Analysis & Preventive Measures for Cracks in Ultra-high Pressure Steam Pipelines
Posted: 06/25/2023 09:55:46 Hits: 42
4.1 The Analysis of test results
Through visual inspection, hardness inspection, composition and metallographic inspection, it is shown that the current performance of the material is consistent with its service time. Compared with the test results in 2006, the overall hardness value dropped significantly, and the spheroidization degree of the metallographic structure was as high as 3.5, indicating that with the extension of use time, the comprehensive mechanical properties of materials decreased, especially the surface strength decreased more obviously. From the size of the crack, it can be determined that the crack’s initiation point is in the upper part.
4.2 The analysis of force
During the start-up period of the cracked pipeline, when the temperature rises from normal temperature, affected by thermal stress, there is axial displacement. At the same time, it is subjected to a tensile stress of the upper branch through the tee and the axial stress of the horizontal branch. Judging from the deformation of the external insulation iron sheet of the horizontal branch pipe at the support position (the horizontal branch pipe has a displacement of about 100 mm to the south), after the temperature rises, the horizontal branch pipe and the vertical branch pipe give upward tensile stress to the main pipe at the cracked part, forming the upward bending moment at the cracked site. The upper part of the eccentric reducer is the largest geometric discontinuity position, and it is also the position with the greatest stress concentration. Under the joint action of upward tensile stress, it is easy to exceed the material strength and crack.
4.3 Comparative analysis of two cracks in SS pipelines
The cracking of the high-pressure steam pipeline that occurred on June 25, 2006, has several characteristics in common with the cracking of the high-pressure steam pipeline this time.
1) The parts where cracks occur are located in the heat-affected zone of the welding seam at the small end of the eccentric reducer.
2) Cracking occurs in the upper half of the welding seam of the reducer.
3) There is a vertical branch line near the crack point, and the flexible design of the pipeline is the same.
The two cracks can be determined to be cracks in the weak position of the pipeline caused by the temperature difference stress release of the piping system, so the nature of the two cracks can be classified as the same type and the same cause of cracking. The time cracking occurred in 2006, which is nearly 16A from this cracking, so the occurrence of cracking due to material factors can be ruled out.
4.4 Frequent pressure rise and depressurization of the SS pipeline system caused fatigue cracking of welding seams.
The ultra-high pressure steam system was put into use in 1995. After checking the number of complete shutdowns of the device and that of decompression of the SS pipeline, there were a total of 19 times. When the temperature of the SS pipeline is raised and lowered, the welding seam at the cracked part is repeatedly stretched and compressed due to the expansion and contraction of the pipeline, and the welding seam has fatigue damage, which eventually causes fatigue cracking of the welding seam.
4.5 Analysis of temperature rise and fall, pressure rise and fall of SS pipelines
The temperature rise of the cracking furnace is strictly operated in accordance with the operation manual. When the SS is connected to the grid, the pressure rise rate is 2.7 MPa/h, which is less than the 3 MPa/h required by the operation manual, and the heating process meets the requirement for operation. At 20:30 on April 27, the SS pipeline was depressurized urgently due to the production needs of the post-system. Within 1.3 hours, the system pressure drops from 10.17 MPa to 4.96 MPa. The depressurization rate is 4 MPa/h, and the depressurization rate exceeds the operating requirement for 3 MPa/h, causing a load impact on the pipeline.
4.6 Precautions
During the maintenance period of the device, non-destructive testing and investigation of the eccentric reducer of the steam pipes are carried out, and X-ray or conventional ultrasonic, magnetic particle testing and hardness testing are carried out, focusing on checking whether there are cracks. Before the regular inspection of the pipeline, check whether it is necessary to add pipeline supports and hangers, parts with obvious displacement, parts where stress concentration may occur, non-destructive testing of different diameters and ends, and tees in the review plan. Spot-check the metallography of the pipeline, and comprehensively judge the safe use of the pipeline according to the degree of spheroidization and hardness. In the process of starting and shutting down, especially for equipment that has been running for more than 20 years, the temperature rise and fall speed, pressure rise and fall speed should be controlled slowly to minimize the influence of temperature difference stress and load impact on the pipeline.
5. Conclusion
To sum up, various test data and analyses and judgments show that the SS pipeline has been in use for 27 years since it was started in 1995. It has experienced repeated temperature rises and falls, and the part has been repeatedly subjected to tensile and compressive stress, and bainite spheroidization has occurred inside the material, resulting in a material yield point, tensile strength, and impact toughness decreased. The cracking position of the ultra-high-pressure steam pipeline is the welded part of the eccentric reducer and the straight pipe. This part is the stress concentration part of the pipeline. Under the action of the temperature difference stress of the pipeline, this weak part (heat-affected zone of the eccentric reducer) cracks.
Through visual inspection, hardness inspection, composition and metallographic inspection, it is shown that the current performance of the material is consistent with its service time. Compared with the test results in 2006, the overall hardness value dropped significantly, and the spheroidization degree of the metallographic structure was as high as 3.5, indicating that with the extension of use time, the comprehensive mechanical properties of materials decreased, especially the surface strength decreased more obviously. From the size of the crack, it can be determined that the crack’s initiation point is in the upper part.
4.2 The analysis of force
During the start-up period of the cracked pipeline, when the temperature rises from normal temperature, affected by thermal stress, there is axial displacement. At the same time, it is subjected to a tensile stress of the upper branch through the tee and the axial stress of the horizontal branch. Judging from the deformation of the external insulation iron sheet of the horizontal branch pipe at the support position (the horizontal branch pipe has a displacement of about 100 mm to the south), after the temperature rises, the horizontal branch pipe and the vertical branch pipe give upward tensile stress to the main pipe at the cracked part, forming the upward bending moment at the cracked site. The upper part of the eccentric reducer is the largest geometric discontinuity position, and it is also the position with the greatest stress concentration. Under the joint action of upward tensile stress, it is easy to exceed the material strength and crack.
4.3 Comparative analysis of two cracks in SS pipelines
The cracking of the high-pressure steam pipeline that occurred on June 25, 2006, has several characteristics in common with the cracking of the high-pressure steam pipeline this time.
1) The parts where cracks occur are located in the heat-affected zone of the welding seam at the small end of the eccentric reducer.
2) Cracking occurs in the upper half of the welding seam of the reducer.
3) There is a vertical branch line near the crack point, and the flexible design of the pipeline is the same.
The two cracks can be determined to be cracks in the weak position of the pipeline caused by the temperature difference stress release of the piping system, so the nature of the two cracks can be classified as the same type and the same cause of cracking. The time cracking occurred in 2006, which is nearly 16A from this cracking, so the occurrence of cracking due to material factors can be ruled out.
4.4 Frequent pressure rise and depressurization of the SS pipeline system caused fatigue cracking of welding seams.
The ultra-high pressure steam system was put into use in 1995. After checking the number of complete shutdowns of the device and that of decompression of the SS pipeline, there were a total of 19 times. When the temperature of the SS pipeline is raised and lowered, the welding seam at the cracked part is repeatedly stretched and compressed due to the expansion and contraction of the pipeline, and the welding seam has fatigue damage, which eventually causes fatigue cracking of the welding seam.
4.5 Analysis of temperature rise and fall, pressure rise and fall of SS pipelines
The temperature rise of the cracking furnace is strictly operated in accordance with the operation manual. When the SS is connected to the grid, the pressure rise rate is 2.7 MPa/h, which is less than the 3 MPa/h required by the operation manual, and the heating process meets the requirement for operation. At 20:30 on April 27, the SS pipeline was depressurized urgently due to the production needs of the post-system. Within 1.3 hours, the system pressure drops from 10.17 MPa to 4.96 MPa. The depressurization rate is 4 MPa/h, and the depressurization rate exceeds the operating requirement for 3 MPa/h, causing a load impact on the pipeline.
4.6 Precautions
During the maintenance period of the device, non-destructive testing and investigation of the eccentric reducer of the steam pipes are carried out, and X-ray or conventional ultrasonic, magnetic particle testing and hardness testing are carried out, focusing on checking whether there are cracks. Before the regular inspection of the pipeline, check whether it is necessary to add pipeline supports and hangers, parts with obvious displacement, parts where stress concentration may occur, non-destructive testing of different diameters and ends, and tees in the review plan. Spot-check the metallography of the pipeline, and comprehensively judge the safe use of the pipeline according to the degree of spheroidization and hardness. In the process of starting and shutting down, especially for equipment that has been running for more than 20 years, the temperature rise and fall speed, pressure rise and fall speed should be controlled slowly to minimize the influence of temperature difference stress and load impact on the pipeline.
5. Conclusion
To sum up, various test data and analyses and judgments show that the SS pipeline has been in use for 27 years since it was started in 1995. It has experienced repeated temperature rises and falls, and the part has been repeatedly subjected to tensile and compressive stress, and bainite spheroidization has occurred inside the material, resulting in a material yield point, tensile strength, and impact toughness decreased. The cracking position of the ultra-high-pressure steam pipeline is the welded part of the eccentric reducer and the straight pipe. This part is the stress concentration part of the pipeline. Under the action of the temperature difference stress of the pipeline, this weak part (heat-affected zone of the eccentric reducer) cracks.
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