Dynamic Analysis Of An 8000 HP Steam Turbine Operating Near Its Second Critical Speed.

LecturePg. 79-92A rotordynamics analysis of an 8000 hp steam turbine which drives an ethylene compressor is presented. This machine characteristically exhibited increasing vibration levels with increasing speed, in the upper portion of the speed range. It was suspected that this ramping vibration was due to operation near a critical speed. The purpose of this analysis was to identify the source of this high vibration with the present rotor and hearings and then to determine the best solution to the problem. The computer analysis determined that the original rotor/bearing system was operating near the second critical speed. The predicted peaks were within nine percent to thirteen percent of miming speed (8400 cpm). The optimum solution was to shorten the shaft at the governor end and replace the bearings. The shaft modification consisted of cutting off the governor worm gear assembly and replacing it with an electronic governor disk. This new bearing design was also a four-shoe tilting pad journal bearing, but it had longer pads and higher preload than the existing design. Because there was concern about coupling gear tooth lock-up in the original design, a dry, flexible-element coupling design was evaluated. The coupling change was also of interest because of its potential effect on moving the second critical speed. However, in this ease, the analysis determined that the rotor vibration characteristics were very close with either the existing gear coupling or the dry coupling. Therefore, the justification fo r changing couplings would be that the dry coupling would not be susceptible to drive tooth lockup and unbalance eccentricity due to tooth wear