A butterfly closure for a c-type liquid tank

Abstract

This invention discloses a butterfly-shaped head for a type C liquid tank, belonging to the field of pressure vessel head technology. It includes an integrally formed spherical section, a transition arc section, and a straight edge section, which are smoothly connected sequentially. The transition arc section is a variable curvature arc structure, with its radius of curvature gradually increasing uniformly from the end closer to the spherical section to the end closer to the straight edge section. A circular transition buffer zone is provided at the junction of the spherical section and the transition arc section, and a stress-relieving chamfer is provided at the junction of the straight edge section and the transition arc section. This invention significantly optimizes stress and greatly improves safety. By employing a composite structure of a variable curvature gradient surface combined with a circular transition buffer zone, this invention completely solves the stress concentration defects caused by abrupt curvature changes in traditional heads, reducing the overall stress concentration factor of the head from above 3.0 to below 1.5, eliminating local stress peaks, and effectively preventing the initiation and propagation of fatigue cracks under alternating loads.

Description

Technical Field

[0001] This invention relates to a butterfly head, and more particularly to a butterfly head for a type C liquid tank, belonging to the field of pressure vessel head technology. Background Technology

[0002] As a core pressure-bearing and sealing component at the end of a pressure vessel, the butterfly head combines the advantages of the good pressure-bearing stability of an ellipsoidal head with the ease of processing of a flat head. With its simple forming process, low manufacturing cost, adaptability to large-diameter tank assembly, and mature welding technology, it is widely used in various atmospheric and medium-low pressure vessels, especially in the field of C-type liquid tanks. Currently, conventional butterfly heads in the industry are designed and manufactured strictly in accordance with the national standard GB / T 25198-2010 "Pressure Vessel Heads". The standard structure consists of three parts: a spherical section, a transition arc section with equal curvature, and a straight edge section. The industry-standard parameters are: the radius of the spherical section Ri = 0.9Di (Di is the inner diameter of the head), the radius of curvature of the transition arc section is fixed at r = 0.17Di, and the straight edge section is a standard flat structure, mainly used for butt welding and positioning sealing between the head and the tank body. It is a key structure to ensure the overall sealing performance and pressure-bearing stability of the pressure vessel.

[0003] Type C liquid tanks, as specialized pressure-bearing equipment for storing and transporting media such as LPG and light chemical oils, operate under more complex and harsher conditions than ordinary stationary chemical storage tanks. They are subjected to a complex environment of alternating pressure shocks, vehicle vibrations, continuous media corrosion, and fluctuating outdoor temperature and humidity. These stringent operating conditions amplify the inherent structural defects of traditional standard butterfly heads, leading to frequent operational hazards and a significantly shortened service life, failing to meet the demands of high-end, high-safety, and long-life industrial applications. Extensive engineering practice and finite element simulation tests have verified that existing traditional butterfly heads have four core technical defects in Type C liquid tank applications, as follows:

[0004] First, the fixed curvature structure leads to severe stress concentration. Traditional standard butterfly heads use fixed curvature parameters for both the spherical and transition arc sections, resulting in abrupt curvature changes at the junction of the two curved surfaces without a smooth transition buffer structure. Under continuous pressure from the internal medium, large areas of high stress concentration form in these curvature abrupt areas, with the overall stress concentration coefficient of the head generally exceeding 3.0, far surpassing the safety design threshold for pressure vessels. When the C-type liquid tank is subjected to alternating internal pressure and transport vibration loads for extended periods, micro-fatigue cracks will first develop in the high stress concentration areas. As the equipment continues to operate, these cracks will continue to expand and extend, eventually causing localized deformation, leakage, or even overall cracking and failure of the head, significantly shortening the equipment's service life and posing a major safety hazard.

[0005] Secondly, the uniform wall thickness structure exhibits poor stress matching and insufficient local pressure-bearing redundancy. During pressure operation, the stress distribution in each structural section of the end cap varies significantly. The transition arc section, serving as the connecting area between the spherical and straight-edge sections, is the core part of the end cap with the highest stress load and the most complex stress conditions. The internal pressure load, shear load, and alternating impact load it bears are far higher than those of the spherical and straight-edge sections. However, traditional butterfly-shaped end caps adopt a uniform wall thickness design throughout, failing to differentiate the structural design based on the actual stress field distribution of the end cap. This results in a severe lack of pressure-bearing redundancy in the high-stress transition arc section, while the low-stress spherical and straight-edge sections suffer from wasted wall thickness redundancy. During long-term operation, the transition arc section is prone to problems such as local wall thinning, elastic deformation, and plastic deformation, leading to poor overall pressure-bearing stability and fatigue resistance, and a persistently high failure rate.

[0006] Third, the superposition of welding stress and internal pressure stress amplifies the risk of failure. The connection area between the straight edge section and the transition arc section is the core joint area for the butt welding of the head and the tank body, and it is also the weakest point in terms of structural mechanical properties. During the welding and assembly process of the head, the high temperature of welding will generate irreversible residual stress. This residual stress will form a multiple superposition effect with the internal pressure stress of the medium and the vibration and impact stress during equipment operation, further amplifying the local stress peak. This can easily cause defects such as microcracks and weld bead cracking at the weld joint, which in turn can lead to the leakage of flammable and explosive LPG media. At best, this will cause equipment shutdown and media loss; at worst, it will cause major safety accidents such as combustion and explosion, seriously threatening production and transportation safety.

[0007] Fourth, there is a lack of dedicated protective structures, resulting in weak adaptability to complex operating conditions. Traditional standard butterfly heads are formed using only a single steel base material and do not have a dedicated protective structure for the special service conditions of type C liquid tanks. The inner wall of the head is in long-term contact with LPG oil and gas media, making it prone to chemical corrosion and erosion wear; the outer wall of the head is exposed to the outdoor environment for a long time, and is significantly affected by wind and rain erosion, alternating high and low temperatures, oxidation aging, and fatigue damage from transportation bumps. The overall corrosion resistance, aging resistance, fatigue resistance, and impact resistance of the head are limited, making it unable to meet the stable operation requirements of type C liquid tanks throughout their entire life cycle. This leads to high maintenance frequency, high repair costs, and poor overall economic efficiency.

[0008] Currently, conventional optimization and improvement methods for addressing the aforementioned defects of traditional butterfly-shaped heads are limited to adjusting a single parameter, mainly including increasing the fixed radius of curvature of the transition arc and thickening the head wall overall. However, these optimization methods have fundamental technical drawbacks and cannot solve the head failure problem at its root. First, thickening the head wall overall will significantly increase the consumption of steel raw materials, significantly increasing the equipment manufacturing cost. At the same time, the thickened head structure will occupy the effective storage volume inside the tank, reducing the medium storage capacity of the C-type liquid tank and sacrificing the performance of the equipment. Second, simply fine-tuning the fixed curvature parameter cannot eliminate the problem of abrupt curvature changes between the spherical section and the transition section. The core defect of stress concentration still exists, and it can only slightly reduce the stress peak, with extremely limited optimization effect. Third, the lack of differentiated structural design makes it impossible to specifically strengthen high-stress areas, and the problems of uneven stress distribution and insufficient local pressure bearing capacity of the head are not fundamentally solved. In summary, existing traditional butterfly-shaped heads and conventional optimization technologies cannot simultaneously meet the multiple requirements of Type C liquid tanks for high safety, high durability, high volumetric utilization, low cost, and low maintenance. Therefore, developing a specialized butterfly-shaped head for Type C liquid tanks with optimized structure, uniform stress distribution, strong pressure bearing capacity, fatigue and corrosion resistance, and suitability for industrial mass production is a pressing technical challenge in this field. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing technologies, such as stress concentration due to abrupt curvature changes in standard butterfly-shaped heads, mismatched stress in structures with equal wall thickness, easy failure due to superimposed welding stress, poor adaptability to complex working conditions, and low cost-effectiveness of conventional optimization schemes. This invention provides a butterfly-shaped head for C-type liquid tanks. The invention corresponds to multi-dimensional structural optimization design. Through the synergistic design of limiting optimization parameters for the spherical section, gradient parameters for the variable curvature transition section, differentiated variable wall thickness structure, integrated circular arc buffer structure, stress-relieving chamfer at the welding end, and double-layer dedicated protective structure, it achieves uniform stress distribution, enhanced pressure resistance in key areas, stress relief during welding, and comprehensive improvement in fatigue and corrosion resistance. Without reducing the effective volume of the tank or significantly increasing production costs, it greatly improves the operational safety and service life of C-type liquid tanks.

[0010] The objective of this invention can be achieved by adopting the following technical solution:

[0011] A C-type liquid tank butterfly head includes an integrally formed spherical segment, a transition arc segment, and a straight edge segment, which are smoothly connected sequentially. The transition arc segment adopts a variable curvature arc structure, and the radius of curvature of the transition arc segment gradually increases uniformly from the end closer to the spherical segment to the end closer to the straight edge segment. A circular arc transition buffer is provided at the junction of the spherical segment and the transition arc segment, and a stress-relieving chamfer is provided at the junction of the straight edge segment and the transition arc segment. This invention eliminates the curvature abruptness defect of traditional heads by using a continuous curved surface with variable curvature, and achieves multi-dimensional stress relief in conjunction with the circular arc transition buffer and the stress-relieving chamfer, fundamentally improving the stress concentration problem of the head and adapting to the complex service conditions of C-type liquid tanks under alternating pressure and media corrosion.

[0012] As a preferred structure of the present invention, the ratio of the spherical radius Ri to the inner diameter Di of the head in the spherical segment is 0.92~0.95. Compared with the traditional spherical parameter of 0.9Di in the national standard, this optimized parameter can make the stress distribution of the spherical segment more uniform, improve the overall pressure-bearing stability of the spherical segment, avoid local stress accumulation in the spherical segment, and adapt to the long-term pressure-bearing operation requirements of medium and low pressure C-type liquid tanks. The transition arc segment is set as a gradually changing curvature structure, wherein the ratio of the minimum curvature radius rmin of the transition arc segment to the inner diameter Di of the head is 0.20~0.22, and the ratio of the maximum curvature radius rmax of the transition arc segment to the inner diameter Di of the head is 0.25~0.28. Moreover, the transition arc segment maintains a uniform linear curvature change throughout, without abrupt inflection points or curvature drops, completely solving the technical defect of excessively high stress concentration coefficient in traditional fixed curvature transition segments.

[0013] As a preferred structure of the present invention, the end cap adopts a differentiated variable wall thickness structure design. The wall thickness of the transition arc segment is 15% to 20% thicker than that of the spherical segment and the straight edge segment, and the wall thickness of the spherical segment and the straight edge segment are consistent. This structure matches the actual stress field distribution law of the end cap, and locally strengthens the transition arc segment where the stress is most concentrated and the load-bearing capacity is the largest, while maintaining the conventional wall thickness in low-stress areas. Without increasing the overall material consumption or reducing the effective volume of the tank, the pressure-bearing capacity of key parts is significantly improved. Among them, the wall thickness of the spherical segment and the straight edge segment are strictly selected according to the national standard for the design pressure of C-type liquid tanks to ensure the pressure-bearing safety of the equipment foundation. At the same time, the variable wall thickness area of ​​the end cap is prepared by an integral spinning molding process, resulting in a continuous and smooth wall thickness transition without abrupt steps or stress dead corners, high molding precision, and good structural integrity.

[0014] As a preferred structure of the present invention, the radius of the arc transition buffer is set to 0.3 to 0.4 times the minimum radius of curvature of the transition arc segment, and the arc transition buffer, the spherical segment, and the transition arc segment are integrally spun using a seamless process. By adding a dedicated arc transition buffer, a multi-level smooth transition structure is formed between the spherical segment and the variable curvature transition arc segment, effectively eliminating the curvature difference between the two curved surfaces, completely eliminating the local stress peak at the surface connection, further reducing the overall stress concentration coefficient, and solving the problem of fatigue cracks easily initiating at the connection position of traditional end caps.

[0015] As a preferred structure of the present invention, the stress-relieving chamfer is a rounded chamfer with a chamfer radius set to 0.5 to 0.6 times the wall thickness of the straight edge section, and the chamfer surface smoothly transitions to the outer surface of the straight edge section and the outer surface of the transition arc section. The junction of the straight edge section and the transition arc section is the core area for welding the head and the cylinder, where residual stress is easily generated during the welding process. The present invention, by setting a special rounded chamfer to relieve stress, can effectively disperse and dissipate welding residual stress, avoid the superposition and accumulation of welding residual stress and internal pressure stress during equipment operation, and eliminate safety hazards such as weld joint cracking and media leakage.

[0016] As a preferred structure of the present invention, the inner wall of the end cap is provided with a corrosion-resistant reinforcing layer, and the outer wall is provided with an anti-fatigue protective layer, forming an internal and external composite protection system. The corrosion-resistant reinforcing layer on the inner wall is a nickel-based alloy coating with a thickness controlled between 0.8 and 1.2 mm. This nickel-based alloy coating possesses excellent resistance to acid and alkali corrosion and oil and gas corrosion, and can resist the micro-corrosion and erosion damage caused by LPG media to the inner wall of the end cap for a long period. The anti-fatigue protective layer on the outer wall is an epoxy zinc-rich coating with a thickness controlled between 1.0 and 1.5 mm. This coating has strong adhesion, excellent anti-aging properties, and excellent resistance to alternating loads, and can effectively resist outdoor wind and rain erosion, oxidation aging, and fatigue damage caused by transportation bumps, significantly improving the end cap's adaptability to complex working conditions and its service life.

[0017] Beneficial technical effects of the present invention:

[0018] 1. Significant stress optimization effect and greatly improved safety level. This invention adopts a composite structure of a variable curvature gradient surface combined with a circular arc transition buffer zone, which completely solves the stress concentration defect caused by the abrupt curvature change of the traditional head, reducing the overall stress concentration coefficient of the head from more than 3.0 to less than 1.5, eliminating local stress peaks, effectively avoiding the initiation and propagation of fatigue cracks under alternating loads, preventing safety accidents such as media leakage and head cracking, and significantly improving the operational stability of C-type liquid tanks.

[0019] 2. Strong stress matching and excellent pressure-bearing durability. This invention adopts a variable wall thickness differentiated design based on the actual stress field distribution of the end cap, specifically strengthening the area with the highest stress bearing capacity. This improves the pressure-bearing performance of the transition arc section by more than 20%, effectively solving the problems of local deformation, wall thinning, and plastic failure in traditional equal wall thickness structures. The overall resistance of the equipment to internal pressure, impact, and alternating loads is greatly improved, and the service life is increased by more than 35% compared to traditional end caps.

[0020] 3. Balancing economy and practicality, suitable for industrial production. Compared to traditional overall thickening and large curvature radius optimization solutions, this invention only locally thickens and optimizes the structure, without requiring a large amount of additional raw materials, effectively controlling production costs; the overall structure and spinning and welding processes are fully compatible with existing production lines, without the need to modify production equipment, with high forming accuracy and strong assembly versatility, suitable for large-scale mass production.

[0021] 4. Wide range of operating conditions adaptability and low maintenance cost. The inner and outer double protective layers form a composite protection system. The inner wall resists LPG medium corrosion, while the outer wall resists outdoor oxidation, wind and rain erosion and fatigue damage, effectively reducing the probability of end cap aging and corrosion failure. This significantly reduces the frequency and cost of later equipment inspection and maintenance, and can be stably adapted to various medium and low pressure LPG storage and transportation complex operating conditions for a long time. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall structure of the C-type liquid tank butterfly head of the present invention;

[0023] Figure 2 for Figure 1 A magnified schematic diagram of part A in the middle.

[0024] In the diagram: 1-spherical segment, 2-transition arc segment, 3-straight edge segment, 4-circular arc transition buffer zone, 5-stress relief chamfer. Detailed Implementation

[0025] To enable those skilled in the art to understand the technical solution of the present invention more clearly, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0026] like Figures 1-2 As shown, the butterfly-shaped end cap for a C-type liquid tank provided in this embodiment is integrally spun from steel and includes a spherical segment 1, a transition arc segment 2, and a straight edge segment 3 that are smoothly connected in sequence. The transition arc segment 2 is a uniformly variable curvature arc structure, with the radius of curvature gradually increasing from the connecting end of the spherical segment 1 to the connecting end of the straight edge segment 3. An integrally formed arc transition buffer zone 4 is provided at the connection between the spherical segment 1 and the transition arc segment 2, and a stress-relieving chamfer 5 with a rounded corner structure is provided at the connection between the straight edge segment 3 and the transition arc segment 2.

[0027] In this embodiment, the inner diameter Di of the end cap is set to 2000mm, the radius Ri of the spherical segment 1 is 1860mm, and Ri / Di=0.93, which is within the preferred value range; the minimum radius of curvature of the transition arc segment 2 is rmin=420mm, rmin / Di=0.21, the maximum radius of curvature is rmax=520mm, and rmax / Di=0.26, and the curvature changes uniformly and gradually throughout the entire process without any abrupt inflection points.

[0028] Wall thickness parameter configuration: The spherical section 1 and the straight edge section 3 have a uniform wall thickness of 16mm, which meets the design standard of 1.6MPa medium and low pressure C-type liquid tank; the wall thickness of the transition arc section 2 is set to 19mm, with a local thickening ratio of 18.75%, which is within the optimal thickening range of 15%~20%. The variable wall thickness area is formed by integral spinning, and the wall thickness transition is smooth and continuous, without steps or stress dead corners.

[0029] Buffer and chamfer parameter configuration: The radius of the arc transition buffer 4 is 130mm, which is 0.31 times the minimum curvature radius of the transition arc segment, to achieve a smooth transition of the curved surface; the radius of the stress relief chamfer 5 is 8mm, which is 0.5 times the wall thickness of the straight edge segment, to effectively disperse the residual welding stress.

[0030] Protective layer configuration: The inner wall of the end cap is coated with a 1.0mm thick nickel-based alloy corrosion-resistant reinforcement layer, suitable for long-term contact with LPG media; the outer wall is coated with a 1.2mm thick epoxy zinc-rich anti-fatigue protective layer, suitable for complex outdoor operating environments.

[0031] To verify the performance of this invention, the end cap of this embodiment was assembled into a standard C-type liquid tank with a design pressure of 1.6 MPa, and finite element stress simulation analysis, hydrostatic strength test, and 100,000-cycle alternating load fatigue test were conducted. The test results show that the maximum stress value of the end cap of this invention is only 128 MPa, and the stress concentration factor is as low as 1.42, which is far superior to the existing standard end caps. After 100,000 cycles of alternating load, the overall structure of the end cap remains intact, without cracks, deformation, or leakage, and its sealing and pressure bearing performance is excellent. Compared with traditional end caps, the overall service life of the equipment is increased by more than 35%, and the comprehensive safety and durability performance is significantly improved.

[0032] This invention breaks through the design limitations of traditional fixed curvature, uniform wall thickness heads. Through multi-structure collaborative optimization, it achieves performance upgrades. First, it adopts a uniform variable curvature transition arc structure to replace the traditional fixed curvature structure, allowing the overall surface curvature of the head to change continuously and gradually. This eliminates the defect of abrupt curvature changes between the spherical and transition sections, achieving continuous and uniform stress distribution and significantly reducing the stress concentration factor. Second, based on the stress distribution law of the head, a differentiated variable wall thickness design is adopted. Only the high-stress transition arc section is locally thickened and strengthened, while the low-stress spherical and straight-edge sections maintain conventional wall thickness. This maximizes the load-bearing capacity of key pressure-bearing areas without increasing overall material consumption or reducing the effective volume of the tank. Simultaneously, the spherical side arc transition buffer achieves a smooth transition of surface stress, and the chamfer on the cylinder side effectively disperses residual welding stress, avoiding the risk of stress superposition failure in both directions. Finally, the inner and outer double-layer special protective coatings are adapted to internal corrosion and external environmental erosion conditions, respectively, improving the head's fatigue resistance, aging resistance, and corrosion resistance, making it suitable for the complex alternating conditions of LPG storage and transportation.

[0033] In summary, in this embodiment, the butterfly-shaped head for C-type liquid tanks exhibits significant stress optimization and a substantial improvement in safety. This invention employs a composite structure combining a variable curvature gradient surface with a circular arc transition buffer zone, completely resolving the stress concentration defects caused by abrupt curvature changes in traditional heads. This reduces the overall stress concentration coefficient of the head from over 3.0 to below 1.5, eliminating local stress peaks and effectively preventing the initiation and propagation of fatigue cracks under alternating loads. This eliminates safety accidents such as media leakage and head cracking, significantly improving the operational stability of C-type liquid tanks. It also demonstrates strong stress matching and excellent pressure-bearing durability. Based on the actual stress field distribution of the head, this invention adopts a variable wall thickness differential design, specifically strengthening the area with the highest stress bearing capacity. This improves the pressure-bearing performance of the transition arc segment by over 20%, effectively solving the problems of local deformation, wall thinning, and plastic failure in traditional equal-wall-thickness structures. The overall resistance to internal pressure, impact, and alternating loads is significantly improved, and the service life is increased by over 35% compared to traditional heads. It balances economy and practicality, making it suitable for industrial production. Compared to traditional overall thickening and large curvature radius optimization solutions, this invention only involves localized thickening and structural optimization, requiring no additional raw materials and effectively controlling production costs. The overall structure and spinning / welding processes are fully compatible with existing production lines, requiring no modification to production equipment. It boasts high forming precision, strong assembly versatility, and suitability for large-scale mass production. It has a wide range of operating conditions and low maintenance costs. The inner and outer double protective layers form a composite protection system. The inner wall resists LPG medium corrosion, while the outer wall resists outdoor oxidation, weathering, and fatigue damage, effectively reducing the probability of end cap aging and corrosion failure. This significantly reduces the frequency and cost of subsequent equipment inspection and maintenance, ensuring long-term stable adaptability to various complex operating conditions in medium and low-pressure LPG storage and transportation.

[0034] If certain terms are used in the specification and claims to refer to specific components, those skilled in the art will understand that hardware manufacturers may use different names to refer to the same component. This specification and claims do not distinguish components based on differences in name, but rather on differences in function. The term "comprising" as used throughout the specification and claims is an open-ended term and should be interpreted as "comprising but not limited to." "Approximately" means that within an acceptable margin of error, those skilled in the art can solve the technical problem and substantially achieve the technical effect within a certain margin of error.

[0035] It should be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a product or system comprising a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a product or system. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the product or system that includes that element.

[0036] The foregoing description illustrates and describes several preferred embodiments of the present invention. However, as previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A butterfly-shaped end cap for a type C liquid tank, characterized in that, It includes an integrally formed spherical segment (1), a transition arc segment (2), and a straight edge segment (3), which are smoothly connected in sequence; the transition arc segment (2) is a variable curvature arc structure, and the radius of curvature of the transition arc segment (2) gradually increases uniformly from the end near the spherical segment (1) to the end near the straight edge segment (3); an arc transition buffer zone (4) is provided at the junction of the spherical segment (1) and the transition arc segment (2), and a stress relief chamfer (5) is provided at the junction of the straight edge segment (3) and the transition arc segment (2).

2. A butterfly-shaped end cap for a type C liquid tank according to claim 1, characterized in that, The ratio of the spherical radius Ri of the spherical segment (1) to the inner diameter Di of the head is 0.92~0.

95.

3. A butterfly-shaped end cap for a type C liquid tank according to claim 1, characterized in that, The ratio of the minimum radius of curvature rmin of the transition arc segment (2) to the inner diameter Di of the head is 0.20~0.22, the ratio of the maximum radius of curvature rmax of the transition arc segment (2) to the inner diameter Di of the head is 0.25~0.28, and the transition arc segment (2) maintains a uniform linear curvature change throughout its entire length.

4. A butterfly-shaped end cap for a type C liquid tank according to claim 1, characterized in that, The end cap is a variable wall thickness structure. The wall thickness of the transition arc segment (2) is 15% to 20% thicker than that of the spherical segment (1) and the straight edge segment (3). The wall thickness of the spherical segment (1) is the same as that of the straight edge segment (3).

5. A butterfly-shaped end cap for a type C liquid tank according to claim 4, characterized in that, The variable wall thickness area of ​​the end cap is prepared by an integral spinning molding process, resulting in a smooth and continuous wall thickness transition without abrupt changes or stress dead angles.

6. A butterfly-shaped end cap for a type C liquid tank according to claim 1, characterized in that, The radius of the circular arc transition buffer (4) is 0.3 to 0.4 times the minimum radius of curvature of the transition circular arc segment (2).

7. A butterfly-shaped end cap for a type C liquid tank according to claim 6, characterized in that, The circular arc transition buffer zone (4) is seamlessly integrated with the spherical section (1) and the transition circular arc section (2) by spin forming.

8. A butterfly-shaped end cap for a type C liquid tank according to claim 1, characterized in that, The stress-relieving chamfer (5) is a rounded chamfer with a chamfer radius of 0.5 to 0.6 times the wall thickness of the straight edge segment (3).

9. A butterfly-shaped end cap for a type C liquid tank according to claim 8, characterized in that, The chamfered surface of the stress-relieving chamfer (5) is smoothly connected to the outer surface of the straight edge segment (3) and the outer surface of the transition arc segment (2).

10. A butterfly-shaped end cap for a type C liquid tank according to claim 1, characterized in that, The inner wall of the head is provided with a corrosion-resistant strengthening layer, and the outer wall is provided with a fatigue-resistant protective layer; the corrosion-resistant strengthening layer is a nickel-based alloy coating with a coating thickness of 0.8~1.2mm; the fatigue-resistant protective layer is an epoxy zinc-rich coating with a coating thickness of 1.0~1.5mm.

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