Net Positive Suction Head (NPSH)


Net positive suction head (or NPSH) has been defined in many ways, some of
which can be confusing to someone who is just learning the fundamentals of
centrifugal pumps.

Simply stated, net positive suction head (NPSH) is a measure of the available
pressure at any given datum point. The pressure is expressed in feet absolute
(rather than in pressure gauge reading, or PSIG, that we are all familiar with), and
the datum point can be either one of the suction nozzle centerline, the impeller
centerline, or the top of foundation, depending upon the specific circumstances.

And here is where the confusion often comes from: the conversion of the pressure
into a unit of feet absolute, and how it is determined from a selected datum point.
Adding to the confusion are the two terms:

  • Net positive suction head available (NPSHA)
  • Net positive suction head required (NPSHR)

NPSHA refers to the available suction pressure at a pump site, whereas NPSHR  
refers to the suction pressure required by a pump at the impeller centerline so
that it can deliver its intended capacity. For a pump to function properly, its NPSHR
should be lower than the NPSHA; there should be sufficient available pressure to
push a specific amount of flow rate into the impeller. Higher flow rate requires
higher pressure to push the liquid into the impeller. Once the liquid is
pushed
inside the impeller, the pump can now do its task of imparting kinetic energy to
add pressure to the liquid.

If liquid is coming from an elevated tank, NPSHA is calculated from the equation:


           2.31 x  ( Ps - Pv )
NPSHA = ------------------------  +  Hs  -  Hf
           Specific gravity

where:

Ps   =  absolute pressure in a
closed tank, in PSIA (or gauge pressure + 14.7), or
Ps   =  absolute atmospheric pressure in an
open tank (= 14.7 PSIA at sea level)
Pv    =  vapor pressure of liquid, in PSIA
Hs   =  vertical distance between the liquid surface and impeller centerline, in feet*
Hf    =  friction loss from tank to suction of impeller, in feet

* If the tank is below the impeller centerline (underground), Hs has negative sign.
The vertical distance should include the drawdown, or the decrease in the liquid
surface level when the pump is in operation.

To calculate the friction loss, it is necessary to convert the suction fittings such as
elbows, valves, etc., into their equivalent pipe length. [**]  The estimated friction
loss can then be taken from published friction loss table. [**]  Loss from suction
strainer should also be added to the friction loss.

[**] Select data  are available on request.


Using suction pressure  gauge

If suction pressure gauge reading can be obtained, the NPSHA can be calculated
from the equation:


            2.31 x  ( Pg + Pa - Pv )
NPSHA = -------------------------------  +  He  -  Hv  - Hf
                Specific gravity

where:

Pg   =  pressure measure at gauge, in PSIG
Pa   =  14.7 at sea level (conversion to absolute pressure, or PSIA)
Pv    =  vapor pressure of liquid, in PSIA
He   =  vertical distance between pressure gauge and impeller centerline, in feet*
Hv   =  velocity head at point of gauge connection, in feet
Hf    =  friction loss, from gauge connection to suction of impeller, in feet

*  
He has negative sign if the pressure is located below the impeller centerline


R: 0210-NEPO
C: basics, design
F: NPSH definition


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Introduction to NPSHA
Impeller NPSHR
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