Analysis and Solution of Reliability Problems with a Steam Turbine - Feed Water Pump Train In a Nuclear Power Plant

摘要

A large high-speed single-stage boiler steam generator feed water pump at a nuclear power plant had a history of high vibration, premature wear and failures. Numerous attempts at balancing and other field modifications produced minimal improvements. The steam turbine driver of the pump was also a maintenance problem with high vibration due to unstable operation and subsynchronous vibration. A systematic approach was used to address the causes of the problems. A complete train torsional and lateral rotor dynamics analysis was used to understand the system and to design new bearings for increased reliability. It was found that the steam turbine bearings were the cause of unstable vibration due to excessive cross-coupled bearing stiffness. New optimized tilting pad journal bearings eliminated the subsynchronous vibration problem and also controlled synchronous vibration. The pump analysis included the effects of two throttle bushings and two impeller wear rings resulting in a six-bearing system. It was found that the wear rings were significantly stiffer than the original lightly loaded plain journal bearings. This caused the journal bearings to be ineffective and diminished the bearing damping. New stiffer preloaded tilting pad journal bearings were designed that effectively control rotor motion of the pump. This eliminated the high vibration and allowed full-load, full-speed operation. The train torsional analysis showed that there were no significant torsional resonances in the operating speed range.