SUBJECT : Reductions in centrifugal pump NPSHR (net positive suction head required 12-1)

The curve that came with your pump shows the NPSH required for any given impeller size and capacity. This number was determined by pumping cold water through the pump while reducing the suction head until the pump showed a reduction in discharge head of three percent (3%), due to the low suction head and any formation of bubbles within the pump. This point is called "the point of incipient cavitation".

Please take a look at the pump curve shown in the next drawing. It demonstrates that if you had a 13 inch (330 mm) impeller and you wanted to pump 300 gpm (68m3/hr.) you would need at least 10 (3 meters) feet of NPSH. If you are pumping hot water or some hydrocarbons you can, in some cases, operate with a lower NPSH required than shown on the pump curve.

If you reference Technical paper Volume #9, Number #12 , you will learn that we used a similar reduction when we were calculating the suction specific speed number (SSS) of the impeller.

The NPSH reduction chart, will show you how to calculate this reduction. As you use this chart please keep the following in mind:

Using the chart is not very complicated:

Find the temperature of your product and proceed either up or down to the vapor pressure of your product in either psia. or kPa. (100 kPa = 1 atmosphere)

From this point follow along or parallel to the sloping lines to the right side of the chart where you can read the NPSH reduction in feet or meters.

Example #1:

Example #2 :

If your calculations show that you have a potential cavitation problem you have several choices:

If you will refer to my Technical paper Volume #1 Paper number #3 you will see that I have covered the above subjects in good detail.

Here are a couple of more thoughts on the subject:

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