Heuristic for Maximum Pump impeller End of Curve Motor Power 1

[Sawsan311]

Dear All,

For a design of a new pump, it is highly recommended to keep room for future expandability. In fact API 610 section 6.1.4 is also indicating the head rise from rated condition by 5 % with regards to potential impeller increase.

In absence of available vendor data during the early design stage, if we want to design the pump motor for the maximum end of curve operation at maximum impeller in order to ensure that no pump replacement would occur in future upon upgrading pump impeller, is there any rule of thumb which can be used in conjunction with affinity law when estimating the motor power demand for the maximum impeller. If we know a typical rule for correlating the rated impeller with the maximum impeller within a horizontal OH pump type, we can estimate a preliminary prorated value of upgraded impeller end of curve motor power load based on the approximation from the calculated motor power for the rated impeller.

Are there any rules of thumb available for the above?

Thanks

Regards,

 

[LittleInch]

25%??

AFAIK,there's no reliable way to judge rated impellor to max impellor. Each pump will be different.

Affinity law is about speed of the impellor.

Unless you really think you're going to get an increase in flow or head this is not a good way to proceed unless you like wasting money.

Bigger motor needs bigger cables, starters and bigger starting current. And all because you might one day perhaps need a bigger impellor?? Really?

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[pmover]

what's wrong w/ asking the pump mfg or using the pump HP curve at max impeller size?

 

[LittleInch]

API 610 6.1.4 says (my highlight).

"Pumps shall be capable of at least a 5 % head increase at rated conditions by replacement of the
impeller(s) with one(s) of larger diameter or different hydraulic design, variable-speed capability or use of a blank
stage.
This requirement is intended to prevent a change in selection caused by refinement of hydraulic requirements
after the pump has been purchased. It is not intended to accommodate future expandability. If there is a future
operating requirement, it should be specified separately and considered in selection."

All this means is that there is a little bit of leeway if required to fine tune your pump.

My point is that I think your statement " it is highly recommended to keep room for future expandability" is not correct or commonly applied.

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Also: If you get a response it's polite to respond to it.

 

[Sawsan311]

Dear All,

thanks for the feedback, I do agree that accommodating future expansion in design may lead to oversized motor with mandated high start-up current, however, this is really an important factor to be able to upgrade your impeller without facing future motor power limitations. This will also aid in eliminating the requirement for additional pumping units and cope with systems where variation in backpressure is expected due to downstream conditions.

My question was quite clear, in case no pump curve is available from vendor and you need a quick estimate of a rise in pump impeller from rated impeller, what would be an estimate. I can deduce from your responses that there is none and pump vendor will anyways need to be approached.

Regards,

 

[LittleInch]

You would be better off with a VFD if you're that uncertain over downstream conditions.

But 25% is a fairly good number. Most pumps have a fairly limited range of impellor sizes if it's one stage.

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Also: If you get a response it's polite to respond to it.

 

[Sawsan311]

Thanks MR. LittleInch , if we consider roughly 25%, knowing the end of curve motor consumption at the rated impeller, can we approximate the maximum impeller end of curve motor as 1.25 times the rated impeller power ignoring the fact that the higher impeller would operate over a wider curve and extends to more than the end of curve flow of the rated impeller.

Thanks

Regards,

 

[LittleInch]

It's as good a number as I can think of.

Some pumps you will be at the low end of the impellor diameter, others at the bigger end.

So you will be somewhere between 30 to 10%.

25% is as good as you're going to get. IMHO.

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Also: If you get a response it's polite to respond to it.

 

[Sawsan311]

Thanks LittleInch for your response,

taking the 25% approximate margin for maximum impeller diameter, we can approximate the End of curve power as 1.25^3 with a margin towards run out as additional 20% considering original rated point is close to BEP (80-110%). Do you agree that this would correspond to roughly 2.4 multiplier with regards to maximum impeller end of curve power

 

[LittleInch]

Yes, that looks about right, but highlights why I don't think it is a good idea. If your motors are then 2.5 to 3 times bigger than you need them then that is a huge extra cost.

I would guess 90-95% of all pumps in a plant stay exactly as they are, so to add this much extra cost in the vague potential possibility that one pump would need a bigger impellor seems madness to me...



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
A good reference about pumps :

https://mcnallyinstitute.biz/06-html/6-04.html

Pierre