Rotor Dynamics of Overhung Rotors: Hysteretic Dynamic Behavior

Overhung-configuration rotors are commonly used in the oil, gas and process industries. Examples of this type of equipment include power turbines, Fluid Catalytic Cracking (FCC) expanders, turbochargers and pipeline boosters. Generally, in overhung-configuration rotors, the mass concentration is near the bearing on the overhung end, so the rotor dynamics behavior of these overhung-configuration rotors is different than other equipments that have their mass concentrations between the bearing spans, such as multistage compressors. Among the more important characteristics that directly affect the rotor dynamics of the overhung rotors are gyroscopic effects on the higher modes and the fluid-film journal bearings. Gyroscopic effects are more significant in overhung configurations because of the relatively large overhung mass. These rotors also have a short bearing span and a relatively stiff shaft, so the first two modes are characterized by rigid body motion, as long as the bearing supports are rigid, as in most pipeline boosters. For pipeline boosters it would be typical to describe them as subcritical machines. If the bearing supports are not rigid, as at the disc end of power turbines and FCC expanders, then the first mode can be amplified, and it would not be unusual to describe them as supercritical machines. This paper will assume that the bearing supports are rigid, as in most pipeline boosters. A phenomenon observed in overhung rotors is known as the synchronous thermal instability or “Morton Effect”. The Morton Effect occurs when synchronous vibration produces non-uniform heating of the shaft under the bearing, leading the shaft end to develop a thermal bow. It is typical for this to happen on the overhung end of the rotor, where there is more unbalance to react with any thermal bow. The paper examines the hysteretic dynamic behavior observed in an overhung rotor mounted on tilting pad journal bearings, presenting a series of analysis using state-of-the-art rotor dynamics programs, and comparing analytical results with measurements, handling possible variables associated with synchronous “hysteresis” vibration.