Vortices in Centrifugal Pumps and the Resulting Damage 3

[AngloEngineer]

Please could someone let me know where I can find information on the different types of vortices that can be generated inside centrifugal pumps, their nature, what causes them, what damage they can do and what can be done to prevent or fix their presence?

This is purely for my own learning and development as an engineer and I have no immediate application to use as an example where vortices are a problem.

Thanks for your help.

 

[SNORGY]

I would have referred you to Karassik's book (The Pump Handbook) ... but I came across this link via Google, which ought to be a good start.

 

[pennpiper]

Vortices in Centrifugal Pumps and the Resulting Damage
should read:
Cavitation in Centrifugal Pumps and the Resulting Damage

Just terminology.

prognosis: Lead or Lag

 

[AngloEngineer]

Pennpiper, could you please elaborate.

 

[HPost]

Cavitation and vortices are not the same thing at all.

Cavitation is caused by reducing the pressure to point below the fluid vapour pressure.

Vortices are rotating regions of fluid with the most damaging vortices being the ones created behind the impeller that can and do lead to seal damage.

Other smaller vortices created within the pump are termed recirculation and are less damaging but still troublesome none the less.

HPOST CEng MIMechE

 

[JJPellin]

The linked article from SNORGY looks like a very good reference for vortexing that occurs outside a pump. A vortex inside a pump would usually be associated with recirculation. It is most damaging if it results in localized erosion or cavitation damage. I attached a picture of some erosion/corrosion damage that may be associated with vortex formation within a pump. We commonly see evidence of cavitation in the vibration of pumps running at very low flow that we associate with suction recirculation cavitation.

Johnny Pellin

 

[AngloEngineer]

JJPellin, thanks for the picture, there's some serious stuff going on there, very interesting.

Thanks for your post SNORGY, and Pennpiper thank you for yours, I am genuinely interested in any elaboration on your statement that you might have, you'll see why in the details below.

JJPellin, Relating to what you said regarding vortices resulting in localised erosion and cavitation damage. Please could you elaborate on the link between vortices and cavitation. I only ask this because my understanding was exactly as HPost stated, also bearing in mind that increase in temperature from a number of factors can have an impact also; although in the last few minutes I found an excerpt from the book that SNORGY references, page 172, that refers to the formation and collapse of vortices in a certain location producing noise and cavitation. I am genuinely interested and not trying to trip you up, just incase you thought I was by asking for clarification.

Thanks again everyone.

 

[HPost]

There needs to be a clear distinction between cavitation that occurs through a lack of adequate NPSH and errosion that occurs through localised fluid acceleration.

The damage shown in Jonny's picture is on the cone. With a double shrouded impeller this would not be caused by cavitation as the there would be minimal flow and pressure in that region. Even with an open impeller, the pressure recovery that leads to cavitation would not tend to be in that area. If this damage was through cavitation, the impeller would be scattered around the system, assuming that it's not made of something super hard.

The damage in the picture would suggest some localised fluid acceleration that has eroded the pump casing.

The smooth finish suggests that the damage has been done by fast moving fluid. The explosive erosion in cavitation is more like pitting and has a very rough surface.

HPost

 

[JJPellin]

I guess I would say that any damage associated with vortexing inside a pump would relate to velocity. The picture I attached was naphthenic acid corrosion on 12 chrome. I classify it as erosion / corrosion because it occurs in areas where the velocity is high enough to strip away the corrosion product to keep exposing fresh metal. Areas of extreme turbulence or vortex formation can produce the high velocity needed for this to occur.

Cavitation is a phenomenon associated with pressure. But, as we all know, a velocity increase results in a pressure decrease. If any point in a vortex has a high enough velocity to drop the pressure below the vapor pressure, then vapor bubbles can form. If the surroundind fluid is at lower velocity and higher pressure, then there is ample opportunity for the vapor bubbles to collapse. That is cavitation.

In my pump from the picture, we added swirl breaks to the surface of each head. This produces a stable pattern of turbulence, but prevents the formation of large vortices. That is why we have straffing around our heater stacks. That is why golf balls have dimples.

Johnny Pellin

 

[DubMac]

So those short curved tears on the cover of my golf balls are due to cavitation??....Interesting. Maybe I should swing harder.

 

[tarrassac]

Dear all,
thank you for this mostly interesting thread!