Changes in rotor response characteristics based diagnostic method and its application to identification of misalignment

Misalignment is one of the commonly encountered faults that generate excessive rotor vibration. The diversity of its characteristic frequency brings great trouble for fault diagnosis. Especially when misalignment-induced vibration manifests it as only synchronous (1x) component, properly diagnosing this fault becomes more difficult, since other faults often show the similar frequency signature. To extract discriminative information from the view of fault development, a diagnostic method is proposed with comprehensive utilization of changes in rotor static and dynamic response characteristics. First, rotor static and dynamic responses are deduced, the average rotor centerline (ARC) plot and orbit plot are introduced separately to represent these responses. The results indicate that ARC position changes considerably and 1x-orbit synchronously becomes more flattened with the increase of misalignment, when all machine operating conditions remain constant. Combination of these symptoms can be used to better distinguish misalignment from other 1x vibration problems related to changes in unbalance force, e.g. rotor unbalance and temporary thermal bend. Subsequently, the combination plot and its feature trend plot of ARC and 1x-orbit are developed to fully describe the changes in rotor static and dynamic responses with time and their correlation. Finally, the proposed method was successfully applied to 1x vibrations of a NOx compressor and a flue gas expander.