Method for predicting, testing and evaluating service life of urban gas polyethylene pipe

Abstract

The invention belongs to the technical field of polyethylene pipe service life prediction, and particularly relates to an urban gas polyethylene pipe service life prediction test evaluation method. On the basis of establishing an indoor accelerated aging life prediction model by adopting hot air aging tests at different temperatures and an Arrhenius formula extrapolation method, a medium and a low-temperature environment correction coefficient are introduced in combination with the aging performance change rule of the urban gas PE pipelines in the field saline-alkali areas and frozen earth areas; the method is used for correcting an indoor life prediction model, and the aging performance and the service life of the gas PE pipeline are accurately speculated.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene pipe service life prediction technology, specifically relating to a test evaluation method for predicting the service life of urban gas polyethylene pipes. Background Technology

[0002] Polyethylene (PE) pipes are lightweight, easy to install, have good toughness, are corrosion-resistant, and are low in cost, making them widely used in urban natural gas transmission and water supply. However, as an organic polymer material, PE pipes age more and more with increasing service time. Especially for PE pipes used for urban gas, their physical and mechanical properties gradually deteriorate under the combined effects of temperature, pressure, and environmental media during long-term operation, thus directly affecting their service life.

[0003] For PE pipelines used in urban gas transportation with a certain service life, a life evaluation method is established, which can be used to evaluate the service safety, reliability, and remaining life of PE pipelines used in urban gas transportation. With the increasing use of PE pipes, many studies on life prediction of PE pipes under different influencing factors have been conducted both domestically and internationally. Domestic scholars' failure analysis of PE pipes mainly focuses on welding failure, mechanical properties, and aging.

[0004] In general, research methods for predicting the lifespan of thermoplastics fall into two main categories: one is simulation calculation based on theoretical foundations such as linear elastic fracture mechanics (LEFM), elasto-plastic fracture mechanics (EPFM), and crazing mechanism (CM); the other is analysis and extrapolation prediction of lifespan based on stress-failure curves (usually obtained from long-term hydrostatic tests or aging tests). While simulation calculations are quick and inexpensive, they consider limited influencing factors in the calculation and analysis process, and theoretical models are mostly based on ideal conditions, failing to fully reflect the material's damage process. Long-term hydrostatic tests or aging tests are too time-consuming, only considering the impact of single medium type, single environmental factor (such as temperature and time), and single failure mode (toughness or brittleness) on the aging performance of polymer materials, and cannot clearly define the material's aging mode and failure form.

[0005] Another common method for predicting the lifespan of non-metallic materials such as thermoplastics and rubber, as well as pipes, is: hot air aging tests at different temperatures + Arrhenius equation extrapolation. This involves using equipment such as ovens to conduct thermal aging tests on samples in hot air for different durations. By testing the performance of the test samples, the performance changes under different conditions are obtained. Based on this, the Arrhenius equation is used to analyze and extrapolate the lifespan of the thermoplastic. However, existing methods typically evaluate the aging performance of pipes not yet in service, and the lifespan prediction models and evaluation results do not consider the impact of the service environment on the performance of urban gas PE pipelines. Therefore, it is necessary to establish an indoor accelerated aging lifespan prediction model, comprehensively consider the aging performance changes of urban gas PE pipelines in saline-alkali areas and permafrost areas, introduce correction coefficients for the medium and low-temperature environment, and use these to correct the indoor lifespan prediction model to make accurate research and predictions on the aging performance and lifespan of gas PE pipelines. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for predicting and evaluating the service life of urban gas polyethylene pipes. This method, based on an indoor accelerated aging life prediction model established through hot air aging tests at different temperatures and the Arrhenius formula extrapolation, incorporates the aging performance variation patterns of urban gas PE pipes in saline-alkali and permafrost regions. It introduces correction coefficients for the medium and low-temperature environment, which are then used to correct the indoor life prediction model, enabling accurate predictions of the aging performance and service life of gas PE pipes.

[0007] The technical solution provided by this invention is as follows:

[0008] A method for predicting and evaluating the service life of urban gas polyethylene pipes includes the following steps:

[0009] In a simulated service environment, polyethylene pipe samples are subjected to accelerated aging treatment under preset temperature and pressure conditions. The simulated service environment includes, but is not limited to, saline-alkali environment and frozen soil environment. The preset temperature is higher than the operating temperature range of the polyethylene pipe. The medium environment used in the saline-alkali environment includes, but is not limited to, water, soil solution and corrosive liquid.

[0010] The performance parameters of the polyethylene pipe samples at different aging stages were tested, including but not limited to mechanical strength, creep performance and stress cracking resistance.

[0011] Based on the variation law of the performance parameters, the service life of the polyethylene pipe is analyzed and extrapolated using the Arrhenius formula;

[0012] By introducing correction coefficients for the medium and low temperature environment, the indoor accelerated aging life prediction model is corrected, and the expected service life of the polyethylene pipe under actual service environment is obtained.

[0013] Preferably, the test evaluation method for predicting the service life of urban gas polyethylene pipes includes the following steps:

[0014] S1. Obtain dumbbell-shaped tensile standard polyethylene specimens;

[0015] S2. Define the test content, including:

[0016] Hot air accelerated aging performance test;

[0017] Accelerated aging performance test in a medium environment using hot air;

[0018] Aging performance test under low temperature environment;

[0019] S3. For the specimen obtained in step S1, test the elastic modulus of the specimen under different conditions, including the initial modulus of the specimen, and determine the range of the fitted strain region of the specimen.

[0020] S4. Plot the change of sample modulus exposure time at different temperatures, simulate and establish the linear formula of modulus with exposure time at different temperatures, plot the Arrhenius relationship between test temperature T and test time t when the modulus drops to 50%, and fit the linear relationship between the two.

[0021] S5. Substitute the known service temperature T of the pipe into the linear relationship to obtain the service life t corresponding to that temperature;

[0022] S6. Obtain the mathematical equations for the exposure time and modulus of the gas-fired polyethylene pipe in a water environment, and perform medium correction to obtain the life equation of the gas-fired polyethylene pipe in a saline-alkali service environment; substitute the known service temperature of the pipe into the life equation to obtain the service life in a saline-alkali service environment.

[0023] S7. Obtain the mathematical equations for the exposure time and modulus of gas-fired polyethylene pipes under low-temperature environmental conditions, and perform temperature correction to obtain the life equations for gas-fired polyethylene pipes under low-temperature environmental conditions; substitute the known service temperature of the pipe into the life equations to obtain the service life under low-temperature environmental conditions.

[0024] In the above technical solution:

[0025] The Arrhenius formula used takes into account the impact of temperature changes in the service environment on the lifespan of the polyethylene pipe;

[0026] The correction factors for media and cryogenic environment are determined based on actual data from the field service environment.

[0027] Based on the above technical solution, the indoor accelerated aging life prediction model can be modified by comprehensively considering the aging performance changes of urban gas PE pipelines in saline-alkali areas and permafrost areas, and introducing correction coefficients for the medium and low-temperature environment. Specifically, the service life of the polyethylene pipe is analyzed and extrapolated using the Arrhenius formula. The introduction of correction coefficients for the medium and low-temperature environment modifies the indoor accelerated aging life prediction model, yielding the expected service life of the polyethylene pipe under actual service conditions.

[0028] Specifically, in step S3): test samples are cut from the flexible composite pipe section and processed into dumbbell-shaped tensile standard specimens. Different specimen types are selected according to the different pipe wall thicknesses.

[0029] Specifically, in step S3):

[0030] When the pipe wall thickness is ≥5mm, Type I specimens are selected: minimum total length 150mm; end width 20±0.2mm; parallel section length 60±0.5mm; parallel section width 10±0.2mm; maximum radius 60mm; distance between markings 50±0.5mm; distance between clamps 115±0.5mm; tensile test gauge length 50mm; tensile rate 10mm / min; fitted strain range 0~0.5%; Type I specimens are shown in Table a, Appendix Figure 1 Here is a structural diagram of a type I dumbbell sample:

[0031] Table a. Type 1 sample size is in millimeters.

[0032] symbol illustrate size A Minimum total length 150 B End width 20±0.2 C Parallel section length 60±0.5 D Parallel section width 10±0.2 E Large radius 60 F Distance between markings 50±0.5 G Distance between clamps 115±0.5 H Wall thickness Pipe wall thickness

[0033] When the pipe wall thickness is <5mm, Type II specimens are selected: minimum total length is 115mm; end width is 25±1mm; parallel section length is 33±2mm; parallel section width is 6±0.4mm; small radius is 14±1mm; large radius is 25±2mm; distance between gauge marks is 25±2mm; distance between wall thickness fixtures is 80±5mm; tensile test gauge length is 25mm; tensile rate is 5mm / min; fitted strain range is 0-1%; Type II specimens are shown in Table b, Appendix Figure 2 Here is a structural diagram of the Type II dumbbell sample:

[0034] Table h type 2 sample size is in millimeters.

[0035] symbol illustrate size A Minimum total length 115 B End width 25±1 C Parallel section length 33±2 D Parallel section width 6+0.4 E Small radius 14±1 F Large radius 25±2 G Distance between markings 25±2 H Distance between wall thickness fixtures 80±5 I Bi thick Pipe wall thickness

[0036] The slope is the elastic modulus E of the sample (see appendix). Figure 4 ).

[0037] Based on the above technical solutions, appropriate treatment methods can be selected according to the pipe wall thickness.

[0038] Specifically, in step S4), a hot air accelerated aging performance test is carried out at least three temperatures with an interval of 10-20°C between the temperature points and at least four different time periods.

[0039] Specifically, in step S4), accelerated aging performance tests of hot air were conducted at temperatures T1, T2, and T3, with a temperature interval of 15°C between the temperature points, for four different time periods, and the results were obtained as follows:

[0040] Y = a1X + b1 (1)

[0041] In the formula:

[0042] X-exposure test duration;

[0043] Y-elastic modulus of gas-fired polyethylene pipe;

[0044] Y = a²X + b² (2)

[0045] In the formula:

[0046] X—Exposure test time;

[0047] Y—Elastic modulus of gas-fired polyethylene pipe;

[0048] Y = a³X + b³ (3)

[0049] In the formula:

[0050] X—Exposure test time;

[0051] Y—Elastic modulus of gas-fired polyethylene pipe;

[0052] Calculate the test time t for each condition T1, T2, and T3 when the modulus E of the gas-fired polyethylene pipe decreases by 50%. 50 ;

[0053] Calculate the 1 / T value for each test temperature (in K), and the ln(1 / t) value for the suggested time (in hours). Fit the two sets of data using a linear function to obtain the Arrhenius equation for the temperature-time of the gas-fired polyethylene pipe:

[0054] ln(1 / t)=A*(1 / T)+B (4)

[0055] In the formula:

[0056] T—Temperature (K);

[0057] t—the time (h) at which the failure pressure reaches 50% of the threshold;

[0058] Where a1, b1, a2, b2, a3, b3, A, and B are constants obtained from the fitting process.

[0059] Specifically, in step S6), a hot air accelerated aging performance test is conducted in a brine environment at at least four different time periods at at least one temperature to obtain the mathematical equation for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe in the brine environment:

[0060] Y = a y X+b y (5);

[0061] In the formula:

[0062] X—Exposure test time;

[0063] Y—Elastic modulus of gas-fired polyethylene pipe;

[0064] The medium correction factors at temperatures T1, T2, or T3 are respectively: F a =b y :b1、F a =b y :b2 or F a =b y :b3.

[0065] Specifically: F a Substituting into equation (4), we obtain the Arrhenius equation for the temperature-time of the gas-fired polyethylene pipe, and thus the life equation for the gas-fired polyethylene pipe in a saline-alkali environment:

[0066] ln(F a / t)=A*(1 / T) + B (6);

[0067] In the formula:

[0068] T—Temperature (K);

[0069] t—the time (h) at which the failure pressure reaches 50% of the threshold;

[0070] Where A and B are constants obtained from the fitting process.

[0071] Specifically, in step S7), at least four different time periods at at least one low temperature are used to conduct low-temperature environmental aging performance tests to obtain the mathematical equation for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe under low-temperature conditions:

[0072] Y = a⁵X + b⁵ (7)

[0073] In the formula:

[0074] X—Exposure test time;

[0075] Y—Elastic modulus of gas-fired polyethylene pipe;

[0076] The temperature correction factors for temperatures T1, T2, or T3 are respectively: Ft =b5:b1、F t =b5:b2 or F t =b5:b3.

[0077] Specifically: F t Substituting into equation (4), we obtain the life equation for gas-fired polyethylene pipes in low-temperature service environments:

[0078] ln(F t / t)=A*(1 / T)+B (8)

[0079] In the formula:

[0080] T—Temperature (K);

[0081] t—the time (h) at which the failure pressure reaches 50% of the threshold;

[0082] Where A and B are constants obtained from the fitting process.

[0083] More specifically:

[0084] In step S2

[0085] 1) Conduct accelerated aging performance tests of PE pipes at three temperatures (T1, T2, and T3), with a 15°C interval between the three temperature points, and four different time periods (t1, t2, t3, and t4), using hot air to analyze the performance change law of PE pipes and establish an indoor test life prediction equation.

[0086] 2) Considering the impact of saline-alkali environment, a hot air accelerated aging performance test was carried out in a saline environment with 1 group T1 and 4 different time periods (t1, t2, t3, t4).

[0087] 3) Considering the impact of low temperature environment, conduct one set of performance change tests at 4 different time periods (t1, t2, t3, t4) for T5;

[0088] In step S3:

[0089] Based on the tensile property test data of the polyethylene dumbbell-shaped sample, the stress-strain curve (see attached) was selected. Figure 3 Linear fitting is performed on the initial part;

[0090] In step S4:

[0091] Plot the variation of sample modulus with exposure time at different temperatures, and simulate to establish a linear formula for modulus with exposure time at different temperatures. Plot the Arrhenius relation between test temperature (T) and test time (t) when the modulus decreases to 50%, and fit to obtain a linear relationship between the two.

[0092] (1) Linear fitting of exposure time and modulus at temperature T1:

[0093] Table 1 shows the modulus corresponding to different exposure times under T1.

[0094] Exposure time (h) Modulus (GPa) <![CDATA[t 11 ]]> <![CDATA[E 11 ]]> <![CDATA[t 12 ]]> <![CDATA[E 12 ]]> <![CDATA[t 13 ]]> <![CDATA[E 13 ]]> <![CDATA[t 14 ]]> <![CDATA[E 14 ]]>

[0095] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 1 to obtain the mathematical equation (Equation 1) for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe under condition T1:

[0096] Y = a1X + b1 (1)

[0097] Under temperature T1, when the modulus of the gas-fired polyethylene pipe reaches 50% of the threshold (50% of the elastic modulus of the original sample, 1 / 2*E), y When the GPa is 1, substituting into equation (1) will give the corresponding exposure time X as t1 h (t1 / 24 days, t1 / 8760 years).

[0098] (2) Linear fitting of exposure time and modulus at temperature T2:

[0099] Table 2 Modulus corresponding to different exposure times under T2

[0100] Exposure time (h) Modulus (GPa) <![CDATA[t 21 ]]> <![CDATA[E 21 ]]> <![CDATA[t 22 ]]> <![CDATA[E 22 ]]> <![CDATA[t 23 ]]> <![CDATA[E 23 ]]> <![CDATA[t 24 ]]> <![CDATA[E 24 ]]>

[0101] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 2 to obtain the mathematical equation (Equation 2) for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe under T2 conditions:

[0102] Y = a²X + b² (2)

[0103] Under temperature T2, when the modulus of the gas-fired polyethylene pipe reaches 50% of the threshold (50% of the elastic modulus of the original sample, 1 / 2*E), y When the GPa is 1000 g / kg, substituting into equation (2) will give the corresponding exposure time X as t2 h (t2 / 24 days, t2 / 8760 years).

[0104] (3) Linear fitting of exposure time and modulus at temperature T3:

[0105] Table 3 Modulus corresponding to different exposure times under T3

[0106] Exposure time (h) Modulus (GPa) <![CDATA[t 31 ]]> <![CDATA[E 31 ]]> <![CDATA[t 32 ]]> <![CDATA[E 32 ]]> <![CDATA[t 33 ]]> <![CDATA[E 33 ]]> <![CDATA[t 34 ]]> <![CDATA[E 34 ]]>

[0107] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 3 to obtain the mathematical equation (Equation 3) for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe under T3 conditions:

[0108] Y = a³X + b³ (3)

[0109] Under temperature T3, when the modulus of the gas-fired polyethylene pipe reaches 50% of the threshold (50% of the elastic modulus of the original sample, 1 / 2*E), y When the GPa is 1000 g / kg, substituting into equation (3) will give the corresponding exposure time X as t3 h (t3 / 24 days, t3 / 8760 years).

[0110] (4) Establish the Arrhenius equation

[0111] The test time t when the modulus E of the gas-fired polyethylene pipe decreases by 50% under conditions T1, T2, and T3. 50 The corresponding parameters are shown in Table 4.

[0112] Table 4 Time-temperature parameters at different simulation temperatures

[0113] type Simulated temperature 1 Simulated temperature 2 Simulated temperature 3 T(℃) <![CDATA[T1]]> <![CDATA[T2]]> <![CDATA[T3]]> T(K) <![CDATA[T1+273]]> <![CDATA[T2+273]]> <![CDATA[T3+273]]> 1 / T(K) <![CDATA[1 / (T1+273)]]> <![CDATA[1 / (T2+273)]]> <![CDATA[1 / (T3+273)]]> <![CDATA[t 50 (h)]]> <![CDATA[t1]]> <![CDATA[t2]]> <![CDATA[t3]]> ln(1 / t) <![CDATA[ln(1 / t1)]]> <![CDATA[ln(1 / t2)]]> <![CDATA[ln(1 / t3)]]>

[0114] For the test temperature 1 / T (K) and time ln(1 / t) (h) in Table 4, the two sets of data were fitted with a linear function to obtain the Arrhenius formula for temperature-time of the gas-fired polyethylene pipe (Equation 4):

[0115] ln(1 / t)=A*(1 / T) + B (4)

[0116] In step S5, lifetime estimation:

[0117] When the service temperature of the pipe is known to be T, it is substituted into the above formula (4) to calculate the service life t corresponding to that temperature.

[0118] In S6:

[0119] At temperature T1, the modulus under saturated brine immersion conditions is linearly fitted to the logarithm of the exposure time:

[0120] Table 5 shows the modulus corresponding to different exposure times under T1 (saturated saline environment).

[0121] Exposure time (h) Modulus (GPa) <![CDATA[t 11 ]]> <![CDATA[E y11 ]]> <![CDATA[t 12 ]]> <![CDATA[E y12 ]]> <![CDATA[t 13 ]]> <![CDATA[E y13 ]]> <![CDATA[t 14 ]]> <![CDATA[E y14 ]]>

[0122] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 5 to obtain the mathematical equation (Equation 5) for the relationship between the exposure time and modulus of gas-fired polyethylene pipes in a saline environment at temperature T1:

[0123] Y = a y X+b y (5)

[0124] Considering the influence of the saline environment, and in accordance with formula (1), the service life in a saline-alkali environment needs to be reduced by the medium, with a medium correction factor F.a =b y :b1. Alternatively, formula (2) or formula (3) can be used as needed.

[0125] F a Substituting into equation (4), we obtain the life equation for gas-fired polyethylene pipes in saline-alkali environments:

[0126] ln(F a / t)=A*(1 / T)+B (6)

[0127] In S7:

[0128] Modulus under lower temperature T5 conditions is logarithmically fitted to exposure time:

[0129] Table 6 Modulus corresponding to different exposure times under T5

[0130] Exposure time (h) Modulus (GPa) <![CDATA[t 51 ]]> <![CDATA[E 51 ]]> <![CDATA[t 52 ]]> <![CDATA[E 52 ]]> <![CDATA[t 53 ]]> <![CDATA[E 53 ]]> <![CDATA[t 54 ]]> <![CDATA[E 54 ]]>

[0131] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 6 to obtain the mathematical equation (Equation 7) for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe at temperature T5:

[0132] Y = a⁵X + b⁵ (7)

[0133] Since the test medium is hot air, and considering the influence of low-temperature environment, the service life needs to be reduced by temperature. Based on formula (1), the temperature correction factor F t =b5:b1. Alternatively, formula (2) or formula (3) can be used as needed.

[0134] F t Substituting into equation (4), we obtain the life equation for gas-fired polyethylene pipes in low-temperature service environments:

[0135] ln(F t / t)=A*(1 / T)+B (8)

[0136] Specifically, the applicable temperature for the hot air accelerated aging performance test is 60–105℃.

[0137] Specifically, the applicable concentration for the hot air accelerated aging performance test in a salt water environment is 0 to saturation concentration, and the applicable temperature is 60 to 105℃.

[0138] Specifically, the applicable temperature for the low-temperature environment effect aging performance test is -30 to 0℃.

[0139] The beneficial effects of this invention are:

[0140] 1. Existing technologies typically only consider the aging performance of pipes under non-service conditions, while this invention, by introducing correction coefficients for the medium and low-temperature environment, can more accurately reflect the aging performance changes of PE pipes under actual service conditions, especially for gas PE pipelines in saline-alkali areas and permafrost areas.

[0141] 2. This invention improves the existing indoor accelerated aging life prediction model by introducing a correction coefficient for service environment factors, and can more accurately assess the aging performance and service life of urban gas PE pipelines.

[0142] 3. By modifying the indoor accelerated aging life prediction model and combining it with the influence of on-site service environment factors, this invention can more accurately predict the service life of PE pipes, thereby improving the reliability of the prediction results.

[0143] 4. The technical solution of the present invention is applicable to the life assessment of PE pipes under various complex environmental conditions, and can be widely applied to urban gas pipeline systems under different geographical regions and climatic conditions, thereby improving the universality and practicality of the assessment method.

[0144] 5. By accurately predicting the aging performance and service life of PE pipes, this invention helps to plan maintenance and replacement schedules in advance, avoid sudden failures caused by pipe aging, and reduce maintenance costs and safety hazards.

[0145] 6. This invention can provide data for the safe operation of urban gas systems. By timely detecting potential aging problems, it helps prevent safety accidents such as gas leaks and ensures public safety. Attached Figure Description

[0146] Figure 1 This is a structural diagram of a type I dumbbell sample.

[0147] Figure 2 This is a structural diagram of a type II dumbbell sample.

[0148] Figure 3 These are stress-strain curves of samples with different exposure periods.

[0149] Figure 4 It is a linear fit of the initial segment of the stress-strain curve of samples with different exposure periods.

[0150] Figure 5 This is the Arrhenius relation diagram of a gas-fired polyethylene pipe under simulated conditions. Detailed Implementation

[0151] The principles and features of the present invention are described below. The embodiments given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0152] Unless otherwise specified, the test methods used in the embodiments of this invention are conventional methods; unless otherwise specified, the materials and reagents used are commercially available.

[0153] Example 1:

[0154] Life prediction of gas-fired polyethylene pipes

[0155] The elastic modulus E of the original sample was obtained through testing. y =0.724 GPa.

[0156] (1) Linear fitting of exposure time and modulus at 75℃:

[0157] Table 7 Modulus corresponding to different exposure times at 75℃

[0158] Exposure time (h) Modulus (GPa) 168 0.6924 336 0.6875 672 0.6829 1344 0.6787

[0159] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 7 to obtain the mathematical equation (Equation 9) for the relationship between the exposure time and modulus of gas-fired polyethylene pipes at 75℃:

[0160] Y= -0.000010771X+0.69216 (9)

[0161] In the formula:

[0162] X—Exposure test time

[0163] Y—Elastic modulus of gas-fired polyethylene pipe

[0164] At a temperature of 75℃, when the modulus of the gas-fired polyethylene pipe reaches the threshold of 50% (50% of the elastic modulus of the original sample, 0.362GPa), the corresponding exposure time X can be obtained by substituting it into equation (9), which is 30643h (1277 days, 3.5 years).

[0165] (2) Linear fitting of exposure time and modulus at 90℃:

[0166] Table 8 Modulus corresponding to different exposure times at 90℃

[0167]

[0168]

[0169] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 8 to obtain the mathematical equation (Equation 10) for the relationship between the exposure time and modulus of gas-fired polyethylene pipes at 90℃:

[0170] Y = -0.00004399X + 0.6461 (10)

[0171] In the formula:

[0172] X—Exposure test time

[0173] Y—Elastic modulus of gas-fired polyethylene pipe

[0174] At a temperature of 90℃, when the modulus of the gas-fired polyethylene pipe reaches the threshold of 50% (50% of the elastic modulus of the original sample, 0.362GPa), the corresponding exposure time X can be obtained by substituting into equation (10), which is 6456h (269 days).

[0175] (3) Linear fitting of exposure time and modulus at a temperature of 105℃:

[0176] Table 9 Modulus corresponding to different exposure times at 90℃

[0177] Exposure time (h) Modulus (GPa) 168 0.6274 336 0.6145 504 0.5871 672 0.5507

[0178] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 9 to obtain the mathematical equation (Equation 11) for the relationship between the exposure time and modulus of gas-fired polyethylene pipes at 90℃:

[0179] Y = -0.0001533X + 0.6593 (11)

[0180] In the formula:

[0181] X—Exposure test time

[0182] Y—Elastic modulus of gas-fired polyethylene pipe

[0183] At a temperature of 105℃, when the modulus of the gas-fired polyethylene pipe reaches the threshold of 50% (50% of the elastic modulus of the original sample, 0.362GPa), the corresponding exposure time X can be obtained by substituting it into equation (11), which is 1939h (81 days).

[0184] (4) Establish the Arrhenius equation

[0185] The test time t when the modulus E of the gas-fired polyethylene pipe decreases by 50% under conditions of 75℃, 90℃ and 105℃ 50 The corresponding parameters are shown in Table 10.

[0186] Table 10 Time-temperature parameters at different simulation temperatures

[0187] type Simulated temperature 1 Simulated temperature 2 Simulated temperature 3 T(℃) 75 90 105 T(K) 348 363 378 1 / T(K) 0.00287 0.00275 0.00265 <![CDATA[t 50 (h)]]> 30637.7 6456.7 1939.0 ln(1 / t) -10.3300 -8.7729 -7.5699

[0188] For the experimental temperature 1 / T (K) and time ln(1 / t) (h) in Table 10, the two sets of data were fitted using linear functions, as shown in the appendix. Figure 5 As shown, the Arrhenius formula for temperature-time of gas-fired polyethylene pipes (Equation 12) is obtained:

[0189] ln(1 / t)=-12566.04*(1 / T)+25.75 (12)

[0190] In the formula:

[0191] T — Temperature (K)

[0192] t—Time (h) when the failure pressure reaches 50% of the threshold.

[0193] (5) Lifespan estimation:

[0194] When the service temperature of the pipe is known to be T, it is substituted into the above formula (12) to calculate the service life t corresponding to that temperature.

[0195] Table 11 Time-temperature parameters at different simulation temperatures

[0196]

[0197] Example 2

[0198] Lifespan prediction of gas-fired polyethylene pipes under saline-alkali environmental conditions:

[0199] At a temperature of 75℃, the modulus under saturated brine immersion conditions was linearly fitted to the logarithm of the exposure time:

[0200] Table 12 Modulus at 75℃ for different exposure times (saturated saline environment)

[0201] Exposure time (h) Modulus (GPa) 168 0.6531 336 0.6500 672 0.6453 1344 0.6360

[0202] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 12 to obtain the mathematical equation (Equation 13) for the relationship between the exposure time and modulus of gas-fired polyethylene pipes in a saline environment at 75℃:

[0203] Y= -0.00001433X+0.65513 (13)

[0204] In the formula:

[0205] X—Exposure test time

[0206] Y—Elastic modulus of gas-fired polyethylene pipe

[0207] Considering the influence of the saline environment, and in accordance with formula (9), the service life in a saline-alkali environment needs to be reduced by the medium, with a medium correction factor F. a =Y y :Y1=0.65513 / 0.69216=0.9465.

[0208] F a Substituting 0.9465 into equation (12) yields the life equation for gas-fired polyethylene pipes in saline-alkali service environments:

[0209] ln(0.9465 / t)=-12566.04*(1 / T)+25.75 (14)

[0210] In the formula:

[0211] T — Temperature (K)

[0212] t—Time (h) when the failure pressure reaches 50% of the threshold.

[0213] Given the service temperature of the pipe, substitute it into the above formula (14) to calculate the service life corresponding to the salt water environment at that temperature.

[0214] Example 3

[0215] Lifespan prediction of gas-fired polyethylene pipes under low-temperature environmental conditions:

[0216] Modulus was linearly fitted to the logarithm of exposure time at a low temperature of -20℃:

[0217] Table 13-20℃ corresponding to different exposure times Modulus

[0218] Exposure time (h) Modulus (GPa) 168 0.5991 336 0.5956 672 0.5903 1344 0.5785

[0219] Linear fitting was performed on the two sets of data (exposure time (h) and modulus (GPa) in Table 13 to obtain the mathematical equation (Equation 15) for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe at a temperature of -20℃:

[0220] Y = -0.00001731X + 0.60178 (15)

[0221] In the formula:

[0222] X—Exposure test time

[0223] Y—Elastic modulus of gas-fired polyethylene pipe

[0224] Because the test medium is hot air, and considering the influence of low-temperature environments, the service life needs to be reduced by temperature. The temperature correction factor F... t =Y5:Y1=0.60178 / 0.69216=0.87.

[0225] F t Substituting into equation (12), we obtain the life equation for gas-fired polyethylene pipes in low-temperature service environments:

[0226] ln(0.87 / t)=-12566.04*(1 / T)+25.75 (16)

[0227] In the formula:

[0228] T — Temperature (K)

[0229] t—Time (h) when the failure pressure reaches 50% of the threshold.

[0230] Given the service temperature of the pipe, substitute it into the above formula (16) to calculate the service life corresponding to that low temperature.

[0231] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for predicting and evaluating the service life of urban gas polyethylene pipes, characterized in that, Includes the following steps: In a simulated service environment, polyethylene pipe samples are subjected to accelerated aging treatment under preset temperature and pressure conditions. The simulated service environment includes, but is not limited to, saline-alkali environment and frozen soil environment. The preset temperature is higher than the operating temperature range of the polyethylene pipe. The medium environment used in the saline-alkali environment includes, but is not limited to, water, soil solution and corrosive liquid. The performance parameters of the polyethylene pipe samples at different aging stages were tested, including but not limited to mechanical strength, creep performance and stress cracking resistance. Based on the variation law of the performance parameters, the service life of the polyethylene pipe is analyzed and extrapolated using the Arrhenius formula; By introducing correction coefficients for the medium and low temperature environment, the indoor accelerated aging life prediction model is corrected, and the expected service life of the polyethylene pipe under actual service environment is obtained.

2. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 1, characterized in that, Includes the following steps: S1. Obtain dumbbell-shaped tensile standard polyethylene specimens; S2. Define the test content, including: Hot air accelerated aging performance test; Accelerated aging performance test in a medium environment using hot air; Aging performance test under low temperature environment; S3. For the specimen obtained in step S1, test the elastic modulus of the specimen under different conditions, including the initial modulus of the specimen, and determine the range of the fitted strain region of the specimen. S4. Plot the change of sample modulus exposure time at different temperatures, simulate and establish the linear formula of modulus with exposure time at different temperatures, plot the Arrhenius relationship between test temperature T and test time t when the modulus drops to 50%, and fit the linear relationship between the two. S5. Substitute the known service temperature T of the pipe into the linear relationship to obtain the service life t corresponding to that temperature; S6. Obtain the mathematical equations for the exposure time and modulus of the gas-fired polyethylene pipe under the medium environment, and perform medium correction to obtain the life equation of the gas-fired polyethylene pipe under the salt-alkali service environment; substitute the known service temperature of the pipe into the life equation to obtain the service life under the salt-alkali service environment. S7. Obtain the mathematical equations for the exposure time and modulus of gas-fired polyethylene pipes under low-temperature environmental conditions, and perform temperature correction to obtain the life equations for gas-fired polyethylene pipes under low-temperature environmental conditions; substitute the known service temperature of the pipe into the life equations to obtain the service life under low-temperature environmental conditions.

3. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 2, characterized in that, In step S3): Test samples are cut from the flexible composite pipe section and processed into dumbbell-shaped tensile standard specimens. Different specimen types are selected according to the different pipe wall thicknesses.

4. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 3, characterized in that, In step S3): When the pipe wall thickness is ≥5mm, type I specimens are selected: minimum total length is 150mm; end width is 20±0.2; parallel section length is 60±0.5; parallel section width is 10±0.2; maximum radius is 60; distance between markings is 50±0.5; distance between clamps is 115±0.5; tensile test gauge length is 50mm; tensile rate is 10mm / min; fitted strain range is 0~0.5%. When the pipe wall thickness is <5mm, type II specimens are selected: minimum total length is 115mm; end width is 25+1mm; parallel section length is 33±2mm; parallel section width is 6+0.4mm; small radius is 14+1mm; large radius is 25±2mm; distance between markings is 25±2mm; distance between wall thickness fixtures is 80±5mm; tensile test gauge length is 25mm; tensile rate is 5mm / min; fitted strain range is 0~1%. The slope is the elastic modulus E corresponding to the sample.

5. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 2, characterized in that: In step S4), a hot air accelerated aging performance test is carried out at least three temperatures with an interval of 10-20°C between the temperature points and at least four different time periods.

6. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 5, characterized in that, In step S4), accelerated aging performance tests were conducted using hot air at temperatures T1, T2, and T3, with a 15°C interval between the temperature points, for four different time periods. The results were as follows: Y = a1X + b1 (1) In the formula: X—Exposure test time; Y—Elastic modulus of gas-fired polyethylene pipe; Y = a²X + b² (2) In the formula: X—Exposure test time; Y—Elastic modulus of gas-fired polyethylene pipe; Y = a³X + b³ (3) In the formula: X—Exposure test time; Y—Elastic modulus of gas-fired polyethylene pipe; Calculate the test time t for each condition T1, T2, and T3 when the modulus E of the gas-fired polyethylene pipe decreases by 50%. 50 ; Calculate the 1 / T value for each test temperature (in K), and the ln(1 / t) value for the suggested time (in hours). Fit the two sets of data using a linear function to obtain the Arrhenius equation for the temperature-time of the gas-fired polyethylene pipe: ln(1 / t)=A * (1 / T)+B (4) In the formula: T—Temperature (K); t—the time (h) at which the failure pressure reaches 50% of the threshold; Where a1, b1, a2, b2, a3, b3, A, and B are constants obtained from the fitting process.

7. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 6, characterized in that, In step S6), a hot air accelerated aging performance test is conducted in a brine environment at at least four different time periods at at least one temperature to obtain the mathematical equation for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe in the brine environment: Y=a y X+b y (5); In the formula: X—Exposure test time; Y—Elastic modulus of gas-fired polyethylene pipe; The medium correction factors at temperatures T1, T2, or T3 are respectively: F a =b y :b1、F a =b y :b2 or F a =b y :b3.

8. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 7, characterized in that: F a Substituting into equation (4), we obtain the Arrhenius equation for the temperature-time of the gas-fired polyethylene pipe, and thus the life equation for the gas-fired polyethylene pipe in a saline-alkali environment: ln(F a / t)=A * (1 / T)+B (6); In the formula: T—Temperature (K); t—the time (h) at which the failure pressure reaches 50% of the threshold; Where A and B are constants obtained from the fitting process.

9. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 6, characterized in that, In step S7), at least four different time periods at at least one low temperature are used to conduct aging performance tests on the effects of low-temperature environment, and the mathematical equations for the relationship between the exposure time and modulus of the gas-fired polyethylene pipe under low-temperature environment are obtained: Y = a⁵X + b⁵ (7) In the formula: X—Exposure test time; Y—Elastic modulus of gas-fired polyethylene pipe; The temperature correction factors for temperatures T1, T2, or T3 are respectively: F t =b5:b1、F t =b5:b2 or F t =b5:b3.

10. The method for predicting and evaluating the service life of urban gas polyethylene pipes according to claim 9, characterized in that: F t Substituting into equation (4), we obtain the life equation for gas-fired polyethylene pipes in low-temperature service environments: ln(F t / t)=A * (1 / T)+B (8) In the formula: T—Temperature (K); t—the time (h) at which the failure pressure reaches 50% of the threshold; Where A and B are constants obtained from the fitting process.

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