Cavitation after a RO

[aatara]

Hello, I have a 316 SS 30" (Schedule XS) pipe after a multiorifice plate in a water cooling systems of a power plant. I have signs of cavitations (sounds). I´d like know the problems associated and the erosion-corrosion rate.

Should I test the pipe unde UT to determine the evolution of the wall thickness?

How can effect this problems to the hidraulics of the RO?

Thanks

 

[25362]

Are you sure the noise is produced by cavitation, and not by flow-induced pulsations caused by vortexes past the multi-orifice plate? Cavitation should be produced by imploding water vapour bubbles. I suggest you visit Igor J. Karassik's Pump Handbook for a review of possible liquid noise sources.

 

[aatara]

Thank you for your answer,

Sadly, I don´t have this book and I don´t know if here in Spain, I can buy it.

The noise is similar to litle stones transport by the flow (I read that this is the typical noise of the cavitation). How can I difference the noise of the cavitation and the flow-induced pulsation?

The flow-induced pulsation can be dangerous?

 

[25362]

Let's start by saying that noise is not my field of expertise and may be other experts would add their opinion in the next future. El libro mencionado es editado por la McGraw-Hill. En el capitulo 8.102 se explica lo siguiente:

"When pressure fluctuations are produced directly by liquid motion, the sources are fluid dynamic in character. Potential fluid dynamic sources include turbulence, flow separation (vorticity), cavitation, water hammer, flashing, and impeller interaction with the pump cutwater. The resulting pressure and flow pulsations may be either periodic or broad-band in frequency and generally excite either the piping or the pump itself into mechanical vibration. These mechanical vibrations can then radiate acoustic noise into their environment.". It continues detailing the pulsation sources in pumps.
On point 4. it mentions, I quote: "Flow-induced pulsations caused by periodic vortex formation when flow is past obstructions and side branches in the piping system." Then it explains flow mechanisms in a centrifugal pump. Continua diciendo:
"Most of these unstable flow patterns produce vortices by boundary layer interaction between a high-velocity and a low-velocity region in a fluid field, e.g., by flow around obstructions or past deadwater regions or by bidirectional flow. The vortices, or eddies, are converted to pressure perturbations as they impinge on the sidewall and may result in localized vibration excitation of the piping...

It continues in developing a frequency-related number called the Strouhal number S[sub]n[/sub] = fD/V
f in Hz; D, in m; V, in m/s.

For flow through restrictions the excitation frequency f = 0.2V/D-0.5V/D
which would resut in S[sub]n[/sub] = 0.2-0.5.
Cavitation noise is, on the other hand, generally a "broad-band" noise.

I personally think that these are not water vapour "implosions", since cooling water contains dissolved air. If there are low pressure areas in the flow stream, these would result in air release rather than the formation of water vapour bubbles collapsing downstream (cavitation). But I'm just speculating.

I tried to bring your attention to the possibilities of noise not produced by cavitation when cooling water crosses a perforated plate, and that probably there is no danger of erosion to the 316 SS pipe wall. I'm sorry I couldn't be of more help. Buena suerte.

 

[25362]

I forgot to suggest that by putting your query through the Mechanical Acoustics/Vibration Engineering forum you may get straight and to-the- point advice. Good luck!