Condensate Pump Cavitation After Cooler Re-Route
A condensate pump draws from a vented hotwell at 55 °C and delivers to a deaerator through a recently re-routed discharge line. The original route ran about 12 m with two elbows; the new route adds 20 m of pipe and six elbows to clear a new pipe rack, and the discharge control valve was moved closer to the pump. Since the change, the pump runs with a loud rattling noise, discharge pressure fluctuates between 480 and 560 kPa, and the motor current swings. The pump is a centrifugal unit rated at 40 m³/h, 600 kPa head, NPSH required 3.5 m. The hotwell level is normal and the suction strainer is clean. Flow to the deaerator has dropped about 15% at the same control valve position.
1. Determine whether the symptoms point to cavitation on the suction side or a discharge-side problem, and justify your conclusion with the available data.
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The symptoms are ambiguous at first glance, so reason from the data. Cavitation at the impeller produces rattling noise and fluctuating discharge pressure/current, which matches. But the suction side has not changed: hotwell level is normal, the strainer is clean, and the water temperature is 55 °C. At 55 °C the vapor pressure of water is about 15 kPa absolute, so available NPSH depends on hotwell elevation and suction losses, which are unchanged. The change was entirely on the discharge side, so a suction cavitation cause is unlikely unless the hotwell temperature rose (it did not).
The more probable diagnosis is that the added discharge resistance (20 m more pipe, six more elbows, control valve moved close to the pump) raised the system curve so the pump now operates far to the left of its best efficiency point, at low flow and high head. At low flow, recirculation and internal turbulence at the impeller inlet can cause cavitation-like noise and pressure fluctuation even with adequate NPSH, and the pump may be near shutoff where the head curve is flat and unstable. The 15% flow drop at the same valve position confirms the system curve moved. Before condemning the pump, verify suction pressure at the pump flange with a gauge; if NPSH available is comfortably above 3.5 m, the problem is discharge-side/operating-point, not suction cavitation.
2. Propose the operating and piping fixes, and state what measurements you would take to confirm the diagnosis.
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Measurements: suction pressure at the pump flange (convert to NPSH available using the 55 °C vapor pressure), discharge pressure, flow, and motor current; compare the operating point to the published pump curve. Also check for a vortex in the hotwell and confirm water temperature.
If NPSH available is adequate, fix the system curve and operating point: re-route or enlarge the discharge line to reduce fittings and length, or relocate the control valve so it is not immediately at the pump discharge (give at least several diameters of straight run). If the pump must run at reduced flow, install a minimum-flow recirculation line or trim the impeller so the pump operates nearer BEP at the required flow. Operate with the discharge valve throttled only as a temporary measure and monitor for overheating at low flow.
If NPSH available is marginal, lower the hotwell temperature, raise hotwell level, or enlarge/straighten the suction line. After changes, re-test flow and pressure at the design condition and record the new operating point.