel pipe and separate the crack defect along the axial direction. Use measuring instruments to measure and record the original crack defect size and the crack defect size (length and depth) after failure.

[0083] S6. Determine the delayed failure pressure threshold for different defect sizes based on different defect sizes, test pressures, and holding times.

[0084] This invention presents a test system and method for a step-by-step constant load full-scale test system for defective steel pipes. It can accurately obtain the time-related delayed failure pressure and service life of one or more defects at a level below the direct failure pressure, taking into account factors such as the steel pipe's structure, dimensions, and material properties. This provides data support for defect repair and replacement of gas pipelines, ensuring their safe operation. Currently, test methods for delayed creep failure in pipelines are all small-scale tests, which cannot truly reflect the actual performance and operating status of the pipe material. This method, verified through full-scale testing, fills the aforementioned gaps and deficiencies in existing technologies, providing a reliable test method for accurately predicting the delayed failure pressure and service life of steel pipes. Using a constant load full-scale test method to predict the time-related failure pressure of defective gas pipelines, and thus determine the empirical threshold for delayed pipeline fracture, is currently the best approach. Typically, in burst tests of defective pipelines, a single defect is fabricated on the steel pipe. However, in tests investigating the correlation between different defect sizes and failure pressure, multiple full-scale tests with varying defect sizes are required. This new method allows for the simultaneous fabrication of multiple defects on the steel pipe, followed by pressure holding to observe the changes in different defects under the same pressure. This method significantly reduces experimental research costs. Furthermore, since several crack defects can be of different sizes, tests can be conducted simultaneously on defects of varying dimensions to determine the critical defect size, greatly reducing the number of tests and lowering costs.

[0085] The above are merely preferred embodiments of the present invention and are not intended to limit the present 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 method for a step-by-step constant load full-scale test system for defective steel pipes, characterized in that, A step-by-step constant load full-size test system for defective steel pipes is adopted, including a pressure controller (1) and a measuring device for measuring crack defects. The measuring device is installed at the crack defects on the surface of the sealed steel pipe. The two ends of the sealed steel pipe are respectively connected to a first pipe and a second pipe. The first pipe is sequentially equipped with a first pressure measuring device (2), a first water valve (3) and a first pump (4). The second pipe is sequentially equipped with a second pressure measuring device (5), a second water valve (6), an air vent valve (11) and a second pump (7). The first pressure measuring device (2) and the second pressure measuring device (5) are both set close to the end face of the sealed steel pipe. The first input port of the pressure controller (1) is connected to the first pressure measuring device (2), the second input port of the pressure controller (1) is connected to the second pressure measuring device (5), the first output port of the pressure controller (1) is connected to the first water valve (3), the second output port of the pressure controller (1) is connected to the first pump (4), the third output port of the pressure controller (1) is connected to the second water valve (6), the fourth output port of the pressure controller (1) is connected to the second pump (7), the fifth output port of the pressure controller (1) is connected to the exhaust valve (11), and the third input port of the pressure controller (1) is connected to the measuring device. The method includes the following steps: The two ends of the test steel pipe are sealed to form a sealed steel pipe. According to the test requirements, multiple crack defects of different sizes are processed on the surface of the sealed steel pipe. The nominal distance between the defects around the surface of the steel pipe is equal and far enough that they will not interact with each other or at the end of the steel pipe. The second pump (7) and the second water valve (6) are shut off by the pressure controller (1), and the first pump (4) and the first water valve (3) are opened to inject liquid into the sealed steel pipe. At the same time, the exhaust valve (11) is opened to extract the gas in the sealed steel pipe and the exhaust valve (11) is closed. The first pressure measuring device (2) collects the liquid pressure flowing through the first pipeline and transmits it to the pressure controller (1). When the test pressure of the sealed steel pipe increases to the expected pressure value, it is maintained at the expected pressure value for at least 2 days, wherein the expected pressure value is 80% of the direct failure pressure. The expected pressure value is increased by an increment and maintained for at least 2 days until the sealed steel pipe leaks, wherein the increment is at least 5% of the direct failure pressure. During the test, the measuring device collects the crack defect temperature, crack size and crack deformation of the sealed steel pipe in real time and transmits the collected data to the pressure controller (1). The first pump (4) and the first water valve (3) are shut off by the pressure controller (1), and the second pump (7) and the second water valve (6) are opened to extract the liquid in the sealed steel pipe. The second pressure measuring device (5) collects the liquid pressure flowing through the second pipe and transmits it to the pressure controller (1).

2. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 1, characterized in that, An air inlet valve (12) is provided on the first pipeline. The air inlet valve (12) is located between the first water valve (3) and the first pump (4). The sixth output port of the pressure controller (1) is connected to the air inlet valve (12).

3. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 1, characterized in that, The measuring device includes a temperature measuring device (8), a strain measuring device (9), and a CMOD measuring device (10), all of which are connected to the pressure controller (1).

4. The method for the step-by-step constant load full-scale test system for defective steel pipes according to claim 3, characterized in that, The temperature measuring device (8) is a patch-type temperature sensor with a range of -60℃ to 50℃. The temperature measuring device (8) is installed 500mm away from the tip of the crack defect in the sealed steel pipe along the axial direction of the sealed steel pipe. The CMOD measuring device (10) is a clamp-type extensometer. There are two clamp-type extensometers, which are located at the center of the defect and at one-quarter of the length of the defect, respectively.

5. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 3, characterized in that, The strain measuring device (9) is a strain gauge, and the range of the strain measuring device (9) shall not be less than 20000με. The strain measuring device (9) is located at the tip and middle of the crack defect in the sealed steel pipe. Four strain gauges are distributed at the tip of the crack defect in the sealed steel pipe and two strain gauges are distributed in the middle part of the crack defect in the sealed steel pipe; each strain gauge has three channels.

6. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 1, characterized in that, Both the first and second pipelines are made of hard stainless steel or are high-pressure flexible hoses with a rated capacity of 100MPa; both the first pressure measuring device (2) and the second pressure measuring device (5) are pressure sensors with a range of 0MPa-40MPa.

7. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 1, characterized in that, Direct failure pressure It is obtained through formula (1); （1） （2） （3） （4） Where T is the pipe wall thickness. The outer diameter of the pipe. For Charpy impact energy, For elastic modulus, For failure circumferential stress, For rheological stress, For yield strength, For tensile strength, The cross-sectional area of ​​the impact specimen. The length of the crack. It is a swelling factor.

8. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 1, characterized in that, Defects are removed using wire cutting or tool cutting. The wire cutting method includes the following steps: A 6 mm wide notch is machined on the surface of the sealing steel pipe, and the depth of the notch is approximately 30% of the total depth of the sealing steel pipe. A 0.15 mm wide slit is introduced using electrical discharge machining; The cutting method includes the following steps: The sealing steel pipe surface is directly cut using a 50.8 mm diameter disc; The last 0.75 mm of the defect depth is machined using a 0.15 mm thick square profile tool.

9. The method for a step-by-step constant load full-scale test system for defective steel pipes according to claim 1, characterized in that, A sealed steel pipe is formed by sealing and welding a pipe cap to the test steel pipe.

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