Practical Approach for Solving Vibrations of Large Turbine and Generator Rotors - Reconciling the Discord between Theory and Practice

Abstract

The purpose of this paper is to illustrate a different perspective in viewing and solving vibration problems in large rotating machines regarding the commonly seen discord  between theoretical predictions of dynamic behavior, especially the standard predicted and expected "fixes" to many vibration problems, versus observed operation in ractice when unexpected vibration problems still remain or arise anew. The paper will also discuss the key root causes behind this discord with regard to large turbine and generators rotors, and behind unexpected or unexplainable vibration in operation, usually after a major outage. In short, the primary cause in a substantial portion of such cases is the presence of “significant”, axially distributed mass eccentricities inherent to individual rotors, or compound eccentricities from misaligned rotors or bearings. These cases require a different approach versus the methodology traditionally utilized for diagnosing and resolving "unbalance responses” in general, on a variety of rotating machines of different sizes and operating speeds. The paper also presents and describes an improved rotor balancing approach when dealing with such cases. These problems should be ideally resolved in service shops, and when balancing significantly eccentric rotors in balancing facilities, it is necessary to apply a new balancing method using 2N+1 balancing planes, where "N" is the highest mode reached in operation.