DTL: In Situ Laser Surfacing
A new laser welding technique is repairing power station turbine rotor blades in minutes, rather than days, thanks to advances in the way metals fuse together. It allows flaws from wet steam erosion on the blades of low-pressure steam turbines to be repaired in place, without the need for the turbine to be dismantled and the blades removed.
The in situ laser surfacing process was developed by several Australian research institutions and power station operators. Some utilities have reported that current blade repair or replacement costs $250,000 per turbine per day in downtime, or up to $2.5 million in total per turbine.
A typical low-pressure steam turbine rotor has 180 last-row blades, each about a meter long. If they cannot be repaired to the level of their original aerodynamic precision, it costs approximately $10,000 to replace each blade.
The new joining process uses high-power laser energy to fuse a metal alloy powder to the turbine blade's surface. "The work can be done without the need to deblade the rotor," says Nazmul Alam, head of manufacturing and infrastructure technology at the Commonwealth Scientific and Industrial Research Organisation (CSIRO, Clayton, South Victoria). "It only has to be removed from its casing. The laser is a portable unit that is taken to the power station, and the operation is performed by a robotic arm."
However, Alam says the process is technically not welding. "The laser supplies a high-energy stream into which a metal alloy powder is directed," he explains. "The laser fuses the metal to the blade's surface."
According to Alam, conventional repair methods have used tungsten inert gas welding, but the heat is difficult to regulate and blades can suffer thermal damage. The new process has been applied at the Torrens Island power station, which is owned by TRUenergy (Adelaide). Ralph Villarosa, the company's asset strategy manager, says the initial field trial on six blades was successful. It recently completed a second trial on 17 blades. Villaros claims that the technique "has significant commercial application."