Wind power generation has the characteristics of mature technology, high reliability, low cost and remarkable scale benefits. It can be operated on the grid or off-grid. It is the fastest-growing new energy technology in China. The tower is the most important supporting structure of the wind turbine. It is designed to take into account the economics of manufacturing, transportation and hoisting. The tower is usually divided into several sections, and the adjacent sections of the tower are connected by bolts. 10.9 high strength bolts, threaded and formed. High-strength bolts used for tower connection are subject to the influence of alternating load due to the harsh working environment. High local strain and stress are easily generated on the thread, which will always cause the bolt to be in a state of fatigue stress. The literature [1] points out Fatigue fracture of bolts is the most important failure mode of wind turbine tower bolts. The literature [1-2] pointed out that the fatigue life analysis must be carried out in the selection of wind turbine tower bolts. The current method is to use the full-life SN curve method combined with the rain flow counting method and the Palmgren-Miner criterion. The key task of this method is to determine the relationship between the flange load and the bolt stress, and determine a match. Bolt fatigue rating SN curve.
The Wind Turbine Certification Guide, the GL Specification [3], has an assessment of the fatigue life of the tower connection bolts. The Palmgren-Miner criterion can usually be used to determine the relationship between the flange load and the bolt stress, and then pass the Eurocode 3 specification [4] ], determine the relationship between the fatigue life of the high-strength bolt and the stress amplitude, so that it can be judged whether the fatigue life of the selected high-strength bolt meets the requirements of the GL specification. The method does not consider the influence of the gap between the adjacent flanges of the tower when calculating the bolt stress amplitude. However, in many projects in the country, there will still be a certain gap between the adjacent flanges after the tower is completed, and the gap is The effect of bolt fatigue life is difficult to calculate by conventional engineering algorithms. Combining with the actual situation of a wind farm, the author analyzes the influence of the flange clearance on the stress amplitude of the connecting bolts through ANSYS software, and proposes corresponding improvement measures to ensure that the wind turbine will not affect the safety of the tower connecting bolts due to fatigue failure. Stable operation.
1 Factors affecting the fatigue life of bolts
The factors affecting the fatigue life of bolts can be generally divided into three aspects: stress state, manufacturing condition and use environment. Among them, the stress condition can be attributed to the stress concentration and the average stress amplitude at the time of bolt connection; the manufacturing condition depends on the purity and strength of the raw material, the heat treatment process of the bolt, the processing method of the thread and the anti-corrosion measures of the bolt; The impact of bolt fatigue life is mainly corrosive medium and ambient temperature. These factors are related to each other and affect each other. In the analysis of bolt fatigue life, the influence of these factors must be considered comprehensively.
For the 10.9 high-strength bolt used in the tower connection, the raw material is generally selected from 42CrMo, and the thread is formed after the heat treatment. After the completion of the production, the surface of the bolt is treated with Dacromet. The literature [5] proposes that the tower connecting bolts need to bear the alternating load during the operation of the unit, so the stress state of the bolt is the most important factor affecting the fatigue life. In China, the tower flange is machined by CNC Machine tool with high precision, which can ensure that the bolt mounting hole on it meets the design requirements of the drawing, which can effectively control the eccentricity and forced assembly during the bolt installation process. In addition, the tower After the lifting of the cylinder is completed, the bolts are pre-tensioned in the order of diagonal lines, so that the bolt pre-tightening force can be evenly distributed during the installation of the tower bolts, so under normal operating conditions, the stress concentration factor of the tower connecting bolts can be Control is within the scope of the specification. At present, the height of the tower used by the MW-class wind turbine is relatively high, generally about 80 m, the diameter of the tower cylinder section is relatively large, and the diameter of the bottom cylinder section is generally ≥ 4.2 m, and the tower cylinder section and the connecting flange are grouped. For welding, since the effect of the heat-affected zone will have a certain influence on the flatness of the flange, there are many projects where there will be a certain gap between the flange joint faces after the tower is hoisted. The literature [6-7] believes that the root cause of bolt fatigue failure is damage accumulation. In order to ensure the fatigue life of bolts, the stress amplitude of bolts during use should not exceed the allowable stress amplitude of bolt design. In the following, the reverse balance flange used in a project is taken as an example to calculate the influence of the flange clearance on the fatigue life of the bolt by calculating the magnitude of the bolt stress.
2 Reverse Balance Flange Introduction
2.1 Structural features
The flanges currently used for tower connection are mainly in the form of L-shaped flanges and reverse balanced flanges. The traditional L-shaped flanges are forged flanges, which are formed by the ingot ring process and then processed by CNC machine tools. The required dimensions of the drawings; and the reverse balance flange is generally welded by the barrel section, the connecting plate and the ribs. The principle of the reverse balance flange is shown in Figure 1.
Compared with the traditional L-type flange, the reverse balance flange has low cost, short machining cycle, strong flange bearing capacity and reasonable force, which can effectively improve the stress condition and welding of flange and tower welds. Deformation. However, compared with the L-type flange, the contact surface of the reverse balance flange is relatively small, and it is difficult to eliminate the gap between adjacent flanges. Therefore, the machining accuracy and the welding precision requirements of the flange and the barrel pair are compared. high.
2.2 Introduction to parameters
The reverse balance flange used in a project has experienced the phenomenon of individual bolt breakage after running for about two years. The bolts used in the wind place are tested. The results show that there is no problem with the bolt quality and meet the procurement specifications. . In order to find out the cause of the bolt breakage as soon as possible, the relevant technicians went to the site to conduct a field investigation and found that there was a gap between the flange joint faces where the joint bolts broke, and the maximum gap exceeded 1 mm, as shown in Fig. 2.
In the analysis and calculation, the flange between the first tower and the second tower is selected. The outer diameter of the cylinder is D0=4 109mm, the wall thickness of the joint is t=26 mm, and the number of bolts is n. =148, the M36 bolt of class 10.9 is used, the length is 480 mm, the pre-tightening force of the bolt is 510 kN, the maximum bending moment of the flange is 7 186 kN·m, and the load distribution factor of the bolt is p=0.081 7. Tower cylinder and flange material Q345E, bolt pre-tightening factor is 1.2.
3 Calculation of bolt stress amplitude without considering flange clearance
3.1 Calculation of the allowable stress amplitude of bolts
Using the Palmgren-Miner criterion, the stress amplitude of the bolt is calculated by the VDI2230 [8] and Eurocode3 specifications, regardless of the flange clearance. According to the VDI2230 specification, the number of nominal cycles NA = 2 × 106, the nominal fatigue limit stress range ΔσA of the bolt can be obtained by the formula (1), and the bolt fatigue limit stress range recommended by the Eurocode 3 specification and the GL specification is 71. (1) The calculated fatigue limit stress range is more conservative and closer to engineering practice. In addition, when performing bolt fatigue life analysis, when the bolt specification exceeds the M30 and GL specifications, the influence of the bolt reduction factor Ks should be considered when constructing the SN curve, Ks=(30/d) 0.25, d is the bolt. The nominal diameter, so the fatigue limit stress range ΔσA of the bolt can be obtained by equation (2) at 2 × 106 cycles, and the relationship between the bolt stress range ΔσR and the number of stress cycles NR is as shown in equations (3), (4). ) shown.
The SN curve of the bolt is shown in Figure 3.
The limit stress cycle number of the tower connecting bolt is designed according to the 10th power of 10, considering the material safety factor Ym is taken as 1.1, the broken factor Yb is taken as 1.15, and for the M36 bolt, according to the formula (2)~(4) The allowable stress amplitude of the bolt at 107 stress cycles is 17.2 MPa.
3.2 Calculation of the actual stress amplitude of bolts
The VDI2230 will be used to find the stress amplitude of the bolt under the alternating load. The GB/T16823-1 can be used to find the thread stress cross-sectional area of ​​the M36 bolt AS=817 mm2. The flange wall can be obtained by using the formula (5). The bending section modulus W0 can be obtained by the formula (6), and the additional stress Fa of the bolt can be obtained by the formula (7). The stress amplitude Δσ on the bolt can be obtained by the formula (8). 4.7 MPa, equations (5) to (8) are as follows:
Through the above calculations, it can be concluded that the stress amplitude acting on the bolt is smaller than the allowable stress amplitude of the design, so when there is no gap between the flange joint faces, the M36 bolt selected in this project can satisfy the fatigue life of the wind turbine generator set. Claim.
4 Calculation of bolt stress amplitude considering flange clearance
4.1 Model assumptions
Assume that there is a maximum gap of 1 mm on the wall of the flange cylinder and a length of 500 mm. The gap between the upper and lower flanges is symmetrical, as shown in Figure 4.
Since the reverse balance flange between the first tower and the second tower is selected, the established finite element model includes the flange connecting bolt, the upper and lower flanges, and the first tower and the second section. The tube section of the tower is shown in Figure 5. At the center of the flange connection of the second tower and the third tower, a Mass21 mass unit is used to establish a corresponding mass point. The mass point is connected to the top of the second tower through the connecting unit RBE3, and the load is applied to the mass point. , passed to the tower through the RBE3 unit. The upper and lower flanges are connected to the tower by binding, and the bottom of the first tower is fixed.
In the finite element model of the reverse balance flange, the gap at the maximum clearance of the flange is 1 mm, and after the bolt preload is applied, the maximum clearance of the flange is about 0.3 mm, as shown in Fig. 6.
4.2 Description of finite element analysis results
Under the preload of the bolt, the maximum tensile stress on the outer surface of the cylinder wall at the flange gap is about 169 MPa, as shown in Figure 7, the joint between the flange face and the cylinder wall and the connection between the reverse balance flange and the tower. There is a large concentration of stress.
Under the action of external load, when there is no gap between the flange joint faces, as shown in Figure 8, the minimum stress at the bolt compression is 413.1 MPa, the maximum stress at the bolt tension is 422 MPa, and the bolt stress range is 8.9 MPa. The stress amplitude is 4.45 MPa. This result is basically consistent with the results calculated by VDI2230 in Article 3.2 of this paper.
When there is a gap between the flange joint faces, under the external load, the maximum stress of the bolt is generated at the maximum flange gap, and the stress is 435.4 MPa. The stress cloud diagram is shown in Fig. 9. When the external load causes the bolt at the flange gap to be pressed, the minimum stress of the bolt does not occur at the maximum gap. The stress cloud diagram is shown in Figure 10. The bolt stress at the maximum gap is 399.6 MPa. According to the above finite element calculation, under the external load, the maximum stress of the bolt at the maximum flange gap is 435.4 MPa, the minimum stress
It is 399.6 MPa and the stress amplitude is 17.9 MPa, which is about 4 times the bolt stress amplitude when there is no gap between the flange joint faces. According to the previous calculations, the allowable stress amplitude of this project is 17.2 MPa. When there is a gap between the flanges, the stress amplitude of the bolt at the maximum flange clearance has exceeded the stress amplitude allowed by the bolt design of this project. According to the Eurocode 3 specification, It is estimated that when there is a gap of 1 mm between the flange joint faces, the number of stress cycles of the bolt is reduced by about 18%, which causes a sharp drop in the fatigue life of the bolt.
5 Conclusion
From the above analysis, it is known that when there is no gap between the flange joint faces, the relationship between the flange load and the bolt stress is determined by the Palmgren-Miner criterion, and the fatigue life and stress amplitude of the high-strength bolt are determined by the Eurocode3 specification. The relationship is relatively accurate and is basically consistent with the finite element calculation results. However, when there is a gap between the flange joint faces, the engineering algorithm cannot calculate the stress amplitude of the bolt at the flange gap, and the calculation must be performed by the finite element.
In addition, if there is a gap between the flange joint faces, not only will the stress amplitude of the bolts in this area increase sharply, but also affect the fatigue life of the bolts; and the rainwater will also enter the gap between the joint faces of the tower flanges. Inside the tower, it is easy to cause the bolt to rust, which will also have a very adverse effect on the fatigue life of the bolt. Therefore, the gap between the flange joint faces must be eliminated during the tower lifting process. For L-shaped flanges, the gap can be filled by adding gaskets before the bolts are pre-tensioned. The compressive strength of the gaskets must not be lower than the compressive strength of the flange material, and the elastic modulus should be maintained with the flanges. Consistent. For the reverse balance flange, because the contact area is relatively small, it is not convenient to install the gasket. In the manufacturing process, the processing quality must be strictly controlled. If there is still a gap after the tower is hoisted, the bolt can be pre-tightened. Appropriate sanding of the rib joint faces at the gaps to ensure that the gap between the flange joint faces can be eliminated after the bolts are preloaded.
references
[1] Zhu Lingjing, Ling Ting, Liu Haibo, et al. Analysis of high strength bolt fracture of wind turbine tower[J]. East China Science and Technology (Academic Edition), 2015(2): 250-251.
[2] Du Jing, Ding Shuai, Wang Xiuwen, et al. Fatigue Evaluation of High Strength Bolts in MW Stage Wind Turbine Tower Ring Flange Connection[J]. Mechanical Design, 2014, 31(1): 75- 79.
[3] Guideline for the Certification of Wind Turbine [S]. Germanischer Lloyd Wind Energize GMbH, Germany, 2010.
[4] Eurocode 3, Design of Steel Structures, Part 1-9: Fatigue Strength of Steel Structure [S]. PREN, 1993-1-9, 2005.
[5] He Yulin, Wu Dejun, Hou Haibo, et al. Fatigue Life Analysis of High Strength Bolts of 42CrMo Fan Tower Flanges[J].Hot Working Technology,2012,41(4):1-4.
[6] Wang Ziqin. Analysis of Stress and Strain and Fatigue Life of Bolts[J]. Aviation Manufacturing Technology, 2001(4): 44-46.
[7] Zhang Long, Zhao Huijie. Design of high strength bolt strength under combined load[J]. Heavy Industry & Lifting Technology, 2013(1): 3-4.
[8] VDI2230 Part1, Systematic Calculation of High Duty Bolted Joints Joints With one Cylindrical Bolt [S]. Verein Deutsche Ingenieure, Germany, 2003.
handrail accessories & balustrade
Stainless Steel Railing Project,Acrylic Stair Railing,Acrylic Balcony Railing,Acrylic Interior Stair Railings
Foshan City, Nanhai District Huidexing Stainless Steel Products LTD., , https://www.huidexing.net