Flash and cavitation of valves and methods to prevent cavitation damage main technical properties of valves

Flash and cavitation of valves and methods to prevent cavitation damage main technical properties of valves

Often can see regulating valve, reducing valve and other throttle valve disc and seat parts of the internal wear marks, deep groove and pits, which are mostly caused by cavitation. Cavitation is a form of material failure when the pressure and temperature of liquid reach critical value, which is divided into flash and cavitation two stages. Flash is a very rapid transition process when the fluid flows through the regulator due to the seat and
Often can see regulating valve, reducing valve and other throttle valve disc and seat parts of the internal wear marks, deep groove and pits, which are mostly caused by cavitation.
Cavitation is a form of material failure when the pressure and temperature of liquid reach critical value, which is divided into flash and cavitation two stages.
Flash is a very fast transformation process, when the fluid flows through the regulator, due to the valve seat and valve disc formed a local contraction of the flow area, local resistance, so that the fluid pressure and speed change.
When the pressure is P1 of fluid flowing through the orifice, velocity suddenly a sharp increase in static pressure plummeted, after the hole pressure P2 in the fluid in the case of saturated steam pressure before Pv, part of the fluid into gas vaporization, bubbles, formation of gas liquid two phase coexistence phenomenon, called flash stage, it is a system phenomenon.
Regulator cannot avoid flash unless system conditions change. And when the downstream pressure of the liquid in the valve rises back again, and is higher than the saturation pressure, the increased pressure compresses the bubble, so that it suddenly bursts, known as the cavitation stage. During cavitation, the saturated bubble is no longer present and rapidly explodes back into liquid state. Because the volume of bubbles is mostly larger than the volume of the same liquid. So the bubble burst is a transition from a large volume to a small volume.
Cavitation in the process of bubble burst when all the energy concentrated on the point of rupture, resulting in thousands of Newtons of impact, shock wave pressure up to 2 ×103 MPa,** more than the fatigue failure limit of most metal materials. At the same time, the local temperature is up to several thousand degrees Celsius, and the thermal stress caused by these hot spots is the main factor to produce cavitation damage.
Flash produces erosion damage, forming smooth wear marks on the surface of parts. Like sand sprayed on the surface of the part, the surface of the part is torn, forming a rough slag hole like outer surface. In the high pressure differential conditions, very hard disc and seat will be damaged in a very short time, leakage, affect the performance of the valve. At the same time, in the cavitation process, the bubble burst released huge energy, causing the vibration of internal parts, producing noise up to 10 kHz, the more bubbles, the more serious the noise.
Methods for preventing cavitation damage
Regulating valve flash is not preventable, can do is to prevent the destruction of flash. In the design of regulating valve, the factors affecting flash damage mainly include valve structure, material properties and system design. Cavitation damage can be prevented by zigzagging path, multistage decompression and porous throttling valve structure.
1) Valve structure
Although the valve structure has nothing to do with flash, it can restrain the damage of flash. The angular valve structure with medium flowing from top to bottom can prevent flash damage better than the spherical valve. Flash damage is caused by high velocity saturated bubbles impacting the surface of the valve body and corroding the surface of the valve body. Because the medium in the angular valve flows directly to the center of the downstream pipe inside the valve body, rather than directly impacting the body wall like the spherical valve, the destructive force of the flash is weakened.
2) Material selection
In general, materials with higher hardness are more resistant to flash and cavitation damage. Hard materials are commonly used to manufacture valve bodies. Such as power industry often choose chromium molybdenum alloy steel valve,WC9 is one of the commonly used anti-corrosion materials. If the downstream Angle valve is equipped with high hardness of the material pipeline, the valve body can choose carbon steel material, because ** in the downstream part of the valve body only flash liquid.
3) Tortuous path
One way to reduce pressure recovery is to pass the flow medium through a throttle with a zigzagging path. Although this zigzag path can have different forms, such as small holes, radial flow path, 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) Multi-level decompression
Each stage of multistage decompression consumes part of the energy, making the inlet pressure of the next stage relatively low, reducing the differential pressure of the next stage, low pressure recovery, and avoiding the generation of cavitation. A successful design allows the valve to withstand a large differential pressure while maintaining the pressure after the contraction above the saturated pressure of the liquid, preventing the production of liquid cavitation. Therefore, for the same pressure drop, one-stage throttle is more likely to produce cavitation than multi-stage throttle.
5) Porous throttling design
Orifice throttling is a comprehensive design scheme. The use of special seat and valve disc structure form, make high-speed liquid through the valve seat and valve disc each point of pressure is higher than the temperature of saturated steam pressure, and the use of convergence jet method, so that the liquid kinetic energy of the regulating valve due to mutual friction and converted into heat energy, so as to reduce the formation of bubbles. On the other hand, the bubble rupture occurs in the center of the sleeve, avoiding direct damage to the seat and disc surface.
The main technical performance of valve strength performance
The strength performance of the valve refers to the ability of the valve to bear medium pressure. The valve is a mechanical product that bears internal pressure, so it must have sufficient strength and stiffness to ensure long-term use without rupture or deformation.

The sealing performance

Valve sealing performance refers to the valve sealing parts to prevent media leakage ability, it is the most important technical performance indicators of the valve. There are three sealing parts of the valve: the contact between the opening and closing parts and the valve seat two sealing surface; Packing and valve stem and packing box matching; Joint of body to bonnet. One of the former leakage is called internal leakage, which is usually said to be lax, it will affect the ability of the valve to cut off the medium. For the block valve class, internal leakage is not allowed. The latter two leakage is called external leakage, that is, media leakage from the valve to the valve outside. Leakage will cause material loss, pollution of the environment, serious will also cause accidents. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.

The flow of medium

Medium through the valve will produce pressure loss (pressure difference before and after the valve), that is, the valve has a certain resistance to the flow of medium, medium to overcome the resistance of the valve will consume a certain amount of energy. From the consideration of energy saving, design and manufacture of valves to reduce the valve resistance to the flow medium as much as possible.
Opening and closing force and opening and closing moment

Opening and closing force and torque are the forces or torques that must be applied to open or close the valve. Close the valve, the need to make the open-close part and send a form a seal between the two sealing surface pressure, but also overcome between stem and packing, the valve stem and between threads of the nut, valve rod end bearing friction and other parts of the friction force, and therefore must exert closing force and close moment, in the process of opening and closing, the valve is required for opening and closing force and the open-close torque changes, Its maximum value is at the final moment of closing or the initial moment of opening. Valves should be designed and manufactured to reduce closing force and closing torque.

Opening and closing speed

Opening and closing speed is expressed as the time required to complete an opening or closing action of the valve. General valve opening and closing speed is not strict requirements, but some conditions have special requirements for opening and closing speed, such as some requirements for rapid opening or closing, in case of accidents, some requirements for slow closing, in case of water strike, which should be considered when choosing the valve type.
Movement sensitivity and reliability

This refers to the valve for medium parameter changes, make the corresponding response to the degree of sensitivity. For throttle valve, pressure reducing valve, regulating valve and other valves used to adjust the parameters of the medium as well as safety valve, trap valve and other valves with specific functions, its functional sensitivity and reliability are very important technical performance indicators.

The service life of the

It represents the durability of the valve, is an important performance index of the valve, and has great economic significance. Usually in order to ensure the sealing requirements of the number of times to express, can also be expressed by the use of time.