Methods to prevent cavitation damage of high pressure valve - Shanghai liquid ammonia stop valve manufacturer
Time: 2019-04-01 22:53:07 Source: Yonglong Valve Reading: zero second
Methods to prevent cavitation damage of high-pressure valve -Shanghai liquid ammonia stop valve manufacturer
It is often seen that the disc, valve seat and other parts of throttle valves such as control valves and pressure reducing valves produce wear marks, deep grooves and pits, which are mostly caused by cavitation.
Cavitation is a kind of destruction form of materials when the pressure and temperature of liquid reach the critical value, which can be divided into two stages: flashing and cavitation.
Flash evaporation is a very fast transition process. When the fluid flows through the control valve, the valve seat and disc form a locally contracted flow area, resulting in local resistance, which changes the pressure and speed of the fluid.
When the fluid with pressure P1 flows through the orifice, the flow rate suddenly increases sharply and the static pressure drops suddenly. When the pressure P2 behind the orifice reaches the saturated steam pressure Pv under the condition of the fluid, part of the fluid vaporizes into gas, generating bubbles, forming the coexistence of gas and liquid phases, which is called the flash stage. It can be seen that it is a systematic phenomenon.
The regulating valve cannot avoid flashing unless the system conditions change. When the downstream pressure of the liquid in the valve rises back and is higher than the saturation pressure, the rising pressure compresses the bubble, causing it to burst suddenly, which is called the cavitation stage. In the process of cavitation, the saturated bubbles no longer exist, but rapidly explode back to the liquid state. Because the volume of bubbles is mostly larger than that of the same liquid. Therefore, the explosion of bubbles is a change from large volume to small volume.
In the process of cavitation, when the bubble breaks, all the energy is concentrated at the breaking point, producing an impact force of several thousand Newtons. The pressure of the shock wave is up to 2 × 103 MPa, which greatly exceeds the fatigue failure limit of most metal materials. At the same time, the local temperature is as high as several thousand degrees Celsius, and the thermal stress caused by these hot spots is the main factor that causes cavitation damage.
Flash evaporation causes erosion damage and forms smooth wear marks on the surface of parts. As sand is sprayed on the surface of the part, the surface of the part will be torn to form a rough slag hole like outer surface. Under the severe conditions of high pressure difference, the extremely hard valve disc and valve seat will also be damaged in a short time, resulting in leakage, affecting the service performance of the valve. At the same time, during cavitation, bubbles burst and release huge energy, causing vibration of internal parts and generating noise up to 10 kHz. The more bubbles, the more serious the noise.
Methods to prevent valve from cavitation damage
The flash in the control valve cannot be prevented. What can be done is to prevent the damage of flash. In the design of control valve, the main factors affecting flash failure are valve structure, material performance and system design. For cavitation damage, the valve structure of zigzag path, multi-stage decompression and multi orifice throttling can be used to prevent it.
1) Valve structure
Although the valve structure has nothing to do with flash, it can inhibit the damage of flash. The angle valve structure with medium flowing from top to bottom can prevent flash damage more than the spherical valve body. Flash damage is caused by high speed saturated bubbles impacting the valve body surface and corroding the valve body surface. Since the medium in the angle valve flows directly to the center of the downstream pipeline inside the valve body, instead of directly impacting the body wall like the ball valve, the destructive force of flash evaporation is greatly reduced.
2) Material selection
Generally, materials with high hardness are more resistant to flash evaporation and cavitation. Materials with high hardness are generally used to manufacture valve bodies. For example, the power industry often selects chrome molybdenum alloy steel valves, and WC9 is one of the commonly used anti-corrosion materials. If pipes with high hardness are installed downstream of the angle valve, the valve body can be made of carbon steel, because only the downstream part of the valve body has flashing liquid.
3) A zigzag path.
One way to reduce pressure build-up is to make the flowing medium pass through a throttling element with a zigzag path. Although this zigzag path can have different forms, such as small holes, radial flow paths, etc. But the effect of each design is basically the same. This zigzag path can be used in the design of various components to control cavitation.
4) Multistage decompression
Each stage in multi-stage decompression consumes a part of energy, making the inlet pressure of the next stage relatively low, reducing the pressure difference of the next stage, reducing the pressure recovery and avoiding cavitation. A successful design can make the valve bear a large pressure difference while keeping the pressure after flow contraction higher than the saturation pressure of the liquid to prevent liquid cavitation. Therefore, for the same pressure drop, primary throttling is more likely to produce cavitation than multi-stage throttling.
5) Multi hole throttling design
Multi orifice throttling is a comprehensive design scheme. The special structure of valve seat and disc is adopted to make the pressure of high-speed liquid passing through each point of valve seat and disc higher than the saturated steam pressure at this temperature, and the convergent jet method is adopted to make the kinetic energy of the liquid in the control valve change into heat energy due to mutual friction, thus reducing the formation of bubbles. On the other hand, the bubble rupture occurs in the center of the sleeve, avoiding direct damage to the valve seat and disc surface.
Shanghai Ammonia Valve Factory/Shanghai Yonglong Valve Factory Main business: ammonia stop valve, emergency Shut off valve , electric cut-off valve, electric ball valve, pneumatic ball valve, pneumatic emergency cut-off valve, hydraulic emergency cut-off valve, gas emergency cut-off valve, liquid ammonia emergency cut-off valve, ammonia emergency cut-off valve, ammonia emergency cut-off valve, air drain valve, ammonia cut-off valve, air check valve, flame retardant vent cap, flame arrester, flow limiting valve, fuse valve, pull off valve, electro-hydraulic valve, Liquid ammonia stop valve and other mechanical supporting valves.
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