CAVITATION DIAGNOSTICS

[PUMPDESIGNER]

Would like input on the following statement I downloaded from a website.

"If impellers in your centrifugal pumps look like "iron worms" have eaten through the center of the impeller, there is suction side cavitation. If you notice damage around the outer diameter of the pump impeller, and in the casing, this is probably discharge cavitation."

I will start off by saying that I disagree with the statement. I believe suction side cavitation damage location on the impeller will be located where the cavitation bubbles are collapsing due to increasing pressure. This may be near the center, in the middle, or on the outside edge of the impeller. Therefore I conclude that cavitation damage observed on the outside edge of an impeller is NOT as the article states "this is probably discharge cavitation".

Does anyone have firm opinions or educated guesses?
I would like contradiction to my opinion if I am wrong.
Richard Neff
Irrigation Craft

 

[d23]

PUMPDESIGNER:

I would not question your personal statement. I would ask what will cause pitting in the eye assuming the web site referenced has case history of that.

 

[Kawartha]

PUMPDESIGNER,

My understanding is that suction recirculation will cause cavitation damage on the high pressure side of the impeller vane, near the leading edge of the vane (to observe this type of cavitation, use a small mirror to look at the inside of the vane). Once suction recirculation starts, damage can extend further into the pump suction (upstream)with further reduced flow.

Low NPSH cavitation damage would be expected to occur on the reverse side of the vane, or the low pressure side. This is one of the main methods of evaluating cavitation - what side of the impeller vane is damaged?

Discharge recirculation damage normally affects the high pressure side of the impeller vane and possibly parts of the volute or diffuser.

 

[d23]

PUMPDESIGNER:

Once again Kawartha is correct. Try this link at McNally Institute.

http://www.mcnallyinstitute.com/CDweb/troubleshooting/pump-troubleshooting/pt009.htm

 

[PUMPDESIGNER]

I'm not moving forward here, stuck I suppose.
If cavitation is caused by insufficient NPSHr, then where the bubbles collapse is where the damage will occur, wherever that happens to be. If that happens to be near the outside edge of an impeller then that is where it is.

I have seen minor cases where the minor cavitation resulted in cavitation bubbles collapsing early in the impeller, damage occurred closer to the eye. But I think also that I have seen severe cases of suction cavitation where the bubbles were so severe that they persisted all the way through the impeller and many collapsed on the outside edge of the impeller and caused damage there also.

I went through all of McNally's stuff (I have his CD). I could not find anything.

I have considered Kawartha's statement about leading edge versus trailing edge. That could be key but am not sure.

I have considered re-circultation often, but never have understood it. How does re-circulation cause cavitation? Turbulence yes, other problems yes. Cavitation caused by re-circulation escapes me. I have even recently examined an incredible high tech video showing re-circulation at a pump discharge. But how can that cause cavitation? Richard Neff
Irrigation Craft

 

[PUMPDESIGNER]

Sorry, I looked again at McNally's stuff and found his information on that. I still however do not understand. His explanation is perhaps good, but I cannot see it. I like McNally a lot so that is not the problem. I will have to study this to get it. Richard Neff
Irrigation Craft

 

[PUMPDESIGNER]

OK, I just found some information on the subject.
My opinion (fast changing) at this time is as follows:
Kawartha's idea that the leading or trailing edge of the vane is important.
However the situation is not quite that simple.
On severely damaged impellers cavitation from insufficient NPSHr may cause damage throughout the impeller and the volute case. I think that cavitation on impellers with minor damage may be more diagnostic based on leading or trailing edge of vane.

I found an article that helped me on re-circulation. Turbulence causes eddys, which are restrictions, that in turn cause increased fluid velocities, that in turn cause pressure drops and cavitation. In fact,re-circulation is capable of producing huge cavitation perhaps even exceeding what NPSHr problems can do.

Now I am ready to help the 3 Stooges install that plumbing system better.

I think I am weak on this because I work almost exclusively with low energy pumps that are less vulnerable to re-circulation. Richard Neff
Irrigation Craft

 

[d23]

PUMPDESIGNER:

I was looking for a good explaintion. I would guess we found the same link. About half way down this page is a "Recir Cavitation" explaintation. If this is different from what you found you may want to look at it.

http://www.osmonics.com/products/Page785.htm

David

 

[Kawartha]

PUMPDESIGNER,

Just want to clarify. My understanding of leading and trailing edge of the impeller vane is the suction end (liquid enters impeller at the leading edge) and discharge end of the vane. Note that in my post, I have defined the side of the vane as high or low pressure side. This is an important part of identifying the type of cavitation observed. Hope I have not misinterpreted your comments.

 

[Kawartha]

d23 & PUMPDESIGNER,

I've been trying to come up with a simple example to help explain (really to myself) the complicated concept of suction recirculation. If you think the following is B.S. or helpful, let me know. Expand or modify as you see fit.

Think about a 10 lane highway with bumper-to-bumper traffic in each lane. The traffic must converge into 8 then 6 lanes. The continually inflow of traffic pressures the downstream traffic until somebody says "I can't continue" and reverses direction, then more of the cars follow. Keep in mind this is occuring in a centrifuge. So energy is being added continuously which results in extreme pressure being exerted on the cars reversing direction of flow. This results in very high velocity in the reverse or upstream direction and of course their ultimate destruction (in our case the high velocity converts to low pressure, and creation of vapour).

This scenario helps explain the fact that suction recirc. with additional reductions in flow rate, result in cavitation upstream of the impeller.

It also helps to understand the concept that high suction specific speed pumps have a high tendency for suction recirculation. High suction specific speed pumps are designed for low NPSHr. This means that they have a large suction diameter. So instead of a 10 lane highway, we start with a 12 or 14 lane highway, necking down to 8 or 6 lanes.

 

[d23]

Kawartha

I see where you are going here. The one thing I would add is:

"One picture is worth a thousand words"

It would be nice to see side by side pics that shows the two.

 

[PUMPDESIGNER]

d23
Yep, that osmonics page is the one I found useful.

Kawartha
I am glad you clarified, I had completely misunderstood you.

d23
It would be great to have some photos. I have many and I would like some opinions. I will post some on a site I run and then start a new thread to let you know the URL and get your opinions. Thank you in advance for your help. Richard Neff
Irrigation Craft

 

[BobPE]

pumpdesigner:

just to throw some fresh air on to the subject....that website you quoted in your first post actually used the term "iron worms?"

Am I the only one that finds that funny and at the same time scary? LOL

BobPE