Flash evaporation is the phenomenon that when the static pressure of incompressible fluid from the contraction section to the valve outlet decreases to equal to or lower than the saturated steam pressure of the fluid at the valve inlet temperature after throttling through the regulating valve, part of the liquid vaporizes and forms a gas-liquid two-phase phenomenon behind the valve. The occurrence of flash evaporation makes the liquid flow no longer increase with the increase of pressure drop, resulting in blocking flow. Flash also causes gas-liquid two-phase flow, and gas and liquid flow through the valve core and downstream pipeline at the same time, causing scouring. Its characteristic is that the valve core presents a smooth and polished shape.

Cavitation is the phenomenon that when the fluid passes through the regulating valve, the static pressure at the contraction section decreases to equal to or lower than the saturated steam pressure of the fluid at the inlet temperature of the valve, part of the liquid vaporizes to form bubbles, and then the static pressure returns to the saturated steam pressure, and the bubbles break back to the liquid phase. The whole process of bubble generation and rupture is called cavitation. Cavitation erosion is the erosion of materials by cavitation. Cavitation or cavitation has a great impact on the control valve core. Different from flash scouring, cavitation scouring makes the valve core and downstream pipeline present a rough surface similar to cinder.

In order to avoid the occurrence of flash, the following measures are taken:

1、 Increase material hardness. Select cemented carbide as the valve core, or weld the hard material at the position where flash may occur to improve the hardness of the material and reduce scouring.

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

3、 Select the appropriate type and flow direction of control valve. The pressure recovery coefficient is different for different control valves and flow directions. The control valve and flow direction with large pressure coefficient can prevent blocking flow. For example, for liquids that are easy to vaporize, ball valves or butterfly valves with high pressure recovery should not be selected, but single seat regulating valves with low pressure recovery can be selected.

The measures adopted for cavitation erosion mainly include:

1、 Control the pressure drop so that cavitation does not occur. For example, the multi-stage pressure reducing regulating valve is used to divide the pressure drop of the regulating valve into several stages. The pressure drop of each stage ensures that the pressure at the contraction point is not lower than the saturated steam pressure of the liquid, so as to eliminate the generation of bubbles and prevent cavitation erosion.

2、 Reduce cavitation effects. Similar methods are adopted to prevent the occurrence of flash, such as increasing the hardness of the material and reducing the flow rate, so as to reduce the impact of cavitation.

3、 Reasonably distribute pipeline pressure and increase downstream pressure. From the point of view of the design process, increase the downstream pressure of the regulating valve, so that the flow contraction is also increased accordingly, so as to prevent cavitation. For example, install the regulating valve at the position with high static pressure at the downstream, and add a flow limiting orifice, etc.