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

 

[JJPellin]

I must agree with Artisi. Entrained air does not cause the same sort of damage In fact, the introduction of air is occasionally deliberate in order to soften the collapse of the bubbles and reduce the cavitation damage. Entrained air can cause a noise and will cause a loss of performance. But, not damage.

Johnny Pellin

 

[QualityTime]

Here it is....i have posted this before. If you think about it you are compressing air bubbles whether you are cavitating or ingesting air into the pump. so there has to be some damage. Why would you have only damage when cavitating and not when ingesting air.

 

[QualityTime]

I am not sure why my last post did not append the article. The other thing you would note is that with ingestion of air from a cooling tower the number of bubbles is nowhere near the number of bubbles you would get when you have cavitation

 

[Artisi]

Suggest you take the time to read the first link and would see that the article by Mackay is shown to be incorrect.

I quote part of the article as follows,

"Your statement: "This is because the dissolved air or nitrogen will not implode and should not cause any material damage to the impeller" is, in my opinion, precisely correct. There is a gross, semantical error in the Pump Zone article by Ross Mackay(which is addressed and critiqued in the reader response section) regarding the basic difference between having CONDENSABLE bubbles in the suction fluid and NON-CONDENSABLE bubbles in the suction fluid (such as air, nitrogen, etc.). The former can cause classical "cavitation" and resulting damage. The latter can cause air-binding (or loss of prime)."

You should also read the other articles written by Allen Budris who is probably one of the leading pump guys round today and vastly more experienced than Mackay.

 

[QualityTime]

The following comes from the second article referred to in the first link that you provided. Please note the word....seldom....

Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with liquid. A centrifugal pump can handle air in the range of ½ % by volume. If
the amount of air is increased to 6%, the pump starts cavitating. The cavitation condition is also referred to as Air binding. It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of capacity.

 

[Artisi]

We are now splitting hairs, what does "seldom" mean, every other case,1 in 10 or 1 in 1,000,000, most likely never. Seldom is used here to cover your a_rse as nothing in this world is 100% certain.

As I said previously, the oportunity to read any research information that addresses damage form air entrainment would be most welcome.

As for air handling with centifugal pumps, that is a completely new discussion which we could discuss in a new posting if anyone is interested in starting it up.

 

[QualityTime]

I gave you the paper....so I do not know what you want

I would suggest that the amount of air that actually gets ingested and the size of the air particles have an affect on the damage to the impellor. If you have ever looked inside a cooling water tower you will know what I mean

With true cavitation you get a lot of small bubbles

I think what you will find with all the "experts" they all have different opinions and not all of them are correct. Even though they write a paper does not mean they are right. I have run across a ASCE article on fine air bubble diffusion paper authored by a well known professor in California. It was written by his students. I phoned him up about it and he advised that the paper had the incorrect conclusions...and... yet the paper was published

 

[QualityTime]

This is from the McNally Institute

http://www.mcnallyinstitute.com/01-html/1-3.html#Air

ingestion. Please note the word...SELDOM....not....NEVER...

Air ingestion (Not really cavitation, but acts like it)

A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air gets into as system in several ways that include :

Through the packing stuffing box. This occurs in any packed pump that lifts liquid, pumps from a condenser, evaporator, or any piece of equipment that runs in vacuum.
Valves located above the water line.
Through leaking flanges.
Pulling air through a vortexing fluid.
If a bypass line has been installed too close to the suction, it will increase the temperature of the incoming fluid.
Any time the suction inlet pipe looses fluid. This can occur when the level gets too low, or there is a false reading on the gauge because the float is stuck on a corroded rod.
Both vaporization and air ingestion have an adverse affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization can both occur, they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.

 

[Artisi]

The article writen by Ross Mackay is not a study of the effects of air entrainment on damage in centrifugal pumps' it is an article in a magazine which I an others disagree with and adds nothing except confusion to the cavitation / air entrainment discussion.

However, my intention in replying to your statement was to point out that I and it appears that others do not believe the statement "air entrainment results in the same damage as cavitation".
As these postings can be read widely and can be found with Google and other searches, it is important to point out and clarify errors. If you want to accept seldom that's ok but the blanket statement "entrained air causes the same damage as cavitation" is incorrect.

 

[QualityTime]

Everybody is entitled to their own opinion and I respect that.

 

[mjpetrag]

I tend to agree with QualityTime that there was entrained air getting into the pump suction. The lowest I've seen the pressure in the suction get was around 3# while it was surging. That would give me about 40' of NPSHa, and I have a hard time believing that a double suction pump wouldn't be able to handle that, even far out to the right of its curve.

Thank you all for the replies.

-Mike

 

[QualityTime]

Hi Artisi, I thought I would add this as food for thought:

I think the reason why "seldom" might be used in the McNally papers is because it may have to do, as I said earlier in my posts, the size and quantity of the air bubbles being collapsed and they did not want to go into listing what the size or quantities the bubbles can be without getting any damage to the impeller. It is not an exact science.

In a cooling tower, the entrained air bubbles tend to be very large and relatively speaking "not that numerous". But they are numerous enough to affect the pumping capacity or cause problems to pump seals etc. The same can be said for subsurface or surface vortexes that are sucked into a pump suction. They are "large" and "not that numerous". So relatively speaking there is not that many bubbles collapsing.

If the cavitation is severe, there will be numerous small bubbles collapsing and the damage to the impeller will be "quickly" seen.

I think the POINT of the MacKay article is that the collapsing bubbles, whether it is due to air entrainment or it is due to cavitation, ACTS ON THE SAME AREA of the impeller. How FAST the impeller gets pitted, to me, is dependant on the size of the air bubbles and the quantity of air bubbles being collapsed. I have seen pumps where there is slight cavitation and there is no damage to the pump impeller after years and years of operation.

 

[Artisi]

mjpetrag, post 3 asked this very same question regarding air entrainment.

QualityTime, agreed, everyone is entitled to their opinion, the advantage of living in a democracy (or so they say).

I think Art Montemayor summed up the difference between cavitation and entrained air in this reply

http://www.cheresources.com/invision/topic/10445-pump-impeller-damage-due-to-entrained-nitrogen/

".... the basic difference between having CONDENSABLE bubbles in the suction fluid and NON-CONDENSABLE bubbles in the suction fluid (such as air, nitrogen, etc.). The former can cause classical "cavitation" and resulting damage. The latter can cause air-binding (or loss of prime)."

 

[QualityTime]

This is an interesting article

http://www.lawrencepumps.com/newsletter/news_v04_i12_dec07.html

 

[QualityTime]

One of these days Artisi and I are going to have a world wide summit showdown on pump impeller damage due to air entrainment LOL!!!

 

[mjpetrag]

hahahah...sorry artisi, you were the first one to say air entrainment. your credit is due.

-Mike

 

[mjpetrag]

Just something else I'd like to mention. We took out the rotating assembly of one of these pumps a few months ago. Now I wasn't involved when the assembly was rebuilt so I couldn't see what kind of shape the impeller was in, but I did see the condition of everything else. The only noticeable damage was on the lips where the stuffing boxes sit. It just looked as if the water cut through the metal along the circumferences. I'm guessing this is just normal wear damage of a high capacity pump that's been in service for 30+ years. The damage was similar to the seat of a gate valve that was used for throttling.

Impeller damage I would imagine wasn't evident since I didn't see damage on the volute. I would think the seal would suffer the most damage in an air entrained environment since you don't get the implosion of air bubbles, but you still get the vapor lock effect of entrained air and the possibility of dry running the seal. However, with the limited amount of pumps I've seen running with entrained air, the seals still last for a relatively long time (MTBR on the seals of these pumps is around 2-3 years)

-Mike

 

[Artisi]

Yes we could if air entrainment caused the same damage as cavitation .

The article from Lawerence Pumps is very good, however it doesn't say that entrained air causes damage, what it does say is that a collection of air in the impeller eye can result in cavitation by the pumped liquor being vapourised as flow is reduced due to the lowering the pressure at the impeller eye - same as throttling the inlet.

"The gas collecting at the pump inlet will restrict flow onto the impeller and often will cause a sufficient pressure reduction at the impeller blade inlet for cavitation to occur, even though the pump appears to have sufficient suction pressure. "

 

[Artisi]

Hi mjpetrag,

I don't really need the kudos of being first in with a possible solution to the problem, but what I find interesting, not only in this thread but in many others is, that the solution is flagged very early in the postings but completely ignored as people are looking for involved academic solutions to a simple problem, a bit like this one has developed into.

You last posting is interesting that no damage was evident which seems to put to rest the argument that entrained air causes the same damage as cavitation.

 

[QualityTime]

Hi:

I reread McKay's article more closely (and talked to him)and he is saying:

Air entrainment defines a variety of conditions where
the VAPOR BUBBLES ARE ALREADY in the liquid BEFORE it
reaches the pump.

So he is saying that air entrainment includes (condensable) VAPOR bubbles (and non condensible air bubbles) and he gives examples of how you can get air entrainment that have vapor bubbles.....Note that the vapor bubbles are not as a result of cavitation.

Makes sense to me because that is what is causing the classical gravelly cavitation sound and there are a number of articles on the website talking about condensible and non condensible air bubbles being present.

As far as for the above mentioned cooling tower it sounds like to me it has not been "air entrained" for a very long time so there could very well be no damage. I would be curious to know if he did hear "cavitation" noise


Please read the article again:

http://www.practicalpumping.com/articles/pdfs/Cavitation or Not.pdf