When an inner flange is connected to a pipeline, stress concentration is a critical factor affecting the safety and reliability of the system. Stress concentration typically occurs in areas of abrupt geometric changes or uneven load distribution, such as the flange-to-pipe joint, weld edges, or around bolt holes. If not effectively controlled, local stress may far exceed the material's yield strength, leading to fatigue crack initiation, seal failure, or even structural fracture. Therefore, comprehensive measures must be taken throughout the entire process of design, manufacturing, installation, and maintenance to reduce the risk of stress concentration.
Optimizing structural design is the primary step in avoiding stress concentration. The geometry of the inner flange should avoid abrupt changes as much as possible; for example, using smooth-transition rounded corners instead of right angles reduces distortion of force flow lines. For high-pressure or large-diameter pipelines, weld neck flanges can be selected, as their neck design can distribute bolt preload and reduce stress peaks at the sealing surface edges. Furthermore, the difference in wall thickness between the flange and the pipeline must be controlled within a reasonable range to avoid localized stress concentration due to abrupt changes in stiffness. If a change in wall thickness is unavoidable, a gradual transition structure should be used to achieve a smooth stress distribution.
Manufacturing accuracy directly affects the degree of stress concentration. The sealing surface of the inner flange must be kept highly flat, and its surface roughness must meet standard requirements to reduce local stress concentration when the gasket is under pressure. The machining of bolt holes must be strictly controlled in terms of position to avoid uneven bolt preload distribution due to hole misalignment, which could lead to flange warping or sealing surface deformation. For welded flanges, the weld reinforcement must be ground flush with the base material, and the weld toe should be smoothly rounded to eliminate sharp corner defects. Furthermore, defects such as cracks and porosity must be avoided during welding, as these defects can become sources of stress concentration.
The installation process is crucial for controlling stress concentration. When the inner flange is connected to the pipeline, coaxiality and perpendicularity must meet specifications to avoid additional stress caused by forced alignment. The bolt preload sequence should follow a symmetrical, staggered principle, tightening gradually and evenly to prevent localized flange overload. For high-temperature or low-temperature conditions, thermal expansion differences must be considered, with appropriate gaps provided or expansion joints used for compensation to prevent flange deformation due to thermal stress. If there are vibration sources in the piping system, vibration damping devices should be added at the flange connection to reduce the impact of dynamic loads on sealing performance.
Material selection and heat treatment processes are essential for reducing stress concentration. Inner flange materials must possess sufficient strength and toughness to resist localized high stress. For environments prone to stress corrosion, corrosion-resistant alloys or surface protection treatments are necessary. After manufacturing, welded flanges require heat treatment to eliminate residual welding stress and improve material microstructure. For example, overall high-temperature tempering can reduce residual stress and mitigate the impact of stress concentration on fatigue life. For complex structures, localized heat treatment or vibration aging techniques can be used to specifically eliminate stress in critical areas.
The design of the sealing structure must balance sealing performance and stress distribution. The selection of gasket type and material must match the characteristics of the medium and pressure/temperature conditions. For example, spiral wound gaskets are suitable for high-temperature and high-pressure conditions; their multi-layered structure can disperse pressure and reduce stress concentration at the sealing surface edges. For non-metallic gaskets, the compression rate must be controlled to avoid gasket creep or sealing surface damage due to excessive compression. Furthermore, the sealing surface form (e.g., raised face, recessed face) must be coordinated with the gasket type to ensure uniform pressure distribution.
Regular inspection and maintenance are the last line of defense against the deterioration of stress concentration. Regularly inspect critical areas such as welds and bolt holes of flanges and pipelines using non-destructive testing techniques (such as ultrasonic testing and magnetic particle testing) to promptly detect and repair defects such as cracks. Monitor bolt preload changes to prevent uneven stress on the flanges due to loosening. For operating pipeline systems, strain gauges or vibration analyzers can be used to monitor stress states, assess stress concentration risks, and provide a basis for maintenance decisions.
When connecting inner flanges to pipelines, avoiding stress concentration requires a multi-dimensional approach involving design optimization, manufacturing control, installation specifications, material handling, sealing design, and maintenance inspection. By reducing geometric abrupt changes, controlling manufacturing defects, uniform load distribution, improving material properties, and strengthening monitoring and maintenance, the risk of stress concentration can be significantly reduced, ensuring the long-term safe and reliable operation of flange connections.
