Francis, Kaplan and Pelton Runners: Selection and Reliability

The runner converts hydraulic energy into torque at the shaft. Its geometry cannot be selected from the turbine type alone: net head, flow rate, speed, tailwater level, sediment load and operating regime define the appropriate hydraulic profile. A sound decision starts with the unit’s actual condition, including transients, off-design operation and inspection history.

Francis, Kaplan and Pelton runners serve different hydraulic ranges, but all require the same discipline: a clear specification, controlled manufacturing, non-destructive testing, geometric verification and balancing. On older units, restoring the existing runner may be the most economical option; in other cases, a replacement with an updated profile can reduce losses, vibration and unplanned outage risk.

Head and flow define the runner type

Francis turbines are generally applied to medium heads, with radial inlet flow and predominantly axial discharge. Blade count, inlet and outlet angles, blade curvature and passage width depend on specific flow and speed. A Francis runner for higher head typically prioritizes acceleration control, structural strength and cavitation margin; at lower head, flow passage and capacity become more significant.

Kaplan turbines use axial flow and adjustable blades, making them suitable for low heads and high flow rates. Coordination between guide-vane opening and blade pitch is essential to maintain efficiency across a broad load range. Pelton turbines are impulse machines for high heads and comparatively lower flows: free jets strike double buckets, and bucket shape, splitter geometry and surface finish determine the quality of energy transfer.

• Francis: assess net head, flow per unit, speed, submergence and draft-tube pressure.
• Kaplan: verify flow range, blade-pitch mechanism travel, blade sealing and guide-vane/blade coordination.
• Pelton: size jet diameter, number of jets, peripheral speed and flow distribution among injectors.
• For every type: consider sediment, starts and stops, partial-load operation and vibration limits.

Hydraulic profile, blades and damage mechanisms

The hydraulic profile must guide flow with low losses while avoiding separation, recirculation and pressure concentration. In a Francis runner, small changes at the leading edge or in blade thickness can change incidence angle and increase pressure pulsations. In a Kaplan runner, twist and pitch along the blade must match the velocity distribution; excessive tip clearance increases leakage and reduces efficiency.

In a Pelton runner, splitter and bucket geometry must divide the jet without excessive impact while keeping the flow attached until discharge. Wear at the splitter, edges or impact surfaces changes the effective section and can create imbalance between buckets. For every runner type, the mechanical design must also check centrifugal stress, fatigue, stress concentration at transitions and compatibility with the shaft and hub.

Cavitation occurs when local pressure falls below vapor pressure, forming and collapsing bubbles near metallic surfaces. Damage appears as pitting, material loss and a rough finish, often at outlet regions, suction sides, blade tips or edges exposed to pulsations. Sediment, particularly sand, causes abrasive erosion: particles remove material by impact and sliding, accelerating profile loss.

“The original geometry is only the starting point; the unit’s actual hydraulic condition must guide the engineering decision.”

— Braz Hydro Engineering

Repair or replacement: a risk-based decision

Repair is appropriate when damage is localized, the base metal retains acceptable thickness and properties, and the original geometry can be safely restored. Assessment should combine visual inspection, thickness measurement, hardness testing where applicable, crack evaluation and operating-history review. Welding over a large area without understanding residual stress, composition and metallurgical condition may only conceal the problem.

Replacement is usually more appropriate when there are recurring cracks, deformation, widespread profile loss, deep corrosion, accumulated fatigue or incompatibility between the runner and current plant conditions. A new runner may also be specified for uprating, provided that the shaft, bearing, generator, draft tube, guide apparatus and cavitation limits are checked together. Expected gains must be weighed against manufacturing lead time, scheduled outage duration and adaptation risk.

• Map damage by coordinates, depth, extent and proximity to welds or discontinuities.
• Compare measured geometry with the approved drawing or a reconstructed three-dimensional model.
• Define acceptance criteria, welding procedure, heat treatment and machining sequence.
• Confirm that repair will not change mass, center of gravity, clearances or assembly natural frequency.
• Record before-and-after condition with photographs, thickness maps and test reports.

Stainless steels, overlays and manufacturing

Hydraulic runners commonly use martensitic stainless steels or other alloys selected to combine mechanical strength, weldability and erosion resistance. Selection depends on the application, base metal, water quality and manufacturing process. Stainless steel overlays can restore thickness and improve cavitation resistance, but they do not replace profile control or correct an unsuitable hydraulic cause.

Hardfacing can be applied to areas exposed to severe abrasion, particularly when sediment loading justifies the solution. Material, hardness, dilution and thickness must be compatible with the substrate and final finishing process. Excessive or uneven deposition can create stress, cracking, mass imbalance and flow disturbance; every layer therefore requires machining and verification against defined tolerances.

Manufacturing may follow customer drawings, technical specifications, three-dimensional models and existing interface standards. When documentation is incomplete, reverse engineering begins with dimensional surveying, scanning or coordinate measurement, material identification and reconstruction of hydraulic surfaces. The objective is not to copy wear blindly: it is to distinguish nominal geometry from accumulated deformation and validate interfaces with the shaft, hub and stationary components.

Quality control before delivery

Non-destructive testing should be defined according to material and manufacturing stage. Visual and dimensional inspection may be complemented, where appropriate, by liquid penetrant, magnetic-particle, ultrasonic and radiographic testing in critical areas. After welding, control should cover consumable identification, preheat, interpass temperature, deposition sequence and inspection of repaired zones.

Geometric control compares point clouds or coordinate measurements with the approved model. Diameter, height, thickness, angles, pitch, blade position, concentricity, runout and functional clearances should be checked. The report should state datums, instruments, resolution or uncertainty, deviations and final disposition, providing traceability for assembly and future inspections.

Balancing, assembly and operating decisions

Even a hydraulically efficient runner can generate vibration if mass distribution is uneven or if eccentricity and distortion are present. Balancing must consider the actual assembly configuration, correction planes and operating speed. Material removal or addition must be documented without compromising minimum thickness, stressed regions or surface protection.

During assembly, interface control is as important as blade finish. Shafts, keys, flanges, hubs, rings, pitch mechanisms and seals should be checked before closure. After start-up, vibration, draft-tube pressure, power, flow, noise and behavior at different loads should be compared with the baseline; any deviation should inform the maintenance plan, not merely the commissioning report.

For plant owners, the specification should include approved drawings, materials, welding procedures, NDT criteria, tolerances, balancing, corrosion protection, packing and final documentation. Local manufacturing can reduce logistics interfaces and simplify field support, while also meeting Brazilian financing and regional-origin requirements where applicable. The real value is a traceable, repairable component suited to the unit’s life cycle.

Practical takeaways for plant owners

Choosing between Francis, Kaplan and Pelton begins with the combination of head and flow, but it ends only when hydraulics, structure, materials, manufacturing and operation are treated as one system. Repair, overlay or replacement should be evidence-based decisions that consider risk and outage cost, not merely the visible appearance of damage.

• Measure the runner’s actual condition and document damage before defining the solution.
• Validate hydraulic profile, cavitation, sediment and interfaces before approving a new design.
• Require material, welding, NDT and dimensional-control traceability.
• Include balancing and field verification in the scope, not as later activities.
• Use operating history to adjust inspections, overlays and future uprating work.