A stainless solution

The Tammerkoski rapids flow through the center of the venerable Finnish city of Tampere, running four hydroelectric power plants. The site is where the city was born and industrial production began over two hundred years ago. An ongoing rehabilitation project will bring the rapids’ two channels to the highest standards and worthy of the culturally and historically valuable site.

"We’re working at a heritage site, so all new structures and equipment must be of the highest quality and durable," says Project Manager Seppo Tattari of Tampereen Energiantuotanto Oy, the energy company executing the rebuild project. The design life of the rebuilt channels is 200 years.

For the past one hundred years, water flow in the channels has been controlled by structures and equipment that have reached the end of their service lives. The old floodgates are largely built from wood and would take hours to open.

"Fault scenarios are totally different today from a century ago, and today’s power plants are secured by many backups," explains engineer-in-charge Paavo Hassinen of Pontek Oy, the engineering company responsible for design and structural engineering in the rebuild project.

Maximize lifespan

Design and engineering for the project got underway in 2005. Hassinen elaborates on the new sluice and flood gates, "At first the gates were to be made from carbon steel, apart from some moving parts which would be complicated to coat with paint, so stainless steel was chosen." The stainless choice was two standard austenitic grades.

Very soon after the project launch a team from Outokumpu came calling, led by Sales Manager Seppo Koli. He recounts: "By all aspects this project presented an optimal application for our lean duplex stainless steel LDX 2101." Convincing Pontek was easy. Duplex LDX 2101 became the material of all the main structural parts of the gates, replacing carbon steel.

"We must hand it to Outokumpu how we ended up with this material," Hassinen confesses. The Tammerkoski rapids redesign and engineering is Pontek’s largest stainless steel project to date.

Benefits

The benefits of duplex LDX 2101 in the application are many. Hassinen mentions the high strength of the material (duplex stainless steel is almost twice as strong as austenitic stainless steel), which has brought them multiple benefits by enabling them to use thinner materials. “The gates are lighter than conventional ones,” he continues, “so we need less hydraulic force to operate them. The lighter weight also brings savings in production.”

Hassinen particularly acknowledges the price stability of the duplex stainless steel (which owes to low nickel content): “This is a long process, and the duplex ensures that our stainless material cost won’t run out of hand.”

Owing to its high corrosion resistance, duplex LDX 2101® necessitates no coating and so ensures maintenance-free operation. The Tammerkoski gates will, however, be painted for aesthetic reasons, to match them with the stone structures and the culturally sensitive environment.

Seppo Koli lists other benefits of duplex LDX 2101®: adequate resistance to both pitting and universal corrosion, excellent resistance to stress corrosion cracking, and good machinability and weldability. The material is widely available and there are good references from its use worldwide. In the Tammerkoski application, the high surface hardness of the material protects the gates against wear caused by ice and floating debris.

Furthermore, not only does stainless steel retain its integrity throughout service life, the material is fully recyclable once the application’s life is over. As such, stainless steel helps to build sustainable society – an important consideration for public sector players including municipal power companies. Up to 90 percent of Outokumpu’s stainless special grades are made out of recycled steel.

Exact requirements

"The very narrow tolerances of the gate components make the manufacture an exacting task," says Tero Purhonen of Naaraharju Oy, a Finnish company specializing in the assembly of heavy steel components and in charge of the fabrication of the Tammerkoski sluice and flood gates.

The manufacture is a smooth collaborative effort by Naaraharju and Outokumpu’s Plate Service Center in Jyväskylä, Finland. The service center cuts duplex plates supplied by Outokumpu’s hot rolled plate plant in Degerfors, Sweden into ready-to-assemble components, by either plasma or water-jet cutting as agreed with Naaraharju in each case to optimize assembly. The service center delivers the components to Naaraharju on just-on-time basis for welding and surface finishing.

Purhonen reports that the welding of duplex LDX 2101 does not differ from the welding of other duplex stainless steels, but pickling – surface finishing – requires special attention. "Outokumpu always gives us technical assistance when questions arise, and we do what they recommend," he says, acknowledging the close cooperation: "We’ve had good service."