The most common assumption in pump selection is this: if the available suction head exceeds the required suction head (NPSHa > NPSHr), there is no cavitation. That is wrong, and the reason is hidden in the definition of NPSHr.
NPSHr is already a cavitating point
The published NPSHr value is the point at which the pump's head has dropped by 3%. By the time that value is reached, cavitation has long since begun and part of the blading is operating under a vapour blanket. Cavitation inception lies significantly above that value.
A margin is therefore required. ANSI/HI 9.6.1 defines the recommended NPSH margin according to the application, the pump design and the distance of the operating point from the best efficiency point (BEP) [1]. The margin varies by application — a hydrocarbon process pump and a water/wastewater pump do not take the same margin.
As the operating point moves away from BEP
At part load the flow does not meet the blade at the right angle; separation and recirculation appear at the suction edge. The local minimum pressure drops far below the mean suction pressure. The same pump that runs cleanly at BEP can cavitate at 50% flow.
The consequence for the analysis is this: checking cavitation at a single operating point is not enough. The part-load end of the curve has to be checked separately.
Modelling
Cavitation models derive vapour production and condensation rates from a reduced form of the Rayleigh-Plesset equation; the "full cavitation model" of Singhal and co-workers adds dissolved gas and turbulent pressure fluctuation to these [2].
Points to watch in practice:
- Vapour pressure depends on temperature. Water at 20 °C and water at 80 °C do not pose the same problem; in boiler feed pumps this is decisive.
- Steady state misleads. The instantaneous minimum pressure is well below the mean; the question of cavitation inception usually demands a transient solution. See the note on steady versus transient.
- Use the result comparatively, not absolutely. Cavitation models are weak at predicting inception in absolute terms, but reliable on the question of which of two blade geometries cavitates later. See the note on significant digits.
The same mechanism in valves
In butterfly and ball valves the throat pressure falls below vapour pressure during throttling. The danger there is not loss of efficiency but the erosion caused by bubbles collapsing downstream — the damage is often found not in the valve itself but in the pipe a few diameters further on.
References
- Hydraulic Institute. ANSI/HI 9.6.1 — Rotodynamic Pumps Guideline for NPSH Margin. Hydraulic Institute, Parsippany, NJ.
- Singhal, A. K., Athavale, M. M., Li, H. & Jiang, Y. (2002). Mathematical basis and validation of the full cavitation model. Journal of Fluids Engineering, 124(3), 617–624. DOI: 10.1115/1.1486223