7. Failure modes
Cavitation
| PT | EN | ES |
|---|---|---|
| Cavitação | Cavitation | Cavitación |
(Defined in §1 Hydraulics. Listed here for cross-reference.)
Vapor-bubble formation and collapse in low-pressure regions. Symptoms: gravelly noise, vibration, head and efficiency loss, rapid impeller wear. Cause: NPSHa < NPSHr, often due to clogged suction strainer, fluid temperature rise, or process upset that exceeds suction-side capacity.
Distinguishing cavitation from other failures: cavitation noise is random in frequency and “shotty”; bearing failure noise is more deterministic, increasing with rotation rate.
Recirculation
| PT | EN | ES |
|---|---|---|
| Recirculação interna | Internal recirculation | Recirculación interna |
Damaging flow reversal inside the impeller when operating well below BEP (typically < 50% BEP). Liquid that should flow outward through the impeller channels instead recirculates back, generating shear, vortex shedding, and impact loads on the impeller.
Distinguishing recirculation from cavitation: both produce noise, but recirculation noise is steady (single tone), while cavitation noise is random. Recirculation occurs at low flow; cavitation can occur at any flow if suction conditions are inadequate.
Mitigation: do not operate centrifugal pumps continuously below 70% BEP. For variable-flow services that must drop below 50% BEP, install a bypass loop to recirculate flow back to suction tank when process demand is low.
Dry running
| PT | EN | ES |
|---|---|---|
| Operação a seco | Dry running | Funcionamiento en seco |
Pump operating without fluid in the casing. For mechanical seals, dry running burns the faces in seconds because there is no fluid film for heat dissipation. For positive-displacement pumps, dry running causes gear-on-casing or rotor-on-casing metal-to-metal contact within the same timescale.
Mitigation:
- Level interlocks on suction tanks
- Flow switches that detect zero flow
- Pressure switches on discharge that trigger shutdown if no pressure builds up
- For mechanical seals: dual-seal arrangements with pressurized barrier fluid
Water hammer
| PT | EN | ES |
|---|---|---|
| Golpe de aríete | Water hammer | Golpe de ariete |
Pressure wave caused by sudden flow change (valve closure, pump trip). Pressure spike can reach 5-10× normal operating pressure and damage piping, casings, and check valves.
Joukowsky’s equation estimates the peak:
ΔP = ρ × c × Δv
Where:
ρ = fluid density (kg/m³)
c = wave celerity in fluid + pipe (≈ 1.000-1.300 m/s for water in steel pipe)
Δv = velocity change (m/s)
For a 3 m/s flow stop in water-in-steel pipe, ΔP ≈ 30-40 bar — far above typical pump rated pressures.
Mitigation:
- Slow-closing valves (motorized actuators with 30-60 s ramp)
- Surge tanks / accumulators at vulnerable points
- Pressure-relief valves sized for the surge volume
- Pump soft-stop sequences (controller ramps speed down rather than tripping)
Misalignment
| PT | EN | ES |
|---|---|---|
| Desalinhamento | Misalignment | Desalineación |
Coupling-to-driver concentricity error. Two flavors:
- Parallel misalignment: shaft centerlines offset (TIR > 0.05 mm)
- Angular misalignment: shaft centerlines at an angle (> 0.05 mm/100 mm coupling diameter)
Both transmit cyclic loads to bearings, accelerating wear. Vibration spectrum shows a strong 2× rotation peak.
Cold versus hot alignment: thermal growth shifts the alignment after the pump warms up. For hot pumps (thermal-oil at 250 °C), the cold- alignment must be deliberately offset so that hot operation lands within tolerance. The offset is supplier-specific; pull from the IOM manual.
Imbalance
| PT | EN | ES |
|---|---|---|
| Desbalanceamento (impeller) | Imbalance | Desequilibrio |
Mass imbalance in the rotating assembly. Causes vibration peak at 1× rotation in the spectrum.
Causes: impeller damage (broken vane, erosion), accumulated deposits on impeller, manufacturing defect, replacement impeller not field-balanced.
Mitigation: dynamic balance per ISO 21940-11 (formerly ISO 1940-1). Field balance kits exist for pumps that cannot be sent to a balance shop.
Bearing failure
| PT | EN | ES |
|---|---|---|
| Falha de mancal / rolamento | Bearing failure | Fallo de cojinete |
Most common pump-component failure. Causes:
- Improper lubrication: too much grease causes overheating; too little causes wear; wrong type causes incompatibility
- Contamination: water ingress through the breather, particulates from process side
- Misalignment: external loading exceeds bearing capacity
- Overspeed: bearing exceeds maximum design rpm
- Electrical erosion (PT: erosão elétrica; ES: erosión eléctrica): VFD-induced shaft currents pit the bearing races. Mitigated by insulated bearings on the non-drive end OR shaft grounding rings.
Seal failure
| PT | EN | ES |
|---|---|---|
| Falha de selo / vedação | Seal failure | Fallo de sello |
Mechanical seal failure modes:
- Face wear (normal): typical 2-5 year life depending on duty
- Thermal shock: rapid temperature change cracks the face
- Crystallization: fluid solidifies between faces, plucks face material
- Cavitation behind face: vapor pockets erode face seating
- Dry running (see above)
- Chemical attack: incompatible seal-face or O-ring material
Diagnosis: dismantle and inspect faces. Wear pattern tells the story — even circumferential wear is normal; fracture indicates thermal shock; crystalline deposits indicate fluid issue; pitted faces indicate cavitation or vapor lock.
Erosion
| PT | EN | ES |
|---|---|---|
| Erosão | Erosion | Erosión |
Material loss from impingement of solid particles or high-velocity fluid on impeller, casing, or wear rings. Common in:
- Slurry duty
- Seawater with sand
- Pulp & paper white-water
- High-pressure deluge or fire-water service
Mitigation: hardened materials (CD4MCu, ceramic-coated wear rings), oversize pump for lower velocities, replace impellers on inspection schedule.
See also
- Hydraulic fundamentals — for cavitation theory
- Mechanical seals — for seal-failure mitigation
- Materials — for erosion-resistance choices
- pump-engineering-handbook §1.2 (selection)