Condensate Pump Cavitation After a Hotwell Control Change
A 210 MW coal-fired unit has two 100% condensate extraction pumps, one running and one standby. The running pump is rated 850 m³/h with NPSHr 6.5 m. After a hotwell level controller modification, the hotwell is being held 1.8 m below the pump suction centreline instead of the original 1.3 m. At full load the hotwell water temperature is 55°C, the suction pipe friction loss is 0.5 m, and the barometric pressure is 100 kPa. Operators report a rattling noise, vibration rising from 3.1 to 7.2 mm/s, an 8% fall in pump flow, and fluctuating discharge pressure. The suction strainer is clean, and the standby pump shows the same symptoms when started. The maintenance manager must decide whether to raise hotwell level, reduce suction losses, throttle discharge, or add a booster pump before the next unit outage.
1. Compute the available NPSH and diagnose the problem. Explain why the standby pump showing the same symptoms matters.
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Convert pressures to head. NPSHa = (p_atm − p_v)/(ρg) − static lift − friction. At 55°C, p_v ≈ 15.7 kPa; with ρ ≈ 988 kg/m³, (100,000 − 15,700)/(988 × 9.81) ≈ 8.7 m. Subtract 1.8 m static lift and 0.5 m friction: NPSHa ≈ 6.4 m. NPSHr is 6.5 m, so the margin is negative and the pump is cavitating. The 8% flow loss, rattle and 7.2 mm/s vibration fit.
The standby pump behaving the same way rules out a single-pump fault such as a worn impeller or a partly closed valve. It points to a common suction condition - the lowered hotwell level. Do not throttle suction to reduce flow: that lowers NPSHa further and may collapse the duty point.
2. Evaluate the four options and recommend an immediate and a next-outage action.
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Options ranked: (i) Restore hotwell level to 1.3 m - raises NPSHa to about 6.9 m, a margin of 0.4 m; near-zero cost, but check the level controller so the old set point is not reintroduced. (ii) Reduce suction friction by replacing the strainer or an elbow, worth 0.2–0.3 m; do at the next opportunity. (iii) Add a booster pump - effective but needs an outage, space and capital. (iv) Throttle discharge - may quieten the noise but does not restore duty and wastes energy.
Do (i) immediately and verify with a vibration reading and flow check. If the margin remains below 0.5 m at full load, do (ii). Keep the standby pump isolated if it also cavitates, because cavitation damages the impeller.
3. What checks would you make to rule out air ingress, and what operating limits would you set?
Read the worked answer
Rule out air ingress through the gland seal, flange joints, hotwell make-up and the condenser vacuum decay test; air ingress raises dissolved oxygen and can mimic cavitation. Fit a temporary pressure gauge at the pump suction and log NPSHa at three loads.
Set operating limits: hotwell level not below 1.3 m, pump flow not above 850 m³/h, vibration alarm at 4.5 mm/s and trip at 7.1 mm/s for the machine class, and a minimum NPSH margin of 0.5 m. Record the limits in the operating log and raise a modification note so the reduced level set point cannot be reintroduced.