Cavitation and flashing
Cavitation is a kind of destruction form material reaches a critical value in the liquid pressure and temperature, is divided into two phases, flashing and cavitation. Flash is a very fast process, when the fluid flows through the valve, the valve seat and valve to form local contraction flow area, the local resistance, the pressure and the speed of the fluid changes (see Figure 1). When the pressure is P1 fluid flowing through the throttle hole, the velocity suddenly increased dramatically, static pressure suddenly dropped, when the hole pressure of P2 in the fluid situation where the saturated vapor pressure of Pv, part of the fluid vaporized into gas, air bubbles, the formation of gas-liquid two-phase coexistence phenomenon, known as the flash stage, so it is a kind of system phenomenon. Valve can not avoid the flash vaporization, unless the system conditions change. But when the downstream pressure of the valve in the liquid and rise again, and higher than the saturation pressure, elevated pressure compressed air bubble, which burst, called the cavitation stage. The cavitation process in saturated bubble does not exist, but quickly turned back to liquid blasting. Because of the bubble volume is much bigger than the liquid volume in the same big. Therefore, the bursting of the bubbles from the large volume change to small volume. Cavitation is a shift from liquid to saturation, liquid, it is different from the flash evaporation phenomenon.
The bursting of bubbles cavitation process all the energy is concentrated at the rupture point, impact force generated thousands of Newton, the pressure of shock waves as high as 2 × 103 MPa, considerably more than most metal material fatigue damage limit. At the same time, the local temperature of up to several thousand degrees Celsius, these hot spots caused by thermal stress is the main factor to produce the destructive effects of cavitation. The flash to produce erosion, worn smooth formation on the surface of parts. Cavitation as sand spraying on the surface of parts, the parts surface tear, slag hole is formed on the external surface of the rough like. In the pressure differential conditions, air valve and valve seat will be destroyed, in a very short time effect of leakage, the valve performance. At the same time in the process of cavitation, cavitation bubble burst release enormous energy, causes the vibration of internal parts, noise, up to 10 kHz bubble more noise, the more serious.
Method to prevent cavitation damage
Flash valve adjustment is not prevented. Can do is to prevent flash damage. The valve design influence factors flash damage main valve structure, material properties and system design. For cavitation damage, the valve structure can adopt the zigzag path, multi-stage decompression and porous throttle to prevent.
1) valve structure. Although the valve structure has nothing to do with the flash, but can inhibit the damage of flash. The angle valve flow direction of medium structure from top to bottom to prevent flash damage more than spherical valve. Flash damage is saturated surface bubble impact valve high speed, and the surface of the valve body caused by corrosion. Because the angle valves direct the flow valve inside the media center of the downstream pipeline, not like the spherical valve directly hit the wall, so greatly weakened the destructive power of flash.
2) material selection. In general, high hardness materials can resist more flash and cavitation damage. For high hardness materials general manufacturing valve. As the power industry often uses chromium molybdenum alloy steel valves, WC9 is one of the common corrosion resistant material. If the angle pipe downstream of the valve with high hardness materials, the valve body can use carbon steel materials, because only in the valve body lower part is flashing liquid.
3) system design. Flash phenomenon is decided by the system design, figure 2 for heating water drain valve will flash water discharge to the condenser system. The flash appeared in the pneumatic control valve and a condenser pipe, flash damage will only appear in this area. Figure 2b) flash phenomenon in the downstream and the valve of condenser. So the condenser with respect to the pipeline must have a larger volume, in order to prevent the bubble surface materials of high velocity impact. Thus helps prevent flash damage good system design.
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