Study on tee fittings for oil and gas pipeline

Tee fitting is an important and large quantity pipe fitting in oil and gas transmission pipeline engineering. In pipeline construction, it is used to change the direction of pipeline, change the size of pipe diameter, branch pipeline, locally strengthen and realize special connection. In recent years, oil and gas pipeline construction has entered the peak of development. With the development of pipeline industry, tee fittings tend to develop in the direction of high strength, large diameter, thick wall and high performance. The working pressure of tee pipe fittings in China’s west to east gas transmission project reaches 12MPa, the tensile strength reaches 625mpa and above, the outer diameter reaches 1422mm, and the maximum wall thickness exceeds 50mm. The production process of tee is complex, and there are many factors affecting quality and performance. In the actual service process, the tee bears not only internal pressure, but also bending moment, torque and axial force. Due to the complexity of the geometric structure and external load of tee pipe fittings, it often becomes a dangerous part with high stress concentration in the pipe system. In a sense, the quality and bearing capacity of tee will directly affect the integrity and safe operation of the whole pipe system. Therefore, it is necessary to analyze and summarize the manufacturing method, forming process, design standards and performance requirements of tee pipe fittings in the field of oil and gas transportation.

Overview of tee fittings

The structure of tee pipe fittings for oil and gas transmission pipeline is shown in Figure 1, in which main pipe refers to the part through which the fluid passes, that is, the pipeline with holes. Branch: the tubular portion that intersects the main pipe. Abutment shoulder: the area on the branch and main pipe located in the longitudinal plane (the plane determined by the axis of branch pipe and main pipe) and close to the intersection line. Abdomen: the area of the shell near the intersection of the intersection line and the transverse plane (the plane perpendicular to the main axis and containing the branch axis). The ends of the main pipe and branch pipe of the tee are of the same size, which is called equal diameter tee. When the end size of the branch pipe is smaller than that of the main pipe, it is called a reducing tee.

Fig.1 structural diagram of tee pipe fittings
The manufacturing processes of tee pipe fittings mainly include casting, welding, hydraulic bulging and hot extrusion. Casting tee is to melt the metal into a liquid meeting certain requirements and pour it into the tee shaped mold. After solidification, the blank with tee shape is obtained, and then subsequent machining is carried out to obtain the cast tee pipe fittings. The advantage of casting tee is that it can obtain the blank of pipe fittings with complex shape and thick wall thickness. However, the disadvantage is that the subsequent machining hours are long and the material waste is serious. Especially for the large-diameter thick wall tee pipe, it has a large size, and a lot of materials have to be cut during processing, with a large amount of consumables and high production cost. In addition, the quality of cast tee is not easy to guarantee, and the defects are not easy to check. Leakage accidents often occur after the project is put into operation, so it is not suitable to be used in petroleum and chemical pipelines. The welded tee pipe fitting is directly welded with the branch pipe after opening the saddle shaped welded junction on the pipe blank, which has the advantages of convenient manufacturing and strong field flexibility. However, the welding process of tee pipe fitting is more complex than that of general steel pipe, and the welding surface is on a saddle shaped irregular space curved surface, so the welding quality is difficult to guarantee. The biggest weakness of welded tee is that the high stress area is just located at the weld position of the intersection line between branch pipe and main pipe, which is easy to leave potential accidents when used in high-pressure transportation lines such as oil and natural gas pipelines. The hydraulic bulging of tee pipe fittings is a pressure processing method to press the pipe blank into the mold cavity through internal liquid pressurization and axial force to compensate for the bulging of branch pipes. Its advantages are saving materials, reducing mold costs, and reducing subsequent machining, assembly and welding. However, due to the low forming force of bulging method in China, and the control of internal pressure and axial feed is not strict, it is only suitable for the production of tee fittings with thin wall, low strength and small diameter. Hot extrusion tee is the main technology for manufacturing tee fittings of high-strength and large-diameter oil and gas transmission pipeline in China. It is to heat the whole pipe blank to an appropriate temperature above the pipe recrystallization temperature and put it into the die. The pipe blank is hot extruded under the action of the press. The pipe blank is plastically deformed along the geometry of the die cavity under the action of radial force to form a tee branch pipe. Due to the lack of weld, the hot extrusion tee has good surface finish, smooth shape change, no sudden change in wall thickness distribution and good overall strength.

Research progress of tee pipe fittings for oil and gas transportation

The application history of hot extrusion tee pipe fittings in the field of oil and gas transportation is not long. In the 1970s, the United States formulated the national standard of hot extrusion tee. China began to develop and gradually apply hot extrusion tee after 1980s. In recent years, the construction of high-pressure, large-diameter and long oil and gas pipelines has entered a golden period. The construction of pipeline projects such as the first and second lines of West to east gas transmission, Central Asia and China Myanmar has brought huge development space to pipe fittings. At the same time, there are higher and higher requirements for the quality and performance of tee pipe fittings.

Forming process of tee pipe fittings for oil and gas transportation

The hot extrusion process of tee pipe fittings for oil and gas transportation is generally divided into four steps: blank making, flattening, bulging and drawing.
Blank making: select appropriate steel pipe or steel plate to curl into pipe. Due to the small outer diameter of tee pipe fittings produced by seamless steel pipe, at present, high-strength and large-diameter tee pipe fittings for oil and gas transmission pipeline are generally rolled into pipes with appropriate thickness and then welded into tee blank pipes.
Flattening: heat the tube blank to 50-100 ℃ above Ac3 as a whole, leave the furnace for a constant temperature time ≥ 10min, press it into an ellipse on the press, and place the weld on the top of the long shaft. Liu Limin’s research shows that when the minimum aspect ratio is greater than 0.47, the influence of the aspect ratio of the flattened tee blank section on the tee bulge can be ignored in a certain range. When the height width ratio of the tube blank section is less than 0.47, it is easy to collapse during flattening, and it is not suitable to place and accurately locate on the die during subsequent bulging.
Bulging: heat the whole tube blank to 50-100 ℃ above Ac3, and leave the furnace for a constant temperature time of ≥ 30min. Bulge it on the press for many times until the length of the branch pipe meets the requirements. The initial pressure temperature is not less than Ac3 + 50 ℃, and the final pressure temperature is not less than Ac3-50 ℃. The bulging process of tee pipe fittings for oil and gas transmission pipe is characterized in that the main deformation area is concentrated in the middle of one side of the pipe blank, while other parts are basically not deformed. Before bulging, the heated workpiece is cooled with water on the side without bulging, so that the materials in different areas of the workpiece can show different yield strength due to different temperatures, which is conducive to the flow of materials in the predetermined deformation area during the forming process. The bulging process is the key to the tee forming process. The height and shape of the bulging directly determine the forming difficulty of the subsequent process, and even the success or failure of the whole tee forming process. Li Hongda and Liu Limin used DEFORM-3D finite element software to optimize forming process parameters of Φ1219mm tee pipe fittings. The research shows that different die inlet fillet has a certain influence on the bulging process. When the fillet at the inlet of the die is too small, the bulging deformation force is too large, and it is easy to accumulate materials in the pipe fittings. With the increase of the die inlet fillet, the deformation force required in the bulging process decreases, but too large inlet fillet will make the final bulging height unable to meet the design requirements. When the inlet fillet is R125, it can not only meet the requirements of the lifting height, but also meet the process requirements of subsequent processes. The simulation and test results of Huang Jian and others on WPHY-80 show that during unpacking, the forming part is heated to 950 ℃, locally quenched to 120 ℃, and the quenching range is about 65%, which can effectively avoid forming defects. Liu Limin and other numerical simulation results show that when the cooling area is 63% for the first time and 55% for the second time, the bulging effect is better. Drawing: preformed holes shall be opened on the drum of pipe blank. The opening size shall be in accordance with the standard requirements, with a machining allowance of 20-30mm. The periphery of the opening shall be trimmed with a grinder, deburred and heated in the furnace. The heating temperature shall be 50-100 ℃ above Ac3, and the constant temperature time shall be ≥ 10min. Then, it shall be hot drawn with a special die on the hydraulic press for multiple passes. The simulation results show that three pass drawing is better than two pass drawing because of its small forming load and small thinning rate of branch pipe wall thickness. The drawing effect mainly depends on whether the drum height is ideal or not.
After drawing, the initial tee shaping shall be carried out, and the surface quality and dimension shall be checked. When the diameter roundness of the pipe end fails to meet the technical requirements, it shall be corrected before heat treatment.
In the process of hot extrusion, the tee needs to be heated and cooled for many times. The mechanical properties of the pipe body and weld are greatly changed compared with those before forming, and its tensile and impact mechanical properties can not meet the standard requirements. The final heat treatment is the key to make the tee body and weld meet various mechanical property indexes. Quenching + tempering process is usually adopted for the heat treatment of oil and gas pipeline tee. The heat treatment temperature and holding time are determined by the material and wall thickness of tee body, and the heat treatment characteristics of weld are considered at the same time.

Design of tee pipe fittings for oil and gas transportation

The design of large diameter tee has always been one of the technical difficulties in pipeline engineering, MSS SP-75-2008 and ISO15590.1-2009 there is no reliable design calculation formula in these two standards. With the increasing diameter and pressure of oil and gas pipeline, the design problem of tee with branch pipe diameter close to or equal to main pipe diameter becomes more prominent. Due to the geometric discontinuity, there is stress concentration at the corner of the intersection line of tee pipe under the action of internal pressure. The experimental results of ellyin et al. Show that all specimens begin to yield when the corner pressure is much less than the overall yield pressure in the longitudinal plane. For the tee with thin main pipe wall thickness, the highest stress point in the abdomen reaches the yield value after the forming area of the shoulder, while for the tee with large main pipe wall thickness, the inner wall at the corner of the shoulder and the outer wall of the abdomen reach the yield value at the same time. Zhao Shubing’s research shows that under the internal pressure of the tee, the maximum equivalent stress occurs on the inner side of the shoulder and the maximum deformation occurs on the outer side of the abdomen. Reasonable selection of the fillet radius of the branch pipe inlet can improve the stress concentration of the tee. Ma Yehua and others developed the tee optimization analysis module by using the finite element analysis technology, and obtained the optimal tee fillet radius. The optimal fillet radius increases with the increase of branch radius, as shown in Figure 2.

Figure.2 Relationship between internal and external fillet radius of tee with fillet
MSS SP-75-2008 is the design standard commonly used in the design of long-distance pipeline fittings in China, which stipulates that the structural design of fittings can use two methods: mathematical analysis and verification test. The design method of tee in mathematical analysis method adopts equal area reinforcement calculation method. The so-called reinforcement refers to the need to supplement the strength due to the decrease of the bearing strength after the opening of the pipeline. Equal area reinforcement means that the area of the reinforcement material shall be greater than or equal to the area of the material lost in the opening of the pipeline. Verification test method is a method to analyze, screen and study the test results on the basis of a large number of blasting tests, so as to determine the structural dimensions such as tee wall thickness.
The standards adopted for the design and calculation of tee for domestic oil and gas transportation mainly include GB/T 50251 « code for design of gas transmission pipeline engineering », GB/T 50253 « code for design of oil transmission pipeline engineering », SY/T 0510 « steel butt welded pipe fittings » and ASME B31.3 « process piping », the above standards adopt the design method of equal area reinforcement. Wang Gaofeng et al. Compared the calculation methods of tee opening reinforcement in the above four standards: ASME B31.3, there are many restrictions on the calculation of opening reinforcement of drawn tee, and the provisions on the curvature radius of the outer contour of the outlet are added. The example calculation shows that the design calculation of GB/T 50251 and GB/T 50253 is more strict. Zhao Shubing and others compared and analyzed the calculation methods of GB/T 50251 and GB/T 50253, and found that GB/T 50251 « code for design of gas transmission pipeline engineering » has more strict requirements for reinforcement, and its reinforcement area is related to the wall thickness of the connecting process pipeline. In the calculation of equal area reinforcement, the preconditions and data selection have certain assumptions and margins. The analysis of the actual tee burst test pressure of the second west to East Gas Pipeline shows that the results calculated by the equal area reinforcement formula are conservative, and the larger the diameter of the branch pipe is, the more the wall thickness is. For X80 large diameter tee of West East Gas Transmission Line 2, the design wall thickness calculated by mathematical analysis method is close to the limit of material hot working capacity. In the future construction of larger diameter or larger penstock, if the mathematical analysis method is still adopted, the materials and factory processing capacity selected for the current X80 tee production will not meet the requirements of project construction quality. Based on a large number of physical performance and blasting tests of X80 series tees, Liu Yingrong et al. Used the verification test method to obtain a series of optimized wall thickness of X80 tees, which reduced the expected physical wall thickness of large-diameter tees by 20% compared with the calculation results, and established corresponding technical conditions and database, so as to guide the structural design of high-strength large-diameter tees for pipeline engineering. At present, this achievement has been applied in the construction of the third and fourth West to east gas pipeline. The verification test method for determining the wall thickness of tee pipe fittings is listed in Q/SYGJX105 « technical conditions of DN350 and above pipe fittings for oil and gas transmission pipeline engineering » and Q/SYGJX106 « technical conditions of DN400 and above pipe fittings for oil and gas transmission pipeline engineering ».

Basic requirements for tee pipe fittings for oil and gas transmission

Chemical composition of tee pipe fittings for oil and gas transmission

The hot extrusion tee is heated and deformed repeatedly during the forming process. Heat treatment must be carried out after forming. The main pipe is formed by welding and connected with the main pipe by welding. Therefore, high temperature plasticity, appropriate hardenability and good weldability are the basic requirements for the chemical composition of hot extrusion tee. Table 1 lists the chemical composition of domestic TE555 and Russian K55 steel tee, as well as the maximum allowable value of chemical composition of X80 steel tee pipe fittings specified in Q/SYGJX106 and China Russia east line.

Mechanical properties of tee pipe fittings for oil and gas transportation

Due to the complexity of the structure and forming of X80 hot extrusion tee, it is necessary to take samples from the branch pipe, abutment shoulder, main pipe and welded joint of the tee for tensile and impact tests.

Table.1 common chemical composition (mass fraction) of X80 steel tee pipe fittings

When the wall thickness of tee steel pipe is greater than 25mm, the tensile sample can be either the full wall thickness rectangular tensile sample or the full wall thickness uniformly layered rectangular tensile sample. For cylindrical tensile specimens and Charpy impact specimens, 1/2 of the wall thickness or 25mm from the outer surface of the tube shall be selected.
The specified temperature of Charpy impact test shall be conducted according to the design temperature of pipe fittings minus 5 ℃, which can also be determined by the manufacturer and the purchaser through negotiation. The impact test temperature of X80 hot extrusion tee in low temperature environment of China Russia east line station is as low as -45 ℃.

Conclusions and recommendations

At present, only manual welding method can be used for the welding of X80 tee pipe blank pipe for oil and gas transportation, because there is no submerged arc welding wire meeting the requirements in China, but the welding production efficiency and product quality stability of manual welding method are low, resulting in the weld becoming a weak link on the tee pipe. In order to meet the needs of field girth welding construction, the alloy content of tee for oil and gas transmission pipe is generally low. With the continuous increase of wall thickness, the microstructure of the tee from the surface to the center after quenching is different, and the yield strength and Charpy impact absorbed energy at the center are significantly reduced, resulting in the potential risk of brittle instability and cracking. In addition, in terms of production technology, the current design wall thickness of high-pressure and large-diameter tee has greatly exceeded the current production capacity of domestic tee fittings. Therefore, it is urgent to carry out research on new manufacturing methods and new production processes of tee fittings.

Authors: Hu Meijuan, Liu Yinglai, Ji Lingkang, Ma Qiurong, Wang Peng

Source: China Tees Manufacturer – Yaang Pipe Industry (www.epowermetals.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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