*4.2. Opening and Closing Characteristics*

According to the flow coefficient diagram (Figure 13a), with the increase of opening, the flow performance increases, pressure loss decreases, and flow coefficient increases. However, in the range of 80–100% opening, the flow coefficient has experienced a process of first rising and then falling, and the flow performance is the best around 90% opening (Figure 13b). By comparing the inner surface shapes of 80%, 90% and 100% open diaphragms, it can be seen that the central area of the inner surface of 90% open diaphragms is close to the plane, while the central area is concave when the valve orifice is fully open. It can be inferred that the flow performance is better when the central area of the inner surface of diaphragms is horizontal than that when the valve orifice is concave. It can be guessed that the central area will be approximately planar when the diaphragm is manufactured, or a 10% pre-clearance will be given when the diaphragm is installed to ensure the best flow performance when the diaphragm is fully open.

By analyzing the local resistance coefficient diagram (Figure 13c), we can know that the resistance effect of the valve is obvious when the valve is small. The relationship between outlet flow rate and valve opening shows that the linearity of the middle section of flow control is better but the linearity area is not large. In the two opening areas of 0–20% and 80–100%, the flow characteristics of the valve are not good. The reason is that the small opening can easily cause jet flow or even cavitation, and blocking flow is easy to occur, which results in a small flow variation range and a large flow rate. When opening, the throttling effect caused by the change of valve opening is not obvious and the flow control characteristics are not good because the diameter of valve opening is similar to the diameter of pipeline at the entrance of river basin. In the middle opening range, we can know that the flow characteristics are approximately linear, and the flow control performance is good.

(**a**) Flow coefficient *Cv* and *Kv* under different openings of profile E

(**b**) Flow rate at different openings of profile E

**Figure 13.** The results of the flow characteristics at different openings of profile E.
