Condenser-Water Pump Cavitation After Strainer Fouling
A 1,200-gpm condenser-water pump takes suction from a cooling-tower basin. The pump centerline is 4 ft above the basin water level. At 85°F, the suction piping and fittings produce 8 ft of head loss when clean; the manufacturer's NPSH required at 1,200 gpm is 18 ft. After two seasons, the pump cavitates at design flow. A gauge shows the suction strainer has a 14-ft head loss. The tower water is now 95°F. Atmospheric pressure is 14.7 psia. The operator proposes throttling the discharge valve to stop the noise.
1. Compute NPSH available clean and fouled, at 85°F and 95°F.
Read the worked answer
Atmospheric head = 14.7 × 2.31 = 33.96 ft.
Vapor pressure of water: at 85°F about 0.596 psia, or 0.596 × 2.31 = 1.38 ft; at 95°F about 0.815 psia, or 1.88 ft.
NPSH_A = p_atm/γ - p_v/γ + static head - suction friction
Clean, 85°F:
NPSH_A = 33.96 - 1.38 + (-4) - 8 = 20.6 ft
Fouled, 85°F:
NPSH_A = 33.96 - 1.38 - 4 - 14 = 14.6 ft
Fouled, 95°F:
NPSH_A = 33.96 - 1.88 - 4 - 14 = 14.1 ft
With NPSH_R = 18 ft, the clean pump has only about 2.6 ft margin at 85°F. The fouled pump is short by about 3.4–3.9 ft, so cavitation is expected.
2. Diagnose the problem and evaluate the proposal to throttle the discharge valve.
Read the worked answer
The strainer fouling has raised suction loss from 8 ft to 14 ft, and the warmer tower water has reduced the vapor-pressure margin. Available NPSH is below required NPSH, so the pump cavitates. Throttling the discharge valve would reduce flow and lower NPSH_R somewhat, but it wastes energy, may not restore design condenser flow, and does not address the root cause. It is at best a temporary measure. The correct action is to clean or replace the strainer, remove debris from the basin, verify basin level, and confirm the suction piping is not air-bound. After cleaning, recheck NPSH_A and pump noise/vibration.
3. What instrumentation or operating changes would you recommend to prevent recurrence?
Read the worked answer
Install a differential-pressure transmitter across the suction strainer with an alarm at a small increase above the clean pressure drop, so operators clean it before NPSH margin is lost. Add basin-level and water-temperature indication, and trend pump suction pressure and vibration. Consider a larger or duplex strainer if debris loading is normal, and include strainer cleaning in the tower maintenance schedule. Do not rely on discharge throttling as the permanent fix.