Condition Monitoring, Troubleshooting and Reliability in Rotating Machinery

11Precise Coupling Properties are Required to Accurately Predict Torsional Natural Frequencies

By Robert X. Perez

Introduction

Torsional vibration is the twisting oscillation of a drive train about its axis, which occurs to some degree in rotating equipment trains, composed of reciprocating compressors, engines, turbines, and electric motors. Torsional vibration can be detrimental to rotating equipment because the related torsional stresses are superimposed on the static torque already present in power transmission shafts. The combined stresses can result in extremely high levels that can lead to catastrophic failures. Strain gauges and telemetry equipment can be used to measure torsional stresses in rotating equipment during operation. This brief case study describes a real torsional problem that arose from inaccurate coupling properties that resulted in poor torsional modeling results.

Case Study

An analytical torsional analysis of a two motor-driven compressor drive train was performed to ensure there would be acceptable margins (+/-10%) between mechanical and electrical torsional excitations and the frequency of the train’s torsional modes of vibration at its design speed.

The natural gas compressor drive train consisted of an 8000 Hp, 720 rpm synchronous motor directly coupled to a 6 throw reciprocating compressor (see Figures 11.1 and 11.2). During the torsional vibration analysis, various coupling and compressor-mounted flywheel assembly configurations were evaluated ...