8. ROTOR DYNAMICS AND N.P. DESIGN CRITERIA

 The greatest effort in the design of centrifugal compressors mainly for high pressure applications is at present devoted to problems connected with the lateral stability of the rotor. The stability problem concerns the compressor in all its components, since, as we shall see in detail, all basic parts of the machines contribute to stability: rotor,bearings, oil seals, coupling and all fluodynamic parts such as impellers, diffusers, return channels.

Theoretical prediction and experimental investigation methods made available in recent years have contributed much to the progress in this field. We are referring here to the availability in industry of large computers capable of carrying out very elaborate calculation programmes, and of electronic equipment for detection of vibrations and pressure pulsations (non-contact probes, key-phasors, pressure transducers, real time analysers, etc.) which have allowed more accurate diagnoses.

The measure of mechanical behaviour of a compressor is given by the amplitude and frequency of the rotor vibrations.

The rotor vibration amplitude must not cause contact between rotor and small clearance stator parts (labyrinths), overloading of oil seals, and fatigue stress in the bearings. The frequency of the vibrations is a very important element in evaluating stability of the system.

The vibration may have a frequency corresponding to the machine rotation (synchronous vibration) or a different frequency (asynchronous vibration). Usually in rotating machines both types of vibration can be present.

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