Suction pluggage cavitation
Suction pluggage cavitation
(OP)
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?
Thoughts?
-Mike





RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
Johnny Pellin
RE: Suction pluggage cavitation
-Mike
RE: Suction pluggage cavitation
Artisi asked a very good question about air entrainment. Many of us may have been thinking only about vortexing. But, if the turbulence with the restricted screens is drastically worse, air could still be entrained and carried into the pumps even without a vortex forming. What is the flow rate expected down each of these inlets. If the velocity in the 20 inch line is high enough, air bubbles could be carried all the way down to the pumps.
Johnny Pellin
RE: Suction pluggage cavitation
The local BAC rep had set the working water level to "what they have always set it to". I started looking into it closely and I called the BAC factory in the US to send me literature on what the working water level should be because their shop drawings did not show what it was supposed to be. I found a numerical discrepancy in their publications and drawings that they sent to me. In short one publication had a higher wroking water level than what was advised in another publication and my tower working water level was set to the lower level. After I discussed it with them they advised that there was an indeed an error and they had to talk to their local rep to raise the working water level in the cooling tower. Problem solved. It caused some havic on my pumps. You have to wonder how many installations were installed incorrectly in the past!!!
I had to get the BAC technical support lady in the US to talk to senior people in thier factory to get it straightend out and come back with an answer for me. You have to remember that as staff change over time, especially in a manufacturing environment, people forget the engineering behind what they manufacture
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
The TWL should be just slightly below the invert of the cooling tower overflow pipe
The TWL should be just slightly below the fan discharge beach plate.
The low water level should only be about 1" below the TWL. Do you have a float operated water inlet valve or is the water level controlled by level probes?
RE: Suction pluggage cavitation
There is a lot of junk on the screens from what I can see, but the water is murky and there may be a lot of stuff plugging up the screens below the surface. Would these symptoms point to a suction blockage on the screens?
-Mike
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
-Mike
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
-Mike
RE: Suction pluggage cavitation
The pump is still under a flooded suction. There will be no cavitation
You are still ingesting air
Do you know what the working water level in the tower is supposed to be.....check with the manufacturer
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
-Mike
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
The solution in this case to get rid of the entrained air was to clean the cooling tower screens in order to get the working water level up to thier proper level.
If it was cavitation you don't have enough npsha and you have to reduce the head loss in the suction piping.
Too many people confuse cavitation with entrained air and if you are not trained to know the difference you could chase a solution that will be costly and does not work. I am glad that you have solved your problem
RE: Suction pluggage cavitation
Yes you are 100% correct regarding the confusion between cavitation and entrained air, and I must disagree with your statements that 1)they sound the same (in some cases it may be so) and 2) causes the same damage (no it does not).
I recommend the following links, 3 of the many references which refute the erroneous statement that entrained air causes the same damage as cavitation. Unfortunately, this statement has been repeated on a number of occasions by well meaning but ill-informed writers simply regurgitating the same incorrect information.
http:/
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If you have any information on detailed studies undertaken on the damage from entrained air, I would be pleased to see and read this information.
RE: Suction pluggage cavitation
Johnny Pellin
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
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
RE: Suction pluggage cavitation
ht
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.
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
Thank you all for the replies.
-Mike
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
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:/
".... 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)."
RE: Suction pluggage cavitation
htt
RE: Suction pluggage cavitation
RE: Suction pluggage cavitation
-Mike
RE: Suction pluggage cavitation
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
RE: Suction pluggage 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. "
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
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:
h
RE: Suction pluggage cavitation
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.
RE: Suction pluggage cavitation
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."
RE: Suction pluggage cavitation
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." htt