# Calculating NPSHa for a pump

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.

• 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 . – blacksmith37 Jul 28 '18 at 17:14