Calculate NPSH available at the pump suction, check margin against NPSH required from the pump curve, and apply the affinity laws for a speed change.
Standard method — stated scope limits apply
NPSH Available
Affinity Laws (Speed Change)
Pump Duty Power (New Duty Point)
Unlike the affinity laws above (which SCALE a power you already know), this calculates absolute power from first principles — for a new duty point with no prior operating data to scale from.
Enter suction conditions or affinity law inputs.
About This Calculation
NPSH (Net Positive Suction Head) governs whether a pump cavitates — whether the liquid at the impeller eye stays above its vapor pressure or flashes to vapor bubbles that then collapse violently downstream, eroding the impeller and destroying performance. NPSH available (NPSHa) is what the actual installation provides, from source pressure, elevation, and suction-line friction loss; NPSH required (NPSHr) is what the specific pump needs at that flow, from its manufacturer’s curve. The pump only runs safely if NPSHa comfortably exceeds NPSHr — and because both terms depend on temperature and flow, a pump that’s fine at design conditions can still cavitate at a different operating point.
Frequently Asked Questions
How much NPSH margin is actually enough?
A commonly used minimum is 0.6 m (2 ft) or 10% of NPSHr, whichever is larger — but critical services (boiler feed pumps, for example) often specify more, since the Hydraulic Institute’s figures are a general minimum, not a guarantee against all cavitation risk.
What is the most common real-world cause of inadequate NPSH?
A suction strainer or filter that’s partially clogged, adding friction loss that wasn’t accounted for in the original design — NPSHa silently degrades over time as the strainer fouls, until the pump starts cavitating at a flow it previously handled fine.
Does raising liquid temperature affect NPSH?
Yes, significantly — vapor pressure rises sharply with temperature, which directly reduces NPSHa. This is why boiler feedwater and other hot, near-saturation services are especially sensitive to NPSH margin.
Can a pump be damaged by cavitation even if it still pumps fluid?
Yes — cavitation can be happening (with real impeller erosion accumulating) well before it’s severe enough to visibly affect flow or head. Audible “gravel in the pump” noise is a late symptom, not an early-warning sign.