Failure analysis of atmospheric relief diaphragm in the low pressure steam turbine

In this study, the causes of failure and prevention methods for the atmospheric relief diaphragm in a low-pressure steam turbine were evaluated. The atmospheric relief diaphragm is designed to vent and burst at its center when the pressure in the exhaust housing exceeds 0.7 barG due to steam. However, visual inspection revealed that failure occurred along the flange mounting, rather than at the center of the diaphragm. Fatigue failure was identified as the cause, based on SEM analysis. Internal pressure fluctuations in the turbine caused bending along the fixed edge of the diaphragm. To confirm the occurrence of fatigue failure, finite element analysis was performed using a maximum internal pressure of 0.023 barG and a frequency of 60 Hz. The maximum deformation was 18.8 mm, which is less than the distance between the diaphragm and the knife, 30 mm. However, the maximum stress was 23.7 MPa, which exceeded the fatigue strength of A1050, 15 MPa, with stress concentration at the diaphragm edge, confirming edge fatigue failure prior to central knife failure. To identify an alternative material, finite element analysis was conducted using higher fatigue strength than A1050 under the same conditions. While the maximum stress was similar to that of A1050, it was below the fatigue strength of the alternative materials. Consequently, to prevent unexpected failures, it is recommended to use materials with higher fatigue strength than A1050 to enhance fatigue life of the diaphragm.