Pump Tut - WHY MACHINES FAIL TO FOLLOW TEXTBOOK STATED RULES, AS ILLUSTRATED BY SHAFT CRACK AND ROTOR SELF EXCITED VIBRATION CASES IN CRITICAL MACHINERY

Abstract

This tutorial demonstrates some aspects of rotating machinery diagnostics where there is an apparent difference between the actual response of a machine and its expected behavior for a certain type of malfunction as typically found in industry analyst training courses. This tutorial concentrates on two of the most devasting malfunctions: shaft cracks and fluid film instabilities. The seriousness of these two malfunctions demands the immediate attention by the analyst to quickly determine before the potential catastrophic and terminal results. The reason of selecting these two is because both are serious and potentially terminal problems. From another perspective these malfunctions are on opposite ends of the vibration based diagnostics path. The shaft crack grows slowly and is a forced vibration problem related to the condition of a single machine part (rotor), whereas fluid film induced instabilities are invariably an instantaneous phenomenon that causes self-excited vibrations of high enough to trip the machine. However, a common factor for both malfunctions is that a maintenance experts say these malfunctions are relatively rare phenomena. As a result, to identify faults correctly, diagnostic engineers must rely on analyst training courses and published case history literature, rather than on personal experience. For both malfunctions we start with a short presentation of the diagnostics methodology, as used in the service organization the authors are employed, followed by a discussion of the practical implementation of this methodology along with selected machinery case studies.