Cavitation in a Cooling-Water Pump at High Flow
An open sump supplies a centrifugal pump rated 120 m³/h at 45 m head to a cooling circuit. At the minimum sump level the pump centerline is 3.8 m above the water surface. Suction piping is DN150, equivalent length 12 m, with two 90° elbows and a strainer; calculated suction loss is 2.4 m at 120 m³/h and rises with the square of flow. Water temperature is 38 °C (vapour pressure 6.6 kPa, density 993 kg/m³); barometric pressure is 101.3 kPa. The pump NPSHr curve reads 3.0 m at 120 m³/h and 5.2 m at 150 m³/h. Operators report gravelly noise and vibration only when the discharge valve is wide open; flow then reaches about 150 m³/h. To stop the noise, the night shift has been throttling the suction valve. The discharge control valve is currently 100% open and the process needs 140–150 m³/h.
1. Calculate NPSHa at 120 m³/h and at 150 m³/h and compare it with NPSHr. What conclusion follows?
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Available NPSH
NPSHa = (p_atm − p_v)/(ρg) − z − h_f.
Pressure head: (101.3 − 6.6) kPa = 94.7 kPa. With ρg = 993 × 9.81 ≈ 9,741 Pa/m = 9.741 kPa/m, the pressure head is 94.7/9.741 ≈ 9.72 m.
- At 120 m³/h: NPSHa = 9.72 − 3.8 − 2.4 = 3.52 m. NPSHr = 3.0 m, so the margin is only 0.52 m.
- At 150 m³/h: suction loss = 2.4 × (150/120)² = 3.75 m. NPSHa = 9.72 − 3.8 − 3.75 = 2.17 m. NPSHr = 5.2 m, a deficit of about 3.0 m.
Conclusion
The pump is cavitating whenever flow approaches 150 m³/h. The NPSHr curve rises steeply near runout while NPSHa falls because suction loss grows with flow squared, so the two curves cross well before the required duty. Even at 120 m³/h the 0.52 m margin is thin; typical practice is to keep 0.6–1.0 m above NPSHr, especially with a strainer that can foul. The noise is therefore a symptom of a hydraulic limit, not a mechanical defect. Throttling the suction valve lowers flow and NPSHr, which quiets the pump, but it also adds suction loss and drives NPSHa down; the cavitation damage continues.
2. Evaluate the possible fixes and recommend a combination that can deliver 140–150 m³/h. Quantify where possible.
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Options
- Enlarge the suction line to DN200. For the same flow, friction loss scales roughly with 1/d⁵: (150/200)⁵ ≈ 0.237, so 3.75 m becomes about 0.89 m at 150 m³/h. NPSHa rises to 9.72 − 3.8 − 0.89 ≈ 5.03 m, still just below the 5.2 m NPSHr at 150 m³/h but far better. Replace the strainer with a low-loss type and use long-radius elbows to gain more.
- Lower the pump or raise the minimum sump level. Every metre of reduced elevation adds one metre of NPSHa. Lowering the pump by 1 m together with the DN200 suction gives about 6.0 m available, a safe margin.
- Reduce flow to 120 m³/h with a VFD or impeller trim. This restores margin but does not meet the stated 140–150 m³/h process need, so it is only an interim measure.
- Add a booster pump or a pressurized suction source. Effective but costly and adds a failure mode.
Recommendation
Enlarge the suction to DN200 and lower the pump by about 1 m, or equivalently raise the minimum operating level. Verify the strainer pressure drop and the sump vortexing at minimum level; a vortex can draw air even when NPSHa looks adequate. Recalculate NPSHa at the maximum temperature, not only at 38 °C, because vapour pressure rises quickly with temperature. If the pump must run at 150 m³/h before the piping change, use a temporary flow limit and monitor vibration.
3. What should be said about the night shift throttling the suction valve, and what operating rule should be issued?
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Why suction throttling is the wrong remedy
Throttling the suction valve increases the suction-side loss, so NPSHa falls further. The pump may sound quieter because flow and NPSHr drop, but the cavitation is being masked, not cured. The valve also creates a local low-pressure region downstream of the throttle, which can flash and cause erosion, and it can pull air past seals or flange gaskets. Damage to the impeller, wear rings, and mechanical seal continues, and the pump may lose prime or suffer a sudden capacity collapse.
Operating rule
- Suction valves are to be locked fully open during operation; only the discharge valve or a VFD may be used to control flow.
- Set a maximum flow of about 120 m³/h until the suction modification is complete, and mark it on the control valve or DCS.
- Add a low-suction-pressure alarm or vibration trend, and record any gravelly noise as an event, not a nuisance.
Communication
Explain to the shift team that the noise reduction was a false signal and that the pump was being damaged. Do not blame individuals for a workaround made necessary by a system with no margin. Provide an interim operating limit and a date for the permanent fix. After the modification, verify NPSHa at maximum flow and minimum level before returning to 150 m³/h.