Hydropower plant Lago di Tomé – high pressure pipeline DN 400 for environmentally friendly electricity production

1 Introduction

The Alpine lake known as the Lago di Tomé lies in the municipality of Lavizzara in the Vallemaggia district of the Swiss Canton of Ticino (Figure 1). The Tomé mountain stream flows from Lake Tomé into the valley of the same name and from there into the River Maggia, on the left bank just before the village of Broglio. The section between the Alpine lake, at the upper end of the valley at a height of 1,692 m above sea level, and the point where it joins the Maggia close to the village of Broglio, approximately 680 m above sea level, and hence with a geodetic head of more than 1,000 m, is of extreme interest in terms of using the water power for the production of electrical power.

The client for the construction and operation of the hydropower plant at Broglio is the newly founded joint stock company CEL Lavizzara SA.

2 The project for a small hydropower plant

2.1 General design

The new Lago di Tomé hydropower plant consists of the following components:

• Water catchment and de-sanding basin,
• High-pressure underground pipeline,
• Power plant building,
• Tail race channel for returning the water.

2.2 Water catchment

The inlet structure has been built about 120 m beneath the outflow of the Lago di Tomé lake into the mountain stream and embedded in the stony topography of the river course. Clad in natural Ticino stone slabs, the inlet building blends into the environment. The water flows through a rectangular opening beneath the level of the water into the inlet structure, meaning that no flotsam can get into the de-sanding basin (Figures 2 and 3).

2.3 New DN 400 high pressure pipeline

When selecting the appropriate material for the pressure pipeline, the clinching factors were on the one hand the tough installation conditions in the mountains and on the other hand the very high pressures of up to 100 bars with the almost exactly 1,000 m height difference. What was needed was a flexible piping system which nevertheless had to offer outstanding mechanical and static strength characteristics.

Measured against these requirements, the triedand- tested vonRoll ECOPUR ductile iron pipes with reinforced coating to EN 545 [1] are the optimum solution for the pressure pipeline because of their technical performance. With the innovative vonRoll PUR coating technology, ECOPUR pipes ensure a very high hydraulic effectiveness (PUR roughness: k ≤ 0.01 mm) (Figure 4) and, with the pore-free PUR coating, they provide perfect external protection in all soils. Because of the consistently rocky terrain, the robust ROCK protective cover was also used, which had already been applied to the cast iron pipes in the manufacturer’s works.

The deflectible HYDROTIGHT push-in joint makes assembly in steep terrain rapid and safe (Figure 5).

The vonRoll ECOFIT type fittings required have an epoxy coating to EN 14901 [2] with the enhanced requirements in accordance with RAL – GZ 662 [3]. The relatively low weight of vonRoll ECOPUR full-protection pipes was very helpful for the company constructing the pipeline in a steep and inaccessible environment because the pipes had to be transported by helicopter in order to be installed in the place of use (Figure 6).

Over the entire 3,200 m length of the pipeline route, with a height difference of just about 1,000 m, there were various and challenging installation situations to be mastered in mountainous terrain with stony, rocky ground. In order to preserve the natural appearance of the environment as far as possible, the pressure pipeline was installed underground along almost its entire length, with one exception: the pipeline crosses a stream aboveground via a pipe bridge. The almost natural reconstruction of the route was done with excavation material and natural rocks and boulders (Figures 7 and 8).

Depending on the elevation, the new pressure pipeline was designed for pressure ratings from PFA 10 bars to PFA 80 bars with ECOPUR pipes in various wall thickness classes from K 7 to K 15 with restrained push-in joints of the HYDROTIGHT type. In the upper area to PFA 16 bars, friction locking push-in joints push-in joints were sufficient to absorb the forces while also offering a large degree of flexibility with shortened pipes in the trench.

In the areas with higher pressures – up to PFA 80 bars in the lowest part of the pipeline – ECOPUR full-protection pipes were provided with welding beads in the factory and restrained with the HYDROTIGHT positive locking thrust protection system Fig. 2805 (Figure 9).

With the steepness of the terrain and because of the high operating pressures, reinforced concrete thrust blocks were positioned at points of directional change to take up the forces occurring.

Instead of using shortened pipes processed on site, the manufacturer – vonRoll hydro (suisse) ag – supplied short pipes in precisely defined lengths with welding beads applied in the factory, which are fully protected with polyurethane. This made the expensive application of welding beads and subsequent repair of the external protection at the installation site unnecessary.

In the high pressure area, as from PFA 63 bars, standard push-in fittings in wall thickness class K 12 were no longer sufficient for absorbing the forces occurring. For this application, vonRoll hydro (suisse) ag produced special fittings in wall thickness classes K 14 and K 15.

At regular intervals, inspection openings were also arranged along the pressure pipeline. These are designed as DN 400/400 double socket tees with flanged branches, also in a special design up to K 15 and with flanges for the pressure ratings necessary in each case up to PN 63 (Figure 10).

The lowest and steepest area of the pressure pipeline with the highest pressures – from 80 bars as far as the turbine house where it is almost 100 bars – was laid with welded steel pipes. For the transition area from ductile cast iron to steel, with PFA = 80 bars, once again a special fitting – flanged socket (E), with PN 100 bar flanges – was used (Figure 11).

Parallel to the pressure pipeline, and in the same trench, empty pipes were also laid for the supply cable to the equipment at the water catchment area and also for the control cable for communication between inlet and power plant (Figure 12).

2.4 Power plant building and return channel

The location of the power plant building just before the point where the Tomé mountain stream flows into the Maggia is an optimum compromise between various influencing parameters. Among other things these include the geodetic height of fall and optimum access to the equipment during the construction work and for subsequent operation and maintenance with the least possible impact on the environment.

The power produced by the twin-jet Pelton turbine is fed into the 16 kV network of the Società Elettrica Sopracenerina SES. After going through the turbine, the water is returned to the Tomé mountain stream via a tailrace channel with a cross-sectional area of 1.5 m x 1.5 m and a length of 13 m and this stream later flows into the River Maggia (Figure 13).

3 Some facts about the Lago di Tomé hydropower plant

• Water catchment area 3 km²
• Height of the Lago di Tomé Alpine lake 1,692 m above sea level,
• Water catchment capacity level 1,686 m above sea level,
• Power plant building, turbine shaft 704.55 m above sea level,
• Net head height 945 m,
• Average water volume supplied 3.10 million m³/year,
• Nominal size of pressure pipeline DN 400, effective length 3,110 m
• System output of power plant 2.05 MW,

• Net energy production 6.5 million kWh/year.

Bibliography

 

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