Hydropower is currently Sweden’s largest source of renewable energy while also providing around 45% of the country’s electricity generation. That means ensuring the safety and reliability of the dams that serve the generating turbines is crucial. One of the most important plants is the 156MW Långå hydroelectric station served by the Lossen dam, situated in Härjedalen in the middle-west of Sweden along the tributaries of the river Ljusnan.
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The Lossen dam was built in - and is m long with a maximum reservoir capacity of 500 million m3. A breach of this dam would have major consequences for the downstream area, since it would cause secondary dam failures on all dams down to the Baltic Sea.
To avoid potential loss of life and the impact on the Swedish economy, the Lossen dam was the subject of a major upgrading and improvement program in the previous decade. This included two hydraulically-operated emergency flood gates, installed upstream in , which is where Outokumpu’s duplex stainless steel has played an important role.
One of the major challenges facing the designers of the emergency flood gates was to reduce their weight down so that they could be operated by the hydraulic system. This is why duplex stainless steel was selected, as it has a higher strength than conventional constructional steel and no corrosion allowance is needed.
The advantage of duplex stainless steel is that it combines the best properties of both main types of stainless steel – ferritic and austenitic. The result is a range of alloys that provide corrosion resistance, cost competitiveness and high strength.
A lean duplex stainless steel features lower amounts of expensive alloying elements, such as nickel and molybdenum, providing an especially cost-effective solution. So, by specifying Outokumpu’s Forta LDX , it was possible to reduce the wall thickness of the flood gates by over 30%, with a corresponding and very significant saving in weight.
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Forta LDX was used to manufacture the water-facing side of the flood gates. Since corrosion resistance is not so important for the dry side, this was constructed from painted carbon steel. A special dissimilar welding process was used to attach the stainless plate to the carbon steel structure.
Another important reason for selecting stainless steel was that the gates are very bulky. That meant they had to be transported from the fabricator to the dam with the water side facing the trailer floor. This would have caused significant damage to any coating, requiring a further coating to be applied on site. The fabricator estimated the cost for on-site coating to be far more expensive than selecting stainless steel, in a range of €200-300k.
The advantage of stainless steel surface is that if the surface is scratched it benefits from the self-healing properties of the microscopically thin chromium oxide that forms immediately. This ensures continuous protection against corrosion, so there was no issue in handling the transportation of the gates.
The latest inspection of the Lossen dam gates was carried out in August . After over eight years in service the stainless steel was found to still be in brand new condition.
In most cases, stainless steel is overlooked for this type of application, since the corrosion issues may not be considered as severe. However, when a designer looks further to consider the whole lifetime of the dam equipment, including all aspects of operations and future maintenance costs, then stainless steel quickly becomes a competitive option.
The key benefits are:
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