Optimal turbine-bucket tip clearance promotes top power, efficiency

Excessive turbine-bucket tip clearance costs you performance, began Lloyd Cooke of Liburdi Turbine Services Inc, Dundas, Ont, Canada, but insufficient clearance is conducive to damage caused by rubbing of the buckets against shroud blocks. Your goal is to provide the optimal clearance for maximum efficiency and power output.

To illustrate the importance of tip clearance, Cooke said that a gap of 40 mils between bucket tips and shroud blocks decreases power output by 1%—or about 400 kW for the typical 40-MW Frame 6B gas turbine. This could penalize a base-load engine upwards of $1 million annually, possibly more, depending on the cost of power.

Perhaps the best way to optimize tip clearance, Cooke continued, is to specify a high-strength casting (single crystal or directionally solidified alloy) for first-stage turbine buckets and apply an “engineered blade tip” of different material during airfoil manufacture.

Latter is designed for superior oxidation resistance and sometimes contains embedded abrasive particles to grind into abradable coatings on shroud blocks, thereby creating the optimal clearance. The tip is applied as a powder metallurgy or sintered pre-form material and fused to the tip in a vacuum furnace.

 

It’s not uncommon for a turbine bucket to suffer rub wear and high-temperature oxidation at the tip, Cooke told the group. Original alloys and conventional coatings are unable to prevent oxidation in high-performance engines. To compensate for metal loss—which can open tip clearances by as much as 30 to 60 mils—application of an oxidation-resistant weld alloy was recommended by the metallurgist.

He suggested a nickel-based alloy with higher aluminum content than the original casting—specifically Liburdi’s L3667—which is applied using an automated welding process. More than 15 years of experience with this material, Cooke said, shows repaired buckets can achieve a full 24,000-hr service interval with no metal loss.

Buckets are then repairable for additional service—possibly up to more than 100,000 hours. A series of photos illustrating repairs using conventional weld alloy, total tip replacement using conventional weld alloy, and repairs using L3667 supported Cooke’s recommendation.

To minimize Stage 2 and 3 clearances, he presented the details on the company’s honeycomb seal mod for outer shroud blocks and the advanced cutter teeth it welds to buckets. Examples of these solutions were from Liburdi’s work on 7EA and 7FA engines.

Cooke pointed out some of the deficiencies of the OEM’s original cutter-tooth design, which was conducive to friction heating of the rail and transfer of rail material to the honeycomb seal. He showed how the Liburdi pre-grooving solution with a stellite cutter installed on one quarter of the buckets provided sufficient clearance between honeycomb and rail on both sides and top surfaces to prevent overheating.

Next topic: Shroud blocks. Mature engines suffer turbine-casing distortion over time, Cooke continued. Casing radial dimensions, and tip clearances, are tighter at split lines because of the rigidity of bolting flanges. Inspections reveal that shroud blocks at top and bottom dead center usually are not rubbed because there is excessive tip clearance at these points and close to them.

Thus, the strategy of simply restoring bucket tip heights is defeated by shroud rubs incurred because of out-of-round casings. The Liburdi solution here is to tailor shroud-block thicknesses to accommodate casing distortion by forming a close-to-cylindrical arc for the turbines blades. Cooke explained how casing measurements are taken to determine shroud-block radial adjustments.

The composition of shroud coatings was the final subject of the presentation. Early shroud-block coatings, Cooke said, were dense MCrAlY or ceramic thermal barrier coatings (TBCs). Both were not truly abradable and contributed to bucket-tip metal loss when there was contact between the airfoils and the shroud.

Today’s shroud coatings typically are abradable—that is, friable and sacrificial during a rub. But they must be oxidation resistant as well. High-porosity TBCs applied by air plasma spray can be used for internally cooled F-class blocks. But they are not appropriate for uncooled E-class blocks because of their high hardness.

A high-porosity MCrAlY top coat applied to uncooled E-Class blocks by air plasma spray is recommended because it is both abradable and oxidation resistant. The first pare of the two-part coating consists of a MCrAlY bond coat for oxidation resistance and adhesion. The 25% to 35% porosity top coat is achieved with polyester sacrificial fill.

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