# Research of Modal Analysis for Impeller of Reactor Coolant Pump
## 4 Discussion
### Improved Design of the RCP Impeller  
Due to the inconstancy of the hydraulic load around RCP impeller, natural frequencies of the impeller vary within a certain range. Therefore, in order to prevent resonance during normal operation, it is necessary to keep a large gap between the natural frequency of the impeller and the frequency of fluctuating pressure. Although redetermining the blade numbers of the impeller and vane diffuser can change both the node-diameter number of resonant mode and the excitation frequency, it may have a huge influence on the hydraulic performance of the RCP. Likewise, if the impeller hub structure is modified, other hydraulic components that cooperate with the impeller should be redesigned.  
Natural frequencies of the impeller can be adjusted by modifying blade shape. There are several methods that can be employed, but a few of them are not recommendable for some reasons. For instance, the change of blade outlet edges affects the head, and the change of blade three-dimensional twist shape affects the efficiency. In the above chapter, there is a conclusion that hydraulic load affects natural frequencies. As can be seen from Figure 5 (red circle), the pressure difference between the pressure surface and suction surface at the inlet of the blade is larger than those in other areas. Therefore, cutting the blade inlet edges might be a viable way to alter natural frequencies. Besides, this method will not significantly affect hydraulic performance.  
Cutting the blade inlet edges is brought forward to modify the impeller. The maximum indentation is in the middle of the edges, and its value is one tenth of the inlet diameter. The impact of this measure on natural vibration modes of the impeller was analyzed, and the performance of the pump was calculated. Figure 6 shows natural vibration modes of the impeller before and after the improvement. All three natural frequencies increase, and especially, one nodal diameter natural frequency is more than twice the original value. The three nodal diameters mode natural frequency of the new impeller is 16.3% larger than that of the original impeller, so this measure can effectively prevent hydraulic resonance.  
It can be seen from Figure 5 that pressure difference between two sides of inlet edges is larger than that of other areas. So hydraulic load stress is concentrated in the middle of inlet edges. Figure 7 shows the stress distribution diagrams. After improvement, the maximum stress is reduced, and the structural strength of the blade is enhanced. Figure 8 shows the hydraulic performance curves before and after improvement. Cutting blade inlet edges have little effect on hydraulic performance. In cavitation performance, though Net Positive Suction Head (NPSH) of the pump is reduced from 61 m to 52 m. Because of the high-pressure (15.5 MPa) environment inside RCP, NPSH of the new design still meets performance requirements.  
In summary, cutting the blade inlet edges is a solution of altering natural frequencies of the impeller without affecting hydraulic performance greatly. Since natural frequencies of the underwater impeller are subject to hydraulic load, relieving the stress concentration may increase natural frequencies. Therefore, the area where the load is concentrated has a great influence on the self-vibration model of the impeller, and redistributing hydraulic load can be used to improve the dynamic characteristic of the underwater rotating system.