| dc.description.abstract | Lecture 4: The gas flow entering from the inlet flange of a centrifugal compressor may induce radial forces on the rotor, by directly impinging onto shaft or by generating asymmetric pressure fluctuations in the gas volume surrounding the shaft. This aerodynamic excitation represents a potential source of radial vibrations for the rotor; it is characterized by a broad-band, low frequency spectrum and its effects are typically observed to increase when the operating point moves towards the top right part of the operating envelope, i.e. at high operating speed and high inlet flowrate. The severity of the vibrations caused by this airborne excitation depends on the thermodynamic parameters of the inlet gas (flowrate, velocity, density) and on the mechanical design of the compressor (geometry of the suction volume, stiffness of rotor and journal bearings). While the former are generally constrained by process requirements, the latter can be optimized in order to minimize the induced vibrations. Moreover, a very effective way to mitigate this phenomenon is the adoption of Inlet Guide Vanes at compressor suction that can protect the shaft from exciting forces; on the other hand the installation of IGVs increases the cost and the mechanical complexity of the design, and may also have a non-negligible impact on performances, causing a pressure loss. For this reason it is useful to define a screening criterion to decide whether to install IGVs, based on the predicted sensitivity of the rotor to inlet gas flow excitation. Such criterion was developed basing on physical considerations and then validated through application to a set of recorded references from compressors tested in full load. The outcome allowed to build a diagram based on compressor geometry and operating data, where a boundary line separates the safe area (no significant vibration expected) from the area where the application of IGVs is suggested. | en |
