By: Turkka Kulmala

Challenge: New turbine technology for thermal power plants poses extreme manufacturing challenges.

Solution: Advanced application knowledge and tooling will boost productivity without sacrificing quality and security.

Society is craving greener energy, and suppliers are drawing on new technology to meet the demand.

The patented dampening device eliminates vibrations when performing the critical finishing operation.

The European Union’s COMTES 700 project aims at turbine technology utilizing technology beyond today’s ultra super critical (USC) steam generation — pressures exceeding 300 bar and temperatures exceeding 700°C — to achieve a net efficiency of 50 to 52 percent (lower heat value, LHV). The efficiency of many of the outdated thermal plants still in use today may be as a low as 25 percent. E.ON, a power supplier participant in the project, is planning to have its first USC plant up and running in 2015.

Integrated Gasification Combined Cycle Technology

Another approach to improve the sustainability balance of coal-based energy production is integrated gasification combined cycle technology (IGCC), where coal is combusted first after conversion to synthesis gas. The benefits include both improved efficiency and decreased emissions.

For equipment suppliers, both of these new technologies represent a major manufacturing challenge as well as a great opportunity to be a part of tomorrow’s energy technology.

Turbine components — shafts, discs, blades — call for close dimensional tolerances and excellent surface finish. Complex designs require good accessibility and often also long tool overhangs. Shafts and discs are mainly machined by turning, while casings and blades primarily require milling and boring operations.

It is essentially in the high-pressure turbine where advanced heat-resistant alloys such as alloy 625 are needed. These materials may reduce the tool life as much as eight times in relation to traditional metal-cutting applications. General recommendations for them include low cutting speeds, good stability and generous coolant supply. Efficient tooling is obviously required to improve tool life and thus productivity.

Both steam and gas turbines include very unique components on the one hand and standard serial-production components on the other. Shafts and casings, for example, may require months of demanding machining operations. Ruining such a valuable workpiece is simply not an option, and extremely rigid process reliability is therefore required.

Turbine Blades & Shafts

A steam turbine shaft can be more than 10 metres long and require many machining operations with long tool overhangs.

Turbine blades make a typical example of the latter category: smaller workpieces machined in larger numbers — obviously to stringent requirements as well, but much less is at stake with a single component.

Turbine shafts involve a whole range of operations from heavy turning to finishing with very high surface and tolerances. For turbine casings, milling the casings halves in a gantry-type machine is the first machining stage. With both halves combined, the complete casing assembly will be worked in a vertical lathe.

Grooving is a typical operation for both steam and gas turbine components, particularly shafts. Productive grooving requires efficient chip control to avoid jamming and the resulting production stops and poor surface finish. Efficient chip control solutions include stepped geometry with a chip divider, using a tool installed upside down and also high-precision coolant supply.

Damped parting blades offer a remedy to severe vibration problems, particularly at long overhangs. “Hockey-stick-type” 90 degree internal grooving inserts provide accessibility, especially for facing and back-facing operations.

Turbine blades are manufactured nearly as mass production, utilizing advanced NC programming in 4- to 5-axis machines. Maximal productivity without sacrificing quality is aimed at by dedicated tool grades, geometries and methods.

Dual Competence

Sandvik Coromant works in close cooperation with turbine component manufacturers to optimize the tooling solutions for these demanding machining operations.

Sandvik Materials Technology supplies turbine components with near net shape powder metallurgy components utilizing hot isostatic pressing. This technology offers major advantages in various turbine applications, including shorter delivery times and reliable material properties.

Summary

Modern steam and gas turbine components from high-performance alloys require dedicated tooling solutions and methods. Damped tools, dedicated geometries and grades as well as a high-precision coolant supply offer productive solutions.

Originally published in Metalworking World 1.2011, a business magazine published by Sandvik Coromant.