Condensate Pump Cavitation After Heat Exchanger Retubing
A building heating loop uses a centrifugal condensate pump rated 40 L/s at 35 m head, drawing from a receiver vented to atmosphere at 101 kPa. The pump sits 4.2 m above the receiver water level. Water temperature rose from 65 C to 82 C after a heat exchanger was retubed and the loop was rebalanced. Operators report a rattling noise, erratic discharge pressure swinging between 180 and 260 kPa, and vibration at the bearing housing rising from 2.1 to 7.8 mm/s over two weeks. Suction piping is 100 mm with two long-radius elbows and one fully open gate valve; the strainer differential is 12 kPa. NPSHr from the curve at duty flow is 3.5 m. The pump has run 14 months since last overhaul, and the mechanical seal shows slight weeping.
1. Determine whether the pump is at risk of cavitation and identify the controlling variables.
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Compute NPSH available at the pump suction flange.
Vapor pressure of water at 82 C is about 51 kPa absolute. Atmospheric pressure is 101 kPa. Static lift is negative 4.2 m. Suction line losses: strainer 12 kPa, plus elbows, gate valve, and 100 mm pipe friction at roughly 40 L/s. Estimate line loss at about 8 kPa for fittings and pipe, giving total suction loss near 20 kPa, or about 2.0 m of head.
NPSHa = (Patm - Pvapor)/(rho*g) - static lift - suction losses = (101 - 51) kPa converted to head (about 5.1 m) - 4.2 m - 2.0 m = approximately -1.1 m.
This is far below the 3.5 m NPSHr, so the pump is deeply cavitating. The controlling variables are water temperature (vapor pressure), receiver elevation relative to the pump, and suction line losses. The retubing raised loop temperature, which raised vapor pressure from about 25 kPa at 65 C to 51 kPa at 82 C, removing roughly 2.6 m of margin. The strainer loading adds further loss. The rattling, pressure swing, and rising vibration are consistent with cavitation, not bearing wear alone. The seal weep is a secondary consequence of shaft vibration and unstable running.
2. Recommend corrective actions and state how you would verify them.
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Priorities, in order of effectiveness:
- Reduce water temperature at the pump. If the process allows, blend or subcool the condensate so the pump sees 70 C or less. Every 10 C reduction in temperature recovers roughly 1 m or more of NPSHa in this range.
- Lower the pump or raise the receiver. Dropping the pump by 2 m recovers 2 m of NPSHa directly. This is a permanent fix if space allows.
- Reduce suction losses. Clean or replace the strainer, shorten the suction run, remove the gate valve if it is not needed, and consider increasing suction pipe to 125 mm to cut friction.
Verification: recalculate NPSHa against the pump curve at actual duty flow and confirm at least 1 m margin above NPSHr. After changes, confirm noise and pressure swing stop, vibration falls below 4.5 mm/s, and seal weep stops. Trend vibration weekly for a month. Inspect the impeller for erosion and the seal faces for damage before returning to service, since 14 months of cavitation may have already caused material loss.