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
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
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