Compute NPSHa from the suction system and check the cavitation margin against NPSHr per ANSI/HI 9.6.1
Understand NPSHa vs NPSHr, the cavitation mechanism, vapor pressure, and the ANSI/HI margin rules
Use absolute pressures, never gauge. For an open tank, Psabs = atmospheric.
Cavitation occurs when local pressure at the impeller eye drops to the liquid's vapor pressure: vapor bubbles form and then implode violently in the higher-pressure region, eroding metal and collapsing head. NPSH margin is the guard band that keeps that from happening.
Thresholds follow ANSI/HI 9.6.1. NPSHr always comes from the pump vendor's certified curve — confirm the final margin against it.
NPSHa is the Net Positive Suction Head available — the absolute pressure energy (above the liquid's vapor pressure) present at the pump suction, expressed in feet of liquid. It is a property of the suction system: NPSHa = (Ps_abs − Pv) × 2.31 / SG + Hs − Hf, where Ps_abs is the absolute pressure on the liquid surface, Pv is the vapor pressure at pumping temperature, SG is specific gravity, Hs is the signed static suction head (positive when flooded, negative for a lift), and Hf is the suction-line friction and entrance loss.
NPSHa (available) is what the suction system delivers and is calculated by the engineer. NPSHr (required) is what the pump needs to avoid cavitation at a given flow, and it comes only from the manufacturer's certified test curve. To run safely NPSHa must exceed NPSHr with margin: if NPSHa ≤ NPSHr the pump cavitates.
ANSI/HI 9.6.1 guidance is a margin of at least about 3 ft and a ratio NPSHa/NPSHr of at least about 1.1 for general service. Hot water above 300°F and hydrocarbons near their bubble point typically warrant a larger margin (ratio ≥ 2.0). Always confirm the final margin against the pump vendor's certified curve.