On the Predicted Performance of Oil Lubricated Thrust Collars in Integrally Geared Compressors

Abstract

Integrally geared compressors (IGCs) comprise of single stage impellers installed on the ends of pinion shafts, all driven by a main bull gear (BG) and shaft system. When compared to single shaft multistage centrifugal compressors, the benefits of IGCs include better thermal efficiency, reduced footprint and simple foundation, dispensing with a high speed coupling, as well as better access for maintenance and overhauls. In IGCs the compression of the process gas induces axial loads on the pinion shafts that are transmitted via thrust collars (TCs) to the main drive shaft and balanced by a single thrust bearing. The TCs, located on either side of pinion gears, slightly overlap with the BG outer diameter to form lentil-shaped lubricant-wetted regions. Archival literature on the design and optimization of TCs is scant, in spite of their widespread usage as they are comprised of simple geometry mechanical elements. This paper presents an analysis of the hydrodynamic film pressure generated in a lubricated TC due to the rotation of both thrust collar and bull gears and specified taper angles for both bodies. The model solves the Reynolds equation of hydrodynamic lubrication to predict the operating film thickness that generates a pressure field reacting to impellers’ thrust loads; these forces being a function of the pinion speed and the process gas physical properties. The model also predicts performance parameters such as power loss and axial stiffness and damping force coefficients. A parametric study brings out the taper angles in the TC and BG that balance the transmitted load with a lesser friction factor and peak pressure, along with large axial stiffness and damping.