10.2 Troubleshooting and Repairing Pumps
Key Takeaways
- Check the system before the pump: valve line-up, suction level, strainer condition, rotation direction, and venting explain most no-flow calls.
- A pump that runs but delivers nothing is usually not primed, has a closed or plugged suction, is running backwards, or is air bound.
- Wear ring clearance opens with service, and internal recirculation past worn rings reduces capacity and discharge pressure at unchanged power.
- Running a centrifugal pump far from its best efficiency point raises radial load on the shaft and shortens bearing and seal life.
- High vibration at twice running speed with elevated axial vibration points to misalignment rather than unbalance.
Check the system before the pump
The single most productive habit in pump troubleshooting is refusing to blame the pump first. Before a wrench comes out, confirm:
- Valve line-up. Suction valve fully open, discharge valve in the intended position, bypass and recirculation valves correct, and no closed block valve on a minimum-flow line.
- Suction supply. Tank level above minimum, no vortex forming, suction pressure at the expected value.
- Strainer and filter condition. Differential pressure across the suction strainer.
- Rotation direction. Especially after any electrical work. A centrifugal pump run backwards still moves some liquid, so the symptom is reduced flow rather than none, which makes it easy to miss.
- Venting and priming. A high point in the casing or suction line holding vapor stops a centrifugal pump completely.
- Instrumentation. Is the gauge or transmitter reporting the problem actually working?
Symptom-to-cause table
| Symptom | Likely causes |
|---|---|
| Runs but no discharge | Not primed, air bound, suction valve closed, suction line or impeller plugged, rotation reversed, suction lift beyond capability, discharge head exceeds pump capability |
| Insufficient flow or pressure | Partially plugged suction or strainer, air leak into suction, worn wear rings or impeller, speed too low, trimmed impeller, entrained gas, reversed rotation |
| Loses prime after starting | Air leak on the suction side, suction level falling, vortexing, vapor binding from a hot or volatile liquid |
| Draws excessive power | Speed too high, fluid heavier or more viscous than design, rubbing internals, bent shaft, packing gland over-tightened, mechanical damage |
| Overheats or seizes | Operating at or near shutoff with no minimum flow, dry running, lubrication failure, excessive preload from misalignment or thermal growth |
| Noisy, sounds like gravel | Cavitation from insufficient NPSH available |
| Noise plus erratic delivery | Air entrainment through a suction leak or vortex |
| Vibration at 1x running speed, radial | Unbalance — fouled, worn, or damaged impeller |
| Vibration at 2x running speed with high axial | Misalignment |
| Vibration at vane pass frequency | Excessive volute-to-impeller clearance, or operation far off the best efficiency point |
| Seal leaks quickly after replacement | Shaft runout, pipe strain, plugged flush, dry running, or the wrong seal for the service |
| Bearings fail repeatedly | Misalignment, pipe strain, off-BEP operation, lubrication error, or stray shaft current |
Operating point matters
A centrifugal pump is designed to run near its best efficiency point (BEP). Pushed far to the right of BEP, it may cavitate and the motor may overload. Throttled far to the left, toward shutoff, three things happen:
- Radial load on the shaft rises, deflecting the shaft and loading the bearings and seal.
- Internal recirculation begins at the impeller eye and discharge, causing noise and erosion that looks like cavitation.
- Temperature rises, because the absorbed power has nowhere to go but into the liquid.
A pump that fails bearings and seals repeatedly and is always found throttled at 30 percent of design flow has an operating problem, not a mechanical one. The fixes are a minimum flow recirculation line, a trimmed impeller, or a speed change, not a heavier bearing.
Internal clearances
Wear rings are sacrificial close-clearance rings that limit leakage from the high-pressure discharge side back to the suction eye. As they wear, the leakage path opens and capacity and discharge pressure fall while power stays roughly constant — efficiency collapses.
Measure the ring clearance at overhaul with a micrometer and telescoping gauge or with feeler gauges. A widely used rule of thumb is to renew the rings once clearance reaches about twice the original design clearance. The equipment manual governs.
Open and semi-open impellers have no front wear ring; the equivalent function is the impeller-to-casing (or wear plate) clearance, which is set with the adjustment at the bearing housing and must be re-set after any bearing or seal change. Too much clearance loses performance; too little rubs.
Disassembly and repair
- Lockout, drain, and vent. Confirm zero energy and zero pressure, and know what fluid is in the casing.
- Record the as-found condition: alignment readings, coupling gap, impeller clearance, and any shim values. You need these to reassemble.
- Match-mark the casing halves, coupling hubs, and any component whose orientation matters before separating.
- Disconnect the coupling and remove the driver or spacer. Do not use the coupling as a puller.
- Support the rotating element as it comes out; a wet-end rotor is heavy and easily bent.
- Inspect everything, not just the failed part: shaft runout, sleeve condition, keyway, bearing seats, seal chamber bore and face squareness, wear ring surfaces, impeller vane erosion and balance, and the casing for erosion and corrosion.
- Reassemble clean, with new gaskets, new O-rings, a new mechanical seal, and correctly mounted bearings per Chapter 7.
- Set impeller clearance and check shaft runout after assembly, before the pump leaves the shop.
Post-repair verification
- Confirm rotation direction before coupling the driver.
- Align the machine to the thermal targets, and record the readings.
- Vent and prime before starting.
- Start against a partly open discharge where the procedure calls for it, and open to the operating point promptly rather than leaving the pump near shutoff.
- Take baseline vibration and temperature readings once the pump is at operating condition, so the next investigation has something to compare against.
A centrifugal pump was recently worked on electrically. It now delivers reduced flow and lower discharge pressure, but is not noisy and shows no vibration change. What should be checked first?
A pump repeatedly fails bearings and seals. Investigation finds it is normally operated throttled to about 30 percent of its design flow. What is the mechanism?
During overhaul, a pump's wear ring clearance is measured at slightly more than twice the original design clearance. What does this mean for performance, and what is the usual action?