Turbine developments |
LP blade erosion: repair proves better than replacement
A project at a 4 x VVER-440 nuclear power plant has demonstrated the effectiveness of EthosEnergy’s repair procedures 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. The solid bar-nose restores the aerofoil geometry whilst offering superior erosion protection for future operation
Jon Twiggs Facility Operations Director, Steam Turbine Repair Centre, EthosEnergy, UK (
jon.twiggs@
ethosenergy.com, +44 (0) 1905 459570)
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’ electricity, so the plant is in great demand.
Due to severe steam erosion on the leading edges of the last stage blades (Figure 1), the power plant was forced to replace L-0 blades approximately every 6-8 years, with erosion apparent after as little as 18 months, and by the three-year mark becoming a significant cause for concern.
Above: Figure 1. Severe steam erosion on leading edge of LP last stage blade
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.
Above: Figure 2. Shroud erosion removed
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 be competitive.
Above: Figure 3. Shroud welding in progress
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 their 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
26 | July/August 2022|
www.modernpowersystems.com
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 Jet Hete 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 welded to allow reprofiling.
Figure 2 shows shroud erosion removed and Figure 3 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
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