Cavitation Studies on a Model of Primary Sodium Pump

Indira Gandhi Centre for Atomic Research for their Fast Breeder Reactor had a requirement of two primary sodium pumps, with relatively medium head and comparatively large flow rate. The pumps had to be vertical bowl type, with inverted suction, drawing the relatively cold liquid sodium from the reactor pool, and forcing it over the fuel rods. The flow involved was 4.3 M³/sec. per pump and the head involved was 75 m. The available NPSH was 16.02m. The choice of the right speed was the first hurdle to be overcome. The size of the pump governed the cost of the reactor. Hence it was essential to make the pump as compact as practicable, by designing it to run at the highest possible speed. On the other hand the required NPSH of the pump increased with speed, there by reducing the safety cushion between required NPSH and available NPSH. Increasing the available NPSH beyond 15.55 m to accommodate higher speeds also increased the cost of the reactor phenomenally. Coupled with the requirement of head, flow rate, efficiency, the application demanded a certain erosion life of the pump impeller. Concluding from the previous experience and available literature on the subject, it was necessary to limit the size of the cavitation bubble patch, during operation under available NPSH, to meet erosion life requirement. The paper describes how the choice of speed was made and what were the criteria applied for deciding the value of 3% head drop NPSH. It also describes how a closed circuit test rig was installed for the purpose of testing a model pump and how the usual performance and NPSHR tests paint erosion tests and visual cavitation tests were carried out on a geometrically similar, smaller model pump, to decide that the prototype pump built from the model will meet all the requirements of head, flow rate, efficiency, 3% head drop NPSHR, 0% head drop NPSHR, Paint erosion NPSHR, and the cavitation patch size to meet desired erosion life criteria, in the laboratories of Kirloskar Brothers Limited.