2.2.2. Effect of Stem Loading Area

By analyzing the deformation of diaphragm (Figure 4) under different loading areas, it can be seen that when the valve stem is loaded in a large area, there is almost no stress at the center of the diaphragm and the maximum displacement is not reached. On the contrary, the maximum displacement occurs directly below the edge of the convex platform. When the maximum displacement of the valve stem is input, the center of the diaphragm bottom surface is depressed. It shows that the displacement input cannot be transmitted well. Moreover, due to the concave surface under the diaphragm, the diameters above the ridge have undergone two shrinkage and expansion processes. The continuous change of the diameters will also lead to increased pressure loss and increase the resistance coefficient of the valve. In addition, this kind of depression will also lead to incomplete sealing and easy to leak.

When the diaphragm is loaded in a moderate area, the central area of the lower surface is close to the plane, the boundary of the runner is more uniform, and the sealing performance is better. The diaphragm deflection cannot meet the working requirement when the minimum area load or approximately concentrated load occurs. Even some element grids have been seriously damaged during the simulation iteration process, which leads to the termination of the simulation, indicating that the loading area of the valve stem should not be too small.

By exploring the deformation of diaphragm at different opening, using FEM module in ANSYS WORKBENCH, the geometric model of the deformed diaphragm is obtained, and the shape and size of the diaphragm under different opening are obtained. The flow field boundary at different opening can be obtained by matching the shape of the runner.

(**a**) Displacement Diagram (**left**) and Stress Diagram (**right**) of Large Area (*D*0 *=* 12 mm) Uniform Loading

(**b**) Displacement Diagram (**left**) and Stress Diagram (**right**) of Middle Range (*D*0 = 8 mm) Uniform Loading

(**c**) Deformation deflection (**left**) or even damage (**right**) of diaphragm under small area (*D*0 = 4 mm) loading (When the grid size is not so small (e.g., 2 mm), the state of the diaphragm is shown in the left figure. When the grid size is set 1 mm or smaller, the calculation reports error telling which grid is seriously damaged and stops iteration. The right figure shows the state when the iteration stops, indicating that the diaphragm has been destroyed. The two figures show the two probable situations in reality.)

**Figure 4.** Diaphragm Deformation under Different Loading Area.
