Engineering
Pump NPSHa (Net Positive Suction Head Available) & Cavitation Margin formula
Estimate available suction head from a large reservoir and compare it with a supplied pump requirement and an explicit illustrative margin.
Reservoir-to-pump suction head balance
For steady incompressible flow from a large reservoir with negligible free-surface velocity, use NPSHa=(P_surface-P_vap)/gamma+z-h_loss. Pressures are absolute kPa, gamma is kN/m³, and elevation and losses are metres of liquid head. For an open tank, surface pressure is atmospheric; for a pressurized tank, supply its absolute surface pressure.
Measure z from the pump reference elevation to the reservoir surface: positive for flooded suction and negative for lift. Include all relevant suction losses at the operating flow; do not use a suction-flange static-pressure reading in this reservoir formula without the appropriate velocity-head treatment.
Compare with the manufacturer requirement at the actual flow, speed and liquid. An NPSH3 curve corresponds to a 3% head reduction caused by cavitation, not its complete absence. Required margins depend on the pump and operating conditions. B8=0.5 m is an illustrative chosen target, not a universal safe design allowance.
| Pump Suction Scenario | Elevation z_s (B5) | NPSHa (m) | vs NPSHr = 4.50 m |
|---|---|---|---|
| 60°C Water, Flooded Tank (+2.0 m) | +2.0 m | 8.43 + 2.0 - 1.2 = 9.23 m | +4.73 m above the supplied requirement |
| 60°C Water, Deep Pit Lift (-5.5 m) | -5.5 m (Suction lift!) | 8.43 - 5.5 - 1.2 = 1.73 m | -2.77 m below the supplied requirement |
Net Positive Suction Head Available (NPSHa) in meters
Converts (P_atm - P_vap) pressure margin into meters of liquid head and adds static head z_s minus suction friction h_f.
=IF(COUNT(B2:B6)<>5,"Invalid inputs",IF(OR(B2<=0,B3<0,B4<=0,B6<0),"Invalid inputs",ROUND(((B2 - B3) / B4) + B5 - B6, 2)))Compare the unrounded head with a chosen margin
Passing means only that the entered arithmetic target is met. It does not certify cavitation-free operation or validate the supplied pump curve and losses.
=IF(COUNT(B2:B8)<>7,"Invalid inputs",IF(OR(B2<=0,B3<0,B4<=0,B6<0,B7<0,B8<0),"Invalid inputs",IF(((B2-B3)/B4)+B5-B6>=B7+B8,"Meets chosen margin","Below chosen margin")))Available head minus supplied requirement
This difference is a margin in metres. A negative value means the available head falls below the supplied requirement.
=IF(COUNT(B2:B7)<>6,"Invalid inputs",IF(OR(B2<=0,B3<0,B4<=0,B6<0,B7<0),"Invalid inputs",ROUND((((B2-B3)/B4)+B5-B6)-B7,2)))Try the example
Use the original values or change them to test your understanding.