By David Hughes, Vice President, Day & Zimmermann
Retrofitting nuclear power plant turbines has a positive impact on long-term plant reliability and performance. In the case of the Donald C. Cook Nuclear Generating Station in Michigan, a retrofit has been scheduled to support plant operation beyond the current license period. Achieving the long-term benefits of this retrofit presented some short-term challenges. Turbine retrofit projects are costly, complex and, when not properly managed, can result in lengthy outages. Knowing this, American Electric Power (AEP) employees at the plant were looking for ways to minimize outage time for the upcoming retrofit scheduled for 2016.
|Day & Zimmermann and AEP installed a second bridge crane in the operating Cook nuclear plant in advance of a planned maintenance outage.|
Among the projects to effectively execute the retrofit was the installation of a second bridge crane in the turbine building. The crane would be used to move much of the heavy material needed to perform maintenance on the turbine, while allowing the existing large turbine crane to be free to support other outage activities. Typically, this installation would be planned into the outage and could take up to 10 days to complete. Additionally, standard industry practices include a labor-intensive solution that would involve either the partial removal of the turbine building roof or the removal of the end wall to allow for materials to be brought into the turbine building.
AEP Manager of Projects Scott Dailey and his team wanted to see if they could find a way to eliminate this extra outage time by installing the additional crane while the plant remained online. To do this, they would have to develop a unique approach that minimized risk to the station and maximized efficiency. In 2012, with the retrofit outage still years away, AEP turned to its engineering and crane partners and to its on-site maintenance contractor Day & Zimmermann for solutions.
Innovative Component Rotation Solution
The process of lifting heavy materials while a plant is still online is a high-risk procedure that requires careful planning. The AEP team was keenly aware of the 2013 accident at Arkansas Nuclear 1 in which the stator of the unit’s old generator fell during a maintenance procedure. To prevent a similar incident during the bridge crane installation, AEP began planning for this project in 2012.
AEP and its engineering partner developed an installation solution using a unique method for component rotation in an operating plant that was less invasive to plant structures. The innovative approach used multiple telescoping gantry systems and a hydraulic turntable to safely lift and rotate components for the 250-ton bridge crane. The configuration of the gantry system used in this project was an engineered temporary lift assembly. Two 88-foot girders weighing as much as 110,000 pounds each were lifted 60 feet above the turbine floor and placed in position on the existing crane rails between two operating units.
Because this was a first-of-its-kind application, a proof of concept was required. In January 2014, about six months before the crane’s scheduled installation, the project team, which included AEP, various crane and installation vendors, and Day & Zimmermann, worked with the utility to perform an off-site lifting demonstration. The team simulated each major component lift and positioning movement. The demonstration was done outside which added weather elements to a process that would ultimately be completed indoors. Additionally, the load was intentionally taken beyond the operational limits established to confirm there was sufficient margin to manage potential negative scenarios during installation. ASME NQA-1-2012 requires a dynamic load test equal to 110 percent of the rated load, but the AEP team chose to proceed with a more conservative 125 percent rated load test because of the risk involved with performing these lifts on the turbine deck between two operating units.
The gantry lifts were constantly monitored by LED levels and surveyors at multiple stations to confirm the operation remained within established limits. The stability of the configured system was validated along with the operating team’s ability to control the loads within levelness criteria.
Following the planning phase, AEP worked with its vendors to complete the actual installation. The project was completed on time and under budget with no safety incidents. Collaboration between Day & Zimmermann, AEP and the other project vendors was a key component in delivering a safe and efficient installation. From start to finish, the project took 18,000 work hours.
Keeping the plant online during the installation saved AEP significant time and money. The second bridge crane is now a permanent fixture in the turbine building, and will help AEP save time and money in future outages.