Flashing and cavitation are common, harmful, and frequently misunderstood occurrences in control valves. Here, we’ll answer some FAQs about these phenomena.
What’s the difference between flashing and cavitation?
The words flashing and cavitation are frequently uttered in the same breathe. But they aren’t the same thing.
In a previous article, we described cavitation as 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.
What are the effects of flashing and cavitation on valves and processes?
If not properly controlled, flashing and cavitation can both have deleterious effects on your control valves and your processes.
In terms of processes, flashing reduces the flow, which increases the flow rate and decreases the pressure. This article in Valve Magazine explains this effect well:
“…when a liquid encounters a smaller flow area, the liquid must accelerate to maintain continuity–that is, to retain a relatively constant volumetric flow rate. This is much the same as the way a river tends to meander and run slowly when it’s flowing through a wide plain, but becomes fast-moving rapids or whitewater when the river encounters a narrow canyon.”
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.
In the previous article, we noted that cavitation causes wear and tear on the valve, as well as noise and vibration that can damage the surrounding equipment. Let’s look at these effects in a little more detail.
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.
How can you tell when flashing and cavitation are occurring?
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.
How can you prevent flashing and cavitation?
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.
In their classic Process/Industrial Instruments and Controls Handbook, Gregory McMillan and Douglas Considine note that “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. Here are three system level and two control valve level controls McMillan and Considine describe.
System level controls
- 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 noncondensable 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. Explore several Masoneilan control valves that have multi-stage, variable resistant trim.
Flashing and cavitation are complex problems, and there’s no one-size-fits-all solution. But they’re important issues to deal with — for the success of your processes and the lifespan of your equipment. If you have more questions, or need help selecting the right control valves for your applications, give us a call or shoot us an email. We’d be happy to help.