Francis

Innovative solution with exchangeable vanes allowing for fully hard-coated runners (HVOF), even for small sizes

Introduction

The DynaVec Francis runner consists of separate crown, band and the optimal number of vanes that are bolted together instead of welded as for a traditional runner. This gives the opportunity to a fully hard coating of the wetted surfaces for even small and medium size turbines. All DynaVec runners are customized.

Bolt connections

The runner vanes are machined with flanges both towards the crown and band as illustrated below.

Both the crown and band have grooves for each of the runner vanes, which have flanges that fit into the grooves.

The runner vane flanges are placed in machined grooves in the crown and band as displayed below.

The crown/vane and vane/band are connected by a set of pre-stressed bolts of varying dimension and number based on the geometry and operating conditions. The bolt connections are dimensioned to avoid any sliding or separation of the friction surfaces between the upper flange/crown and lower flange/band. The bolt quality is adjusted to fulfill the stress analysis criteria of the runner. An assembly procedure will follow each turbine to ensure a correct pre-tension of all bolts and to ensure full proof connections.

Similar to the runner hydraulic design, the design of the flanges and distribution and sizes of the bolts are optimized and customized for each project to ensure a safe operation of the runner.

Assembly and disassembly

The bolted runner from DynaVec can easily be assembled and disassembled by standard hand tools. This can easily be performed at site if a lifting arrangement is present. An assembly procedure will follow each delivery. A partly and fully assembled DynaVec runner is shown below. The left figure shows the HPP Cahua runner in Peru partly assembled, while the right figure shows the fully assembled DynaVec runner for the Cahua Power plant

Partly assemble Bolted DynaVec runner

Fully assemble Bolted DynaVec runner

Coating

As the runner can be separated, all wetted surfaces can easily be HVOF coated. The thermal spray coating (HVOF - High Velocity OXY-Fuel) is applied with robots. Typical coating thickness will be between 0.2 – 0.3 mm.

Maintenance

There are two types of maintenance for the bolted runner. The most efficient is the preventive maintenance, where sandblasting and recoating are performed before the coating surface is broken.

The alternative is to reshape the geometry if it is damaged before coating as normally in a planned maintenance schedule. Since the bolted runner can easily be disassemble this will simplify the maintenance and repair compared to a welded runner.

Value proposition

1. Enables manufacturing of fully hard-coated (HVOF) runners

2. Exchangeable runner vanes

3. Allows for a higher sediment concentration in one monsoon period

4. Reduced sediment erosion and hence cavitation

5. Increased energy generation due to no down-time and increased TBO

6. Faster and easier disassembly and maintenance

7. Weighted efficiency is maintained throughout the lifetime of the plant

8. No welding and hence no heat treatment keeping the stresses in the material to a minimum

9. Significantly reduced risk of cracks and cavitation