A project at a four unit VVER-440 nuclear power plant has demonstrated the effectiveness of repairs for free standing low-pressure steam turbine blades. The repair entails butt welding of bar-nose erosion shields made of Stellite 6B to the leading edges of the low-pressure blades that restores the aerofoil geometry whilst offering superior erosion protection, writes Jon Twiggs, Facility Operations Director, Steam Turbine Repair Centre, EthosEnergy, UK.
Blade furnace
In 2011, EthosEnergy was approached to investigate and prepare an engineering assessment as to the feasibility of repairing the last stage blades of 250 MW steam turbines at a four-unit nuclear power plant in eastern Europe. The blades were approximately 1110 mm in length, with lower lacing wire holes and interlocking Z lock tip shrouds.
The four nuclear reactors account for more than 40% of the country’s electricity, so the plant is in great demand. Due to severe steam erosion on the leading edges of the last stage blades, the power plant was forced to replace L-0 blades approximately every 6-8 years. Erosion was apparent after as little as 18 months, and by the three-year mark, it was becoming a significant cause for concern.
The engineering team at the plant had explored various leading-edge protection options but nothing provided the life extension they were looking for. The OEM had settled on supplying blades with a hardened leading edge coupled with a metal spray coating to offer better protection, but this too had limited success.
EthosEnergy was approached to assess whether its technique of incorporating a solid Stellite bar-nose into the leading edge would provide better longevity and in turn reduce the power plant’s operating costs.
The plant had substantial stocks of used blades that could be repaired once a solution had been developed. It gave EthosEnergy, as well as the OEM and other ISPs the opportunity to work on trial blades.
Since the power plant was responsible for such a large share of national electricity production, any alteration to the existing technology and production regime had to undergo a lengthy and rigorous approval process. It would also need to be competitive.
In March 2011 EthosEnergy took receipt of two blades for repair development. During the next couple of years, EthosEnergy repaired several blades which the plant assessed and put through its internal performance analysis programme.
Upon completion of the plant’s internal test programme, the blades were deemed suitable for operation and in September 2014 EthosEnergy received a further six blades for repair. The plant wanted to repair the blades and then install them into one of their units to assess performance in operational conditions. At the same time, they wished to evaluate the performance of blades worked on by the OEM and other ISPs.
The repair process
Prior to starting the repair, it was critical to fully inspect the blades, their aerofoil geometry and stacking position. This was achieved by reverse engineering the blade root and manufacturing a bespoke fixture that replicates how the blades are held in the rotor.
The root fixture is then mounted into a Bohler gauge and aerofoil profile plates are manufactured at various section positions throughout the repair length. These plates are used to check the aerofoil geometry and stacking position before and after the repair to demonstrate no distortion has occurred.
It was also important to check the position of the lacing wire hole and Z lock shroud, which was also done within the gauging fixture. This data would highlight any anomalies upon goods receipt and demonstrate that each blade had been repaired within the technical specification.
The following inspections were also performed prior to the repair: magnetic particle inspection (crack detection); visual inspection (to identify any anomalies); dimensional inspection (chords, max thickness); and hardness check.
Based on the initial engineering assessment, it was agreed that a Stellite bar-nose (basic size 305 mm x 25 mm), machined to the required aerofoil geometry, would be welded to the turbine blade leading edges. It was also agreed that up to 365 mm of erosion below the bar-nose would be repaired using Jethete M190 filler material.
The erosion below the bar-nose seating was dressed out, dye penetrant inspected and welded. This weld was then dressed to profile and checked using profile gauges to ensure the correct geometry had been achieved.
Due to the level of erosion on the tip shroud, it was necessary to undertake an extensive repair to reclaim the geometry and ensure the blades interlocked during installation. The erosion was removed, and the shroud was welded to allow reprofiling.
Figure 2 shows the shroud erosion removed and Figure 3 the shroud welding in progress.
Once the shroud repair was complete the leading edge was prepared in readiness for the Stellite bar-nose. Stellite 6B bar-noses 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 the Stellite bar-nose inclusion.
Once all the 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 particular attention given to the structure of the material within the heat affected zone (HAZ).
A bespoke blade furnace 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.
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. 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 Welder Performance Qualification Record (WPQR) we could determine that the heat treatment processes had not adversely affected the microstructure of the base material.
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 the axial and radial position of each blade and chord widths & maximum thicknesses at various section points throughout the repair length.
Finally, the shroud was machined to reinstate the correct geometry and achieve the required tolerances.
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.
EthosEnergy repaired-blade after 60 months in operation showed that 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 its other units. This time 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.
Whereas the OEM’s blades had been lasting, at most, eight years the blades repaired by EthosEnergy have comfortably surpassed that. Indeed, the blades repaired in 2014 are still in operation.
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 reducing expenditure on repairs. Overall, the result is improved power plant profitability.
This article first appeared in Nuclear Engineering International magazine.