ABSTRACT
This troubleshooting case study reports on the boiler feed water pumps (BFP) at a major coal-fired power plant located south of Melbourne, Australia. That plant housed four large same-configuration steam turbine generating units, each rated at a 500 MW capacity. Each generating unit operated with two 50% BFPs plus a third BFP piped into the system to activate into service if one of the two operating 50% BFPs encountered operating difficulties, which in fact often occurred. These pumps were variable-speed driven and experienced excessive rotor vibration at the normal operating speed range. The consequence of that excessive rotor vibration resulted in an unusually short operating time (10,000 hours) between necessary costly pump rebuilds, where the typical operating time for such pumps between scheduled refurbishment is 40,000 hours. This added a significant annual operating cost to the plant. The author was retained to troubleshoot the rotor vibration problem. After extensive simulation analyses, there was no “smoking gun” uncovered, so the author then travelled to the plant in Australia. On the last day of his one-week scheduled visit to the plant, the author fortunately stumbled on the root cause of the problem while casually observing a plant worker reinstalling one of the BFPs just returned from the pump vender's repair shop. The author observed that the pump journal bearing halves were assembled with thin gasket strips between the bearing sleeve halves and housing halves. While transmitting the specified compression loading to the two bearing sleeve halves, the strips greatly reduced the effective rotor dynamic stiffness of the journal bearings. When that factor was inserted into the author's analysis model, it fully accounted for the vibration critical speed being right in the operating speed range. The gasket strips were replaced with metal shims and the excessive rotor vibration was cured, resulting in a reduction in plant operating costs of half a million dollars per year.
