## 3 Selection of Design Factors for Vibration Reduction  
### Configuring an Analysis Case  
To reduce the vibration of an already installed and operating scrubber, it is essential to select some design factors and reinforce the structure as it is difficult to change the overall design. The design factors were selected within the range that can be reinforced in reality, and are listed in Table 6. However, simultaneously applying all the selected design factors is not cost effective; thus, it is essential to determine and apply the most efficient design factor for vibration reduction. Five cases were investigated in this study. Case 1 was not been structurally reinforced, and the vibration data of the other cases with a change in the design were compared to that of case 1. The part where the upper end of the pump is connected to the pipe was reinforced in Case 2, and the stress distribution caused by the operation of the pump revealed that the stress was concentrated on the connection part of the upper end of the pump (Figure 7). As the stress size was small, it had no effect on the structural safety, but it may affect vibration owing to the occurrence of displacement due to stress. Therefore, this part was reinforced with cast iron, which exhibits an excellent vibration damping ability. Further, in Case 3, as vibration is generated while the impeller pushes out the fluid, the structure of Case 3 was reinforced in the three-axis direction between the impeller and the water tank (Figure 7). In addition, as vibration displacement could be transferred to the water tank, the structure in Case 3 was reinforced using the same cast iron used in case 2. The reinforced structure enabled the reduction of the displacement of the impeller in each direction due to the operation of the pump as much as possible, thus reducing the vibration.  
\begin{table}
\begin{tabular}{c c} \hline
**Case No.** & **Contents** \\ \hline
1 & Original model \\
2 & Pump top reinforcement \\
3 & Structural reinforcement of impeller part \\
4 & Change in the strainer length \\
5 & Water tank structure reinforcement \\ \hline \end{tabular}
\end{table}
Table 6: Design variables.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Data** & **Numerical** & **Analysis** & **Error Rate** \\ \hline Reaction Force (N) & 827.5 & 815.1 & 1.5\% \\ \hline \end{tabular}
\end{table}
Table 5: Comparison of reaction force data of water tank according to pump operation.  
For case 4, the length of the strainer at the bottom of the pump was increased to reduce the flow rate and pressure transferred to the pipe (Figure 7). This was intended to reduce vibration by reducing the loads applied per unit area, and if the flow rate transmitted by the pump is reduced, cavitation that may occur in the pipe can be minimized. Figure 8 shows the pressure image at 4 s, where the fluid in the pump and pipe was in a saturation state. If the pressure is high or low, the image is displayed close to red or blue, respectively. The overall pressure reduced with an increase in the length of the strainer, and the average value of the pump internal pressure can be numerically confirmed using Figure 9. The graph presents the average value of the internal pressure of the pump for 4-7 s, which is the saturation state, and compared the average value of the data. As shown in the image, the pressure reduces by approximately 20.6% with an increase in the strainer length.  
Figure 8: Pressure image according to pump driving force: (**a**) Original model; (**b**) change strainer length.  
Figure 7: Image showing the design variables (case 2-4).