NPSH is the answer to whether a centrifugal pump will cavitate. The available value is a property of the installation: NPSHa = (P_atm − P_vap)/(ρ·g) + z_static − h_friction — the absolute pressure on the supply surface minus the liquid’s vapor pressure, converted to head, plus the static elevation of the surface above the pump centerline (negative for a suction lift), minus the suction-line friction at operating flow. The required value NPSHr belongs to the pump, measured by the manufacturer as the suction head at which total head has already dropped 3% from cavitation.
Temperature is the variable people underestimate: water’s vapor pressure is only 2.339 kPa at 20 °C but 47.39 kPa at 80 °C, so an installation offering 11.11 m of NPSHa cold can shrink to 6.66 m hot with no plumbing change at all — which is why hot services like boiler feed get elevated deaerator tanks, replacing the pressure head the temperature took away. Altitude bites too, at roughly 12 kPa of barometric pressure per 1,000 m.
By the time NPSHa merely equals NPSHr, cavitation is already well developed, so guidance demands daylight between them: commonly cited floors are 0.5–1 m of margin or a 1.1× ratio, more for high-energy or variable-flow services. And because NPSHr rises steeply with flow, the margin belongs checked at maximum flow, not just the nominal duty.