What is NPSH ?

Net Positive Suction Head(NPSH)

A liquid can not be sucked inside the pump. Liquid has to be pushed inside the suction pipe Fig-1. For a liquid to be pushed into a suction pipe some external energy, force or head is required. Atm. pr. provides this head on the free surface of liquid from which the pump draws the liquid.

Theoretically, at sea level the maximum suction lift is 10 m (for water). Actually because of effects of several factors, it is less than this value. Pipeline friction, liquid VP are some of the constraints that determine maximum allowable suction lift.

So what is NPSH?

In Net Positive Suction Head (NPSH)
Net stands for remaining Positive stands for greater then ZERO (pressure) Suction head stands for abs. energy at the pump suction flange in meters of liquid above VP at pumping temperature (in relation to Zero abs. Pr.)
Ref Fig-2 to understand- 

CASE - 1 With zero suc. lift, NPSH is 10m (10-0).

CASE - 2 With 3m suction lift, NPSH is 7m (10-3).

CASE - 3 With 5 m suction head, NPSH is 15m (10+5).

We have ignored other factors like hf losses and VP of liquid involves for simplicity

NPSH Available

It can be defined as the absolute energy in mlc available at pump inlet nozzle and corrected to pump datum minus VP of liquid.

Factors to be considered for NPSH.

1) Atmospheric pr at pumping site converted in mlc.
2) Abs VP of liquid at pumping temp. in mlc.
3) Friction losses in suction pipe in mlc.
4) Suction lift or head in mlc.
5) Pressure head in closed suction vessel in mlc.
This is the characteristic of the system in which the pump operates (site condition). The NPSHA value is to be given to the manufacturer by us when we want to buy the pump.

NPSH Required

This is required or minimum NPSH of the pump (NPSHR)
It can be defined as the positive head in meters abs. necessary at the impeller eye to overcome the internal pressure drop within the impeller and maintain the liquid pressure above its VP and thus ensure stable flow in to the impeller. The pump designer decides the NPSHR value of the pump model.

NPSHR is the function of the impeller eye peripheral velocity in m/s and suction velocity of liquid at pump inlet in m/s. It logically follows that for the same pump when run at 1500 rpm requires less NPSHR then the pump with same discharge and head running at 2900 rpm.

NPSHA must be always more then NPSHR by at least 0.5m.

CAVITATION can occur at any point if NPSHA is less then NPSHR.