Gas turbines offer exceptional flexibility to power generation networks but frequent starts/stops increase the importance of monitoring and maintenance, especially if the site poses a risk of increased corrosion.
For one Californian municipality, the location of two of its 20 MW gas turbine generators presented a significant corrosion problem, being only a mile from the ocean. At this plant, when the turbine was shut down, a daily occurrence, the air inside cooled and condensation formed inside the housing, causing corrosion of exposed metallic parts.
In fact, corrosion combined with daily cycling had led to the loss of a stator blade and its ingestion into the compressor, with subsequent travel through the turbine.
In this situation, the turbine operator is required to consult the approved suppliers’ list for a company that can repair the equipment in the most cost-effective way. Sulzer was called upon to investigate the failure and to complete an overhaul on site. At this point, the extent of the damage was unknown, since the borescope scan had produced limited information.
“Originally, there had been approximately 40 days available for the external works to be completed”, said Tony Dunkle, area sales manager for Sulzer, “but due to internal communication delays within the municipality, when the project was finally given the green light, there were only 28 days left to complete all the work.”
Sulzer’s engineers advised the turbine operator that the best course of action would be to remove the thermal block (the central rotor assembly and hot gas path components) and have it repaired at Sulzer’s service centre in Houston, TX.
The turbine was located in a very confined space. Sulzer’s field service teams worked on site but were only permitted to work during daylight hours. They prepared the damaged thermal block for removal by a mobile crane, but for the crane to reach the turbine a rail system had to be installed to move the thermal block from the turbine area to a location where it could be lifted and placed onto a truck for transport to Houston.
Having removed the damaged unit, it was inspected by Sulzer’s engineers and deemed to be beyond economic repair. This meant it was necessary to procure a new thermal block or complete turbine, but the lead time for obtaining these components from an OEM is typically considerably greater than 28 days.
However, within Sulzer’s inventory there was another thermal block of the same specification. This had been partially repaired but was no longer required. The workshop team in Houston was able to complete the repairs on this thermal block so that it could be used as a replacement. Scheduling was critical, so that when the time came to remove the damaged thermal block, the replacement needed to be waiting nearby, ready to be craned into position. This ensured the fastest turnaround on the project as well as minimising the cost of cranage for the customer, and meant that the 28-day deadline was met.
Building on the success of the project, overhaul of the second unit is being evaluated, with preventive enhancements to improve the durability of the unit. The plan is to install another exchange thermal block but with the addition of specialist coatings to prevent corrosion.
