Failure analysis of an impeller blade

A multi-stage low-pressure centrifugal air compressor used in the utility section of a manufacturing plant tripped. An increase in vibration was recorded prior to the breakdown. Subsequent inspection revealed that a small part of one blade had fractured and separated from the impeller body. The failure occurred in the 3rd stage impeller blade employed in a three-stage compressor with two interstage cooling tanks. The air compressor had been in service for 18 years while the current impeller had been used for 18 months. Impeller was made of 17-4PH steel. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and Vickers hardness testing were used to metallurgically evaluate the broken blade and corrosion deposit obtained from cooling tank. Experimental results indicated that the mode of failure of the impeller blade was corrosion fatigue. Hardness of impeller material was also found to exceed the upper limit required to use it in sulfur bearing environment. Cause of failure can be attributed to intense vibrations in the compressor system responsible for the generation of stresses that led to the fatigue failure of impeller blade already embrittled due to improper heat treatment and exposed to an environment made corrosive due to the presence of sulfur.