Net Positive Suction Head Available (NPSHa)

Cavitation is boiling by pressure drop rather than by heating: if the absolute pressure at the impeller eye falls to the liquid's vapour pressure, bubbles form and then collapse violently downstream, pitting the impeller and making the pump sound like it is passing gravel. NPSHa is the margin you have; NPSHr, from the manufacturer's curve, is the margin the pump demands. Keep NPSHa at least 2–3 ft above NPSHr, more on hot or volatile service. A typical open-tank job: 33.9 ft of atmosphere, 10 ft of flooded suction, 2.5 ft of suction friction, 0.6 ft of vapour pressure gives 40.8 ft available — enormous margin.

The killer variable is temperature. Water at 20 °C has 0.8 ft of vapour-pressure head; at 90 °C it has 24 ft, and at boiling it swallows the whole atmospheric term. Condensate and boiler-feed pumps are therefore designed with flooded suctions and generous suction piping, and a clogged suction strainer — which quietly grows h_f — is the most common single cause of a pump that "suddenly started cavitating" after years of good service. Enter h_s as a negative number when the pump sits above the liquid level.