Hot Spots in Turboexpander Bearings: Case History, Stability Analysis, Measurements and Operational Experience

Vibration induced hot spots in radial fluid film bearings can cause spiral vibrations. The phenomenon is known as Morton effect. Various authors have described the phenomenon of spiral vibrations caused by rubbing in general [1 – 4] and specifically by the differential viscous shearing in fluid film bearings [5 – 8]. The history of the case of a turbo-expander for the cryogenic industry is described. The investigated machine has a single relatively stiff rotor with two overhung impellers. The rotor is supported on two tilting pad bearings, as they are commonly used for these applications. It was designed in line with API 617 7^th edition [11]. During internal factory testing excessive vibration at high speed suddenly developed, in spite of very high damping and comfortable separation margins of the relevant rotor vibration modes. The vibrations evolution in a polar plot had the appearance of a spiral, as they are typically observed in case of vibration induced hot spots. The tilting pad bearings were suspected as the most likely cause for the hot spots. In parallel to the tests, hot spot stability analyses with a rotor dynamic model of the turbo-expander were carried out. The applied method, introduced by Schmied [5] allows the handling of general rotor systems. The hot spot model is based on the theory of Kellenberger [3] using a thermal equation between the shaft’s thermal deflection and the shaft displacement at the hot spot location. The analytical results gave evidence of the instabilities and confirmed the fluid-film bearings as the source of the hot spot. The model was used to assess changes for the solution of the problem. The following measures were studied: Reduction of the oil viscosity, stiffening of the rotor and reduction of the bearing width. The final successfully implemented solution was a combination of lower bearing width and reduced viscosity. Apart from this thoroughly studied case, an overview of further turbo-expanders regarding their hot spot behavior is given.