How accurate or honest is pump head curve ?

[SPP]

Guys,

I am looking to get some real life experience input from members here who had the chance to verify a pump head curve.
Are the pump curves from manufacturer honestly accurate in the upper last 10% of the maximum head ?

I am asking this because I just received a Pentair Whisperflo pump curve table using 50hz electric supply and 2,950 RPPM and it is offering the exact performance with the USA version 60hz at 3,450 RPM. I am confused. Unfortunately I do not have the electrical data to verify BHP. It just does not make sense.

I been looking at SF rating and 1.95 is about the highest on the market for Pentair of 60hz. I am looking at the highest SF of 1.95 for Whisperflo 0.5HP 60hz, no way the 50hz unit can produce similiar performance curve if having the same 1.95 SF. My Pentair Superflo 1.5HP in fact is only a SF 1.10 for a 50hz unit as per name plate, while USA 60hz version is SF 1.65 and I don't have the pump curve of Superflo series.

I am not an electrical engineer but I know enough that a 60hz pump gives better output than a 50 hz one assuming it is a 50/60hz unit. A 2,950 RPM pump must use more energy to produce exact save performance curve of a 3,450 RPM. The manufacturer is the same, the product family is the same, so I doubt the impeller efficiency would be any different.

So what am I missing here ?

Sorry, I don't know how to convert the 50hz curve from PDF to GIF, it will be easy to compare if both are shown as GIF or JPEG

Thanks very much.

 

[mas985]

I would assume that the accuracy of the head curve is only as accurate as the measurement. Since you really don't know how the manufacture measured the pump or even if they measured it at all (i.e. theoretical curves), then I would assume that the head curves are approximate only.

There could be two reasons that the head curves are the same. Either they are too lazy to measure both pumps or they may have designed the 50 hz pump to have the same performance as the 60 hz pump which can be done.

With the same motor and impeller, a 50 Hz pump will have a lower head curve then a 60 hz pump. I am speculation some here but you could get the same output from a 50 Hz pump but the motor would need to be about 1.7x the braking HP of the 60 Hz and the impeller would need to be larger.

 

[JasonLion]

That head curve says right on it "60 cycle, 3450 RPM". I think they just sent you the usual 60 cycle AC USA head curve.
(EDIT)I see now. There is a second graph in the .pdf file. It is exactly the same as the first graph except for the two items I quoted. The .pdf file says "50 cycle, 2950 RPM" and is otherwise identical!(/EDIT)

 

[SPP]

Hi Jason,

The 50hz curve you must download, it is in PDF file below the graph/picture of 60hz curve

 

[mas985]

That head curve says right on it "60 cycle, 3450 RPM". I think they just sent you the usual 60 cycle AC USA head curve.

There is a second head curve which is an attachment below the 60 Hz picture. This one is labeled 50 Hz.

oops posted at the same time as SPP.

 

[SPP]

Hello Mas,

This is what is bothering me. The Affinity Law I been looking at these past few days told me that it is not possible to get same performance of a 3,450 RPM pump from a 2,850 RPM pump if power consumption is the same.

Unless the 50hz version pump is way above 1.9 SF rating, it not possible to have similiar performance with a 60hz unit at 1.9 SF rating. Nothing is free and pumping water is heavy on energy use.

If those 50hz pump curves are not accurate, pool builders in Asia or other 50hz countries like Europe when using 50hz unit can be in trouble. The trouble will be a real legal trouble at higher head loss of 60+ feet.

 

[mas985]

Hello Mas,

This is what is bothering me. The Affinity Law I been looking at these past few days told me that it is not possible to get same performance of a 3,450 RPM pump from a 2,850 RPM pump if power consumption is the same.

Are you sure the power consumption is the same?

Unless the 50hz version pump is way above 1.9 SF rating, it not possible to have similiar performance with a 60hz unit at 1.9 SF rating. Nothing is free and pumping water is heavy on energy use.

If those 50hz pump curves are not accurate, pool builders in Asia or other 50hz countries like Europe when using 50hz unit can be in trouble. The trouble will be a real legal trouble at higher head loss of 60+ feet.

Well the thing about dynamic head is that in some ways it is self correcting. It is nearly impossible for a plumbing system to operate at the top of the head curve. That portion of the head curve is generally reserved for static head because you cannot have high dynamic head loss with very low flow rates although you can get close. So under normal operating conditions, the operating point will never reach the very left side of the head curve.

Let me give you an extreme example of this. Imagine a worst case pool with 1.5" plumbing and 1000 foot runs with a 3 HP pump. The water flow is still 24 GPM at over 103 feet of head. The 0.5 HP pump will have a flow rate of 19 GPM at 69 feet of head. So you can see that the flow rates are not that much different for such a large change in BHP. But that is what happens when you operate near the left side of the head curve. The head curves flatten out here so there is no benefit to higher HBP. The opposite is true on the right side of the curve so larger pipes will allow higher BHP pumps to produce more flow so the change in BHP has a much much bigger effect on that part of the curve.

 

[SPP]

Ooops, "Power Consumption", I mean based on same impeller, out of the two pumps, the 3,450 RPM pump one will have to consume more power or in other words based on same impeller, to get both to perform to its rated RPM , the power consumption can't be the same. Sorry if I wrote it in a confusing way.