Old inlet runner at Cahua Power Plant (Peru) with damage due to erosion in sediment laden waters. Turbine capacity 22 MW. Photo: Ole Gunnar Dahlhaug

When the water flowing through the turbine has a lot of sediment, it will wear down the parts in the turbine and lead to leakage  and reduction in production and efficiency for the power plant. In many places in the world there are very high concentrations of sand, gravel, and debris in the water. In Nepal, for example, such wear and tear will quickly lead to downtime, replacement of parts and this leads to extensive financial costs for the power producer.

When water and sediments enter the space between these parts and expand, leakage in the turbine increases. This means that you do not get to utilize the full power from the water, and thus produce less energy.

Figure 1: Shows a section of a Francis turbine. There are several different types of vanes that guide the water into the turbine. The stay vanes (dark blue) are fixed, the guide vanes (green) can be adjusted, and the runner vanes sit in the rotating runner (red).

New knowledge about how the turbine wears

To better understand what happens in this area, Acharya has looked at the flow pattern around the space between the stationary and rotating parts of the turbine, and how this area is so quickly worn down by sediments.

Figure 2: The figure shows a figure and diagram of the focus area for the project (yellow circles). The space between the stationary part (SV&GV) and the rotating part (RV) of the turbine.

In order to look at what influenced how strongly the turbine was worn by the sediments, lab tests were carried out at Kathmandu University by using a "Rotating Disc Apparatus" (RDA), as seen in figure 3. Here, samples of aluminum were fixed to a plate. The plate was placed in a drum where water and sediments could be added, and then one could watch how the samples were worn down.   

Figure 3. Shows a 3D representation of the RDA setup, with the base plate and the test specimens fixed to it.

Increasing erosion due to height difference  

The results of the experiments showed that with increasing rotation speed, the wear on the metal increased. The position of the rotating and the stationary part in relation to each other had a lot to say. If the rotating part of the turbine was located higher than the stationary part, this would cause significantly more wear than if they were at the same height, or if the rotating part was lower than the stationary part.

Figure 4. The figure on the left shows an increasing weight loss (due to increasing erosion) with an increasing height difference between the stationary and the rotating part of the turbine. The figure on the right shows how there is a correlation between the width of the tracks and the number of rotations. Increasing the number of rotations will significantly increase erosion by increasing the track width.

Improved model

Based on the results of the experiments carried out, the general erosion model has become more specific, so that it becomes more accurate for the operation of Francisturbiner in water with sediments. The "new" model also includes the width and height differences between the rotating and the stationary part of the turbine.

It is quite obvious that the presence of sidewall gaps and flow in such regions is difficult to get around in hydraulic machineries. The design process should therefor ensure sidewall gaps flow as well for efficient machine design. Flow phenomenon associated with sidewall gaps should be well considered and analyze them during the beginning phase of machine design.