Stay vanes refurbishment: factors to consider in decision making

Careful visual inspections can already provide clues, such as signs of wear or deposits on the stay vane surface. However, the first step to quantifying potential gains involves performing 3D numerical simulations.

CFD calculations, which involve simulating fluid flows, help pinpoint potential issues and gauge their extent. They also help predict expected efficiency gains achievable by modifying the geometry of the stay vanes.

The proposed modifications will affect the stay ring itself as well as the rest of the turbine because flow distribution around the runner will be altered. This will also increase draft tube efficiency. This global effect can be confirmed through model testing when specific tests are planned and included in the test program.

Since model tests are expensive, it’s essential to perform a technical and economic analysis before starting. This analysis should include costs for site surveys by an expert, CFD and FEA calculations, model size and prototype drawings, as well as model fabrication and onsite modifications. Unit outage and production losses may also be considered.

These costs will need to be balanced against the value given to the expected efficiency increase over the plant lifespan based on local electricity market prices. Provided that engineering and model test costs are mostly constant, regardless of the prototype size or power output, it's clear that high-power projects, which will yield greater monetary gains, generally benefit more from this type of modification.

From a technical standpoint, two types of modifications are typically considered: re-profiling and adding extensions. Extensions can be made of steel or composites, depending on the needs and options offered by manufacturers performing the work. Each solution has both advantages and disadvantages.

Steel extensions are heavy to handle and require designing and installing a temporary monorail. They must also be adapted to existing and often irregular leading edge geometry. This may involve 3D laser measurements and tests with wooden mock-ups. Next, the heat generated by the welding process induces stresses in the stay vane and must be managed. Welding on a cast-iron stay vane is not recommended.

Opting for a composite material extension implies a slightly more flexible and easier-to-handle hollow shell that can better adapt to uneven geometry. A composite extension poses no risk of corrosion, but it does require selecting the right materials, as some resins absorb more water than others. If the core of the extension is made of resin, the heat generated during curing must also be properly managed.

Re-profiling the stay vanes involves removing material from the leading or trailing edge, using either manual grinding or machining. Since stay vanes are typically under tension when the scroll case is watered, it’s necessary to validate that any local thinning will not compromise structural integrity. Care must also be taken to avoid exposing casting defects such as centerline shrinkage.

Health and safety risks are present in all scenarios, particularly since the work is performed in a confined space where toxic emissions from resin use, welding gases or gouging can pose significant hazards.

Modifying the stay vanes poses several technical challenges that need expert analyses. The potential hydraulic efficiency gains associated with these modifications may justify the work from a financial standpoint. However, the return on investment is normally greater for high-power units. The technical study required to make an informed decision must be thorough, leaving no stone unturned.