Every hydraulic turbine runner is a precision rotating component operating under conditions designed to destroy it — high-velocity water laden with silt and dissolved gases, pressure differentials that cause microscopic cavitation bubble collapses against blade surfaces millions of times per hour, and cyclical mechanical stresses that accumulate fatigue damage across decades of continuous operation. The difference between a runner that delivers its rated efficiency at year 30 and one that requires emergency weld repair at year 8 is not the machine — it is the maintenance programme that tracked its degradation zone by zone, inspection by inspection, and intervened before blade pit depth crossed the weld threshold into replacement territory. OxMaint gives hydro O&M teams structured runner inspection records, cavitation pit depth tracking, balance history, and repair documentation — linked to each runner as a registered asset with its own lifecycle record. To see how runner reliability programmes are structured inside OxMaint, book a 30-minute walkthrough with a hydropower maintenance specialist.
OxMaint · Hydro Turbine Runner Reliability Programs
Francis. Kaplan. Pelton.
Three Runner Designs. One Reliability Challenge: Cavitation Wins If You Stop Watching.
Runner degradation is continuous, silent, and compound. The plants that catch it early spend on maintenance. The ones that miss it spend on replacement.
20–700 m
Francis turbine head range — most cavitation-susceptible of all reaction designs
3–5×
Cost of emergency repair vs planned intervention at early cavitation stage
6 months
Time to severe runner wear in high-sediment rivers without structured O&M
5%
Efficiency loss at Stage 3 cavitation — visible in generation data before failure
Runner Type Profiles
Francis, Kaplan, and Pelton Runners: Different Designs, Different Failure Maps
Each turbine type operates on a different hydraulic principle, runs at different head and flow conditions, and experiences degradation in different locations on the runner. An effective reliability programme starts with knowing exactly where each runner type fails — and why.
Francis Turbine
Mixed-flow reaction | Head: 20–700 m | Most installed globally
How the Runner Works
Water enters radially through wicket gates, turns axially through the runner, and exits via the draft tube. The runner blades are fixed-pitch curved vanes welded between crown and band — creating a complex internal flow that varies with operating head and load.
Where It Degrades
Suction side of blades near trailing edge — leading-edge cavitation
Crown-to-blade junction — interblade vortex cavitation at part-load
Blade inlet edges — silt erosion at high sediment concentration
Band labyrinth seal surfaces — wear from particle-laden leakage flow
Blade-to-crown weld — fatigue cracking from pressure pulsation
Operating Zone Caution
Francis turbines have a designated "banned zone" — operating ranges where cavitation intensity is severe enough to cause measurable blade damage within hours. CMMS tracking of operating hours within the banned zone is a maintenance risk factor that correlates directly with pit progression rate.
Kaplan Turbine
Axial-flow reaction | Head: 2–70 m | Adjustable runner blades
How the Runner Works
Water flows axially through adjustable-pitch runner blades controlled by the blade servomotor — allowing the pitch to be varied in combination with wicket gate opening to maintain optimal efficiency across a wide head and flow range. The on-cam control system coordinates blade and gate movement.
Where It Degrades
Blade tip clearance gap — tip vortex cavitation pitting
Runner chamber inner surface — trailing edge cavitation from blade outer edge
Blade leading edge and suction side — hydro-abrasive erosion
Blade trunnion seals — leakage into blade mechanism causes corrosion
On-cam servo feedback linkage — wear causes off-cam operation and efficiency loss
Unique Maintenance Demand
Kaplan blades are individually removable — each blade has its own trunnion, seal, and operating mechanism. Blade mechanism inspections must be performed per-blade with seal condition, oil leakage, and trunnion clearance recorded individually in the CMMS, not as a single runner-level entry.
Pelton Turbine
Impulse type | Head: 100–3,000 m | Bucket wheel design
How the Runner Works
High-pressure water jets from one to six nozzles strike specially shaped buckets on the runner periphery. Energy transfer is entirely by impulse — the runner operates in air, not submerged. This eliminates reaction-type cavitation but creates distinct erosion and fatigue failure modes concentrated on the bucket and jet surfaces.
Where It Degrades
Bucket splitter ridge — erosion from direct jet impact, particularly in high-sediment rivers
Bucket back face — cavitation from low-pressure region behind jet impact
Bucket trailing edges and outer rim — erosion from high-velocity deflected flow
Bucket root — fatigue cracking from cyclic jet impulse loads
Nozzle needle and seat — the most rapid erosion point in the entire Pelton system
NDT Priority Components
Pelton buckets are individually inspectable without dewatering — each bucket can be examined by dye penetrant or UT while the runner is in position. Crack detection at bucket roots before propagation to the attachment weld is the critical NDT task for Pelton reliability programmes.
Cavitation Staging
The Four Stages of Cavitation Damage — and When OxMaint Triggers Each Response
Stage 1
Surface Pitting
Efficiency loss: <1%
Microscopic pits visible under inspection light
Acoustic monitoring shows elevated cavitation noise
No measurable vibration increase
OxMaint Response: Inspection record created — pit depth measurements logged per zone — monitoring frequency increased
Stage 2
Progressive Pitting
Efficiency loss: 1–2%
Pits deepening — measurable depth at standardised zones
Vibration amplitude slightly elevated above baseline
Efficiency curve 1–2% below original
OxMaint Response: Weld repair scoped and scheduled — planned outage window identified — pit depth trend shows repair timing
Stage 3
Deep Erosion & Cracking
Efficiency loss: 3–5%
Deep pits creating stress concentration sites
Dye penetrant or UT confirms crack initiation at blade edges
Vibration has increased measurably — unit may be operational but risk is high
OxMaint Response: Priority escalation — corrective work order raised — planned outage brought forward before Stage 4
Stage 4
Blade Fracture Risk
Efficiency loss: >5% + safety risk
Crack propagation to critical length — fragment separation risk
Vibration exceeds trip thresholds
Emergency shutdown required — repair cost 3–5× planned intervention
OxMaint Response: Emergency shutdown work order — runner removal sequence activated — post-event root cause analysis documented
Track pit depth zone-by-zone. Know your runner's stage before it tells you.
OxMaint logs cavitation measurements per inspection zone per runner — so deterioration rate drives your repair schedule, not a surprise vibration trip.
Inspection Programme
Runner Inspection Tasks by Type: What to Measure, What to Record, and When
Francis Runner Inspections
Annual — In-Situ
Visual inspection of blade suction faces with unit dewatered — photograph all pit areas with standard measurement scale
Annual — In-Situ
Pit depth measurement at standardised zones using depth gauge — log against previous inspection to calculate deterioration rate
Annual / Condition
Dye penetrant inspection of blade-to-crown welds and trailing edges — document any crack indications immediately
Post-repair
Dynamic balance check after weld repair — balance weight, angle, and residual imbalance logged per repair event
Continuous
Operating hours within banned zone logged in CMMS — correlated with pit depth progression at next inspection
Major Outage
Full dimensional survey of runner — blade profile vs original drawing; labyrinth clearance measurement
Kaplan Runner Inspections
Annual — Per Blade
Individual blade surface inspection — leading edge erosion depth and tip clearance measurement recorded per blade in CMMS
Annual — Per Blade
Blade trunnion seal oil leakage inspection — any water ingress into blade mechanism triggers immediate corrective work order
Annual
Runner chamber inner surface inspection — trailing edge cavitation pitting from blade tips; depth measurement at standard points
Annual
On-cam calibration verification — check that blade angle vs gate opening matches design cam curve across operating range
Major Outage
Blade removal and mechanism inspection — trunnion bearing, oil passages, and servo piston per blade; all seal replacement recorded
Major Outage
Anti-cavitation lip condition on blade tips — measure remaining lip height; replacement threshold triggers fabrication lead time
Pelton Runner Inspections
Per Outage — Per Bucket
Individual bucket visual and dye penetrant inspection — crack detection at bucket root and splitter ridge; each bucket result logged individually
Per Outage — Per Bucket
Splitter ridge erosion depth — measure against original profile; note surface texture change indicating active erosion
Annual
Nozzle needle and seat wear inspection — critical: needle erosion changes jet profile and directly reduces bucket efficiency
Annual
3D scan comparison of bucket profiles vs OEM geometry — deviation greater than 2mm in splitter zone triggers repair scoping
Post-weld repair
Dynamic balance verification — Pelton runners are highly sensitive to bucket-to-bucket weight asymmetry after repair welding
Continuous
Sediment load logging during monsoon / high-silt periods — correlate suspended solids concentration with erosion rate at next inspection
Weld Repair Programme
Runner Weld Repair: What the CMMS Must Record to Protect Structural Integrity
01
Pre-Repair NDT
Before any weld material is applied, dye penetrant or ultrasonic testing must establish the full extent of cracking and pit depth. Weld repair applied over undetected cracks — even hairline indications — will fail under operating stress. Pre-repair NDT results must be attached to the repair work order in OxMaint before repair commencement is authorised.
02
Repair Material Specification
Weld filler material must match or exceed the parent runner material in hardness and cavitation resistance. The most common runner materials are CA6NM stainless and 13/4 Cr-Ni — each requiring matched consumables. Repair procedure, filler specification, and welder qualification must be logged per repair event; material changes vs previous repairs must be flagged.
03
Heat Control During Welding
Excessive inter-pass temperature during weld repair causes HAZ (heat-affected zone) brittleness and distortion of blade profile geometry. Inter-pass temperature limits — typically 150°C maximum for CA6NM — must be enforced and recorded during repair. Post-weld heat treatment (stress relief) is required for repairs above a minimum weld volume; PWHT cycle must be documented.
04
Post-Repair NDT and Profile
Post-repair dye penetrant inspection confirms weld integrity. Profile measurement confirms blade geometry is within drawing tolerance — deviation affects hydraulic efficiency and introduces new stress concentration points. Both results are attached to the completed repair work order in OxMaint, creating the documentary record needed for engineering sign-off and next-inspection baseline.
05
Dynamic Balancing After Repair
Any weld repair that changes the runner's mass distribution requires dynamic balancing before return to service. An imbalanced runner introduces vibration that accelerates bearing wear and can re-initiate cracking at blade-crown welds. Balance correction weight, angle, and measured residual imbalance must all be logged in the unit's asset history — and compared at each subsequent balancing event to track long-term trend.
06
Repair History Accumulation Limit
Each repair adds heat-affected zone material and changes local stress distribution. Extensive accumulated weld repairs reduce runner fatigue life and can accelerate future cavitation in repaired zones. OxMaint tracks cumulative repair volume per zone per runner — providing the data for an engineering assessment of whether the next damage event warrants repair or runner replacement.
Frequently Asked Questions
What is the difference between cavitation damage and silt erosion on a turbine runner?
Cavitation damage occurs when water pressure locally drops below vapour pressure, forming microscopic vapour bubbles that collapse against blade surfaces with tremendous localised force — creating smooth-edged pits that start on the low-pressure suction face of blades. Silt erosion is abrasive material removal by hard mineral particles in the water stream — producing rougher, grooved surface texture often concentrated on leading edges and high-velocity zones. In practice, both mechanisms frequently act together — silt erosion removes the protective oxide layer, making the base metal more susceptible to cavitation, and vice versa. OxMaint inspection records distinguish between pit morphology types per zone, enabling repair decisions matched to the actual damage mechanism.
How often should a Francis turbine runner be inspected for cavitation?
Annual in-situ visual inspection with pit depth measurement at standardised zones is standard practice for Francis runners in moderate-sediment environments. In high-sediment rivers — particularly during monsoon seasons — inspection intervals should be shortened to 6 months, with operating hours within the cavitation "banned zone" tracked continuously. The inspection frequency should be dynamic: if pit depth measurements show deterioration rate exceeding design expectation, the next inspection must be brought forward rather than deferred to the calendar date.
Can Kaplan turbine blades be inspected individually without removing the runner?
Yes — the adjustable-blade design of Kaplan turbines allows individual blades to be rotated to a position accessible from the draft tube without full runner removal, enabling inspection of leading edge erosion, tip clearance, and trunnion seal condition on a per-blade basis during planned dewatering outages. Full internal mechanism inspection — trunnion bearing, blade servo piston, oil passages — requires blade removal and is typically performed at major overhaul intervals. In OxMaint, each Kaplan blade is registered as a sub-asset of the runner, with its own inspection record and condition history separate from the other blades.
What are the signs that a Pelton bucket needs weld repair or replacement?
The principal indicators are: splitter ridge erosion depth exceeding 2mm from original profile measured by depth gauge or 3D scan; dye penetrant indications at bucket root or attachment weld; visible changes in bucket back-face surface texture indicating active cavitation; and asymmetric water spray patterns from individual buckets suggesting profile deviation. Any crack indication at the bucket root is treated as an immediate repair requirement — crack propagation to the attachment weld creates a risk of bucket detachment, which is a catastrophic failure mode. Book a demo to see how OxMaint structures Pelton bucket inspection records.
How does operating a Francis turbine in its banned zone affect maintenance intervals?
The banned zone — specific head and load combinations where cavitation intensity is severe — accelerates blade pit depth progression by a factor of 3 to 10 compared to optimal operating range. Every hour spent in the banned zone should be logged in the CMMS and used to adjust the next inspection date: a unit that has accumulated 200 hours in the banned zone since its last inspection carries meaningfully more risk than one that has operated within the optimal range for the same period. OxMaint allows operating regime tracking to be linked directly to runner inspection records, giving maintenance engineers the operating context they need to interpret pit depth measurements correctly.
OxMaint · Hydro Turbine Runner Reliability Programs
Every Runner Has a Story Written in Pit Depths and Repair Records.
Make Sure Yours Is Being Written Down.
OxMaint structures runner reliability programmes for Francis, Kaplan, and Pelton turbines — with per-zone cavitation tracking, weld repair documentation, balance history, and operating regime logging — so your maintenance team never misses the early intervention that prevents the emergency replacement.
