Pipe life prediction method

Abstract

To predict the life of a pipe earlier.SOLUTION: A life prediction method of a pipe formed of stainless steel through which high-temperature fluid flows, includes the steps of: polishing a surface of the pipe; collecting a replica sample from the surface of the pipe after polishing; observing the replica sample with a scanning electron microscope or a laser microscope to obtain a generation amount of minute voids in the replica sample; and predicting the service life of the pipe based on the generation amount of minute voids.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a method for predicting the life of a pipe. [Background technology]

[0002] The piping through which high-temperature fluids flow in boilers and other equipment is subject to the pressure and heat of the high-temperature fluid. Therefore, in order to perform maintenance on the piping at the appropriate time, it is necessary to predict the lifespan of the piping, until creep damage occurs and the piping becomes unusable. Various non-destructive testing methods are sometimes used to predict the lifespan of piping.

[0003] As one of such non-destructive inspection techniques, for example, Patent Document 1 discloses a configuration for performing flaw detection inspection using ultrasonic flaw detection or the like on a target portion of a pipe.

[0004] Another non-destructive testing method is to measure the outer diameter of a pipe. This method focuses on the fact that pipes used for a long period of time expand and deform due to the effects of pressure and heat, and measures the outer diameter of the pipe to determine the expansion of the pipe surface and estimate the pipe's lifespan. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-6801 Summary of the Invention [Problem to be solved by the invention]

[0006] However, since the outer diameter of pipes made of materials with high strength at high temperatures only changes measurably toward the end of the pipe's lifespan, it is difficult to determine the end of the pipe's lifespan earlier using methods that measure the outer diameter of the pipe.

[0007] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a method for predicting the life of a pipe that can predict the life of the pipe at an earlier stage. [Means for solving the problem]

[0008] In order to solve the above-described problems, a method for predicting a lifespan of a pipe according to the present disclosure is a method for predicting a lifespan of a pipe made of stainless steel through which a high-temperature fluid flows, the method comprising: If the period from when the pipe began to be used until the end of its lifespan is taken as 100%, the pipe will be replaced at 60-80% of its lifespan. The method includes the steps of polishing the surface of the pipe, taking a replica sample from the surface of the polished pipe, observing the replica sample with a scanning electron microscope or a laser microscope to obtain the amount of microvoids, which are extremely small defects of 1 nm to 1 μm, occurring in the replica sample, and predicting the service life of the pipe based on the amount of microvoids occurring. [Effects of the Invention]

[0009] According to the method for predicting the lifespan of a pipe disclosed herein, the lifespan of a pipe can be predicted earlier. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view showing an example of a pipe to be subjected to life prediction by a pipe life prediction method according to an embodiment of the present disclosure. [Figure 2] 1 is a diagram showing the occurrence of microvoids, voids, and microcracks in the above-mentioned pipe. FIG. [Figure 3] FIG. 10 is a diagram showing the correlation between the size of voids generated in steam pipes with different life ratios and the void number density. [Figure 4] 1 is a flowchart illustrating the steps of a method for predicting a pipe life according to an embodiment of the present disclosure. [Figure 5] 1 shows an example of an image of a replica sample observed with a scanning microscope. [Figure 6] This is an example of an image of the surface of an actual pipe observed with a scanning microscope. [Figure 7]The following shows an example of specific conditions for a test specimen when creating master data using the above-described method for predicting the life of a pipe. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment for carrying out a method for predicting a pipe life according to the present disclosure will be described with reference to the accompanying drawings. However, the present disclosure is not limited to only this embodiment.

[0012] (Plumbing) The method for predicting a pipe life in this embodiment predicts the life of a pipe through which a high-temperature fluid flows. As shown in FIG. 1 , the pipe 1, the life of which is to be predicted, has a circular cross-sectional shape and extends continuously in a cylindrical shape in the extension direction. The pipe 1 is used in equipment through which high-temperature steam flows as a high-temperature fluid, such as a boiler. The pipe 1 is used, for example, in a power generation plant. The pipe 1 is made of stainless steel that is resistant to corrosion even when exposed to high-temperature steam. The pipe 1 is, for example, a tubular material having an outer diameter of approximately 30 mm to 40 mm and a wall thickness of approximately 3 mm to 4 mm, made of high-strength stainless steel. The pipe 1 is affected by the heat and pressure of the high-temperature fluid flowing through the pipe 1.

[0013] The inventors have discovered that in a pipe 1 made of stainless steel through which such high-temperature fluid flows, microvoids 101 occur on the surface of the pipe 1 much earlier than the end of the pipe's life when creep damage and the like would occur. Here, microvoids are extremely small defects of 1 nm to 1 μm in size that occur from the middle of the pipe's life onwards. Hereinafter, defects of this size will be referred to as "microvoids" in this specification.

[0014] Specifically, as shown in FIG. 2 , according to the findings of the present inventors, if the period from the start of use of the pipe 1 until creep damage or the like occurs and the pipe 1 reaches the end of its life is defined as 100%, no voids are generated in the pipe 1 until the life ratio reaches 50%. Then, as time passes and the life ratio reaches approximately 60%, microvoids 101 begin to generate in the pipe 1 near the surface 1f facing the outer side Dro in the radial direction Dr. As time passes, when the life ratio reaches approximately 80%, voids 102 larger than the microvoids 101 are generated, and the range in which the voids 102 are generated also expands from the surface 1f to the inner side Dri in the radial direction Dr. As time passes, when the life ratio reaches approximately 90%, multiple voids 102 connect to each other, resulting in the generation of microcracks 103. Thus, when many voids 102 and microcracks 103 have generated (e.g., when the life ratio reaches 80% or 90%), the outer diameter of the pipe 1 begins to change to a measurable level. On the other hand, at the stage where only microvoids 101 have occurred (for example, when the life ratio is 60%), the outer diameter of the pipe 1 does not change measurably.

[0015] Here, conventionally, the size of voids detectable by optical microscopes or ultrasonic flaw detection is, for example, 0.2 μm or larger. As shown in FIG. 3, the size of voids 102 at a stage where the life ratio is about 80% is about 1 μm to 20 μm, and the occurrence of voids 102 is detectable by observation with an optical microscope. In contrast, the size of microvoids 101 that occur at a stage where the life ratio is about 60% is 1 nm or larger and 1.0 μm or smaller, and it is difficult to detect microvoids 101 by observation with an optical microscope. For this reason, it was conventionally believed that no observable damage occurs inside pipe 1 until the outer diameter of pipe 1 begins to change at a measurable level. However, the inventors have discovered that microvoids 101 occur earlier than the time when voids 102 of a size detectable by observation with an optical microscope or ultrasonic flaw detection occur.

[0016] (Pipe life prediction method) In the pipe life prediction method S10 according to this embodiment, which will be described below, the life of the pipe 1 is predicted by detecting the occurrence of microvoids 101 that are difficult to detect using an optical microscope. As shown in Fig. 4, the pipe life prediction method S10 includes a step S11 of polishing the pipe, a step S12 of taking a replica sample, a step S13 of acquiring the amount of microvoids that occur, a step S14 of predicting the service life of the pipe, and a step S21 of creating master data.

[0017] In the pipe polishing step S11, the surface 1f of the pipe 1 is polished. The polishing of the surface 1f of the pipe 1 is preferably performed by a method that enables detection of microvoids 101 while suppressing the detachment of precipitates deposited on the surface 1f. For example, CMP (Chemical Mechanical Polishing) using colloidal silica or electrolytic polishing using oxalic acid is used to polish the surface 1f of the pipe 1.

[0018] In step S12 of collecting a replica sample, a replica sample is collected from the surface 1f of the pipe 1 after polishing. The replica sample is collected without destroying the pipe 1 after polishing. The replica sample is collected using a known replica method. For example, a liquid resin is applied to the surface 1f of the pipe 1, and the resin is cured to obtain a replica sample with the surface 1f of the pipe 1 transferred thereto. Alternatively, a replica sample with the surface 1f of the pipe 1 transferred thereto can be obtained by heating and pressurizing a solid polymer onto the surface 1f of the pipe 1. When a replica sample is collected from the surface of the pipe 1 after polishing, if microvoids 101 have occurred on the surface 1f of the pipe 1, the microvoids 101 will be transferred to the replica sample.

[0019] In step S13, which acquires the amount of microvoids, the replica sample collected in step S12 is observed with a scanning electron microscope (SEM) or a laser microscope, and the amount of microvoids in the replica sample is acquired. Microvoids 101 of the above-described size can be clearly observed with a scanning electron microscope or a laser microscope. FIG. 5 shows an example of an image of the replica sample observed with a scanning microscope. For comparison, FIG. 6 shows an example of an image of the surface 1f of an actual pipe 1 observed with a scanning microscope. As shown in FIGS. 5 and 6, by observing the replica sample obtained as described above with a scanning electron microscope or a laser microscope, the occurrence of microvoids 101 similar to that in the actual pipe 1 can be observed. In step S13, the amount of microvoids 101 in the replica sample is acquired, for example, as a void number density, which indicates the number of microvoids 101 per unit area.

[0020] In step S14 of predicting the useful life of the pipe, the useful life of the pipe 1 is predicted based on the amount of microvoids generated obtained in step S13. In this embodiment, in step S14, the amount of microvoids generated obtained in step S13 is compared with data showing the correlation between the amount of microvoids generated and the useful life of the pipe 1, thereby predicting the useful life of the pipe 1. In step S14, the generation state (generation amount) of microvoids 101 in the actual pipe 1 is compared with master data described below, thereby predicting the useful life of the actual pipe 1. Specifically, of the multiple data included in the master data, data whose generation amount of microvoids 101 obtained in step S13 is closest to the generation state (generation amount) of microvoids 101 in the actual pipe 1 is identified. Then, the useful life associated with the identified data is acquired as the useful life of the pipe 1.

[0021] When master data is used in step S14, the master data is created by performing step S21 in advance before performing step S14 of predicting the service life of the pipe 1. In step S21 of creating the master data, the master data to be used in step S14 of predicting the service life of the pipe 1 is created. The master data is created using a test specimen that simulates the pipe 1, rather than the actual pipe 1. In this case, multiple test specimens are used that simulate multiple pipes 1 with different usage conditions. The test specimens have the same material, diameter dimensions, and wall thickness as the pipe 1. Creep tests are performed on the multiple test specimens and observations are made to obtain the data necessary to create the master data.

[0022] The test on the test specimen is a creep test in which a predetermined temperature and stress are applied to simulate the flow of high-temperature fluid. During this test, for example, the time elapsed from the start of the test is varied in multiple stages, and the state of microvoids 101 generated on the surface of the test specimen is observed at each stage. Furthermore, the temperature and applied stress during the test are changed, and the state of microvoids 101 generated on the surface of the test specimen is observed at each stage. The test is continued until the test specimen finally breaks due to creep damage. In this way, tests are conducted on multiple test specimens that simulate multiple pipes 1 in different usage conditions.

[0023] It is preferable to carry out the test on the test specimen under conditions that are more severe than the temperature and stress caused by the high-temperature fluid flowing through the actual pipe 1. This allows the master data to be created in a shorter time.

[0024] When observing the test specimen, first, the surface of the test specimen is polished using the same polishing method as in step S11 for polishing the pipe. Then, a replica sample of the surface of the test specimen is obtained in the same manner as in step S12 for collecting replica samples. Furthermore, similar to step S13 for obtaining the amount of microvoids 101 generated, the obtained replica sample of the test specimen is observed with a scanning electron microscope or a laser microscope, and the amount of microvoids 101 generated in the replica sample is obtained. In this manner, multiple pieces of data are obtained from the surfaces of multiple test specimens simulating multiple pipes 1 with different usage conditions. Furthermore, for example, the life ratio (T / Tb) of each test specimen is calculated based on the elapsed time T from the start of the test to the time of observation for each test specimen and the elapsed time Tb from the start of the test to fracture. The elapsed time T during observation is the time when the replica sample is obtained, and is the time when the amount of microvoids 101 generated is obtained. Then, replica samples are obtained from the surface of the test specimen according to the life ratio (T / Tb). This allows data on the change over time in the amount of microvoids 101 generated in the test specimen to be obtained. That is, the correlation between the change over time in the amount of microvoids 101 generated and the time (life) elapsed until rupture occurs is acquired as master data.

[0025] FIG. 7 shows an example of the specific conditions for the test specimen when creating the master data in step S21. As shown in FIG. 7, a creep test was conducted, for example, at 750°C and with a pressure of 61 MPa applied to the test specimen. The test was interrupted every 1000 hours from the start of the test, and replica samples were taken from the surface of the test specimen. The condition of the replica samples (surface condition of the test specimen) was observed using a laser microscope. The test specimen fractured due to creep damage 11,911 hours after the start of the test. Therefore, master data was created by correlating the number of microvoids 101 generated with the life ratio at 7000, 8000, and 9000 hours from the start of the test until fracture.

[0026] Note that Figure 7 is merely an example of the specific conditions for the test specimen when creating the master data, and the test conditions for the test specimen when creating the master data can be selected appropriately depending on the type of pipe 1 whose lifespan is actually to be improved and the environment in which it is used.

[0027] In this way, the obtained amount of microvoids 101 generated is compared with the master data to predict the service life of the pipe 1.

[0028] (Action and effect) In the pipe life prediction method S10 configured as described above, the surface 1f of the pipe 1 is polished, and a replica sample is collected from the polished surface of the pipe 1. If microvoids 101 are present on the surface 1f of the pipe 1, the microvoids 101 are transferred to the replica sample. The replica sample with the transferred microvoids 101 is observed with a scanning electron microscope or a laser microscope, thereby enabling the amount of microvoids generated in the replica sample to be obtained with high accuracy. Based on the newly acquired knowledge that some microvoids 101 occur earlier than when they grow to a size that can be detected by observation with an optical microscope or flaw detection testing using ultrasonic flaw detection, the microvoids 101 are observed and the amount of microvoids 101 generated is obtained. This allows the service life of the pipe 1 to be predicted at an earlier timing than when the outer diameter of the pipe 1 is measured, observation with an optical microscope, or flaw detection testing using ultrasonic flaw detection. Therefore, the life of the pipe 1 can be predicted earlier. This makes it possible to predict the mid-term life of the pipe 1, which was previously difficult to evaluate. Furthermore, since it is possible to predict the lifespan in the medium term, it is possible to efficiently plan various types of construction work, such as replacement work for the pipe 1 during inspection.

[0029] Furthermore, in step S14 of predicting the useful life of the pipe, the useful life of the pipe 1 is predicted based on the actual amount of microvoids generated and master data showing the correlation between the amount of microvoids generated and the useful life of the pipe 1. Because the master data is created in advance, the correlation between the amount of microvoids 101 generated and the useful life of the pipe 1 can be grasped with high accuracy in advance. As a result, the useful life of the pipe 1 can be predicted more easily and accurately.

[0030] Additionally, replica samples are obtained from the surfaces of multiple test specimens that simulate multiple pipes 1 in different usage states. Then, the amount of microvoids generated in the replica samples of the multiple test specimens is obtained to create master data. By creating master data in this manner, it is possible to obtain data that is close to the generation state of microvoids 101 in actual pipes 1 in different usage states. By predicting the service life of the pipe 1 based on the master data obtained in this manner, it is possible to predict the service life of the pipe 1 with higher accuracy.

[0031] (Other embodiments) The above describes in detail the embodiments of the present disclosure with reference to the drawings, but the specific configuration is not limited to this embodiment, and design changes and the like are also included within the scope that does not deviate from the gist of the present disclosure.

[0032] In the above embodiment, the procedure of the method S10 for predicting the life of the pipe 1 has been described, but the details of the work content and procedures in each step can be changed as appropriate.

[0033] For example, the present invention is not limited to predicting the service life of the pipe 1 using master data showing the correlation between the amount of microvoids generated and the service life of the pipe. In this case, the step S14 of predicting the service life of the pipe may be performed without performing the step S21 of creating the master data. In this case, in the step S14 of predicting the service life of the pipe, data on the actual amount of microvoids 101 generated when the pipe 1 was measured in the past is stored in advance in a storage device or the like, and the stored data is used as data showing the correlation between the amount of microvoids generated and the service life of the pipe 1. Therefore, the service life may be predicted by comparing the amount of microvoids 101 generated with the stored data.

[0034] Furthermore, even when the step S21 of creating the master data is performed, the step S21 of creating the master data may be performed only once or may be performed each time when performing multiple periodic inspections of the piping 1. In other words, the same master data created in advance may be used for each periodic inspection of the piping 1, or new master data may be created each time the piping 1 is inspected.

[0035] Furthermore, in the step S21 of creating the master data, the master data is not limited to being created by conducting tests on a plurality of test specimens that simulate a plurality of pipes 1 with different usage conditions. In the step S21 of creating the master data, for example, the master data may be created by simulation instead of actual testing.

[0036] Furthermore, the high-temperature fluid flowing inside the pipe 1 is not limited to steam. It may be a high-temperature gas or liquid that can cause damage such as corrosion to the pipe 1 made of stainless steel.

[0037] The polishing method in step S11 for polishing the pipe is not limited to CMP using colloidal silica or electrolytic polishing using oxalic acid. It is preferable to test different types of abrasives and polishing methods on the pipe 1 and test specimens in advance, and then apply the polishing method that is most suitable for the pipe 1 to be predicted.

[0038] <Additional Notes> The method S10 for predicting the life of the pipe 1 according to the embodiment can be understood, for example, as follows.

[0039] (1) A first aspect of the method S10 for predicting the service life of a pipe 1 is a method S10 for predicting the service life of a pipe 1 made of stainless steel through which a high-temperature fluid flows, and includes the steps of: a step S11 of polishing the surface 1f of the pipe 1; a step S12 of taking a replica sample from the surface 1f of the pipe 1 after polishing; a step S13 of observing the replica sample with a scanning electron microscope or a laser microscope to obtain the amount of microvoids generated in the replica sample; and a step S14 of predicting the service life of the pipe 1 based on the amount of microvoids generated.

[0040] The inventors have found that in a pipe 1 made of stainless steel and through which a high-temperature fluid flows, microvoids 101 are generated over time in the radial direction Dr of the pipe 1 near the surface 1f due to the influence of the temperature and pressure of the high-temperature fluid. These microvoids 101 begin to generate earlier than the time when the outer diameter of the pipe 1 changes measurably. Furthermore, the microvoids 101 are much smaller than voids that can be detected by observation with an optical microscope or flaw detection testing using ultrasonic flaw detection. In other words, the microvoids 101 are generated earlier than the time when voids grow to a size that can be detected by observation with an optical microscope or flaw detection testing using ultrasonic flaw detection.

[0041] In this method S10 for predicting the lifespan of a pipe 1, the amount of microvoids generated in a replica sample is obtained based on the newly obtained knowledge as described above. This allows the service life of the pipe 1 to be predicted at an earlier stage than by measuring the outer diameter of the pipe 1, observing with an optical microscope, or performing flaw detection testing using ultrasonic flaw detection. Therefore, the lifespan of the pipe 1 can be predicted at an earlier stage.

[0042] (2) The second aspect of the method S10 for predicting the life of a pipe 1 is the method S10 for predicting the life of a pipe 1 of (1), and further includes a step S21 of creating master data in advance, which creates master data showing the correlation between the amount of microvoids generated and the useful life of the pipe 1, and in a step S14 of predicting the useful life of the pipe 1, the useful life of the pipe 1 is predicted based on the master data and the amount of microvoids generated.

[0043] By creating the master data in advance, it is possible to grasp with high accuracy in advance the correlation between the amount of microvoids 101 generated and the service life of the pipe 1. As a result, it is possible to more easily and accurately predict the service life of the pipe 1.

[0044] (3) The third aspect of the method S10 for predicting the lifespan of a pipe 1 is the method S10 for predicting the lifespan of a pipe 1 of (2), and in the step S21 for creating the master data, replica samples are obtained from the surfaces of multiple test bodies that mimic multiple pipes 1 with different usage conditions, the amount of microvoids generated in the replica samples of the multiple test bodies is obtained, and the master data is created.

[0045] By creating the master data in this way, it is possible to obtain data that is close to the state of occurrence of microvoids 101 in actual pipes 1 under different usage conditions. By predicting the useful life of the pipe 1 based on the master data obtained in this way, it is possible to predict the useful life of the pipe 1 with higher accuracy. [Explanation of symbols]

[0046] 1...Piping 1f…Surface 101...Microvoid 102...Void 103...Microcracks Dr…Radial direction Dri…inside Dro...outside S10...Pipe life prediction method S11: Pipe polishing process S12: Replica sample collection process S13: A step of acquiring the amount of microvoids generated S14: Process for predicting the service life of pipes S21: Process for creating master data T: Elapsed time during observation Tb: Time elapsed from the start of the test to fracture

Claims

1. A method for predicting the life of a pipe made of stainless steel through which a high-temperature fluid flows, comprising: a step of polishing the surface of the pipe at a time when the life ratio is 60 to 80%, assuming that the period from when the pipe starts to be used until the life of the pipe reaches 100%; taking a replica sample from the surface of the polished pipe; a step of observing the replica sample with a scanning electron microscope or a laser microscope to obtain the amount of microvoids, which are extremely small defects of 1 nm to 1 μm, generated in the replica sample; and predicting the service life of the pipe based on the amount of microvoids generated.

2. The method further comprises a step of creating master data in advance, the master data indicating a correlation between the amount of microvoids generated and the service life of the pipe; 2. The method for predicting a service life of a pipe according to claim 1, wherein the step of predicting the service life of the pipe predicts the service life of the pipe based on the master data and the amount of microvoids generated.

3. A method for predicting the lifespan of a pipe as described in claim 2, wherein in the process of creating the master data, replica samples are obtained from multiple test specimens simulating multiple pipes with different usage conditions, the amount of microvoids generated in the replica samples of the multiple test specimens is obtained, and the master data is created.

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