Ductile cast-iron pipes - backbone of modern drinking water power plants
More and more communities in the Alps want to utilise the potential energy that resides in the sources situated at high altitude by having a drinking water power plant.
The plans usually become concrete when a pipeline system which has already become outdated is due to be replaced. In the past year, two Austrian communities demonstrated the ideal way to implement such a project. The key aspect in both cases was the choice of optimum pipe material. Both of the developers opted for ductile cast-iron pipes from the traditional Tyrolean manufacturer TRM, which display their benefits in full particularly when they are used for drinking water power plants.
Even in the Alps, with their plentiful supply of water, the number of streams on which it is today possible to construct new small-scale hydroelectric power plants has become clear and easy to understand. In contrast to this, the potential for drinking water power plants is far from being exhausted. In particular those communities that are blessed with lots of water and obtain their drinking water in part from great heights find that in some cases they have the optimum conditions for this. The two communities of Stainach-Pürgg in Styria and the community of Bach in Tyrol's Lech Valley recently showed how this potential can be exploited.
Old pipeline requiring action
For Stainach-Pürgg, the original springs are extremely important. They safeguard the main thrust of the supply of drinking water to the community. The karst spring was established and the source feed line constructed back in the 1960s. At the time, an Eternit pipeline was laid underground, but in the last few years its condition clearly left something to be desired. The old pipeline with a DN125/150 diameter repeatedly sprung leaks in recent years, leading to fractures of the pipe. The worst of the damage was repaired using PE pipes. There was therefore a need to take action. Four pressure breaker shafts were installed between the highest point of the pipeline and the high-level reservoir from which the drinking water enters the distribution pipeline. One study that was prepared in advance ultimately concluded that the most economical solution was to use a section of pipeline which should enable a drop height of almost 300 m. So enough of a drop to make it possible to operate a power plant efficiently with a feed rate of up to 30 l/s. "What gave me a bit of a headache right from the outset was the question of how a high-pressure pipeline can be laid in this steep terrain. In the upper section in particular, it is so steep that when we carried out inspections we found ourselves climbing more than walking along the planned route of the pipeline. In addition, the terrain is more or less undeveloped. There was only one single possibility for accessing the marked-out route. Ultimately, three pipe depots were created from where the pipes were then transported to their respective installation site using a digger," recounts Manfred Semmler, the project manager from e2, the planning office that was commissioned to carry out the work.
Cast-iron pipe is the ideal medium
On the other hand, the question of which pipe material should be used for the 1,020 m long section of pipe was very quickly answered. Semmler says: "It was clear to us that, with the pressure conditions that exist, ductile cast iron from TRM was the only possible choice. We decided to opt for the restrained VRS®-T connections as the entire pipeline was to be run over the steep terrain with a DN200 diameter in a spiral form – and we were therefore not supposed to require any fixed points set in concrete." He also points out that, in extremely inaccessible terrain, there would scarcely have been any possible way of delivering bedding material. This was yet another reason why the people in charge made the decision to use TRM cast-iron pipes. Ultimately, bedding material is not absolutely essential when laying the cast-iron pipes. Another important quality advantage for the pipe material which is used is its pipe inside coating made from cement mortar. The primary purpose of this is to protect the pipe from the corrosive properties of the water, but it also helps to ensure the pipe has absolutely no leaks at all. Particularly with pipes that carry drinking water, it is absolutely imperative that no foreign materials whatsoever are able to get into the pipeline. In addition, the planner also cites the extreme stability and durability of the ductile cast-iron pipes from TRM. "In this project, we tried to install high-quality components wherever possible. Ultimately, this is a project which should also help to benefit the next generation," says the project manager.
Laying of pipeline under adverse conditions
Nevertheless, the omens for the work to lay the pipeline were not good. The persistent rain last summer had saturated the ground, and some hillsides were even threatening to slip down. In view of these adverse conditions and the difficult topography of the terrain, it was simply astounding that just two men from the construction company commissioned to carry out the work were able to construct the entire pipeline in just eight weeks. All of the works were carried out using just a conventional 27 tonne chain excavator. In the end, the new pipeline was laid underground in the route of the old pipeline with a coverage of 1.0 to 1.40 m above it.
Profitability over the long term
When the construction works began to draw to a close in the autumn, it was time for the electrical machinery equipment to be installed. A 1-nozzle Pelton turbine which is designed for a bottleneck capacity of 75 kW was used. All water-carrying parts are of course designed to be fully suitable for use with drinking water. The new drinking water power plant was switched on for the first time in November of last year. And this was successful; the small-scale power plant has been running continuously without any interruption since the autumn of last year. The grand opening ceremony followed at the end of November. "The continuous feed rate from the original sources does of course very much benefit the plant. This has so far never fallen below 27 l/s. This means that the operators can generally expect to achieve an annual working output of 600,000 kWh," says Semmler. For the community in the Enns Valley, the investment of around 620,000 euros is definitely not chicken feed. But with the guaranteed feed-in tariff of 10.55 cents/kWh for 13 years, the project will pay dividends over the long term. The people in Stainach-Pürgg are convinced of this. Moreover, today there is no longer any need to worry that precious drinking water will be lost on its way down the valley. In its new pipeline, it is safe and secure.
Milestone for Tyrolean community
The situation was not dissimilar for the small community of Bach in Tyrol's Lech Valley. However, initial plans for a dedicated drinking water power plant were on the table there back in the 1990s. Although at the time they were deemed to be uneconomical, judicious planning and a collective will to cooperate then helped the project to make the breakthrough. The central element of the construction project was the costly construction of a forest road which allows people to gain access to the route of the existing pipeline for the first time. The most important source of drinking water in this community, which is situated at an altitude of more than 1,000 m, is considered to be the forest spring. It is established at 1620 m above sea level, its feed rate is around 30 l/s, with the annual average hovering around 28 l/s. The community has been getting the majority of its drinking water from this source for around 70 years. An entire forest was cleared for the construction of the well chamber and pipeline in the 1940s. Whereas the natural landscape of this area has slowly recovered, the technical condition of the pipeline has steadily gone downhill. "The old pipeline was no longer state-of-the-art – and was also in need of remediation. It was made on the one hand from old steel pipes and on the other hand from asbestos pipes and in some places it had been repeatedly 'patched'," explains mayor Egon Brandhofer.
Forest road as the key to success
It was time for the old pipeline to be replaced. For the people from the community who were responsible for this, this was an appropriate opportunity to pick up an old idea again: the construction of a drinking water power plant. The more intensively planners and the local community consequently looked at the project, the more obvious it became that the whole enterprise would only be possible if sufficient access to the area of the construction site could be ensured. The basic requirement was therefore for a forest road to be constructed. The construction of the 1.5 km long forest road subsequently proved to be very demanding. Rocks, steep terrain crossings and the inaccessibility of the area presented a number of challenges for the construction company. In general, the new pipeline route did not envisage any major deviations from the existing route. But what was important here was to ensure that a sufficient gap from the old pipeline was maintained. After all, operation of this pipeline had to be maintained in the meantime. The old pipeline could only be removed once the new one was up and running. The route of the new pipeline extends over a total length of 2.2 kilometres.
Pipes guarantee operational reliability
There was no doubt among the people in charge of the project about which pipe material should now be used for the new pipeline: Only DN200 cast-iron pipes from the company TRM were considered. First of all the criterion of operational reliability was the primary concern, as the mayor emphasised, but the issue of durability also played a major role. It was a stated aim of the community in the Lech Valley to lay a pipeline which would be in service over many decades – and would not have any failures, fractures or leaks. In addition, with drinking water pipelines especially there is also always the issue of hygiene criteria. In this regard in particular, the TRM pipes made from cast iron with the cement mortar lining comprising blast furnace cement or Portland cement offer a particularly high level of safety. In addition, this inner lining guarantees a completely neutral taste - a key prerequisite for contact with drinking water.
Pipe trench is cut out
The laying of the high-pressure pipeline in difficult terrain began on 2 June last year. The most important tool for the construction team very quickly became the rock milling machine, which excavated a 40 cm wide pipe trench from the rocky ground beneath. Of the route of the pipeline over a distance of 2.2 km, around 1.2 km needed to be cut using the milling machine. In the steeper section, blasting operations were also unavoidable. The cutting operations delivered a not inconsiderable benefit in particular for the ballast bedding for the pipes: The excavated material which was prepared with a particle size of 0.22 mm was ideally suited to bedding down the pipes in a narrow pipe trench. In order to protect them from any possible mechanical defects, they were wrapped in a fleece lining.
Anchoring on the rock in the steepest section
While cast-iron pipes from the pressure class K9 with pressure resistance of PN40 were fitted in the top section, pipes from class K11 / PN63 were used in the section at the bottom, some parts of which are very steep. The very last section of the pipeline runs over an almost vertical wall of rock with a gradient of 70° to 75° - this was the most challenging part for the team laying the pipeline: In this area shortly before the machine house it was no longer possible to lay the pipeline underground. It was secured with rock anchors and clamps on the fittings facing the slope. This meant that each pipe was securely fixed and anchored to the rock. The topography of the terrain resulted in one high point for venting and one low point for draining the pipeline. The complicated pipe-laying work was completed at the end of August.
Revenues benefit community
The new power house is situated around 70 metres below the high-level reservoir. From there, the water that has passed through the turbine is pumped back up to the high-level reservoir, depending on the particular requirements. The machine house now contains a drinking water turbine that rotates with a design output of 155 kW. It ensures that an average of around 700,000 kWh of power is generated each year. "The revenue from the drinking water power plant is intended to benefit the community directly; we want to use it to fund the construction of the new community hall building," says mayor Brandhofer. Both the community of Bach and the community of Stainach-Pürgg have gone to considerable time and effort to exploit the hydro power potential of their drinking water. And they have done this with great success. In both cases, it has proved possible to deliver modern eco-friendly power plants which are integrated perfectly into the infrastructure for the relevant supply of drinking water. A primary prerequisite for the success of these projects was the construction of a high-quality and extremely durable high-pressure pipeline made from ductile cast-iron pipes from TRM. Ultimately, both projects have been constructed for the benefit of future generations.