Failure Analysis of Impeller Blade Cracking in a Mechanical Vapour Recompressor (MVR) Fan

F2 - Vibrations and Failure Analysis

A Mechanical Vapour Recompressor (MVR) fan installed in a dairy processing plant has a problem with cracking impeller blades. The impeller is a centrifugal type with a two sets of equally spaced blades – one set spanning from inlet to discharge and the other a shorter intermediate set. The shorter blades exhibited failure from two different fatigue regimes over a two year operating period. The first failure, occurring after approximately 150 hours of service following commissioning, was due to oscillatory motion of the impeller in the second mode of torsional vibration, excited by harmonics of the variable speed drive controller. The second failure was due to coupled bending and first mode torsion, due in part to radial clearance in a rolling element bearing. The crack growth mechanisms for both failures were determined by electron microscopy, the first failure being due to a high load - high cycle environment and the second due to high load - low cycle environment. This paper describes the steps taken to identify the possible mechanisms for loading, the testing undertaken to determine system variables, and presents recommendations for avoidance of further failures.