STALL INDUCED AERODYNAMIC FORCING AND ROTOR VIBRATIONS IN A MULTISTAGE CENTRIFUGAL COMPRESSOR

LectureThe Oil & Gas industry is looking with increased interest at solutions for improving operating flexibility of centrifugal compressors. The stable operation of a compressor stage or machine is generally limited at the left of operating range by the occurrence of a local aerodynamic unsteady phenomenon, the rotating stall, which usually precedes the surge. Rotating stall could cause, depending on the actual operating conditions, severe sub-synchronous vibrations to the rotor which may compromise rotordynamic behavior, preventing the machine from operating at very low flow rates. An accurate characterization of rotating stall phenomena, and their impact on rotordynamic stability, may represent an important step forward in centrifugal compressor design and performance predictability, insofar as it allows to correctly predict the real operating range of the machine. In recently published works the authors presented a procedure which allows reconstructing the pressure unbalance due to the diffuser rotating stall, to estimate the rotating force acting on the shaft and, by means of a rotordynamic model, the vibration at the bearings. In addition to this, a criterion to scale the rotating force coming from model test conditions up to fullscale machine conditions has been developed and successfully validated. In this framework a thorough work has been performed to apply the aforementioned procedure to an LNG multistage compressor. Firstly, the stages which equip the machine were tested as single scaled-down stages in a model test rig, in order to fully characterize their dynamic behavior while approaching the left limit and operating in stall condition. Then, the full scale machine has been equipped with dynamic pressure probes in different locations along the gas flow path, and has been tested according to ASME PTC-10 standard; this allowed to capture the stall inception and its evolution and finally to get the rotating pressure pattern acting on the rotor. A noticeable agreement was obtained between the force resulting from the pressure field integration and the one obtained through a proper scaling of the test data. Finally, the calculated stall force has been used as an input in a rotordynamic model of the whole compressor: the predicted Subsynchronous Vibration (SSV) estimated at the displacement probe location has been compared with the measured value showing some differences which can be related to the proximity of the first rotor mode