Wachel's Equation - Origin And Current Evaluation Of API 617 Rotor Stability Screening Criteria

Abstract

To satisfy API standard 617 (2002) stability requirements, the design of centrifugal compressors must be analyzed against the Level 1 screening criteria for self-excited subsynchronous instability of the rotor lateral motion. Two criteria are given. The first compares a measure of anticipated excitation to the excitation required to predict an zero logarithmic decrement. The second compares the subject machine to a historically developed demarcation between successful and proven problematic machines. Failure to satisfy either of these two criteria requires further stability investigation be performed according to a Level 2 analysis. The two Level 1 criteria can be shown to be related. The prescribed excitation calculation employed for a Level 1 analysis has been derived from an empirical equation first published by Wachel and von Nimitz (1981.) This paper discussed the origins of Wachel’s Equation and illustrates by example the relationship between the two API criteria. The need to consider additional distinctions between rotors intended for different services when screening for Level 1 acceptance is shown.