Failure analysis and preventive measures of induction bend
Induction Bend is an important component of oil and gas transmission pipeline. Due to the particularity of bend manufacturing process and structure, the working condition of bend is worse than that of straight pipe in the transportation process, which is the weak link of the transportation pipeline, and the failure accident often occurs. In view of the above problems, the author makes a comprehensive analysis and research on the manufacturing process, quality influencing factors, quality control, failure reasons, etc. through the specific failure case analysis, it demonstrates the impact of solid particle erosion, material structure defects and mechanical damage on the failure of the bend, and puts forward the corresponding preventive measures.
Induction bend refers to the pipe fittings made by simmering the pipes in the hot state (i.e. higher temperature). The medium frequency induction electric heating pipe bender is a common Induction bending pipe unit, which can realize the continuous operation of heating, bending and cooling. The bending process (induction bend) is actually a continuous local quenching integral forming process, which is mainly composed of three parts: medium frequency induction heating system, cooling water system and pushing system.
According to the different methods of bending moment application, the pipe bender can be divided into push bending type and pull bending type. Now, push bending process is mostly used to complete the pipe bending. The working principle of the pushing process is that after the straight pipe is cut, the induction heating coil is sleeved on the steel pipe to be bent part through the bend pushing machine, the pipe head is clamped with the mechanical arm, and the medium frequency current is introduced into the induction heating coil to heat the steel pipe. When the steel pipe is heated to a certain temperature (generally 970-1020 ℃ austenitic structure area), the rear end of the steel pipe is pushed by mechanical thrust, and the front section of the steel pipe is fixed in the required radius length (generally the radius is 6D) mechanical cantilever rotating fixture. Since the rear end of the steel pipe continuously pushes the steel pipe, the bend is formed on the fixed track. At the same time, in the bending process, the steel pipe is cooled quickly with coolant after being heated, so that the bending process can be completed by heating, pushing, bending and cooling. Generally, there are five most commonly used bending angles of 30 °, 45 °, 60 °, 90 ° and 180 ° for the Induction bend, and other angles such as 15 ° can also be included according to the needs of the project (for the push Induction bend process, any angle can be produced, but it is rarely needed in the actual project). The conventional materials of Induction bends are stainless steel, alloy steel, carbon steel, non-ferrous metals and plastics, etc. Production standards mainly include butt welded joints of marine steel pipes (GB / T 10752-1995), steel induction bends for oil and gas transmission (SY / T 5257-2012) and steel seamless butt welded pipe fittings (GB / T 12459-2005), etc. As the Induction bend has good comprehensive performance, it is widely used in oil and gas transmission engineering of petroleum pipeline .
During the pushing process of the Induction bend, the following situations are caused:
- ① If heating system failure;
- ② Cooling water circulation system failure;
- ③ Pushing system failure;
- ④ Power failure.
Factors affecting bend quality
There are many factors that affect the quality of the Induction bend, mainly in the performance of the main pipe and the Induction bend process. On the one hand, the chemical composition, mechanical properties and structure of the main pipe directly determine the internal quality of the bend; On the other hand, it is necessary to make a proper simmer process, that is, to make a proper heating temperature, width of heating belt, feeding speed, cooling speed after bending and other process parameters, so as to ensure that reheating has no obvious impact on the original performance of the main pipe, and to obtain qualified bends that meet the technical performance requirements. The influencing factors are as follows:
- (1) The influence of Induction bending process on product quality. For induction bend, the bending process can be generally divided into two types: quenching (high temperature heating and fast water cooling), no tempering and quenching + tempering. Each Induction bending process has certain scope of application and limitations. In the specific bending process of Induction bend, it is necessary to select different Induction bending processes according to the performance requirements, materials, pipe diameter and wall thickness, Induction bending equipment and control technology of Induction bend, and formulate appropriate Induction bending process parameters, so as to ensure the bending of qualified bends .
- (2) Matching of process parameters of Induction bend. The process matching of the Induction bending process directly affects the quality of the bend. For example, in the Induction bending process, the pushing speed of the main pipe determines the high temperature holding time and quenching cooling speed of the bending heating section, and ultimately affects the structure and performance of the bend. Therefore, the production process should ensure the overall matching of all process parameters, and prevent the adjustment of individual process parameters.
- (3) The influence of tyre bending accuracy. During the manufacturing of bent tire, the corresponding bent tire shall be selected according to the bending pipe diameter, and the specification and size of bent tire shall be controlled within a certain tolerance range.
- (4) Influence of header quality. The main pipe is the basic guarantee for the quality of the Induction bend. The product quality of the main pipe itself is particularly critical, especially the chemical composition and grain size of the main pipe. The internal continuity of the main pipe must also meet the requirements of non-destructive testing. In contrast, the mechanical properties and internal continuity of the main pipe can only improve the pass rate of the bend, and have little impact on the quality of the Induction bend, while the chemical composition and grain size are the premise to ensure the production of the qualified bend. In the simmer process, with the increase of heating temperature, the grain size increases, and the influence of grain size on the toughness of the material is greater than that on the strength .
- (5) The influence of the surface quality of the main pipe. The quality of the Induction bend will also be affected by the defects or defects of the outer surface of the main pipe, such as corrosion pit, drop pit, scratch and hard block.
- (6) Influence of pollution on the surface of main pipe during simmering. As for the Induction bend, the main pipe should not be polluted by low melting point metal, especially copper ion. Once the main pipe is polluted by copper ion, the X70 and X80 steel grade bends will be basically scrapped. In serious cases, cracks will appear throughout the whole wall thickness during the bending process.
- (7) The influence of the level discontinuity of the main pipe on the quality of the bend. In general, the main pipe for simmering can meet the technical requirements of nondestructive testing, but for the discontinuity of defect level (such as small area base metal layering, weld porosity, slag inclusion, etc.), it will expand in the process of simmering heating and push bending, from defect to defect, so the discontinuity of defect level of the main pipe should be strictly controlled.
Failure of pipe bends
During the operation of oil and gas transmission pipeline, due to the influence of external environment, pipe quality, corrosion, construction quality and other factors, failure accidents of oil and gas transmission pipeline often occur, resulting in fire, explosion, poisoning and other adverse events, even causing major casualties, economic losses and environmental pollution [3-4]. bend is an essential part of the connection part of oil and gas transmission pipeline. Because of its special structure, it often bears more complex stress and harsh working conditions, and its failure risk will increase. The common failure causes of bends for oil and gas transportation mainly include service environment, defects of raw materials and mechanical damage [5-6].
For the bends required by low-strength pipelines, although they are simple to make, with large wall thickness and reliable quality, they can better meet the stress requirements of pipelines and the needs of pipeline engineering construction, but in actual use, when the bends are connected with the straight pipe section, the gap between the butt joints and the amount of staggering are difficult to meet the requirements of design and construction, which requires more difficult on-site treatment, while affecting the welding quality Quantity.
Take the 45 ° Induction bend of 20 steel with specification of DN50 mm × 8 mm 6D sch160 as an example, there is leakage during service (as shown in Figure 1). The base metal of the 45 ° Induction bend is 20 steel seamless steel pipe with a wall thickness allowance of 2 mm. The design pressure of the bend is 25 MPa, and the design temperature is – 20 ~ 100 ℃. The average content of s in the delivered gas is 4000 mg / m3. When the bend is cut open, it can be seen that there is an obvious erosion groove on the outside of the bend body at the front side of the fluid flow direction, and the puncture point is just at the bottom of the erosion groove (as shown in Figure 2). Through testing, the chemical composition and mechanical properties of the failed bend meet the requirements of SY / T 5257-2012 and GB / T 8163-2008, but the grain size is relatively coarse. There are a large number of solid particles and corrosion products formed deposits under the inner wall area near the bayonet. The particle shape in the deposit is irregular, and the maximum particle size is more than 100 μ M. The results of spectral analysis show that the solid particles are rich in O, Si, C, Fe, Al, K, Ca and other elements. The main components of the corrosion products are o, Fe, C and s with a mass ratio of 1.3%. It shows that the sediments are mainly the mixture of sand solid particles and corrosion products. The thickness of the thinnest part on the outside of the bend near the spigot is 0.57 mm, and the wall thickness of the bend is in circular arc shape (as shown in Figure 3). It shows that under the action of corrosion and erosion, the outer wall thickness of the eroded part of the bend is continuously thinning.
Fig.1 Morphology of pierced and leaky bend
Fig.2 Erosion morphology of inner wall of pipe
Fig.3 Wall thickness reduction of outer arc side of pipe bend
Combined with the analysis of bend Induction bending process, due to the fluctuation of power supply voltage or the accidental interruption of cooling system, or the uneven temperature of heating induction coil, the local heating peak temperature at the defect is too high, and the bend appears overburning phenomena such as grain coarseness, grain boundary oxidation and melting, and the outer arc side of the bend bears the maximum axial tensile stress and axial tensile stress during the bending process Under the joint action of local high temperature and overburning, multiple intergranular cracks occur on the outer arc side of the bend, and the crack propagation direction is perpendicular to the axial tensile stress. Due to the rapid advancement of the main pipe, the defect has not penetrated the wall thickness . At the same time, the impact force of the bend on the outer arc side of the pipeline becomes larger, and the main pipe is reheated during the simmering process, which is the thinnest weakness of the whole pipeline. The tensile stress of the outer arc side is the largest , and the wall thickness is the largest thinning, and the pipeline leakage usually occurs here . Under the superposition of the above reasons, the thinning of the bend wall caused by fluid erosion and corrosion is the main reason for the bend leakage.
During the operation of a Φ 273 mm × 9 mm gas transmission pipeline in a project, the bend burst. The bend material is l360nb, the specification is 6D 90 ° Bend PN100 – Φ 273 mm × 9 mm, the design operation pressure of the pipeline is 9.9 MPa, the operation temperature is 20-37 ℃, the flow is 40000 m3 / D, and the actual maximum operation pressure is about 6.5 MPa. Due to the huge energy during the burst, some parts of the pipe body on the outer arc side of the bend are blown up, and the shape of the burst bend is shown in Figure 4. Through the magnetic particle testing of the outer surface of the bend, it is found that there are about 1 ~ 300 mm long transverse cracks on the outer arc side of the bend close to the fracture area, and several transverse microcracks with a length of 6 ~ 12 mm (as shown in Figure 5). A series of tests show that there are obvious differences in the material properties at different positions of the bend, the strength of the central axis is low, the elongation is high, and the properties of the straight pipe are similar. The tensile and yield strength of the inner and outer arc sides are much higher than the central axis, and the elongation is much lower than the central axis, which is lower than the requirements of GB / T 9711-2017 (see Table 1). The impact toughness test results at different positions of the bend correspond to the tensile performance test results. The impact energy at the inner and outer arc sides is far lower than that at the central axis, which is lower than the requirements of SY / T 5257-2012 induction bend for oil and gas transportation (see Table 2). The structure of the inner arc side and outer arc side of the bend is obviously different from that of the central axis. The structure of the inner arc side of the bend is coarse pearlite + ferrite, the structure of the outer arc side of the bend is martensite, and the structure of the central axis and straight section is fine-grained pearlite + ferrite (as shown in Fig. 6-fig. 9).
Figure.4 Appearance of l360nb Φ 273 mm × 9 mm pipe bend after bursting
Figure.5 Surface cracks on the outer arc side of pipe bend
Table.1 Test results of tensile properties at different positions of pipe bends
Table.2 – Charpy impact test results of different positions and directions of 5 ℃ pipe bend
Fig.6 Metallographic structure of inner arc side of pipe bend
Fig.7 Metallographic structure of outer arc side of pipe bend
Fig.8 Metallographic structure of pipe bend axis
Fig.9 Metallographic structure of straight pipe section
Based on the above analysis, there is a brittle and hard martensitic structure layer on the outer arc side of the bend, which is an important factor leading to crack initiation. When the crack propagates to the critical point, the bend will burst.
In addition, through the element detection of the fracture surface, it is found that the Cu element is higher than other positions, but still within the standard range. To some extent, the enrichment of Cu will weaken the grain boundary strength , and the transverse microcrack source may be produced under the external arc tensile stress during the hot simmering process.
During pigging of X60 steel Φ 457 mm × 7.1 mm oil and gas transmission pipeline of a project, the bend burst, and the pig protruded from the outer arc side of the burst bend. The appearance of the burst bend is shown in Figure 10. When the bend is cut open, it can be found that there are many mechanical scratches on the inner wall of the outer arc side of the bend, and the propagation direction of the crack coincides with one scratch on the inner wall (as shown in Figure 11). Through inspection, the physical and chemical performance test results of the burst bend material are in accordance with the requirements of cdp-s-pc-pl-016-2011 / B technical specification for induction bend of oil and gas pipeline engineering  and cdp-s-pc-pl-017-2011 / B technical specification for induction bend of oil and gas pipeline engineering . The metallographic analysis of the mechanical damage shows that there is obvious tissue deformation on the scratch surface, and cracks can be seen at the tissue deformation (as shown in Figure 12).
Figure.10 Appearance of cracked X60 steel grade Φ 457 mm × 7.1 mm bend
Figure.11 Mechanical scratches and cracks on the inner wall of the outer arc side of the burst bend
Fig.12 Structure and crack of mechanical scratch on inner wall of outer arc side of burst bend
It can be seen from the data provided on site that during the pigging process, the 2 × pig with a steel brush is stuck in the pipeline when it passes through the bend. At this time, the 3 × pig is sent to continue the pigging operation, and the gas injection pressure continues to rise. When the 2 × pig passes through the bend, the 3 × pig then passes through the outer arc side of the bend. Based on this analysis, the failure process of the bend is as follows: the thrust of the high pressure gas acting on the surface of the 2 × pig overcomes the friction resistance between the 2 × pig and the bend, which makes the 2 × pig pass through the bend, but leaves multiple mechanical scratches with different depths on the inner wall of the bend, and produces cracks at the bottom of the scratch. When the 2 × pig passes through the bend, the pressure between the 2 × pig and the 3 × pig drops instantly. The 3 × pig, driven by a large amount of high-pressure gas, comes to the bend quickly. The sharp impact force and impact energy make the bottom of the scratch crack on the inner wall of the outer arc side of the bend burst, and the 3 × pig flies out from the crack. The mechanical scratch damage is the important cause of the bend burst. In the later stage of oil and gas transportation, due to the fluctuation of pressure, the axial load fluctuates, and the crack gradually expands until it is cracked through, and the bend does not leak until it is used [12-13].
In view of the above-mentioned medium erosion, quality defects and mechanical damage, the following preventive suggestions and measures are proposed for the prevention of bend failure:
- (1) Purify the conveying medium. Such as setting sand screen or screen to reduce the particle composition in natural gas and prevent sand from entering the natural gas pipeline.
- (2) Strictly control the bending process and operation procedures, strengthen the quality inspection, and avoid defects such as inclusions, cracks and poor organization.
- (3) Pay attention to the reasonable selection of maintenance tools and the stability of pipeline operation pressure in site construction to avoid the generation of impact energy such as artificial mechanical damage or cavitation.
- (4) Strengthen the quality control of the manufacturing process of the main pipe of the Induction bend, especially put forward strict requirements for the composition and grain size in the steel plate purchase or production process, and transfer the quality control to the front end.
Source: Network Arrangement – China Pipe Bend 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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