NPSHr (Net Positive Suction Head Required) is the minimum pressure at the suction port of the pump to keep the pump from cavitating, which will cause cavitation erosion, noisy slow operation, and damage to the pump. The NPSHr is a function of the pump and must be provided by the pump manufacturer.
NPSHa (Net Positive Suction Head Available) at the suction port of the pump.
NPSHa is a function of the system that must be calculated prior to ordering a pump to assure the selected pump will operate correctly.
If the NPSHr is greater than the NPSHa, the liquid vaporizes. The vapor bubbles collapse (implode) as they encounter increased pressure when exiting from the impeller, leading to cavitation that sounds like marbles or grinding inside the pump.
The NPSHr is unique to each pump depending on the pump design, pump size, RPM, and operating conditions.
The NPSHr is provided on the pump curve provided by Steel & O’Brien.