Forming of Flanged Conical Head
What is a conical head?
Conical head is the head whose shell surface is conical. It is easy to unload viscous liquid and materials containing solid particles, but its mechanical properties are poor, and the discontinuous stress at the joint of cylinder or nozzle due to shape mutation is large. In order to reduce the discontinuous stress, circular arc transition with folded edge structure or local thickening structure can be used at the large end or small end of the conical shell. The Chinese pressure vessel design code requires: when the half cone angle is greater than 30 °, the large end of the conical shell shall be of the folded structure; when the half cone angle is greater than 45 °, the small end of the conical shell shall be of the folded structure. Due to the large wall thickness of the conical head and inconvenient manufacturing, it can only be used when the process needs.
The conical head without flange is a section of cone.
Because the cone is directly connected with the cylinder, the shell shape at the connection is abrupt and discontinuous, resulting in large local stress. This stress depends on the size of the semi apex angle α of the cone. The greater the α, the greater the stress; otherwise, the smaller.
There are three restrictions on the non folded conical head in the design code:
(1) The non folded conical head is only suitable for the case of the cone half apex angle α ≤ 30 °;
(2) When α is more than 30 °, the type of folded cone must be adopted, otherwise, the stress analysis method must be used for calculation;
(3) The butt weld at the joint of conical head without flange must adopt full penetration structure.
The flanged conical head consists of three parts: cone, folded edge and cylinder.
It is mostly used when the half apex angle of cone is more than 30 degrees. Because the larger the α, the greater the stress of the cone and the larger the required wall thickness, the more difficult the machining is. Therefore, the half top angle of conical head with folded edge is generally no more than 45 ° unless it is specially needed. In addition, the larger the inner radius r of the folded edge is, the better the stress state of the head is. Therefore, the design regulation has made the following restrictions: the inner radius r of the folded edge should not be less than 10% of the inner diameter DN of the large end of the cone and 3 times of the cone thickness.
There are three forms of flanged conical head:
- One is to fold both ends (i.e. large end and small end);
- The other is to fold the large end but not the small end;
- The third is to fold the large end but not the small end.
There are four methods for flanging conical head:
- One is to press and flanging integrally. After the blank cone is manufactured, the flanging end is pressed;
- Second, split stamping and flanging. After cutting the flap, press the folded end (or segment) and then assemble and weld;
- The third is rolling flanging. After the blank cone is manufactured, the flanging end is rolled with special equipment;
- The fourth is spinning flanging. After the blank cone is manufactured, the big end is spun. If the small end needs flanging, the small end will be pressed or rolled again.
In addition, there are also model tapping flanging.
Integral stamping flanging
Integral stamping flanging is carried out on hydraulic press (or hydraulic press).
The stamping flanging of large end of flanged conical head is shown in Fig.1.
The blank cone to be flanged is placed between the upper die and the lower die of the press, and the shape of the upper die and the lower die is the same as that of the flanging of the conical head;
The diameter of the lower die is slightly larger than the outer diameter of the large end of the flanged conical head. After starting the hydraulic press, with the downward movement of the upper die, the blank cone gradually deforms and flanging is completed.
Figure.1 1 – upper die; 2 – workpiece; 3 – lower die
The blank cone to be flanged is placed between the lower die and the blank holder ring of the press, and the shapes of the lower die and the blank holder ring are the same as those of the flanging of the conical head; the upper die is a general stamping die for the head, and the diameter is the same as the inner diameter of the small end of the conical head.
After starting the hydraulic press, with the downward movement of the upper die, the blank cone gradually deforms and flanging is completed.
Figure.2 1 – upper die; 2 – blank holder; 3 – blank
The advantages of integral stamping flanging are high production efficiency, wide range of wall thickness and low cost of mass production.
The disadvantage is that the mould cost is high and the processing is difficult. Because of the different specifications of the conical head, the number of the mould increases. When the single piece is produced, the manufacturing cost will increase greatly due to the processing of the die.
With the increase of the size of conical head, the cost increases, and a large tonnage press is needed.
In very special cases, the conical head (including the variable diameter section) at both ends is formed by integral stamping, and the whole plate (which can be connected to the plate) is annular blanking. The large end of the blank cone is first pressed and then the small end is punched by using the upper die and the lower die.
Large end and small end need stamping flanging for many times, but small end flanging is difficult.
Split stamping flanging
The split stamping and flanging is carried out on the hydraulic press (or hydraulic press), which is used for the flanging of the folded conical head after the split forming and then splicing. The process equipment (except the mould) and the forming method are the same as those of the stamping convex head which is formed by dividing and then splicing, so it will not be repeated.
However, if the length of the flanged conical head is large, it needs to be formed in sections and sections according to the diameter and thickness of the big and small ends. The flap of the folded section is pressed and flanged, and the flap of the non folded section is pressed.
Rolling flanging is carried out on a special flanging machine, which can complete the flanging of large end, small end or large and small end of conical head at the same time. As spinning and stamping can complete large end flanging of conical head, special flanging machine is mainly used for small end flanging of conical head, as shown in Fig.3.
The blank cone is placed on two idlers placed at a certain angle on the flanging machine. The angle between the two idlers is adjustable. During operation, the two idlers can rotate and play a supporting role.
When working, the auxiliary wheel rotates to stabilize the cone.
The supporting wheel is equivalent to the lower die, and its shape is similar to the folded edge shape.
After the flanging machine is started, the pressure wheel and supporting roller roll to complete the small end flanging as the blank cone rotates on the roller and roller.
Figure.3 1-pressure wheel; 2-supporting roller; 3-idler; 4-auxiliary wheel; 5-billet cone
Spinning is only applicable to the large end flanging of conical head. Flanging is similar to the second step of “two-step” spinning forming of convex head, that is, the blank cone is placed between the upper roll (rolling roller) and the lower roll (profiling) of the spinning machine. After the press is started, the two rollers rotate and cooperate with each other until the large end flanging of the conical head is completed.
If the small end needs flanging, stamping or rolling shall be adopted. The stamping and rolling flanging method is the same as above.
The advantages of spinning flanging are low cost (reduce the number and cost of dies), suitable for a wide range of diameters, without wrinkles.
The disadvantage of spinning flanging is the low efficiency of mass production and small thickness range.
Model tapping flanging
The model tapping flanging is mainly used for large end flanging of conical head with large diameter and thin wall thickness (such as the top folded conical head of atmospheric storage tank). The shape of the mold (model) is the same as that of the conical head. When flanging, it is difficult to ensure the flanging quality, especially the corner radius of the transition section, due to the uneven heating temperature and striking force of the plate Therefore, the method of model beating flanging is seldom used in the manufacture of pressure vessel flanged conical head.
The forming of conical head is difficult for cylinder section in terms of deformation size. It is easy to form hemispherical head, elliptical head and dished head. In fact, due to the particularity of conical head shape, it is difficult to control the forming process of hemispherical head, elliptical head and dished head.
Therefore, in addition to forming equipment, forming method, relevant forming technical requirements (such as butt misalignment of welded joints, edges and corners of welded joints, surface quality of welded joints, roundness, process thinning, shape and size deviation, heat treatment for restoring material properties, heat treatment for restoring material supply status, and welding requirements for restoring material supply status) and technical measures for conical head forming Besides, the following points should be noted:
(1) The requirements of marking accuracy and blanking accuracy are high.
Conical head is different from convex head and cylinder section, so there are many factors affecting the shape and size, and it is difficult to control.
Therefore, in the manufacturing process, it is necessary to accurately calculate the unfolding size, accurately draw lines and blanking, and at the same time, the groove processing, pre bending and forming operation should be strictly controlled, otherwise the shape and size will be out of tolerance.
(2) Various sizes restrict each other.
If the inner diameter of large end, small end and length deviation of conical head are too large, the assembly with cylinder will be affected. If one dimension is changed, the other size will change accordingly. If the inner diameter of large end is too large, the length of large end may be shortened in order to ensure assembly with cylinder.
It is difficult to correct this situation by orthopedic methods.
(3) The concentricity is out of tolerance.
It is difficult to control the concentricity of conical heads, especially those with folded edges.
The concentricity of the conical head affects the concentricity and straightness of the vessel. If the concentricity and straightness of the vessel are strictly required, in order to ensure the concentricity and straightness requirements, the misalignment of class B welded joints of the cylinder connected with the conical head may exceed the tolerance.
Because of the large rigidity of the conical head, it is difficult to correct the local correction and the out of tolerance of concentricity.
(4) Thickness of transition section of flanged conical head.
The thickness of the transition section is easy to be thinned by flanging of the conical head with folded edges. If it is hot flanging, the thinning amount is greater, especially at the large end transition section.
Insufficient thickness of transition section may cause insufficient strength or stiffness of conical head.
Therefore, the appropriate steel thickness should be selected to meet the minimum thickness requirements after the transition section is formed, but attention should be paid to the problem of steel thickness jumping.
(5) It is difficult to weld the longitudinal weld of conical head formed by split section, which belongs to class a welding joint.
Vertical welding (vertical welding) is generally adopted for longitudinal weld of segmented conical head, which is difficult to weld and the energy absorbed by impact is not easy to guarantee.
Therefore, for vessels with high impact absorption energy or low temperature vessels, tooling should be used to weld longitudinal welds to avoid vertical welding.
(6) Section assembly welding.
When the conical head is long, it needs to be assembled and welded after being made into sections (more than two sections). From the surface, it is similar to the cylinder assembly welding, but due to the particularity of the conical shell structure,
There are at least three differences between the welding ratio and the shell forming and welding ratio
- First, the shape and size of the conical shell should be controlled more strictly than that of the cylinder section;
- Second, the conical shell needs vertical assembly or horizontal assembly of tooling;
- Third, the longitudinal or circumferential welds (belong to a or B type welding joints) generally adopt manual welding, which is difficult to realize automatic welding.
(7) Welding and NDT of conical head and cylinder without flange.
The conical head without flange is directly welded with the cylinder (or equipment flange), and the longitudinal curvature of the shell at the joint is discontinuous, and the welded joint requires full penetration welding, which is not only difficult to assemble and control, but also difficult to control the welding quality and local stress. It is also not convenient for radiographic testing and ultrasonic testing, especially for ultrasonic testing.
Therefore, the quality of welding joint between conical head and cylinder is the key to control the welding quality of the whole vessel.
Picking steel—Cutting—Welding—Heating—Spinning—Stamping—Trimming—Defect detection—QC—Warehousing—Transportation
Source: China Pipe Caps Manufacturer – Yaang Pipe Industry (www.steeljrv.com)