## Pump efficiency estimate based on temperature

## Pump efficiency estimate based on temperature

(OP)

Hi,

I'm working on a project to estimate pump efficiency where pump temperature, pressure, and flow are the only readily available data. I came across a formula in "Forsthoffer's Best Practice Handbook for Rotating Machinery" that relates pump efficiency to temperature difference. In this book (link below) the author says pump head is calculated from data in (m*kgf)/(kgM) OR (ft*lb_f)/(lb_M). This doesn't make sense to me as I've only ever seen head calculated in units of length and I can't find information online.

Has anyone seen head measured in the units described in the handbook or understand why the author would have used this method?

https://books.google.ca/books?id=9l3F3qovu9EC

Page 77

(I can post a screenshot of the link if it cannot be accessed)

Any comments would be appreciated!

Thanks,

I'm working on a project to estimate pump efficiency where pump temperature, pressure, and flow are the only readily available data. I came across a formula in "Forsthoffer's Best Practice Handbook for Rotating Machinery" that relates pump efficiency to temperature difference. In this book (link below) the author says pump head is calculated from data in (m*kgf)/(kgM) OR (ft*lb_f)/(lb_M). This doesn't make sense to me as I've only ever seen head calculated in units of length and I can't find information online.

Has anyone seen head measured in the units described in the handbook or understand why the author would have used this method?

https://books.google.ca/books?id=9l3F3qovu9EC

Page 77

(I can post a screenshot of the link if it cannot be accessed)

Any comments would be appreciated!

Thanks,

## RE: Pump efficiency estimate based on temperature

## RE: Pump efficiency estimate based on temperature

However I did see it earlier and it's far from clear what M stood for or where the kgf came from. Missing the page 76 is fairly crucial.

Anyway you shouldn't need it.

If you take the assumption that the inefficiency is translated to temperature difference, just follow the energy.

You can calculate shaft power at 100% eff using flow, head and density.

You can presumably obtain actual shaft power.

You can calculate the energy rise due to temperature increase.

Then you have a difference

Or just play around with the efficiency until the shaft power required matches the actual shaft power, but then you do need to account for other losses in the pump like bearing friction etc.

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## RE: Pump efficiency estimate based on temperature

I'm assuming (without having read the book), that he's showing the derivation and using this to keep units in some sort of rational progression as he explains what he's doing.

## RE: Pump efficiency estimate based on temperature

Please see the attached picture with the pages being referenced.

Image is from books.google.ca

http://files.engineering.com/getfile.aspx?folder=f...

## RE: Pump efficiency estimate based on temperature

Not sure if it's actually correct....

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

## RE: Pump efficiency estimate based on temperature

@georgeverghese

On page 76 it states that "pump efficiency is low at low flows" which makes sense based on pump performance curves. But to me it doesn't seem like the author is saying the equation can only be used in cases when the pump is running at low flow. Do units given for calculating pump head make sense to you?

@LittleInch

Thank you for the suggestion!

Unfortunately due to a fluid coupling between the motor and pump, calculating the shaft power is very difficult based on my discussions with the coupling vendor. Hence obtaining actual shaft power as you suggested doesn't seem feasible at this time.

@TenPenny

The author doesn't seem to show derivation of his equations, units, or constants which is making things a little more difficult.

@LittleInch

I agree. I don't think pump head is the correct term. Without the derivation is hard to understand the rational and meaning.

## RE: Pump efficiency estimate based on temperature

If you can't accurately get shaft power in its not really possible to guage efficiency.surely?

Remember - More details = better answers

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

## RE: Pump efficiency estimate based on temperature

With such poor editing, it would be impossible to understand what it is supposed to say.

In the Goulds literature, temp rise in F is calculated by 5.09 * (shut off HP) / (gpm * SG * SH).

Which uses hp input instead.

## RE: Pump efficiency estimate based on temperature

I do have access to the pump curve. I want to compare the calculated efficiency to the ones from the tested pump curve provided by the manufacturer.

The actual shaft power is difficult with the torque converter

@TenPenny

Do you have a copy of that literature you could post here?

## RE: Pump efficiency estimate based on temperature

Remember - More details = better answers

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

## RE: Pump efficiency estimate based on temperature

## RE: Pump efficiency estimate based on temperature

In my experience in doing these calcs on past projects ( which were to compute the temp rise at low flow, unlike your case where you want the back calculate eff from temp rise), you get a perceptible, measurable temp rise only at low flows coincident with low eff for centrifugal pumps( which is what this book says also). At higher flows, the dT is perhaps measurable but we are talking about very small dT here (perhaps less than 0.5degC). Which makes getting accurate dT values rather challenging at higher flows. Hence, this approach for extracting pump eff is possibly approximate only at low flows , at or near min flow up to near dead head.

## RE: Pump efficiency estimate based on temperature

Here, pump developed head is in metres, eff is pump hydraulic eff expressed as a fraction, and Cp is in kJ/kg/degC.

Temp rise across the pump dT = (g.h/(1000.Cp)).((1/eff)-1), where g is usually 9.807m2/sec.

If pump head is expressed as Nm/kg, which is the same as J/kg or kgf.metres / kg, then use this value directly in the numerator and delete multiplication by g. Most pump vendors do not use Nm/kg for expressing head; Nm/kg is usual practice for expressing centrifugal compressor head only.

So for example, when we have a developed head of 100metres at eff=0.6 for water with a Cp of 4.18kJ/kg/degC, then dT=0.16degC, which is hardly measurable with run of the mill devices.

## RE: Pump efficiency estimate based on temperature

I'm intrigued as to why you have a fluid coupling though??

Remember - More details = better answers

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

## RE: Pump efficiency estimate based on temperature

I wish people would stick to SI units, it makes things much more clear.

## RE: Pump efficiency estimate based on temperature

@LittleInch

I'm not sure if I misunderstood what your asking but we use the fluid coupling to control speed of the pump. Its considered a VSD that transfers the motor power to the pump, hydro-dynamically.

@TenPenny

Thanks for attaching the page! Apparently there is a large range for the temperature profile on the pump casing I'm working on which I didn't realize.

@georgeverghese

You bring up a good point about the equation only being usable in low flow conditions. I'll do some testing with what data I have but I don't know if our devices can measure to an accuracy better than 0.16degC.

Also, thanks for taking the time to derive the equation! I rearranged it and it aligns with the equation I found on this website: www.pumptraining.com/temperature-rise

## RE: Pump efficiency estimate based on temperature

Still don't understand though why you want to try and find efficiency? with a fluid coupling in the way it's not like you can complain that you're spending more on power than you thought so what's the point?

Doing a pump test for flow rates at 0, 20%, 40% etc to create your pump curve to compare to the as tested version may show some difference which implies wear or damage, but if you can't get an accurate shaft power then you can't get efficiency. Temperature rise is too low to accurately measure and who runs their pump at low flow?

So maybe you need to break the news to someone that unless they fit a torque measurement device with a speed counter on the pump inlet shaft then it's not really feasible to get efficiency with only pressure, flow and temperature measurements.

But WHY do they want to know????

Remember - More details = better answers

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