Low pressure gas turbine rotors (LPT) designed to meet American Petroleum Institute 616 prescriptions should target specific rotordynamics criteria and supposedly past prototype testing. General modeling guidelines are reported by the API specification. Because of its generality, it does not mention some typical phenomena specific to overhung rotor with flexible supports subject to considerable thermal load. This can lead to skipping important design considerations. The aim of this paper is to deliberate key areas of design, manufacturing, testing and diagnostics of overhung rotors for oil and gas applications (gas turbines and expanders). These topics address vital factors affecting lateral vibrations over the entire rotor life. Considerations are made on bearings design and their condition monitoring, basic vibration modes and possible interactions with casing and supporting structure. It reports experiences on potential sources of unbalance, interactions between turbine rotor and other rotors of the shaft-line, rotor stability and specific cross-coupling effects. Most of these items are discussed through the outcome of the several sensitivity studies and experimental results. One of the main outcomes is a confirmation on the modeling criteria selected for LPT rotor. The bending mode frequency predictions can be considered satisfactory with prediction error up to 7%. One of the main results coming from the sensitivity studies reveal that variations of static load and hydrodynamic characteristics are most significant for coupling end bearings dynamic behavior. These values could be significantly different from design conditions when thermal conditions are not well predicted or due to poor quality of the shaft line alignment. The paper, at its conclusion reports a statistic on the root cause of high vibrations in such rotors: despite of the type (synchronous, sub-synchronous, etc), in 65% of all the considered population, it is the residual or developed unbalance — most probably at coupling side.

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