[Shut off valve technology] Harmful phenomena of cavitation (cavitation) and evaporation (flashing)
Time: January 9, 2020 10:59:35 Source: Yonglong Valve Reading: zero second
[Shut off valve technology] Harmful phenomena of cavitation (cavitation) and evaporation (flashing)
The generating principle of valve noise can be divided into the following categories: mechanical, aerodynamic and hydrodynamic. Mechanical noise may be caused by vibration, resonance of internal parts of the valve, misalignment of moving parts, or excessive clearance. The cause of aerodynamic noise is that the mechanical energy of compressible medium flow is converted into acoustic energy. As the medium depressurizes, the energy released accelerates the flow of the fluid - usually faster than the speed of sound - and creates noise. Hydrodynamic noise is generated by fluid flow. The specific situations are as follows: collision of turbulence with valve and pipe wall, cavitation (cavitation) and evaporation (flashing).
Evaporation (flash evaporation)
In addition to collapsing and disappearing, bubbles may also become larger. The result is that the liquid with bubbles will quickly change into steam containing tiny droplets, which is called flashing phenomenon. The damage caused by flash evaporation is completely different from that caused by cavitation. It may make the parts appear smooth grooves. The mechanism of this damage is similar to that of sand blasting. Downstream of the pulse contraction, the fluid contains a lot of steam and a lot of fine droplets. As the liquid evaporates, its volume will increase significantly, so the downstream flow rate may reach hundreds of feet per second, and high-speed droplets may wash away the valve parts. Damage caused by flash evaporation usually does not occur in an instant like cavitation.
The occurrence of flash evaporation is determined by various parameters of the fluid, and can not be completely eliminated by the design. But the destructive effect of flash evaporation can and must be completely eliminated. In addition to the above methods to improve the durability of valve components, hard coatings can also be coated on the surface of the valve body and corrosion resistant gaskets can be provided; And a shut-off valve with a protective cage valve core.
Cavitation
Cavities always move downstream. When the cross-sectional area of the flow channel becomes larger, the flow rate of the medium decreases and the pressure increases. In this case, cavitation will occur, and its internal pressure is the same as that of the steam in the flow channel, but lower than that of the surrounding medium. The vacuoles then collapse inward and disappear. The formation of cavitation, the blocking flow caused by cavitation, and the subsequent collapse and disappearance of cavitation downstream are the so-called cavitation phenomena. When a bubble collapses, it will make a popping sound. The noise made by countless bubbles collapses is like countless crushed stones passing through the valve. This kind of noise is very loud, and people who are affected by it for a long time may even suffer from hearing loss. Not only that, when the bubble collapses, a shock wave will be generated and the valve may be seriously damaged.
Solution
The method of reducing noise is to adopt cage structure and Shut off valve The working condition sets the fit clearance. To reduce aerodynamic noise, the most important and efficient way is to use perforated control parts for the valve, such as perforated plug or perforated cage valve element (sleeve). Another method is to reduce the medium flow rate at the outlet. In order to achieve this goal, the most common measure is to use the cage valve core with holes, orifice plate or diffuser, and use their choke effect to increase the outlet port pressure. If the noise is very serious, it is necessary to use all the above measures at the same time.
Another method that has been verified in use is to use thermal spraying, diffusion, plasma nitriding and other technologies to coat part or all of the surface of the stop valve plug and valve seat with stellite protective layer to make the hardness of the surface layer of parts within the range of CA.0.1mm reach 950HV, or use heat treatment to make the hardness reach 55HRC. The core design of anti bubble valve is multi-stage cock. The principle of preventing cavitation of multi-stage cocks is that the pressure drop at each stage of the cocks is controlled below the critical value. However, the disadvantage of this design is that it is difficult to effectively ensure the effective choking of all stages of cocks at the beginning of opening the valve. For this reason, we have adopted a surface forming multi-step cock with holes. Its structure has both active characteristics, which can provide different resistance according to the valve opening, and passive characteristics, that is, the choke effect of the cage valve core and orifice plate with holes.
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