Z.Liang，Y.Xiao and L.Y.Cui.
Keywords: CNG cylinder, corrosion defect, strength analysis, FEM, equivalent stress-strain.
Abstract:
Internal surface corrosion defects are prone to happen in the in-car CNG (Compressed Natural Gas, referred to as CNG) cylinder, as affected by its internal medium, and the defacts are difficult to be obserbed and checked. The difficulty of preventing and repairing in time lead to a frequency of dangerous accidents. Therefore, it is significant to analyze the strength and safety of in-service CNG cylinders with internal corrosion defects. In this paper, 30CrMo was used as materials of CNG cylinder, and numerical simulation was used to analyze the equivalent stress and displacement in the defect area of the pressurized cylinder. The results are as follows. Under the influence of inner pressure, the cylinder of CNG cylinder expands and deforms outward, and the stress concentration occurs along the axial direction (Y-axis) in defect area, and the maximum stress is reached at the deepest depression. With the increase of internal pressure load,the shape of defect turns into a circle from the initial ellipse, thus the overall strength of the cylinder is weakend. Inner corrosion defects have a serious impact on the strength of in-car CNG cylinders in-service.
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Strength Analysis of the In-car GNG Cylinder with an Inner Corrosion Defect in Service Z. Liang, Y. Xiao, L. Y. Cui. School of Mechatronic Engineering, Southwest Petroleum University, Chengdu, 610500, China, 594741554@qq.com Abstract Internal surface corrosion defects are prone to happen in the in-car CNG (Compressed Natural Gas, referred to as CNG) cylinder, as affected by its internal medium, and the defacts are difficult to be obserbed and checked. The difficulty of preventing and repairing in time lead to a frequency of dangerous accidents. Therefore, it is significant to analyze the strength and safety of in-service CNG cylinders with internal corrosion defects. In this paper, 30CrMo was used as materials of CNG cylinder, and numerical simulation was used to analyze the equivalent stress and displacement in the defect area of the pressurized cylinder. The results are as follows. Under the influence of inner pressure, the cylinder of CNG cylinder expands and deforms outward, and the stress concentration occurs along the axial direction (Y-axis) in defect area, and the maximum stress is reached at the deepest depression. With the increase of internal pressure load,the shape of defect turns into a circle from the initial ellipse, thus the overall strength of the cylinder is weakend. Inner corrosion defects have a serious impact on the strength of in-car CNG cylinders in-service. Keywords: CNG cylinder, corrosion defect, strength analysis, FEM, equivalent stress-strain. Introduction With the development of society and the increasing awareness of environmental protection of the whole nation [1], the emission of vehicles has already been of high demand. Moreover, with the popularity of using gas to replace oil in modern automobile, an increasing number of users began to join the rank. Incar CNG cylinder is a high-pressure vessel, and the medium in it is flammable and explosive gas, which can lead to serious consequences once an accident occurs. To ensure the safety of in-car CNG cylinders, China has published regulations such as Periodic inspection and evaluation of steel gas cylinders for the on board storage of compressed natural gas as a fuel [3], Gas Cylinders Safety and Technical Supervision Regulation for Vehicles [4], and Gas Cylinder Filling Licensing Regulation[5]. However, there are few studies on strength analysis and safety evaluation of in-car CNG cylinders in service. In order to reduce dangerous accidents, personnel and property safety, the CNG cylinder research becomes particularly urgent [6]. In 1992, American National Standards Institute (ANSI) published the ANSI/AGANGV2-1992 which is vehicle compressed natural gas cylinder standard, which classified CNG cylinders into four categories for the first time: (1) Type CNG-1: metal (steel or aluminum) cylinder; (2) Type CNG-2: metal (steel or aluminum) hoop-wrapped composite cylinders with steel liner. (3) Type CNG-3: metal (steel or aluminum) full fiber winding composite layers cylinder; 1 (4) Type CNG-4: plastic liner full winding cylinder [6]. Due to the influence of internal and external media, the inner and outer surface of the cylinder is prone to corrosion defects, which is also the main reason for cylinder scrap and dangerous accidents. Outer corrosion of gas cylinder is generally atmospheric corrosion, and most are uniform corrosion, can be observed by eyes [7]. Inner corrosion is mainly caused by impurities contained in the medium, however, is difficult to find and predict. Therefore, it is very important to analyze the strength of CNG cylinder with inner corrosion defect and inspect the inner surface of the cylinder regularly to ensure safety of the cylinder. Building Analysis Models FEA (Finite Element Analysis) is a method using mathematical approximation to get rid of the constraints of many difficult field operations by simulation instead of the actual situation. In this paper, a numerical model of in-car CNG metal cylinder with inner corrosion defect was established and its mechanical response and strength change were analyzed. Fig. 1 The construction of CNG gas cylinder with an inner corrosion defect Building physical model. Analyse a in-car CNG cylinder in actual working conditions. The structure is shown in Fig. 1 with geometric parameters being marked. As the wall thickness in the middle of the cylinder is relatively thin, the corrosion defect of the inner surface has a serious impact on the safety of the cylinder. In order to improve the accuracy of the analysis results, the whole model with the same size is used to simulate. Considering the rationality of corrosion defect, an in-car CNG cylinder analysis model with ellipsoidal inner corrosion defects is established, as shown in Fig. 2. 2 Fig.2 Simplified analysis model of the in-car CNG cylinder Setting material properties. Considering the nonlinearity of the selected CNG-1 gas cylinder material, the true stress-strain curve of the 30CrMo steel which is CNG cylinder liner is shown in Fig. 3[8]. The specific material parameters are shown in Table 1. Fig. 3 True stress-strain curve of 30CrMo steel Table 1 Material parameters of the In-car gas cylinder Brand name Elastic modulus Poisson's ratio Yield strength Strength limit Extended rate Gas storage pressure 30CrMo 200GPa 0.29 ≥680MPa 800-880MPa ≥13% 20MPa Dividing the mesh. Since the CNG cylinder is a regular structure with good symmetry, a quadric tetrahedral mesh (C3D10) is used to ensure the transmission of the unit node force. Appropriate unit size is selected 3 to ensure accurate results, as shown in Fig. 4. The total number of grids is 148939 and that of nodes is 264016. Fig. 4 Grid pattern of the in-car CNG gas cylinder Constraining boundary conditions. To ensure that the cylinder end surface does not move axially, keep the cylinder fixed, but does not limit the shape change of the cylinder. Applying loads. Most foreign steel cylinder design companies have designed CNG cylinders with a working pressure of 20 MPa. However, because the high-pressure gas in the in-service CNG cylinder is in motion for a long time, the inner pressure will easily exceed 20 MPa, or even reach 26 MPa. To assess the safety of cylinder operation with a greater extent, using more dangerous conditions to analyze the effect of inner pressure on the strength of CNG cylinders. Step1: From 0MPa, linearly increase to 20MPa (design pressure); Step2: Following Step1 , increase from 20MPa to 26MPa (maximum pressure). Analysis results Inner surface of CNG cylinders In the process of increasing inner pressure, the stress distribution nephogram under different load is shown in Fig. 5, and the stress law of CNG cylinder is obvious. The stress of the thin walled cylinder is slightly higher, and the shape of the cylinder also changes. Affected by inner pressure, the cylinder expands outwards. The inner corrosion defect is also seriously affected by the inner pressure. The stress in the defect area is much higher than other parts, and the stress is concentrated along the axial direction (Yaxis), but the stress is lower along the circumferential direction (perpendicular to the Y-axis). The ratio of long axis to short axis of elliptical defect decreases, the eccentricity reduces, and the shape of elliptical defect tuns to circular from its initial ellipse shape. 4 Fig.5 Von Mises stress of the inner surface of CNG cylinders In order to study the stress variation law in the defect area, stress curves corresponding several paths under each load is extracted are shown in Figure. 6-8. In Fig. 6, along the circumferential path1, the stress curves on both sides of the defect are symmetrically distributed, and a high stress region is formed at depression of the inner corrosion defect. The stress extremum of the defect center under each load is about two times the stress stability value far away from the defect. With the increase of load, the stress values of all points increase on the path 1. When the inner pressure exceeds 23.7 MPa, the increase of inner pressure has little effect on the stress peak at defect center. Fig.6 Equivalent stress curve crossing an inner defect in path1 5 In Fig. 7, along the axial direction（path 2） of the cylinder, the stress curves on both sides of the defect are perfectly symmetrical. And the stress slightly reduced at the edge of the inner corrosion defect. However, the closer to the center of the defect, the steeper the stress. The stress extreme still reached at the deepest point of the depression, and stress concentration occurs. As the inner pressure load increases, the stress values increase at each point on path 2. Fig.7 Equivalent stress curve crossing an inner defect in path2 Fig.8 Equivalent stress curve along inner defect edge in path3 6 Connect the defect edge node to establish Path3, and the stress curves under the path as shown in Fig. 8. It is obvious that the stress is symmetrically distributed at the edge of the defect. The stress reaches a maximum at the two axial poles (point A and point C) and reaches a minimum at the two circumpolar poles (points B and D). The stress changes rapidly and uniformly along the edge of the defect, and the entire edge stress is similar to a rounded rectangle. The edge stress of the defect increases with the increase of the inner pressure load, but the change is not obvious after the inner pressure exceeds 23.7 MPa. Outer surface of CNG cylinders Due to the thinning of the wall thickness of the inner defect, the strength is extremely weakened. During operation, it is affected by the inner pressure load, and a certain degree of shape change also occurs in the outer region of the cylinder corresponding to the inner defect, as shown in Fig. 9. Different inner pressure have different degrees of depression in the corresponding areas on the outer surface of the cylinder. The larger the load, the more obvious the depression. Fig.9 Von Mises stress of the outer surface of CNG cylinders In order to study the deformation degree of the position corresponding to the inner defect, the radial displacement nephogram of the outer wall of the cylinder is shown in Fig. 10. Under several loads, the radial displacement of the cylinder part is larger than that of the two ends. With the increase of load, the radial displacement of the cylinder increases, the cylinder expands outward, and the displacement difference between the cylinder and the end increases. The depression of the outer wall region corresponding to the inner corrosion defect is also more obvious. 7 Fig.10 Radial displacement nephogram of the outer surface of the CGN cylinder To quantitatively analyze the depression shape of the defect corresponding to the outer wall, the radial displacement curves of the path 4 under several loads are shown in Fig. 11. The radial displacement on the path 4 is greater than 0 under inner pressure. With the increase of inner pressure, the radial displacement increases, while the cylinder section expands outwards more seriously, which lead to a more serious deformation. The radial displacement fluctuation occurs in the range of 0.015m on both sides with the defect as the center, which is "V" type. Besides， with the inner pressure decreases, the range of influence is narrowed, and the radial displacement away from the defect remains stable. Fig.11 Radial displacement curve crossing inner defect edge in path4 8 Conclusions The strength of in-service CNG cylinders with inner corrosion defect was studied by finite element software, and the influence of internal pressure on the strength was analyzed. The following conclusions are proposed. Under the influence of inner pressure, the cylinder of CNG cylinder expands and deforms outward. The stress of the inner corrosion defect area is much higher than that of other parts, and the tendency of stress concentration occurs along the axial direction (Y-axis), which makes maximum stress reaches at the deepest point of the depression, and the shape changes from ellipse to the circle. The stress reaches to maximum at the two poles of the axial direction, while the minimum stress happens at the two poles of the ring at the edge of the defect. Moreover, the stress is symmetrically distributed, changes rapidly and uniformly along the edge of the defect. The internal pressure load increases, which weakens the overall strength of the cylinder. The radial displacement of the cylinder section is greatly different from that of the end. Different inner pressure loads, the outer wall area corresponding to the inner corrosion defect occurs with different degrees of depression. The larger the load, the more obvious the depression. Inner corrosion defects have a serious impact on the strength of in-car CNG cylinders in-service. Reference [1] ‘Safety Inspection and Quality Management Analysis of CNG Cylinders for Vehicles’, H. Jiang, China New Technologies and New Products,14, pp147-148, 2018. [2] ‘Safety Inspection and Quality Management of In-car CNG Cylinders’, Q. Wu, Internal Combustion Engine & Parts,24, pp75-76, 2017. [3] ‘Periodic inspection and evaluation of steel gas cylinders for the on board storage of compressed natural gas as a fuel’, GB/T 19533-2004. [4] ‘Gas Cylinders Safety and Technical Supervision Regulation for Vehicles’, TSG R0009-2009. [5] ‘Gas Cylinder Filling Licensing Regulation’, TSG R4001-2006. [6] ‘Overview of research on compressed natural gas cylinders for vehicles’, H. X. Zhang, H. S. Jiang, PetroChemical Equipment, 14, pp5-8, 2011. [7] ‘Gas cylinder defect classification and failure analysis’, Y. S. Hou, H. S. Jiang, Heilongjiang Science and Technology Information, 7, pp64, 2015. [8] ‘Application of Finite Element Analysis in Vehicle Wound Composite CNG Cylinder’, Y. J. Guan, Y. W. Zhou, B. Yang, G. F. Chang, Chemical Engineering & Equipment, 2, pp231-234, 2018. 9