Cavitation in a Hot-Well Pump
A centrifugal pump rated 60 m³/h at 45 m head draws from an open concrete sump through a 100 mm line with a foot valve and two bends. The water is process return at 80 °C, with vapour pressure 47.4 kPa absolute and density 971 kg/m³. At the normal sump level the pump centreline is 3.5 m above the water surface, and the measured suction-line loss at 60 m³/h is 1.8 m. The pump curve shows NPSH required of 2.5 m at 60 m³/h and 4.1 m at 75 m³/h. Operators report rattling and falling head at high demand, and the pump has already lost one mechanical seal. The proposal is to fit a bigger motor because “the pump is not delivering”.
1. Calculate the available NPSH at 60 m³/h and at 75 m³/h and compare it with the required values.
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The static head available from atmospheric pressure is (p_atm − p_v)/(ρg) = (101.3 − 47.4) × 10³/(971 × 9.81) = 53.9 kPa/9.525 kPa/m ≈ 5.66 m.
At 60 m³/h: NPSHa = 5.66 − 3.5 (lift) − 1.8 (loss) = 0.36 m. The pump requires 2.5 m, so the margin is −2.14 m: cavitation is certain.
At 75 m³/h: suction loss scales approximately with Q², so h_f ≈ 1.8 × (75/60)² = 2.81 m. NPSHa = 5.66 − 3.5 − 2.81 = −0.65 m, which is physically impossible at the pump inlet. The required NPSH has also risen to 4.1 m, giving a deficit of about 4.75 m. This explains why the rattling and head loss are worse at high demand: the available margin falls while the required margin rises, so the two curves move apart. The seal failure is consistent with operation in cavitation.
2. Evaluate the proposal to fit a bigger motor.
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A bigger motor cannot fix a suction-side problem. The motor only determines whether the pump can be driven; it does not change NPSHa, and it does not change the pump curve. If anything, a larger motor invites higher flow, which increases suction loss and required NPSH, making cavitation more severe. The pump may also run out along its curve, drawing more power and moving further from the duty point.
The correct interpretation is that the pump is not delivering because the inlet is starved, not because it lacks power. Adding motor capacity would increase energy use and may damage the impeller through erosion and the seal through vibration. The proposal should be rejected.
3. Recommend a ranked set of corrective measures, with at least one quantified option that restores an adequate margin.
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First, reduce the static lift or make the suction flooded. Raising the sump level by 3 m, for example, gives NPSHa = 5.66 + 3 − 1.8 = 6.86 m at 60 m³/h, a margin of 4.36 m; even 1.5 m of raising gives 5.36 m and a margin of 2.86 m.
Second, reduce suction loss. Replacing the 100 mm line with 150 mm reduces loss roughly as 1/d⁵, to about 1.8 × (100/150)⁵ ≈ 0.24 m. Combined with lowering the pump by 1.5 m, NPSHa becomes 5.66 − 2.0 − 0.24 = 3.42 m, comfortably above 2.5 m. A larger suction line also reduces the risk at 75 m³/h.
Third, consider cooling the water if the process allows it; at 60 °C the vapour pressure falls sharply and NPSHa rises. Fourth, restrict operation to 60 m³/h with a flow-control valve or a trimmed impeller until the suction is modified. Finally, if none of these is possible, select a low-NPSH impeller or a different pump. The foot valve should be replaced with a low-loss arrangement, and the seal should be checked for air ingress after the modifications.