NPSHa = (P_suction – P_vap) x 10.2 + static head – suction loss. Hold margin above NPSHr of at least 1 m or redesign suction.
Offer versus demand
NPSHa combines suction pressure, vapor pressure, static elevation, and suction losses into metres of head. NPSHr comes from the vendor curve at the operating flow and rises steeply past best efficiency.
The 1 m rule and when to want more
Hold NPSHa – NPSHr at 1 m or more for water-like service. Hot, flashing, or high-suction-energy duties deserve more, and any margin under half a metre is a stop-work condition.
Fixes that actually work
Raise the supply level, upsize the suction line one size, remove strainers and sharp elbows near the eye, reduce speed with a VSD, or select a lower-NPSHr pump. Throttling the discharge to cut flow also cuts NPSHr.
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.
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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