Laser fast turbine repairs
"Laser cladding" revolutionizes maintenance time for turbines.
With laser cladding technology, turbine components can be maintained and repaired in days rather than weeks. For operators, this means less downtime - and less loss of production. We have visit one of the world's most modern turbine service plants.
by Hubertus Breuer
The steam turbine cannot stay put for long. After it has found its long way from a methanol plant in Saudi Arabia to Nuremberg with the help of low-loading trucks and a freight plane, its rotor is propped up at the turbine factory of Siemens AG in Nuremberg, Germany. Or, to be more precise, it is located in a factory hall in the enclosed 'Laser Cladding Center' (LCC), which opened in 2016. Its specialty: Laser cladding for the maintenance and repair of turbines. Using traditional welding and coating methods, this used to take several weeks. Now it's just days - and the result is better than ever.
An orange-colored robot arm rises above the eight-meter long steel rod weighing over 20 tons, fitted with control wheels, blades, bearing and sealing areas - also known as a 'rotor'. At its end a nozzle is directed at the turbine. "The new shortened maintenance time for turbines are revolutionary," says Thorsten Scheller, Director of Product Management at Siemens Power Generation Services, standing at the open gate of the LCC. "Here, we can save our customers a considerable amount of money, as turbine downtime is expensive. In a refinery, e.g., one million euros can be easily lost per day."
Cooperation with Fraunhofer-IWS
Steam and gas turbines as well as compressors – in power plants, the chemical industry, in mines, refineries, district heating or waste incineration plants –, have to be overhauled regularly. In addition to quality and reliability, the decisive factor for this service today is speed. This is why operators try to maintain their turbines only when necessary. These days, remote monitoring systems can help to provide accurate information about their condition. Once maintenance cannot be postponed any longer, every day counts. To speed things up, cool air can be blown into the turbine, which shortens the cooling time by up to 50%. But once it arrives for maintenance at a repair workshop, the process could hardly be accelerated until recently.
The new method of laser deposition welding, also known by its acronym LMD for 'Laser Metal Deposition', is one of numerous innovative technologies that are grouped under the umbrella of Additive Manufacturing. It also includes the well-known ‘Selective Laser Melting’ (SLM), in which a laser welds a component layer by layer from a power bed in a small printing chamber. Laser Metal Deposition, on the other hand, currently is mainly used for surface welding. At the LCC, for example, to repair damage to a turbine component’s surface. Nozzles integrated into the manufacturing head blow metal powder into the laser beam. A melt pool forms on the surface and creates a metal layer. "Around 2014 it became clear to us that we should use LMD for turbine service," says Scheller. "This is why, in 2015, we teamed up with the Fraunhofer Institute for Material and Beam Technology IWS in Dresden to develop the LCC.”
Fraunhofer-IWS is an expert for laser deposition welding, especially for the innovative design of the laser heads being used (see interview). Siemens, on the other hand, like no other company knows the requirements that a modern turbine maintenance system must meet. This enabled Fraunhofer-IWS to contribute to a feasibility study, which then led to the German equipment manufacturer GTV planning and building the LCC according to Siemens’ and Fraunhofer IWS’ specifications.
No two turbines are alike
The LCC consists of two separate working areas - a large one, in which the turbine rotor from Saudi Arabia is waits for maintenance, and a smaller one with a tilting table, in which smaller components such as valve spindles, which control the steam pressure in turbines, or stator parts, are repaired. Welding of filigree turbine parts did not really exist before, since such parts cannot really take manual welding. "We recently repaired a valve spindle for a German waste incineration customer using LMD in just seven days," says Roland Wexler, a former welder who now supervises the LCC. "A new valve spindle would have meant a delivery time of several weeks, - a no-go if detected only during the ongoing overhaul of a turbine."
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Paper on additive manufacturing
During the last years, a new revolutionary way of manufacturing – Additive Manufacturing (AM) has emerged in the industry and is considered as a game-changer. This technology enables OEMs to manufacture and repair gas turbine components faster and at the same time with enhanced functionality and performance. Currently Siemens Power Generation is using this technology for prototyping, manufacturing, repair of gas turbine components, and spare part manufacturing. Industrialization and current field experience of additively manufactured gas turbine components will be discussed is this paper.
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Wexler is not bothered by the fact that the new system practically has replaced the job he once trained for. "Without the LCC we would have much fewer orders - with it our order books are full. It wouldn't make any sense to protest against that," he says over the noise of a ventilation fan. "Even more, because I know what precise welding requires, the possibilities offered by the LCC are particularly convincing to me," he says.
This is also made possible by the end-to-end digitalization and thus automation of the welding process. The melt pool is constantly monitored, and the laser power adjusted as required – it’s a so-called 'closed loop' process. This makes coatings possible close to the desired result, which in turn reduces post-processing. Similarly, many of the more complex parts, such as turbine casings or stator parts, are first 3D-scanned to create a digital twin. The virtual twin in turn allows complex welding processes to be simulated using Siemens NX- software. This in turn accelerates actual repair time - "with results exactly matching our simulations," Scheller emphasizes.
However, the LCC is no boring assembly line. No two turbines are alike. The geometry is always different, depending on the number of stages, extractions and the type of exhaust system. In addition, some turbines are up to 40 to 50 years old. The oldest machines still in operation date to the 1920s; and there are more than 50,000 active Siemens steam turbines worldwide. Because of this large global market, the company is also cooperating with LCC sites in India, Brazil and in the future in the USA. Siemens is also developing mobile LCC units - "this also helps to save time and money," says Scheller. In addition, demand continues to grow: The LCC is increasingly servicing not only steam, but also compressors and gas turbines.
Enough talk. From his control desk, Wexler presses a button that allows the door to the large working area of the LCC to squeakily close. He checks the settings for the Saudi Arabian rotor, then starts the welding process, which starts at its bearings. Cameras film the process and stream the images to a computer screen. Just a day later, Wexler checks the results with a microscope. Some of the coatings are so thin and fine-grained that they cannot be seen with the naked eye. He is satisfied. But not surprised.
World Record Laser
The Fraunhofer Institute for Material and Beam Technology IWS in Dresden, Germany, developed the ‘Laser Cladding Center’ (LCC) together with Siemens. Holger Hillig, a Fraunhofer expert for laser cladding and system technology, was part of the project. A short conversation about the perspective of laser deposition welding.
Most people with an interest in technology, when they hear of 3D-metal-printing, think of ‘Selective Laser Melting’, a technique in which a laser forms a component out of a powder bed. Laser deposition welding, on the other hand, is much less well known.
Holger Hillig: In laser deposition welding, layers of metallic powder or wire can be applied to any component. Compared to other welding processes, the quality and precision of the layers is particularly high. Even materials that usually cannot be welded can also be processed. This technology is being used e.g. for corrosion and wear protection or for the repair of components. In comparison, generative welding, which creates new components by building up material layer-by-layer, is still in development. At the LCC in Nuremberg, the main focus is currently on machining surfaces and repairing damaged parts of components.
How important are welding heads for the development?
Holger Hillig: Very important. Depending on whether you want to apply very fine structures or work over large areas, you need a wide variety of laser heads. For example, we developed a welding head with a rectangular laser, which currently holds a world record with a 45mm weld bead width – this is relevant when it comes to coating large surfaces, for example for the corrosion protection coating of large hydraulic cylinders for offshore drilling rigs. It is also important to control the laser heads flexibly and precisely. The system configuration, movement of the robot arm, welding parameters, laser power and material feed must be precisely coordinated - for example, to master the challenge of pulling welding beads with a width of only one tenth of a millimeter and for the repair of turbine blades.
What about the new materials?
Holger Hillig: So far, we've generally used materials that weren't developed specifically for the new manufacturing processes. But that is beginning to change. We are working on this in the Center for Additive Manufacturing Dresden, which was set up together with Dresden Technical University. This is very exciting, because new materials can be used, for example, under harsher conditions or higher temperatures. This can enormously increase the life and efficiency of machines and systems.
How do you see the future of laser deposition welding?
Holger Hillig: Laser deposition welding has a great future, especially for the maintenance and repair of large and complex turbine components such as those serviced at the LCC. And one day it may be may lead to building or ‘printing’ large parts completely generatively.
Hubertus Breuer is a German science & technology journalist.
Combined picture and video credits: Hubertus Breuer and Siemens AG
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