Comco Inc

Turbine Blade Regeneration Using MicroBlasting

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Courtesy of Comco Inc

Goal: Clean Turbine Blades for Refurbishment
Whether you’re refurbishing landbased gas or jet-engine turbines,  MircoBlasting is the ideal method of cleaning turbine blades. When turbines are split in two for maintenance, hot path components and combustion components are pulled out for repair. These parts take in and distribute cooling air within a turbine through multiple small holes, which can number more than 140 per component and be as small as 0.08 mm (.003″) in diameter. The holes are often present in airfoils and shrouds of many first- and second-stage components. Because of the complex geometry of these parts, MicroBlasting is ideal for cleaning coating and contaminants from them.

Why MicroBlasting?
Hot path and combustion components in turbine blades need to be protected from the high temperatures to which they are subjected. To achieve this the blades are coated with layers of a thin diffused metallic and/or sprayed ceramic coatings to the external surfaces of the components. Many of these coatings are created from a powder mix consisting of aluminum oxide, halogenated salts and free aluminum flake with some catalysts. It’s then placed in a furnace at temperatures of up to 1093°C. The process creates a metal gas that alloys with the base material. This treatment allows the blades to withstand higher operating temperatures. Unfortunately, the process also creates a significant challenge when the blade needs to be regenerated.

Turbine blades have many small holes that easily become plugged.

The blade surface is engineered to be tough enough to withstand extreme temperatures, meaning in the best of circumstances it’s difficult to remove. This is further complicated by hot air baking oxidization and contaminants into the part. To remove the diffused coating, these parts are commonly cleaned in an industrial grit blasting cabinet and then dipped into a chemical bath. This two-step process can result in uneven surface texturing, plugged air holes, and missed spots. In order to remove the coating completely and produce a uniform surface finish both steps may need to be repeated 5-6 times. The hard to reach areas of the blade are problematic. They do not see direct impingement from the grit blast stream and may not get good chemical movement and activation. This is where MicroBlasting can help.

Removing the Protective Layer
To avoid the need for multiple iterations of the cleaning process many turbine manufacturers have added in a MicroBlasting step.  With the inclusion of this process in between the coarse grit blasting and chemical bath, the proper removal and surface finish can be achieved in 1-2 cycles.  In addition to saving time, MicroBlasting also reduces abrasive consumption and chemical dependence.
 
The precision nature of MicroBlasting means that it will not replace grit blasting for bulk removal of the coating.  The pinpoint focus of the MicroBlasting abrasive stream is ideally suited for removing the coating from recessed and protected areas of the blade as well as inside the small air passages.

The precision in MicroBlasting comes from the small nozzles and fine abrasives that are used. With a uniform abrasive stream focused through a nozzle as small as 0.018”, the operator can easily target the coated areas and plugged air holes. The effective abrasive for this application is 25-50 micron aluminum oxide, with is a sharp cutting abrasive. For particulary difficult coatings silicon carbide, our most aggressive media, is required.

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Turbine Blade Regeneration Using MicroBlasting

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