Stability Improvement for Upgraded Four-Stage Centrifugal Compressor Rotor-Bearing System

As the plant capacity is increased, the equipment in the plant also needs to be upgraded. Replacing with a new machine is not always the right economical decision when alternative options for upgrading and retrofitting the exiting machine exist. Upgrading the machine not only increases the performance thus reducing the power consumption, but it also increases the system reliability, reducing the downtime and thus helping the plant operate more efficiently. The compressor used for this work was in operation, and due to prolonged operation, the impellers had degraded primarily due to fouling caused by process contaminants. This multi-stage centrifugal compressor was subsequently upgraded to increase the inlet volume flow rate by 20% with the objective of retaining the same casing and rotor shaft. The redesigned compressor stages has better performance with reduced power consumption. The redesigned four-stage centrifugal compressor operating at design speed of ~8856 rpm may be prone to instabilities caused by aerodynamic cross-coupling and hydrodynamic bearing modification. The authors in the present study are exploring any instabilities in the rotor-bearing system that may be caused due to the upgrade of the compressor for higher volume flow rate. To meet the new operational requirements, the bearings were redesigned. The stability analysis was performed considering the destabilizing effects of the aerodynamic excitations, based on which modifications were performed on the design to stabilize the multi-stage centrifugal compressor and to conform to satisfy the API Level I stability criteria. The modifications performed on the rotor/bearing system resulted in reduced vibration amplitudes for the required level of rotor unbalance conditions. These modifications also significantly eliminated the instability that was initially observed, and the redesigned compressor not only met the requirements for the change in operating conditions but also fully complied with API 617 standards.