Centrifugal Compressor Train Torsional Vibration Anal-ysis; The Effect of Shaft-Coupling Penetration Ratio

When designing typical rotating equipment, torsional natural frequencies (TNFs) shall be studied to identify the possible resonances. To appropriately compute the TNFs and torsional modes, the effect of the shaft penetration in the coupling hub, known as the shaft penetration factor (SPF), on the torsional stiffness calculation must be considered. In this paper, a com-prehensive code for the torsional vibration analysis of a centrifugal compressor train is de-veloped by the rotor-dynamics team of the OTCE company. The torsional analysis of an elec-trocompressor train is then modeled, based on API 617 criteria in order to examine the effect of the SPF value on the torsional output of the model. Two assumptions regarding the pene-tration of the shaft in the coupling hub are considered. Assumption #1 is “end-to-end connec-tion of the shaft and the coupling,” and assumption #2 is “1/3 penetration of the shaft in the coupling hub.” The effects of these assumptions on the torsional behavior of a real centrifu-gal compressor train, designed by OTCE, are investigated. As the results show, considering 1/3 penetration instead of end-to-end connection causes the value of 2nd and 4th unrigid TNFs to increase by 17% and 12%, respectively. The corresponding mode shapes also show notice-able differences between the two assumptions. From the Campbell diagram, it is observed that the intersections of the 2X line with the 2nd and 4th unrigid TNFs fall near and inside the API separation margin. While modeling the compressor using the 1/3 penetration assump-tion, the mentioned intersections are kept away from the critical speed range. Consequently, it can be said that the flexible couplings’ stiffness precise modeling is a crucial point to cor-rectly address the compressor train torsional natural frequencies. Minor uncertainties in the SPF modeling may lead to noticeable differences in the equipment’s torsional behavior.

Centrifugal Compressor Train Torsional Vibration Anal-ysis; The Effect of Shaft-Coupling Penetration Ratio Keywords:

Centrifugal Compressor , Torsional Vibration , Flexible Coupling , Torsional Nat-ural Frequencies.