Stress analysis and strength evaluation of weld of high pressure industrial pipeline

As the “blood vessel” of chemical industry, industrial pipeline plays an important role in chemical production. Most industrial pipelines are welded and installed on the construction site. Due to the complicated conditions on the construction site, the construction quality is easily affected. In addition, during the regular inspection process, it is often encountered that the on-site industrial pipelines do not conform to the design drawings. All the above reasons may lead to accidents in the operation of pressure pipelines, resulting in loss of personnel and property. How to ensure the safe operation of industrial pipelines has always been the key research direction of scholars.

With the progress of science and technology, more and more scholars use finite element analysis software to carry out stress analysis and strength evaluation on industrial pipelines, and the finite element analysis method has also been recognized by the academic circle. Light Yong and others adopt finite element method to carry out stress analysis and strength evaluation on ultra-high pressure pipeline, and decompose the calculation results and classify the stress. Bai fang et al. studied the residual stress of X80 steel multilayer weld bead by combining simulation and experiment. Bai Hui and others studied the influence of thermal radiation on the temperature and stress distribution of the hydrogenator hot box. Considering the thermal radiation, the temperature field distribution of the Hot Box will be more uniform and the stress will be significantly reduced. Xu Jinsha et al. studied the influence of three factors of thermal expansion coefficient, temperature difference and thickness of glass lining on the resistance of glass lining to temperature difference and sudden change, and verified the numerical simulation results through experimental research. Wu Xinli and others used Sysweld to establish a welding model to simulate and analyze the welding residual stress of the butt weld between the tube plate and the annular support plate after different heat treatment processes, at the same time, the simulation calculation magnitude of stress is compared with the blind hole method test magnitude of stress, and its mechanical properties are verified through the simulation test. Huo Yufeng and others established a three-dimensional transient thermoelastic-plastic simulation model of steel slag tank, and analyzed the temperature field of steel slag tank during slow cooling, the stress and plastic strain of typical parts such as tank and vertical and horizontal stiffener are also analyzed. Su literature and others use finite element analysis software to carry out failure analysis and improvement design on leakage of condensation kettle with jacket under complex load. Huang Yun et al. used finite element method to study the influence of size and position changes of single and double cracks on the stress of oil and gas pipelines, and obtained corresponding formulas by fitting. Zhang Guowei and others conducted stress analysis on the outlet gas pipeline of dry gas compressor based on thermal-structural coupling. When the temperature difference of the pipeline is too large, the outer wall is under great stress. Wu Ying and others established a finite element model according to the actual operation condition of the pipeline for typical smooth dent defects on the pipeline, and obtained the variation law of stress with various parameters, the nonlinear regression analysis method is used to fit the calculation results. Shuaijian et al. analyzed the stress and deformation of the pipeline under the action of occupied load, and local bending and elliptical deformation occurred under the uneven action of overlying soil and tamping foundation.
It can be concluded from the above literature that scholars at home and abroad have done a lot of research on pipeline stress analysis, however, most of the researches ignore the stress and strength analysis of pressure pipelines under single and complex loads under temperature, with relatively large limitations. This paper studies a high temperature and high pressure pipeline in a chemical enterprise, establishes a three-dimensional physical model of pipe fittings and weld joints, and uses finite element analysis method, the variation of stress field and temperature field with time under design and service conditions of pipelines and welds is studied, and stress assessment is carried out based on analysis and design standards to determine the safe application range.

Finite element model

Physical model and material properties

The pipeline is an industrial pipeline in use by a chemical enterprise, and the weld pipe fitting elbow is found at the joint between the pipeline incomplete filled groove and the tee Weld, as shown in the red box in Figure 1(a). After field measurement, the difference between weld bead height and pipe surface is about 10mm. The overall design pressure of the pipeline is 44.8Mpa, the design temperature is 260℃; The working pressure is 38Mpa, and the working temperature is 110℃. The outer diameter of the three-way pipe is 241.3mm, the inner diameter is 125.1mm; The outer diameter of the elbow is 241.3mm, and the inner diameter is 140.9mm. The welding coefficient of pipeline weld is 0.85. Since this paper mainly studies the stress distribution of the weld at the joint of tee and elbow, only the tee and elbow of the pipeline are selected for finite element analysis. The physical model of tee, elbow and weld is completely established according to the size of the design drawing, as shown in Figure 1 (B).
The material of tee and elbow is SA-182F11 cl2. the material of weld is consistent with that of pipe fittings. The thermal expansion coefficient, elastic modulus, thermal conductivity coefficient, Poisson’s ratio, density and other characteristic parameters of pipe fittings materials are selected according to the material characteristics in ASME-SECTION II PART-D.
20210206234549 30047 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Figure.1 physical and three-dimensional physical model of piping system

Unit selection and grid division

In the pipeline structure analysis, the finite element model selects the 8-node hexahedral structure analysis unit Solid185 unit. Solid185 unit is used to construct three-dimensional solid structure model, which has the functions of plasticity, superelasticity, stress stiffening, creep, large deformation and large strain. For the finite element model of pipeline temperature field analysis, the isoparametric unit Solid 70 unit of 8 nodes is selected. Solid 70 unit can be used for three-dimensional steady-state or transient thermal analysis, and can compensate the heat flow loss caused by constant velocity field mass transport. The number of grid units in the pipeline physical model is 387 040, and the total number of grid nodes is 413 987. Grid refinement the weld joint of the three-way pipe and elbow to ensure that the calculation accuracy requirements are met. The overall grid details of the pipeline are shown in figure 2.
20210206234703 46644 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Figure.2 Piping system mesh model

Boundary constraints

The boundary conditions of the pipeline stress analysis model are shown in figure 3. The end face A of the elbow applies full displacement constraint, the inner surface B of the pipeline applies internal pressure in the vertical surface direction, the two end surfaces C and D of the three-way pipe respectively apply the outward pulling force of the vertical end surface. The pressure applied on the end face of the tee can be determined by the Lame equation, and the formula of the Lame equation is as follows:

20210206234925 33890 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline

In the formula: P is the design pressure or working pressure, Mpa;K is the ratio of the outer diameter of the pipe at the end face to the inner diameter of the pipe; P2 is the pressure on the end face of the three-way pipe.
20210206235000 77525 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Fig.3 Boundary conditions
The specific setting of boundary conditions of pipeline stress analysis model under design and service conditions is shown in Table 1.
Table.1 boundary conditions of stress analysis model

Type Elbow end A Pipeline inner wall surface B pressure/Mpa Three-way pipe end face C and D pressure/Mpa
Design conditions Total displacement constraint 44.8 30.54
Service condition Total displacement constraint 38 25.91

Table 2 boundary conditions for temperature analysis the temperature of the inner surface of the pipe temperature field analysis model is set as the design temperature under the design condition and as the working temperature under the service condition. Since the outer surface of the pipe is not insulated, the natural convection heat transfer coefficient is set according to the environmental conditions. In order to obtain the relationship between the stress field and temperature field of pipeline and weld with time, the total calculation time is 1000s and the time step is 50s under the unsteady condition. The specific setting of boundary conditions of pipeline temperature model under design and service conditions is shown in Table 2.
Table.2 boundary conditions for temperature analysis

Type Internal surface temperature/ Surface heat transfer coefficient / (w / m2. ) Ambient temperature/
Design conditions 260 30 25
Service condition 110 30 5

Result analysis

Temperature distribution

Figure 4 shows the temperature distribution of the pipeline at 50s and 1000s. It can be seen from Figure 4 (a) that the maximum temperature of the inner wall of the pipeline is 110 ℃, the minimum temperature of the outer wall of the pipeline is 34.2 ℃, and the temperature difference between the inner and outer walls of the pipeline is 75.8 ℃. The lowest temperature occurs on the inside of the bend. According to Fig. 4 (b), the minimum temperature of the outer wall of the pipeline is 104.9 ℃ and the temperature difference between the inner and outer wall of the pipeline is 15.1 ℃ at 1000s. At 1000s, the overall temperature distribution of the pipeline is more uniform than that at 50s, the temperature difference between the inner and outer walls of the pipeline decreases by 60.7 ℃, and the lowest temperature moves from the inner side of the elbow to the tee interface.
20210207001157 85880 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Fig.4 temperature distribution under operating conditions
Figure.5 also shows the distribution of pipeline temperature at 50s and 1000s under design conditions. It can be seen from Figure 5(a) that the highest temperature of the inner wall of the pipeline is 260℃, the lowest temperature of the outer wall of the pipeline is ℃ at 50s, and the temperature difference between inside and outside the pipeline is ℃. The lowest temperature also occurs inside the elbow. From figure 5 (B), the lowest temperature of the outer wall of the pipeline at 1000s is ℃, and the temperature difference between the inside and outside of the pipeline is ℃. At 1000s, the overall temperature distribution of the pipeline is more uniform than that at 50s. The temperature difference between the inner and outer walls of the pipeline is reduced by ℃, and the lowest temperature is moved from the inside of the elbow to the tee interface, which is similar to that under service conditions.Figure 5 also shows the temperature distribution of the pipeline at 50s and 1000s under the design condition. It can be seen from Figure 5 (a) that the maximum temperature of the inner wall of the pipeline is 260 ℃, and the minimum temperature of the outer wall of the pipeline is 50.5 ℃ at 50s, and the temperature difference between the inner and outer wall of the pipeline is 209.5 ℃. The lowest temperature also appears on the inside of the bend. From Fig. 5 (b), it can be seen that the minimum temperature of the outer wall of the pipeline is 245.8 ℃ at 1000s, and the temperature difference between the inside and outside of the pipeline is 14.2 ℃. At 1000s, the overall temperature distribution of the pipeline is more uniform than that at 50s, the temperature difference between the inner and outer walls of the pipeline decreases by 195.3 ℃, and the lowest temperature moves from the inner side of the elbow to the tee joint, which is similar to that under service conditions.
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Fig.5 temperature distribution under design conditions
Figure 6 shows the relationship between the minimum temperature and time of the pipeline at each time under two working conditions. It can be seen from the figure that with the increase of time, the pipeline temperature increases continuously. Under the working condition, the heating speed before 300s is faster, the temperature after 300s is gradually stable, and the temperature after 700s is basically no longer changing. The temperature reaches ℃ at 1000s, and the pipeline temperature has reached equilibrium. Under the design condition, the heating speed before 400s is faster, and the heating speed between 400s and 800s is slower. After 800s, the temperature basically does not change, and the temperature reaches ℃ at 1000s.Figure 6 shows the relationship between the minimum temperature of the pipeline at each time and the time under the two conditions. It can be seen from the figure that with the increase of time, the temperature of the pipeline increases continuously. Under the operating conditions, the heating rate is faster before 300s, and the temperature is gradually stable after 300s. After 700s, the temperature basically does not change. At 1000s, the temperature reaches 104.9 ℃, and the pipeline temperature has reached the equilibrium state. Under the design conditions, the heating rate is faster before 400s, and slower between 400s and 800s. After 800s, the temperature basically does not change, and the temperature reaches 245.8 ℃ at 1000s.
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Fig.6 variation of pipeline temperature with time under two working conditions

Stress distribution

Figure 7 shows the stress distribution of the pipeline at 50s and 1000s under service conditions. It can be seen from Figure 7 (a) that at 50s, the maximum stress of the pipeline is 430.1mpa, and the maximum stress point appears on the outer surface of the elbow; the stress level of the weld is obviously higher than that of both sides of the weld, and the maximum stress at the weld is 305mpa, and the maximum stress point appears on the outer surface of the weld. It can be seen from Figure 7 (b) that at 1000s, the maximum stress of the pipe is 380.8mpa, and the maximum stress point appears on the inner surface of the elbow; the maximum stress of the weld is 210.8mpa, and the maximum stress point appears on the outer surface of the weld. Comparing with Figure. 7 (a) and (b), it is found that with the increase of time, the maximum stress of the pipeline decreases by 49.3mpa, the maximum stress of the weld decreases by 94.2mpa, the maximum stress point of the pipeline transfers from the outer surface of the elbow to the inner surface, and the maximum stress point of the weld basically does not change.
20210207001637 18540 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Figure.7 stress distribution under service conditions
Figure.8 shows the stress distribution of the pipeline at 50s and 1000s under design conditions. As can be seen from figure 8(a), at the moment of 50s, the maximum stress of the pipeline is 684.3Mpa, and the maximum stress point appears at the inner surface of the elbow; the stress level at the weld is obviously higher than that on both sides of the weld. The maximum stress at the weld is 529.8Mpa, and the maximum stress point appears on the outer surface of the weld. It can be obtained from Figure 8 (B) that at 1000s, the maximum stress of the pipeline is 454.2Mpa, and the maximum stress point appears at the outer surface of the elbow; The maximum stress at the weld is 259.2Mpa, the maximum stress point appears at the outer surface of the weld. Compared with Fig. 8(a) and (B), it is found that with the increase of time, the maximum stress of the pipeline decreases by 230.1Mpa, and the maximum stress at the weld decreases by 270.6Mpa, the maximum stress point of the pipeline is transferred from the inner surface of the elbow to the outer surface, and the maximum stress point of the weld is basically unchanged.Figure 8 shows the stress distribution of the pipeline at 50s and 1000s under the design condition. It can be seen from figure 8 (a) that at 50s, the maximum stress of the pipe is 684.3mpa, and the maximum stress point appears on the inner surface of the elbow; the stress level of the weld is obviously higher than that of both sides of the weld, and the maximum stress at the weld is 529.8mpa, and the maximum stress point appears on the outer surface of the weld. It can be seen from figure 8 (b) that at 1000s, the maximum stress of the pipe is 454.2mpa, and the maximum stress point appears on the outer surface of the elbow; the maximum stress of the weld is 259.2mpa, and the maximum stress point appears on the outer surface of the weld. Comparing with figure 8 (a) and (b), it is found that with the increase of time, the maximum stress of the pipeline decreases by 230.1mpa, the maximum stress of the weld decreases by 270.6mpa, the maximum stress point of the pipeline transfers from the inner surface of the elbow to the outer surface, and the maximum stress point of the weld basically does not change.
20210207001732 89958 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Fig.8 stress distribution under design conditions
Figure 9(a) shows the change of the overall maximum stress of the pipeline with time. In both working conditions, the maximum stress of the pipeline decreases with the increase of time; under the design working condition, the pipeline stress drops before 400s and the displacement penetrates, 400s Compared with the change of the pipeline stress afterwards, the maximum stress of the pipeline basically does not change after 400s; under the operating conditions, Figure 9(b) shows the change of the maximum stress of the weld with time, and the maximum stress of the weld decreases with the increase of time; Under design conditions, before the decrease in the weld stress, 300s before the weld stress decreases, after 300s the weld stress decreases, 500s decreases before the weld stress decreases, and after 500s the weld stress decreases The amplitude decreases, and the weld stress decreases the most after 500s. There is basically no change. Comparing Fig. 6 and Fig. 9, it is found that the stress change of the pipeline and the weld seam corresponds to the temperature change of the pipeline. A higher temperature change rate means a higher stress change rate, and a lower temperature change rate means a smaller stress change rate.
20210207002133 40517 - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
Figure.9 variation of maximum stress of downtube track and weld with time under two working conditions

Strength evaluation

The strength evaluation and evaluation of the stress concentration of the two working conditions downtube and weld respectively are based on the evaluation strategy of elastic stress analysis method in steel pressure vessel-analytical design standard JB4732-1995 (confirmed in 2005), the stress concentration is classified and evaluated by using the linearized principle. Firstly, through the maximum stress node, and set the linearized path along the shortest distance of wall thickness; Secondly, due to geometric discontinuity, therefore, the stress of the film obtained by taking the cross section along the intersection is the primary local film stress PL, corresponding to the primary local film stress intensity sⅱ; the bending stress in the discontinuity zone of the overall structure of the pipeline should be classified into the category of secondary stress, in order to meet the JB4732-1995 (confirmed in 2005) according to the requirements of “steel pressure vessel-analysis and design standard” on the evaluation of stress intensity step by step, the film stress plus bending stress plus secondary stress shall be treated in accordance with SⅣ. The stress evaluation strength results are shown in Table 3.

Table.3 stress evaluation results

Working condition Evaluation path Stress combination type Time/S Calculated value of stress intensity/Mpa Allowable value of stress intensity/Mpa Evaluation results
Service condition Weld S 50 124.7 176 Qualified
1000 125.6 Qualified
S 50 266.3 352 Qualified
1000 155.1 Qualified
Pipeline S 50 140.3 176 Qualified
1000 149.2 Qualified
S 50 390.7 414 Qualified
1000 252.9 Qualified
Design conditions Weld S 50 145.8 176 Qualified
1000 144.3 Qualified
S 50 526.9 352 Qualified
1000 200.2 Qualified
Pipeline S 50 143.3 176 Qualified
1000 165.1 Qualified
S 50 397.7 414 Qualified
1000 375.3 Qualified

From the stress assessment results in table 3, it can be concluded that both the pipe fittings and weld joints meet the stress strength requirements under the service conditions; Under the design conditions, the pipe fittings meet the stress strength requirements, the stress of the welding seam at the moment of 50s is assessed as unqualified, indicating that cracking and other failure may occur during the initial heating process of the welding seam of the pipeline under the design condition, which is easy to cause loss of personnel and property.

Analysis and discussion

Through temperature analysis, stress analysis and strength evaluation of high-pressure pipeline and weld, after obtaining the analysis results, it is found that there are problems in the design and use of the pipeline: under the design conditions of the pipeline, stress analysis is carried out on the pipeline and Weld to obtain the stress distribution. Due to incomplete filled groove of the pipeline weld, stress concentration exists at the weld; Linearized strength evaluation is carried out during the heating process of the pipeline weld, it is found that the strength evaluation of the weld is unqualified at the initial 50s of heating up, which does not meet the strength requirements. The reason for this situation is that in the initial heating stage of the pipeline, the temperature difference between inside and outside the pipeline is relatively large, the secondary stress level in the pipeline is relatively high, plus the phenomenon of high-strength mechanical load and local stress concentration, this leads to unqualified strength evaluation in structural discontinuity such as weld seam. In order to ensure the safe operation of the pipeline under design conditions, repair welding should be carried out at the weld to make the weld surface flush with the pipeline surface. When the weld is filled, on the one hand, the weld strength is improved; On the other hand, the structural discontinuity at the weld of the pipeline can be eliminated and the influence of stress concentration can be reduced.

Conclusion

Taking a high-temperature and high-pressure pipeline and weld seam in a chemical enterprise as the research object, based on the elastic analysis strategy, the stress intensity of the pipeline and the high-stress area at the weld seam is evaluated, and the temperature distribution and stress distribution of the pipeline and the weld seam are analyzed. The following conclusions:

  • Under the operating conditions, the pipeline temperature continues to rise with the increase of time, the pipeline temperature is 104.9℃ at 1000s, and the temperature difference between inside and outside is 15.1℃; under the design conditions, the pipeline temperature rises continuously with the increase of time, and the pipeline temperature at 1000s is 245.8℃, the temperature difference between inside and outside is 14.2℃; the temperature difference between inside and outside the pipeline decreases with time.
  • The pipeline has stress concentration phenomenon at the elbow and weld. Under the working conditions, the maximum stress of the elbow is 430.1Mpa and the maximum stress of the weld is 305Mpa; under the design conditions, the maximum stress of the elbow is 684.3Mpa. The maximum stress of the seam is 529.8Mpa; the maximum stress of the pipeline and the weld decreases gradually with time.
  • Perform linearized strength evaluation during the heating process of pipelines and welds. Under working conditions, pipe fittings and welds meet the stress intensity requirements; under design conditions, pipe fittings meet the stress intensity requirements, and the pipe welds are at 50s. The stress is assessed as unqualified and does not meet the strength requirements.

In order to ensure the safe operation of the pipeline under the design conditions, repair welding should be carried out at the weld, so that the weld surface is flush with the pipeline surface.
Authors: han shoupeng, zhang pugen, xu jinsha, wang shenghui, xu hongtao

Source: China Industrial Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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stress analysis and strength evaluation of weld of high pressure industrial pipeline - Stress analysis and strength evaluation of weld of high pressure industrial pipeline
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Stress analysis and strength evaluation of weld of high pressure industrial pipeline
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This paper studies a high-temperature and high-pressure pipeline in a chemical enterprise, establishes a three-dimensional physical model of pipe fittings and welds, uses the finite element analysis method to study the stress field and temperature field changes with time under the design and service conditions of the pipeline and welds, and carries out stress assessment based on the analysis and design standards to determine the safe use range.
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