Flashing refers to the phenomenon that when the static pressure from the constriction section to the valve outlet decreases to be equal to or lower than the saturated vapor pressure of the fluid at the valve inlet temperature after the incompressible fluid is throttled through the control valve, part of the liquid vaporizes to form gas-liquid two-phase flow behind the valve. Flashing makes the liquid flow no longer increase with the increase of pressure drop, resulting in blocked flow. Flashing also causes gas-liquid two-phase flow, The gas and liquid flow through the valve core and the downstream pipeline at the same time, causing erosion, which is characterized by the smooth and polished shape of the valve core.  

Cavitation refers to the phenomenon that when the fluid passes through the control valve, the static pressure at the constriction section decreases to be equal to or lower than the saturated steam pressure of the fluid at the valve inlet temperature, part of the liquid vaporizes to form bubbles, and then the static pressure returns to the saturated steam pressure, and the bubble breaks back to the liquid phase. The whole process of bubble generation and breaking is called cavitation. Cavitation is the erosion of materials by cavitation. The occurrence of cavitation or cavitation will cause great erosion damage to the regulating valve core. Different from flash flushing, cavitation flushing makes the valve core and downstream pipeline have rough surfaces similar to cinders.  

In order to avoid flash evaporation, the measures adopted mainly include:

1、 Improve material hardness. The hard alloy shall be selected as the valve core, or the hard material shall be welded at the position where flashing may occur to improve the material hardness and reduce scouring.  

2、 Reduce fluid velocity. Design a reasonable flow path to reduce the flow rate of the downstream fluid, so as to reduce the scouring speed. For example, set an expansion pipe downstream of the control valve to reduce the flow rate.  

3、 Select appropriate regulating valve type and flow direction. Different regulating valves and flow directions have different pressure recovery coefficients. Selecting a regulating valve and flow direction with large pressure coefficient can prevent blocked flow. For example, for liquids that are easy to vaporize, it is not appropriate to select ball valves or butterfly valves with high pressure recovery, but single seat control valves with low pressure recovery.  

The measures adopted for the occurrence of cavitation mainly include:

1、 Control the pressure drop to prevent cavitation. For example, multistage pressure reducing control valve is used to divide the pressure drop of the control valve into several stages. The pressure drop of each stage ensures that the pressure at the constriction is not lower than the saturated steam pressure of the liquid, so as to eliminate the generation of bubbles and prevent cavitation.  

2、 Reduce the impact of cavitation. Adopt similar methods to prevent flash evaporation, such as increasing material hardness and reducing flow rate, so as to reduce the impact of cavitation.  

3、 Reasonably distribute the pipeline pressure and increase the downstream pressure. From the point of view of design process, increase the downstream pressure of the control valve, so that the flow constriction will also be increased accordingly, thus preventing cavitation. For example, the regulating valve is installed at the downstream position with high static pressure, and flow limiting orifice is added.