Is there an ideal impeller vane number?
Randal FermanCentrifugal pump design and manufacturing has been around for over 200 years. If there were an ideal impeller vane number, it seems likely someone would have discovered it by now. However, just because an ideal vane number has not yet been established is not sufficient reason to consider it doesn’t exist.
Single stage pump and first stage, high suction performance impellers tend to have fewer vanes – often 3, 4 or 5. Multistage pump series stages, pipeline pumps and large, high efficiency pumps tend to have 7 or more impeller vanes. Vane number might be dictated by special application requirements. Such is the case with solids handling impellers which often have just 2 or 3 vanes and sometimes only 1.
I recall an anecdote I heard decades ago from a senior hydraulic engineer that only 3-vane impellers are found across the entire range of specific speeds. The problem, especially for high speed, high head pumps, is a 3-vane impeller produces significantly less head than, say, a 7-vane impeller. Head rise to shutoff and pressure pulsations are also issues.
But thinking about this quasi-factual anecdote, I’ve occasionally considered that an ideal vane number does indeed exist. If there is a magic vane number, what is it? If there could be only one impeller vane number for all pumps, what would it be? Pump engineers think these kinds of thoughts.
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37 comments
Bob,
It depends upon how it is done. If the objective is to cut the flow in half, then I suggest ‘side-cutting’ to trim down the B2 dimension (impeller outlet), starting from the eye shroud side, to half the width. The impeller eye has to be adjusted so that the velocity triangles are ‘correct.’
This is relatively easy to do if you have a semi-open impeller. If it is an enclosed impeller, you might be better off creating a new one or just finding another pump.
Another option is to design a new impeller and have it cast or 3D printed in either plastic or metal, and finish machined.
Let me know if you need help with this.
Best,
Randal
Mr Ferman,
What is the effect of the impeller internal volume on flow rate? If I decrease the impeller volume by a half and keep the same rotational speed will I decrease the overall pump flow rate by a half?
Thanks,
Bob
Erich,
Another comment if I may. Your inquiry starts with “Is there are set rule….” In the practice of pump hydraulic design, I seldom find a “set rule.” The design space is multi-dimensional and it is often the case that a desired performance characteristic can be achieved with a variety of different hydraulic geometries. Of course, the more that constraints are placed on the problem and when design optimization is done, the range of possible geometry variations is reduced. On the other hand, in the industrial world, it is often the case that a “workable” design is all that is needed and that opens the door to a wider range of possible solutions.
Randal
Erich,
Glad you asked! Not because I can directly answer your question about the 16×16 gravel pump. But you may be saving yourself some grief. In the world of pump design, an assumption that one can simply scale up a working design may yield unanticipated and undesirable results.
What I would do, as a minimum, is develop the suction velocity triangles for the two successful designs, and then compare those with the proposed new design. To do this, you need to know a design flow, for each case. I would also consider velocity magnitudes – those are always a consideration and with solids handling equipment: you do not want to find that you are too frequently replacing/repairing components due to an excessive rate of erosion.
Developed total head versus flow is relatable to the impeller discharge velocity triangles and I would compare those as well.
I hope this helps,
Randal
Mr. Ferman,
Is there a set rule regarding impeller size (diameter) vs suction and discharge diameter for solids handling recessed 4 vane impeller style gravel pumps. I have built a 6×6 gravel pump utilizing 10" diameter flat-plate 4 vane recessed impeller and the pump performed with great suction power. I have built the same style pump in 8×8 w/20" impeller and it too created tremendous suction. I am interested in building a 16×16 gravel pump and am trying to figure out my impeller diameter based upon the other 2 successful pumps. My rpm will not exceed 1800. Any advise would be appreceated.
Erich