Suction pluggage cavitation 4

[mjpetrag]

I have a cooling tower with a 2' water level sitting 20' above 3 pumps that routinely swing from 190' to 130' about every 30 seconds. There are suction screens with a bunch of junk on them right in the pan. The suction screens sit on top of 3 respective 48" pipes and below that are vortex breakers for each suction. These 3 pipes feed the suction header to the pumps. The question I have is a loss issue. The difference in water height is about 6" across the screens. So that means a 1/2'(.22 psi) loss in the suction piping is causing the cavitation? It surprises me that the pumps are so close to the NPSHr. Unfortunately, I don't have NPSHr curves for these pumps because they were built before NPSHr curves were imposed on pump curves. I have a hard time believing there is another pluggage downstream of the vortex breaker causing an additional pressure drop, but maybe that is the case.

Thoughts?

-Mike

 

[Artisi]

Had a good read of the article and have a couple of things to disagree with.
1. How do bends in the suction line cause air entrainment. Air entrainment can only be introduced at the source.
Poorly positioned bend can cause turbulence resulting in a mis-match of flow onto the impeller blade with resultant noise and vibrations etc. I accept that poorly positioned bends could result in a vortex being formed and may causes poor flow conditions into the impeller eye.

2. Pumping of fermenting or foaming agents / liquors.
If this is the case, please explain why pumps installed in froth floation handling (which can contain more froth than product) in the mineral dressing area of mineral process plants don't show signs of cavitation.
Like-wise in the paper industry, paper stock can be extremely aerated and in 15 years have never seen any cavitation damage from aerated paper stock.

Sorry but can not accept that entrained air can cause the same damage as cavitation.

 

[Artisi]

For those interested in cavitation / entrained air the following is an interesting article by Joe Evan PhD - it tends to be long winded but describes nicely the formantion and collapse of cavitation bubbles in a pump.

http://www.pumped101.com

This gives the the lead page -
navigate to Basic Hydraulics - centrifugal pumps
then
Cavitation - A Largely Misunderstood Phenomenon

On page 6, air within the pumped liquor is also addressed:

" -- Unlike an air bubble, which simply dissolves into the surrounding water as the pressure increases, the vapor bubble actually changes state from gas to liquid."

 

[Artisi]

Think I might hang up my hat after this post as I can find no real evidence of entrained air causing cavitation like damage, other than a couple of articles which all closely resemble each other.

For interest I searched out the following article which again mentions entrained air and it having no effect in cavitation like damage:

"It may be interesting to note that cavitation itself is a source of nuclei in many facilities. This is because air dissolved in the liquid will tend to come out of solution at low pressures, and contribute a partial pressure of air to the contents of any macroscopic cavitation bubble. When that bubble is convected into a region of higher pressure and the vapor condenses, this leaves a small air bubble that only redissolves very slowly, if at all."

http://caltechbook.library.caltech.edu/22/1/chap5.htm