Rotordynamic Evaluation of Centrifugal Compressor Using Electromagnetic Exciter

Abstract

Since heavier gases exert larger effects on rotordynamic stability, stability evaluation is important in developing or designing high-pressure compressors. To evaluate the rotor stability during operation, an excitation test using a magnetic bearing is the most practical method. In stability analysis, labyrinth seals can produce significant cross-coupling forces, which particularly reduce the damping ratio of the first forward mode. Therefore, forward modes should be distinguished from backward modes in the excitation test. One method that excites only the forward modes, not the backward modes (and vice versa), is the use of a rotating excitation. In this method, the force is simultaneously applied to two axes to excite the rotor in circular orbits. Two trigonometric functions, i.e., cosine and sine functions, are used to generate this rotation force. Another method is the use of a unidirectional excitation and a mathematical operation to distinguish the forward whirl from the backward whirl. In this method, a directional frequency response function that separates the two modes in the frequency domain is obtained from four frequency response functions by using a complex number expression for the rotor motion. In this study, the latter method was employed to evaluate the rotor stability of a high-pressure compressor. To obtain the frequencies and damping ratios of the eigenvalues, the curve fitting based on system identification methods, such as the prediction error method, was introduced for the derived frequency response functions. Firstly, these methods were applied to a base evaluation under a low-pressure gas operation, in which the stability mainly depends on the bearing property. Using the obtained results, the bearing coefficients were estimated. Next, the same methods were applied to stability evaluations under high-pressure gas operations. The destabilizing forces were also estimated from the test results and compared with the calculation results.

Development of Scallop Cut Type Damper Seal for Centrifugal Compressors

“…A high-pressure centrifugal compressor [11] was used to test the effect of the damper seal in a full-scale machine. The Table 2 Conditions for CFD analysis point with carbon dioxide gas operation, the outlet pressure is 19.6 MPa (abs) for an inlet pressure of 6.08 MPa (abs).…”

Experimental Investigation on Rotordynamic Characteristics and Rotor System Stability of a Novel Negative Dislocated Seal

“…Lee [32] introduced a complex number to describe the motion status of the rotor, used the direct frequency response function method to identify parameter, and separated positive and negative frequency precessions. Takahashi et al [33] compared traditional frequency response function recognition with direct frequency response function recognition in the complex field and found that the advantages of recognition with complex field. Although the positive and negative precession can be effectively separated, the model of this method is relatively simple and is only suitable for the occurrence of a single peak in the resonance region.…”

Rotor-System Log-Decrement Identification Using Short-Time Fourier-Transform Filter

“…Rotor stability identification methods for rotor-bearing systems have been investigated by many researchers, and they estimated rotor stability during the shop test based on both frequency and time domain signal analysis [1,2]. Takahashi et al [1], Kocur and Cloud [2], and Pettinato et al [3] did a plenty of work on shop rotor dynamic testing by using the electromagnetic bearing as a shaker to excite the rotor system. Cloud [4] developed the Multiple Output Backward Autoregressive (MOBAR) method for time domain signals and carried out significant work on the rotor stability investigation.…”