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Control Valve Cavitation Cause and Solution

Control Valve Cavitation cause and solution

Cause of Caviation

When the fluid pressure is reduced to the saturated vapor pressure or lower, flash evaporation or bubbles will occur. In most control valves ( figure 5), the inlet pressure is P1, velocity is V1. When the fluid passes through the plug necking area, the velocity is increased to Vvc. According to the principle of conservation of energy, the fluid pressure suddenly drops to Pvc. When Pvc is equal to or less than the liquid saturated vapor pressure Pv, the liquid will be gasified and bubbles will be produced, so that flash evaporation occurs.

After the fluid passes through the plug, the pressure starts to be restored and the kinetic energy is transferred into potential energy again. When the pressure is restored to the downstream pressure, which is expressed as P2 and the velocity is V2. When the restored pressure exceeds the saturated vapor pressure Pv, the bubbles formed will be broken and cavitation will occur. This kind of energy release will increase the partial stress to be above 200000PSI ( 1400MPa) and the stress will rapidly destroy the solid plug.

 

Solution to Cavitation

The labyrinth control valve can effectively eliminate the damage caused by failure of control of fluid velocity.

First, the fluids are scattered into many small flow channel. Thus, even the bubbles are formed, their volume is very small and the energy is not sufficient to produce stress that can damage materials. Secondly, the flow velocity is maintained at the lowest level. Thus, the partial pressure will not be reduced to be lower than the fluid vaporizing pressure. Therefore, cavitation will not occur.

The damage caused by cavitation is a typical signal that indicates failure of control of flow velocity. As is mentioned above, the adoption of materials of high hardness, insulating sleeve or downward orifice will ony eliminate a small amount of faults in the valve caused by cavitation. The high flow velocity will cause cavitation and damage the plug, and the solution to cavitation is to adopt the labyrinth cage as shown in figure 6.

According to the fluid evaporation pressure, the flow velocity can be achieved through the following formulas:

V=

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