Methods for monitoring the lifespan of metal pipes in combined heat and power units

Abstract

This invention relates to a method for monitoring the lifespan of metal pipes in combined heat and power (CHP) units. The purpose of this invention is to address the inaccuracy of existing online monitoring methods based on limited measuring points. Each pipe is divided into multiple sections. Boiler operating parameters and the internal pressure of each section are collected. Based on the operating parameters, the outlet steam temperature and midpoint wall temperature of each section are obtained. The outer surface wall temperature is obtained based on the outlet steam temperature and midpoint wall temperature. The inner wall temperature of each section is obtained based on the midpoint wall temperature and outer surface wall temperature. The fatigue life loss rate of each section is obtained based on the outer surface wall temperature, inner wall temperature, and internal pressure. The creep life loss rate is obtained based on the outer surface wall temperature. The sum of the fatigue life loss rate and creep life loss rate is taken as the total loss rate of each section, thereby achieving the monitoring of pipe lifespan. This invention is used for monitoring pipe lifespan.

Description

Technical Field

[0001] This invention relates to the field of monitoring. Background Technology

[0002] The boiler piping system is the most problematic component in the long-term operation of combined heat and power (CHP) units, especially the main steam piping and reheat steam piping, which operate under high temperature and high pressure. Under these harsh conditions, the metal material of these pipes gradually suffers creep and fatigue damage under continuous high temperature and alternating stress, eventually leading to pipe rupture, shutdowns, and even safety accidents, resulting in significant economic losses and safety risks. Therefore, accurate monitoring of the lifespan of pipe metal is a core requirement for ensuring the safe and economical operation of power plant equipment.

[0003] Currently, pipeline life monitoring relies on the following methods:

[0004] Limited-point online monitoring: Strain gauges are attached to the pipe surface or fiber optic grating sensors or temperature sensors are installed to monitor stress-strain changes or temperature changes in real time during operation and calculate the life loss at local locations. However, boiler heating surface piping systems are large and complex. Due to cost, space, and installation limitations, monitoring points can only cover a very small portion of the locations. The vast majority of pipe locations without monitoring points are in monitoring blind spots, and their true lifespan is completely unknown. The system can only reflect the health status of the monitoring point, not the entire pipe, and cannot obtain the true, non-uniform temperature distribution of the pipe (especially the various tube panels and sections of the heating surface), leading to inaccurate lifespan monitoring and monitoring results that deviate from reality. Summary of the Invention

[0005] The purpose of this invention is to solve the problem of inaccurate life monitoring in existing online monitoring methods based on limited measuring points, and to propose a method for monitoring the life of metal pipes in combined heat and power units.

[0006] A method for monitoring the metal life of pipelines in combined heat and power (CHP) units, the method comprising the following:

[0007] Step 1: Divide each pipe into multiple pipe sections and collect the boiler's operating parameters and the internal pressure of each pipe section;

[0008] Step 2: Obtain the outlet steam temperature and intermediate point wall temperature of each pipe section according to the working parameters, and obtain the outer surface wall temperature according to the outlet steam temperature and intermediate point wall temperature of each pipe section.

[0009] Step 3: Obtain the inner wall temperature of each pipe section based on the wall temperature at the midpoint and the outer surface wall temperature.

[0010] The fatigue life loss rate of each pipe section is obtained based on the outer surface wall temperature, inner wall temperature, and internal pressure of each pipe section. The creep life loss rate of each pipe section is obtained based on the outer surface wall temperature of each pipe section. The sum of the fatigue life loss rate and creep life loss rate of each pipe section is taken as the total loss rate of each pipe section, thereby realizing the monitoring of pipeline life.

[0011] Preferably, the operating parameters include the outlet temperature of each pipe section and the inlet and outlet smoke opacity of each pipe section.

[0012] Preferably, there are two ways to express the outlet steam temperature of each pipe section. The first way is:

[0013] ,

[0014] In the formula, For the first Steam temperature at the outlet of each pipe section For the first Steam temperature at the outlet of each pipe section and All are coefficients. For the first The outlet temperature of each pipe section For the first The length of each pipe section;

[0015] The second type is:

[0016] ,

[0017] In the formula, The influence of smoke temperature changes on the first The coefficient of change of steam temperature in each pipe section , For the first The current outlet temperature of each pipe segment. For the first The outlet temperature of each pipe segment at the previous moment;

[0018] There are two ways to represent the wall temperature at the midpoint. The first way is:

[0019] ,

[0020] In the formula, For the first The wall temperature at the midpoint of each pipe section, For the first The wall temperature at the midpoint of each pipe section, and All are coefficients. For the first The outlet temperature of each pipe section For the first The length of each pipe section;

[0021] The second type is:

[0022] ,

[0023] In the formula, The slope of the wall temperature change at the midpoint caused by the change in smoke temperature.

[0024] Preferably, the outer surface wall temperature is expressed as:

[0025] ,

[0026] In the formula, For the first The outer surface wall temperature of each pipe section.

[0027] Preferably, the inner wall temperature of each pipe section is expressed as:

[0028] ,

[0029] In the formula, For the first The inner wall temperature of each pipe section.

[0030] Preferably, the creep life loss rate is expressed as:

[0031] ,

[0032] In the formula, For the first Creep life loss rate of each pipe section The sampling time interval, For the fracture time, , It is a constant. Absolute temperature , For creep performance, , and All of these are material performance coefficients. For stress.

[0033] Preferably, the process for obtaining the fatigue life loss rate of each pipe section is as follows:

[0034] Based on the internal pressure of each pipe section, the stress caused by the internal pressure in each pipe section is obtained;

[0035] Based on the outer surface wall temperature and inner wall temperature of each pipe section, the stress caused by temperature in each pipe section is obtained;

[0036] Calculate the sum of the stress caused by internal pressure and the stress caused by temperature in each pipe segment at the same time, and use it as the principal stress of each pipe segment. Obtain three maximum principal stresses and three minimum principal stresses from each preset sampling period, calculate the difference between every two principal stresses among the three maximum principal stresses, calculate the difference between every two principal stresses among the three minimum principal stresses, and calculate the stress amplitude based on all the differences.

[0037] The amplitude is corrected to obtain the corrected stress amplitude. Combined with the preset stress amplitude and cycle number relationship table, the corresponding cycle number is obtained.

[0038] The fatigue life loss rate of each pipe section is obtained based on the number of cycles and the theoretical number of cycles.

[0039] Preferably, the stress caused by internal pressure in each pipe section is expressed as:

[0040] ,

[0041] In the formula, The circumferential stress is caused by internal pressure. For thin film stress intensity, The normal stress caused by internal pressure, , and The stress concentration factor is the internal compressive stress. The axial stress is caused by internal pressure. , For the first The internal pressure of each pipe section, The inner diameter of the pipe. For pipe wall thickness;

[0042] The temperature-induced stresses in each pipe section include radial thermal stress and circumferential thermal stress. The radial thermal stress is expressed as:

[0043] ,

[0044] In the formula, The circumferential thermal stress is caused by the radial temperature difference. Axial thermal stress caused by radial temperature difference Normal thermal stress caused by radial temperature difference and The stress concentration factor is the radial thermal stress due to the temperature difference. The coefficient of linear expansion is 1 / 3. For elastic modulus, For correction factor, Poisson's ratio, Radial temperature difference, , For structural coefficients, For the thickness of the cylinder, The rate of temperature change of the medium in the boiler drum. , and For the coefficient related to heat transfer, For time;

[0045] Circumferential thermal stress is expressed as:

[0046] ,

[0047] In the formula, Circumferential thermal stress caused by circumferential temperature difference Axial thermal stress caused by circumferential temperature difference Normal thermal stress caused by circumferential temperature difference and The stress coefficient is the axial thermal stress coefficient. For coefficients, This represents the maximum circumferential temperature difference.

[0048] The principal stresses are expressed as:

[0049] .

[0050] Preferably, the difference between any two principal stresses among the three maximum principal stresses is expressed as:

[0051] ,

[0052] In the formula, for and The difference, for and The difference, This is the second maximum principal stress. for and The difference, The third maximum principal stress, For the first maximum principal stress,

[0053] The difference between any two principal stresses among the three minimum principal stresses is expressed as:

[0054] ,

[0055] In the formula, for and The difference, for and The difference, For the second minimum principal stress, , The third minimum principal stress, For the first minimum principal stress,

[0056] The stress amplitude is expressed as:

[0057] ,

[0058] In the formula, The stress amplitude, , , , ;

[0059] The corrected stress amplitude is expressed as:

[0060] ,

[0061] In the formula, The corrected stress amplitude, The elastic modulus at room temperature, It is the elastic modulus at the highest medium temperature.

[0062] The beneficial effects of this invention are:

[0063] This invention divides the pipeline into multiple pipe segments. Based on the internal pressure and operating parameters of each pipe segment, it calculates the fatigue life loss rate and creep life loss rate of each pipe segment, thereby calculating the total loss rate of each pipe segment. Therefore, this application can monitor the life of each pipe segment, accurately locate the pipe segment with problems, and achieve accurate monitoring of pipeline life. Attached Figure Description

[0064] Figure 1 A flowchart for a method of monitoring the lifespan of metal pipes in combined heat and power (CHP) units.

[0065] Figure 2 This is a schematic diagram of the first tube section of the final stage reheater;

[0066] Figure 3 Steam temperature diagram for each pipe section;

[0067] Figure 4 Here is a flowchart for slope calculation;

[0068] Figure 5 This is a diagram showing the wall temperature of each pipe section. Detailed Implementation

[0069] 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0070] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0071] Example:

[0072] A method for monitoring the metal life of pipelines in combined heat and power (CHP) units, the method comprising the following:

[0073] Step 1: Collect the boiler's operating parameters and the internal pressure of each pipe section under different operating conditions;

[0074] Step 2: Obtain the outlet steam temperature and intermediate point wall temperature of each pipe section according to the working parameters, and obtain the outer surface wall temperature according to the outlet steam temperature and intermediate point wall temperature of each pipe section.

[0075] Step 3: Obtain the inner wall temperature of each pipe section based on the wall temperature at the midpoint and the outer surface wall temperature.

[0076] The fatigue life loss rate of each pipe section is obtained based on the outer surface wall temperature, inner wall temperature, and internal pressure of each pipe section. The creep life loss rate of each pipe section is obtained based on the outer surface wall temperature of each pipe section. The sum of the fatigue life loss rate and creep life loss rate of each pipe section is taken as the total loss rate of each pipe section, thereby realizing the monitoring of pipeline life.

[0077] Further, the operating parameters include the outlet temperature of each pipe section and the inlet and outlet smoke opacity of each pipe section.

[0078] Further specifying, there are two ways to express the outlet steam temperature of each pipe section. The first way is:

[0079] ,

[0080] In the formula, For the first Steam temperature at the outlet of each pipe section For the first Steam temperature at the outlet of each pipe section and All are coefficients. For the first The outlet temperature of each pipe section For the first The length of each pipe section;

[0081] The second type is:

[0082] ,

[0083] In the formula, The influence of smoke temperature changes on the first The coefficient of change of steam temperature in each pipe section , For the first The current outlet temperature of each pipe segment. For the first The outlet temperature of each pipe segment at the previous moment;

[0084] There are two ways to represent the midpoint metal wall temperature. The first way is:

[0085] ,

[0086] In the formula, For the first The wall temperature at the midpoint of each pipe section, For the first The wall temperature at the midpoint of each pipe section, and All are coefficients. For the first The outlet temperature of each pipe section For the first The length of each pipe section;

[0087] The second type is:

[0088] ,

[0089] In the formula, The slope of the wall temperature change at the midpoint caused by the change in smoke temperature.

[0090] Specifically, taking the inlet and outlet steam temperatures of a certain pipe section of a component with different pipe panels as an example, calculate the outlet steam temperature, the intermediate point wall temperature, and the outer surface wall temperature of that pipe section.

[0091] Calculate the thermodynamics of 100% BMCR, 75% THA, and 50% THA. Based on the thermodynamic calculation results, extract the data required for the wall temperature calculation of the corresponding components, including: flow rate, inlet pressure and temperature, outlet pressure and temperature, inlet and outlet flue gas temperatures, convective heat transfer coefficient and radiative heat transfer coefficient, and direct radiative heat absorption.

[0092]

[0093] To accommodate the range of steam temperature variations on-site, the total heat absorption needs to be adjusted.

[0094] There are three scenarios: The first scenario is where the flue gas temperature remains constant, but the heat absorption temperature changes (the working fluid temperature at the header inlet remains constant, while the working fluid temperature at the header outlet changes).

[0095] Upper limit benchmark: +20℃ based on the steam temperature at the header outlet; lower limit benchmark: -50℃ based on the steam temperature at the header outlet (the upper and lower limits are selected to include the maximum and minimum values ​​of the field operating data for the pipe outlet section). Wall temperature calculation is performed using a coefficient of 1.2. The calculation results for the unbalanced steam temperature, intermediate point wall temperature, and outer surface wall temperature of each pipe section are exported. Example: Steam temperature calculation for the first pipe of the final reheater, where a schematic diagram of the first pipe structure of the final reheater is shown below. Figure 2 As shown.

[0096]

[0097] These two sets of numbers can be used to obtain the slope of the working fluid temperature for each segment. When the temperature range changes from 537℃ to 607℃, the linear change of the slope from K1 to K11 is given by the formula K=(K11-K1) / (T2-T1)(T0-T1)+K1, as follows. Figure 3 and 4 As shown;

[0098] ,

[0099] ,

[0100] The intermediate point wall temperature is calculated using this method, but the relationship between the wall temperature at 9.1m and the inlet steam temperature needs to be fitted. This is based on the formula for the effect of changes in inlet temperature on the wall temperature at 9.1m, such as... Figure 5 As shown;

[0101] .

[0102] The second type: The flue gas temperature remains constant, but the heat absorption changes (the temperature of the working fluid at the inlet of the header changes, and the temperature of the working fluid at the outlet of the header changes).

[0103] Step 5: Perform thermodynamic calculations for inlet working fluid temperatures of +20℃ and -60℃, and then calculate the wall temperature based on the results to obtain the changes in inlet working fluid temperature, header outlet pipe section temperature, steam temperature, and intermediate point wall temperature under BMCR conditions.

[0104] (1) With the difference between the working medium temperature at the outlet pipe section and the working medium temperature at the inlet pipe section remaining constant, To`-Ti=To-476.7, the steam temperature slope can be shifted to obtain the steam temperature change of each section (the steam temperature does not need to be calculated using the wall temperature program, but can be calculated by shifting the original formula).

[0105] To = (To' - Ti) + 476.7

[0106] To—the temperature value entered in the formula, To` = the steam temperature of the outlet section of the pipe.

[0107] (2) The wall temperature at the intermediate point calculated by the program shows a linear relationship with the wall temperature at 9.1m when the inlet working fluid temperature Ti changes. Under BMCR conditions, Tb9.1 = 0.8108Ti + 190.34, and the slopes of the wall temperatures at Tb34.1, Tb48.4, and Tb57 can be shifted to obtain the wall temperature changes in each segment. However, the wall temperature at 68.4ft needs to be corrected according to the change in the inlet working fluid temperature, resulting in Tb68.4 = (0.0135To - 9.675) × 11.4 + Tb57 + (Ti - 476.7) / 20.

[0108] The corrected wall temperature at 75% load is calculated as T68.4 = (0.0133To - 9.582) × 11.4 + Tb57 + (Ti - 476.7) / 14.1.

[0109] 50% load T68.4 = (0.0142To - 10.341) × 11.4 + Tb57 + (Ti - 476.7) / 10.

[0110]

[0111] The third type: Operating conditions where heat absorption remains constant but flue gas temperature changes.

[0112] Under the same load conditions, with the same tube panel, the temperature fluctuation of the same tube is caused by the flue gas temperature. Therefore, the wall temperature needs to be corrected according to the influence of the flue gas temperature on the change of the working fluid outlet temperature.

[0113] For BMCR operating conditions, the inlet and outlet working fluid temperatures are referenced. The inlet and outlet flue gas temperatures are increased by 50°C and decreased by 50°C respectively. The program calculates the steam temperature and intermediate point wall temperature, and then compares them with the reference temperature to obtain the temperature difference between the steam temperature and the intermediate point wall temperature of each section.

[0114] Method: The change in flue gas temperature of 100℃ is used to correlate the change in steam temperature in each section with ΔTo = To2 - To1, and the change in wall temperature at the midpoint is also considered. Then, the change in flue gas temperature of 100 / 7.222 = 13.8℃ is used to correlate the change in outlet steam temperature of 1℃, and the changes in steam temperature in each section and the wall temperature at the midpoint are used as the slope.

[0115] To2 is the working fluid temperature of the outlet pipe section at the next time.

[0116] To1 represents the working fluid temperature of the outlet pipe section at the previous moment.

[0117] Example: Steam temperature under 100% load imbalance

[0118] ,

[0119] ,

[0120] Calculate the outer surface wall temperature of the tube and find the relationship between (To-Tm) and (Tm-Tf).

[0121] Example: The outer surface wall temperature of the first pipe. Find the relationship between (To-Tm) and (Tm-Tf).

[0122] ,

[0123] ,

[0124] Find To - Tm = 0.07(Tm - Tf) + 0.2;

[0125] The changes in To–Tm and (Tm-Tf) for other tubes in the final stage reheater and superheater are also linear and identical, so a single formula can be used. Each tube in the partition screen needs to be calculated individually.

[0126] The formula for calculating the inner wall temperature of a pipe is: TI = 2Tm - To.

[0127] Interpolation calculations were performed on the unbalanced steam temperature and intermediate point wall temperature under these three loads to obtain the unbalanced steam temperature and intermediate point wall temperature as a function of flow rate.

[0128] This embodiment can also calculate the remaining life of each pipe segment based on the total loss rate of each segment using existing technology, thereby enabling early warning.

[0129] Further defining the process, the process for obtaining the fatigue life loss rate of each pipe section is as follows:

[0130] Based on the internal pressure of each pipe section, the stress caused by the internal pressure in each pipe section is obtained;

[0131] Based on the outer surface wall temperature and inner wall temperature of each pipe section, the stress caused by temperature in each pipe section is obtained;

[0132] Calculate the sum of the stress caused by internal pressure and the stress caused by temperature in each pipe segment at the same time, and use it as the principal stress of each pipe segment. Obtain three maximum principal stresses and three minimum principal stresses from each preset sampling period, calculate the difference between every two principal stresses among the three maximum principal stresses, calculate the difference between every two principal stresses among the three minimum principal stresses, and calculate the stress amplitude based on all the differences.

[0133] The amplitude is corrected to obtain the corrected stress amplitude. Combined with the preset stress amplitude and cycle number relationship table, the corresponding cycle number is obtained.

[0134] The fatigue life loss rate of each pipe section is obtained based on the number of cycles and the theoretical number of cycles.

[0135] Further defining the stress caused by internal pressure in each pipe section, we express it as follows:

[0136] ,

[0137] In the formula, The circumferential stress is caused by internal pressure. For thin film stress intensity, The normal stress caused by internal pressure, , and The stress concentration factor is the internal compressive stress. The axial stress is caused by internal pressure. , For the first The internal pressure of each pipe section, The inner diameter of the pipe. For pipe wall thickness;

[0138] The temperature-induced stresses in each pipe section include radial thermal stress and circumferential thermal stress. The radial thermal stress is expressed as:

[0139] ,

[0140] In the formula, The circumferential thermal stress is caused by the radial temperature difference. Axial thermal stress caused by radial temperature difference Normal thermal stress caused by radial temperature difference and The stress concentration factor is the radial thermal stress due to the temperature difference. The coefficient of linear expansion is 1 / 3. For elastic modulus, For correction factor, Poisson's ratio, Radial temperature difference, , For structural coefficients, For the thickness of the cylinder, The rate of temperature change of the medium in the boiler drum. , and For the coefficient related to heat transfer, For time;

[0141] Circumferential thermal stress is expressed as:

[0142] ,

[0143] In the formula, Circumferential thermal stress caused by circumferential temperature difference Axial thermal stress caused by circumferential temperature difference Normal thermal stress caused by circumferential temperature difference and The stress coefficient is the axial thermal stress coefficient. For coefficients, This represents the maximum circumferential temperature difference.

[0144] The principal stresses are expressed as:

[0145] .

[0146] Further specifying, the difference between any two principal stresses among the three maximum principal stresses is expressed as:

[0147] ,

[0148] In the formula, for and The difference, for and The difference, This is the second maximum principal stress. for and The difference, The third maximum principal stress, For the first maximum principal stress,

[0149] The difference between any two principal stresses among the three minimum principal stresses is expressed as:

[0150] ,

[0151] In the formula, for and The difference, for and The difference, For the second minimum principal stress, , The third minimum principal stress, For the first minimum principal stress,

[0152] The stress amplitude is expressed as:

[0153] ,

[0154] In the formula, The stress amplitude, , , , ;

[0155] The corrected stress amplitude is expressed as:

[0156] ,

[0157] In the formula, The corrected stress amplitude, The elastic modulus at room temperature, It is the elastic modulus at the highest medium temperature.

[0158] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A method for monitoring the lifespan of metal pipes in combined heat and power (CHP) units, characterized in that, The method includes the following: Step 1: Divide each pipe into multiple pipe sections and collect the boiler's operating parameters and the internal pressure of each pipe section; Step 2: Obtain the outlet steam temperature and intermediate point wall temperature of each pipe section according to the working parameters, and obtain the outer surface wall temperature according to the outlet steam temperature and intermediate point wall temperature of each pipe section. Step 3: Obtain the inner wall temperature of each pipe section based on the wall temperature at the midpoint and the outer surface wall temperature. The fatigue life loss rate of each pipe section is obtained based on the outer surface wall temperature, inner wall temperature, and internal pressure of each pipe section. The creep life loss rate of each pipe section is obtained based on the outer surface wall temperature of each pipe section. The sum of the fatigue life loss rate and creep life loss rate of each pipe section is taken as the total loss rate of each pipe section, thereby realizing the monitoring of pipeline life.

2. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 1, characterized in that, The operating parameters include the outlet temperature of each pipe section and the smoke density at the inlet and outlet of each pipe section.

3. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 2, characterized in that, There are two ways to express the outlet steam temperature of each pipe section. The first way is: ， In the formula, For the first Steam temperature at the outlet of each pipe section For the first Steam temperature at the outlet of each pipe section and All are coefficients. For the first The outlet temperature of each pipe section For the first The length of each pipe section; The second type is: ， In the formula, The influence of smoke temperature changes on the first The coefficient of change of steam temperature in each pipe section , For the first The current outlet temperature of each pipe segment. For the first The outlet temperature of each pipe segment at the previous moment; There are two ways to represent the wall temperature at the midpoint. The first way is: ， In the formula, For the first The wall temperature at the midpoint of each pipe section, For the first The wall temperature at the midpoint of each pipe section, and All are coefficients. For the first The outlet temperature of each pipe section For the first The length of each pipe section; The second type is: ， In the formula, The slope of the wall temperature change at the midpoint caused by the change in smoke temperature.

4. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 3, characterized in that, The outer surface wall temperature is expressed as: ， In the formula, For the first The outer surface wall temperature of each pipe section.

5. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 4, characterized in that, The inner wall temperature of each pipe section is expressed as follows: ， In the formula, For the first The inner wall temperature of each pipe section.

6. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 4, characterized in that, Creep life loss rate is expressed as: ， In the formula, For the first Creep life loss rate of each pipe section The sampling time interval, For the fracture time, , It is a constant. Absolute temperature , For creep performance, , and All of these are material property coefficients. For stress.

7. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 5, characterized in that, The process for obtaining the fatigue life loss rate of each pipe section is as follows: Based on the internal pressure of each pipe section, the stress caused by the internal pressure in each pipe section is obtained; Based on the outer surface wall temperature and inner wall temperature of each pipe section, the stress caused by temperature in each pipe section is obtained; Calculate the sum of the stress caused by internal pressure and the stress caused by temperature in each pipe segment at the same time, and use it as the principal stress of each pipe segment. Obtain three maximum principal stresses and three minimum principal stresses from each preset sampling period, calculate the difference between every two principal stresses among the three maximum principal stresses, calculate the difference between every two principal stresses among the three minimum principal stresses, and calculate the stress amplitude based on all the differences. The amplitude is corrected to obtain the corrected stress amplitude. Combined with the preset stress amplitude and cycle number relationship table, the corresponding cycle number is obtained. The fatigue life loss rate of each pipe section is obtained based on the number of cycles and the theoretical number of cycles.

8. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 7, characterized in that, The stress caused by internal pressure in each pipe section is expressed as follows: ， In the formula, The circumferential stress is caused by internal pressure. For thin film stress intensity, Normal stress caused by internal pressure, , and The stress concentration factor is the internal compressive stress. The axial stress is caused by internal pressure. , For the first The internal pressure of each pipe section, The inner diameter of the pipe. For pipe wall thickness; The temperature-induced stresses in each pipe section include radial thermal stress and circumferential thermal stress. The radial thermal stress is expressed as: ， In the formula, The circumferential thermal stress is caused by the radial temperature difference. Axial thermal stress caused by radial temperature difference Normal thermal stress caused by radial temperature difference and The stress concentration factor is the radial thermal stress due to the thermal difference. The coefficient of linear expansion is 1 / 3. For elastic modulus, For correction factor, Poisson's ratio, Radial temperature difference, , For structural coefficients, For the thickness of the cylinder, The rate of temperature change of the medium in the boiler drum. , and For the coefficient related to heat transfer, For time; Circumferential thermal stress is expressed as: ， In the formula, Circumferential thermal stress caused by circumferential temperature difference Axial thermal stress caused by circumferential temperature difference Normal thermal stress caused by circumferential temperature difference and The stress coefficient is the axial thermal stress coefficient. For coefficients, This represents the maximum circumferential temperature difference. The principal stresses are expressed as: 。 9. The method for monitoring the metal life of pipelines in a combined heat and power unit according to claim 8, characterized in that, The difference between any two principal stresses among the three maximum principal stresses is expressed as: ， In the formula, for and The difference, for and The difference, This is the second maximum principal stress. for and The difference, The third maximum principal stress, For the first maximum principal stress, The difference between any two principal stresses among the three minimum principal stresses is expressed as: ， In the formula, for and The difference, for and The difference, For the second minimum principal stress, , The third minimum principal stress, For the first minimum principal stress, The stress amplitude is expressed as: ， In the formula, The stress amplitude, , , , ; The corrected stress amplitude is expressed as: ， In the formula, The corrected stress amplitude, The elastic modulus at room temperature, It is the elastic modulus at the highest medium temperature.

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