The installed base of advanced gas turbines is increasing rapidly due to increasing fuel costs and environmental regulations. Higher temperatures, as well as mechanical stresses, require the implementation of advanced technology components and refurbishment procedures.
Advanced gas turbine components are subjected to high mechanical and severe environmental loads. Among others, creep, thermomechanical fatigue and hot corrosion are main damage mechanisms that lead to degeneration of hot section components.
Life extension by the implementation of advanced refurbishment procedures will significantly reduce operational costs. Generically, refurbishment activities consist of stripping and reapplication of the coating, rebuilding of the geometry by welding and rejuvenation of the material condition by appropriate heat treatments.
If the component’s wall thickness is reduced by e.g. hot corrosion or excessive cracking due to thermal strains, conventional repair procedures are not sufficient to guarantee the component’s reliability. Advanced refurbishment techniques are needed to extend the component’s life. To limit potential damage reoccurrence during a next turbine exposure, it is essential to analyse and determine the cause damage and respective applicable damage mechanisms.
Advanced refurbishment procedures include changes in materials, coatings or designs and should reduce potential risks of failure or reoccurrence.
To successfully develop and implement advanced refurbishment techniques, dedicated in-house research and test methods are essential.
Advanced vane repair
In many advanced gas turbines, life of hot section components is limited or reduced due to the high operational temperatures and stresses. Some critical components such as first stage rotating and stationary components show extensive operational degradation. Based upon a detailed damage assessment, comprising of a series of detailed metallurgical inspections, it has been determined that the extensive and critical cracks are caused by an unequal temperature distribution in the vane airfoil.
The thermal strains in combination with the (originally applied) “brittle” MCrAlY overlay coating and the poor properties of the base alloy, leads to this type of extensive cracking, even with a limited number of start/stops. Conventional refurbishment techniques are insufficient and result in rejection of these components and in financial consequences.
Sulzer Elbar has successfully developed effective refurbishment procedures which have resulted in full refurbishment of components where severe operational damages is evident.
Key to success is the continuous development of advanced refurbishment procedures. This results in a fully refurbished component by reducing thermal strains and applying higher grade materials with superior/improved thermomechanical properties.
All advanced refurbishment procedures are fully tested and QA approved before actual application. In the case of the vane as shown, a dedicated test stand was developed to verify the refurbishment process and operational start/stop conditions.
Sulzer Elbar has designed a test stand for the selection of nickel base super-alloys with the best LCF properties. In this test, a tapered test piece is repeatedly heated and cooled in quick succession. The temperature gradients result in thermal strains. After a particular number of thermal cycles, the material starts to crack. The crack length as a function of the number of cycles is recorded per base material. Typical results of the test are shown in. Evidently the selected patch compared to vane base material shows a superior LCF resistance.
The degraded vane material was outside technical limitations for restoration and therefore the complete leading edge had to be replaced. Custom made replacement leading edges were manufactured and joined by laser fusion welding. Sulzer Elbar has extensive in-house capability on laser fusion welding which resulted in a high quality, crack free joint with superior mechanical properties and limited distortion.
To further extend operational life after refurbishment, a dedicated and improved coating scheme was selected and a TBC top coating was applied to limit the amplitude and the differences in thermal strains over the entire vane. By means of this, the LCF resistance is significantly improved. To ensure the optimal coating adherence, dedicated in-house testing was carried out.
The vanes which have been refurbished by the use of the advanced refurbishment procedure as noted, have been installed into the gas turbine and taken into successful operation. No indications or damages were observed.
Sulzer Elbar offers many similar types of advanced component refurbishment solutions demonstrating that a combination of knowledge in material science, skilled workmanship and highly developed in-house processes create a high added value as well as lowest cost of ownership to operators and owners of gas turbines, steam turbine and compressors. It is the pre-requisite to extend component life, in a reliable and cost effective way.
René Vijgen
Sulzer Elbar |