FAILURE ANALYSIS OF THE IMPELLERS OF A FEED PUMP

An eight-stage feed pump in a urea-manufacturing plant started showing a high overall vibration level 3 years after commissioning. The level continued to remain high for about 2 years. One day the pump seized a nd had to be taken out of service. Visual examination and NDE showed t hat three pump impellers were broken and most of the impellers had one or more vanes with cracked trailing edges. Chemical examination of im peller material did not show any serious irregularity regarding its ch emical composition. Metallographic examination confirmed that the impe llers were not subjected to proper heat treatment. A scanning electron microscopic examination of the crack in the trailing edge revealed st riations. It confirmed fatigue to be the mechanism of cracking. Severe pressure pulsations caused cyclic loading, which was mainly responsib le for crack initiation and its initial propagation. Further propagati on led to breakage of the impeller. Measures to reduce the pressure pu lsations and improve the fatigue strength of the impellers were sugges ted.