Globe Type Control Valve CV-8407

The words flashing and cavitation are frequently uttered in the same breath. But they aren’t the same thing.
Cavitation having two phases: first low localized pressure causes bubbles to form, and then the pressure recovers causing the bubbles to collapse. The first phase in this process — when the liquid evaporates to vapor — is flashing. The second phase — when the vapor bubbles collapse — is cavitation.

In terms of processes, flashing reduces the flow, which increases the flow rate and decreases the pressure. 
In an industrial process, the bubbles created by flashing get in the way of the liquid, reducing the flow while increasing the flow rate. The reduced capacity is often referred to as “choked flow.”
Flashing can also cause severe damage to your valves, mostly in the form of erosion of the valve plug. It’s important to note that this damage occurs irrespective of the liquid media flowing through the valve — even valves used for clean water applications can be damaged by flashing.

We noted that cavitation causes wear and tear on the valve, as well as noise and vibration that can damage the surrounding equipment. The damage caused by cavitation happens when the bubbles collapse. This can cause high-pressure shock waves in the system and also cause streams of fluid (often called “micro-jets”) to hit the surface of the valve at high velocities. The result is pitting, most often of the valve body and trim. Over time, this corrosion can cause major surface damage, which makes the valve even more susceptible to further corrosion from cavitation as well as from chemical damage caused by the media moving through the valve.

All processes should be designed in a way that reduces flashing and cavitation in predictable areas. And, if they do happen unexpectedly, hopefully you’ll find out about it before you see the telltale signs of erosion and pitting.
One way you can determine if you might have a problem is just to listen. Flashing makes a hissing sound, and cavitation makes a popping sound. Both of these sounds can be quite loud, comparable to a snow blower.

Finally, given the serious damage that both flashing and cavitation can cause, it’s important to protect your valves and your processes by preventing these phenomena from occurring.
There is no single best method for controlling the problems caused by cavitation.” Controls can be implemented on both the system and the control valve levels, and they recommend system level controls whenever possible.
·        Put the valve in a high-pressure area. This will increase the differential between the fluid pressure and the vapor pressure, making it less likely the fluid pressure will fall low enough for flashing to occur. You can do this by putting the valve as far upstream as possible.
·        Use a downstream restriction device, like an orifice plate or a second valve, to increase the backpressure. This will increase fluid pressure and reduce velocity. This approach is recommended for on-off service only, and you’ll likely still need to account for cavitation at the downstream device.
·        Introduce a non-condensable gas into the flow stream. For processes that can tolerate a gas, this can prevent the violent collapse of bubbles that causes the pitting corrosion.
Control valve level controls
·        Choose materials that will minimize cavitation damage. There is no material 100% resistant to cavitation damage. But, in general, harder materials and those that are corrosion resistant will stand up better than others.
·        Use an anti-cavitation trim design. Anti-cavitation trim designs can handle cavitation in a number of ways, such as pressure drop staging, tortuous-path treatment, dividing the flow into multiple paths, and redirecting the flow away from the surface.