Failure analysis and fatigue performance assessment of titanium diaphragms used in sonic soot blowers

Abstract

Sonic soot blower, utilizing an oscillating diaphragm, generates acoustic waves for cleaning of equipment such as boiler and furnace to increase their efficiency. Based on earlier research by NRI (Niroo Research Institute), one of the main reasons disrupting the regular function of sonic soot blower is the failure of the diaphragm. The failure of circular Ti-6Al-4V sheets employed as the diaphragms is investigated by using SEM images. Based on these images, fatigue failure is found in the governing failure mechanism. A modification to the arrangement of the blower is proposed by introducing a gap between the diaphragm and its seat to improve the fatigue performance. A computational fluid dynamics (CFD) analyses are performed using the Ansys CFX software for estimation of pressure distribution on the diaphragm during service loading, and the results are introduced to the finite element (FE) model developed by the Abaqus software for calculation of stresses induced in the diaphragm. Variation of Mises stress at the center of the diaphragm, where the maximum stress developed, is extracted from the FE model for one period as a stress spectrum for one operation interval. A stress-life approach is employed to assess the fatigue life of the diaphragm subjected to estimated stress spectra. According to the prediction, increasing the gap size by 0.1 mm results in extension of life up to 2.75e10 spectrum passes, which are significantly higher than the desirable lifetime (5 years) of the diaphragm. Further increase of gap size leads to higher fatigue life, however, considering the decrease of efficiency as the gap size is increased, the gap size of 0.1 mm would be the optimum configuration.