Cavitation and flashing that occurs in valves and how to prevent
Posted: 02/22/2019 14:30:10 Hits: 381
It can be frequently seen that the disc and the seat of valves such as the regulating valves and the pressure reducing valves have abrasion, deep grooves and pits inside, which are mostly caused by cavitation.
Contents
♦ Definition of Cavitation
♦ Definition of Flashing
♦ How to Prevent
• Valve structure
• Material selection
• Structure with a tortuous path
• Multi-stage decompression
• Porous throttling design
Cavitation is a form of damage that occurs when a material's pressure and temperature reach a critical value. It can be divided into two stages: flashing and cavitation. Flashing is a very rapid transition process - When the fluid flows through the regulating valve, local pressure is generated due to the locally contracted flow area of the valve seat and the valve flap, causing the pressure and velocity of the fluid to change.
When the fluid with pressure flows through the orifice, the flow rate increases suddenly and sharply, and the static pressure suddenly drops. When the post-hole pressure reaches the saturated vapor pressure of the fluid, part of the fluid vaporizes intogas, and generates bubbles. The formation of a gas-liquid two-phase coexistence phenomenon, which is called the flashing stage, can be seen as a systematic phenomenon.
The regulator valve does not prevent flashing unless the system conditions change. When the downstream pressure of the liquid in the valve rises again and is higher than the saturation pressure, the increased pressure compresses the bubbles, causing them to suddenly burst, this is called the cavitation stage. The saturated bubble no longer exists during the cavitationprocess, but is rapidly blasted back to the liquid state. Since the volume of the bubbles is larger than the volume of the same liquid, the blasting of bubbles is a transition from a large volume to a small volume.
During the cavitation process, all the energy is concentrated on the fracture point, causing a large impact force whose pressure is as high as 2 × 103 MPa, which greatly exceeds the fatigue limit of most metal materials. At the same time, the local temperature is several thousand degrees Celsius, and the thermal stress caused by hot areas is the main factor causing cavitation damage.
Flashing produces erosion damage and creates a smooth wear scar on the surface of the part, just like sand is sprayed onto the surface of the part, the part is torn to form a rough, slag-like outer surface. In the harsh conditions of high pressure difference, valve flaps and valve seats that are extremely hard will also be damaged in a short time, causing leakage and affecting the performance of the valve. At the same time, the bubbles burst during cavitation releases huge energy, causing vibration of internal parts, generating noise of up to 10 kHz. The more bubbles, the louder the noise.
How to Prevent
Flashing in the regulating valve is not preventable, all that can be done is to prevent the destruction of flashing. The factors affecting flashing damage in the design of the regulating valve are mainly valve structure, material properties and system design. For cavitation damage, it can be prevented by a porous throttle valve structure with a tortuous path and multi-stage decompression.
1. Valve structure
Although the valve structure is independent of the flashing generation, it can suppress the destruction of it. An angular valve structure that uses a medium to flow from top to bottom is more resistant to flashing damage than a spherical valve body. Flashing destruction is caused by high velocity saturated bubbles impinging on the surface of the valve body and corroding the surface of the body. Since the medium in the angular valve flows directly to the center of the downstream pipe of the valve body, and does not directly impact the body wall like a spherical valve, the destructive force of flashing is greatly reduced.
2. Material selection
In general, materials with high hardness are more resistant to flashing and cavitation damage. High-hardness materials are generally used in valve bodies. For example, the chrome-molybdenum alloy steel valve is often used in the power industry. WC9 is one of the commonly used corrosion-resistant materials. If the pipe with high hardness is installed downstream of the angular valve, the valve body can be made of carbon steel because the flashing liquid only appears in the downstream part of the valve body.
3. Structure with a tortuous path
The flow medium passes through a throttle set containing a tortuous path is one way to reduce pressure recovery. Although such a tortuous path can have different forms, such as small holes, radial flow paths, and the like, the effect of each design is basically the same. This tortuous path can be utilized in a variety of component designs that control the occurrence of cavitation.
4. Multi-stage decompression
Each stage of the multi-stage decompression consumes a portion of the energy, so that the inlet pressure of the next stage is relatively low. The cavitation is avoided since the pressure difference of the next stage is reduced. A successful design allows the valve to withstand large pressure differentials while maintaining the pressure after being contracted above the saturation pressure of the liquid, preventing liquid cavitation. Therefore, for the same pressure drop, the primary throttling is more likely to generate cavitation than the multi-stage throttling.
5. Porous throttling design
Porous throttling is a comprehensive design. One the one hand, by using a special seat and disc configuration, the pressure of each point of high-speed liquid passing through the seat and the flap is higher than the saturated vapor pressure at that temperature, and the method of converging injection is used to make the kinetic energy of the liquid in the regulating valve convert into heat energy by rubbing, thereby reducing the formation of bubbles. On the other hand, the rupture of the bubble occurs in the center of the sleeve, avoiding direct damage to the valve seat and the surface of the flap.
