Feed Pump Cavitation After a Deaerator Temperature Excursion
A 250,000 lb/h feedwater system draws from a 15 psig deaerator whose storage tank water level is 22 ft above the centerline of two 100% centrifugal feed pumps. Each pump is rated 1,200 gpm at 1,050 ft head and 3,600 rpm, with a required NPSH of 16 ft at rated flow. The suction header is 10 in; each pump branch is 6 in. At rated flow the branch friction loss is 4.5 ft. The deaerator pressure gauge reads 15 psig, but the storage-tank temperature indicator reads 262°F. During a 1,500 gpm emergency demand, one pump trips on low discharge pressure and the running pump cavitates. The deaerator vent condenser is fouled, and a high-temperature condensate return enters the storage tank below the water line. The plant proposes to raise the deaerator pressure to 25 psig to push more water to the pumps.
1. Calculate NPSH available at normal rated flow and at 1,500 gpm, stating assumptions, and compare with NPSH required.
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Use NPSHa = static head + tank pressure head - vapor pressure head - suction friction. At 262°F the vapor pressure is about 36 psia versus 29.7 psia in the tank, so the net vapor-pressure penalty is about 6.3 psi, or 15 ft at a specific gravity near 0.96. At normal flow: 22 - 4.5 - 15 = about 2.5 ft. At 1,500 gpm, friction scales roughly as flow squared: 4.5 x (1500/1200)^2 = about 7.0 ft, so NPSHa = 22 - 7.0 - 15 = about 0 ft. Both values are far below the 16 ft required. If the tank were truly saturated at 250°F, the pressure and vapor terms would cancel and NPSHa would be 17.5 ft normal and 15 ft emergency, still inadequate at 1,500 gpm. The pumps are cavitating because available NPSH is below required NPSH.
2. Explain why raising deaerator pressure to 25 psig will not restore NPSH margin and may worsen the problem.
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In a direct-contact deaerator, raising pressure also raises the saturation temperature. If the water reaches the new saturation temperature, tank pressure head and vapor pressure head rise together and cancel, so NPSHa remains essentially static head minus suction friction. The 262°F reading at 15 psig already shows the water is above saturation, meaning it is flashing in the suction path; raising pressure may suppress flashing briefly but does not correct the fouled vent condenser or the high-temperature return. Once the deaerator heats to the new saturation temperature, the margin is still only about 17.5 ft at normal flow and 15 ft at 1,500 gpm, below the 16 ft required. Higher pressure can also increase flashing of the hot condensate return and does not address the 6-inch branch losses.
3. What immediate and permanent corrective actions would you take?
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Immediate: verify the pressure gauge with a calibrated instrument, cross-check the temperature, restore venting, divert or isolate the high-temperature condensate return, reduce feedwater demand to the normal rated flow, and avoid running at 1,500 gpm. If the running pump continues to cavitate, transfer to the spare pump only if its suction path is independent and proven. Permanent: repair the vent condenser, relocate or desuperheat the high-temperature return, enlarge the suction branch or reduce pump speed, and raise the deaerator or lower the pumps to provide at least 3 ft of NPSH margin above the 16 ft required. Add pressure and temperature transmitters with a low-NPSH alarm and verify the suction piping for air pockets and closed valves.