Feedwater Pump Cavitation at High Fire
A deaerator operates at 5 psig and supplies 230°F feedwater to a centrifugal boiler feed pump. The pump centerline is 8 ft below the deaerator outlet, and the suction line is 4-inch with a clean strainer pressure drop of 2 psi and piping friction of 1.5 psi. The pump manufacturer requires 7 ft NPSH. At high fire the pump develops cavitation noise and the discharge pressure oscillates; the flow is 120 gpm. The suction strainer differential gauge reads 8 psi, and the deaerator pressure is steady at 5 psig. The operator throttles the pump discharge to reduce noise. The boiler is a 20,000 lb/h, 100 psig unit and the plant is at peak load. The feedwater temperature is 230°F.
1. Calculate NPSHa with the clean strainer and with the observed strainer differential. What does the result show?
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Use NPSHa = pressure head at the deaerator + static head − suction-line friction − strainer loss. At 5 psig, pressure head is about 5 × 2.3 = 11.5 ft. Static head is 8 ft. Piping friction is 1.5 psi × 2.3 = 3.5 ft. Clean strainer: 2 psi × 2.3 = 4.6 ft. NPSHa = 11.5 + 8 − 3.5 − 4.6 = 11.4 ft; margin over the 7 ft NPSHr is 4.4 ft.
With the observed 8 psi strainer drop: 8 × 2.3 = 18.4 ft, so NPSHa = 11.5 + 8 − 3.5 − 18.4 = −2.4 ft. That is below the required 7 ft and even negative, so cavitation is expected. The steady deaerator pressure does not rule out cavitation; the loss is on the suction side.
2. Diagnose the problem and list immediate corrective actions. Why is throttling discharge not a proper fix?
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The strainer is clogged, and the pump is cavitating. Throttling the discharge reduces flow and may reduce the required NPSH, but it does not clean the strainer and can cause recirculation, overheating, and thrust problems; it also hides the real problem.
Immediate actions: reduce boiler load if possible, open the pump recirculation line if provided, stop the pump if vibration or noise is severe, and clean or replace the suction strainer. Verify deaerator level and pressure, check for a closed suction valve or collapsed lining, and inspect the impeller for damage. After cleaning, confirm strainer differential is near 2 psi and restart with the discharge valve throttled only for normal startup. Monitor vibration, noise, and discharge pressure.
3. What permanent changes and monitoring would ensure adequate NPSH margin?
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Permanent improvements: replace the 4-inch suction line with a larger line if calculations show marginal NPSH, minimize elbows and fittings, and raise the deaerator or lower the pump. Add a differential-pressure gauge and alarm across the strainer and a preventive-maintenance schedule for cleaning. Consider a low-NPSH impeller or a booster pump.
Keep feedwater temperature and deaerator pressure within design; higher temperature reduces NPSH margin. Install flow, pressure, and vibration monitoring and trend the data. Recalculate NPSHa at maximum flow and worst-case strainer fouling, and require at least a few feet of margin above NPSHr. Do not rely on discharge throttling as a cure.