Pump Case and Impeller design 1

[Marken]

I am working with a manufacture and we are having our casting and impellers founded overseas. I notice when we received the cases and impeller the cases were machined on the inside to match the contour of the impeller but the contour did not extend to the tip of the impeller. Should this contour extend all the way to the tip of a full diameter impeller?

 

[JJPellin]

IThe short answer is yes. I assume you are talking about an open-faced impeller. I have seen this several times with vertical volute pump from one particular manufacturer. If the pump was purchased with a full diameter impeller, the tips of the impeller vanes would hang off into the volute area of the case. I consider this a design defect with this particular model. We were able to prove by performance testing that this defect made it impossible to get full performance out of the pump with the maximum diameter impeller. That part of the vane that is hanging out without a proper gap to the surface of the case does not produce full head and flow as predicted by the published curves for the pump or by the affinity laws for the increase in impeller diameter. We modified our pump specifications to require that the surface in the pump case is the full diameter of the face of the impeller. This basically prevents us from buying this particular model of pump. As far as I know the manufacturer never corrected this defect. I suppose they didn’t have enough customers complaining to justify the pattern changes. Perhaps now that the economy is slowing down, they might be more open to the idea of fixing this.

Johnny Pellin