ArticleAnalysis and Solution of Reliability Problems with a Steam Turbine-Feed Water Pump Train in a Nuclear Power Plant
by Malcolm Leader, P.E. & Larry Pope, P.E.
This is an excellent case study paper for obtaining a basic understanding of rotor dynamics and bearing analysis. The paper is intended for an intermediate level analyst or engineer that has a thorough understanding of the basics of vibration analysis and has an interest in a more in depth understanding of the dynamics of rotors and bearings in operating machinery. There are illustrations of the equipment and plots of vibration data and modeling analysis results for the discussion. This is a more practical paper, but does provide the basic formula for the calculations discussed. This is a descriptive paper in that there is good discussion of what varying the stiffness and damping of the bearing will do for the rotor dynamics of the system.
Comanche Peak is a pressurized water nuclear power plant with two 1,160 megawatt generators. In each unit, steam generator feed water is provided by two single stage, double-suction, double-volute pumps. Each pump is driven by a six-stage steam turbine. The pumps have a rated capacity of 19,800 GPM and 2,322 feet of head at 5,200 RPM with a suction pressure of 375 PSIG. The pumps were originally supplied with plain sleeve bearings and throttle injection type seals. The turbine drivers have a rated capacity of 14,000 HP at 5,740 RPM, and originally supplied with elliptical bearings. Each train typically operates at about 4,900 RPM delivering about 17,000 GPM of flow. All four trains are required to operate reliably, without interruption, for eighteen months between plant outages since an unplanned outage results in reduced power and significant financial loss. Performing maintenance on a steam generator feed water pump or turbine to address high vibration or bearing temperature conditions requires reducing plant power to half in order to take a pump off line.
Figure 1 is a cross-section of the pump rotor. Six “bearings” are indicated including the wear rings and pressure break down seals. The calculated rotor weight is 798 pounds including the external weights from the impeller, sleeves, thrust disk, and ½ coupling. For both rotor models, the rotor polar inertias were matched with the data supplied by each manufacturer.
The turbines are coupled to the pumps with flexible disk spacer couplings with 12 inches between shaft ends. Several different couplings were used over the years with similar results. The relative shaft displacement is currently monitored at each bearing with a pair of orthogonal eddy-current probes. Shaft axial position is also monitored to protect against thrust failures. Figure 2 is a cross-section of the turbine rotor as modeled on the computer. The calculated turbine rotor weight is 7,498 pounds including the external weights from the blades, thrust disk, turning gear and ½ coupling. This rotor has wheels integral with the shaft forging.
Since initial startup of the plant, both pumps and turbines have intermittently experienced a variety of problems. These include difficulty in maintaining shaft alignment, classic indications of oil whirl vibration on the turbine bearings, high bearing metal temperatures on some pump bearings, persistent concerns with higher than expected vibration and, intermittent periods of widely fluctuating vibration in the pumps. Not every train experienced all of these conditions; however, no train was without some problems.”
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