Acoustic & Mechanical Dynamics Issues for High Horsepower, High-Speed Compressors in Gas Transmission Service
The first half of the paper presents natural frequency predictions for the suction bottle cantilever mode of a large, high-speed compressor which has dual nozzles, and 48-inch diameter bottles. It shows how the prediction varies widely with modeling assumptions, using both a beam model and three-dimensional finite elements. The results show it is necessary to account appropriately for flexibility of the reinforcement, using shell elements and for internal stiffening of the bottle offered by the baffles to correctly predict the suction bottle cantilever frequency. It is also shown that with an appropriate joint flexibility factor, it is possible to predict the mode accurately using a beam model. A convenient method for control of vibrations at this mode, should it be excited, is presented and demonstrated. The second part of the paper presents predictions of acoustic natural frequencies in the bottle of a large high-speed reciprocating compressor. The paper compares the predictions using 3D acoustic modeling with predictions based on plane wave acoustics. The paper shows that all modes predicted by plane wave acoustics are reproduced closely by the 3D analysis. It further shows that cross-modes are predicted by the 3D analysis for which there is no evidence in the plane wave analysis. These cross-modes occur at a frequency corresponding to high measured vibrations and strains in the suction bottles, thus making such cross-modes an important factor to consider in future acoustic design analysis for high-speed reciprocating compressors. The results also suggest the need for supplementary criteria on pulsation amplitudes within the compressor manifold system of such large systems.
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