Manufacturing problems and technical analysis of pressure vessel head

The manufacturing of pressure vessel heads, influenced by process operation and material characteristics, will produce certain quality problems, such as bulging, cracking and other defects, which seriously reduce the performance of the equipment. Based on this, the article analyzes the problems in the manufacturing of pressure vessel head and proposes relevant technical solutions to the problems.

20220529154629 39508 - Manufacturing problems and technical analysis of pressure vessel head

Pressure vessel is a closed equipment to store a certain amount of liquid or gas through pressurized processing to meet the needs of industrial production, daily life and so on. The selection of pressure vessel materials must have stability and meet the work requirements of the special operating environment to ensure that the equipment in the process of use will not produce the problem of breakage. From the point of view of the overall structure of the pressure vessel, the most vulnerable position is the head, because the pressure vessel body belongs to one compression process, the head belongs to the tank sealing parts, which presents certain differences between the parameters and the tank, such as the pressure-bearing parts at the head need to go through multiple thermoplastic processes to ensure that the parts meet the needs of equipment use. However, in the manufacturing process of head parts, influenced by factors such as process and operation behavior, the part body faces serious quality problems, which indirectly increases the failure rate of pressure vessels. In order to reduce the generation of processing defect problems, it is necessary to analyze the possible problems in the manufacturing process of head components and then, develop relevant prevention initiatives to improve the quality performance of head components.

1. Problems in manufacturing of head of pressure vessel

1.1 Head bulging problem

The bulging problem in the manufacturing process of head components generally occurs in the hot forming process of components. If the tangential force is greater than the internal stress of the parts, the instability effect of the parts will be generated and the raw materials will be bulged. In addition, when the raw materials of head parts are heated in the furnace, if uneven heating occurs (the upper temperature of material is lower than the lower temperature), the parameters of compressive value of each area of metal material will form an error, at this time, the material cannot form a balanced volume increase because of heating, which leads to the increase of transition area at the smooth corner of bend in the later compression molding process, and when the metal tissue shows a relatively soft phenomenon, the stress in the hard value area is higher than the stress in the hard value area. When the metallic structure is relatively soft, the stress generated in the hard value area is higher than that in the soft value area, which results in bulging phenomenon. Generally speaking, the bulging area of head parts is generally in the transition zone of bending corners.

1.2 Head cracking problem

Cracking of metal materials is mainly caused by the internal stress not being able to meet the demand of external environmental pressure, which leads to the phenomenon of cracking of the material body. In the manufacturing process of pressure vessel head components, the main links for crack generation are in stamping process and spinning process. From the viewpoint of internal stress generation forms, the stresses generated in the head components are mainly divided into two kinds.

  • (1) Distortion of the lattice inside the metal material, which hardens the original metal soft tissue and generates excess stress.
  • (2) The tensile deformation of inner and outer surface of metal material has large parameter difference, which leads to high variability of pressure value that the part can bear. Generally speaking, the cracking area of head parts is generally at the welding seam and port. When the stress of material cannot take up the external force, such as the gap force generated by stamping of parts, if the stress is not concentrated or the cross-section surface is not smooth, it will cause the local stress to be smaller than the external stress, which will cause the cracking of parts.

1.3 Head thinning problem

In the process of pressure vessel head shaping, the part will be subject to tensile force and friction force because it is a compression process, at this time, there will be certain differences between the mechanical parameters on the top and bottom of the head part, and the forces on the bottom are relatively small, which causes the wall thickness thinning value on the bottom to be relatively small. As for the straight edge area of the head part, the thickness of the part will be increased because the force in the upper area has comprehensive characteristics and the force in the edge area is greater than the extension tension in the corner transition area. In the process of compression, the corner transition area is subjected to the external force brought by the mold impact and the ductile force formed by the internal stretching of the material, and under the joint action of these two stresses, the degree of wall thinning will increase. The causes of the head thinning problem are mainly divided into the following three points.

  • (1) The failure to control the strength of the drum pressing process in the specific implementation process, resulting in the unevenness of the wall in the thinning process, but under the setting of the system parameters, the thinning process will take the thinnest part of the wall as the benchmark, which will lead to the wall thinning over the standard.
  • (2) Mold, material surface is not smooth, then the friction coefficient generated during the compression molding of the part will increase, reducing the stretching coefficient of the raw material, if the material stretching effect is not obvious, need to increase the compression force, which causes the parameters of the part after molding can not meet the expected setting requirements.
  • (3) The edge pressure is too large, and the tensile degree that the material can bear in the fixed space will show certain limitations, and if the tangential degree of freedom of the material is less than the maximum value that the internal stress can bear, it will cause the problem of thinning of the material beyond the limit.

2. Technical solution of pressure vessel head manufacturing problem

The manufacturing process of pressure vessel head is affected by the environment of action forces, which will generate different types of defect problems, such as part pulling, scratching, cratering, bulging, over-burning, etc. For this reason, it is necessary to find a reasonable solution to the problem, analyze the main causes of different defects and develop a more accurate solution in combination with the performance of head components.

2.1 Technical solution to the bulging problem

To avoid the problem of residual stress inside the head material, secondary stretching treatment can be carried out to improve the toughness value of the material by using the force of primary and secondary stretching to offset the residual stress. From the viewpoint of mechanical properties of metal materials, within a certain parameter range, the stress of the material body is linearly related to the creep force, which makes the material body produce two kinds of yield strengths, upper and lower. When secondary stretching is applied to the material, the residual stresses generated by primary stretching will be optimized by the secondary process, and the residual stresses will be offset by external forces, which can effectively improve the quality performance of the material. In addition, in order to prevent the change of material pressure value caused by uneven temperature heating, the material can be turned over and heated in the furnace, or the heating equipment can be used to balance the upper and lower temperatures, so as to effectively avoid the phenomenon of hardening of metal tissues and reduce the problem of bulging of head parts.

2.2 Technical solution of cracking problem

If the stress value generated at the crack of the head part is difficult to bear the pressure value inside the tank equipment during the use of the pressure vessel, the head part will be destroyed along the crack and the tank explosion will cause serious harm to the surrounding environment. Considering the causes of cracks, multi-category operational initiatives need to be adopted to exclude the cracking problem. For the cracks generated at the straight edge of the head part, the material processing form should be controlled through the control of the spinning speed or pressure, so that the material processing parameters can be effectively restrained within the controllable range and the stress value generated in the head part can be guaranteed to meet the overall compression molding requirements. In the control of cutting process, the material edge and mouth should be polished, especially in the transition area of bending corners, and the parameters of the material processing surface should be measured to ensure that the parameters of the material processing surface meet the requirements to be processed to avoid the phenomenon of port cracks. To avoid the impact of external forces and internal stresses on the welded joints, strict welding process coefficients must be set for the bearing pressure of the parts, and in the actual welding, uniform speed must be ensured and the cooling speed slowed down to improve the stability of the welded positions. Generally speaking, for the fine welding treatment of the head parts, it is necessary to adopt the layer-by-layer welding process with low current to disperse the concentrated stress in the center of the weld and avoid the crack differentiation effect in the center. At the same time, the welding process should be prevented from biting, the welding slag should be cleaned up in time, and the second heat treatment should be carried out in the weld area.

2.3 Technical solution of head thinning problem

If the wall thickness is thinned beyond the standard, the internal stress value will be less than the base value set by the system in the process of using, and it will not be able to make the vessel carry more pressure under the fixed volume. For this reason, when thinning the whole part, it is necessary to take into account whether the thickness of the material body meets the requirements for processing, i.e. whether the material has enough thickness for subsequent processing, such as the width of the crimp, during the processing. In addition, each process associated with the processing of head parts must be measured, such as the crimping process, cracking control process, etc. The parameters available for process execution must meet the needs of head equipment manufacturing. It is also necessary to analyze the professionalism of the operators to ensure that the job staff has the basic skills and professionalism of process implementation, etc., and that they can treat their jobs correctly to ensure that the process operations undertaken will not cause industrial production errors due to human problems.

3. Conclusion

To sum up, the manufacturing of pressure vessel head components requires high mechanical characteristics. In order to avoid the defective problems brought about by process operations, the essential causes of the defective problems must be analyzed from multiple angles, and then a precise solution strategy must be formulated by adopting appropriate process behaviors to improve the application performance of the equipment. It is expected that in the future development process, the technical department as well as the management department will strictly control the application of raw materials and develop a more refined manufacturing plan to solve the defect problem of head equipment from both internal and external aspects.

Author: Lin Zhongla

Source: China Head Manufacturer – Yaang Pipe Industry (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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