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Analysis of the application of double slope high neck forged flanges in steel pipe towers

With the improvement of high-voltage transmission voltage levels, steel pipe towers have achieved unprecedented development in the transmission line industry. The main connection method for the main materials of steel pipe towers is flange connection, and the main applications of steel pipe towers are rigid flanges, flexible flanges, and high-neck forged flanges. The double slope high neck forged butt-welded flange has high stiffness. The article analyzes the influencing factors such as flange thickness, neck height, and upper end width of the neck.

20230505131447 10797 - Analysis of the application of double slope high neck forged flanges in steel pipe towers

1. Design of node connection for steel pipe tower

Various steel pipe tower nodes have complex stress, structure, and high processing quality requirements. The node x forms of steel pipe towers mainly include three types: docking nodes, cross nodes, and complex nodes, which are subdivided according to different parts of the tower, as shown in Table 1.

Table.1 Node Types

Node Type  Serial Number Node Category Connection Method
Docking Nodes Main Material Docking Flange Node Flange Connection
Extra Long Diagonal Joint Flange Node
Intersection Node Cross Diagonal Connection X Node Intersecting Welding Or Plug-In Plate Connection
Connection K Node Between Diagonal And Main Materials
T-Node For Connecting Horizontal And Main Materials
Inverted K-Node For Diagonal And Horizontal Beams
Inverted Y Node For Connection Between Tower Leg Diagonal Member And Main Material
Complex Nodes Other Complex Nodes, Etc Specific Research

The above table shows that the main connection methods for steel pipe tower nodes include flange connection, intersecting welding, or plug-in plate connection. For docking nodes, a flange connection is generally used; For cross nodes, both intersecting welding and plug-in connections can be used.

2. Flange connection

A flange is a component that connects shafts and connects pipe ends; a flange connection refers to a detachable connection consisting of flanges, gaskets, and bolts connected as a combination sealing structure. Due to their reasonable structure, reliable force transmission, high efficiency, convenient maintenance, and low cost, flanges have become the most commonly used connection form in steel pipe towers.

The usual flange connections for steel pipe towers can be divided into flexible flanges, stiff flanges, and high-neck forged flanges. Flexible flanges can reduce welding workload, facilitate installation, and ensure the flatness of the flange plate more easily. However, due to the lack of stiffening ribs, the flange undergoes significant deformation under force, and the bolt forces are complex. Flexible flanges without stiffeners can be used for structures that require convenient construction, a beautiful structural appearance, and low requirements for structural deformation and building deflection. Stiffened flanges are welded with many stiffeners on the flange plate. The flange has high stiffness, and the deformation of the flange after being subjected to force is small. The bolt force is simple, but the welding workload is large, and it takes work to ensure the quality of the weld seam. At the same time, the construction is more complex than non-stiffened flanges. The high-neck forged flange combines the advantages of both traditional stiff flanges and flexible flanges. It not only has the characteristics of high-stress stiffness and small deformation of stiff flanges but also is convenient for factory processing and on-site installation like flexible flanges. In foreign countries, such as the United Kingdom and the United States, especially Japan, it is not only used on crossing towers but also common in conventional voltage-grade steel pipe towers. It has been widely used in high-voltage grade lines in China, and the technical conditions are mature.

The types of high neck forged flanges include butt-welded flanges and flat-welded flanges. Butt welding flanges are generally made using forging and rolling processes, with only one butt weld seam and a small amount of welding. The weld seam should undergo ultrasonic testing and have no delamination defects. The welding quality can be easily guaranteed as long as the welding process is appropriate. Flat welding flange refers to a type of flange that is connected to a container or pipeline using a fillet weld. Simple structure, low material consumption, two fillet welds, large welding volume, and difficult to control weld quality. And the two fillet welds are far apart, resulting in uneven stress and poor sealing. Therefore, priority should be given to high neck forged butt-welded flanges.

Single slope high neck forged butt-welding flange will weaken the entire flange due to the smallest neck section. And the double slope high neck forged butt-welding flange has a certain slope on both the inner and outer sides of the weld seam, which is conducive to load transmission and diffusion, to some extent avoiding this problem. Moreover, its stiffness is larger than that of the single slope high neck forged butt welding flange, which is conducive to maintaining the overall rigidity of the flange, making the bolt force transmission uniform, and reducing the number of bolts. It is recommended to use double slope high neck forged butt welding flanges for steel pipe towers.

3. Stress characteristics of double slope high neck forged butt welding flanges

Although the double slope high neck forged butt welded flange does not have the same stiffening ribs as traditional stiffened flanges, its stress characteristics are similar to those of traditional stiffened flanges. Under tensile load, the double slope high neck forged butt welded flange directly transfers tensile force to the flange plate through the flange diameter. However, the flange plate is thick and stiff enough to transfer tensile force to the bolts directly. The flange plate edge will not generate prying force, but the flange plate edge will detach, and the limited strain of the bolts mainly causes the detachment gap of the nodes. The relative deformation of the entire node is relatively small, and the node connection of the main material joints of domestic transmission line steel pipe towers is fully applicable.

4. Analysis of factors affecting the bearing capacity of double slope high neck forged flanges

The analysis of the stress mechanism of the double slope high neck forged flange shows that the main parameters affecting its bearing capacity include flange plate thickness, neck height, neck width, neck slope, etc. When the design bearing capacity of the forged flange does not meet the usage requirements, the flange parameters can be adjusted according to the following principles to improve the node bearing capacity:

  • (1) Within a limited slope range, when the width of the root and top of the flange neck is fixed, the higher the flange neck, the greater its stiffness, and the easier the upper and lower flanges are to detach. The rotation of the flange plate itself is smaller, but this effect is relatively small.
  • (2) Within a limited slope range, when the top width and height of the flange neck are fixed, the larger the root width of the flange neck, the easier the flange is to detach, and the smaller the rotation of the flange itself.
  • (3) The thickness of the flange plate is the most important factor determining whether the upper and lower flanges will eventually detach. The larger the thickness, the easier it is for the flange to detach, and the smaller the rotation of the flange plate itself.

Author: Wang Enlai

Source: China Forged Flanges Manufacturer – Yaang Pipe Industry (

(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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