NPSHa formula - h(sp) +/- h(s) - h(f) - h(vp)

h(sp)- static pressure head (absolute) applied to the fluid

h(s)- elevation difference between fluid level in reservoir to pump inlet

h(f) - friction loss in suction piping

h(vp)- cap or pressure of the liquid at a stated temperature

How do I determine whether h(s) is a positive or negative ?

I’ve researched that-

Pump below reservoir , h(s) is a positive and vice versa but I don’t know the reason behind it ... I’ve been told that if it’s not good for the pump, u have to substract away from the NPSHa formula which makes sense for h(f) and h(vp). and if it’s good for the pump, you have to add.

am I right to say it’s because the fluid loses more energy as it needs to fight against gravity , that is why h(s) is negative if the pump is above the reservoir, this means more work for the pump.

and if pump is below reservoir, the fluid doesn’t lose much energy.

  • $\begingroup$ I don't understand the question as stated. However a liquid pump must have a positive NPSH to operate properly . Otherwise , cavitation is likely which seriously deteriorates the pump performance. and may damage it There are special cases like a jet ( boost) pump that appear not to conform . $\endgroup$ Commented Jul 28, 2018 at 17:14

2 Answers 2


The reason h(s) is positive when the pump is below the reservoir fluid level is that there is a pressure created by the height difference which contributes to the net work done.

When the pump is above the fluid level, then the pump has to apply work to get the fluid into the pump before delivering on it so that work reduces the net work done.


It's wrong and same wrong information has been disseminated in a whole variety of books and has just endlessly been copy/pasted over & over

Dynamical pressure is not pressure, it's kinetic energy. Only way how it can be turned down into pressure again is if you brake it down, which isn't what happens in the inlet of the pump. Only relevant thing for cavitation is static pressure.

Just imagine if you had a very strongly converging, nozzle, at the inlet, fluid would accelerate and pressure in it would drop at the exit side of the nozzle, which would lead to cavitation. But by using dynamic head as well, you get same thing as when there is no nozzle. Which is clearly wrong

Reason why this error has gone below the radar for so long is because dynamic head is typically negligible.


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