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
I suspect that theoretical calculations would indicated that an infinite number of vanes of zero thickness would be the optimum for a fluid with zero viscosity. As viscosity rises and both the frictional losses due to vane surface friction and the tolerable stress on the vane material are take into account, the desired number of vanes will go down. The optimum number of vanes for a real fluid will be influenced by the viscosity of the fluid which drags on both sides of each vane, causing both head loss and stress on the vane. So, the optimum number of vanes is going to depend on the viscosity of the fluid you need to move. Some really smart (not I) engineer could calculate the optimum number of vanes, but the results will come to a small set of practical choices; more than 2 and probably less than 16. Pump manufacturers are not going to give the freedom to choose any number of vanes you like (unless CNC machining to customize impellers takes over) and, the better pump manufacturers have done their own tests to determine best impeller design for fluids of different viscosities. They should offer guidance with that choice.
Dear Jimmy,
Unless you can find documentation indicating the impeller vane number, that information is essentially impossible to know, short of direct observation. The most reliable observation would be to look at the impeller exit perimeter because sometimes an impeller will have partial length vanes that might not be visible from the suction inlet side.
Good luck,
Randal
PUMP:13 DURCO GR2 MK3 ANSI3 FML SGL AFLA.
How do I know how may vanes the impeller/pump if I do not have the pump in front of me? The pump information is above
Hi Randal,
Your replys which I have read so far had been highly accurate and helpful, thank you! Here is a non standard question which perhaps you could help with. I have a spinning tube or it could also be a cone shape that I want water drawn up in 2 different ways. One to achieve the highest pressure possible and the second for the highest volume. I played with some axial flow pump impeller designs but as there is no central shaft and the drive needs to come from the spinning tube. Figuring out an effective way to do this is tricky as there is no diffuser or I have not figured out a way to integrate one yet. I could perhaps bolt the assembly to the floor and have the shaft stationary with the diffuser connected on the other side of the impeller but there are probably better ways to do this!
If you have some creative ideas it would be nice to touch base.
Cheers
Che
Randal,
Thank you for responding so quickly. The pump in question has an enclosed “double-sided impeller” so a new impeller with the same external dimensions that can be “dropped in” is probably the way to go. This is a special pump for an Aerospace application that is being adapted for another system.
Although I am an engineer this is outside of my area so I have been doing research into the design of turbo pumps that use centrifugal pumps. Although I have found a lot of material (Including the Affinity laws) the information on the impeller seems to be limited to the diameter and the rotational speed with some information on the number of blades. I have not been able to find anything on the internal volume of the impeller which should have an impact on the flow volume. Do you have any references? Also, I may be able to share some more information if I can contact you directly. Is it possible to email you? My email should be in the information I supplied with this reply. Thanks Again. Bob