CFD-Aided Hydraulic Development of a Steam Generator Feed Pump

A rerated single-stage, double-suction nuclear reactor steam generator feed pump, rated at 8,850 hp¹ for two-pump operation and 13,300 hp for emergency single-pump operation, underwent fluid dynamic design improvements to achieve satisfactory operability and performance, especially at flow rates below the rated flow of 13,865 gpm² when running at 4850 rpm. An unsteady-flow CFD solution was conducted at 66% of rated flow via an unstructured RANS code called CRUNCH CFD to provide guidance in a) removing axial shuttling of the rotor; and, b) improving the performance curve of head versus flow rate so as to eliminate hydraulic instabilities at flow rates well below the design value, which could lead to excessive pressure pulsations, vibrations, and also the axial rotor shuttling (a). To aid in this effort, steady-flow solutions via the same CFD code were obtained at this and several other flow rates. The as-built diffuser entry throat area between vanes was found to be nearly 10% larger than intended, and the impeller exit throat areas about 4% oversize. Two new diffusers were designed with 14% less throat area than the as-built original diffuser, the difference being in the vane shape and flow-path area distributions. CFD analysis of these two diffusers showed one to have better performance at low flow, enabling it to be chosen without resorting to building and testing both diffusers. Similarly, CFD solutions of two types of pressure-pulsation-reducing “vee” cuts of the impeller OD enabled a choice of which of these cuts should be implemented in the final design — again without test of both cut types. In addition to these improvements, the radial gap between the impeller and diffuser shrouds — called “Gap A” — was reduced by more than half in an effort to isolate the zones adjacent to the impeller from unsteady backflow emerging from the diffuser. The final design incorporating these features was then analyzed via steady-flow CFD, and then the pump was tested at both its rated speeds and operating flow ranges. The test results agreed very closely with this CFD solution. Moreover, the pump achieved satisfactory performance over the flow range of interest, and has demonstrated the same during operation in the power plant.