Split Case Pump Cavitation - Trouble Shooting

[nationalpump]

We have a split case pump in for repair and there is excessive cavitation damage to one side of the casing around the wear ring fit. The impeller vanes on both sides of the impeller have 1" diameter holes at suction. There is relatively no damage to the discharge portion of the casing nor the rest of the impeller. The suction line is 14" and this pump is directly at the end of a header system. I am wondering what would cause such devastating cavitation damage to only one side of a split case pump. I am sure there is more information I can offer although I would like to know your thoughts before going into excessive detail. I am guessing that excessive turbulance on one side of the case has something to do with it. Thanks for giving this some thought.

 

[vpl]

Is it possible to see a picture?

Patricia Lougheed

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[Artisi]

More than likely approach pipework - photo of the damage and of the pipework leading upto the pump might assist.

 

[JJPellin]

As noted, more information would be helpful. Why do you believe it is cavitation damage? The way you describe the case damage, it sounds like a problem we have been seeing lately with double suction impellers in radially split pumps. The damage is confined to the surface between the wear ring and the discharge volute on one or both sides. We have been seeing this in heavy vacuum gas oil service with CA6NM cases. I suspect a corrosion-erosion mechanism. I think we are getting localized high velocity in this area that is stripping away the pacifying corrosion products and accelerating the corrosion. I suspect some sort of turbulence or vortex shedding down the shrouds on each side of the impeller. We are modifying a pump with full diameter shrouds and swirl breaks down these faces. We are also hard-surfacing the affected area with HVOF TC.

Your description of holes through the impeller vanes tends to fit better with suction problems, as Artisi noted. But, you may have more than one problem. Do the holes in the impeller vanes appear to be originating from the visible side of the vane, or the back side?

A picture would be very nice. A description of the case and impeller materials and the process fluid would be good, too.


Johnny Pellin