CST16 - A Systematic Method For Determining the Source of Unbalance in a Rotor Assembly

Abstract

Unbalance is present in rotor systems due to manufacturing and assembly tolerances which results in synchronous vibration during operation. Adherence to API guidelines relative to acceptable levels of rotor vibration is important to ensure a satisfactory design. These vibration levels are a function of the cumulative effect of unbalance of all rotor components. When elevated vibration levels are observed during full speed operation, it is important to identify the contribution of the major rotor components to the overall vibration levels in order to take proper corrective action to bring the rotor into API compliance. A real world example is presented which involves a multistage compressor with dry gas seals which demonstrated elevated vibration levels during Factory Acceptance Testing. The coupling and dry gas seal assemblies were independently clocked 180 degrees. From vector calculations, the contribution of each component to the overall vibration was assessed. From this data, the magnitude and orientation of component unbalance was determined from the rotor dynamic analysis. In this instance, it was found that the coupling had the greatest influence. After the coupling was rebalanced, the compressor was retested and the vibration satisfied API requirements.