Overview of manufacturing and inspection technology for three-precision welded ball valves. With the adjustment of energy structure and energy regional focus, China's oil and natural gas pipelines have achieved rapid development. In the pipeline construction represented by the first line and the second line of the West-East Gas Pipeline, many of the switch regulating valves are all welded ball valves that are matured and applied internationally. Due to the large demand for all-welded ball valves in China in recent years, the advancement of all-welded ball valve technology in China has been greatly promoted. At the same time, due to the high pressure, material and environmental requirements of the pipeline system in China, it is also all-welded. Ball valves put forward higher requirements. At present, the requirements for long-distance pipelines for ball valves are: uniform force, reliable sealing, compact structure, light weight and high strength.
    This paper introduces the manufacturing process and welding process of all-welded ball valve, in order to provide reference for the development of the industry.
1 construction and use of all-welded ball valve
    The all-welded ball valve described in this paper uses a single spherical weld, and the welding uses a fully automatic welding process. The ball valve has the characteristics of uniform force, reliable sealing, compact structure, light weight and high strength. It is a technical and economical pipeline ball valve, which is widely used at home and abroad, and breaks the pattern of domestic pipeline valves in the form of tubular manufacturing. .
1.1 Ball valve structure design
    The difference between this type of ball valve and the traditional three-stage ball valve is that the way of closing the ball is by welding. Therefore, the inner and outer linings are important during the welding process. In addition, the installation of the ball valve on the pipeline is also by means of a welded connection.
    The seat seal construction is generally a double seal, ie an upstream and downstream double seal. The valve seat insert has excellent corrosion resistance, wear resistance and good elasticity, and has a high sealing performance. The structure of the valve seat is shown in Figure 2.
    The principle of the valve seat double seal construction is as follows.
    (1) Sealing principle of the upstream side valve seat: Due to the upstream force, the valve seat is pressed by the ball to form a seal, as shown in Figure 3. The pressure on the seat is F:
     Where p is the pressure inside the pipe;
F s - spring force.
    (2) Sealing principle of the downstream side valve seat: The valve seat is sealed by the ball due to the pressure in the valve body, as shown in Figure 4. Under the pressure F is:
    Where p is the pressure inside the pipe;
F s - spring force.
The stem is sealed with a GT (fluorinated rubber and Teflon) seal that is highly resistant to rotation and is double-sealed. An emergency seal grease can be injected between the double seals for emergency repair. The sealing structure of the valve stem is shown in Figure 5.
1.2 Ball valve use and maintenance
    Once the seat is in need of an emergency repair, it can be repaired in the manner shown in Figure 6, without the need to damage the welded structure. Static electricity generated by parts inside the valve body (ball, seat ring, etc.) is discharged to the valve body through springs and washers. From an operational point of view, due to the trunnion support, it is possible to operate with less force. The trunnion portion is a Teflon bearing with low frictional resistance. The valve seat is a floating valve seat, and the force that the valve seat is pressed by the ball is small relative to the fluid pressure.
    In terms of maintenance management, ball valves generally do not require maintenance and maintenance. In the event of a leak in the seal, a sealant can be injected for emergency repair. Emergency repair of seat and stem seals with IKSS-3L sealant.
2 for producing welded ball valve
2.1 Ball valve main components of the material
    (1) The valve body mainly uses forgings;
    (2) The ball mainly uses surface-treated forgings;
    (3) 18Cr-8Ni stainless steel forgings are used for the seat ring;
    (4) The valve stem needs to be surface treated;
(5) The sealing materials mainly include butyl (harmonic) nitrile rubber and epichlorohydrin rubber.
2.2 Welding materials and welding process
    The ball valve welding process uses fully automatic welding.
    (1) Body welding
GTAW (tungsten gas shielded welding) + SAW (submerged arc welding)
    (2) stem welding
GTAW (tungsten gas shielded welding) + GMAW (molten gas shielded welding)
    (3) take over welding
GTAW (tungsten gas shielded welding) + SAW (submerged arc welding)
3 full welding ball valve detection
    Since the installation of the all-welded ball valve is directly welded to the pipeline, the test items that need to be performed are also very different from the ordinary ball valve. The more specific testing requirements are as follows.
3.1 bending test
    The bending test of the ball valve simulates pipeline deflection in different settlement areas. The test process is shown in Figure 7.
    Test method: (1) Connect the valve and apply a bending moment of 4200 kN·m to measure the stress of the valve and the pipe. (2) Test the sealing performance of the valve under the nominal working pressure of the air medium. (3) Measure the opening and closing torque of the valve.
    The test is based on the following criteria: (1) The stress at each part of the valve is below the yield strength. (2) No external leakage or seat leakage. (3) There is no abnormality in the operating torque.
3.2 valve tensile and compression test
    The valve tensile and compression test process is shown in Figure 8.
    Test method: (1) The pipe connecting the valve is loaded with a tensile force and a compressive force of 19900 kN. (2) A gas pressure of 0.4 to 0.7 MPa was applied to conduct external leakage, valve seat leakage, and torque measurement test. (3) Measure the stress of the valve and the sleeve.
    The test is based on the following criteria: (1) The stress at each part of the valve is below the yield strength. (2) No external leakage or valve seat leakage. (3) There is no abnormality in the operating torque.
3.3 flush test
    The flush test is a test that simulates the oil and gas transportation process. The test process is shown in Figure 9.
    Test method: (1) A nitrogen gas of a nominal working pressure is injected into the piping of the flushing test apparatus. (2) Open the valve and let the fluid pass through the valve seat. (3) Check the valve seat for any abnormalities.
    The test is based on the following criteria: (1) No abnormalities were found in the valve seat. (2) The sealing performance of the valve seat is not abnormal.
3.4 repeated pressurization operation test
    The repeated pressurization operation test mainly simulates the operation and durability of the valve during actual operation.
    Test method: (1) First close the valve, apply a nominal working pressure to the valve seat, and then fully open the valve. Repeat this operation 200 times. (2) Check the valve seat for leaks at a pressure of 0.4 to 0.7 MPa and a nominal working pressure.
    The test is based on the following criteria: (1) No leaks were found in the valve seat. (2) There is no abnormality in the valve seat of the valve.
3.5 low temperature repeated operation test
    The low-temperature repeated operation test simulates the operation of the valve in the northwest and northeastern regions in the extreme cold conditions in winter.
    Test method: (1) The ball valve was cooled to -45 ° C and the switch was repeated 200 times. (2) After repeating the switch, check the valve seat for leakage at 0.4 to 0.7 MPa and the nominal working pressure.
    The test is based on the following criteria: (1) No leaks were found in the valve seat. (2) There is no abnormality in the valve seat of the valve.
3.6 Foreign body occlusion test
    The foreign matter occlusion test simulates the internal debris during the oil and gas transportation process, and requires the ball valve to operate normally even when the debris enters the ball valve.
    Test method: (1) Place 100 mL of sand, rust and scale of different meshes at the contact between the valve seat and the ball. (2) Perform 250 switchings. (3) Check the valve seat for leaks at a pressure of 0.6 MPa and 7 MPa for 50 times per switch. (4) Inject the sealing grease into the seat to perform an emergency repair test.
    The test is judged according to the following conditions: (1) The switch is 150 times, and the valve seat has no leakage. (2) Switch 200 times, less than 180mL/min leakage. (3) Switching 250 times, less than 625mL/min leakage. (4) There is no leakage of the valve seat after the sealing grease is injected into the seat. (5) The operating torque is not changed and the operation is good.
3.7 blowdown test
    Test method: (1) Close the valve, place 1L of foreign matter in front of the valve seat, apply 7MPa of air pressure on the side of the foreign object, open the valve 16%, and blow foreign matter into the valve seat. (2) The seat leakage test was performed under a pressure of 0.6 MPa and 7 MPa.
    The test is based on the judgment: the valve seat has no leakage, and the operating torque is not abnormal.
3.8 seismic test
    Simulated earthquakes were tested on the vibration test bench using different loads and different amplitudes and vibration frequencies.
    Test method: (1) Loading the extension rod, which is equivalent to the earth pressure bending load generated when the foundation landslide occurs. (2) Measure the stress on the valve and the extension rod. (3) The sealing test was carried out at a force of 11.3 MPa. (4) Perform an operating torque test.
    The test is based on the following criteria: (1) The stress at each part is below the yield strength. (2) The valve has no leakage and no abnormality. (3) There is no abnormality in the operating torque.
4 conclusion
    The all-welded ball valve with spherical structure has the advantages of uniform force, compact structure, light weight and high strength. The double-piston effect of the seat structure and the special sealing rubber material can effectively enhance the reliability of the seal and ensure the life of the ball valve for 30 years. Through a series of tests of the valve, the requirements of the valve under special working conditions can be guaranteed.
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