Natural Gas Overhung Pipeline Compressor Rotordynamic Design, Test Validation, and Consideration of Morton Effect

This study examines a natural gas pipeline compressor design and its potential for “Morton effect” instability. The compressor is designed to utilize a rotor with two tandem stages arranged in an overhung configuration. An overhung rotor design allows for increased compressor efficiency by lowering the inlet losses in the compressor, eliminating a balance piston, and minimizing leakage by having only one dry gas seal. Comprehensive rotordynamic analysis was performed on the rotor-bearing system accompanied by an iterative design optimization. The rotordynamic unbalance response and stability characteristics were good and met the API requirements. Another important aspect “Morton effect” for overhung rotor was also carefully examined because the rotor design has large overhung weight, long overhung length and high bearing surface speed making it subject to a high Morton effect (ME) risk. Bearing optimization, for example, using offset pivot tilting pad bearing and adjusting bearing clearance was performed to minimize the ME risk through the ME analysis. Eventually a test rotor was built to experimentally validate the rotordynamic performance of the worst cases rotor-bearing design, which had minimum clearance at the bearings. The rotor was operated up through its trip speed. Testing was performed to validate that the rotor design is rotordynamically sound and there is no ME within the operating speed range of the rotor. An attempt was then made to induce ME by altering the bearing configuration from the intended design to one where ME would be more likely. Larger amounts of unbalance were applied. Some additional amplitude and phase change was observed; however, this did not become the classic ME instability.