Bull Run Fossil Plant Main Steam Piping Creep Evaluation

Bull Run Unit 1, rated at 950 MW, is the first of four fossil supercritical power plants at TVA; the unit went into commercial operation in 1967. The boiler, built by Combustion Engineering (CE), has a radiant reheat twin divided furnace with tangential-fired burners for burning coal. The unit’s maximum continuous rating (MCR) is 6,400,000 lbs/hr of main steam flow, with a design temperature of 1003°F and pressure of 3840 psig. Through the end of November 2003, the unit had a total of 589 cumulative starts and 253,343 operating hours. In 1986 TVA located and repaired extensive cracking in the mixing link headers (27 of 32 saddle welds cracked) downstream of the superheater outlet headers. Visible sag was also noted at the mid-span of the mixing headers. Since that time through 2003, additional cracking of girth welds in the mixing link headers was discovered, followed by cracking in the main piping girth welds at the connections to the mixing headers and at one of the connections to the turbine. From 1988 through 2003 several elastic analyses which were performed were unable to explain the observed girth weld cracking and sagging in the piping. In October 2003, TVA contracted with Structural Integrity Associates (SI) and BW Roberts Engineering Consulting to perform elastic and creep analyses of the Bull Run main steam piping system to determine the most likely contributing factors to noticeable creep sagging and cracking problems in the mixing header link piping and main steam piping girth welds, and, to develop recommendations to mitigate additional cracking and creep/sagging. The evaluations concluded that improper hanger sizing along with longer-term hanger operational problems (non-ideal loads/travel, topped/bottomed out hangers) contributed to the observable creep sagging and girth weld cracking. The elastic and creep piping analyses performed to address these issues are described in this paper.