Turbine developments |
were manufactured from 3/8” thick plate and individually fitted to each blade. They were welded into position using Inconel 82 filler material and polished for inspection. A dye penetrant inspection was used to check for surface defects and volumetric examination was performed by radiography.
Figure 4 shows Stellite bar-nose inclusion. Once all blades had passed inspection, they had to be stress relieved. Critical to the success of the weld repair is the development of the appropriate heat treatment process. As rotating parts suffer from cyclic stressing in a hostile environment it is essential to control the material hardness and metallurgical structure of both the components and weld filler material, with attention given to the materials structure within the heat affected zone (HAZ). A bespoke blade furnace (Figure 5) is used for stress relieving turbine blades. The design enables only the repair zone to be stress relieved, eliminating the need to apply any heat to the blade root.
Above: Figure 4. Stellite bar-nose inclusion
Numerous trials were conducted to calculate the correct positioning of the blades within the furnace to ensure sufficient heat deposition was maintained throughout the repair zone. The final heat treatment procedure was documented and followed to ensure repeatability throughout the repair cycle.
After all blades had been successfully stress relieved, they were polished and inspected to ensure the repair had been completed successfully. First, blades were hardness checked across various locations to ensure the material was within the required hardness range. Using the sample data taken prior to the repair in conjunction with the relevant WPQR we could determine that the heat treatment processes had not adversely affected the microstructure of the base material.
Above: Figure 5. Blade furnace
The final inspection methods were dye penetrant to ensure no surface defects had propagated during stress relieving and then a full set of dimensional data was captured and cross referenced against the ‘goods-in’ figures. This data covers axial and radial position of each blade and chord widths & max thicknesses at various section points throughout the repair length.
Finally, the shroud was machined to reinstate the correct geometry and achieve the required tolerances.
Figure 6 shows the repaired blade. Success achieved
Once the blades had been repaired, the power plant installed all six blades into one of their units to test the repair performance over a prolonged period. They also had a small quantity of new blades coated and induction hardened by other sub-suppliers to establish which option offered the greatest level of erosion protection. Figure 7 shows the condition of an EthosEnergy repaired blade after 60 months in operation. There is very light erosion on the leading edge. In contrast, the other repaired blades were severely eroded.
In 2016, EthosEnergy repaired another four blades to enable the plant to undertake a second performance test in one of their other units. Only very light erosion was found to be present on the leading edges after three years of operation.
Reflecting this success, in 2020 the power plant placed an order with EthosEnergy for the repair of 860 blades, the company’s largest ever order for blade repairs.
Z lock shroud was CNC machined to reproduce the correct profile. This was critical to ensure the blades can interlock during installation
The blades repaired in 2014 are still in operation. Whereas the OEM’s blades had been lasting, at most, eight years the blades repaired by EthosEnergy have comfortably surpassed that.
Fully repaired leading edge incorporating Stellite bar-nose for enhanced erosion protection
Repairing the blades is estimated to achieve a saving of about 70% relative to new blades. Also, the longer life of the EthosEnergy repaired blades increases periods between outages, boosting power plant output, and reduces expenditure on repairs. Overall, the result is improved power plant profitability.
Above: Figure 6. Repaired blade
Above: Figure 7. Condition of repaired blade after 60 months operation. Very light erosion on leading edge
28 | July/August 2022|
www.modernpowersystems.com
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