Feedwater Pump Cavitation at a Low Deaerator
A deaerator operates at 5 psig (about 227°F) and sits with its normal water level 8 ft above the two feedwater pumps. Each pump is rated 100 gpm at 150 psig with a required NPSH of 10 ft. The suction line is 3 in. with 25 ft equivalent length, two elbows, and a strainer showing 3 psi pressure drop. At 80 gpm the pump is noisy, discharge pressure swings between 120 and 145 psig, and flow fluctuates. The standby pump is set to auto-start on low discharge pressure. The operator throttles the pump discharge to smooth the flow.
1. Calculate approximate NPSH available and determine whether cavitation is likely.
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Because the deaerator water is at saturation at 5 psig, the pressure at the water surface is essentially the vapor pressure of the water. The available NPSH is therefore set mainly by static head minus suction-line losses. Static head is 8 ft. The strainer drop alone is 3 psi × 2.31 ft/psi ≈ 6.9 ft, plus elbow and pipe friction and velocity head. NPSHA ≈ 8 - 6.9 - friction/velocity ≈ 1 ft or less. The pump requires 10 ft. With NPSHA far below NPSHr, cavitation is very likely; the noise, fluctuating flow, and swinging discharge pressure are consistent with it.
2. Explain why throttling the discharge and auto-starting the standby pump may not solve the problem.
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Throttling discharge reduces flow and may reduce the pump's required NPSH slightly, but it does not fix a suction system that is already below NPSHr at 80 gpm. It can also move operation away from the best-efficiency point and create recirculation. Auto-starting the standby pump is worse: two pumps drawing from the same suction line increase velocity and friction loss, lowering NPSHA further. The standby should not be regarded as a cure for a suction-limited installation. The root cause is insufficient absolute pressure above vapor pressure at the pump suction.
3. Recommend corrective actions, ranking them by practicality.
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The most direct fix is to increase NPSHA: raise the deaerator storage tank so the normal level is much higher above the pumps, or relocate the pumps lower. Increasing suction line size from 3 in. to 4 in. and reducing equivalent length will cut friction. Replace or clean the strainer and eliminate unnecessary fittings. If the process allows, lower the deaerator operating pressure/temperature, which increases subcooling margin, or install a low-NPSH pump. Verify the deaerator level control is not operating near the bottom of the storage range. After changes, confirm NPSHA exceeds NPSHr with margin at maximum flow, not just at 80 gpm.