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# shape of bhp vs flow curve for radial, axial, mixed flow pumps4

## shape of bhp vs flow curve for radial, axial, mixed flow pumps

(OP)
In Mark's handbook (old edition) I see typical curves for radial and axial flow pumps. The axial flow gives requires max bhp at min flow and decreases as flow increases.  The radial flow gives min bhp at min flow and increases as flow increases.

Is this true in general?

I have seen a curve which started low bhp at low flow, increased to peak bhp near bep, then started decreasing again. Would that be a mixed flow?

Do bhp vs flow curves for fans have identical behavior (to pumps) for radial, axial, mixed?

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

(OP)
Come on you guys.... give it a try. The simple question is:

What is the general shape of BHP vs flow curve for axial, radial, and mixed flow pumps?

True or False?
Is it monotonic increasing for radial flow?
Is it monotonic decreasing for axial flow?
Is there a maximum point in the middle of the curve for mixed flow?

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

Electricpete!

Your statement on both radial and axial pumps is true. An axial pump consumes more power in the shutoff condition and should not be run in that condition for a long time as you suggested in other post. The power consumption decreases as you open up the valve.

In radial flow pumps it is the other way round, that is why you are always suggested to start the pump in shut off condition so that initial torque on motor will be low.

As for mixed flow pumps, forgive me, I have no practical experience.(But even for some radial flow pumps there is a maximum point on the BHP curve in the middle, not exactly I mean)

True, Fans and pumps behave in a same way.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

(OP)
Thanks for all of the above responses.

I had a talk with a semi-knowledgeable pump engineer on this topic. He walked me through various types in a Worthington pump catalog and I believe that basically he agreed with the statements I made above.

From my limited understanding, most of the single-stage pumps will fall in the pure radial or pure axial categories which will have the steadily rising or falling BHP curve with no peaks or valley's in the range of the curve (BEP and beyond to runout). Most of the mutli-stage pumps will have a mixed characteristic with peaks, valleys and wiggles in the operating range.  Does this paragraph sound correct to you guys?

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

electricpete

with respect to your question about fans: They generally behave in a similar way to pumps because the work at low pressures (usually) and so the air behaves as an incompressible fluid. The info I have is as follows:

Centrifugal fans:
backward curved blade:power rises to max at the middle of flow range then falls at highest flow rates, this is known as a non-overloading power characteristic.

Reference - Air conditioning systems design commissioning and maintenance, Roger Colby Legg ISBN 0 7134 5644 2. I think its out of print now.

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

(OP)
Thanks Shaun.  It seems your discussion for fans is pretty consistent with Vanstoja's discussion for pumps, in terms of dependence on blade curve.

If I can paraphrase both of you are saying that:
- backwards curved blades gives bhp-vs-flow curve with  peak in the middle and low on high/low flows.
- forwards curved blades gives bhp-vs-flow curve continuously increasing.
I also understood van to say that the backwards curve was common on the low/med specific speed which generally corresponds to high/med actual speed.

As you recall my observation was that:
- pure axial flow (common in single stage) gives bhp-vs-flow curve continously decreasing
- mixed flow (common in multi-stage) gives bhp-vs-flow curve with peak in the middle and low on high/low flows.
- pure radial flow (common in single stage) gives bhp-vs-flow curve continously decreasing

I have not looked closely at the curvedness of the blades for pumps curves I checked. Something does not quite add up. One thing is that your formulations do not provide any room for a pump with continuously decreasing bhp vs flow, but I have seen that. I will review a little more and see if I can find an example.

In the meantime, any other comments?

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

electricpete,
There is an excellent book on the subject: Design of Centrifugal and Axial Pumps by Stepanoff, also the Pump Handbook by Karassik, Fraser, Messina and Krutzsch.
Left the Stepanoff book at home (different country) but as far as I recall... there is a math derivation showing that decreasing power vs. flow characteristic for axial pumps.
In layman's terms (I hope)... an axial pump will require more power at very low flows (with constant speed) because the impeller will create eddies... the efficiency goes down the drain so to create very little flow you require an awful lot of power.
As the flow increases so does the efficiency and more than compensates for the increased flow...therefore the power requirements go down.
Do I make sense of am I full of myself?
Cheers.
a.

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

Just a word of warning:

### RE: shape of bhp vs flow curve for radial, axial, mixed flow pumps

To sow,
I wouldn't doubt that mixed flow pumps in the high speed end would approach axial pump' rising characteristic toward shutoff. I just looked up Mixed Flow pump range in the Hydraulics Institute Standard and was a bit surprised to find that officially it extends from 4200 to 9000 (RPM-GPM-Ft). They don't recognize the so-called Francis-Vane pumps in the 2000-4200 range and just lump them in with radial pumps from zero to 4200. Francis vane pumps which generally predominate in the nuclear power industry main coolant pumps behave much more like mixed flow then radial pumps with lots of three-dimensional, secondary flow action not seen very much in mostly two-dimensional radial flow pumps below 1500 specific speed. Perhaps we need an official category recognizing the in-between Francis-vane pumps.

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