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Steam Cavitation Cause in Control Valve Chamber
When the fluid pressure drops to the saturated vapor pressure or lower, flashing or bubbles will occur. In most control valves (Figure 5), the inlet pressure is P1 and the speed is V1. When the fluid passes through the plug throat area, the velocity increases to Vvc. According to the theory of conservation of energy, the fluid pressure suddenly drops to Pvc. When Pvc is equal to or less than the saturated vapor pressure of the liquid, Pv, the liquid will be vaporized and bubbles will be generated, thereby flashing.
After the fluid passes through the plug, the pressure begins to recover, and the kinetic energy is converted into potential energy again. When the pressure returns to the downstream pressure, it is expressed as P2 and the speed is V2. When the recovered pressure exceeds the saturated vapor pressure Pv, the formed bubbles will burst and cavitation will occur. This energy release will increase the partial stress above 200,000 PSI (1400 MPa), and the stress will quickly destroy the solid plug.
Steam Cavitation Solution in Control Valve Chamber
The labyrinth control valve can effectively eliminate the damage caused by the failure of flow rate control.
First, the fluid is dispersed into many small flow channels. Therefore, even if bubbles are formed, the volume of the bubbles is very small, and the energy is insufficient to generate stress that can damage the material. Second, keep the flow rate to a minimum. Therefore, the partial pressure does not drop below the vaporization pressure of the fluid. Therefore, cavitation does not occur.
Damage caused by cavitation is a typical signal, indicating that the flow rate cannot be controlled. As mentioned above, with high hardness, the material of the insulating sleeve or the downward orifice will only eliminate a small number of failures in the valve caused by cavitation. High flow rate will cause cavitation and damage the plug. The solution to cavitation is to use a labyrinth cage as shown in the following figure
