On-Site Machining for Safe, Predictable Hydropower Outages

During a planned outage, every hour of unavailability should be tied to a defined activity, an acceptance criterion and a technical owner. When the work involves a shaft, cover, ring, seat or other large component, removing the part from the powerhouse can extend the schedule, logistics and equipment exposure. On-site machining makes it possible to restore critical surfaces in place, provided that measurement, sequencing and quality control are planned with the discipline of a manufacturing operation.

Braz Hydro combines field engineering, component manufacturing, turbine repair and commissioning for Francis, Kaplan and Pelton units. The objective is not simply to complete a mechanical task, but to return the assembly to its functional geometry, clearances and operating condition. That requires close coordination among operations, safety, maintenance, engineering and the field-service team.

Outage planning starts before the shutdown

The first step is to turn symptoms and maintenance history into a verifiable intervention plan. Vibration records, leakage, ring wear, cavitation erosion, bearing temperatures and contact events provide useful hypotheses, but they do not replace dimensional inspection. Approved drawings, tolerances, materials, welding procedures and previous intervention records should be assembled before mobilization.

The outage window should be divided into milestones: isolation and lockout, draining, disassembly, inspection, repair decision, execution, final measurement, reassembly, static tests and start-up. Each milestone needs defined dependencies and alternatives. For example, a wear measurement may determine whether the solution is machining only, welding followed by machining, or component replacement.

• Confirm access, lifting capacity, lighting, ventilation and a protected work area for portable equipment.
• Issue the lockout and tagout plan, risk assessment, work permits and emergency response plan.
• Pre-fabricate supports, tooling, reference fixtures and consumables compatible with the existing material.
• Define inspection points, instruments, calibration standards and acceptance criteria before work starts.
• Review physical progress, dimensional deviations, operating constraints and the critical path every day.

For large components, logistics preparation is part of the engineering. Portable boring machines, lathes, milling systems, welding equipment and optical instruments must arrive with suitable interfaces, power, fixturing and protection. A readiness meeting with the plant before the outage reduces improvised decisions once the equipment is open.

In-situ machining restores surfaces without excessive dismantling

On-site machining is appropriate when removing the component is unnecessary, risky or incompatible with the available outage window. Shafts can receive diameter and seat restoration; covers and rings can have faces, bores or sealing surfaces corrected; and bearing seats and coupling areas can be aligned to a common reference. The method depends on access, material, assembly stiffness and the required accuracy.

Before cutting, the team establishes datums and measures runout, concentricity, roundness, flatness and axial position. On a shaft, reaching the nominal diameter is not enough: the relationship between the seat, adjacent journals, coupling and hydraulic centreline must also be verified. The portable machine must be installed with adequate stiffness, and its own geometry should be checked through independent measurements.

The cutting strategy considers minimum material removal, a symmetrical sequence where applicable, temperature control and surface finish. Intermediate measurements prevent the recovery limit from being exceeded and allow the operation to be adjusted when deformation or unexpected deviation appears after opening. The final report should record before-and-after readings, instruments used, surface condition and any engineering-approved change.

“Repair accuracy starts with the correct reference; removing more material does not correct a poorly defined geometry.”

— Braz Hydro Engineering

Welding, hardfacing and cavitation repair need a controlled method

Cracks, material loss and cavitation damage should not be treated by simply depositing metal. The area must be cleaned, mapped and examined to confirm the extent of the damage. Engineering then defines the consumable, joint preparation, preheat, interpass temperature, deposition sequence and any heat treatment, considering the base metal and residual stresses in the component.

On runner blades, covers, rings and hydraulic passage surfaces, the repair must restore strength, profile and finish at the same time. Excess weld metal is removed by machining or controlled finishing while respecting minimum thickness and the approved profile. Visual examination, liquid penetrant, magnetic-particle or ultrasonic testing is selected according to the material, geometry and expected failure mode.

Hardfacing can protect areas exposed to erosion, abrasion or localized wear, but the deposit must be compatible with the surface function. An unsuitable layer may introduce cracking, change clearances or make future machining difficult. For cavitation, the team should also investigate hydraulic conditions, operation away from the best-efficiency region, air admission, vibration and coating condition; repairing the cavity without addressing recurrence reduces the life of the intervention.

• Map and photograph each damaged area before removing material.
• Confirm the material and qualified welding procedure for the application.
• Control heat input, pass sequence and deformation during deposition.
• Machine or grind to the specified profile without creating notches or abrupt transitions.
• Perform non-destructive testing and release the area only after reviewing the results.

Alignment, reassembly and commissioning complete the work

Geometric restoration must continue through reassembly. Optical systems, precision levels, dial indicators and three-dimensional measurement tools can verify centreline, elevations, radial displacement, parallelism and concentricity. The technique is selected for the unit configuration and must account for the possibility that casing references have moved during the operating life.

After the shaft, runner, cover, rings, bearings and seals are installed, the team checks static clearances, freedom of rotation, interference and fastener torque. Kaplan units require attention to blade-pitch mechanisms and synchronization; Francis units require checks of the relationship between runner, guide apparatus and draft tube; in Pelton units, the relative position of jet, nozzle and bucket is critical. All readings should be compared with approved drawings and the agreed baseline.

Commissioning begins with static tests and proceeds to manual or low-speed rotation where applicable, no-load tests and a progressive increase in output. Vibration, temperatures, oil pressure, leakage, guide-vane opening, blade position and governor response are monitored against defined limits. Operational release should follow only after data review, closure of outstanding items and updating of maintenance documentation.

Practical results for the next outage

Well-executed field service reduces transport and dismantling, but its main value is technical control of the repair decision. An OEM that manufactures components and also performs repairs can relate observed wear to recovery capability, replacement alternatives and life-cycle impact. For the operator, this turns a planned or unplanned outage into a documented, repeatable intervention.

• Define scope and acceptance criteria from drawings, measurements and unit history.
• Mobilize qualified personnel, tooling, consumables and calibrated instruments before shutdown.
• Use in-situ machining when it preserves references, reduces risk and meets the required tolerances.
• Address the likely cause of cavitation and wear, not only the visible material loss.
• Deliver dimensional records, test results, alignment data, commissioning records and O&M recommendations.