Application and development of titanium pipe fittings
The standardized production of titanium pipe fittings has sprung up in China in recent years, which is also a revolution in the application of titanium pipe. It greatly improves the service life of titanium pipe and promotes the application of titanium. In this paper, the development history of standard titanium pipe fittings in China, the working situation of titanium pipe fittings and the titanium pipe fittings used in the past are analyzed and reviewed in detail, and the advanced technology of elbow production – Elbow extrusion process is introduced. The titanium elbow produced by this process can ensure uniform wall thickness. The birth and development of titanium pipe fitting standards at home and abroad, and the production and standardization of titanium pipe fittings in China are introduced.
Comparison of production processes of titanium pipe fittings
Table of Contents
Comparison of production processes of titanium pipe fittings
- Working conditions of titanium pipe fittings first, let’s analyze the working conditions of titanium pipe fittings
- Common production technology of titanium pipe fittings
- Standards and specifications for titanium pipe fittings
In the 1970s, China began to use titanium in the chemical system of civil industry. Titanium, as a corrosion-resistant structural material used in chemical plants, has established its position. Moreover, as an ideal material in the pipeline for transporting corrosive media in chemical industry, the service life of the pipeline depends on the pipe, and titanium pipe fittings have attracted more and more attention of engineers and technicians, In particular, the standardization of titanium pipe fittings is more important.
Working conditions of titanium pipe fittings first, let’s analyze the working conditions of titanium pipe fittings
Titanium pipeline mainly transports highly corrosive dangerous media. When the media with corrosive media passes through titanium pipe fittings, the media will have a certain pressure, and the bearing pressure of each part of various pipe fittings is different. The three most commonly used pipe fittings (elbow, tee and reducer) are used for analysis.
Titanium elbow is one of the important pipe fittings of various pipeline systems. It not only changes the flow direction of medium, but also improves the flexibility of pipeline. When the medium passes through the elbow, as shown in Figure 1, the medium with pressure directly rushes the back of the elbow, and the medium flows along the back to the outlet. It can be seen that the back of the elbow bears both great pressure and serious erosion corrosion, indicating that the load on the back is greater than any part.
Fig.1 working condition of titanium elbow
The working condition of titanium tee is shown in Figure 2, which is similar to that of titanium elbow. When the medium passes through the titanium tee, the bearing pressure and scouring corrosion at the intersection of branch and straight road of straight tee are greater than those at other parts. The pressure relief and shunting state of the branch pipeline is the main pipeline.
Fig.2 operation of titanium tee
The working condition of the titanium reducer is shown in Figure 3. When the medium passes through the titanium reducer, the medium often flows from the big end to the small end. Due to the gradual reduction of the cross-sectional area, the pressure is increased at the cone of the reducer, and the inner surface of the cone bears both great pressure and serious erosion and corrosion.
Fig.3 work of titanium reducer
The above analysis shows that titanium pipe fittings are extremely important components in titanium pipes, which directly affect the service life of titanium pipes.
Common production technology of titanium pipe fittings
Multi weld titanium elbow
At the beginning of the use of titanium pipes in China, there was no domestic manufacturer to produce standard titanium pipe fittings, so people had to adopt multi weld type (commonly known as “shrimp waist” type) titanium elbow, see Figure 4, and its processing technology was complicated. Usually, the pipe is cut into multi-section oblique mouth and welded, or the sheet metal is rolled into multi-section leaf shape and then welded, with a large amount of welds. Because the geometry of the welded joint is discontinuous, it will produce high stress concentration. Therefore, strict regulations must be made on the working pressure and working temperature of this kind of pipe fitting. The weld will greatly reduce the corrosion resistance, easy to leak and poor appearance. The inner surface is a folded surface, which increases the transmission resistance of the pipeline, and the weld on the back is seriously scoured and corroded, which reduces the service life.
Figure.4 “shrimp waist” titanium elbow
Titanium welded tee
As shown in Figure 5, the titanium welded tee is made by opening holes on the straight pipe and welding the branch pipe directly. Because the processing performance of titanium material is not as good as that of other materials, the processing of the intersection line at the welding point is also quite difficult. More importantly, the right angle is generated at the weld, which seriously affects the flow conductivity of the medium and increases the transmission resistance of the pipeline, the curve welding of the intersecting line is difficult, and the erosion corrosion at the right angle increases, resulting in early damage at the right angle.
Fig.5 welding right angle tee
Compression welded titanium elbow
1. Upper die 2. Blank 3. Lower die
Figure.6 stamping of titanium elbow
In order to improve the defects of multi weld titanium elbow, the pressure half welded titanium elbow is adopted. Compared with the multi seam “shrimp waist” type, it has fewer welds, and the welds are not scoured and corroded head-on, but scoured in the direction, so the corrosion resistance is better.
Titanium cast elbow
People tried to get rid of the weld and develop a titanium seamless elbow, resulting in a cast elbow. Although it appears as a seamless elbow, the wall thickness (at least 5mm) cannot match the pipe wall thickness (2 ~ 4mm), and the surface finish is poor, which increases the transmission resistance. More importantly, there are a large number of defects such as pores produced by casting, which seriously affect the corrosion resistance and service life. It is not suitable for the use of butt welded titanium pipes. Most of them are used in small-diameter bearing pipe fittings. The titanium pipe fittings produced by this process have high cost and can not be recognized by users.
Punched titanium elbow
Some people use the stamping method to punch the titanium elbow, which seems to meet the standard requirements in appearance, but in essence, the processing process is to punch the pipe blank in the stamping die on the punch. In the forming process, the back of the elbow is pulled, forcing the back to thin, and the abdominal pipe wall is pressed and thickened, resulting in uneven wall thickness or wrinkling. During use, the back of the elbow is subject to erosion and corrosion, and the back wall is thinner, so the back will have early damage. The wall thickness is difficult to guarantee, and its pressure resistance and service life can not meet the wall thickness tolerance requirements in the standard. Try not to use the titanium elbow produced by this process.
How to improve the service life of titanium pipeline? Increasing the service life of titanium pipe fittings has become a technical problem in the field of pipeline design technology.
Elbow extrusion technology is a new steel elbow production technology introduced into China by Japan in the middle and late 1980s. Because this process can realize continuous production and the wall thickness of the produced elbow is uniform, it quickly replaces the traditional process. Seamless titanium pipe fittings such as seamless titanium tee and seamless reducer have been developed successively, and the maximum diameter of seamless pipe fittings can reach Φ 219mm and meet the requirements of ASTM B363-95.
The processing of push titanium elbow takes seamless titanium pipe as blank, and the blank with diameter less than the finished diameter is pushed and extruded on the special push extruder. As shown in Figure 7, the forming die is fixed on the main machine, and there is a movable thrust to push the pipe blank forward from right to left. The blank is heated and protected during the forming die, and the pipe blank is expanded during the forming process, The stress analysis in the forming process shows that the stress in different parts is different, but they are all under two-way compression and always under tension, that is, axial compression, radial compression and circumferential tension. In the whole deformation process, the diameter gradually increases, the bending, the length shortens, and the thickness basically remains unchanged. The experimental results of network method show that the deformation of elbow mainly occurs in the lower part (see Fig.8). Draw a uniform grid on the blank. During the forming, it can be observed that the grid changes little during the forming of the back of the elbow. The more serious the abdominal deformation is, the grid is elongated in the circumferential direction and compressed in the axial direction, while the total area of the grid remains unchanged, indicating that there is no obvious change in the thickness direction.
Figure.7 forming stress diagram of titanium elbow
a) Before forming b) After forming
Fig.8. Experimental results of mesh change before and after seamless extrusion titanium elbow forming
Standards and specifications for titanium pipe fittings
China’s titanium pipe fitting standards and specifications titanium pipe fitting standards first appeared in American aerospace standards. The only international standard for titanium pipe fittings in the chemical industry is ASTM B363 . It is also a document often followed in the international trade of titanium pipe fittings. ASTM B363 was established in 1961, first modified in 1978, and then modified many times (such as 83 and 87). The latest version is 1995, which makes the standard more perfect. In 1994, Northwest Nonferrous Metals Research Institute issued the enterprise standard Q/XB1507-94 titanium butt welded seamless pipe fittings. It is warmly welcomed by the design department. The introduction of this standard represents the beginning of the standardization of titanium pipe fittings in China. It is the main reference document for the production, distribution, design and construction of titanium pipe fittings in China.
In order to better guide the standardized production, design, selection, procurement, distribution, installation and construction of titanium pipe fittings in China, the State Administration of petroleum and chemical industry issued titanium butt welded seamless pipe fittings on July 13, 1999.
Drafted by the Research Institute, the standard number is HG/T 3651-1999, which has been implemented since June 1, 2000. The dimensional tolerance of pipe fittings specified in this standard is the same as American national standard ANSI B16.9-1986 pipe fittings for butt welding of forged steel and Chinese national standard GB12459-90 seamless pipe fittings for butt welding of steel. The requirements of pipe fittings, material properties and surface quality of finished pipe fittings are the same as American Society for testing and Materials ASTM B363-95 pipe fittings for butt welding of seamless and seamed titanium and titanium alloy.
Titanium is a rare metal. Because of its good corrosion resistance and wide application, but its value is high, the selection of titanium pipe fittings is more important. From the above analysis and comparison, the following conclusions can be drawn:
- In order to ensure the service life of the pipeline, seamless pipe fittings shall be preferred for titanium pipe fittings, and push process shall be selected for elbow production as far as possible to ensure uniform wall thickness and smooth surface.
- Shrimp waist elbow and tee directly welding straight pipe in the pipeline shall be avoided. The titanium pipeline connected by this method will seriously affect the service life of titanium pipeline.
- Titanium pipe fittings have the industry standard HG/T 3651-1999, and the design, procurement and construction shall be carried out in accordance with the standard.
Author: Dai Hongwei
Source: China Titanium Pipe Fittings Manufacturer: www.titaniuminfogroup.com
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- American Society for testing and materials, ASTM B363—95: Standard Specification for Seamless and Welded Unalloyed Titanium and Titanium Alloy Welding Fittings.