Flow induced fatigue of intrusive pipe fittings such as thermowells

Pipe fittings in many industries are typically in circular cross-section of various axial styles: typically straight, stepped or tapered. They are fixed at one end (cantilevered) and the free end is typically inserted half way into the pipe. Fluid flow causes various forces leading to fatigue failure in some situations. The most common failure types are due to vortex shedding induced resonance or varying drag forces, which can be analysed using analytical models. Thermowells are intrusive fittings of particular interest as failure may result in loss of containment. This paper develops analysis of vortex shedding resonance and varying drag forces for general straight, tapered and stepped thermowells. Detailed testing of thermowell natural frequencies was performed to confirm the accuracy of the natural frequency calculations used; finite element analysis (FEA) was also performed for comparison. Case studies of a retrospective design assessment and a failure analysis are also presented.