The minimum operating values that can be reached at the pump suction end are limited by the onset of cavitation. Cavitation is the formation of vapour-filled cavities within liquids where the pressure is locally reduced to a critical value or where the local pressure is equal to or just below the vapour pressure of the liquid. The vapour-filled cavities flow with the current, and when they reach a higher pressure area, the vapour contained in the cavities condenses.
The cavities collide, generating pressure waves that are transmitted to the walls. These, being subjected to stress cycles, gradually become deformed and yield due to fatigue. This phenomenon, characterised by a metallic noise produced by the hammering on the pipe walls, is called incipient cavitation.
The damage caused by cavitation may be magnified by electrochemical corrosion and a local rise in temperature due to the plastic deformation of the walls. The materials that offer the highest resistance to heat and corrosion are alloy steels, especially austenitic steel. The conditions that trigger cavitation may be assessed by calculating the total net suction head, referred to in technical literature with the code NPSH (Net-Positive Suction Head).
The NPSH represents the total energy (expressed in metres) of the liquid measured at suction under conditions of incipient cavitation, excluding the vapour pressure (expressed in metres) that the liquid has at the pump inlet.