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Pipe Weld Improvements Saves Power Plant Costs
Article Posted On 10/23/2009
Most power plants around the country are operating well beyond their original design life of 30 to 40 years. This is necessitated by the prohibitive cost of building new facilities. In order for these older plants to provide reliable power, it is necessary to find ways to prevent the failure of their aging tubes and pipes. This can be a challenge, however, because each power generation station has pipes made from different kinds of steel welded together in places which are prone to breaks.
With funding in part from PITA, Lehigh University Professors John DuPont and Joachim Grenestedt are leading a team of Lehigh researchers from the Departments of Materials Science and Engineering and Mechanical Engineering to address the problem of failures in these "dissimilar welds." They are working in collaboration with PPL Corporation. Headquartered in Allentown, PA, PPL controls or owns more than 12,000 megawatts of generating capacity in the United States, sells energy in key U.S. markets, and delivers electricity to about 4 million customers in Pennsylvania and the United Kingdom.
In generating stations, both carbon steel piping and stainless steel piping are used. Inexpensive carbon steel pipes are generally used at locations of low stress and temperature where strength and corrosion resistance are not a primary concern. Other locations within the plant operate under higher temperatures and more corrosive atmospheres that require the use of stronger and more corrosion resistant stainless steels. At some point within the plant, these two types of alloys must be welded directly together, and a "dissimilar weld" is formed. Unfortunately, the sharp change in alloy composition that occurs across these connections leads to dissimilar weld failures, which are a common cause of forced outages for power plants. The cost associated with them is significant, as a single failure can cost a power company up to $850,000 per day in downtime and lost revenue.
DuPont and Grenestedt are creating a method for avoiding failures by developing design methods that can be used to establish the optimal change in composition required across the weld joint and developing fabrication methods with the required changes in alloy composition. Their method creates a "graded transition joint" that eliminates the sharp changes in composition associated with dissimilar welds, thus eliminating the source of failure.
With this approach, small pipes are fabricated that match the carbon steel composition on one end, the stainless steel composition on the other end, and then have a gradual change in composition across the pipe. The small transition pipe can then be inserted between the carbon steel and stainless steel pipes, and two similar welds can be made in place of the one failure-prone dissimilar weld. Different types of transition joints are currently being fabricated and tested with the goal that PPL Corporation will eventually install the graded pipes into its plants for long-term service testing. If successful, these transition joints can lead to significant savings associated with eliminating costly dissimilar weld failures. The results are also expected to be applicable to nuclear power generation plants.
