How is the NPSH(R) determine? 5

[417]

Hi, I would like to know about the test or procedure to determine the NPSH of a centrifugal pump. How is the NPSH curve deing determine? What is mean by 3% and 0% head drop?

Regards,
Echo 417

 

[MJCronin]

417, (lovely handle..)

I suggest that you consult the all time clasic..."The Pump Handbook" by Igor Karassic. In this book, the standard manufacturer's test for establishing NPSHr for the particular pump casing/impeller/speed combination is explained.

The test, as done by most manufacturers, is performed to the Hydraulic Institute Standards.

Get a beer.... and a sandwich....Go to

http://www.pumps.org

and begin to read. Get your employer to purchase the entire group of HI Standards..... the one you want is 9.6.1

By the way, within this standard, HI has recently established a guideline describing the suggested margin between the NPSHR and NPSHA. It varies for the type of centrifugal pump type and service....... a lot of questions are answered



Good Luck !!!

MJC

 

[tstead]

Here's a short answer:

The pump is run and it's head is measured. Then, the suction pressure is slowly decreased (this can be accomplihed through the use of a valve or vacuum chamber).

The head is constantly measured as the suction pressure is decreased. When the head has dropped by 3% then that is recorded as the NPSHr for that flow.

The test is run again at a couple of different points and a curve is developed from that data.

Likewise for 0% head drop criteria - but 0% drop is usually only seen with critical service applications. One of the problems with this measurement is that normal fluctuations occur in measurement (you know, the +/- tolerances..) and it's difficult to discern between the head having dropped from the low NPSHa and the normal fluctuations in the pressure gauges. So, more expensive test loops are used - which raises costs but also accuracy...

So, whenever you see a performance curve in a catalog, the NPSH curve is actually the 3% curve. I know of no pump manufacturer that publish a 0% NPSHr curve as part of their standard catalog curves.

So, the moral of the story is if you are sufficiently close to the NPSH values on the catalog curve, but still below them, you are already cavitating.

Some manufacturers use a rule of thumb of adding half a meter to the NPSHr or 10%, whichever is greater. But the topic of NPSH goes even further which I have not seen adequately discussed in the literature and that is one of impeller material selection.

The NPSHr curves are generally generated using an impeller of the manufacturer's standard material; however, it is well-known (amoing pump designers but not pump salespersons) that impeller material selection can greatly affect the NPSH characteristics of your pump. For example, if you were to measure the NPSH using a ductile iron, bronze, and stianless steel impeller, you would get 3 different NPSH curves using the same test loop, the same instrumentation, and the same pump casing.

Of course now, if you were to slow the pump down or if the NPSHr was already pretty low, the measured values might very well be within the accuracy of the gauges so you will never see it - but get to values of around 25 feet and you will see some change.

But realistically speaking, smaller diameter impellers also are not as susceptible to this problem - it's when you get into splitcase pumps that you need to really pay attention to it.

So, one might do well to ask the pump manufacturer for an NPSH curve based on their specific impeller material; however, you would probably get a few blank stares and maybe even a chuckle from the ill-informed salesman.

 

[kenelms]

The question of npsh required by a pump /impeller design have been well covered here and will continue to be a subject of interest and study. An archive article appearing on the pumpzone site by terry henshaw is well reading.Much art as well as science still exists in this area ie cavitation notably.

 

[electricpete]

tstead - what is the physical mechanism that causes short-term pump performance to depend on impelleter material.

( can understand long-term degradation effects from cavitation, but I'd expect this would not show up during a short-duration test).

 

[tstead]

Electricpete:

Surface roughness and casting quality. With lower tip speeds (<40 m/s), there is not that much of a problem. Over about 40 m/s though, the surface roughness of the impeller tip can cause excessive recirculation and eddy currents at the impeller tips causing the fluid to flash at that point. This is akin to the same phenon. that causes NPSH to increase as you get close to shutoff.

 

[stanier]

Suggest another site worth a visit is

http://www.pumpline.com

 

[Kawartha]

tstead,

I find your comments regarding NPSHr for different material impellers very interesting.

If what you are saying is correct, then in many cases the NPSHr for a bronze impeller may in fact be less than the NPSHr for a 316 SS impeller. Since the finish on bronze impellers is generally superior to 316 SS impellers (316 SS is much more difficult to work). Likewise, the NPSHr for Cast Iron impellers may be greater than for 316 SS impellers, assuming that the surface finish of the cast iron is of low quality.

Do you have any typical test results for NSPHr tests for identical pumps with different material impellers?

 

[electricpete]

just to make sure I'm not out in left field... when you say &quot;eddy currents&quot;, you're talking about fluid currents (not electric), right?

 

[vanstoja]

NPSH tests need to be run at not less than 3 flowrates to be able to plot a curve of NPSH in feet vs flow in GPM. One of the flowrates should be the design point (rated) flow and the other selected flowrates should include maximum flow , (possibly) minimum non-recirculating flow and any other expected operating flows between the extremes. It helps to have already run a head-flow performance curve with suction pressure much higher than NPSHR at the design flowrates. If you need to estimate the expected NPSHR at design flow, Stepanoff's sigma (NPSHR/Head) equation :
Sigma=6.3n_s^1.33/10^6
is a place to start using design rated head and flow from the head-flow performance test. Since head vs suction pressure curves plotted to determine 3% head drop may have different slopes at different flowrates, it is wise to start the NPSH test at the highest flowrate where the slope may be great. Start with a suction pressure that is high enough to provide no head drop for three step reductions in suction pressure and do this for every flow so that the zero head drop condition is truly met. When you lower suction pressure to a level that causes head to drop about 1%, reduce the suction pressure increments to improve accuracy of determination of the 3% head drop suction pressure. Continue past the 3% head drop till the curve reaches a &quot;knee&quot; beyond which the head will start to nosedive toward zero head. The slope variation with flowrate is more pronounced for higher specific speeds above 2000 RPM-GPM-Ft. At lower specific speeds flowrate has a lesser effect on slope of the head vs suction pressure curves. Sometimes there at &quot;steps&quot; in these curves which may signify alternate blade cavitation where the lowest pressure in the channels switches from suction side of one blade to pressure side of the adjacent blade. How many points it takes to get decent NPSHR data depends a lot on experience of the testers. Some 20+ suction pressure settings have been run for each flowrate in 2800 specific speed pumps we have had NPSH tested with several impeller modifications.