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ENGINE & TURBINE TECHNOLOGY


fluid and non-destructive evaluation (NDE) penetrant fluid. Tese contaminants are removed via chemical or thermal means in the “degrease” process. From this point, extreme care is taken to ensure contaminants are not re-introduced to the substrate that would jeopardise the bonding of the coating. Te next step is to prepare the surface to accept the new protection system using grit profiling. Tis process roughens the target surface of the component, creating an “anchor-tooth pattern” for the coating to mechanically bond to. Tis profile is attributed to the type and size of blast material used in this process, which will depend on both the substrate and the coating to be applied. However, in all cases care should be taken to use virgin grit as opposed to re-used grit to prevent contamination, which can result in a poor-quality bond or even diffusion of contaminants into the base material. At this point the equipment involved


starts to become more complicated – and for good reason. Te application of both base and top coats requires considerable accuracy and precision to ensure the right amount of coating is applied to the correct areas and with the specified characteristics. Industrial robot arms, controlled by positioning software, work in conjunction with custom holding fixtures to give a consistent application.


OPTIMISING QUALITY Robotic application can, if done properly, provide a leap forward in quality control and consistency when compared to manual processes. It is important to have a thorough understanding of the fundamentals of coating application prior to program development. Without these fundamentals, robotic programming may result in a false sense of quality. Once applied, the base coat


in some cases requires heat treatment – the temperature, duration and type of furnace will depend on the coating and the substrate material. Once again, accuracy in all aspects of this process is crucial in achieving a successful outcome. Following any heat treatment process, it is essential that a NDE is completed to ensure that no voids opened during the heat treatment process. Tis will typically be a penetrant inspection using red dye or even fluorescent dye to detect even the slightest defect.


FINAL DIMENSIONS


When applicable, a top coat, typically a thermal barrier coating (TBC), is applied in a similar quality-controlled manner as the bond coat. After this application is complete, it is important to carefully remove any overspray and polish the coating so that it meets the specified surface roughness. Te final quality inspection should identify any areas that may need minor repairs and confirm that all the required specifications have been met. Following the coating inspection, test fitting or dimensional checks should be performed to ensure that the coating has not pushed the dimensions of the component out of specification. If a third party is being used, they should be involved with this process. For components with cooling channels, any change in flow rate can lead to decreased turbine efficiency, overheating of components, and even failure. Terefore, it is critical that flow checks


are performed once more to ensure coating, grit or any other foreign matter has not caused the component’s cooling air to flow below its specified rate. During these post- coating processes and any further handling of coated components, it is important to ensure that the coating remains protected and in pristine condition until the component is reinstalled. Tis is particularly important for brittle TBCs.


Turbine liner sections benefit from thermal barrier coatings


20 www.engineerlive.com


IMPROVING PERFORMANCE Te performance of an industrial gas turbine is dependent on specialised


Gas turbine components would quickly overheat without specialised coatings


protection systems, such as TBCs, without which the base materials would quickly overheat and fail; efficiency is maintained by abradable coatings between the blade tip and shroud; hardface coatings reduce wear mechanisms on the substrate; anti-corrosion protection improves durability and prolongs the service life of the machine. In each case, a specialised coating enhances the performance of a component, but each one is different and the processes to apply them vary as well. Only through years of experience and expertise in the metallurgical properties, the application technology and quality control procedures, can a reliable and durable protective system be realised.


FINDING THE BEST SERVICE With such a complex procedure that demands precision and attention to detail throughout, the application of specialised coatings requires a considerable amount of expertise and experience to achieve the best results. Tis also needs to be coupled with rigorous quality control systems and a very close partnership with the component repair engineers upstream of the process. Working with an experienced service


provider, such as Sulzer, can offer peace of mind when it comes to refurbishing gas turbines. By continuously striving to improve procedures to perfect repair processes, Sulzer aims to maximise productivity and keep operating costs to a minimum.


Garret Haeglin is with Sulzer. www.sulzer.com


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