Francis Turbine Runners: Design, Materials, Cavitation Repair and Replacement

Quick Answer

A Francis runner is the rotating wheel of a mixed-flow reaction turbine. Water enters it radially through the wicket gates, gives up its pressure and swirl energy across the blades, and leaves axially into the draft tube. European small-hydro guidance places Francis turbines at roughly 25–350 m of net head, between Kaplan and Pelton machines; large purpose-built units are designed for higher heads. The runner has three main parts — the crown, the band and the blades between them — plus labyrinth seals that limit leakage. Most modern runners are 13Cr-4Ni soft martensitic stainless steel, designated CA6NM in ASTM A743 and A487 (11.5–14% chromium, 3.5–4.5% nickel, 0.4–1.0% molybdenum, 0.06% carbon maximum), cast in one piece or fabricated by welding blades to the crown and band, then heat treated, CNC machined, inspected and balanced. Runners wear out through cavitation, hydro-abrasive (silt) erosion and fatigue cracking; fatigue cracks typically start where blade trailing edges meet the crown or band. Repair means gouging to sound metal, building up with matching 410NiMo or austenitic weld metal, and grinding to templates; tungsten carbide HVOF coatings protect against silt. To order a replacement, send the head and flow range, output, speed, dimensions or a 3D scan, damage history and water-quality data.

The decision usually arrives after an outage. Blade outlets near the band are pitted like coral, the welders used more filler than last time, and the unit no longer reaches its old output at full gate. Repair the runner again, or replace it?

Both can be right. The answer depends on what is damaging the runner, how fast, and whether the original hydraulic design still suits the way the plant runs today.

Where Francis Turbines Fit

Pelton wheels suit high heads and small flows, Kaplan and propeller turbines low heads and large flows, and Francis turbines the wide band between. Published ranges overlap and vary by source; these come from European small-hydro guidance, and large individually designed units extend them.

nQE is the IEC dimensionless specific speed, n × √Q ÷ E^¾, with n in revolutions per second, Q in m³/s and E = g × H in J/kg.

Fixed blades give a Francis turbine a sharper efficiency peak than a Kaplan. Indian small-hydro guidance gives an operating range of about 65–125% of design head, notes that efficiency can fall to around 75% at 65% of rated head, and warns of vibration and power surges below about 40% of rated discharge. A replacement runner is therefore a chance to re-optimise for the heads and flows the plant sees now.

Anatomy of a Francis Runner

Water reaches the runner through the spiral case, the stay vanes, which carry structural loads and turn the flow, and the wicket gates (guide vanes), which set the flow and the swirl entering the runner. The runner removes that swirl and most of the pressure energy, and the draft tube recovers part of the remaining kinetic energy. The runner passage runs full under pressure, so the runner can sit above or below tailwater within the limits cavitation allows.

Materials: Why 13Cr-4Ni

Older runners were cast in carbon steel or plain 13% chromium steel. Most runners built today use a soft martensitic stainless steel with about 13% chromium and 4% nickel: CA6NM in the ASTM castings specifications, and EN 1.4313 (X3CrNiMo13-4) in wrought form for plate and forgings. The low carbon and the nickel give better toughness and weldability than plain 13% chromium grades; the chromium gives the cavitation and corrosion resistance that carbon steel lacks.

ASTM A743 sets minimum tensile and yield strengths of 755 MPa and 550 MPa for CA6NM castings, with 15% minimum elongation.

Welding behaviour. CA6NM is air-hardening: weld metal and the heat-affected zone cool to fresh martensite, which stays hard and brittle until tempered. Shop practice uses matching 410NiMo filler (AWS E410NiMo or ER410NiMo), preheat of around 100–140°C on heavy sections, a controlled interpass temperature and post-weld heat treatment at around 590–620°C — routine in a workshop, difficult with the runner in the pit, which is why in-place repairs often use austenitic fillers. Some small-hydro runners are aluminium bronze.

How Runners Are Made

Whatever the route, the sequence is similar: hydraulic design (and for larger units a model test), casting or fabrication, heat treatment, CNC machining of blade surfaces, seal diameters and coupling face, non-destructive testing (surface methods plus ultrasonic testing for internal soundness), a dimensional check against templates or a 3D scan, balancing to the grade in the contract, and documentation including material certificates to EN 10204.

Hydro castings are commonly bought to CCH 70-4, the 2014 revision of a European acceptance specification for steel castings of hydraulic machines. It sets acceptance criteria for each NDT method, and the designer defines quality zones on each part within the contract requirements: strict levels where stress is high, such as blade roots and trailing edges, relaxed ones elsewhere.

Three IEC documents shape the contract. IEC 60193 covers model acceptance tests and applies mainly to prototypes above 5 MW or with a reference diameter above 3 m. IEC 60041 covers field acceptance tests, when performance has to be proved on site. IEC 60609-1 is the basis for cavitation pitting guarantees on reaction turbines: it evaluates material lost over a given operating time under defined conditions, and states that a guarantee limiting pitting belongs in the contract whether pitting is expected or not.

What Damages a Francis Runner

Cavitation

Cavitation starts wherever local pressure falls below the vapour pressure of water. Vapour bubbles form in the low-pressure zone and collapse when they reach higher pressure, and repeated collapse pits the metal; a cavitating turbine sounds as though gravel is passing through it. The common forms on a Francis runner:

• Leading-edge cavitation — well above or below design head, flow meets the blade inlet at the wrong angle

• Travelling-bubble cavitation — on the blade suction side towards the outlet, at high load with too little submergence

• Inter-blade vortices — at deep part load, between the blades near the crown

• The draft tube vortex rope — at roughly 40–80% of best-efficiency flow, a cavity precesses below the runner at 0.2–0.4 times its rotational frequency, causing pressure pulsations and power swings rather than pitting

How close a turbine runs to cavitation is measured by the Thoma coefficient, σ: the net positive suction energy, referred to the draft tube outlet, divided by the head. It depends on atmospheric pressure, the water's vapour pressure, the draft tube outlet velocity and how high the runner sits above tailwater. Every runner design has a critical sigma from model tests; the plant sigma must stay above it with margin.

Worked example: altitude and runner setting

The permissible setting above tailwater is Hs = (p_atm − p_v) ÷ (ρ × g) + V² ÷ (2 × g) − σ × H. Take a 120 m net head unit whose required plant sigma (the runner's critical sigma from model tests, plus a margin) is 0.07, with a draft tube outlet velocity of 2 m/s and water at 15°C (vapour pressure about 1.7 kPa):

At sea level the runner can sit about 2 m above tailwater; at 2,000 m it must sit slightly below. Replacement runners at high-altitude plants fall into exactly this trap: a new design with a higher sigma can cavitate in a setting the old runner tolerated.

Hydro-abrasive erosion

Sediment, especially hard quartz, wears the surfaces with the highest velocities: blade outlets, the band, the labyrinths, and the wicket gates and facing plates upstream. IEC 62364 (second edition 2019, with a revision in draft) gives guidelines on erosion rates, hydraulic design, resistant materials and coatings, and operation in sediment-laden water. It notes that erosion and cavitation together do more damage than either alone.

Fatigue cracking

Cracks start where cyclic stress meets a stress raiser: the junctions of blade trailing edges with the crown or band, and weld toes. The cyclic stress comes from rotor-stator interaction — a pressure pulse each time a blade passes a wicket gate — and from unsteady flow at part load. In one published failure analysis, trailing-edge fractures at the crown junction appeared after a single day of deep part-load operation; the authors attributed them to possible resonance, after inter-blade vortex cavitation shifted the runner's natural frequencies. Cracks that keep returning point to a design or operating problem that welding will not fix.

Inspection and Repair at an Outage

At each outage:

• Map pitting and erosion on every blade, the band and the crown, with pit depths and photographs

• Record the weight of filler deposited at each repair; it is the clearest sign of whether damage is accelerating

• Crack-test blade roots and trailing edges by penetrant or magnetic particle methods

• Measure labyrinth clearances round the circumference, and check blade outlet openings against templates

• Inspect the wicket gates, facing plates and draft tube cone for the same damage

A cavitation repair removes damaged metal by arc-air or plasma gouging and grinding, confirms sound metal by penetrant testing, builds up with weld metal, grinds back to the original profile against templates and re-inspects, all to a qualified welding procedure.

Coatings for silt. Where sediment is the main problem, tungsten carbide–cobalt–chromium coatings sprayed by HVOF (high-velocity oxy-fuel) are the established protection for blades, guide vanes, facing plates and labyrinths. They are a few tenths of a millimetre thick and go on without significant heating of the part, but the spray gun needs line-of-sight access, so the inner passages of a one-piece runner can be hard to reach. They resist silt abrasion better than bare stainless steel but do not cure cavitation. Restore the profile before coating.

Repair or Replace?

How much a new runner gains depends on the original design and its condition. One published reverse-engineering study, for example, redesigned a runner that delivered only 70% of its installed capacity so that the unit could use its full capacity.

Specifying a Replacement Runner

A manufacturer can only design for the plant as it really operates. Send:

• Head: minimum, rated and maximum net head, and tailwater levels at minimum and maximum flow

• Flow: rated flow and range, and annual hours at part load

• Output and generator: rated output, frequency, number of poles, rated and runaway speed

• Geometry: runner diameters, blade count, crown and band profiles, seals and coupling — from drawings, or a 3D scan if drawings are lost

• Setting: runner elevation relative to tailwater, site altitude and water temperature

• History and water: damage maps, crack locations, repair filler weights, sediment concentration and particle size

• Interfaces and requirements: shaft coupling, guide bearing, labyrinths and head cover; material, inspection and testing requirements

Worked example: speed and specific speed

For a unit with 120 m net head, 25 m³/s rated flow and 93% turbine efficiency:

P = ρ × g × Q × H × η = 1,000 × 9.81 × 25 × 120 × 0.93 ≈ 27.4 MW

On a 50 Hz grid the generator runs at synchronous speed n = 120 × f ÷ p, where p is the number of poles. Each candidate speed gives a different specific speed (E = g × H = 1,177 J/kg):

All five sit inside the Francis band of about 0.05–0.33. A faster machine means a smaller runner and generator, but a higher specific speed usually brings a higher sigma and a deeper setting. On a replacement, the existing generator fixes the speed, and the new runner is designed for that speed and the existing setting.

Related Parts to Check at the Same Time

• Turbine shaft — coupling faces, the journal at the guide bearing, and seal areas

• Guide bearing housing — clearances, alignment and bearing condition

• Wicket gates and guide vanes — cavitation and erosion on vanes and facing plates, end clearances and bushings

• Labyrinth rings — renew them with the runner if clearances have opened

• Turbine inlet valve — butterfly valves are usual at low and medium heads, spherical valves at high heads; see butterfly vs gate valves

• Generator excitation — on machines with static excitation, field current reaches the rotor through slip rings and carbon brushes; see slip ring and exciter brushes

Common Mistakes

1. Repairing cavitation without asking why it happens. The same pits return at every outage, deeper each time.

   Prevention: Compare operating hours by head and load with the runner's design range and sigma, and change operation or setting before welding again.

2. Ordering a new hydraulic design without checking the setting. A design with a higher critical sigma cavitates where the old one did not, especially at altitude.

   Prevention: Check plant sigma at minimum tailwater and actual site altitude against the new design's model-test sigma.

3. Long hours at deep part load. Pulsations and rotor-stator interaction crack the trailing-edge junctions.

   Prevention: Avoid zones with known pulsations, and share load between units to keep each near best efficiency.

4. Austenitic repairs on structural cracks. Lower-strength weld metal in a highly stressed zone cracks again.

   Prevention: Repair cracks with a qualified matching-filler procedure and tempering; keep austenitic overlays for cavitation build-up.

5. No record of repair filler weights. Nobody can tell whether damage is stable or accelerating.

   Prevention: Weigh consumables at every repair and trend the figure.

6. Ignoring labyrinth clearances. Leakage grows and the new runner is blamed for poor efficiency.

   Prevention: Measure clearances at each outage and renew the rings with the runner.

7. Incomplete enquiry data. The quotation rests on assumptions about head, flow and setting.

   Prevention: Send the full data list above, with a 3D scan if drawings are lost.

Supply from Kasko Makine

Francis turbine runners — stainless steel Francis runners for medium-head hydroelectric plants and high flow rates, with blade geometry optimised for efficiency and reduced cavitation. They are made from premium stainless steel for resistance to corrosion and abrasive sediment, using a combination of advanced casting and precision CNC machining, with a smooth surface finish and tight tolerances to limit flow losses.

Turbine shafts — forged steel, precision machined, with a stepped design of multiple diameter stages for bearings and sealing units, for heavy-duty hydroelectric generation.

Bearing housings and guide rings — high-strength alloy, CNC machined, with a stepped inner diameter for mounting bearings and lubrication systems, supporting large-diameter rotating shafts in power generation equipment and turbine assemblies.

Power generation parts — turbine and generator shafts, including flanged, spigot, drilled and hollow shafts, and components for Francis, Pelton and Kaplan turbines: runners, rings, runner tops, servo-deckels, levers, pull-eyes, wicket gates, guide vanes and blades, supplied to customers throughout Europe.

Engineering support: Send the head and flow range, rated output, speed, runner drawings or a 3D scan, damage history and water-quality data, and we will reply with pricing, lead time and engineering details.

Send your unit data and runner drawings or scan for a quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Slip Ring and Exciter Brushes: Wound-Rotor Motors, Synchronous Machines and Turbogenerators

• WPS, PQR and Welder Qualification to ASME Section IX

• Weld Overlay and Clad Construction

• Material Test Certificates: EN 10204 3.1 vs 3.2

• Butterfly Valve vs Gate Valve

Frequently Asked Questions

Q: What is a Francis turbine runner?
A: A Francis runner is the rotating wheel of a Francis turbine, a mixed-flow reaction turbine used at medium heads, roughly 25 to 350 m in small-hydro guidance. Water enters radially through the wicket gates, gives up its energy across the blades and leaves axially into the draft tube. The runner consists of a crown, a band and fixed blades, with labyrinth seals to limit leakage.

Q: What material are Francis turbine runners made of?
A: Most modern Francis runners are 13Cr-4Ni soft martensitic stainless steel, designated CA6NM in ASTM A743 and A487, with 11.5 to 14% chromium, 3.5 to 4.5% nickel, 0.4 to 1.0% molybdenum and no more than 0.06% carbon. It combines cavitation and corrosion resistance with good toughness and weldability. Older runners were often carbon steel or plain 13% chromium steel, and some small runners use aluminium bronze.

Q: What causes cavitation damage on a Francis runner?
A: Cavitation occurs where local pressure drops below the vapour pressure of water, so vapour bubbles form and collapse against the metal. On Francis runners it appears at the blade inlet edges far from design head, on the blade suction side near the outlet at high load, and between the blades at deep part load. A runner set too high above tailwater, or at a high-altitude site, is more exposed.

Q: How is a cavitated Francis runner repaired?
A: Damaged metal is removed by arc-air or plasma gouging and grinding, sound metal is confirmed by penetrant testing, and the area is rebuilt with weld metal and ground back to the original profile against templates. Matching 410NiMo filler suits cracks and structural repairs but needs preheat and post-weld tempering. For in-place cavitation repairs, many plants use a 309L buffer with a cobalt-alloyed austenitic overlay.

Q: How does sediment damage a Francis turbine, and how is it reduced?
A: Hard particles such as quartz wear the surfaces with the highest water velocities: blade outlets, the band, labyrinth seals, wicket gates and facing plates. IEC 62364 gives guidance on erosion rates, hydraulic design, resistant materials, coatings and operation in sediment-laden water. Tungsten carbide–cobalt–chromium coatings applied by HVOF are the usual protection, alongside sediment management and avoiding operation when concentrations peak.

Q: When should a Francis runner be replaced rather than repaired?
A: Replacement is worth assessing when the weight of repair weld metal rises from outage to outage, when cracks keep returning at the same location, when the unit can no longer reach its rated output, or when the head or flow regime has changed since commissioning. A new hydraulic design can then recover efficiency and capacity, and add erosion-resistant profiles or coatings where the river carries heavy sediment.

Q: What information is needed to order a replacement Francis runner?
A: Send the minimum, rated and maximum net head, tailwater levels, rated flow and part-load hours, rated output, generator frequency, poles and speed, and the runner geometry from drawings or a 3D scan. Add the runner setting, site altitude and water temperature, the damage history, sediment data, interface details for the shaft coupling, guide bearing and labyrinths, and the material, inspection and testing requirements.