NPSH

NPSH stands for Net Positive Suction Head and is the net hydraulic load at the suction of a pump, i.e. the difference between the pressure in a generic point of the hydraulic circuit and vapour pressure of the liquid in the same point, and depends on the characteristic parameters of the system, and on how the pump is installed.
NPSH is an important parameter in the sizing of the hydraulic circuits: if the pressure of the liquid in a given point falls below the vapour pressure, the liquid will boil, with obvious perturbations of the circuit. This may lead to cavitation, which is a harmful phenomenon that occurs when bubbles form, to which are also added any gases dissolved in the fluid. These cavities are transported by the surrounding fluid, and when the pressure increases, they are rapidly reabsorbed. This in turn causes violent inelastic shocks of 1000N/mm2 that damage the solid surfaces with which they come into contact.
If we take the hydraulic circuit shown in figure 1, the NPSH in the 1-1 section will be:
NPSH = P0 + H − Y − Vt
obviously to be resolved with coherent units, where H is the geodetic height between sections 0-0 and 1-1, Y is the pressure drop between sections 0-0 and 1-1, and Vt is the vapour pressure of the liquid at the temperature that occurs in section 1-1.

In a pump, NPSH is distinguished between NPSHA (Net Positive Suction Head (available)) and NPSHR (Net Positive Suction Head (required)), also called total dynamic head (hdt), where NPSHA is the NPSH calculated at the suction inlet of the pump and NPSHR is the minimum NPSH with which the pump can operate without cavitation occurring. NPSHA and NPSHR are two independent quantities, but the relationship that must exist between them is important. In order to prevent cavitation from occurring:
NPSHA > NPSHR

Source: Wikipedia