Failure Prevention of Small-bore Piping Connections
Failures of small bore piping connections continue to occur frequently in power and industrial plants, resulting in degraded systems and unscheduled plant downtime. Fatigue-related failures are generally detected as small cracks or leaks before major pressure boundary ruptures occur. However, in many cases, the leak locations cannot be isolated from the pressure boundary and result in forced plant outages.
Prior research has found that the majority of such failures are caused by vibration fatigue of socket welds. Investigations have demonstrated that socket weld leg size configuration can have an important effect on high cycle fatigue resistance.
Longer legs along the pipe side of the weld greatly increase predicted fatigue resistance. Other potentially important factors influencing fatigue life include weld bead sequence, residual stress, weld root and toe condition, loading mode, pipe size, axial and radial gaps, and materials of construction.
Vibration/fatigue testing
To study the importance of these factors, and to confirm analytical predictions, a large number of socket welds samples were vibration-fatigue tested to failure on a high frequency shaker table. The objectives were to improve the understanding and characterization of socket weld resistance to high cycle fatigue, and to develop appropriate fatigue strength standards for such welds reflecting the effects of factors that prove to be significant. The ultimate goal of this research is to develop recommended design and fabrication practices that can be used to enhance socket weld fatigue resistance in vibration-sensitive locations, as well as provide guidelines for screening out and preventing vibration-fatigue failures.
The testing program was carried out in two phases. The first phase, completed in 1998, investigated the variation in high cycle fatigue resistance as a function of weld leg length, pipe diameter, and piping material. The effects of implementing an additional weld pass, applying post-weld heat treatment, and eliminating the axial gap were also studied. The second phase, which has just been completed, investigated remedial actions for existing socket welds as an alternative to expensive modifications.
The effect of varying the conditions was also studied. Additional data at higher loads was collected to quantify the benefit of increasing weld leg length. Testing results confirmed the benefit of increasing the pipe side weld leg length and determined fatigue strength reduction factors for various socket weld configurations. It was also demonstrated that applying a weld overlay repair to a leaking weld fully restored its fatigue life, and building up a standard Code weld in-situ to a longer weld leg length provided results as good as a new, longer leg weld.
EPRI sponsorship
The Electric Power Research Institute (Palo Alto, CA) sponsored this work, primarily for application in nuclear power plants. Structural Integrity Associates, Inc., an HSB company, has been providing engineering consulting and inspection services to the utility and other industries for over 16 years, primarily in the areas of structural analysis, failure prevention and repair.
Edited by Nick Basta
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