Rotor Stability Criteria for Multi-Stage Centrifugal Compressors
The American Petroleum Standard 617, 7th edition [1], screens new designs of centrifugal compressors using its Figure 1.2-5, which plots mean gas density versus rotor flexibility ratio. The purpose is to avoid rotor instability from causing unacceptable vibrations, typically whirling of the first-forward bending mode. The API figure is based on a plot by Fulton [2,3] of the same variables showing a number of compressors tested near the threshold of instability, thus forming an empirical basis for a Stability Map. This paper develops a method to derive a threshold line, using a set of typical industrial rotors varying in flexibility and running on tilt pad bearings. Instability requires a tangential force (normal to the bending displacement) to act on the rotor in the direction of whirl. The source of this force is taken to be the (tooth type) labyrinth seals inside the compressor. This tangential force is represented as linear function of whirl/spin scaled by gas density. This function is applied to the set of rotors to produce a threshold line. The derivation thus forms a basis for understanding the Stability Map.