Surface work

Australian researchers claim to have devised a method of restoring the surface of power station turbine blades in-situ.

Precisely-shaped, delicate, and extremely expensive, the blades operate in a hostile environment which gradually damages their surfaces — but because they are precision engineered, they are difficult to repair without removing them from the turbine.

The Australian resurfacing technique uses robotics, laser and welding technologies and is most likely to be useful for blades from low-pressure steam turbines, whose surfaces can become damaged by wet steam erosion, where the constant impact of tiny water droplets against the supersonic spinning blades wears away the surface of the metal.

A typical turbine will have 180 last-row turbine blades, each one metre long and costing several thousand pounds. Traditional repairs means the blades have to be removed from their hubs — and often taken off-site — to be repaired. and if it isn't possible to return them to their original state, they must be replaced.

The removal, transport, repair and reinstallation of the blades is a lengthy process, leading to extended downtime for the turbine, and is costly even if replacement isn't required.

The problem was tackled by teams from the Cooperative Research Centre for Welded Structures in New South Wales; the Commonwealth Scientific Research Organisation (CSIRO), and the Industrial Research Institute, Swinburne.

The teams developed a variant on an existing technique called in-situ laser surfacing, where a high-powered laser is used to vaporise a stream of metal particles. Laser surfacing was previously only possible in specialist facilities, but the teams have developed a portable version of the process which is not only usable on-site, but also requires no removal of the blades. The alloy used to resurface the blade is typically stronger than the original material, so resurfacing effectively gives the blade a second lifetime.

The major innovation of this technique is in the design of the system's nozzle. The powdered alloy is delivered coaxially to the laser, and both are focused on to a single point on the surface.

'The work can be done without the need to deblade the rotor,' said CSIRO lead researcher Nazmul Alam. 'The laser is a portable unit that is taken to the power station, and the operation is performed by a robotic arm.' He added: 'Technically, this isn't a welding process. The laser fuses the alloy to the blade's surface.'

Lasers are an ideal option for applying the necessary heat to the alloy, because they deliver precise, localised energy. In other techniques, such as tungsten inert gas (TIG) welding, the heat is difficult to regulate, which can lead to warping of the blade. The development of the focused coaxial delivery of alloy and energy allows the device to work front-on to the blade surface rather than only from above, which allows the system to be used in-situ.

A spin-out company, Hardwear, has been formed to commercialise the technology, which has been tested at a power station in Adelaide. Further trials are scheduled for later this year.