1. Pump Cannot Restart (Locked Rotors)
Possible causes include solid binding, elastomer swelling, collapsed clearance, or insufficient cold-start torque.
| Probable cause | Corrective action |
|---|---|
| Solids settled in pump casing after shutdown | Lock out and depressurize the unit, open the front cover, and clean the wet end before attempting restart. |
| Rotor rubber swelling or rotor tight against casing | Check fluid temperature, chemical composition, and elastomer compatibility; replace swollen lobes and review material selection. |
| Long fibers, metal fragments, or plastic sheets jam pump | Clean the pump and inlet flange, remove wrapped solids, and review upstream screening, maceration, or cutting protection. |
| Rotor axial tightness or seal nut adjustment incorrect | Review seal nut and rotor axial adjustment. Use the 1/6-turn alignment rule in the maintenance guide before final tightening. |
| Motor power insufficient for cold-start viscosity | Review startup viscosity, temperature, and torque margin; increase motor or reducer margin if required. |
- Solids settled in pump casing after shutdown
- Corrective action
Lock out and depressurize the unit, open the front cover, and clean the wet end before attempting restart.
- Rotor rubber swelling or rotor tight against casing
- Corrective action
Check fluid temperature, chemical composition, and elastomer compatibility; replace swollen lobes and review material selection.
- Long fibers, metal fragments, or plastic sheets jam pump
- Corrective action
Clean the pump and inlet flange, remove wrapped solids, and review upstream screening, maceration, or cutting protection.
- Rotor axial tightness or seal nut adjustment incorrect
- Corrective action
Review seal nut and rotor axial adjustment. Use the 1/6-turn alignment rule in the maintenance guide before final tightening.
- Motor power insufficient for cold-start viscosity
- Corrective action
Review startup viscosity, temperature, and torque margin; increase motor or reducer margin if required.
2. Pump Cannot Self-Prime (Suction Loss)
Check for air leakage, wrong rotation, suction blockage, a dry casing, or increased slip from worn rotors.
| Probable cause | Corrective action |
|---|---|
| Wrong rotation direction | Correct motor phase sequence and confirm rotor direction matches the intended suction line. |
| Inlet pipe blocked | Clean inlet pipe, basket, strainer, gasket openings, and any collapsed hose sections. |
| Inlet leakage or loose inlet bolts | Inspect flange gasket, hose joint, threaded connection, and inlet bolt tightness for air ingress. |
| Air trapped in pump or pipeline | Vent trapped liquid or gas, remove high points where practical, and restore a filled priming path. |
| Rotor rubber layer damaged or worn | Inspect rotor tips and coating; replace damaged lobes if internal slip prevents priming. |
- Wrong rotation direction
- Corrective action
Correct motor phase sequence and confirm rotor direction matches the intended suction line.
- Inlet pipe blocked
- Corrective action
Clean inlet pipe, basket, strainer, gasket openings, and any collapsed hose sections.
- Inlet leakage or loose inlet bolts
- Corrective action
Inspect flange gasket, hose joint, threaded connection, and inlet bolt tightness for air ingress.
- Air trapped in pump or pipeline
- Corrective action
Vent trapped liquid or gas, remove high points where practical, and restore a filled priming path.
- Rotor rubber layer damaged or worn
- Corrective action
Inspect rotor tips and coating; replace damaged lobes if internal slip prevents priming.
3. High Vibration & Heavy Noise (Cavitation)
Piping vibration and metallic crackling can come from cavitation, suction starvation, gas entrainment, or drive misalignment.
| Probable cause | Corrective action |
|---|---|
| Excessive speed or suction lift | Reduce speed, shorten suction lift, or improve flooded suction where practical. |
| Suction line restricted | Check clogged gaskets, strainers, elbows, and undersized suction pipe. |
| Drive shaft misalignment | Verify coupling alignment and baseplate rigidity. |
| Hot or volatile fluid vapor pressure | Review fluid temperature, vapor pressure, and available NPSH against the duty. |
- Excessive speed or suction lift
- Corrective action
Reduce speed, shorten suction lift, or improve flooded suction where practical.
- Suction line restricted
- Corrective action
Check clogged gaskets, strainers, elbows, and undersized suction pipe.
- Drive shaft misalignment
- Corrective action
Verify coupling alignment and baseplate rigidity.
- Hot or volatile fluid vapor pressure
- Corrective action
Review fluid temperature, vapor pressure, and available NPSH against the duty.
4. Mechanical Seal & Isolation Chamber Leaks
Seal failure often shows first as fluid at the injection hole (indicator drain). Check the seal faces, elastomers, and isolation-chamber oil condition.
| Probable cause | Corrective action |
|---|---|
| Worn seal faces | Inspect silicon carbide or tungsten carbide faces for grit wear and scoring; replace the seal set if faces are damaged. |
| Failed seal O-rings | Check EPDM, FPM, or other elastomers for swelling, cracking, or chemical attack; match compound to the fluid. |
| Dry-run overheating | Verify isolation-chamber oil level and review dry-run duration, speed, and shutdown logic — empty running is controlled exposure, not open-ended duty. |
- Worn seal faces
- Corrective action
Inspect silicon carbide or tungsten carbide faces for grit wear and scoring; replace the seal set if faces are damaged.
- Failed seal O-rings
- Corrective action
Check EPDM, FPM, or other elastomers for swelling, cracking, or chemical attack; match compound to the fluid.
- Dry-run overheating
- Corrective action
Verify isolation-chamber oil level and review dry-run duration, speed, and shutdown logic — empty running is controlled exposure, not open-ended duty.
5. Gearbox Overheating & Abnormal Gear Noise
Overheating or gear clash can indicate low oil level, wrong lubricant, bearing wear, gear scuffing, or process-media contamination from a failed seal path.
| Probable cause | Corrective action |
|---|---|
| Incorrect or depleted gear oil | Top up or replace oil with the specified grade and inspect for contamination or water cut. |
| Gear scuffing or bearing wear | Open the gearbox for inspection if timing noise, heat, or vibration persists after oil correction. |
| Seal leakage into gearbox path | Check isolation-chamber drains and oil condition for process-media contamination before blaming gears alone. |
- Incorrect or depleted gear oil
- Corrective action
Top up or replace oil with the specified grade and inspect for contamination or water cut.
- Gear scuffing or bearing wear
- Corrective action
Open the gearbox for inspection if timing noise, heat, or vibration persists after oil correction.
- Seal leakage into gearbox path
- Corrective action
Check isolation-chamber drains and oil condition for process-media contamination before blaming gears alone.
6. Why Does a Rotary Lobe Pump Shaft Break?
A rotary lobe pump shaft carries drive torque and can also experience bending loads. Breakage may follow a sudden overload or a crack that grows under repeated loading. A broken shaft alone does not establish a material defect: review the operating history, installation, and fracture evidence before selecting replacement parts.
| Possible cause | Checks before repair |
|---|---|
| Excessive differential pressure or torque overload | Review existing inlet/outlet pressure records, discharge valve positions, bypass or relief activity, motor trips, and startup conditions. Confirm the pressure protection and torque limits for the installed configuration; do not raise protection settings simply to keep running. |
| Rotor binding or a seized component | After isolation, inspect for trapped solids, settled product, swollen elastomers, lost running clearance, and damaged bearings or timing gears. Find the source of resistance before replacing the shaft; repeated starts can add further damage. |
| Coupling misalignment or mounting distortion | Have the motor-to-pump alignment, coupling condition, baseplate, mounting feet, and pipe supports checked. Correct pipe strain or mounting movement before final alignment, using the installed equipment specifications. |
| Bearing wear or loss of shaft support | Inspect bearing condition, fits, lubrication, and support components for wear or damage. Review abnormal heat, vibration, seal wear, and rotor contact before the failure. Shaft support reduces deflection but cannot prevent every overload or fatigue failure. |
| Repeated loading and fatigue at a local stress concentration | Record the break location relative to keyways, shoulders, and bearing seats, together with starts, stops, reversals, vibration, and prior repairs. Qualified fracture examination may be needed to distinguish fatigue from a single overload; a photo alone is not a confirmed diagnosis. |
| Corrosion, surface damage, or a material/manufacturing issue | Record process exposure, pitting, scoring, and any previous machining or repair. Check the specified shaft material, heat treatment, and available inspection records. Use appropriate material or laboratory examination where needed rather than assuming a stronger steel grade will remove the cause. |
- Excessive differential pressure or torque overload
- Corrective action
Review existing inlet/outlet pressure records, discharge valve positions, bypass or relief activity, motor trips, and startup conditions. Confirm the pressure protection and torque limits for the installed configuration; do not raise protection settings simply to keep running.
- Rotor binding or a seized component
- Corrective action
After isolation, inspect for trapped solids, settled product, swollen elastomers, lost running clearance, and damaged bearings or timing gears. Find the source of resistance before replacing the shaft; repeated starts can add further damage.
- Coupling misalignment or mounting distortion
- Corrective action
Have the motor-to-pump alignment, coupling condition, baseplate, mounting feet, and pipe supports checked. Correct pipe strain or mounting movement before final alignment, using the installed equipment specifications.
- Bearing wear or loss of shaft support
- Corrective action
Inspect bearing condition, fits, lubrication, and support components for wear or damage. Review abnormal heat, vibration, seal wear, and rotor contact before the failure. Shaft support reduces deflection but cannot prevent every overload or fatigue failure.
- Repeated loading and fatigue at a local stress concentration
- Corrective action
Record the break location relative to keyways, shoulders, and bearing seats, together with starts, stops, reversals, vibration, and prior repairs. Qualified fracture examination may be needed to distinguish fatigue from a single overload; a photo alone is not a confirmed diagnosis.
- Corrosion, surface damage, or a material/manufacturing issue
- Corrective action
Record process exposure, pitting, scoring, and any previous machining or repair. Check the specified shaft material, heat treatment, and available inspection records. Use appropriate material or laboratory examination where needed rather than assuming a stronger steel grade will remove the cause.
For a failure review, send the pump nameplate, drive and gearbox details, actual operating speed if known, normal and startup liquid conditions, pressure/current records, and a timeline of noise, vibration, trips, or lockup. Photograph the assembly and both fracture faces with their locations identified. Preserve the fragments and avoid grinding, polishing, welding, or forcing the fracture faces together before examination.
Before returning the pump to service, review the replacement specification and inspect the coupling, bearings, timing gears, rotors, and seals for associated damage. Confirm alignment, clearances, lubrication, and pressure protection against the installed pump documentation. A new shaft does not resolve an uncorrected loading or installation problem.