Pump Station Duty Point and Cavitation Margin
A contractor is commissioning a booster pump station that lifts water from a sump at 4 m above the pump centreline to a reservoir. Design duty is 90 L/s at 55 m head. The selected pump curve at rated speed gives 55 m at 90 L/s, with NPSHr of 6.5 m at that flow. Site measurements show the sump water surface can drop to 1.8 m above the pump centreline during the dry season, water temperature reaches 38 °C (vapour pressure about 6.6 kPa), and the suction line has 3.2 m of friction loss at design flow. Atmospheric pressure at site is 101 kPa. The contractor wants to accept the station because the pump 'meets the duty point'.
1. Determine whether the pump has adequate NPSH margin under the worst-case sump level, and state the engineering conclusion.
Read the worked answer
Available NPSH: NPSHa = (p_atm − p_vap)/(ρg) + z − h_f. With ρ ≈ 993 kg/m³ at 38 °C: (101000 − 6600)/(993×9.81) ≈ 9.69 m. Then NPSHa = 9.69 + 1.8 − 3.2 = 8.29 m. Required is 6.5 m, so NPSH margin = 8.29 − 6.5 = 1.79 m (about 27% of NPSHr).
That is thin for a station where the sump level varies and flow may drift above design. A common design rule is to keep margin ≥ 1.0–1.5 m or ≥10–30% above NPSHr at the run-out flow, whichever is more onerous. Since the margin is only marginally acceptable and depends on a low sump level, the correct conclusion is conditional acceptance: the pump meets the duty point but the installation has limited cavitation protection. The contractor should either raise the minimum operating level, reduce suction losses (larger suction pipe or shorter run), or obtain the NPSHr curve at higher flows to confirm margin is maintained. Meeting the duty point alone does not demonstrate acceptable suction performance.
2. The contractor proposes trimming the impeller to reduce power. Explain how trimming affects the operating point and NPSHr, and what check is needed before accepting it.
Read the worked answer
Trimming reduces both head and flow for a given speed; using affinity relations, head falls roughly with the square of the diameter ratio and flow with the ratio. The system curve is unchanged, so the intersection moves to a lower flow and head. Power falls, which is the intent.
Two checks are required before acceptance:
- Duty check - confirm the new intersection still delivers at least 90 L/s at the required pressure, including any future demand. A trim that drops below the required flow is not acceptable.
- NPSH check - NPSHr generally falls with reduced flow for a given impeller, but the pump now operates at a different point on its curve. Obtain the manufacturer's NPSHr for the trimmed impeller at the new flow and re-run the NPSHa calculation. If the new flow is lower, suction friction also falls, which helps.
Also verify the pump does not operate at shut-off or far left of best efficiency point, since recirculation and vibration can damage the pump. The trim should be justified by a revised duty point, not by power savings alone.