STEAM TURBINE REPAIRS | BALANCE OF PLANT
LP blade erosion: repair proves better than replacement
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
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
Jon Twiggs
Facility Operations Director, Steam Turbine Repair Centre, EthosEnergy, UK
the last stage blades (see Figure 1 below), 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 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 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. U
Above, figure 1: Severe steam erosion on leading edge of LP last stage blade
www.neimagazine.com | September 2022 | 27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49
