Coatings increase turbine efficiency

Thermal barrier coatings can significantly increase the turbine efficiency of turbomachinery by allowing higher firing temperatures while reducing component thermal fatigue, warpage, oxidation and cracking, says thermal spray coatings supplier Thermaspray.

Thermaspray MD Dr Jan Lourens explains that wear control coatings can prolong the life of critical turbomachinery parts tenfold and corrosion prevention coatings can dramatically reduce corrosion damage, while providing a smooth aerodynamic surface on compressor blades and stator assemblies.

Thermally sprayed coating solutions also ensure protection from wear, erosion and corrosion for a range of components found in the turbomachinery and rotating equipment industries. The application of a thermally-sprayed coating leads to extended service life, reduced maintenance, increased uptime and production, and ultimately lower operational and ownership costs.

He says thermal spraying comprises different procedures, such as high velocity oxy-fuel and flame- or wire-arc and plasma. In these processes, a fine powder – usually metallic, or non-metallic powders such as ceramics – is fed through a chamber by a gaseous carrier, which is then ignited. The powder is melted and is then deposited onto the surface of the component being coated.

Thermal spray coatings for the turbomachinery industry include thermal barrier coatings, wear control coatings, corrosion prevention coatings, high temperature environment coatings, oxidation resistant coatings and solid-particle erosion resistant coatings. Lourens adds that control technologies are essential to modern high-performance, high-quality industrial turbines.

“The ability to ‘tailor design’ thermal spray powders and spray them to a designated hardness range using carefully monitored microstructural control has had a revolutionary impact on the service life of turbomachinery,” he says.

Abradable thermal spray coatings, he explains, which are also known as clearance control or seal coatings, are successfully used in steam turbines and various other types of turbomachinery applications to reduce leakage gaps between stationary and rotating parts.

“The abradable thermal spray coatings readily and sacrificially wear away when in contact with a rotating part; the resulting debris created by the abraded coating is soft, relative to the rotating surface, and fine enough to exit the system without causing erosion on other components of the engine. Abradable coatings can be applied by the flame (combustion) spray process or the plasma spray process,” Lourens explains.

He points out that abradable thermal spray coatings are highly effective in reducing emissions and fuel consumption in turbomachinery. The coatings comply to two conflicting requirements; they must be abradable, but equally mechanically stable in the harsh operating conditions of a gas turbine.

“Therefore, the ideal solution for gas turbines, and more recently steam turbines, is abradable coatings, which allow rotating compressor- or turbine blades to cut their own gas seal inside their casings, minimising losses and improving fuel efficiency,” Lourens points out.

Turbine components exposed to corrosion at high temperatures, such as 1 000 ºF or higher, degrade faster than at lower temperatures and are also subjected to cracking due to thermal fatigue and cycling.

He says high temperature-resistant coatings diffuse into substrate, creating a nearly impenetrable surface which can reduce scaling and cracks due to thermal cycling. High temperature oxidation, a typical condition found in gas turbines, is mostly responsible for premature failure of ‘hot section’ components.

“Oxidation resistant coatings impede oxygen penetration of the component surface while providing a sacrificial layer capable of protecting the part between overhauls. Solid particle erosion is most responsible for premature turbine failure and solid particle erosion coatings are specifically designed and tested for this environment and have proven effective in extending the life of critical steam turbine parts,” he points out.

“Thermaspray has enjoyed significant growth in the turbomachinery and rotating equipment market over the past six years and today these sectors make up almost a quarter of our business,” he affirms.

Lourens explains that the full range of coatings available from Thermaspray can be found at its facilities in Olifantsfontein, in Johannesburg, and is also available from its joint venture company, thermal spray coatings supplier Surcotec, in Cape Town. Thermaspray’s large top-loading spray booth has the capacity to handle shafts of up to 5 500 mm in length and up to 6 000 kg in weight.

He explains that Thermaspray, in its joint venture with Cape Town-based Surcotec, has a large portfolio of engineering and thermal spray coating solutions that extend component life cycles to assist original-equipment manufacturers and end-user clients across Southern Africa in reducing costs and increasing production.

“Thermaspray’s proven reputation of quality solutions and expertise has established strong business relationships with our customers who trust our precision- engineering capabilities and skills set to provide optimum solutions tailored to their individual requirements,” Lourens concludes.