Altitude correction to pump performance curve

[tkall]

My question relates to submersibles and is not necessarily an NPSH question. If a pump performance curve has been determined at, for example, sea level and the pump will be used at a higher elevation, say 8000 ft above MSL, is is necessary/correct to de-rate the performance curve? I need an accurate curve so I don't want to simply neglect the difference in atmospheric pressure.
Tom

 

[dcasto]

ChemE rule 12, always use absolute pressures, it's just easier to keep everything staight.

 

[TD2K]

I'm not sure what you are asking tkall, pump curves are almost always show the differential pressure, usually in feet of fluid versus flow rate. That's not going to change between say sea level and 8000 feet though the absolute pressures will change.

 

[tkall]

I will try to clarify my question. If the performance curve for a pump was developed at sea level, will actual performance at 8000 ft above sea level match the performance curve. I tested this in my test tank and it appears that due to the lower atmospheric pressure(and the numbers seem to work out)that the pump curve is shifted down...or to the left depending on how you look at it. I couldn't find anything in any of my textbooks addressing this issue, maybe because it is so elementary that everyone but me knows it. I appreciate the comments.
Tom

 

[tkall]

There should not be any difference because the discharge is at the same elevation...right? Why then does the pump manufacturer indicate the elevation of the test center where the curve was developed? Obviously I am kind of confused

 

[wimple]

Hi

I guess the NPSHr is given by Mfr accounting for 3% losses of head. In case the altitude increase, the NPSHa will tend to decrease.

One need to check if at higher altitude, NPSHa is still above (NPSHr + margin). If not, pump is likely to experience cavitation, leading to additionnal % decrease in head (I guess difficult to predict). This is not normal operation. Perf. curves are given for Normal Operation.

My opinion only

Rgds
Wimple

 

[athomas236]

It is common to correct fan performances for altitude because of the impact of altitude on the density of air but have not seen a similar correction for pumps.

Regards,

athomas236

 

[BigInch]

NPSHa and Power requirements will change, however the pump curve will remain the same as it was at sea level.

As was pointed out above, its a differential head, so there can be no effect. Changes in densities of liquids at lesser atmospheric pressures (even if they might be appreciable) will not affect the pump head curve, as pump differential head imparted to any liquid is constant (when neglecting viscosity affects), however the suction and discharge pressures would change, if a change in density was significant (although that is highly unlikely).

Of course any pressures read with a differential pressure gage relative to atmospheric pressure would change due to altitude difference.

So, recalculate NPSHa and Power requirements. Power requirements are increased at higher altitudes, the specifics vary with different types of drivers.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[wimple]

BigInch

I agree with you.
What about the following scenario : the pump is quoted at sea level. There is sufficient NPSH margin for operation. The design of the pump is frozen in these conditions.

Then the pump is relocated (at higher altitude) leading to lower NPSH margin.

Is it possible that this lead to an increase of "permanent cavitation" ?
Therefore what is the definition of permanent cavitation (is it 3% losses of head) ? Is it possible to agree with Pump Manufacturer, to redefine pemranent cavitation limits, let say 5% head losses and then revisit the Pumps curves?

What do you think?

Thanks
Rgds
Wimple

 

[BigInch]

"Is it possible that this lead to an increase of "permanent cavitation"?"

Yes, of course, if the NPSHa at installation altitude is lower than the pump's published NPSHr (defined at 3% head loss with cool water at <presumedly> sea level atmospheric pressure).

At some new altitude, the only thing that appears to change would be the atmospheric pressure. We have to ask, what effects if any that would have on the pump head curve and the NPSHr curve. It wouldn't appear to have any effect on the pump curve. Now, would NPSHr be affected? At any flowrate, would the onset of cavitation be any different when at altitude than when at sea level? If you had a closed system at sea level and at altitude, no. Any supposed difference would have to be attributed to a change in atmospheric pressure. If you use absolute pressure units, it seems apparent to me that the only difference would be accounted for when solving for NPSHa, so nothing else to consider. NPSHa is less, margin is less, so using the NPSHa solution method covers us quite well.

Yes you could redefine the NPSHr curve onset of cavitation for some arbitrary difference in %head loss, 3%, 5%, 10%... , but why do that when the normal NPSHa method already considers this and tells us everything we really need to know. We would wind up with an infinite number of pump curves for just one pump for all altitudes(??) We could just as easily say we should define pump curves for all possible combinations of fluids, each with different vapor pressures, or alternatively, for one fluid at all its possible vapor pressures. Much easier to just do one for cool water at sea level and remember to operate it within the limits of NSPHa > NPSHr.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/