Analys av orsaken till läckagefel i en reducerande tee-kropp
En anläggning för behandling av naturgas hade ett läckagefel i en reducing tee body. Nondestructive testing, physical and chemical inspection, metallographic analysis, and energy spectrum analysis were used to analyze the cause of the failure of the leaking pipe fittings. The results show that the tee was improperly controlled by the molding process, and under axial pressure, the inner surface appeared to be deformed along the radial direction of the main pipe, and the transition between the branch pipe and the main pipe appeared to be wavy and raised, and cracks were produced on the inner surface, and cracks sprouted and expanded to form penetrating cracks. Eventually, in pressure testing, the tees have insufficient pressure-bearing capacity, and leakage occurs at the crack.
Oil and gas in petrochemical production are transported by pipelines of different metal materials and different structural shapes. The internal pressure of the piping system served by the Rördelar. in these pipelines is usually more than 0.1 Mpa. Suppose there are serious manufacturing defects or extremely harsh medium scouring corrosion and other factors in use. In that case, it may cause a fire or explosion, resulting in huge economic losses and adverse social impacts. Therefore, in the operation process of pressure pipelines conveying flammable products, it is necessary to pay great attention to this, strictly implement the requirements of the relevant national standards, timely detection and elimination of major pipeline defects, and effectively prevent the occurrence of pipeline failure accidents.
1. Overview of the failure
In March 2019, a leakage accident occurred in a reducing tee used in an oilfield natural gas purification plant. The tee specification is DN100/DN65, the material is L245NS, and its manufacturing standard is GB/T 13401-2017 “Technical Specification for Steel Butt-Weld Pipe Fittings”. The leakage occurred in the first installation of the tee during the test pressure, the maximum test pressure of 0.7MPa, and the target pressure of 6.8MPa. After the leakage, the field staff marked the leakage site and then disassembled and replaced it, and the leakage of the tee samples was sent to the testing organization to analyze the cause of failure.
2. Macro-analysis and penetration testing
The outer surface of the leaking tee is covered with a layer of black paint; according to the yellowish-brown watermarks remaining on the paint surface, it is initially determined that the suspected leakage point of its pressurization process is located in the transition area between the branch pipe and the main pipe of the tee, which is locally polished with an electric grinding wheel, revealing the metallic luster of the outer wall. Still, no defects are found in the appearance.
To further determine the location of the leakage point, the leakage of the tee surface penetration non-destructive testing found that the polished tee branch pipe and the main pipe transition area have a closed crack visible to the naked eye, the length of about 40mm, the crack location is the scene found in the hydraulic leakage defects.
Subsequently, the leaking tee was dissected along the axial direction parallel to the main pipe, and the inner wall was cleaned of corrosion. There are obvious machining traces on the inner surface of the tee; the transition between the branch pipe and the main pipe can be seen in the obvious machining cutter marks angular interface, belonging to the non-circular arc transition, where the inner wall of the branch pipe is a wavy concave-convex mutation of the brown uniform corrosion morphology, and its low-power morphology shown in Fig. 1 and Fig. 2.
Figure.1 Traces of penetration defects on the outer wall of the leaking tee
Figure.2 Low-power morphology of the inner wall at the leakage point of the tee
3. Physical and chemical performance test
3.1 Chemical composition analysis
Sampling in the main part of the tee, according to GB/T 4336-2016 standard, with ARL4460 direct reading spectrometer for chemical composition analysis, the results are shown in Table 1. Chemical composition analysis results show that in addition to the element Mn, the content of the other elements is in line with the standard GB/T 13401-2017 requirements.
Table.1 Chemical composition analysis results (mass fraction) %
|Sample number||Pipe body||GB/T 13401-2017 Requirements|
3.2 Brinell hardness test
Samples were taken from the main part and tested with a BH3000 Brinell hardness tester according to the requirements of ASTM E10-18 standard. The result of the hardness test is HBW145, and the result meets the requirements of GB 13401-2017 standard.
3.3 Metallographic analysis
Take the metallographic analysis specimen from the competent part, according to GB/T 13298-1991, GB/T 10561-2005, GB/T 4335-2015, GB/T 6394-2002 standards, with MEF4M Metallographic analysis with MEF4M metallographic microscope and image analysis system, the organization of the main pipe body is granular bainite + ferrite, grain size of 9.5, the metallographic organization of the pipe body is shown in Figure 3.
Figure.3 The organization of the main pipe body
Along the vertical crack extension direction to intercept the metallographic analysis of specimens. Take the specimen containing the crack tip, polish it with sandpaper, observe it under the light microscope, and find that the crack mainly expands along the bottom of the dent in the concave-convex interface of the inner wall. The direction of crack expansion is relatively straight, the tip of which is rounded and blunt, and the crack is filled with gray material. After an etching by nitric acid-alcohol solution, the matrix organization was ferrite plus pearlite. The ferrite organization was around the crack edge and tip, with an obvious decarburization phenomenon. Specimen local organization along the wall thickness direction there is a slight deformation morphology, carefully observed near the inner wall surface layer of the organization, microscopy can be seen more than one nearly parallel to the axial direction of the branch pipe of the small micro-cracks, crack length is less than or equal to the actual length of 0.3mm, cracks embedded with a large number of gray matters. The cracks and their organization are shown in Figures 4 to 8.
Fig.4 Low magnification morphology of cracks and metallographic sampling site map
Fig.5 Crack morphology of sample cross-section
Fig.6 Cracks and inlays within the cracks
Fig.7 Decarburization of the matrix tissue around the cracks
Fig.8 Crack defects close to the inner surface layer
4. Energy spectrum analysis
The crack inlay was analyzed by XFORD INCA350 energy spectrum analyzer, and its energy spectrum analysis curve is shown in Fig. 9. As can be seen in Fig.9, the gray substance embedded in the crack is rich in Fe and O elements, which is iron oxide.
Fig.9 Energy spectrum analysis curve of non-metallic substances inside the cracks
5. Comprehensive analysis
The chemical composition analysis and Brinell hardness test are carried out on the leakage tee. The test results show that the Brinell hardness test results of the leakage tee meet the GB 13401-2017 standard requirements, and the Mn content in the chemical composition does not meet the standard requirements. Metallographic analysis results show that the main pipe body organization is granular bainite + ferrite + pearlite.
From the penetration test and metallographic analysis, it is shown that the tee’s leakage occurs due to cracks through the full wall thickness locally at the transition between its branch pipe and the main pipe. Low-magnification morphology observation shows that the tee internal surface layer organization under the microscope has obvious deformation characteristics along the radial direction of the main pipe, macro unprocessed branch pipe, and the main transition part of the inner wall of the local metal. There is a steep wave-like concave-convex phenomenon, combined with the characteristics of the tee molding process, can be determined; this is due to the tee molding process control is not appropriate; that is, in the tee branch pipe molding stage, the tee blanks in the non-normally matched internal pressure and the axial stress produced by the role of the non-matching axial stress under the action of non-coordinated strain and formation. Low-power analysis shows that the tee leakage point crack originates from the concave part of the wavy mutation on the inner surface of the tee, the crack is filled with a large amount of gray material, and the tissue on both sides of the crack edge is typical decarburization morphology, which is distinctly different from the tissue of the rest of the tee matrix. In summary, the multiple cracks in the leakage area existed before the final heat treatment of the finished tee, i.e., the cracks were generated in the tee molding stage. When there are cracks in the tee, its subsequent normal heat treatment process, due to the heating temperature is usually above the normalizing temperature, located in the oxidizing atmosphere containing cracks tee in the furnace for a long time in the holding process, the cracks on both sides of the surface are relatively rough, the oxidation layer is relatively loose at the base body of the carbon atoms through the diffusion mechanism, will be with the furnace high temperature atmosphere of oxygen atoms continue to oxidation reaction, combined with the generation of carbon dioxide and release, resulting in its local tissue dehiscence. She was released, resulting in local decarburization of the organization and, ultimately, the formation of the observed decarburization layer phenomenon.
X-ray spectroscopy analysis shows that the gray substances within the cracks are iron oxides, combined with the decarburization characteristics of the organization around the cracks, which further indicates that the oxides within the cracks are generated in the high temperature environment of the tee during the heat treatment stage of the production process.
The failure of the tee using the cold extrusion molding method of manufacturing production in the cold forming process, the tee’s branch pipe is attached to the main tube ends and the inner wall of the high-pressure effect so that the main tube on the metal in the outer mold bound to the branch pipe mold flow and formation. Tee cold extrusion molding process is the plastic deformation of metal materials processing process; in this process, the material will be due to a relatively short period to produce intense deformation, internal dislocations occurring proliferation, the movement of the formation of dislocations entanglement and lattice distortion and lead to cold hardening phenomenon, so that the hardness of the material, the internal stress rises sharply, toughness decreases, this time, even in the smaller the external load conditions will also be made to produce its instability cracking. According to the cold extrusion tee molding process characteristics can be seen, tee branch pipe metal all from the blank pipe metal, its main pipe and branch pipe transition region of the metal will produce a large deformation; tee molding process, the reverse flow of metal flow line in this meeting or cross, the material organization continuity of raw materials lower, at the same time, the deformation process material due to produce a large plastic deformation and the deformation of hardening, resulting in the material within the larger internal stresses. This material produces a large internal stress; if the tee molding does not have the corresponding heat treatment, the stress cannot be effectively released, and the risk of instability and cracking will increase dramatically.
In summary, the tee in the molding process, cracks on the inner surface, cracks sprouted and expanded, forming penetrating cracks. In the subsequent heat treatment process, the matrix organization at the edges of both sides of the crack undergoes oxidative decarburization in a high-temperature environment. If such a tee with cracks cannot be detected in the inspection process, and the tee containing crack defects is installed into the piping system, an early leakage accident will occur at the cracks during the pressure testing of the piping system due to the insufficient effective pressure-bearing capacity of the tee containing cracks.
6. Conclusions and recommendations
- 1) The leakage failure of the tee is due to cracks through the wall thickness in its main pipe and branch pipe parts, which are generated in the process of cold extrusion molding of the tee.
- 2) The transition area between the main pipe and the branch pipe of the cold-extruded tee is the weak link of its structure, which is also one of the important key points of tee quality control, and the tee produced by this process should eliminate the stress in time to prevent the occurrence of brittle cracking.
Author: Cai Ke