THERMOWELL FATIGUE LIFE PREDICTION USING FINITE ELEMENT ANALYSIS

Abstract

The objectives of the research involve developing the procedure for fatigue life prediction using the S-N curve for the thermowell material and investigating the effects of fluid velocity, fluid density and cyclic loading on the thermowell structure using the engineering drawing tool, AutoCAD 2022, and simulation software, ANSYS Workbench 21.1. By comparing the results for the proposed fatigue life estimation to Diwakar’s study on fatigue life estimation of thermowells in supercritical operation, the percent error obtained was 0.11%, which indicates that the proposed method is an accurate and reliable tool to perform fatigue analysis. Afterward, the parametric study of the thermowell model was conducted to investigate the effects of fluid velocity, fluid density and geometric dimensions on the fatigue life of a thermowell. There were six load cases investigated for two thermowells, which were designed based on the minimum and maximum dimensional limit of the thermowell performance test code PTC 10.3 TW-2016. The parametric study results showed that the minimum thermowell performed better for all the load cases, which obtained a maximum number of cycles to failure of 8898 cycles for a maximum stress value of 869 MPa. Furthermore, the higher the fluid velocity and density, the higher the likelihood and occurrences of resonance, the lower the number of cycles to failure. Similarly, the higher the natural frequency of the thermowell, the lower the likelihood and occurrences of resonance; hence, the higher the number of cycles to failure. It was observed that the point where the stress value is highest, or the stress concentration zone, is the area between the flange and thermowell neck, which was consistent with the thermowell performance standard codes of practice, PTC 19.3 TW-2016.
 Keywords: Fatigue analysis, parametric study, resonance, stress concentration zone, supercritical, vortex shedding frequency