Often can see the throttle valve, reducing valve disc and seat and other parts of the internal wear marks, deep trenches and pits, most of these are caused by cavitation.

Cavitation is a kind of failure when the pressure and temperature of liquid reach critical value. It is divided into two stages: flash evaporation and cavitation.

Flash is a very fast transition process, when the fluid flow through the regulator valve, due to the seat and disc to form a local contraction of the flow area, local resistance, 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.When the downstream pressure of the liquid in the valve rises again and is above the saturation pressure, the increased pressure compresses the bubble and causes it to burst suddenly, which is called the cavitation stage.In the cavitation process, saturated bubbles no longer exist, but quickly burst back into liquid.Because most bubbles are larger in volume than the same liquid.So bubble burst is a transition from a large volume to a small volume.

In the cavitation process, all the energy is concentrated on the bursting point, generating thousands of newtons of impact force. The pressure of the shock wave is as high as 2 ×103 MPa, which greatly exceeds 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 causing cavitation damage.

Flash produces erosion and damage effect, forming smooth grinding marks on the surface of parts.Like sand sprayed on the surface of the parts, the surface of the parts will be torn, forming a rough slag hole like the outer surface.Under poor conditions of high pressure difference, very hard disc and seat will be damaged in a very short time and leakage will occur, affecting the service performance of the valve.At the same time, in the cavitation process, the bubble bursting releases huge energy, which causes the vibration of internal parts and produces noise up to 10 kHz. The more bubbles, the more serious the noise.

A method of preventing cavitation damage

Flash in the regulator valve can not be prevented, what can be done is to prevent the damage of flash.The main factors affecting flash damage in the design of control valve are valve structure, material performance and system design.Cavitation damage can be prevented by zigzag paths, multi-stage decompression and multi-hole throttling.

1) Valve structure

Although the valve structure has nothing to do with generating flash, it can suppress flash damage.The angular valve structure with the medium flowing from top to bottom can prevent flash damage more than the spherical valve body.Flash damage is caused by high velocity saturated bubbles impacting the valve body surface and corroding the valve body surface.Because the medium in the Angle valve flows directly to the center of the downstream pipe inside the valve body, rather than directly impacting the body wall like the ball valve, the damage of flash is greatly reduced.

2) Material selection

In general, high hardness materials are more resistant to flash and cavitation damage.Hard materials are commonly used to make valve bodies.If the power industry often choose chromium molybdenum alloy steel valves, WC9 is one of the commonly used corrosion resistant materials.If the Angle valve downstream with high hardness of the pipe, its body can choose carbon steel material, because only in the downstream of the valve body has flash liquid.

3) Zigzag path.

One way to reduce pressure recovery is to pass the flowing medium through a throttling piece containing a zigzg path.Although this zigzagging path can take different forms, such as holes, radial flow paths, and so on.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 of multistage decompression consumes a part of energy, making the inlet pressure of the next stage relatively low, reducing the pressure difference of the next stage, restoring low pressure and avoiding cavitation.A successful design allows the valve to withstand a large differential pressure while maintaining a post-shrinkage pressure above the liquid saturation pressure, preventing the formation of liquid cavitation.Therefore, for the same pressure drop, one-stage throttling is more likely to produce cavitation than multi-stage throttling.

5) Multi-hole throttle design

Porous throttling is a comprehensive design scheme.Special seat and disc structure is adopted to make the pressure of high speed liquid passing through the seat and disc at each point is higher than the saturated steam pressure at the temperature, and the method of converging jet is adopted to make the kinetic energy of liquid in the regulating valve converted 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 seat and disc surface.