FULL LOAD STABILITY TEST (FLST) ON LNG COMPRESSOR

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 midspan 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.