Introduction To Rotordynamics Of Pumps Without Fluid Forces

Abstract

This paper is for the user of rotating equipment, Reliability is the key to the bottom line and rotordynamics is often the significant factor in determining reliability. Rotordynamics is a very interesting and complicated subject. The importance of this subject has increased over the last few decades, particularly in smaller equipment such as pumps. As machine speeds have increased and higher flows and efficiencies have become necessary, the side effect has been to introduce new rotordynamics problems. These include critical speeds, unbalance response and rotor stability. An example of a recently seen problem involved the application of variable frequency drives to vertical pumps which induced structural resonances in the system at operating speeds. The object of this paper is to discuss various aspects of rotordynamics that are general and then apply these to some examples. The mathematics will be kept to a minimum and as many helpful “rules of thumb” will be included as this subject allows. Rotordynamics can be a very controversial subject from the nomenclature used to the question of the degree of accuracy needed to model a rotordynamic system. There have been many simplifications and assumptions made in this paper and the author offers some opinions that some people will disagree with, but the approach here is quite conservative and the guidelines will not get anyone into trouble. The approaches and guidance offered here are based on experience of many people and many years of analyzing and testing machinery. The track record of those using these techniques is very good when comparing analytical and actual test data. The paper is broken down into two major sections. The first section covers rotordynamics of machines with the majority of the mass between bearings such as a compressor or a multi-stage pump. All the effects of the various geometrical factors will be discussed for these types of machines. The second section covers overhung mass machines with a particular eye toward the rotordynamics of overhung pumps. A nomenclature page is included and, whenever possible, the symbols used for various factors are identified in the body of the paper…