Cavitation is suction pressure falling to vapor pressure inside the eye. NPSHa measures what the suction offers; NPSHr, read from the pump curve, measures what the pump demands. Hot liquids, long suction runs, and high suction-energy pumps all eat margin first. Fix low margin by raising the source, enlarging suction piping, cutting losses, slowing the pump, or changing the pump.
Formulas
NPSHa
NPSHa = (P_suc - P_vap) x 10.2 + Hs - hL
Pressures absolute in bar; heads in m.
Margin
margin = NPSHa - NPSHr >= 1 m
Below 0.5 m, stop and redesign.
Worked example
Healthy water suction
Inputs: P_suc 2 bara; P_vap 0.023; Hs +3; loss 1; NPSHr 4.
Calculation: NPSHa 22.17 m; margin 18.17 m.
Lesson:
Frequently asked questions
What margin is safe?
At least 1 m; hot service and high suction energy want more.
Why does hot liquid cavitate more?
Vapor pressure rises with temperature and directly eats NPSHa.
How do I fix low NPSHa?
Raise level, upsize suction, reduce fittings, lower speed, or change pump.
Sources and further verification
- Net positive suction head Wikipedia
- Cavitation Wikipedia
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