A runner doesn't fail on the day it's pulled from the water. It fails months earlier, one pit, one hairline crack, one degree of imbalance at a time, while the plant keeps generating and nobody's looking closely enough to notice. Whether it's a Francis runner fighting cavitation at the blade suction side, a Kaplan blade eroding at the hub under sediment-laden flow, or a Pelton bucket splitter wearing thin from a high-head jet, the damage pattern is different but the outcome is the same: a scheduled inspection catches it as a weld repair, a missed one turns it into a runner replacement. Sign up to log runner inspections, NDT results, and repair history against every unit instead of a maintenance log that only gets opened after something breaks.
Why It Matters
Cavitation pitting on a Francis runner can advance over a millimetre a month once it starts, and it rarely announces itself until efficiency has already dropped. The runners that get pulled for an unplanned outage are almost always the ones nobody had recent thickness readings or crack data on.
Three Runners, Three Different Ways To Fail
| Turbine Type |
Primary Damage Mechanism |
Where It Shows Up First |
| Francis (Reaction) |
Cavitation at the blade leading and suction edges, worsened by sediment |
Runner intake region and blade-to-band weld seams |
| Kaplan (Reaction) |
Tip and hub cavitation from moving blades, plus blade-pitch seal wear |
Blade tip clearance and the hub oil-seal assembly |
| Pelton (Impulse) |
Hydro-abrasive erosion from the high-velocity jet on buckets and needle |
Bucket splitter, cut-out edge, and needle tip |
The Four Damage Types Every Inspection Should Rule Out
Cavitation Pitting
Collapsing vapour bubbles punch tiny craters into the blade surface, worst near the suction side and leading edge
Sediment Erosion
Fine silt smaller than 200 microns slips past filtration and sandblasts the runner surface with every pass
Fatigue Cracking
Repeated stress cycles turn a pitted or eroded spot into a hairline crack, usually at a weld or fillet
Imbalance & Vibration
Uneven material loss shifts the runner's centre of mass, and vibration readings climb before anything is visible
Catch Pitting Before It Becomes A Weld Repair
Oxmaint logs NDT results, thickness readings, and vibration trends against every runner, so wear shows up as a curve instead of a surprise during teardown. Sign up for a free trial to bring your turbine fleet online, or book a demo to see it set up for your plant.
A Runner Inspection Cycle That Actually Catches Damage Early
Record A Baseline At Commissioning
Photograph blade surfaces and log wall thickness at known points so every future reading has something to compare against
Run Dye-Penetrant And Ultrasonic NDT Every Outage
Surface cracks and thinning metal rarely show up to the naked eye before non-destructive testing catches them first
Track Vibration Trends Between Outages
A slow climb in vibration readings often flags developing imbalance weeks before it would be caught on a visual walk-down
4
Re-Balance After Any Weld Repair
Adding weld metal to one blade without checking runner balance can introduce the exact vibration problem the repair was meant to fix
How A Pitted Runner Turns Into A Failed One
Early Pitting
Small craters near the leading edge, only visible on close inspection
Efficiency Loss
Flow disruption around pits quietly drags down unit efficiency
Crack Initiation
Stress cycles turn pitted or eroded metal into a hairline crack
Runner Failure
A propagating crack risks a released blade fragment and forced outage
Caught at the pitting stage, the fix is a scheduled weld build-up during a planned outage. Left to reach the crack stage, the same runner can mean an unplanned shutdown, an emergency pull, and weeks of lost generation.
The Payoff
Runners inspected on a fixed NDT and vibration schedule get a planned weld repair during a scheduled outage. Runners inspected only when performance visibly drops usually need a full runner replacement, plus every megawatt-hour lost while the damage went unnoticed.
Frequently Asked Questions
Q
How often should a Francis, Kaplan, or Pelton runner be inspected?
Most plants pair annual dye-penetrant and ultrasonic checks with a full visual inspection at every scheduled outage, moving to shorter intervals for units running high-sediment rivers or high-head sites.
Q
Do all three turbine types need the same maintenance approach?
No. Francis and Kaplan runners, as reaction turbines, are checked mainly for cavitation and blade-seal wear, while Pelton units, as impulse turbines, are inspected mainly for hydro-abrasive erosion on the buckets and needle.
Q
What's the earliest warning sign of runner damage?
A gradual rise in vibration readings or a small unexplained dip in unit efficiency usually appears well before any pitting or cracking is visible on a walk-down inspection.
Stop Losing Runners To Damage Nobody Was Tracking
Oxmaint logs cavitation, erosion, cracking, and vibration data against every Francis, Kaplan, and Pelton unit, so runner decisions are backed by trend data instead of a guess at teardown. Sign up for a free trial to bring your turbine fleet online, or book a demo to see it configured for your plant.
