Feedwater Pump Cavitation From Low NPSH Available
A centrifugal boiler feed pump is rated 400 gpm at 450 ft head, 1,800 rpm, driven by a 75 hp motor. It takes suction from a deaerator operating at 5 psig and 228°F; the pump centerline is 8 ft below the deaerator outlet. Suction piping is 6 in. diameter, about 40 ft long, with six long-radius elbows and a wide-open gate valve. The pump's NPSHr at 400 gpm is 14 ft. At 350 gpm the pump is noisy, discharge pressure swings between 420 and 445 ft, and motor current is 65 A against 78 A FLA. The suction gauge reads 2 psig and fluctuates; the minimum-flow recirculation line is closed. The operator wants to throttle the discharge valve further to stop the noise.
1. Estimate the available NPSH and compare it with the pump's NPSHr. Show the reasoning.
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Use NPSHa = pressure head at the suction surface + static elevation head - vapor pressure head - suction friction losses. The deaerator is at 5 psig and the water is essentially saturated at 228°F, so the absolute pressure head above the water surface and the vapor pressure head nearly cancel. That leaves the static head of about 8 ft, minus suction-pipe friction and fittings. The 6 in. suction line is short but has six long-radius elbows and a gate valve; at 400 gpm the losses are perhaps 2-3 ft, and they increase if the strainer or valve is partly obstructed. The estimated NPSHa is therefore roughly 5-6 ft, well below the 14 ft NPSHr. This is a classic cavitation condition: the pump is asked to operate with less absolute suction energy than it needs. The fluctuating suction gauge and discharge pressure support that conclusion.
2. Diagnose the operating symptoms and explain why throttling the discharge valve further is not a proper fix.
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Noise, fluctuating discharge pressure, fluctuating suction pressure, and low motor current are consistent with cavitation and unstable flow, not normal throttling. The pump is running at 350 gpm, below its rated point, but NPSHa is still below NPSHr because the deaerator is at saturation and the static head is small. Closing the discharge valve further will reduce flow and may reduce the required NPSH somewhat, but it does not correct the fundamental lack of suction head. It can also move the pump toward shutoff, where recirculation heats the water and damage to the impeller, wear rings, and seal can occur. The minimum-flow recirculation line is closed, which is a separate risk if flow drops. Throttling may temporarily quiet the noise by reducing flow, but it can mask the cavitation and allow erosion to continue.
3. Recommend corrective actions, distinguishing immediate operating changes from permanent fixes.
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Immediately, open the minimum-flow recirculation line as required by the pump manufacturer and avoid operating at 350-400 gpm until suction conditions are corrected. Reduce firing rate or use the standby pump if available, and monitor vibration, noise, and suction pressure. Do not rely on discharge throttling as the solution. Check the suction strainer, gate valve position, and any obstruction; verify the deaerator pressure and water temperature. Permanent fixes include increasing NPSHa by raising the deaerator or pump, enlarging and simplifying the suction piping, reducing fittings, adding a booster or inducer pump, or reducing the maximum pump flow to a point where NPSHa exceeds NPSHr with margin. The selected fix should be verified by calculation and a performance test, with the recirculation control restored and the pump operated within its allowable range.