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Screw Compressor Torsional Analysis 191115

The Twists and Turns of Screw Compressors

Reliable operation of rotating machinery requires thorough examination of its vibration characteristics early in the design stage. Usually a lateral rotordynamic study is performed to figure out the critical speeds. What should not be neglected is the torsional modes of these machines. Torsional resonance is notoriously known as the silent killer of rotors, often undetected untill the moment of failure.

Torsional Analysis of a Screw Compressor

KHE was requested to analyze an oil-flooded screw compressor for its torsional modes and their modal proximity to excitation sources. Typically the strongest forcing function for screw compressors are identified as the pocket pass frequency and its harmonics. In this particular case, the rigidity in the screw compressor allowed us to simplify the analysis for the fundamental modes by considering the compressor as a lumped inertia connected to the flexible coupling and the torsionally softer drive train shaft. A rotordynamic model was set up per API 684 recommendations for torsional analysis. The rotor model was cross-checked for inertia and stiffness to match the manufacturer data.

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The first few torsional modes were calculated and plotted against the excitation sources in an interference diagram (Campbell diagram). Results showed an available margin of 7% for the fundamental mode from the male rotor and motor frequency. API stipulates that a minimum separation margin of 10% is required for safe operation of machinery. In this case, a minor reduction in the coupling stiffness was enough to decouple the fundamental mode from excitations, considering the heavily damped response by the virtue of the oil-flooded compressor.
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