2021 RETA Breeze March-April

in a vibrational failure, as the sharp valley at the bottom of the thread acts as a “stress riser”which concentrates applied stress to a single point. In summary, the failure of the pipe nipple was associated with vibration induced fatigue crack formation and propagation. The relatively rapid initiation and progression of the fatigue cracks was likely accelerated by the fact that the nipples are supported on one end and have relatively concentrated mass at the opposite end, forming a cantilever- like structure that imparts significant stress at the thread roots where the fatigue cracks initiated. Another contributing factor is that the frequencies of vibration vary with the speed of the compressor. Even though the compressor itself has been tested and shows acceptable amounts of vibration, the specific weights and lengths of the assemblies pictured above would have a completely different set of resonant frequencies, much like tuning forks of various lengths do. Ideally one could analyze the vibration of the assemblies; In reality this is not possible, as the installation of the vibration sensor would add

2. Increase the mass/strength by designing and manufacturing a replacement fitting for the pipe nipple. Note that the facility has already taken this approach, and the initial results appear to be promising. The following photos are of a part that facility staff had a local machine shop designmake, along with a photo of one after installation (Right page): A few notable features to about this solution: • 304 stainless steel was used. This material has excellent corrosion and good fatigue resistance. • The fitting eliminates the use of a ¼” nipple that was screwed into a ½”x ¼”NPT reducing bushing. • The hole through the fitting was kept very small, providing extreme wall thickness for increased part strength. • Generous fillet radii were provided at each profile change to eliminate stress risers. The second category is to change the physical configuration to reduce

There are several measures that can be taken to reduce the risk of recurrence of this failure. These solutions fall into two main categories: The first category involves adding strength or design features that make the assemblies more resistant to fatigue, in other words, better able to withstand the vibration they are experiencing: 1. Select pipe nipples with rolled threads as opposed to cut threads. This is best explained by the illustration above In summary, rolled thread pipe nipples are better because: • The wall thickness of the nipple is not reduced by the threads • The forming process provides increased metal strength in the threaded area by re-forming the grain of the metal instead of cutting through it. • The bottoms of the threads will have a slight fillet radius, reducing or eliminating the stress riser seen at the bottom of a cut thread.

Solutions

mass and alter the resonant frequencies of the assembly.

Key Take Away

Vibrational fatigue can lead to rapid failure of materials, especially those with stress risers such as threaded sections. The risk of vibrational fatigue is increased for components that are only supported on one end.

22 RETA.com