The disc and seat and other parts of the internal of the control valve and the reducing valve will appear friction, groove and other defects, most of these are caused by cavitation. Cavitation is the whole process of bubble accumulation, movement, division and elimination. When the liquid passes through the valve partially open, the static pressure is less than the saturation pressure of the liquid in the area of increasing velocity or after the valve is closed. At this time, the liquid in the low-pressure area begins to vaporize and produces small bubbles that absorb impurities in the liquid. When the bubble is carried to the area of higher static pressure by the liquid flow again, the bubble suddenly bursts or explodes, we call this kind of hydraulic flow phenomenon valve cavitation.
The direct cause of cavitation is flashing caused by a sudden change of resistance. Flashing refers to the high pressure of the saturated liquid after decompression into a part of the saturated steam and saturated liquid, bubble and the formation of smooth friction on the surface of the parts.
When the bubbles burst during cavitation, the impact pressure can be up to 2000Mpa, which greatly exceeds the fatigue failure limit of most metal materials. Bubble rupture is the main source of noise, the vibration produced by it can produce up to 10KHZ of noise, the more bubbles, the noise is more serious, in addition, cavitation will reduce the bearing capacity of the valve, damage valve inner parts and prone to produce leakage, then how to prevent valve cavitation?
- Multiple-stage pressure reducing
Multistage step-down internal parts, that is, the pressure drop through the valve into several smaller, so that the pressure vena contraction section is greater than the steam pressure, to avoid the formation of steam bubbles and eliminate cavitation.
- Increase the hardness of the material
One of the main causes of valve damage is that the material hardness can not resist the impact force released by the bubble burst. Surfacing or spray welding of stryker alloy based on the stainless steel to form a hardened surface, once damaged, a second time surfacing or spray welding can extend its service life of the equipment and reduce the maintenance cost.
- Porous throttling design
Special seat and disc structure make the flow of liquid pressure higher than the saturated vapor pressure, the concentration of injection liquid in the valve of the kinetic energy into heat energy, thus reducing the formation of air bubbles.
On the other hand, making the bubble burst in the center of the sleeve to avoid the damage directly on the surface of the seat and disc.