NPSHa vs NPSHr

[brokate]

In the reference books there are formulas for calculating the NPSHa that indicate an adjustment to the pressure absolute should be made to account for an increase in elevation (if the pump is to operate at elevation). When pumping water it seems to small as to be negligible. Also, it would seem reasonable that is the NPSHr is established for conditions at sea level then the NPSHr would need to be corrected for the operation at elevation so in the end it would all be a moot point. What is everyone else's take on this?

 

[Artisi]

Why would NPSHr need to be corrected?

 

[brokate]

That is what I was wondering - why do the reference books indicate that the NPSHa should be corrected for the atmopheric pressure due to increase in elevation, but if the NPSHa needs to be adjusted then would not the NPSHr need to be corrected also. But yes, that is what I am asking.

 

[Artisi]

NPSHa changes due to reduced atmospheric pressure at just over 1 metre for every 1000m of elevation.

NPSHr won't change as this is a function of pump design and remains constant for the flow and speed etc under consideration.

 

[brokate]

But if the NPSHr is establish at the point the total head is reduced by 3% due to restricting the suction head, and this is calculated at sea level, would not the NPSHr change also at elevation? I am just trying to figure this out - it seems somewhat incongruous that the elevation would impact the NPSHa and not have any impact on the required. What am I not understanding here? If I am understanding the whole NPSHr thing correctly, it is established by operating a pump at its BEP and then restricting the suction until there is a 3% drop in the Tdh. If this is performed and sea level and the NPSHr is determined and the pump is carried to Leadville, CO - elevation 10,000+ feet and the test is performed there does the NPSHr remain the same?

 

[Artisi]

NPSHr remains the same - what changes or must be allowed for in applying the pump at this altitude (Leadville) is the lower atmospheric pressure.

NPSHa = Ha - Hvpa (+ or - Hst) - Hfs

During pump testing NPSHr is corrected for atmospheric pressure at sea level (not all test facilities are at sea level), temperature using std water (from memory 60 F) and any S.G. correction etc.

 

[BigInch]

To account for the effects of changing atomospheric pressure, simply convert all pressures to absolute pressures. NPSHr (head) converts to an absolute pressure.
Let's consider a suction pressure of 2 psiG. Assume water and a vapor pressure of 0 psia.

At sea level, atmospheric pressure (14.7 psia) converts to a head of 33.9 feet. A suction pressure of 2 psig would convert to 4.61 ft Absolute suction pressure = 14.7 + 2 = 16.7 psia. Total suction head available at sea level = 33.9 + 4.61 = 38.51 ft

At 10,000 feet atmospheric pressure would be about 1/2, so say 8 psia, or some 17 feet. A suction pressure of 2 psig would convert to absolute suction pressure of 2 + 8 = 10 psia and 23.07 feet.

If you need an NPSHr of 25 feet, that converts to 10.83 psia

At sealevel you'd have 16.7 psia, or 38.51 ft, > 25 ft, so you're OK.
At 10,000 ft you'd have an absolute suction pressure of 10 psia, or 23.07 ft < 25 ft, so you're NOT OK.

If vapor pressure was significant, then you'd have to subtract that vapor pressure from the absolute suction pressures above. Vapor pressure always decreases available NPSHa.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

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

 

[brokate]

Artisi & BigInch - Thanks, I appreciate your feedback. But Artisi, if adjustments have to be made to the NPSHr if the tests are not performed at sea level, then doesn't that mean that the elevation affects (I hope that is right, I always get affect and effect mixed up) the NPSHr. And if the NPSHr is affected, then way do we not have to account for the impact of elevation on the NPSHr when considering an installation at elevation. I am beginning to see the picture of the NPSHa as a condition of the required pressure at the pump's suction eye, thus the pressure could hypothetically attain absolute zero at the suction eye. That part of the puzzle is fairly clear to me; where the wheels come off the cart is with the NPSHr being unaffected (I'm putting all my eggs in one basket with this affected - effected thing)by the ambient environment. At one level I understand that the pump is just adding energy to the flow stream, consequently the conditions outside of the pump do not affect what the pump does, i.e., Tdh, BEP, flow and of course NPSHr. But then if the environment is extended to the extremes would not there be changes in the pump performance; if the pump was to operate on the moon would the parameters of the pump performance remain unchanged? That is where the picture gets all scrambled in my little pea sized brain. Anyway, thanks for the feedback, I will keep processing this until some type of picture emerges. In the meantime I will accept the NPSHr as being chiseled in stone and work from there.

 

[FiltrationEng]

Could you describe your pump system in the suction side.

When the fluid level is over the pump suction level, the
NPSH available can be calculated as follow:

NPSHa = Atmospheric Pressure + Static Height – Total Friction Losses (in the suction side) – Vapor Pressure of the fluid at the temperature being pumped

NPSHr, in most of case, is a value that the manufacturer of the pump can give you.

In order to avoid cavitation NPSHa > NPSHr

Regards
Luis Sisniegas

 

[BigInch]

As far as I know NPSHr curve is provided corrected to 14.7 atmospheric pressure and water at 50ºF.

"And if the NPSHr is affected, then way do we not have to account for the impact of elevation on the NPSHr when considering an installation at elevation. "
NO - The only correction that needs to be done is to NSPH A THAT varies with elevation of the pump.

"At one level I understand that the pump is just adding energy to the flow stream, consequently the conditions outside of the pump do not affect what the pump does, i.e., Tdh, BEP, flow and of course NPSHr. THAT is true, but that is a differential energy added to what is available at the pump's elevation (NPSHa) which is affected by atmospheric pressure.

"But then if the environment is extended to the extremes would not there be changes in the pump performance; if the pump was to operate on the moon would the parameters of the pump performance remain unchanged?" The pump peformance (the addition of differential energy) is unchanged. What changed is the part about differential energy added to what initial energy. At the Moon, atmospheric pressure is 0, but the moon is not a good example, as the Moon still has some gravity and would give you some pressure head, if the tank were for instance, elevated. Deep outer space would be a better example, without gravity at all and where NPSHa would total 0 psia - suction loss - vapor pressure, clearly not possible, which would indicate that the tank had better be pressurized and have a diaphram to force the fluid towards the pump.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

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

 

[BigInch]

Well.... it clearly wouldn't work, if there were suction pipe losses. You might use a submersible pump to eliminate those.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

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

 

[brokate]

Thanks again for all the feedback - this is just one of those things that was always rattling around in the back of my mind but I never really got a good handle on it. FiltrationEng - I do not have a specific installation; I have made a lot of pump sets and have always followed the given calculations for determining the NPSHa but I always wondered why the NPSHr was not affected by the elevation and thus require some compensating calculations. Then I came across this site and I knew that there were a lot of folks out there a whole lot smarter than me so that is what prompted my inquiry. It is somewhat comprehensible that the NPSHr is solely a function of the pump regardless of the environment. On the other hand, intuitively, it seems like there should be some interplay between the pump performance and the environment in which it is located; I have witnessed pump performance tests in different locations and it just seems like there should be some impact on the performance at the different elevations. Maybe someday when I am old and gray (or at least older and grayer) I will test a pump in San Diego to determine the NPSHr and then carry the pump to La Paz and test it again just to try to convince myself that it just don't matter when it comes to NPSHr. Thanks again everyone - Mahalo.

 

[BigInch]

Been there, done that. No difference, but nothing like seeing it for yourself. Note the motor or IC drivers efficiency varies due to cooling requirements and the thinner air, so there is some difference in the systems at different altitudes, but not really any difference in pump performance. The pump is isolated from the outside environment by the pressure casing. Kinda like its the same for the guys in a submarine, whether its at the dock or 1000 ft below.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

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

 

[Artisi]

You need to step back from it and re-think what is happening internal of the pump -no need to transport pumps from sea level to high altitude.

First up - pumps don't "suck" that's unless of course they aren't working like you expected - then they really suck while you sort out the problems .

Secondly - in a fully primed and running pump, the pressure at the impeller eye is reduced to below atmospheric pressure - this is a function of design.

Third - the pressure on the pumped product measured at the at the pump inlet must be higher than the eye pressure, this can be from from atmospheric pressure or an overhead source or a combination of both - otherwise flow can't take place.

fourth - at a given flow rate, the reduced pressure at the impeller eye will be "Y" - this is shown as NPSHr, this doesn't change due to atmospheric pressure external to the pump case - therefore the inflow needs to be at a pressure exceeding "Y" NPSHa so that cavitation doesn't take place.

Hence the need to factor in atmospheric pressure into your NPSHa calc's along with temp (vapour pressure), viscocity, friction and entry losses.