What is Flashing And Cavitation and how can it be cured?

What’s the difference between flashing and cavitation?

The two words flashing and cavitation are interchangebly used but they arent the same.

 According the Bernoulli equation - when a fluid pass a valve seat and the fluid velocity increases - the fluid pressure decreases.

Cavitation

If the fluid speed through the valve increases enough, the liquid pressure drops to a level where the fluid may start to boil, bubble or flash. And when the pressure recovers sufficiently downstream of the control valve the bubbles will collapse upon themselves. This collapse causes cavitation.

Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely destructive and may wear out the trim and body parts of a valve in short time.

What are the effects of flashing and cavitation on valves and processes?

If not controlled the flashing and cavitation can cause the trim to damage.

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.

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.

How can you prevent flashing and cavitation?

• 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.