NPSH - Net Positive Suction Head
The NPSH is related to the problem of cavitation. Cavitation means that the liquid pumped vaporizes within the pump which causes breakdown of suction column and head, loud noise, heavy vibration leading to erosion or pitting and possibly destruction of the impellers. The condition for operation without cavitation is:
NPSHa > NPSHr
Where:
NPSHa: NPSH available, of the plant.
NPSHr: NPSH required, of the pump. Lower value of NPSHr means greater the suction capability.
Usually is used a safety allowance of 0.5m.
P: Gas pressure. If the suction tank is open the gas pressure correspond to the atmospheric pressure (1 bar).
Vp: Vapour pressure of the fluid pumped at a given temperature.
ρ: Density of the fluid pumped.
g: gravitational constant 9.81 m/s2
S: Suction head. Vertical distance between suction tank and pump center line. (- ) for operation under suction lift conditions and (+) for operation under flooded suction conditions.
H: Head loss for the maximum flow in pipes, bends and valves on the suction side due to friction.