Condensate Extraction Pump Cavitation During Hotwell Level Excursion
At a 500 MW coal-fired unit, the condensate extraction pump takes suction from the hotwell and discharges to the deaerator. During monsoon operation the hotwell level has been unstable. Design data: pump flow 1,200 m³/h, suction pipe 450 mm ID, equivalent length 45 m, Darcy friction factor 0.018, total minor-loss coefficient K = 5.0. Hotwell water temperature is 55°C, vapour pressure 15.7 kPa(a), density 985 kg/m³, atmospheric pressure 101.3 kPa. The pump centreline is 1.2 m below normal hotwell water level. The pump NPSHr is 8.0 m at the operating flow. The suction strainer is clean and the level transmitter has been calibrated. Last week the pump developed a rattling noise and discharge pressure fluctuation whenever the hotwell fell by about 0.6 m; operators responded by throttling the discharge valve.
1. Calculate the available NPSH at the normal hotwell level. Show the velocity, velocity head, friction and minor losses, and the pressure-head conversion. State the margin over NPSHr.
Read the worked answer
Convert flow: Q = 1200/3600 = 0.3333 m³/s. Area A = π(0.45)²/4 = 0.1590 m², so V = Q/A = 2.096 m/s. Velocity head = V²/(2g) = 2.096²/(2×9.81) = 0.224 m. Friction loss hf = f(L/D)(V²/2g) = 0.018(45/0.45)(0.224) = 0.403 m. Minor loss hm = K(V²/2g) = 5(0.224) = 1.120 m. Total suction loss = 1.523 m. Pressure head = (patm - pv)/(ρg) = (101.3 - 15.7)×10³/(985×9.81) = 8.859 m. NPSHa = 8.859 + 1.2 - 1.523 = 8.536 m. Margin = 8.536 - 8.0 = 0.536 m. Positive but small; a 0.6 m level drop would erase it.
2. If the hotwell level falls by 0.6 m, will the pump cavitate? Explain the symptoms and why throttling the discharge valve is the wrong response.
Read the worked answer
With the level 0.6 m lower, static head is 0.6 m: NPSHa = 8.859 + 0.6 - 1.523 = 7.936 m, which is 0.064 m below NPSHr. Cavitation is therefore expected, and the rattling and discharge-pressure fluctuation are consistent with it. Throttling discharge is the wrong response because it does not restore suction pressure. It may reduce suction loss slightly by lowering flow, but it moves the pump toward low-flow recirculation and raises the temperature of the recirculated water, which increases vapour formation. It also hides the real cause. The correct immediate action is to restore hotwell level and, if necessary, reduce the duty per pump by starting the standby pump. Check for dissolved-gas release, blocked strainer and vent restrictions.
3. Recommend three actions in priority order, identifying one immediate operator action and one design or configuration change.
Read the worked answer
- Immediate operator action: raise the hotwell level above 1.8 m and verify NPSHa from the level and temperature indications. If level cannot be held, reduce unit load or transfer flow to the standby pump rather than throttling discharge.
- Operational/instrumentation action: add a low-level alarm and trip, install a vortex breaker, and trend NPSHa against hotwell level and water temperature so the margin is visible before cavitation starts.
- Design change: increase suction-line diameter or reduce K by removing unnecessary bends and using a full-port strainer; if the margin is still inadequate, lower the pump elevation or raise the hotwell. This ranking restores protection first, prevents recurrence second, and addresses the root hydraulic limitation last.