Is there an ideal impeller vane number?

Randal Ferman

Centrifugal 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

Leonardo,
The article does not address positive displacement vacuum pumps, but your question may have some relationship to centrifugal pump hydraulic design. With these, the more efficient hydraulic passageway shapes are closer to being either square or round, as opposed to having an elongated or ‘flattened’ aspect ratio. Also, having less impeller rotating disc area relative to its normal inlet and outlet areas – this is related to specific speed – tend to be more efficient. Centrifugal pump hydraulic designs which minimize the wetted surface area will tend to be more efficient. I should note that there is often a broad range of design solutions for which the differences in efficiency are small.
Not sure that there is a ‘rule of thumb’ on positive displacement rotary vane vacuum pump designs. It would be a valuable exercise – not a trivial one though – to create an analytical spreadsheet performance model to check out possible design geometries. That said, tribology and mechanical wear are likely to be your largest design and practical concerns with the rotary vane vacuum type of pump.
Interesting question – thank you!
Randal

Randal Ferman

Hi there! First of all, thanks a lot for all the knowledge put in here.

I have a question about fixed displacement unbalanced vacuum vane pumps with oil bath. Specifically I would like to know how the rotor width and diameter relation can change pump efficiency (mantaining a constant volumetric capacity). Is there any rule of thumb for this?

Thank you!

Leonardo Marsaglia

Yu,
The inlet geometry is normally based on velocity triangle calculations.
The exit can be based upon:
1) modeling from pumps of known performance and similar specific speed,
2) textbook or handbook design charts and formulas,
3) velocity triangles with impeller slip factor from textbook or handbook formulas, or
4) some combination of these.
Design experience helps.
I trust this is useful.
Yours,
Randal

Randal Ferman

How much does the height at the inlet and outlet sides of the vane affect the performance when designing the impeller?
If there is a good way to select this, I would like to know.

Yu

Dear Steve,
The optimal number of impeller vanes will depend upon the pump type and application. For instance, the need for low NPSHA suction performance often dictates fewer vanes. Or the need for a rising head characteristic over a wide range of conditions, including parallel pumping operation, may dictate vane number choices. In the realm of engineered-to-order high pressure multistage pumps, it is not uncommon to find a vane numbers for the first stage impeller plus a couple of different impeller vane numbers for the series stage impellers. These choices can be dictated by both suction performance and the specified operating point along the head versus flow curve.
Today, a user could hypothetically design his own impeller and a service, repair, parts replicator shop would be happy to make it and install it in the pump of his choice. I see some degree of that happening when a pump user hires a consultant or engages a pump manufacturer to modify or re-rate a pump.
The significant development I now see is an individual’s or a user’s ability to 3D print a prototype part. This democratization of manufacturing is having profound effects in the making of molds, dies and parts for prototype testing and parts for actual industrial machinery service.
For the most part, the discussion between manufacturer and user regarding impeller vane number mainly occurs in the context of vibration or acoustic excitation frequencies and harmonics and is normally only a concern with the higher head per stage pumps.
Thank you for the comment,
Randal

Randal Ferman

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