In the last 10 years, major centrifugal compressor manufacturers have been investing in developing technologies to improve their products. Following the increasing demand in terms of performance, efficiency and compactness, the current trend in the compressor industry is to increase the “power density”. One big challenge of this “power density” approach is the increase of the rotational speed which may be related to rotordynamic concerns (e.g. crossing of higher rotor modes, stability). Commonly used in the aircraft gas turbines [16], the squeeze film dampers represent an efficient solution to deal with high vibrations and to ensure stable operation for supercritical rotors. In the Oil & Gas Centrifugal Compressors world, SFDs are not so often utilized by the manufacturers but sometimes chosen by the end users as a retrofit solution when high level of synchronous/sub-synchronous vibrations are experienced in the field. The experimental activities described in this paper represent the authors’ Company effort to validate the behavior of a special, integrated SFD type in order to add this component in the available technology portfolio of a centrifugal compressor using it since the design phase. To accomplish this target, the SFD testing was performed originally at the component level and finally at a system level on a “dummy rotor”, specifically designed to mimic the rotordynamic behavior of a real rotor (e.g. running across both the two first rigid modes and the first bending mode). The main objectives of the testing activity were: to check the benefit of using SFDs in order to increase the rotor system damping, to check the SFD overall operational performances, and finally to validate the rotordynamic predictability of this new rotor system. The system level testing program was performed in a high speed balancing bunker where the rotor was equipped with a magnetic exciter able to deliver sub-synchronous excitation. The main test results which will be described in details in the paper are anticipated here. SFDs showed a significant increase in the damping of rigid modes compared to a baseline configuration (rotor running on traditional journal bearings); the SFDs behavior was fully assessed both from rotordynamic viewpoint (rotor and damper housing vibrations) and from operational viewpoint (oil temperature and pressures directly measured in the damper land); finally the rotor modal damping identification techniques are applied to this highly damped rotor system in order to compare the experiment with the relevant predictions. As a conclusion the testing activity provided the authors’ Company with confidence in the use of this integrated SFD technology and enabled a new option for centrifugal compressor design.

The present paper shows the results of a full pressure stability test on a centrifugal compressor for LNG application. The rotordynamic behavior of the compressor has been investigated during the full load test of the entire compression train. A Magnetic Exciter (ME) able to exert a constant rotating force was installed at shaft end (opposite to the coupling) in order to apply sub-synchronous excitation. In addition to bearings measurement location a measurement plane, equipped with vibration probes, has been introduced at compressor mid-span to gain a better understanding of the rotordynamic behavior (in particular for the first mode) of the machine during full load operations. A traditional stability test has been carried out at different compressor operating speeds exciting the rotor by mean of the ME, in order to identify frequency and logarithmic decrement of the first lateral mode the vibration data have been post-processed by a MDOF technique. Moreover Operational Modal Analysis (OMA) has been performed at the same operating speeds without any external excitation. Rotor was naturally excited by the gas flow inside compressor and the vibration signal has been recorded over proper measurement time windows. Power Spectral Density (PSD) of recorded signals shows a broad band excitation with several harmonic components superimposed while the analysis of coherence between different probes highlights the presence of excited modes in the spectrum. A state-space in time domain algorithm (Stochastic Subspace Identification) has been used to post-process the vibration signal. Natural frequency, damping properties and mode shapes at different speeds have been identified for the excited mode. A comparison between these two different identification techniques has been drawn and a confidence factor for OMA approach is defined disclosing new approaches to the compressor stability test.

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The presence of high subsynchronous vibrations and other rotordynamic instabilities in steam turbines can prevent operation at full speed and/or full load. The destabilizing forces generating subsynchronous vibrations can be derived from bearings, seals, impellers or other aerodynamic sources. The present paper describes the case of an 11 MW steam turbine, driving a syngas centrifugal compressor train, affected by subsynchronous vibrations at full load. After the occurrence of anomalous vibrations at high load and a machine trip due to the high vibrations, the analysis of data collected at the site confirmed instability of the first lateral mode. Further calculations identified that the labyrinth seal at the balance drum was the main source of destabilizing effects, due to the high pre-swirl and the relatively tight seal clearance. The particular layout of the turbine, a passing-through machine with a combined journal/double thrust bearing on the steam admission side, together with the need for a fast and reliable corrective action limited the possible solutions. Based on the analyses performed, adjusting the clearance and preload of the journal bearings could not have ensured stable operation at each operating condition. The use of swirl brakes to reduce the steam pre-swirl at the recovery seal entrance would have required a lengthy overhaul of the unit and significant labor to access and modify the parts. The final choice was a drop-in replacement of only the rear bearing (on the steam exhaust side) with a bearing featuring integral squeeze film damper (ISFD) technology. In addition to being a time efficient solution, the ISFD technology ensured an effective tuning of stiffness and damping, as proven by the field results. The analyses carried out to understand the source of the subsynchronous vibrations and to identify possible corrective actions, as well as the comparison of rotordynamic data before and after the application of the bearing with ISFD technology, are discussed.

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