Last year, TurboCare was employed to repair SCC on a 1,100 MW fossil power train at a major utility plant. These plants depend upon turbines that have been in operation more than 30 years with super-critical inlet steam (3,500 psig, 1,000F). A typical configuration consists of high pressure (HP) and intermediate pressure (IP) turbines driving a generator at 3,600 rpm and two separate low-pressure (LP) turbines connect to a 1,800 rpm generator. Each of the LP turbines has a double flow configuration. The stages most at risk from SCC are typically the second and third stages (L-2 and L-3) upstream from the last stage blade.
During the scheduled spring maintenance outage, nondestructive examination revealed nearly 200 indications of SCC in the L-2 and L-3 disks on the two LP rotors. Repair options were prioritized according to the customer's desire to maintain its outage schedule, to the degree possible, and minimize any reduction in power generation.
The L-3 stages were the simplest to repair because the cracking was restricted to the notch area of the disk. Probabilistic analysis found that if the turbine operated as-is, there would be an unacceptably high probability of failure. Using three pins to connect the notch blade directly to the wheel reduced the probability of failure within five years to below 1 percent.
For L-3, the utilization of titanium blade material was considered, since this reduces the blade weight by approximately 40 percent. If all of the blades are made with titanium, the reduction in SCC susceptibility is usually significant enough to prevent a reoccurrence within the life of the rotor. In other applications, titanium is used only at the entrance slot. SCC generally initiates at that location first and in many applications is confined to this area.
Switching to titanium notch blades (43 percent lighter than steel) and pinning them to the wheel brought the probability of failure at this plant to below 0.01 percent in 10 years, and below 0.65 percent in 20 years. The customer opted for the titanium blades. The cracks on the wheel hook fillet radii were removed by local grinding to prevent further crack propagation.
The L-2 stages were a different story. Inspection showed 98 indications of cracking on one rotor and 78 on the other. The cracks were distributed throughout the wheels and had a depth that ranged up to 0.39". The best solution for this condition was designing a longshank replacement blade. All the blades were removed, the wheels were machined to eliminate the cracks, and new blades designed with ultra-longshanks, manufactured, and installed. To reduce the chance of SCC in the future, five titanium blades were installed at the notch. Five more titanium blades were installed 180º opposite from the entrance notch to ensure balance. The job was completed on-site with minimal impact on the outage schedule.
SCC poses a major threat to safe turbine operation. However, as this experience demonstrates, resolving SCC does not need to involve a greatly extended outage. By modeling the turbine's operating conditions and conducting a thorough analysis of the repair options, the correct mix of solutions can be found to minimize costs and downtime, without risking an unplanned future outage, or unnecessarily sacrificing electrical output.
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