Cracking of Welding Seams of Eccentric Reducers in Ultra-high Pressure Steam Pipelines (Part One)
Posted: 01/12/2024 05:10:29 Hits: 7
Abstract: The cracking process of the reducer’s welding seam on the ultra-high pressure steam pipeline of the 220kt/an ethylene plant of PetroChina Dushanzi Petrochemical Company is described. The visual inspection, alloy analysis, on-site metallographic coating, and hardness testing of the reducer’s welding seam are used to find the cause of cracking at the reducer’s welding to formulate maintenance measures for ultra-high pressure steam pipelines, extend the service life of the pipeline, and ensure the safety and economic benefits of the ethylene plant.
The ultra-high pressure steam pipeline is one of the most important pressure pipelines in the ethylene unit, and its long-term smooth operation directly affects the output and efficiency of the ethylene unit. The 220 kt/a ethylene cracking unit of PetroChina Dushanzi Petrochemical Branch was designed by the British SNAM Company and put into use in August 1995. The ultra-high-pressure steam pipeline was designed, installed and put into use at the same time. The ethylene plant has a total of seven cracking furnaces, and the ultra-high-pressure steam generated by each cracking furnace is directed get into the main pipe through a globe valve. On April 28, 2022, during the grid connection operation of No. 4 furnace SS, it was discovered that cracks appeared in the welding seam of the reducer with 273 mm × 168 mm between the high-pressure steam main pipe of the cracking furnace and the tee. Since the device is at a critical stage of construction, the cracked areas must be repaired. It is impossible to sample the cracked area for physical and chemical analysis of the fracture. The cause of this crack can only be analyzed based on on-site inspections and historical failures.
1. Basic situation of ultra-high pressure steam pipeline
The eccentric reducer of the ultra-high-pressure steam pipeline is made from ASTM A335 P22. The pipeline medium is ultra-high-pressure steam. The operating temperature is 525℃ and the operating pressure is 12.5MPa. The specification of the eccentric reducer is DN150 × DN250 mm; the wall thickness of the pipe on the side of the small diameter of the reducer is 23 mm, and that of the large diameter of the reducer is 42mm.
At 1:30 pm on April 28, 2022, a leak was found in the reducer. At 5:20 pm, the SS system pressure dropped to 1MPa. The maintenance personnel removed the pipe insulation and confirmed that the leakage location was at the welding seam of the eccentric reducer of the main pipe of DN250 × DN150 mm. The crack was judged from the crack opening size and appearance. It expanded downward along the fusion line from the upper part, and the crack length was about 240mm. The fracture was completely flat from the outside; at 22:43, the construction workers repaired the cracked part of the welding seam; at 16:14 on April 30, the heat treatment of the welding seam was performed, and the cracking furnace was heated up.
The SS system was put into use in 1995 based on operational performance over the years. After inquiry, it was found that the number of times the device was shut down for maintenance and the number of times the ultra-high pressure steam pipeline was raised and lowered was 19 times in total.
2. The inspection status of ultra-high-pressure steam pipelines
Since the ethylene plant is at a critical stage of construction, the cracked areas must be repaired by removal and then welding. Complete observation and microscopic analysis of the fracture cannot be carried out. On-site visual inspection, alloy analysis, on-site metallographic coating, hardness and other test and analysis items can only be performed.
2.1 Appearance inspection of reducers
The crack is located on the fusion line of the side welding seam of the small end of the SS pipe eccentric reducer. From the size of the crack’s opening and appearance, it can be judged that the crack extends from the upper part downward along the fusion line. The length of the crack is about 240 mm. The fracture is flat viewed from the outside, which has obvious characteristics of great stress cracking. The bottom of the eccentric reducer has obvious upward bending deformation, as shown in Figure 1.
Figure 1 The appearance of locations of cracking of the SS pipe
2.2 Hardness tests
Use a Leeb hardness tester to conduct hardness tests on the welding seam of the crack, reducer, and base metal parts of straight pipes. The weld hardness value is 164 HB, reducer 139 HB, and straight pipe base material 131 HB. The hardness value corresponds to its service time.
2.3 The analysis of the material
Use an alloy analyzer to detect the composition of the cracked straight pipe and reducer. The alloy’s composition is consistent with ASME SA-335 P22.
2.4 Metallographic inspection
Use an on-site metallographic microscope to analyze the metallographic microstructure of the reducer and straight pipes near the cracked site. The results show that the metallographic structure of the reducer is ferrite plus bainite plus carbide, as shown in Figure 2. The spheroidization grade is rated as 3.5 according to DL/T 999-2006 Spheroidization Rating Standard for Power Station Steel, which is moderate spheroidization. The metallographic structure of the base material of the straight pipe section is ferrite plus bainite plus carbide, as shown in Figure 3. According to the DL/T 999-2006 standard, the spheroidization grade is evaluated as 3.0, which is mild spheroidization. The metallographic structure is commensurate with its service time.
Figure 2 The structure of the reducer
The ultra-high pressure steam pipeline is one of the most important pressure pipelines in the ethylene unit, and its long-term smooth operation directly affects the output and efficiency of the ethylene unit. The 220 kt/a ethylene cracking unit of PetroChina Dushanzi Petrochemical Branch was designed by the British SNAM Company and put into use in August 1995. The ultra-high-pressure steam pipeline was designed, installed and put into use at the same time. The ethylene plant has a total of seven cracking furnaces, and the ultra-high-pressure steam generated by each cracking furnace is directed get into the main pipe through a globe valve. On April 28, 2022, during the grid connection operation of No. 4 furnace SS, it was discovered that cracks appeared in the welding seam of the reducer with 273 mm × 168 mm between the high-pressure steam main pipe of the cracking furnace and the tee. Since the device is at a critical stage of construction, the cracked areas must be repaired. It is impossible to sample the cracked area for physical and chemical analysis of the fracture. The cause of this crack can only be analyzed based on on-site inspections and historical failures.
1. Basic situation of ultra-high pressure steam pipeline
The eccentric reducer of the ultra-high-pressure steam pipeline is made from ASTM A335 P22. The pipeline medium is ultra-high-pressure steam. The operating temperature is 525℃ and the operating pressure is 12.5MPa. The specification of the eccentric reducer is DN150 × DN250 mm; the wall thickness of the pipe on the side of the small diameter of the reducer is 23 mm, and that of the large diameter of the reducer is 42mm.
At 1:30 pm on April 28, 2022, a leak was found in the reducer. At 5:20 pm, the SS system pressure dropped to 1MPa. The maintenance personnel removed the pipe insulation and confirmed that the leakage location was at the welding seam of the eccentric reducer of the main pipe of DN250 × DN150 mm. The crack was judged from the crack opening size and appearance. It expanded downward along the fusion line from the upper part, and the crack length was about 240mm. The fracture was completely flat from the outside; at 22:43, the construction workers repaired the cracked part of the welding seam; at 16:14 on April 30, the heat treatment of the welding seam was performed, and the cracking furnace was heated up.
The SS system was put into use in 1995 based on operational performance over the years. After inquiry, it was found that the number of times the device was shut down for maintenance and the number of times the ultra-high pressure steam pipeline was raised and lowered was 19 times in total.
2. The inspection status of ultra-high-pressure steam pipelines
Since the ethylene plant is at a critical stage of construction, the cracked areas must be repaired by removal and then welding. Complete observation and microscopic analysis of the fracture cannot be carried out. On-site visual inspection, alloy analysis, on-site metallographic coating, hardness and other test and analysis items can only be performed.
2.1 Appearance inspection of reducers
The crack is located on the fusion line of the side welding seam of the small end of the SS pipe eccentric reducer. From the size of the crack’s opening and appearance, it can be judged that the crack extends from the upper part downward along the fusion line. The length of the crack is about 240 mm. The fracture is flat viewed from the outside, which has obvious characteristics of great stress cracking. The bottom of the eccentric reducer has obvious upward bending deformation, as shown in Figure 1.
Figure 1 The appearance of locations of cracking of the SS pipe
2.2 Hardness tests
Use a Leeb hardness tester to conduct hardness tests on the welding seam of the crack, reducer, and base metal parts of straight pipes. The weld hardness value is 164 HB, reducer 139 HB, and straight pipe base material 131 HB. The hardness value corresponds to its service time.
2.3 The analysis of the material
Use an alloy analyzer to detect the composition of the cracked straight pipe and reducer. The alloy’s composition is consistent with ASME SA-335 P22.
2.4 Metallographic inspection
Use an on-site metallographic microscope to analyze the metallographic microstructure of the reducer and straight pipes near the cracked site. The results show that the metallographic structure of the reducer is ferrite plus bainite plus carbide, as shown in Figure 2. The spheroidization grade is rated as 3.5 according to DL/T 999-2006 Spheroidization Rating Standard for Power Station Steel, which is moderate spheroidization. The metallographic structure of the base material of the straight pipe section is ferrite plus bainite plus carbide, as shown in Figure 3. According to the DL/T 999-2006 standard, the spheroidization grade is evaluated as 3.0, which is mild spheroidization. The metallographic structure is commensurate with its service time.
Figure 2 The structure of the reducer
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