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
Richard,
A dredge pump is designed to pass solids. The internal passageways are designed to pass a specified minimum object size, and this is rated as ‘minimum sphere’ size. The minimum sphere size affects the impeller and the casing passageway sizes. Particularly in the small-to-medium scale pumps, the choice of impeller vane number and the vane geometry are often the result of design compromises aimed at achieving the specified minimum sphere size. The ‘simplified’ appearance you speak of is probably because the vanes have to have an excessively large passageway opening that can limit the degree of vane curvature and overlap, and limit the vane number as well.
The hydraulic design compromises for a solids handling pump typically results in a steeper H-Q rise to shutoff which can be a desirable performance attribute for many solids handling applications, but with reduced efficiency compared to a non-solids handling pump.
These are excellent questions.
Thank you,
Randal
Dear Randal,
How to choose good vane design for sand dredge pump? (Is this a relevant question?)
I have seen some impellers for sand dredge pumps with similar vane design as for water. But some others have simplified designs (4-vane but shorter ones, similiar to vane design for 6-vane).
What is the purpose of this simplified design? How does it affect the dredge pump performance?
Do you have any experience on this?
Thanks,
Richard
Dear Richard,
The trend for increasing impeller vane number is a reduction in vibration levels. Except, if the new vane pass frequency is near a resonant condition, then vibration levels can be much higher.
For process pump applications it is almost always more desirable to select a single stage pump over a multi-stage pump unless there is a substantial penalty in motor sizing.
Here’s what I understand of your inquiry. Let us say your motor speed is assumed to be approximately 2970 rpm. Then 140 meters head from a single 315 mm diameter impeller is possible, but detailed hydraulic design evaluation of both the impeller and the stationary casing components would be required.
I hope this helps.
Randal
Dear Randal,
All of these informations are really interesting.
I’m fully agree with you about the pump efficiency is not the first priority of petrochemical industry, and I think will be not in near futur. Just due to the capacity reduction of the companies located in Europe…
But for me the rated ratio flow/head is moving for a lower flow rate with similar head.
If increase the vane numbers is occur instability (vibration), does the multistage pump is still the best solution?
Or a vane geometry can easily allow more than 140mCL for 315mm diameter single stage?
Thanks for supporting,
Richard.
Kalpesh,
It depends on the design you are starting with. If the design has a low vane number for the given pump specific speed, then increasing vane number is often effective. Some higher specific speed impeller designs do not respond favorably to an increase in vane number. Often for impeller vane number changes, the vane design itself has to be modified to ensure adequate inlet and outlet areas. In other words, one may not be able to simply change the vane number without also modifying the vane geometry. Sometimes the vane inlet and discharge tips can be adjusted and sometimes an entirely new vane design is necessary.
Besides increasing head, an increase in vane number can possibly introduce instability in the head rise at reduced (throttled) flow conditions. This is potentially a problem when there are pumps operating in parallel.
Good luck with this,
Randal