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
Dear Pritesh,
Because your comment pertains to a post about impeller vane number, it is not clear whether your question is about the number of impellers or the number of vanes. But I will at least give a short answer to both possibilities.
Concerning the number of stages for a multi-stage pump, there is no set number. The number of stages can be either odd or even. It primarily depends upon meeting the required head. If it is a custom design, the manufacturer will usually select a design specific speed that provides an optimal balance of the number of stages, head per stage and efficiency. For very high head, mostly types BB3 and BB4 applications, the manufacturer will deliberately choose a specific speed to limit the maximum number of stages.
Concerning the number of impeller vanes in multi-stage pumps, type BB3 pumps will normally have impellers with odd vane numbers in order to avoid two vanes passing the double volute lips simultaneously in order to avoid reinforcement of pressure pulsation magnitude.
I trust this answers your question.
Best regards,
Randal
In multi stage pump, impellers r always in odd numbers??? If it correct Pl. Give the reason of it.
Dear Danny,
The impeller diameter and pump speed determine the pump total head. So for a given speed, the larger impeller diameter would be expected to generate more head than a smaller one, but both yielding approximately the same volume of flow. The differences in pumping volume based on different vane numbers is generally that the higher vane numbers will yield more flow and some additional total head. This holds true up to a point where increasing the number of vanes may not yield additional flow, but may, in fact, cause a reduction in flow due to vane blockage effects.
I hope this answers your questions.
Best regards,
Randal
Hi,
On a 6-inch centrifugal mixed sand and gravel pump with a large diameter impeller and a small diameter impeller what is the difference in discharge volume? I also would like to know what will be the difference between a 1-vane impeller, a 2-vane impeller and a 3-vane impeller on the same type of pump in terms of discharge capacity.
Thank you for your assistance.
Dear Reese,
Whether changing impeller number is the best solution for either of these two technical issues cannot be answered here in this blog. Changing from 6 to 7 impeller vanes may very well be the answer, but there is some risk that the vibration problem does not resolve.
On the vibration side, there are both excitation and response frequencies. The various harmonics, side bands and possibly other attributes of the frequency spectrum must be considered for both the rotating and stationary components. Expert vibration analysis coupled with rotordynamic and structural frequency analyses may be required.
Concerning the higher capacity, the new system head versus flow conditions will need to be looked at to determine the appropriate pump hydraulic modifications. These could include design changes or alterations to both the impeller and the volute or diffuser.
Of course, if it determined that a different impeller vane number will solve the vibration problem, then the hydraulic modifications would incorporate that.
I hope this helps.
Best,
Randal