6 replies to this topic

[Shahnawaz Makroo]

Dear

I want to avoid cavitaion in RO for flashing liquids?Reason of cavitation is as below:

When liquid reachs to the converging portion(vena contra) velocity increases as result of which pressure falls and some times falls slightly below the vapor pressure of fluid and liquid flashs out.But after pressure recovery in divergent portion condensation of begins and cavitaion comes in to the picture.The line amy fail due to high virations as well.


Regards
SAM

[joerd]

Limit the pressure drop.
There are special designs available that help you avoid cavitation, you can probably find them on the web.

[djack77494]

You could try further cooling of the liquid upstream of the RO, but if you have no control over the temperature or the pressure drop, then there's not much you can do. I think for a high pressure drop, you have no options, but that may not be a big problem. Cavitation is not necessarily disasterous; just design for it.

(If you don't find this response satisfies your needs, then please provide more information. Especially, will there be two phases downstream of the orifice?)

[JoeWong]

The magnitude of Cavitation may be reduced by multiple ROs.

[latexman]

For information on multiple orifices to avoid cavitation see the article by Tung and Mikasinovic in Chemical Engineering, December 12, 1983, pgs 69-71 and by Lin S.H. Hydrocarbon Processing, July 1985, pgs 93-95.

[Zubair Exclaim]

Are you using RO or venturi tube .

if its venturi tube then you may have to increase inlet pressure

Edited by Exclamation, 02 April 2010 - 06:06 AM.

[Himanshu Sharma]

As Precisely pointed out by fellow members ,use of multiple orifice in series will be a practical thing.

However i would like to differentiate between Flashing and cavitation.Pressure differential of interest pertaining to flashing and cavitation is the differential between the valve inlet and the vena contracta. If pressure at the vena contracta should drop below the vapor pressure of the fluid (due to increased fluid velocity at this point) bubbles will form in the flow stream.
Now there are two scenarios

1.If pressure at the valve outlet remains below the vapor pressure of the liquid, the bubbles will remain in the downstream system and the process is said to have "flashed".Flashing damage is normally greatest at the point of highest velocity, which is usually at or near the vena contracta.

2.However, if downstream pressure recovery is sufficient to raise the outlet pressure above the vapor pressure of the liquid, the bubbles will collapse, or implode, producing cavitation. Collapsing of the vapor bubbles releases energy and produces a noise similar to what one would expect if gravel were flowing through the RO. If the bubbles collapse in close proximity to solid surfaces, the energy released gradually wears the material leaving a rough, cinderlike surface.

Crane technical paper 410 is an invaluable resource and covers how to calculate the resistance created by an orifice in a piping system

The orifice calculations will give you the head loss across the orifice for a specified flow. (or the flow for a specified head loss). There is no direct relationship between orifice diameter and flow reduction. The head loss across the orifice will cause a flow reduction in your system but in order to calculate the flow reduction you will need to look at the overall system not just at the orifice. i.e when you reduce the flow you will also reduce the head losses through other fittings and pipes in the system and you have to look at the system as a whole.


Overall pressure loss through an orifice depends on the Beta ratio (orifice diameter compared to pipe diameter). There is a curve in ASME Fluid Meters. An equation that fits the curve within 2% is

% pressure loss = 0.958 + 0.022*Beta - 0.934*Beta^2

For example, at Beta = 0.5, % pressure loss = 0.735, overall pressure loss = 0.735 * differential