Finite Element Analysis of Sealing Performance of Pipeline Mechanical Joint

The sealing performance of the pipe joint is the most critical controlling technical index in the pipeline transportation system. The failure of the pipe joint will directly lead to the leakage of the transport medium, which will cause serious consequences. Therefore, studying the sealing performance of pipe joints has important practical value.

Pipe mechanical joint is a new type of metal pipe connection technology. It is a pipe connection realized by the elastic-plastic deformation of the pipe joint during the crimping process. This connection method has no welding, fast speed, mechanization and automation High, can greatly reduce the labor intensity, etc. m. (1 "= 2.54cm) pipe mechanical joint, analyze the sealing principle of the pipe joint and its influencing factors, and use Marc finite element software 2 to carry out the internal pressure resistance process of the pipe joint Numerical simulation to further study the sealing performance of pipe joints. The research results provide a theoretical basis for the structural design of pipeline mechanical joints.

1 The basic structure of the pipe mechanical joint and its working principle The design structure of the pipe mechanical joint is as shown.

Pipe joint structure diagram The pipe joint is an axisymmetric structure and is composed of 2 outer casings and 1 inner pipe hoop. Its working principle is to insert the two ends of the connected pipe into the pipe hoop, and the inner surface of the pipe hoop is provided with 5 convex ring ribs (the middle bar is the positioning ring when the connected pipe is inserted into the pipe hoop, and each section is connected There are two extrusion rings at the end of the pipe. The section of the extrusion ring is designed to be isosceles trapezoidal. If the extrusion ring can be deformed radially and elastically, the extrusion ring can be tightly clamped to the connected The outer surface of the H. pipe hoop on the pipeline is designed to have a certain taper, and there is a pair of casings on the outside of the pipe box, the casing has an internal taper. The two sleeves can slide left and right relative to the pipe hoop respectively, so the pipe hoop is affected by the radial pressure exerted by the sleeve, and undergoes elastoplastic deformation, which is compressed on the pipe section to be connected, so that the pipe hoop and the connected Securely connect and seal the pipeline.

2 Analysis of the sealing principle of pipe joints The sealing of pipe mechanical joints is a contact type seal in static sealing. It does not require other sealants, gaskets or seal rings, but forms a reliable seal through interference fit between metals.

During the axial advancement of the outer casing by the loader, due to the conical surface of the pipe hoop, the pipe hoop is subjected to radial tightening force and axial shearing force, and at the same time is subject to frictional force and radial tightening The radial shrinkage occurs, so that the gap between the pipe hoop and the connected pipe becomes smaller and smaller until the pipe hoop squeeze ring contacts the inner pipe, and the radial deformation is limited to a certain extent. At this time, the contact stress between the extrusion ring and the pipe is getting larger and larger. When the contact stress reaches a certain level, the normal pressure is increased enough to cause obvious elastoplastic deformation on the surface of the pipe, between the extrusion ring and the pipe Zero clearance begins to form, forming a so-called interference fit, and the sealing effect can be achieved.

Since the two connected pipes are only positioned by the positioning ring, there is no other sealant or sealing ring on the contact surface, so in actual engineering applications, the fluid in the pipe will be from the contact gap of the two connected pipes It overflows and penetrates into the gap between the outer surface of the pipe and the inner surface of the pipe hoop until the pressure in the gap due to fluid filling is equal to the pressure in the pipe. (See), in addition to the function of connecting the pipe, the pipe squeeze ring must also be able to isolate the fluid in the pipe to play a sealing and protective role. Therefore, in order to ensure the sealing performance of the pipeline, the pipe joint must be able to withstand the pressure equal to the internal pressure of the pipeline.

3.1 The influence of the contact stress of the sealing surface on the sealing performance The sealing performance of the pipeline mainly depends on the contact stress of the contact surface between the pipe squeeze ring and the connected pipeline after connection. Contact stress is the normal load on the contact surface. Only a large enough normal load can form a zero-gap interference surface between the ferrule squeeze ring and the connected pipe, and the contact stress still exists when the fluid in the pipe is at the maximum internal pressure. Therefore, in order to ensure the sealing performance of the joint, the structural size of the pipe hoop and the pipe sleeve must be reasonably designed according to the internal pressure conditions of the pipeline, so that after the joint is connected to the pipeline, a large enough elastoplastic deformation is generated on the contact surface of the joint, even in the pipe At the maximum internal pressure, an interference surface is still formed on the contact surface, which can prevent the leakage of the fluid in the sealed tube and achieve a reliable sealing effect.

3.2 The effect of surface state on sealing performance The parameters characterizing the sealing surface performance state include roughness, flatness, surface texture direction and non-continuing state, etc. These parameters all affect the sealing performance of the pipe joint, of which the roughness and surface texture have a greater impact Big. Roughness refers to the difference between the micro peaks and micro valleys and the average value, which is the result of the material cutting method (turning, grinding, cutting). Since the reduction of roughness will increase its production cost, the roughness is generally taken as a compromise of 0.8 ~ 1.6m in processing. Different processing methods cause different surface texture directions, which are divided into unidirectional, multidirectional, Radial and circumferential. Because the mechanical joint of the pipeline is a rotator, the circumferential texture is beneficial to enhance the sealing performance. If the circumferential texture cannot be formed, the processed surface must be made smoother to achieve the same sealing effect.

4 Finite element analysis and results 4.1 Establishment of the finite element model of the pipe joint Marc software is an advanced finite element software that handles the problem of highly combined nonlinear structures, thermal and other physical fields and coupled fields, and its outstanding feature is the ability of nonlinear analysis H.Marc calculation The analysis process is mainly composed of model establishment, mesh division, loading solution and result display. S. A finite element model of the pipe joint is established (as shown).

Finite element model of pipe joints The finite element analysis of the working physical process of pipe joints is a complicated problem. During the work process, plastic strain may be generated locally, and there will be contact between the pipe and the pipe joint, and between the inner and outer parts of the pipe joint. Friction stress will also be generated on the surface 63. Therefore, the entire analysis process includes not only material nonlinearity, state nonlinearity, but also friction, but in order to make the solution process simple and smooth, friction is ignored. Since the pipe joint is an axisymmetric structure, in order to make the solution process easier, a 2D model can be taken from its cross section, where the pipe model parameters are standard pipes, the elastic modulus of 7 is 210GPa, and the Poisson's ratio is 0.3. The pipe joint material is Q345 (16Mn), the material density is 7.85gcm3, the elastic modulus is 210GPa, the Poisson's ratio is 0.3, the yield strength is 345MPa Compression performance, the finite element simulation analysis of the pipe joint in the process of resisting internal pressure is based on the crimping process. During the pressure filling process, a certain pressure load is applied outward from the inner surface of the pipe. The size of the load increases linearly with the entire internal pressure resistance process until the pipe joint is subjected to internal pressure, which causes a gap in the contact surface and the seal fails.

It is the equivalent strain cloud diagram of the pipe joint before and after charging.

(A) is the equivalent strain cloud diagram of the pipe joint before the pressure is filled. It can be seen from the figure that the pipe hoop and the pipe are close to each other, and the extrusion ring of the pipe hoop is embedded in the surface of the pipe. At this time, the contact between the extrusion ring and the pipe There is no gap in the face.

As the internal pressure load increases, the wall thickness of the pipe hoop near the interface of the two connected pipes is relatively thinner, so the expansion deformation occurs first, and the gap between the pipe hoop and the pipe gradually expands, and the gap The area slowly extends from the interface to the sealing surface between the pipe squeeze ring and the pipe (as shown by the arrow in (b)). In practical engineering applications, when the sealing surface formed by the extrusion ring begins to have gaps, a leak channel is formed at the contact surface, which cannot effectively prevent the fluid from flowing out of the tube, resulting in the complete failure of the seal.

Since the internal pressure resistance of the pipe joint mainly depends on the contact stress of the sealing surface between the pipe squeeze ring and the pipe, the contact stress can be analyzed to determine the internal pressure resistance of the pipe joint. Shown is the relationship curve of contact stress and time on the sealing surface. Before charging, there is a larger contact stress. With the linear increase of the internal pressure load (as shown), the contact stress begins to decrease continuously. It should be the gap generation stage; by the 40s, the contact stress reached the minimum, at this time, the tight coupling between the pipe squeeze ring and the pipeline has been unable to achieve close contact, and leakage may occur; afterwards, the contact stress value rises, which may be the pipeline The impact force after the internal fluid leaks or the internal stress caused by the deformation of the pipe joint itself. Therefore, when the minimum contact stress value appears, the pipe joint is most likely to reach the limit of the pressure resistance, and the corresponding internal pressure load is 18MPa. The contact stress and time curve on the sealing surface is numerically simulated by the relationship curve of the pipeline internal pressure load and time The limit internal pressure of the pipe joint is 18MPa. When the internal pressure exceeds 18MPa, the seal fails, and the pipe joint is developed according to the technical requirements of withstanding the maximum internal pressure of 15MPa. Compared with the numerical simulation results, the maximum work of 15MPa The internal pressure value is more conservative. Therefore, according to the above data, the mechanical joints and pipes of the above-mentioned design size can withstand the maximum working internal pressure during the working process, and the sealing performance is reliable.

5 Concluding remarks The design of the mechanical joint of the pipeline uses 0.75 times of the casing (turning down to page 31), generally directly input 1.0 times; if it is in the state of strain hardening, it should not exceed 2.0 times.

When entering the bellows wavelength, the value should be calculated according to the formula n + 2L4.

When entering the fatigue life safety factor and the number of fatigue cycles, the relevant values ​​in 1 should be referred to, taking the factor 15 and the cycle number as 75. When the expansion joint is compressed, the distance of H should not be too small, it must be greater than its contraction If the volume is too small, the deflector will collide with the expansion joint, so that the expansion joint cannot play its due role in expansion.

When inputting data, only input "nominal thickness before forming", not "minimum effective thickness after forming", otherwise the calculation result is that the thickness of the expansion joint is too small and unsafe.

5.4 Calculation result of the program Each stress above 166.5MPa passes the stress check. p = 0.2MPa MPa, no fatigue life check is required.

6 Conclusion According to the influence of each parameter on the performance of the expansion joint, each parameter can be intuitively adjusted to improve the stress situation, and a more qualified expansion joint is designed.

Although the design of the expansion joint is cumbersome, as long as the main performance indicators can be found, according to the relationship between each parameter and the performance of the expansion joint, and in combination with the mold of the manufacturing plant, a qualified expansion joint can be manufactured.