MODERN CAST IRON PIPE TECHNOLOGY IN RESPONSE TO DIFFICULT GROUND CONDITIONS

Autor: Roland Gruber , 10.10.2016

The community of Cortina d’Ampezzo in the Veneto region in northern Italy has been supplied with clean electricity since last summer thanks to a newly constructed small-scale hydropower plant.

The biggest hurdle for the project arose during the planning stage beforehand, and involved the fabrication of the penstock. This is because the only feasible route that it can run along is in the area of a shifting slope which experiences significant earth movements in the months of spring in particular. A solution for these difficult ground conditions was found by working together with the company Tiroler Rohre GmbH (TRM).

By 2014, more than a fifth of the total demand for electricity in Italy was covered by the use of hydropower. As strict environmental constraints mean that new large-scale hydropower projects only tend to be approved in exceptional cases, investors and operators are increasingly focusing on the small-scale hydropower sector. The construction of the Costeana Power Plant on the river of the same name is also thanks to this desire for environmentally friendly energy production using locally available resources. The plant is operated by „Regole d’Ampezzo” in the community of Cortina d’Ampezzo, which is famous primarily for its alpine skiing. The company has public involvement and endeavours to maintain and look after the regional landscape and natural terrain. The construction of this plant means that the operating company now obtains its own electricity from hydropower in its home region for the first time. To generate power, the plant relies on two identical Pelton turbines from the manufacturer “Maierhofer Brida” which are each designed to handle a discharge flow rate of 550 l/s. With a gross head of 103 m, the machines each achieve an output of 500 kW, which allows around 4.5 GWh of green electricity to be generated in a normal year.

NOT A STANDARD PIPELINE
„The most complicated part of the works to construct the power plant was definitely designing the penstock, which is around 1,600 m in total and has a consistent dimension of DN 900,” explains Luca Frasson, who is the TRM sales representative for Italy, and he goes on to say: „And specifically this was because the only possible route for the middle section of the pipeline runs for a length of around 400 m over a shifting slope. Geological surveys which were done in advance of the construction works had revealed that the slope moves by up to 2 cm per month when the snow starts to melt at the end of the winter.” Diverting the pipeline to a different route was not a feasible option due to the very limited amount of space available, which is why the planners had to consider a different approach.

WORKING TOGETHER TO FIND A SOLUTION
A solution to the problem was found by „Regole d’Ampezzo” project manager Roland Bernardi working together with the TRM engineer Massimiliano Fellin. Although the company, which specialises in cast iron pipes, has already demonstrated its qualities in a large number of hydropower projects right across Europe, the penstock for the Costeana Power Plant represented a new challenge even for TRM. Although restrained and ten sion-proof pipe connections very much form part of the standard range of products offered by TRM, a special solution had to be found for the hydropower project in Cortina d’Ampezzo. The decision was made to employ components which had previously only ever been used for pipe systems for artificial snowmaking facilities in skiing areas: expansion joints within a penstock laid in the ground.

EXPANSION PIECES PROVIDE THE NECESSARY COMPENSATION
In order to prevent the penstock from fracturing when the slope moves, a total of three specially made expansion pieces were fitted in the area of the shifting slope. These compensation pieces are designed to be a kind of slipon sleeve in which the pipes can move by up to 800 mm in each case in the longitudinal direction even when they are fully installed. „Although this system has already proved itself to work many times with smaller dimensions of up to DN 300, the developers from TRM had to design special components with a diameter three times the size for the Costeana plant,” explains Luca Frasson. In addition, the intention was that the pipeline should not run linearly but in a “U-shape” on the shifting slope in order to create additional relief for the penstock in the event of any geological shifts. This route of the pipeline was achieved by using four pipe bends with an angle of 22 degrees in each case. As an additional control option, large maintenance manholes and several measuring devices on which the slope movements can be monitored continuously and precisely by GPS were installed on the expansion pieces.

CAST IRON COMBINES MANY ADVANTAGES
The ductile cast iron pipes were laid during a period of hot weather in August of last year and the whole process took around two weeks. The weather god had the best of intentions back then as during the day the thermometer never fell below 30 degrees in the shade. As the penstock is made from steel above and below the shifting slope, expansion pieces had to be used in each case for around 30 m outside of the area subject to shifting. In order to take no risk whatsoever in the event of any slope movements, solid welding joints were made for the transitions to the steel material. When it came to connecting the cast iron pipes, by contrast, the BLS socket system which was developed by TRM itself was used. This is a socket joint which is restrained against longitudinal forces and, as well as being user-friendly to lay, also boasts lots of other persuasive advantages. The individual pipes can be quickly and safely plugged together to form a fully watertight system. This obviates the need for any complicated welding, testing, subsequent surface treatment or the need to leave pipe trenches open unnecessarily. This simple laying saves time and can generally be done by the relevant construction companies themselves. As tractive forces are also absorbed by the joint, there is also no need to construct concrete thrust blocks at elbows. Moreover, no separate filling material is required for embedding the cast iron pipes, as the construction trench can be filled up with the material that has been excavated from it immediately after the pipe has been laid. A laying depth of 2.5 m on average was chosen for the cast iron section of the penstock of Costeana Power Plant, with the pipes having a pressure rating of PN 32. The leak test which was successfully conducted following the completion of the construction works was performed at 1.2 times the operating pressure.

ACID TEST PASSED
The plant has now been producing clean electricity for more than nine months. Following its initial commissioning in the autumn, both the operators and the representatives from TRM keenly awaited the first real acid test for the penstock with the onset of snowmelt – which as expected the pipe system passed with flying colours. In spite of continuous movement of the slope, the power plant pipeline reliably does its work and does not display any leaks. At TRM this successful project is regarded as a positive example in two regards: First the ductile cast iron pipes have proved to be the right solution on difficult terrain, and the customer’s demands were therefore met in full. Second this technical solution provided a valuable addition to the company’s wealth of experience which is bound to prove very useful when it comes to meeting future challenges.

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Built to be used to satisfy the highest material demands. The ductile cast iron pipes from Tiroler Rohre GmbH are also able to withstand heavy landslides and settlement thanks to their restrained and tension-proof design.

photo credits: TRM

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The installation of four pipe bends provided extra relief for the penstock.

photo credits: TRM

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The use of a total of three specially made expansion pieces gave the penstock the flexibility it needs.

photo credits: TRM

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From the water catchment the feed water travels down the penstock over a distance of around 1.6 km to be transformed into hydroelectric power by two identical Pelton turbines, each with an  output of 500 kW.

photo credits: TRM

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Within the three expansion pieces, the pipes can each move by up to 800 mm in the longitudinal direction.

photo credits: TRM

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The cast iron pipe section, which is around 400 m long, was laid over a period of just two weeks last summer. The penstock passed its first acid test at the onset of the snowmelt with flying colours.

photo credits: TRM