Designing the suction so pumps never cavitate: NPSH available vs required, the cavitation margin, vapour-pressure vs temperature, flooded-suction design and a worked NPSHa calculation.
Cavitation is the silent killer of pumps. If the absolute pressure at the impeller eye falls to the liquid’s vapour pressure, vapour bubbles form and then implode — eroding metal, shaking the machine and collapsing head. The defence is a margin: the NPSH available from the suction system must comfortably exceed the pump’s NPSH required across the whole operating range.
Patm = absolute pressure on the suction source (Pa)Pvap = vapour pressure of the liquid at temperature (Pa)hs = static suction head (+ flooded, − lift) (m)hfs = friction + fittings loss in the suction line (m)The design rule is a healthy margin over the pump’s catalogue NPSHr:
Two terms dominate the risk: warm liquid (Pvap rises steeply with temperature, shrinking NPSHa) and suction friction (long, undersized or fitting-heavy suction lines). Both are designed out, not patched later.
| Water temperature (°C) | Vapour pressure (kPa abs) | Vapour head (m) | Effect on NPSHa |
|---|---|---|---|
| 10 | 1.2 | 0.12 | Generous margin |
| 20 | 2.3 | 0.24 | Baseline |
| 40 | 7.4 | 0.75 | Margin shrinking |
| 60 | 19.9 | 2.03 | Flooded suction usually needed |
| 80 | 47.4 | 4.83 | High risk — pressurise / flood |
This is why hot-effluent and condensate pumps are so often flooded-suction with short, generous suction pipework — every metre of vapour head eats directly into NPSHa.
Open tank at sea level (Patm = 101.3 kPa), water at 40°C (Pvap = 7.4 kPa), pump 2.0 m below the tank water level (flooded, hs = +2.0 m), suction friction hfs = 0.8 m. Pump NPSHr = 3.0 m.
Keep suction friction low — compute it for the suction line before fixing the layout.
Reynolds & Bauhm sizes pipework, pumps and valves and closes the head budget as part of a gated, fully documented bespoke design — every figure traceable from flow to fabrication.
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