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01 May 2011

Is there any confusion about NPSH?

Net Positive Suction Head (NPSH) is perhaps the most misunderstood concept in pumping technology.  The main source of confusion stems from multiple uses of the term in different but related contexts.

One usage of the term NPSH is to denote the suction performance characteristic of the pump itself, independent of the system into which it is installed.  In this context the terms often used are ‘NPSH Required’ or ‘NPSHR’.  The usual industry standard criterion for NPSHR is the value of NPSH for which a 3% reduction in pump total head occurs.  The term ‘NPSH3’ is also used for this.  At this NPSH condition cavitation is taking place to the point where the pump head performance is falling off measurably.  NPSHR varies with rate of flow and normally increases at a steep rate of climb beyond the pump’s best efficiency rate of flow.  The pump manufacturer’s NPSHR versus flow curve may include margin above NPSH3.  NPSHR is normally considered the lowest value of NPSH at which the pump can be operated.  It is usual practice to provide a system design margin of NPSH above the pump’s NPSHR.

 

double suction pump profile

The term NPSH also denotes a dynamic liquid property in a given pump intake or piping system and in that context it is often referred to as ‘NPSH Available’ or ‘NPSHA’.  It is the calculated absolute total head less liquid vapor head present at the pump suction nozzle.  NPSHA normally decreases as the rate of flow in the system increases.  The value of NPSHA is independent of the pump.

While there are, in fact, distinct definitions for NPSHR, NPSH3 and NPSHA – and, by the way, there are other NPSH terms – the general term ‘NPSH’ is often used interchangeably to mean any one of those and the reader is left to determine from context the correct technical meaning.  The main idea is to realize whether the context of NPSH is relative to the pump or to the system.

For an independent evaluation of a pump NPSH issues, contact an experienced consulting engineer who can help with your specific application.  See the complete list of engineering consulting services we offer.

This is an update of the May 1, 2011 blog post entitled “NPSH clarification.”

Filed Under: General Interest, Technical Tagged With: 3% head reduction, engineering consulting, engineering consulting services, misunderstood, Net Positive Suction Head, NPSH, NPSH Available, NPSH Required, NPSH3, NPSHA, NPSHR, pump, pump engineering consulting, value of NPSH

About Randal Ferman

Randal Ferman has more than 47 years’ experience in the pump industry. He is a registered Professional Engineer in both California and Texas.

During his 32 years with Flowserve, a global pump manufacturer, his responsibilities covered a broad range of engineering assignments. He is presently Principal, Ekwestrel Engineering LLC, in Austin, Texas, providing services on new and existing pumping equipment and systems.

Randal's clients span a range of industries including oil & gas, electric power generation, water, wastewater, solids handling, pump manufacturing, engineering and construction, energy efficiency, pipeline transportation, and pump service and repair.

Comments

  1. Alliance Concrete Pumps says

    May 13, 2011 at 1:09 pm

    Thank you for this post. For such content you clarified the material very well. Thanks for bringing light to this. I have not been to this site before, but will return. You cover some very interesting material. Cheers, Dave

    Reply
  2. ramy says

    May 28, 2011 at 10:15 am

    please i need to know the inducer uses.

    Reply
    • Randal Ferman says

      June 9, 2011 at 6:27 pm

      Ramy, Inducers are used mainly to lower the NPSH Required by the pump. They are found in a wide variety of applications. There are certain operational and practical limitations depending on the size, rotational speed, and other factors, so inducers are not always the best solution. As with most industrial equipment, successful application experience should count heavily toward any decision to use it. Hope this helps. Randy

      Reply
  3. Scott Boyson says

    April 27, 2012 at 8:27 am

    Nice overview. of NPSH. Personally, I think manufacturers should not add their on margin of safety on published curves for NPSHr. NPSHr has a clear definition of 3% head loss. Safety factors should be left to the system designer or as a note at the bottom of the published curves.

    Not only is the NPSH term often misunderstood but how to measure it in an existing system can be confusing. The issue often revolves around the fact that NPSH is an absolute pressure measurement. In open tank suction conditions, you have one atmosphere of pressure contributing to positive NPSHA. We so often think in gauge pressures or pressure differerential conditions that this tends to be confusing for those that do not work with pumps often.

    Reply
    • Randal Ferman says

      April 27, 2012 at 12:14 pm

      Excellent comment Scott. Production NPSHR test results can vary significantly due to the test conditions and the individual pump configuration. Often the production test is essentially a verification that the pump NPSHR falls below a guaranteed value at one or a few flow conditions. Manufacturers will balance the necessities for publishing a competitive level of NPSHR and meeting that level on test or in the field. Manufacturers would seldom have reason to add margin for the sake of system uncertainties. Notwithstanding, the term NPSH3 is coming into more common usage and clearly represents the NPSH definition for 3% head loss. Best, Randal

      Reply

Trackbacks

  1. Water Pumps Api | Water Pumps Direct says:
    May 16, 2012 at 10:22 pm

    […] Is there any confusion about NPSH […]

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